This PDF file contains the front matter associated with SPIE Proceedings Volume 9551, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.

The spin to charge current conversion (SCCC) due to spin-orbit coupling (SOC) opens the way to manipulate the magnetization by electrical means. SCCC results either from bulk effects, in particular through Spin Hall Effect (SHE) [1-3], or from interfacial effects, for example in our recent experimental discovery of Rashba-Edelstein Effect (REE) [4]. We have investigated the SCCC by SHE in metals such as Pt [1,2], Au and AuW [3]. In the case of 2D systems, we recently demonstrated a large SCCC efficiency in Rashba type Ag/Bi interface [4]. An even larger effect can be even anticipated if topological insulators are used. We will present a practical application of this SCCC to control the magnetization due to the spin-orbit torque induced by SHE [5-8], and focus on the case of Pt/(Co/Ni)3/Al multilayers with perpendicular magnetization. Using both the anomalous Hall Effect and Kerr experiments in patterned Hall bars, we are able to establish a scenario for the reversal mechanism involving domain wall motions and spin-orbit torques induced by SHE in Pt. [1] J-C. Rojas-Sanchez et al. PRL 112, 106602 (2014). [2] J.-C. Rojas-Sánchez et al. SPIE 9167, Spintronics VII, 916729 (2014). [3] P. Laczkowski et al. APL 104, 142403 (2014). [4] J C Rojas Sánchez et al. Nat. Comm. 4:2944 (2013). [5] A. V. Khvalkovskiy et al. PRB 87, 020402 (2013). [6] I. M. Miron et al. Nature 476, 189 (2011). [7] L. Liu et al. PRL 109, 096602 (2012). [8] N. Perez et al. APL 104, 092403 (2014).

The discovery that a current flowing in a heavy metal can exert a torque on a neighboring ferromagnet has opened a new way to manipulate the magnetization at the nanoscale. This “spin orbit torque” (SOT) has been demonstrated in ultrathin magnetic multilayers with structural inversion asymmetry (SIA) and high spin orbit coupling, such as Pt/Co/AlOx multilayers. We have shown that this torque can lead to the magnetization switching of a perpendicularly magnetized nanomagnet by an in-plane current injection. The manipulation of magnetization by SOT has led to a novel concept of magnetic RAM memory, the SOT-MRAM, which combines non volatility, high speed, reliability and large endurance. These features make the SOT-MRAM a good candidate to replace SRAM for non-volatile cache memory application. We will present the proof of concept of a perpendicular SOT-MRAM cell composed of a Ta/FeCoB/MgO/FeCoB magnetic tunnel junction and demonstrate ultra-fast (down to 300 ps) deterministic bipolar magnetization switching. Macrospin and micromagnetic simulations including SOT cannot reproduce the experimental results, which suggests that additional physical mechanisms are at stacks. Our results show that SOT-MRAM is fast, reliable and low power, which is promising for non-volatile cache memory application. We will also discuss recent experiments of magnetization reversal in ultrathin multilayers Pt/Co/AlOx by very short (<200 ps) current pulses. We will show that in this material, the Dzyaloshinskii-Moryia interaction plays a key role in the reversal process.

Spintronics aims to utilize the coupling between charge transport and magnetic dynamics to develop improved and novel memory and logic devices. Future progress in spintronics may be enabled by exploiting the spin-orbit coupling present at the interface between thin film ferromagnets and heavy metals. In these systems, applying an in-plane electrical current can induce magnetic dynamics in single domain ferromagnets, or can induce rapid motion of domain wall magnetic textures. There are multiple effects responsible for these dynamics. They include spin-orbit torques and a chiral exchange interaction (the Dzyaloshinskii-Moriya interaction) in the ferromagnet. Both effects arise from the combination of ferromagnetism and spin-orbit coupling present at the interface. There is additionally a torque from the spin current flux impinging on the ferromagnet, arising from the spin hall effect in the heavy metal. Using first principles calculations, we identify spin-orbit hybridization at the ferromagnet-heavy metal interface as central to the spin-orbit torques present in Co-Pt bilayers. We additionally propose that the transverse spin current (from the spin hall effect) is locally enhanced over its bulk value due to scattering at an interface which is oriented normal to the charge current direction.

Spin orbit coupling at interfaces can give rise to chiral magnetic textures such as homochiral domain walls and skyrmions, as well as current-induced torques that can effectively manipulate them [1-3]. This talk will describe interface-driven spin-orbit torques and Dzyaloshinskii-Moriya interactions (DMIs) in ultrathin metallic ferromagnets adjacent to nonmagnetic heavy metals. We show that the DMI depends strongly on the heavy metal, differing by a factor of ~20 between Pt and Ta [4], and describe the influence of strong DMI on domain wall dynamics and spin Hall effect switching [5]. We present high-resolution magnetic force microscopy imaging of static magnetic textures that directly reveal the role of DMI and allow its strength to be quantified. Finally, we will describe how SOTs can be enhanced through interface engineering [6] and tuned by a gate voltage [7] by directly controlling the interfacial oxygen coordination at a ferromagnet/oxide interface [8]. [1] A. Thiaville, et al., Europhys. Lett. 100, 57002 (2012). [2] S. Emori, et al., Nature Mater. 12, 611 (2013). [3] J. Sampaio, V. Cros, S. Rohart, A. Thiaville, and A. Fert, Nature Nano. 8, 839 (2013). [4] S. Emori, et al., Phys. Rev. B 90, 184427 (2014). [5] N. Perez, et al., Appl. Phys. Lett. 104, 092403 (2014). [6] S. Woo, et al., Appl. Phys. Lett. 105, 212404 (2014). [7] S. Emori, et al., Appl. Phys. Lett. 105, 222401 (2014). [8] U. Bauer, et al., Nature Mater. 14, 174 (2015).

We investigate the impact of tunnel barrier thickness on electron spin dynamics in Fe/MgO/GaAs heterostructures using spin-resolved optical pump-probe spectroscopy. Comparison of the Larmor frequency between thick and thin MgO barriers reveals a four-fold variation in exchange coupling strength, and investigation of the inhomogeneous dephasing time, T₂^*, argues that inhomogeneity in the local effective hyperfine field dominates free-carrier spin relaxation across the entire range of barrier thickness. These results provide additional evidence to support the theory of hyperfine-dominated spin relaxation in GaAs at low temperature and in the presence of an externally applied magnetic field. Further, this work lays the foundation for engineering both the exchange coupling and the free carrier spin dynamics in ferromagnet/semiconductor heterostructures, allowing for the exploration of dissipation and transport in the regime of dynamically-driven spin pumping.

