Direct correlation of Cr 3 d orbital polarization and O K -edge X-ray magnetic circular dichroism of epitaxial CrO ₂ films

Abstract

PACS. 78.20.Ls -Magnetooptical effects. PACS. 75.30.Gw -Magnetic anisotropy. PACS. 78.70.Dm -X-ray absorption spectra.Abstract. -The role of delocalization and hybridization in complex magnetic oxides has been investigated by magnetic oxygen K-edge absorption of circular polarized soft X-rays in epitaxial grown CrO2 as a function of the azimuthal angle at grazing incidence. Unusual strong variations in the typically small X-ray magnetic circular dichroism (XMCD) signal have been observed. Those angular dependencies of the typical CrO2 oxygen K-edge XMCD signal could be quantitatively interpreted in terms of an induced anisotropic Cr 3d orbital magnetism for different spectral regions of the unoccupied density of states. The results strongly suggest a delocalized non-ionic magnetic behavior of the conduction electrons.Introduction. -Recently, half-metallic CrO 2 has attracted a revived research interest due to its remarkable high spin polarization at the Fermi level, theoretically predicted to 100% [1][2][3][4] and verified by superconducting tunneling spectroscopy at 1.8 K [5,6] and spin-polarized photoemission spectroscopy at T = 293 K [7]. The reason for this revival are potential applications of magnetoelectronic and spintronics devices on the basis of half-metallic ferromagnets such as CrO 2 due to its intrinsic high degree of spin polarization [8] and Curie temperature well above RT. The oxygen-mediated magnetic coupling and the half-metallic nature are challenging tasks for theoretical investigations, originated by the presence of oxygen-hybridization, double-exchange, self-doping and correlation effects [2]. CrO 2 has a magnetic moment of 2 µ B /Cr-ion, which is consistent with a fully spin-polarized band structure and also with an ionic 3d 2 configuration. Therefore, transition-metal-oxide absorption spectra have been theoretically modeled in a full ionic ligand field approach [9]. Due to the metastable nature of the CrO 2 surface at T = 293 K and ambient atmosphere, spectroscopic and surface-sensitive measurements have been a challenging problem in the past. Nevertheless, epitaxially grown thin CrO 2 (100) films could be prepared, showing a magnetic easy axis along the in-plane c-axis for an unstrained sample [10,11].

Large hidden orbital moments in magnetite

“…Such a strong hybridization has been observed in many other oxide systems (e.g., see Ref. [41]) and is also present in the O K edge spectra discussed below. If localization were to play an important role here one would also expect visible changes in the Fe L 2,3 edge BR at the Verwey transition of magnetite, which is not the case [5,15,32].…”

“…So the 3d orbital moment projections and also the O K edge XMCD showed both a narrow feature close to the excitation threshold and a broader feature at higher energy with opposite sign [41]. In addition, the angular dependency of the Cr 3d orbital polarization and the O K edge XMCD was also quite comparable.…”

“…Despite the fact that O K edge spectra usually reveal only small effects in the range of less than 4% with respect to the white-line intensity, these spectra are able to indentify magnetic moments at the oxygen site. Due to the nature of K edge spectroscopy only orbital O 2p projections are observed [33,41,57]. This is even more important, because in oxides O 2p electrons are often strongly hybridized with the TM 3d electrons, providing valuable information on the degree of hybridization and the magnetism of the hybridized TM3d-O2p bands.…”

Resonant soft X-ray scattering studies of multiferroic YMn2O5

“…For this reason, no strong XMCD signal due to a possible spin-moment at oxygen is expected. Nonetheless, a clear circular dichroism in the absorption at the O K edge has been observed experimentally in a number of TMO compounds [29,30]. In contrast to the TM L 2,3 edges, however, the circular dichroism in the O K edge absorption was found to be due to orbital moments, not spin-moments.…”

“…In contrast to the TM L 2,3 edges, however, the circular dichroism in the O K edge absorption was found to be due to orbital moments, not spin-moments. These moments are transferred to oxygen by hybridization [29,30]. Based on general symmetry arguments [28], such orbital moments also cause a dichroism in absorption and therefore, again by virtue of the optical theorem, a dependence of the resonant scattering length on the direction of the (magnetic) orbital moment at oxygen.…”