Spin-polarization and x-ray magnetic circular dichroism in GaAs

The combination of angular spin momentum with electronics is a promising successor to charge-based electronics. The conduction bands in GaAs may become spin-polarized via optical spin pumping, doping with magnetic ions, or induction of a moment with an external magnetic field. We investigated the spin populations in GaAs with x-ray magnetic circular dichroism for each of these three cases. We find strong anti-symmetric lineshapes at the Ga L₃ edge indicating conduction band spin splitting, with differences in line width and amplitude depending on the source of spin polarization.     

X-ray magnetic circular dichroism at rare-earth L23 absorption edges in various compounds and alloys

Theoretical interpretations of X-ray magnetic circular dichroism (XMCD) at rare-earth (called R hereafter) L₂₃ absorption edges are reviewed using differing models, depending on the material under investigation. In the first chapter, we present an overview of recent developments for XMCD in XAS with a few general remarks, especially at R atom absorption edges. In Section 2, we first describe basic mechanism of XMCD at L₂₃ edges of R systems, and then we essentially discuss two examples of XMCD spectra in: (i) R₂Fe₁₄B metallic compounds, with the help of a cluster model, and (ii) RFe₂ Laves-phase compounds, using a tight-binding approximation for R 5d and Fe 3d conducting states. A good agreement between theory and experiment for R₂Fe₁₄B suggests that a cluster model provides a valuable method to quantitatively calculate XMCD spectra of R systems, even with quite complicated atomic arrangements. For RFe₂ systems the XMCD spectral shape, especially for the L₂ edge of heavy R elements, is more complicated than those of R₂Fe₁₄B systems, and this is explained by the competition of some different XMCD mechanisms. In Section 3, we focus on special series of Ce systems, related to XAS and XMCD studies at the Ce L₂₃ edges. Two clearly differing cases are interpreted: (i) A well localized 4f¹ system, i.e. CeRu₂Ge₂; (ii) A less localized 4f¹ system, i.e. CeFe₂, with a 3d partner. Then, from a more experimental point of view, we investigate the influence of substitution on the low temperature properties of CePd₃ compounds, i.e. Ce(_Pd1-xMnx)3$\mathrm{Ce}({\mathrm{Pd}}_{1-x}{\mathrm{Mn}}_x{)}_3$ alloys where x   is about 0.03, giving rise to (_CePd3)8Mn$({\mathrm{CePd}}_3{)}_8\mathrm{Mn}$ ordered structure. We give another example: Ce(_Pd1-xNix)3$\mathrm{Ce}({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x{)}_3$ alloys with x taken up to about 0.25. Also the Ce L₂₃ XMCD signal measured in pure CePd₃ demonstrates that in the Ce based dense Kondo materials, only the 4f¹ channel yields a magnetic response.     

Size-dependent magnetism of deposited small iron clusters studied by x-ray magnetic circular dichroism

The size-dependent magnetic properties of small iron clusters deposited on ultrathin nickel films have been studied with circularly polarized synchrotron radiation. With the use of sum rules, orbital and spin magnetic moments have been extracted from x-ray magnetic circular dichroism spectra. The ratio of orbital to spin magnetic moments varies considerably with cluster size, reflecting the dependence of magnetic properties on cluster size and geometry. These variations can be explained in terms of enhanced orbital moments in small clusters.     

软x射线磁性圆二色吸收谱研究铁单晶薄膜的面内磁各向异性

利用软x射线磁性圆二色(XMCD)吸收谱测得Fe/MgO膜不同磁化方向的轨道磁矩和自旋磁矩.实 验表明，沿铁单晶薄膜的不同方向，铁原子轨道磁矩的改变量达到600%以上，而自旋磁矩的 变化约50%，但原子的总磁矩没有如此大的改变.结合常规方法分析了铁薄膜的宏观磁各向异 性性质，半定量地获得磁矩与宏观各向异性能的关系，并对样品的磁矩和磁各向异性能进行 了比较. 关键词： x射线磁性圆二色 磁各向异性 磁性薄膜     

Ferromagnetic nature of (Ga, Cr)As epilayers revealed by magnetic circular dichroism

A systematic study of magnetic circular dichroism (MCD) was carried out in a wave length range 500-960 nm for (Ga_1−x, Cr_x)As epilayers with x=2.38% and 4.59% grown by the low temperature molecular beam epitaxy (LT-MBE) technique. Hysteresis characteristics showed up indeed in the magnetic field dependence of both MCD and magnetization measured by the superconductor quantum interference device (SQUID). The Curie temperature of the (Ga_1−x, Cr_x)As epilayer was determined to be about 12 K by the Arrott approach. The present result provides evidences that there is strong coupling of the Cr spins to the GaAs host band structure in (Ga_1−x, Cr_x)As samples. That affects the critical point of the semiconductor host, and makes the magnetization behavior in a plot of M∼B/T (magnetic field divided by temperature) substantially different from standard superparamagnetism.     

Role of magnetic circular dichroism in all-optical magnetic recording

Using magneto-optical microscopy in combination with ellipsometry measurements, we show that all-optical switching with polarized femtosecond laser pulses in ferrimagnetic GdFeCo is subjected to a threshold fluence absorbed in the magnetic layer, independent of either the excitation wavelength or the polarization of the laser pulse. Furthermore, we present a quantitative explanation of the intensity window in which all-optical helicity-dependent switching (AO-HDS) occurs, based on magnetic circular dichroism. This explanation is consistent with all the experimental findings on AO-HDS so far, varying from single- to multiple-shot experiments. The presented results give a solid understanding of the origin of AO-HDS, and give novel insights into the physics of ultrafast, laser controlled magnetism.     

Spin state transition in LaCoO3 studied using soft x-ray absorption spectroscopy and magnetic circular dichroism

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-L(2,3) edge, we reveal that the spin state transition in LaCoO3 can be well described by a low-spin ground state and a triply degenerate high-spin first excited state. From the temperature dependence of the spectral line shapes, we find that LaCoO3 at finite temperatures is an inhomogeneous mixed-spin state system. It is crucial that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low- or intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance, and inelastic neutron data.