The Rashba effect [1] describes the momentum-dependent spin splitting of the electron states at a surface or interface. It is the combined result of the relativistic spin-orbit interaction (SOI) and the inversion-symmetry breaking. The control of the Rashba effect by an applied electric field is at the heart of the proposed Rashba-effect-based spintronics devices for manipulating the electron spinfor ma- nipulating the electron spin in the semiconductors. The effect is expected to be much stronger in the perovskite oxides owing to the presence of high-Z elements. In this talk, I will introduce the Rashba effect and discuss how the Rashba SOI at the surfaces and interfaces can be tuned by manipulating the two dimensional electron gas (2DEG) by an applied electric field. The effect can be understood in terms of a tight-binding model Hamiltonian for the d orbitals incorporating the effect of electric field in terms of effective orbital overlap parameters [3]. From first principles calculations we see that the Rashba SOI originates from the first few layers near the surface and it therefore can be altered by drawing the 2DEG to the surface or by pushing the 2DEG deeper into the bulk with an applied elec- tric field. These ideas will be illustrated by a comprehensive density-functional study of polar perovskite systems [4]. References [1] E. I. Rashba, Sov. Phys. Solid State 2, 1109 (1960) [2] A. Ohtomo and H. Hwang, Nature 427, 423 (2004); Z. Popovic, S. Satpathy, and R. Martin, Phys. Rev. Letts. 101, 256801 (2008) [3] K. V. Shanavas and S. Satpathy, Phys. Rev. Lett. 112, 086802 (2014); K. V. Shanavas, Z. S. Popovic, and S. Satpathy, Phys. Rev. B 90, 165108 (2014) [4] K. V. Shanavas, J. Electron Spectrosc., In press (2015)

The spin Hall effect (SHE) and the inverse spin Hall effect (ISHE) are well established phenomena in current spintronics research. A third important effect is the current-induced spin polarization, which, within the Rashba model for a spin-orbit coupled two-dimensional disordered electron gas, has been predicted by Edelstein in 1990 and it is referred to as the Edelstein effect (EE). This effect is deeply connected to the above two effects thanks to a constraint dictated by the equation of motion. Less known is the inverse Edelstein effect (IEE), which is the Onsager reciprocal of the EE and according to which a charge current is generated by a non-equilibrium spin polarization. The IEE has been recently observed (Nature Commun. 4, 2944 (2013)) in a hybrid ferromagnetic-metal system. In this talk I provide a precise microscopic definition of the IEE and its description within the Rashba model. It turns out that the effect has a surprisingly simple interpretation when the spin-charge coupled drift-diffusion equations governing it are cast in the language of a SU(2) gauge theory, with the Rashba spin-orbit coupling playing the role of a generalized spin-dependent vector potential. After sketching briefly the derivation of the drift-diffusion equations, the latter are applied to the interpretation of the experiments. The role of spin-orbit coupling due to impurities is also considered, by showing that the strenght of the IEE can be controlled by the ratio of the spin relaxation rates associated to the two type of spin-orbit coupling.

We consider a semi-classical model to describe the origin of the spin-orbit interaction in a simple system such as the hydrogen atom. The interaction energy U is calculated in the rest-frame of the nucleus, around which an electron, having linear velocity v and magnetic dipole-moment μ, travels in a circular orbit. The interaction energy U is due to the coupling of the induced electric dipole p=(v/c)×μ with the electric field En of the nucleus. According to quantum mechanics, the radius of the electron’s orbit remains constant during a spin-flip transition. Under such circumstances, our model predicts that the energy of the system changes by ΔE=1/2U, the factor 1/2 emerging naturally as a consequence of equilibrium and the change of the kinetic energy of the electron. The correct 1/2 factor for the spin-orbit coupling energy is thus derived without the need to introduce the well-known Thomas precession in the rest-frame of the electron.

We report on theoretical investigations and k.p calculations of carrier tunneling, both electrons and holes, in model systems and heterostructures composed of exchange-split III-V semiconductors, involving spin-orbit interactions. The two media are separated-or not-by a thin tunnel barrier made out of a (III-V) semiconductor. In a 2x2 exchange-split band model, we show that, when Dresselhaus interactions are included in the conduction band of two exchange-split semiconductors in contact in the antiparallel states of magnetization, the electrons are differently transmitted with respect to an axis orthogonal to both normal axis of the interface and of the magnetization. The transmission asymmetry (A) between +k// and -k// incidence is shown to be maximal (A=100%) at some points of the Brillouin zone corresponding to a totally quenched transmission at some given incidence angles. More generally, we derive a universal character of the transmission asymmetry A vs. the in-plane incidence wavevector, the reduced kinetic energy and exchange parameter, A being universally scaled by a unique function, independent of the spin-orbit strength and of material parameters. This particular asymmetry feature is reproduced by a more complete 14x14 band model involving coupling with the conduction band. On the other hand, calculations performed in the valence-band of equivalent model heterostructures and including tunnel barriers in both 6x6 (without inversion) and 14x14 k.p band model more astonishingly highlight, the same trends in the transmission asymmetry (A) which is related to the difference of orbital chirality and to the related branching (overlap) of the corresponding evanescent wavefunctions responsible for tunneling current. In both cases of electrons and holes, the asymmetry appears to be robust and persists only when a single electrode is magnetic.

By performing time-resolved optical non-degenerate pump-probe experiments, we study the relaxation dynamics of spin-polarized excitons in wurtzite epitaxial GaN and in nitride nanostructures. Those materials are indeed promising candidates for spintronic applications because of their weak spin-orbit coupling and large exciton binding energy (~ 17 meV and ~ 26meV in bulk GaN, respectively). In epilayers, we show that the high density of dislocations increases dramatically the spin relaxation of electrons and holes through the defect assisted Elliott-Yafet mechanism. That makes the exciton dephasing time very short. In high quality GaN/AlGaN quantum wells, both the exciton-spin lifetime S and the exciton dephasing-time T2 were determined via pump-probe spectroscopy using polarized laser pulses and time-resolved four wave-mixing experiments. The evolution of both quantities with temperature shows that spin relaxation occurs in the motional narrowing regime up to 80 K. Above this threshold, the thermal energy becomes large enough for excitons to escape from the QW. Such measurements demonstrate that GaN-based heterostructures can reach a very high degree of control that was previously mostly restricted to conventional III-V semiconductors and more specifically to the arsenide family.

Ultrafast optical spectroscopy can provide insight into fundamental microscopic interactions, dynamics and the coupling of several degrees of freedom. Pump/ probe studies can reveal the answer to questions like “What are the achievable switching speeds in multiferroics?”, “What is the influence of the crystallographic orientation and domain states on the available switching states?”, and “What is the effect of the hetrostructure on promoting the coupling between the varying field excitations?”. In this presentation, we report on two color (400/800nm) ultrafast pump-probe differential reflectance spectroscopy of BiFeO3-BaTiO3 structures to probe the coupling between optical and acoustic phonons to spin waves. The data presented here is a combination of different transient reflectivity measurements to probe both the carrier and spin dynamics. The (001)-BiFeO3-BaTiO3 thin films were prepared using pulsed laser deposition on vicinal SrTiO3 substrates using La0.70 Sr0.30MnO3 bottom electrodes. Crystal orientation and topography were analyzed by x-ray diffraction and atomic force microscopy. . Our results are important to developing devices on the basis of this material system. This work was supported by the AFOSR through grant FA9550-14-1-0376,NSF-Career Award DMR-0846834, and the Virginia Tech Institute for Critical Technology and Applied Science.

Two-dimensional systems offer a rich array of physical phenomena that include the integer and fractional quantum Hall effects, both of which have been observed in multiple materials systems to date. The mitigation and control of coherence in quantum states in 2D systems is an area of great current interest that is critical for the development of the next generation of solid state electronics based on quantum phenomena. In the first experiments that I will discuss, we investigate the terahertz frequency properties of a high mobility (μ ≥ 106 cm2 V-1 s-1) gallium arsenide two-dimensional electron gas (2DEG) at cyclotron resonance in a perpendicular magnetic field, which results in the formation of a spectrum of Landau levels. Our experiments reveal a strong increase in the decoherence at low temperatures and a power law dependence to the decoherence time from T = 0.4 - 100 K. In the second part of the talk, I will discuss our high fluence, nondegenerate pump-probe spectroscopic experiments of GaAs in the Florida Split Helix magnet at 15 K and 25 T. We model the electronic component of our data with an approximate four level system, from which we have extracted scattering and recombination rates in high magnetic field. We also observe coherent phonons, which were isolated and fitted to a sinusoid with an oscillation frequency of 43.5 GHz at 25 T, which is 3.0% larger than the previously measured zero field frequency.

