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Register a new userPushing the detection limit of Magnetic Circular Dichroism to 2 nm
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Magnetic Circular Dichroism (MCD) is a standard technique for the study of magnetic properties of materials in synchrotron beamlines. We present here a new scattering geometry in the Transmission Electron Microscope through which MCD can be observed with unprecedented spatial resolution. A convergent electron beam is used to scan a multilayer Fe/Au sample and record energy loss spectra. Differences in the spectra induced by the magnetic moments of the Fe atoms can be resolved with a resolution of 2 nm. This is a breakthrough achievement when compared both to the previous EMCD resolution (200 nm) or the best XMCD experiments (approx. 20 nm), with an improvement of two and one order of magnitude, respectively.
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Imaging the magnetic configuration of thin-films has been a long-standing area of research. Since a few years, the emergence of two-dimensional ferromagnetic materials calls for innovation in the field of magnetic imaging. As the magnetic moments are extremely small, standard techniques like SQUID, torque magnetometry, magnetic force microscopy and Kerr effect microscopy are challenging and often lead to the detection of parasitic magnetic contributions or spurious effects. In this work, we report a new magnetic microscopy technique based on the combination of magnetic circular dichroism and Seebeck effect in semiconductor/ferromagnet bilayers. We implement this method with perpendicularly magnetized (Co/Pt) multilayers sputtered on Ge (111). We further show that the electrical detection of MCD is more sensitive than the Kerr magnetometry, especially in the ultra-thin film regime, which makes it particularly promising for the study of emergent two-dimensional ferromagnetic materials.
The difference in the transmission for left and right circularly polarised light though thin films on substrates in a magnetic field is used to obtain the magnetic circular dichroism of the film. However there are reflections at all the interfaces and these are also different for the two polarisations and generate the polar Kerr signal. In this paper the contribution to the differences to the total transmission from the transmission across interfaces as well as the differences in absorption in the film and the substrate are calculated. This gives a guide to when it is necessary to evaluate these corrections in order to obtain the real MCD from a measure of the differential transmission due to differential absorption in the film.
We have investigated the spin and orbital magnetic moments of Fe in FePt nanoparticles in the $L$1$_{0}$-ordered phase coated with SiO$_{2}$ by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe $L_{rm 2,3}$ absorption edges. Using XMCD sum rules, we evaluated the ratio of the orbital magnetic moment ($M_{rm orb}$) to the spin magnetic moment ($M_{rm spin}$) of Fe to be $M_{rm orb}/M_{rm spin}$ = 0.08. This $M_{rm orb}/M_{rm spin}$ value is comparable to the value (0.09) obtained for FePt nanoparticles prepared by gas phase condensation, and is larger than the values ($sim$0.05) obtained for FePt thin films, indicating a high degree of $L$1$_{0}$ order. The hysteretic behavior of the FePt component of the magnetization was measured by XMCD. The magnetic coercivity ($H_{rm c}$) was found to be as large as 1.8 T at room temperature, $sim$3 times larger than the thin film value and $sim$50 times larger than that of the gas phase condensed nanoparticles. The hysteresis curve is well explained by the Stoner-Wohlfarth model for non-interacting single-domain nanoparticles with the $H_{rm c}$ distributed from 1 T to 5 T.
The magnetic circular dichroism of III-V diluted magnetic semiconductors, calculated within a theoretical framework suitable for highly disordered materials, is shown to be dominated by optical transitions between the bulk bands and an impurity band formed from magnetic dopant states. The theoretical framework incorporates real-space Greens functions to properly incorporate spatial correlations in the disordered conduction band and valence band electronic structure, and includes extended and localized electronic states on an equal basis. Our findings reconcile unusual trends in the experimental magnetic circular dichroism in III-V DMSs with the antiferromagnetic p-d exchange interaction between a magnetic dopant spin and its host.
Magneto-optical properties of the ferromagnetic semiconductor GaMnAs are studied in a material specific multi-band tight-binding approach. Two realistic models are compared: one has no impurity band while the other shows an impurity band for low Mn concentrations. The calculated magnetic circular dichroism (MCD) is positive for both models proving that, unlike previously asserted, the observed positive MCD signal is inconclusive as to the presence or absence of an impurity band in GaMnAs. The positive MCD is due to the antiferromagnetic p-d coupling and the transitions into the conduction band.
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