Efficient and dependable characterization methods of magnetic-plasmonic nanostructures
are essential towards the implementation of new nanoscale materials in magneto-optical
applications. Surface magneto-optic Kerr effect (SMOKE) is a powerful characterization
technique, because of its simplicity and high sensitive to even monolayer thick magnetic
materials. It relies on the measurement of polarization and absorption changes of reflected
light in the presence of a magnetic field. While SMOKE has been applied in the past to
investigate the magnetic information of continuous films, there is little work on applying it to
characterize arrays of nanoparticles with variable magnetic and optical properties. Here, we
have used it to investigate the magnetic behavior of nanoparticle arrays made by nanosecond
pulsed laser self-organization. This technique produces an array of single-domain magnetic
nanoparticles with size-dependent magnetic orientation. Nanoparticle arrays of Co and Ni
were prepared on SiO2 substrates. SMOKE measurements were performed for a variety of
different particle sizes and material. Systematic differences in saturation and coercivity were
observed for the different samples. These results demonstrated that SMOKE is a reliable
technique to rapidly characterize the magnetic behavior of nanoparticle arrays.
Laser-induced melting of ultrathin films can lead to self-organized arrays of hemispherical particles. We have applied
this procedure to assemble arrays of Fe nanomagnets on SiO2 substrates. Morphological studies showed presence of
spatial short range order (SRO) in the array. Magnetic properties were studied at room temperature using zero-field
magnetic force microscopy (MFM). The particles upto 55 nm in diameter showed in-plane (≤ 45°), compared to out-of-plane magnetization directions (≥ 45°) for the larger particles. The size-dependent orientation of magnetization for
these hemispherical particles, was attributed to the dominating magnetostrictive energy and a size-dependent residual
strain.
Previous studies on dewetting of ultrathin Co films by nanosecond pulsed laser melting have shown that the films dewet due to a thin film hydrodynamic instability and form a system of ordered nanoparticles with uniform average size and nearest neighbor particle spacing. For Co films less than 8 nm thick, the nanoparticle spacing, λNN was dependent on the initial film thickness, h, and varied as h2. For Co films thicker than 8 nm, the nanoparticle spacing decreased with increasing film thickness, due to a thermocapillary effect generated by the ns laser heating. Here we show the results from investigations on dewetting of Co films that had initially much rougher surfaces with root mean square roughness values, 0.9 < Rrms < 2.8 nm as compared to smoother films examined in prior investigations, for which Rrms ≤ 0.2 nm. Laser induced dewetting of Co films with much large Rrms values generated nanoparticles that were qualitatively similar to those created from smoother Co films. The size distribution of the nanoparticles was monodispersed and there was short range spatial order present in the system from the average nearest neighbor nanoparticle spacing; however, a drastic reduction in the characteristic length scales was observed in the nanoparticulate arrays created from the rougher Co films. This result suggests that knowledge of film thickness and roughness are important towards predicting characteristic length scales from metal film dewetting.
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