We demonstrate the exfoliation of large-area monolayer flakes prepared from bulk MoS₂ crystals. The flakes are first characterized using Raman and photoluminescence measurements. We then utilize time-resolved Kerr rotation (TRKR) measurements to probe the valley dynamics in the monolayer flakes at low temperatures. This technique allows resonant excitation of the excitonic transitions and yields sub-picosecond resolution. We find valley lifetimes of about 40 ps at a temperature of 4 K in monolayer MoS₂ for resonant excitation. With increasing temperatures, we observe a dramatic decrease of the valley lifetimes, indicating that valley dephasing is mediated by phonon-related scattering processes.

Here we study the exciton valley relaxation dynamics in atomically thin MoS₂ by non-equilibrium optical techniques. A spin polarized excitons population is selectively created in a single valley by circularly polarized ultrashort laser pulses resonant with the optical gap, while the subsequent decay of the valley polarization is measured as a rotation of a linearly polarized probe beam due to a transient Faraday effect. We show that the photoinduced valley polarization in monolayer MoS₂ is quenched after few ps due to an efficient intervalley scattering channel and it displays a peculiar bi-exponential behavior. This rapid time scale is in a good agreement with an intervalley scattering mechanism mediated by an electron-hole exchange interaction. Moreover time resolved circular dichroism experiments performed in the same experimental condition confirms the fast valley relaxation dynamics observed with transient Faraday rotation technique.

In my invited talk the fine structure of neutral and charged excitons for GaAs/AlGaAs quantum dots (QDs) grown on (111) plane as well for transition metal dichalcogenides (TMDCs) monolayers will be discussed. These, at first glance, different systems posses similar trigonal symmetry, which makes exciton fine structure and spin dynamics unusual compared with standard low-dimensional semiconductors. The effects of long-range exchange interaction induced mixing of excitons in two valleys of TMDCs and of magneto-induced mixing of bright and dark excitonic states in trigonal QDs are predicted and confirmed experimentally. Manifestations of excitonic spin/valley dynamics in photoluminescence, pump-probe Kerr rotation and spin noise are discussed. The presentation will be based on the following references: [1] G. Sallen, B. Urbaszek, M. M. Glazov, et al., Dark-Bright Mixing of Interband Transitions in Symmetric Semiconductor Quantum Dots, Phys. Rev. Lett. 107, 166604 (2011). [2] L. Bouet, M. Vidal, T. Mano, N. Ha, T. Kuroda, M. V. Durnev, M. M. Glazov, et al., Charge tuning in [111] grown GaAs droplet quantum dots, Appl. Phys. Lett. 105, 082111 (2014). [3] M. M. Glazov, et al., Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides Phys. Rev. B 89, 201302(R) (2014). [4] C. R. Zhu, K. Zhang, M. Glazov, et al., Exciton valley dynamics probed by Kerr rotation in WSe2 monolayers, Phys. Rev. B 90, 161302(R) (2014).

Monolayer transition metal dichalcogenides, MX2 (M = Mo, W and X = S, Se), are direct-gap semiconductors with some interesting properties. First, the low-dimensional hexagonal structure leads to two inequivalent K-points, K and K’, in the brillioun zone. Second, this valley index and spin are intrinsically coupled, and spin-dependent selection rules enable one to independently populate and interrogate a unique K valley with circularly polarized light. Here we probe the degree of circular polarization of the emitted photoluminescence as function of the photo-excitation energy and temperature to elucidate spin-dependent inter- and intra-valley relaxation mechanisms. Monolayer flakes of MoS2 and MoSe2 show a strong depolarization as the excitation energy is increased. However, WS2 maintains significant polarization for high excitation energies, even at room temperature when properly prepared. We discuss the behavior of the polarization in terms of various phonon assisted intervalley scattering processes. This work was supported by NRL and the NRL Nanoscience Institute

Tunnel barriers are key elements for both charge-and spin-based electronics, offering devices with reduced power consumption and new paradigms for information processing. Such devices require mating dissimilar materials, raising issues of heteroepitaxy, interface stability, and electronic states that severely complicate fabrication and compromise performance. Graphene is the perfect tunnel barrier. It is an insulator out-of-plane, possesses a defect-free, linear habit, and is impervious to interdiffusion. Nonetheless, true tunneling between two stacked graphene layers is not possible in environmental conditions (magnetic field, temperature, etc.) usable for electronics applications. However, two stacked graphene layers can be decoupled using chemical functionalization. Here, we demonstrate homoepitaxial tunnel barrier devices in which graphene serves as both the tunnel barrier and the high mobility transport channel. Beginning with multilayer graphene, we fluorinate or hydrogenate the top layer to decouple it from the bottom layer, so that it serves as a single monolayer tunnel barrier for both charge and spin injection into the lower graphene transport channel. We demonstrate successful tunneling by measuring non-linear IV curves, and a weakly temperature dependent zero bias resistance. We perform lateral transport of spin currents in non-local spin-valve structures and determine spin lifetimes with the non-local Hanle effect to be commensurate with previous studies (~200 ps). However, we also demonstrate the highest spin polarization efficiencies (~45%) yet measured in graphene-based spin devices [1]. [1] A.L. Friedman, et al., Homoepitaxial tunnel barriers with functionalized graphene-on-graphene for charge and spin transport, Nat. Comm. 5, 3161 (2014).

We demonstrate visible-light electroluminescence due to d-d transitions in GaAs:Mn based light emitting diodes (LEDs) [1][2]. We prepared p+n junctions with a p+GaAs:Mn layer. At a reverse bias voltage (-3 to -6V), holes are injected from the n-type layer to the depletion layer and accelerated by the intense electric field, and excite the d electrons of Mn in the p+GaAs:Mn layer by impact excitations. We observe visible-light emission E1 = 1.89eV and E2 = 2.16eV, which are exactly the same as the 4T1 -> 6A1 and 4A2 -> 4 T1 transition energy of Mn. Furthermore, by utilizing optical transitions between the p-d hybridized orbitals of Mn atoms doped in Si, we demonstrate Si-based LEDs that continuously emit reddish-yellow visible light at room temperature. The Mn p-d hybrid states are excited by hot holes that are accelerated in the depletion layers of reverse biased Si pn junctions. Above a threshold reverse bias voltage of about -4V, our LEDs show strong visible light emission with two peaks at E1 = 1.75eV and E2 = 2.30eV, corresponding to optical transitions from the t-a (spin-down anti-bonding) states to the e- (spin-down non-bonding) states, and from the e- to the t+a (spin-up anti-bonding) states. The internal quantum efficiency of the E1 and E2 transitions is 3-4 orders of magnitude higher than that of the indirect band-gap transition [3]. [1] P. N. Hai, et al., APL 104, 122409 (2014). [2] P. N. Hai, et al., JAP 116, 113905 (2014). [3] P. N. Hai, et al., submitted.

The electronic structure of topological insulators (TIs) are well described Dirac-like equations, e.g. the BHZ model, with a mass term that changes sign at some interface. This simplistic description includes a pseudo-spin-orbit coupling that is intrinsic to the Dirac Hamiltonian. Consequently, the TIs share common properties with the Dirac equation. Among them, the interference between positive and negative energy bands leads to the relativistic oscillatory motion known as the Zitterbewegung. Here we discuss the ballistic time-evolution (pico and nanoseconds) of wave-packets in TIs in the presence of an external electric field. We show that the guiding center of large wave-packets have a finite motion transversal to the electric field equivalent to side-jump in Rashba GaAs. However, for narrow wave-packets the dynamics change and the guiding center description is not complete. We also discuss the reflection of a wave-packet colliding with the edge of the system and the effects of the edge states. Acknowledgement: We acknowledge support from CAPES, CPNq, FAPEMIG, FAPESP, and NAP Q-NANO from PRP/USP.

