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Register a new userOrigin of Magnetic Circular Dichroism in GaMnAs: Giant Zeeman Splitting versus Spin Dependent Density of States
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We present a unified interpretation of experimentally observed magnetic circular dichroism (MCD) in the ferromagnetic semiconductor (Ga,Mn)As, based on theoretical arguments, which demonstrates that MCD in this material arises primarily from a difference in the density of spin-up and spin-down states in the valence band brought about by the presence of the Mn impurity band, rather than being primarily due to the Zeeman splitting of electronic states.
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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.
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.
Magnetic anisotropies of ferromagnetic thin films are induced by epitaxial strain from the substrate via strain-induced anisotropy in the orbital magnetic moment and that in the spatial distribution of spin-polarized electrons. However, the preferential orbital occupation in ferromagnetic metallic La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) thin films studied by x-ray linear dichroism (XLD) has always been found out-of-plane for both tensile and compressive epitaxial strain and hence irrespective of the magnetic anisotropy. In order to resolve this mystery, we directly probed the preferential orbital occupation of spin-polarized electrons in LSMO thin films under strain by angle-dependent x-ray magnetic circular dichroism (XMCD). Anisotropy of the spin-density distribution was found to be in-plane for the tensile strain and out-of-plane for the compressive strain, consistent with the observed magnetic anisotropy. The ubiquitous out-of-plane preferential orbital occupation seen by XLD is attributed to the occupation of both spin-up and spin-down out-of-plane orbitals in the surface magnetic dead layer.
Strong Rashba effects at surfaces and interfaces have attracted great attention for basic scientific exploration and practical applications. Here, the first-principles investigation shows that giant and tunable Rashba effects can be achieved in KTaO$_3$ (KTO) ultrathin films by applying biaxial stress. When increasing the in-plane compressive strain nearly to -5%, the Rashba spin splitting energy reaches $E_{R}=140$ meV, approximately corresponding to the Rashba coupling constant $alpha_{R}=1.3$ eV {AA}. We investigate its strain-dependent crystal structures, energy bands, and related properties, and thereby elucidate the mechanism for the giant Rashba effects. Furthermore, we show that giant Rashba spin splitting can be kept in the presence of SrTiO$_3$ capping layer and/or Si substrate, and strong circular photogalvanic effect can be achieved to generate spin-polarized currents in the KTO thin films or related heterostructures, which are promising for future spintronic and optoelectronic applications.
The electric-field control of $d$-electron magnetism in multiferroic transition metal oxides is attracting widespread interest for the underlying fundamental physics and for next generation spintronic devices. Here, we report an extensive study of the $3d$ magnetism in magnetoelectric Ga$_{0.6}$Fe$_{1.4}$O$_3$ (GFO) epitaxial films by polarization dependent x-ray absorption spectroscopy. We found a non-zero integral of the x-ray magnetic circular dichroism, with a sign depending upon the relative orientation between the external magnetic field and the crystallographic axes. %By reliably enlarging the limit of the spin and orbital sum rules, which usually holds for materials where the magnetic ions exhibit a unique crystal field symmetry This finding translates in a sign-reversal between the average Fe magnetic orbital and spin moments. Large Fe-displacements, among some of the octahedral sites, lower the symmetry of the system producing anisotropic paths for the Fe-O bondings giving rise to a large orbital-lattice interaction akin to a preferential crystallographic direction for the magnetic orbital moment. The latter may lead to a partial re-orientation of the magnetic orbital moment under an external magnetic field that, combined to the ferrimagnetic nature of the GFO, can qualitatively explain the observed sign-reversal of the XMCD integral. The results suggest that a control over the local symmetry of the oxygen octahedra in transition metal oxides can offer a suitable leverage over the manipulation of the effective orbital and spin moments in magnetoelectric systems.
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