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Conventional susceptometers based on superconducting quantum interference devices monitor the magnetic moment of a magnetized sample which is moved through a superconducting gradiometer coil. The measuring accuracy of 10-8 emu (equals 10-11 Am2) is limited by mechanical degradations and vibrations of the sample holder, its geometrical shape, magnetostrictive phenomena and positioning error of the sample. Also the method of evaluating the magnetic moment of the sample from the depicted magnetic flux is not unique for extended thin layer samples. The l/f-noise from the `static' measuring procedure is commonly compensated by increasing the volume of the magnetized sample and accordingly the signal to noise ratio. However, if thin epitaxial samples of atomic monolayer thickness, quantum well and superlattice structures of semimagnetic constituents on thick diamagnetic substrates (in contrast to ferromagnetically ordered metal superlattices) are investigated, the conventional method fails for an useful analysis. Thus a novel `dynamic' method to modulate selectively the thermal properties of the epi-layer by a light-wave (guided by a magnetically clean glass fiber) is applied. The boundary conditions to find the thermal wave amplitude in the epi-layer are calculated for the extended configuration helium gas- substrate-helium gas-glass fiber. The sample is mechanically fixed. Lock-in technique increases the sensitivity by at least a factor ten. Since the thermal wave modulates the magnetic properties only of the magnetic epi-layer, the dynamic range is enhanced dramatically. The method called thermal wave susceptometry (TWS) is applied to drive an antiferromagnetic phase transition in a 2.5 micrometers EuTe epi-layer on BaF2 substrate in a normally out-of-detection limited field of 10 Gauss. A ferromagnetic EuTe/PbTe magnetic semiconductor superlattice of 5 monolayers EuTe between 15 diamagnetic PbTe monolayers (repeated 400 times) is also investigated by this novel low-intensity (10 mW/cm2) light- modulated TWS-method.
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