Most theoretical studies of chiral magnetism, and the resulting spin textures, have focused on 3D systems with broken bulk inversion symmetry, where skyrmions are stabilized by easy-axis anisotropy. In this talk I will describe our results on 2D and quasi-2D systems with broken surface inversion, where we find [1] that skyrmion crystals are much more stable than in 3D, especially for the case of easy-plane anisotropy. These results are of particular interest for thin films, surfaces, and oxide interfaces [2], where broken surface-inversion symmetry and Rashba spin-orbit coupling naturally lead to both the chiral Dzyaloshinskii-Moriya (DM) interaction and to easy-plane compass anisotropy. I will then turn to systems that break both bulk and surface inversion, resulting in two distinct DM terms arising from Dresselhaus and Rashba spin-orbit coupling. I will describe [3] the evolution of the skyrmion structure and of the phase diagram as a function of the ratio of Dresselhaus and Rashba terms, which can be tuned by varying film thickness and strain. [1] S. Banerjee, J. Rowland, O. Erten, and M. Randeria, PRX 4, 031045 (2014). [2] S. Banerjee, O. Erten, and M. Randeria, Nature Phys. 9, 626 (2013). [3] J. Rowland, S. Banerjee and M. Randeria, (unpublished).

Metallic interfaces between insulating oxides such as STO-LAO or STO-GTO provide a versatile platform to study two dimensional electron liquids. Numerous experiments have observed magnetism and significant spin-orbit effects in such structures. We construct the general free energy governing long-wavelength magnetism in two-dimensional oxide heterostructures, which applies irrespective of the microscopic mechanism for magnetism. This leads, in the relevant regime of weak but non-negligible spin-orbit coupling, to a rich phase diagram containing in-plane ferromagnetic, spiral, cone, and skyrmion lattice phases, as well as a nematic state stabilized by thermal fluctuations.

In electronic systems it is well established that when there is a magnetic field or spontaneous magnetization, the Hall effect, and in some cases the quantum Hall effect appears. We theoretically pursue analogs of these phenomena in magnons (spin waves) and plasmons. In the case of magnons in ferromagnets, the Hall effect or quantum Hall effect requires some kind of a spin-orbit coupling (similar to electronic systems), and we show that the dipolar interaction, as well as the Dyaloshinskii-Moriya interaction, plays the role. By calculating the Berry curvature from the wavefunction, we can calculate thermal Hall effect for magnons in ferromagnets with dipolar interaction. We found that only the magnetostatic forward volume-wave mode exhibits the thermal Hall effect while the backward mode and the surface mode do not. In addition, by introducing some artificial spatial periodicity into the magnet, for example by fabricating nanostructures with two different magnets in a periodic structure or by making a periodic array of nanomagnets, we theoretically find appearance of quantum Hall effect in a certain range of the magnetic field. There appear chiral edge states which propagate along the edge of the magnet in one way. We call this a topological magnonic crystal. In the plasmon case, we should begin with constructing a fundamental band theory, and we theoretically show that on a metal surface with corrugations forming a triangular lattice under the magnetic field, the quantum Hall effect appears. It can be called a topological plasmonic crystal.

We measured transverse magneto-thermoelectric voltage on devices made of a Permalloy (Py) line and a transverse electrode made of platinum (Pt), copper (Cu) or bismuth (Bi). We show that the angular dependence of the voltage is the same for Pt and Cu but different with a Bi electrode. We interpret the angular dependence with Pt and Cu electrode as anomalous and planar Nernst and Righi-Leduc effect on Py. The results obtained with a Bi electrode can be explained as the Nernst effect of the electrode itself which overwhelms the signal coming from the Py.

We report a novel noncollinear magnetic order in individual nanostructures of a prototypical magnetic material, bilayer iron islands on Cu (111) [1]. Spin-polarized scanning tunnelling microscopy reveals a magnetic stripe phase with a period of 1.28 nm, which is identified as a one-dimensional helical spin order. Ab initio calculations reveal reduced-dimensionality-enhanced long-range antiferromagnetic interactions as the driving force of this spin order. Our findings point at the potential of nanostructured magnets to establish noncollinear magnetic order in a nanostructure, which is magnetically decoupled from the substrate. [1] S.H. Phark, J.A. Fischer, M. Corbetta, D. Sander, K. Nakamura, J. Kirschner, Nature Comm. 5, 5183 (2014).

It is well known that the Landau-Lifshitz-Gilbert (LLG) equation for a macroscopic magnetic moment find its limit of validity at very short time scales or equivalently at very high frequencies. The reason for this limit of validity is well understood in terms of separation of the characteristic times between slow (the magnetization) and fast (the environment) degrees of freedom, as pointed-out in the stochastic derivation of the LLG equation first proposed by W. F. Brown in 1963. Indeed, the ferromagnetic moment is a slow collective variable, but fast degrees of freedom are also playing a role in the dynamics, and especially the variation of the angular momentum responsible for inertia. In the last couple of years, the generalization of the LLG equation with inertia (ILLG) has been derived by different means (see list of references). The signature of the inertial regime of the magnetization is the nutation that can be measured by resonance experiments (but it has not been observed up to know). We developed an approach in terms of geometrical phase (defining the corresponding Hannay angle, which is the classical analog to the quantum Berry phase: see references), that has recently been used with success to analogous problems. We calculated the Hannay angle for the precession of the magnetization in the case of the inertial effect, and the corresponding magnetic monopole. This analysis allows the slow vs. fast variable expansion to be calculated in the specific case of pure precession.

We present a combined experimental and theoretical investigation of the magnetic behavior of ultra-thin Cr films grown on oxygen-passivated Fe(001)-p(1×1)O substrates. In all cases, oxygen floats on the metal/vacuum interface, where a monolayer-range oxide with peculiar electronic and structural characteristics is formed. Significant differences with previous experimental realizations of the Cr/Fe(001) heterostructure are thus introduced by the presence of oxygen. However, we show here that the magnetic behavior of our system is characterized by the same AF stacking at the Cr-Fe interface and by a layer-wise AF order in the Cr layer. In addition, we are able to circumvent the issue of chemical mixing at the Cr-Fe interface, characteristic of standard preparations.

Electromagnetic devices rely on electrical currents to generate magnetic fields. While extremely useful this approach has limitations in the small-scale. To overcome the scaling problem, researchers have tried to use electric fields to manipulate a magnetic material’s intrinsic magnetization (i.e. multiferroic). The strain mediated class of multiferroics offers up to 70% of energy transduction using available piezoelectric and magnetoelastic materials. While strain mediated multiferroic is promising, few studies exist on modeling/testing of nanoscale magnetic structures. This talk presents motivation, analytical models, and experimental data on electrical control of nanoscale single magnetic domain structures. This research is conducted in a NSF Engineering Research Center entitled Translational Applications for Nanoscale Multiferroics TANMS. The models combine micromagnetics (Landau-Lifshitz-Gilbert) with elastodynamics using the electrostatic approximation producing eight fully coupled nonlinear partial differential equations. Qualitative and quantitative verification is achieved with direct comparison to experimental data. The modeling effort guides fabrication and testing on three elements, i.e. nanoscale rings (onion states), ellipses (single domain reorientation), and superparamagnetic elements. Experimental results demonstrate electrical and deterministic control of the magnetic states in the 5-500 nm structures as measured with Photoemission Electron Microscopy PEEM, Magnetic Force Microscopy MFM, or Lorentz Transmission Electron Microscopy TEM. These data strongly suggests efficient control of nanoscale magnetic spin states is possible with voltage.

