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Register a new userHard X-ray Photoelectron Momentum Microscopy and Kikuchi Diffraction on (In,Ga,Mn)As Thin Films
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Recent advances in the brilliance of hard-X-ray beamlines and photoelectron momentum microscopy facilitate bulk valence-band mapping and core-level-resolved hard-X-ray photoelectron diffraction (hXPD) for structural analysis in the same setup. High-quality MBE-grown (In,Ga,Mn)As films represent an ideal testing ground, because of the non-centrosymmetric GaAs crystal structure itself and In and Mn doping concentrations of few percent. Here we present results of k-mapping and hXPD for the title compound with 3% In and 2.5 or 5.6% Mn using hard X-ray photons (3 to 5 keV) at beamline P22 at PETRA III (DESY, Hamburg). Numerical processing (difference or ratio images) emphasizes subtle differences of hXPD patterns like the fingerprint-like hXPD-signatures of As and Ga sites. XPD calculations using the Bloch-wave method show a one-to-one correspondence with the measurements. The hXPD results reveal a predominant Ga substitutional site for Mn. Valence band mapping shows that the Fermi energy lies within the valence band and decreases as the Mn concentration increases. The results support the p-d Zener model of ferromagnetism in the title compound. In addition to the shift of the Fermi energy, the band splitting increases with increasing Mn content, which we attribute to an increase of many-body correlations with increasing metallicity of the sample.
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The GALAXIES beamline at the SOLEIL synchrotron is dedicated to inelastic x-ray scattering (IXS) and photoelectron spectroscopy (HAXPES) in the 2.3-12 keV hard x-ray range. These two techniques offer powerful, complementary methods of characterization of materials with bulk sensitivity, chemical and orbital selectivity, resonant enhancement and high resolving power. After a description of the beamline components and endstations, we address the beamline performances through a selection of recent works both in the solid and gas phases and using either IXS or HAXPES approaches. Prospects for studies on liquids are discussed.
We show, by SQUID magnetometry, that in (Ga,Mn)As films the in-plane uniaxial magnetic easy axis is consistently associated with particular crystallographic directions and that it can be rotated from the [-110] direction to the [110] direction by low temperature annealing. We show that this behavior is hole-density-dependent and does not originate from surface anisotropy. The presence of uniaxial anisotropy as well its dependence on the hole-concentration and temperature can be explained in terms of the p-d Zener model of the ferromagnetism assuming a small trigonal distortion.
Magnetite (Fe3O4) thin films on GaAs have been studied with HArd X-ray PhotoElectron Spectroscopy (HAXPES) and low-energy electron diffraction. Films prepared under different growth conditions are compared with respect to stoichiometry, oxidation, and chemical nature. Employing the considerably enhanced probing depth of HAXPES as compared to conventional x-ray photoelectron spectroscopy (XPS) allows us to investigate the chemical state of the film-substrate interfaces. The degree of oxidation and intermixing at the interface are dependent on the applied growth conditions; in particular, we found that metallic Fe, As2O3, and Ga2O3 exist at the interface. These interface phases might be detrimental for spin injection from magnetite into GaAs.
We have used complementary neutron and x-ray reflectivity techniques to examine the depth profiles of a series of as-grown and annealed Ga[1-x]Mn[x]As thin films. A magnetization gradient is observed for two as-grown films and originates from a nonuniformity of Mn at interstitial sites, and not from local variations in Mn at Ga sites. Furthermore, we see that the depth-dependent magnetization can vary drastically among as-grown Ga[1-x]Mn[x]As films despite being deposited under seemingly similar conditions. These results imply that the depth profile of interstitial Mn is dependent not only on annealing, but is also extremely sensitive to initial growth conditions. We observe that annealing improves the magnetization by producing a surface layer that is rich in Mn and O, indicating that the interstitial Mn migrates to the surface. Finally, we expand upon our previous neutron reflectivity study of Ga[1-x]Mn[x]As, by showing how the depth profile of the chemical composition at the surface and through the film thickness is directly responsible for the complex magnetization profiles observed in both as-grown and annealed films.
Using atomic force microscopy, we have studied the surface structures of high quality molecular beam epitaxy grown (Ga,Mn)As compound. Several samples with different thickness and Mn concentration, as well as a few (Ga,Mn)(As,P) samples have been investigated. All these samples have shown the presence of periodic ripples aligned along the $[1bar{1}0]$ direction. From a detailed Fourier analysis we have estimated the period (~50 nm) and the amplitude of these structures.
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