Electron Correlation Enhances Orbital Polarization at a Ferromagnetic Metal/Insulator Interface: Depth-Resolved X-ray Magnetic Circular Dichroism and First-Principles Study

Abstract

The Fe(CoB)/MgO interface is vital to spintronics as it exhibits the tunneling magnetoresistance (TMR) effect and interfacial perpendicular magnetic anisotropy (PMA) simultaneously. To further enhance TMR and PMA for the development of high-density magnetoresistive random access memory, it is essential to clarify the behavior of Fe atoms interfaced with MgO. This study reveals that the spin and orbital magnetic moments of Fe are enhanced at the Fe/MgO interface. The enhancement in the orbital magnetic moment is much more significant than that predicted by the standard density functional theory. Theoretical calculations based on the orbital polarization correction reproduce this enhancement, the origin of which is attributed to the electron−electron correlation resulting from electron localization at the Fe/MgO interface. The present findings highlight the importance of electron−electron correlation at ferromagnet/oxide interfaces, which has often been disregarded in spintronics.

The origin of enhanced interfacial perpendicular magnetic anisotropy in LiF-inserted Fe/MgO interface

“…Figure 2(b) shows the XAS spectra with the background subtracted. The XAS spectrum of the LiF-free Fe/MgO multilayer looks identical to those reported in previous studies [40][41][42][43]; the XAS spectrum has a broad single peak for each of the Fe L 2,3 edges and does not exhibit multiplet features. This is also the case for the spectra with LiF insertion, except that there is a slight variation in XAS spectral line shape because of the spectral overlap between F K and Fe L 2,3 edges.…”

“…m T is the expectation value of the magnetic represents the XMCD integral over the L 2 or the L 3 edges, and S XAS represents the XAS integral over the L 2,3 edges. As the magnetic dipole term was found negligible in the Fe/MgO interface [40], we ignored this term hereafter. n h is the number of 3d holes, which is assumed to be 3.39 in the present study.…”

“…This may call for another mechanism to explain the OMA enhancement. One possibility is that a stronger electron-electron correlation at the Fe/LiF interface enhances the OMA and PMA [40] as the more ionic nature of LiF results in stronger interfacial electron localization with stronger electron-electron correlation. Note that it is possible that better lattice matching just improves the interface quality and thus enhances the OMA and PMA.…”

“…Furthermore, the observed F K-edge XAS spectra look distinct from those of magnesium fluorides [38] and iron fluorides [39], indicating the absence of interlayer mixing between the Fe, LiF, and MgO layers. Despite the spectral overlap between F K-edge and Fe L 2,3 -edge XAS spectra, we subtracted backgrounds in a rather standard way [40]; we subtracted the summation of a double step function and a linear function that bends at the Fe L 3 edge such that XAS intensities become zero in the pre-edge (around 700 eV) and post-edge regions (around 740 eV). The backgrounds are shown by solid curves separately for each spectrum in Fig.…”