Electrical control of magnetism has been demonstrated in multiferroic compounds and ferromagnetic semiconductors, but electrical switching of a substantial net magnetization at room temperature has not been demonstrated in these materials. This goal has instead been achieved in heterostructures comprising ferromagnetic films in which electrically driven magnetic changes arise due to strain or exchange bias from ferroic substrates, or due to charge effects induced by a gate. However, previous work focused on electrical switching of an in-plane magnetization or involved the assistance of applied magnetic fields. In heterostructures made of juxtaposed ferroelectric and ferromagnetic layers, we have shown electrical control with no applied magnetic field of the perpendicular magnetization of small features [1] and of magnetic stripe domains patterns [2]. Here we investigate Ni81Fe19 films on ferroelectric substrates with and without buffer layers of Cu, whose presence precludes charge-mediated coupling. Ni81Fe19 has virtually zero magnetostriction, but sufficiently thin films show large magnetostriction, and thus, on increasing film thickness through the threshold for zero magnetostriction, we have seeked the crossover from charge- to strain-mediated coupling. We will then show that strain associated with the motion of 90°- ferroelectric domain walls in a BaTiO3 substrate, can switch the magnetization of an array of overlying single-domain Ni dots. [1] M. Ghidini, R. Pellicelli, J. L. Prieto, X. Moya, J. Soussi, J. Briscoe, S. Dunn and N. D. Mathur, Nature Communications 4 (2013) 1453. [2] M. Ghidini, F.Maccherozzi, X. Moya, L. C. Phillips, W.Yan, J. Soussi, N. Métallier, M.Vickers, , N. -J.Steinke, R. Mansell, C. H. W. Barnes, S. S. Dhesi, and N. D. Mathur, Adv. Mater.doi: 10.1002/adma.201404799 (2015).

Artificial spin ices are often spoken of as being realisations of some of the celebrated vertex models of statistical mechanics, where the exact microstate of the system can be imaged using advanced magnetic microscopy methods. The fact that a stable image can be formed means that the system is in fact athermal and not undergoing the usual finite-temperature fluctuations of a statistical mechanical system. In this paper we report on the preparation of artificial spin ices with islands that are thermally fluctuating due to their very small size. The relaxation rate of these islands was determined using variable frequency focused magneto-optic Kerr measurements. We performed magnetic imaging of artificial spin ice under varied temperature and magnetic field using X-ray transmission microscopy which uses X-ray magnetic circular dichroism to generate magnetic contrast. We have developed an on-membrane heater in order to apply temperatures in excess of 700 K and have shown increased dynamics due to higher temperature. Due to the ‘photon-in, photon-out' method employed here, it is the first report where it is possible to image the microstates of an ASI system under the simultaneous application of temperature and magnetic field, enabling the determination of relaxation rates, coercivties, and the analysis of vertex population during reversal.

Complex architectures of nanostructures are routinely elaborated using bottom-up or nanofabrication processes. This technological capability allows scientists to engineer materials with properties that do not exist in nature, but also to manufacture model systems to explore fundamental issues in condensed matter physics. Two-dimensional frustrated arrays of magnetic nanostructures are one class of systems for which theoretical predictions can be tested experimentally. These systems have been the subject of intense research in the last few years and allowed the investigation of a rich physics and fascinating phenomena, such as the exploration of the extensively degenerate ground-state manifolds of spin ice systems, the evidence of new magnetic phases in purely two-dimensional lattices, and the observation of pseudoexcitations involving classical analogues of magnetic monopoles. We show here, experimentally and theoretically, that simple magnetic geometries can lead to unconventional, non-collinear spin textures. For example, kagome arrays of inplane magnetized nano-islands do not show magnetic order. Instead, these systems are characterized by spin textures with intriguing properties, such as chirality, coexistence of magnetic order and disorder, and charge crystallization. Magnetic frustration effects in lithographically patterned kagome arrays of nanomagnets with out-of-plane magnetization also lead to an unusal, and still unknown, magnetic ground state manifold. Besides the influence of the lattice geometry, the micromagnetic nature of the elements constituting the arrays introduce the concept of chiral magnetic monopoles, bringing additional complexity into the physics of artificial frustrated spin systems.

Frustration, the presence of constraints/interactions that cannot be completely satisfied, is ubiquitous in the physical sciences as well as in life and a source of degeneracy and disorder which gives rise to new and interesting physical phenomena. In the past years a new perspective has opened in the study of frustration through the creation of artificial frustrated magnetic systems, consisting of arrays of lithographically fabricated single-domain ferromagnetic nanostructures that behave like giant Ising spins. The interactions can be controlled through their geometric properties and arrangement: The degrees of freedom of the material can be directly tuned, but also individually observed. Experimental studies have unearthed intriguing connections to the out-of-equilibrium physics of disordered systems and non-thermal “granular” materials, while revealing strong analogies to spin ice materials and their fractionalized magnetic monopole excitations, lending the enterprise a distinctly interdisciplinary flavor. In this talk we outline the more recent developments and future vistas for progress in this rapidly expanding field. We show how recent results have opened paths to new territories. Higher control, inclusive of genuine thermal ensembles have replaced the earlier and coarser methods based on magnetic agitation. Dynamical versions are now being realized, characterized in real time via PEEM, revealing statistical mechanics in action. This has lead us to afford implementation of new geometries, not found in nature, for dedicated bottom up design of desired emergent properties. Born as a scientific toy to investigate frustration-by-design, artificial spin ice might now be used to open “a path into an uncharted territory, a landscape of advanced functional materials in which topological effects on physical properties can be explored and harnessed.”

I will discuss the collective properties of arrays of single domain nanomagnets called Artificial Spin Ice.1 The shape of each nanomagnet controls the magnetic anisotropy and the elements are closely spaced so dipolar interactions are important. The honeycomb lattice geometry prevents the satisfaction of all dipole interactions. Here I will show direct magnetic imaging studies of magnetic charge flow.2 The magnetic charge is carried by transverse domain walls and the chirality of the domain wall is found to control the direction of propagation.3,4 Injection of domain walls within the arrays with local fields is also explored.5 References 1 Branford, W. R., Ladak, S., Read, D. E., Zeissler, K. and Cohen, L. F. Emerging Chirality in Artificial Spin Ice. Science 335, 1597-1600, (2012). 2 Ladak, S., Read, D. E., Perkins, G. K., Cohen, L. F. and Branford, W. R. Direct observation of magnetic monopole defects in an artificial spin-ice system. Nature Physics 6, 359-363, (2010). 3 Burn, D. M., Chadha, M., Walton, S. K. and Branford, W. R. Dynamic interaction between domain walls and nanowire vertices. Phys. Rev. B 90, 144414, (2014). 4 Zeissler, K., Walton, S. K., Ladak, S., Read, D. E., Tyliszczak, T., Cohen, L. F. and Branford, W. R. The non-random walk of chiral magnetic charge carriers in artificial spin ice. Sci Rep-Uk 3, 1252, (2013). 5 Pushp, A., Phung, T., Rettner, C., Hughes, B. P., Yang, S. H., Thomas, L. and Parkin, S. S. P. Domain wall trajectory determined by its fractional topological edge defects. Nature Physics 9, 505-511, (2013).

