Controlling Orbital Moment and Spin Orientation in CoO Layers by Strain

Abstract

We have observed that CoO films grown on different substrates show dramatic differences in their magnetic properties. Using polarization dependent x-ray absorption spectroscopy at the Co L2,3 edges, we revealed that the magnitude and orientation of the magnetic moments strongly depend on the strain in the films induced by the substrate. We presented a quantitative model to explain how strain together with the spin-orbit interaction determine the 3d orbital occupation, the magnetic anisotropy, as well as the spin and orbital contributions to the magnetic moments. Control over the sign and direction of the strain may therefore open new opportunities for applications in the field of exchange bias in multilayered magnetic films.The discovery of the exchange bias phenomenon in surface-oxidized cobalt particles about 50 years ago [1] marks the beginning of a new research field in magnetism. Since then several combinations of antiferromagnetic (AFM) and ferromagnetic (FM) thin film materials have been fabricated and investigated [2,3], motivated by the potential for applications in information technology. Numerous theoretical [4,5,6,7,8] and experimental [9,10,11,12,13,14] studies have been devoted to unravel the mechanism(s) responsible for exchange biasing. However, no conclusive picture has emerged yet. A major part of the problem lies in the fact that there is insufficient information available concerning the atomic and magnetic structure of the crucial interface between the AFM and FM material. The important issue of, for instance, spin reorientations in the AFM films close to the interface is hardly considered [15,16,17,18,19], and the role of epitaxial strain herein has not been discussed at all.In this paper we study the magnetic properties of CoO thin films epitaxially grown on MnO(100) and on Ag(100), as model systems for an AFM material under either tensile or compressive in-plane stress. Our objective is to establish how the magnetic anisotropy as well as the spin and orbital contributions to the magnetic moments depend on the lowering of the local crystal field symmetry by epitaxial strain. Using polarization dependent x-ray absorption spectroscopy (XAS) at the Co L 2,3 (2p → 3d) edges, we observe that the magnitude and orientation of the magnetic moments in the CoO/MnO(100) system are very different from those in the CoO/Ag(100). We present a quantitative model to calculate how local crystal fields together with the spin-orbit interaction determine the magnetic properties. to 10 −6 mbar. The base pressure of the MBE system is in the low 10 −10 mbar range. The thickness and epitaxial quality of the films are monitored by reflection high energy electron diffraction measurements. With the lattice constant of bulk Ag (4.09Å) being smaller than that of bulk CoO (4.26Å) and MnO (4.444Å), we find from x-ray diffraction that CoO on Ag is slightly compressed in-plane (a ≈ 4.235Å, a ⊥ ≈ 4.285Å), and from reflection high energy electron diffraction (RHEED) that CoO sandwiched by MnO is about 4% expanded in-plane (a ...

Shape-induced phenomena in finite-size antiferromagnets

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Tetragonal BiFeO3 on yttria-stabilized zirconia

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Spin State Transition inLaCoO3Studied Using Soft X-ray Absorption Spectroscopy and Magnetic Circular Dichroism

“…fact essentially different in details. First of all, our set includes a low temperature (20 K) spectrum representative for the LS state, and second, our spectra do not show a pronounced shoulder at 777 eV photon energy which is characteristic for the presence of Co 2+ impurities [39]. The extended temperature range and especially the purity of the probed samples provide the required sensitivity for the spin-state related spectral changes.…”