For a small island of a magnetic material the magnetic state of the island is mainly determined by the exchange interaction and the shape anisotropy. Two or more islands placed in close proximity will interact through dipolar interactions. The state of a large system will thus be dictated by interactions at both these length scales. Enabling internal thermal fluctuations, e.g. by the choice of material, of the individual islands allows for the study of thermal ordering in extended nano-patterned magnetic arrays [1,2]. As a result nano-magnetic arrays represent an ideal playground for the study of physical model systems. Here we present three different studies all having used magneto-optical imaging techniques to observe, in real space, the order of the systems. The first study is done on a square lattice of circular islands. The remanent magnetic state of each island is a magnetic vortex structure and we can study the temperature dependence of the vortex nucleation and annihilation fields [3]. The second are long chains of dipolar coupled elongated islands where the magnetization direction in each island only can point in one of two possible directions. This creates a system which in many ways mimics the Ising model [4] and we can relate the correlation length to the temperature. The third one is a spin ice system where elongated islands are placed in a square lattice. Thermal excitations in such systems resemble magnetic monopoles [2] and we can investigate their properties as a function of temperature and lattice parameters. [1] V. Kapaklis et al., New J. Phys. 14, 035009 (2012) [2] V. Kapaklis et al., Nature Nanotech 9, 514(2014) [3] E. Östman et al.,New J. Phys. 16, 053002 (2014) [4] E. Östman et al.,Thermal ordering in mesoscopic Ising chains, In manuscript.

Quantum dot attached to topological wires has become an interesting setup to study Majorana bound state in condensed matter[1]. One of the major advantage of using a quantum dot for this purpose is that it provides a suitable manner to study the interplay between Majorana bound states and the Kondo effect. Recently we have shown that a non-interacting quantum dot side-connected to a 1D topological superconductor and to metallic normal leads can sustain a Majorana mode even when the dot is empty. This is due to the Majorana bound state of the wire leaking into the quantum dot. Now we investigate the system for the case in which the quantum dot is interacting[3]. We explore the signatures of a Majorana zero--mode leaking into the quantum dot, using a recursive Green's function approach. We then study the Kondo regime using numerical renormalization group calculations. In this regime, we show that a "0.5" contribution to the conductance appears in system due to the presence of the Majorana mode, and that it persists for a wide range of the dot parameters. In the particle-hole symmetric point, in which the Kondo effect is more robust, the total conductance reaches $3e^2/2h$, clearly indicating the coexistence of a Majorana mode and the Kondo resonance in the dot. However, the Kondo effect is suppressed by a gate voltage that detunes the dot from its particle-hole symmetric point as well as by a Zeeman field. The Majorana mode, on the other hand, is almost insensitive to both of them. We show that the zero--bias conductance as a function of the magnetic field follows a well--known universal curve. This can be observed experimentally, and we propose that this universality followed by a persistent conductance of $0.5,e^2/h$ are evidence for the presence of Majorana--Kondo physics. This work is supported by the Brazilians agencies FAPESP, CNPq and FAPEMIG. [1] A. Y. Kitaev, Ann.Phys. {bf 303}, 2 (2003). [2] E. Vernek, P.H. Penteado, A. C. Seridonio, J. C. Egues, Phys. Rev. B {bf 89}, 165314 (2014). [3] David A. Ruiz-Tijerina, E. Vernek, Luis G. G. V. Dias da Silva, J. C. Egues, arXiv:1412.1851 [cond-mat.mes-hall].

We focus on topologically protected crossings of Andreev bound states in spin-polarized normal-superconductor hybrid structures [1]. Such crossings, signaling a change in the ground state fermion parity, became the focus of recent attention as they are regarded to be precursors to Majorana fermions that appear in the long-wire limit. In recent work, we showed how a topological state can be induced from regular or irregular scattering in (i) p-wave superconducting wires and (ii) Rashba wires in proximity to an s-wave superconductor. We also related the topological properties of such nanowires to their normal state properties such as conductance [2]. In the present work, we build on these results and study the correlation between parity-crossings in the superconducting state and the normal state properties of a hybrid nanostructure. Surprisingly, we find that the crossing points as well as their statistics are universal and are described solely by their normal-state properties. We obtain formulae for mean spacing between parity crossings as well as crossing statistics in disordered wires/cavities. We finally discuss under what conditions these crossings signal Majorana fermions. [1] I. Adagideli et al. [2] I. Adagideli, M. Wimmer, A. Teker, Phys Rev B 89, 144506 (2014)

Magnetic noise impacts a wide variety of solid-state devices, from quantum bits in superconductor and semiconductor-based quantum computer architectures to spintronic devices made of metals and semiconductors. Developing a theory of magnetic noise will have great impact in minimizing fluctuations in these devices. Magnetic noise is commonly detected as flux noise in superconducting quantum interference devices (SQUIDs). At low frequencies, SQUID flux noise spectral density decreases with frequency as $1/f^{\alpha}$ with $\alpha=0.5-0.8$ in a wide variety of devices. However, at higher frequencies (above ~1~GHZ) flux noise was found to be Ohmic, i.e. increasing linearly with frequency. This puzzling behavior is not explained by any model of magnetic fluctuations. Here we present a theory for the magnetic noise produced by local charge traps, elucidating the kind of noise that the majority of defects produce in a typical solid-state device. Our numerical renormalization group calculations reveal a deviation from 1/f behavior in the magnetic noise of charge traps in the Kondo regime over a wide range of frequencies. Remarkably, such behavior is not present in the charge noise, which is dominated by single-particle processes, consistent with a mean-field picture. The results show that, when Kondo correlations are present, magnetic noise originating from charge traps has a many-particle character, while charge noise does not. Since Kondo temperatures can be relatively high in charge traps, these findings indicate that electron-electron interactions can lead to a strong contribution to the magnetic noise that has not been captured by current models.

Dielectric metasurfaces with spatially varying birefringence and high transmission efficiency can exhibit exceptional abilities for controlling the photonic spin states. We present here some of our works on spin photonics and spin-photonic devices with metasurfaces. We develop a hybrid-order Poincaré sphere to describe the evolution of spin states of wave propagation in the metasurface. Both the Berry curvature and the Pancharatnam-Berry phase on the hybrid-order Poincaré sphere are demonstrated to be proportional to the variation of total angular momentum. Based on the spin-dependent property of Pancharatnam-Berry phase, we find that the photonic spin Hall effect can be observed when breaking the rotational symmetry of metasurfaces. Moreover, we show that the dielectric metasurfaces can provide great flexibility in the design of novel spin-photonic devices such as spin filter and spin-dependent beam splitter.

In 2014, we were able to report 1-kHz, electrical helicity switching of circular polarization (CP) at low temperatures using dual-electrode spin-LEDs [1] and detection of CP up to RT by operating reversely the spin-LEDs [2]. Those LEDs consisted of n-AlGaAs/undoped-InGaAs/p-AlGaAs double heterojunctions and newly developed crystalline γ-like AlO_x (x-AlO_x) tunnel barriers [3]. Presented here are progresses made in the period 2014-2015, including 1-MHz helicity switching, a high CP value of P_EL = 0.12 at RT, and development of a phenomenological model for spin photodiode (PD), all of which are important towards the realization of spin-photonic devices.

An optical isolator is an important component of an optical network. At present, there is a significant commercial demand for an optical isolator, which can be integrated into the Photonic Integrated Circuits (PIC). A new design of an integrated optical isolator, which utilizes unique non-reciprocal properties of surface plasmons, has been proposed [1]. The main obstacle for a practical realization of the proposed design is a substantial propagation loss of the surface plasmons in structures containing a ferromagnetic metal. The reduction of the propagation loss of a surface plasmon is the key to make the plasmonic isolator competitive with other designs of the integrated isolator. We studied experimentally optical and magneto-optical properties of a Fe plasmonic waveguide integrated with an AlGaAs rib waveguides and a Co plasmonic waveguide integrated with Si nanowire waveguides. It was demonstrated experimentally that by utilizing a double-dielectric plasmonic waveguide it is possible to reduce significantly the optical loss in a plasmonic waveguide. For Fe/SiO2/AlGaAs double-dielectric plasmonic waveguide the low optical loss of 0.03 dB/um is obtained. As far as we know at present it is a lowest optical loss demonstrated for a plasmon propagating at a surface of a ferromagnetic metal. For Co/Ti2O3/SiO2 double-dielectric plasmonic waveguide integrated with a Si nanowire waveguide on a Si substrate the optical loss of 0.7 dB/um was demonstrated. The designs of the plasmonic isolator utilizing a ring resonator or a non-reciprocal coupler were studied. [1] V. Zayets, H. Saito, S. Yuasa, and K. Ando,, Materials 5, 857 (2012).

Optical isolators are one of the essential components to protect semiconductor laser diodes (LDs) from backward reflected light in integrated optics. In order to realize optical isolators, nonreciprocal propagation of light is necessary, which can be realized by magnetic materials. Semiconductor optical isolators have been strongly desired on Si and III/V waveguides. We have developed semiconductor optical isolators based on nonreciprocal loss owing to transverse magneto-optic Kerr effect, where the ferromagnetic metals are deposited on semiconductor optical waveguides1). Use of surface plasmon polariton at the interface of ferromagnetic metal and insulator leads to stronger optical confinement and magneto-optic effect. It is possible to modulate the optical confinement by changing the magnetic field direction, thus optical isolator operation is proposed2, 3). We have investigated surface plasmons at the interfaces between ferrimagnetic garnet/gold film, and applications to waveguide optical isolators. We assumed waveguides composed of Au/Si(38.63nm)/Ce:YIG(1700nm)/Si(220nm)/Si , and calculated the coupling lengths between Au/Si(38.63nm)/Ce:YIG plasmonic waveguide and Ce:YIG/Si(220nm)/Si waveguide for transversely magnetized Ce:YIG with forward and backward directions. The coupling length was calculated to 232.1um for backward propagating light. On the other hand, the coupling was not complete, and the length was calculated to 175.5um. The optical isolation by using the nonreciprocal coupling and propagation loss was calculated to be 43.7dB when the length of plasmonic waveguide is 700um. 1) H. Shimizu et al., J. Lightwave Technol. 24, 38 (2006). 2) V. Zayets et al., Materials, 5, 857-871 (2012). 3) J. Montoya, et al, J. Appl. Phys. 106, 023108, (2009).

In _p+ GaAs thin films, under excitation by a tightly-focused laser, the spatial profile of the spin polarization is monitored as a function of excitation power. It is found that photoelectron diffusion depends on spin, as a direct consequence of the Pauli principle which causes a concentration dependence of the spin stiffness. Thermoelectric currents are also predicted to depend on spin under degeneracy (spin Soret currents), but these currents play a relatively small role in this case. The spin dependence of the mobility is also found weak. Conversely, ambipolar coupling with holes increases the steady-state photo-electron density at the place of excitation and therefore the amplitude of the degeneracy-induced polarization decrease at the place of excitation.

We have designed and fabricated 2D GMR spin valve sensors on the basis of IrMn/CoFe/Cu/CoFe/NiFe nanolayers in monolithic integration for high sensitivity applications. For a maximum signal-to-noise ratio, we realize a focused double full bridge layout featuring an antiparallel exchange bias pinning for neighbouring meanders and an orthogonal pinning for different bridges. This precise alignment is achieved with microscopic precision by laser heating and subsequent in-field cooling. Striving for maximum signal sensitivity and minimum hysteresis, we study in detail the impact of single meander geometry on the total magnetic structure and electronic transport properties. The investigated geometrical parameters include stripe width, stripe length, cross bar material and total meander length. In addition, the influence of the relative alignment between reference magnetization (pinned layer) and shape anisotropy (free layer) is studied. The experimentally obtained data are moreover compared to the predictions of tailored micromagnetic simulations. Using a set of optimum parameters, we demonstrate that our sensor may readily be employed to measure small magnetic fields, such as the ambient (geomagnetic) field, in terms of a 2D vector with high spatial (~200 μm) and temporal (~1 ms) resolution.

To detect magnetic signals coming from the body, in particular those produced by the electrical activity of the heart or of the brain, the development of ultrasensitive sensors is required. In this regard, magnetoresistive sensors, stemming from spin electronics, are very promising devices. For example, tunnel magnetoresistance (TMR) junctions based on MgO tunnel barrier have a high sensitivity. Nevertheless, TMR also often have high level of noise. Full spin polarized materials like manganite La_0.67Sr_0.33MnO₃ (LSMO) are attractive alternative candidates to develop such sensors because LSMO exhibits a very low 1/f noise when grown on single crystals, and a TMR response has been observed with values up to 2000%. This kind of tunnel junctions, when combined with a high Tc superconductor loop, opens up possibilities to develop full oxide structures working at liquid nitrogen temperature and suitable for medical imaging. In this work, we investigated on LSMO-based tunnel junctions the parameters controlling the overall system performances, including not only the TMR ratio, but also the pinning of the reference layer and the noise floor. We especially focused on studying the effects of the quality of the barrier, the interface and the electrode, by playing with materials and growth conditions.

We will present our last development of GMR-based magnetic sensors devoted to sensing application for non-destructive control application. In these first realizations, we have chosen a so-called shape anisotropy - exchange biased strategy to fulfill the field-sensing criteria in the μT range in devices made of micronic single elements. Our devices realized by optical lithography, and whose typical sizes range from 150 μm x 150 μm to 500 μm x 500 μm elements, are made of trilayers GMR-based technology and consist of several circuitries of GMR elements of different lengths, widths and gaps. To obtain a full sensing linearity and reversibility requiring a perpendicular magnetic arrangement between both sensitive and hard layer, the magnetization of the latter have been hardened by pinning it with an antiferromagnetic material. The specific geometry of the design have been engineered in order to optimize the magnetic response of the soft layer via the different magnetic torques exerted on it essentially played by the dipolar fields or shape anisotropy, and the external magnetic field to detect. The smaller dimensions in width and in gap are then respectively of 2 μm and 3 μm to benefit of the full shape anisotropy formatting the magnetic response.

The selective coupling between polarized photons and electronic states in materials enables polarization-resolved spectroscopy studies of exchange interactions, spin dynamics, and collective magnetic behavior of conduction electrons in semiconductors. Here we report on Magnetic Circular Dichroism (MCD) studies of magnetic properties of electrons in crystalline thin films of small molecule organic semiconductors. Specifically, the focus was on the magnetic exchange interaction properties of d-shell ions (Cu²⁺, Co²⁺ and Mn²⁺) metal phthalocyanine (Pc) thin films that one may think of as organic analogues of diluted magnetic semiconductors (DMS). These films were deposited in-house using a recently developed pen-writing method that results in crystalline films with macroscopic long range ordering and improved electronic properties, ideally suited for spectroscopy techniques.

Our experiments reveal that, in analogy to DMS, the extended π-orbitals of the Pc molecule mediate the spin exchange between highly localized d-like unpaired spins. We established that exchange mechanisms involve different electronic states in each species and/or hybridization between d-like orbitals and certain delocalized π-orbitals. Unprecedented 25T MCD and PL conducted in the unique 25T Split Florida HELIX magnet at the National High Magnetic Field Laboratory (NHMFL) will prove useful in probing these exchange interactions.

From the viewpoint of electric circuit theory, the three fundamental two-terminal passive circuit elements, resistor R , capacitor C, and inductor L, are defined in terms of a relationship between two of the four basic circuit variables, charge q, current i, voltage v, and magnetic flux φ. From a symmetry concern, there should be a fourth fundamental element defined from the relationship between charge q and magnetic flux φ. Here we present both theoretical analysis and experimental evidences to demonstrate that a two-terminal passive device employing the magnetoelectric (ME) effects can exhibit a direct relationship between charge q and magnetic flux φ, and thus is able to act as the fourth fundamental circuit element. The ME effects refer to the induction of electric polarization by a magnetic field or magnetization by an electric field, and have attracted enormous interests due to their promise in many applications. However, no one has linked the ME effects with fundamental circuit theory. Both the linear and nonlinear-memory devices, termed transtor and memtranstor, respectively, have been experimentally realized using multiferroic materials showing strong ME effects. Based on our work, a full map of fundamental two-terminal circuit elements is constructed, which consists of four linear and four nonlinear-memory elements. This full map provides an invaluable guide to developing novel circuit functionalities in the future.

The methods for the optimization of the magnetoresistive (MR) sensors are to reduce sources of noises, to increase the signal, and to understand the involved fundamental limitations. The high-performance MR sensors result from important magnetic tunnel junction (MTJ) properties, such as tunneling magnetoresistance ratio (TMR), coercivity (H_c), exchange coupling field (H_e), domain structures, and noise properties as well as the external magnetic flux concentrators. All these parameters are sensitively controlled by the magnetic nanostructures, which can be tuned by varying junction free layer nanostructures, geometry, and magnetic annealing process etc. In this paper, we discuss some of efforts that an optimized magnetic sensor with a sensitivity as high as 5,146 %/mT. This sensitivity is currently the highest among all MR-type sensors that have been reported. The estimated noise of our magnetoresistive sensor is 47 pT/Hz^1/2 at 1 Hz. This magnetoresistance sensor dissipates only 100 μW of power while operating under an applied voltage of 1 V at room temperature.

Compared to purely charge based devices, spintronic lasers offer promising perspectives for new superior device concepts. Especially vertical-cavity surface-emitting lasers with spin-polarization (spin-VCSELs) feature ultrafast spin and polarization dynamics. Oscillations in the circular polarization degree can be generated using pulsed spin-injection. The oscillations evolve due to the carrier-spin-photon system that is coupled for the linear modes in the VCSEL's cavity via the birefringence. The polarization oscillations are independent of the conventional relaxation oscillations and have the potential to exceed frequencies of 100 GHz. The oscillations are switchable and can be the basis for ultrafast directly modulated spin-VCSELs for, e.g., communication purposes. The polarization oscillation frequency is mainly determined by the birefringence. We show a method to tune the birefringence and thus the polarization oscillation frequency by adding mechanical strain to the substrate in the vicinity of the laser. We achieved first experimental results for high-frequency operation using 850 nm oxide-confined single-mode VCSELs. The results are compared with simulations using the spin-flip-model for high birefringence values.

Organic semiconductors are emerging as a leading area of research as they are expected to overcome limitations of inorganic semiconductor devices for certain applications where low cost manufacturing, device transparency in the visible range or mechanical flexibility are more important than fast switching times. Solution processing methods produce thin films with millimeter sized crystalline grains at very low cost manufacturing prices, ideally suited for optical spectroscopy investigations of long range many-body effects in organic systems. To this end, we synthesized an entire family of organosoluble 3-d transition metal Pc’s and successfully employed a novel solution-based pen-writing deposition technique to fabricate long range ordered thin films of mixtures of metal-free (H₂Pc) molecule and organometallic phthalocyanines (MPc's). Our previous studies on the parent MPc crystalline thin films identified different electronic states mediating exchange interactions in these materials. This understanding of spin-dependent exchange interaction between delocalized π-electrons with unpaired d spins enabled the further tuning of these interactions by mixing CoPc and H₂Pc in different ratios ranging from 1:1 to 1000:1 H₂Pc:MPc. The magnitude of the exchange is also tunable as a function of the average distance between unpaired spins in these materials. Furthermore, high magnetic field (B < 25T) MCD and magneto-photoluminescence show evidence of spin-polarized band-edge excitons in the same materials.

We report the demonstration of intrinsic spin Hall effect (SHE) of cylindrical vector beam. Employing a fan-shaped aperture to block part of the vector beam, the intrinsic vortex phases are no longer continuous in the azimuthal direction, and results in the spin accumulation at the opposite edges of the light beam. Due to the inherent nature of the phase and independency of light-matter interaction, the observed SHE is intrinsic. Modulating the topological charge of the vector beam, the spin-dependent splitting can be enhanced and the direction of spin accumulation is switchable.

The change of magnetization (i.e. using the inverse magnetostriction effect) allows to investigate at the nanoscale the effects of thermoelastic and piezoelectric strain of an active track-etched β-PVDF polymer matrix on an electrodeposited single-contacted Ni nanowire (NW). The magnetization state is measured locally by anisotropic magnetoresitance (AMR). The ferromagnetic NW plays thus the role of a mechanical probe that allows the effects of mechanical strain to be characterized and described qualitatively and quantitatively. Due to the inverse magnetostriction, a quasi-disappearance of the AMR signal for a variation of the order of ΔT ≈ 10 K has been evidenced. The coplanarity of the vectors between the magnetization and the magnetic field is broken. A way of studying the effect of the geometry on such a system, is to fabricate oriented polymer templates. Track-etched polymer membranes were thus irradiated at various angles (α_irrad) leading, after electrodeposition, to embedded Ni NWs of different orientations. With cylindrical Ni NW oriented normally to the template surface, the induced stress field in a single Ni NW was found 1000 time higher than the bulk stress field (due to thermal expansion measured on the PVDF). This amplification results in three nanoscopic effects: (1) a stress mismatch between the Ni NW and the membrane, (2) a non-negligible role of the surface tension on Ni NW Young modulus, and (3) the possibility of non-linear stress-strain law. When the Ni NWs are tilted from the polymer template surface normality, the induced stress field is reduced and the amplification phenomenon is less important.

Spintronic lasers offer promising perspectives for novel concepts and characteristics superior to conventional purely charge-based devices. This applies in particular to spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs), which exhibit ultrafast spin and polarization dynamics. Using pulsed spin-injection, oscillations in the circular polarization degree can be generated, which have the potential to be much faster than conventional relaxation oscillations and may exceed frequencies of 100 GHz. The oscillations originate from the coupled carrier-spin-photon system in birefringent VCSEL cavities. The polarization oscillations are independent from conventional relaxation oscillations and thus can be the cornerstone for ultrafast directly modulated spin-VCSELs in the near future. It is possible to switch the oscillations on and off, depending on phase and amplitude conditions of two consecutive excitation pulses. Even half-cycles can be generated, which is the basis for short polarization pulses, only limited by the polarization oscillation resonance frequency. Experimental results of oscillation switching are given using an 850 nm oxide-confined single-mode VCSEL. In order to increase the polarization oscillation frequency, the birefringence has to be tuned to higher values. We demonstrate a method to manipulate the birefringence by adding mechanical strain to the substrate in vicinity of the VCSEL. With this method the polarization oscillation frequency can be tuned over a wide range. The results are compared to the theory with simulations using the spin-flip-model.