Abstract
ABSTRACT We have developed hard X-ray photoelectron spectroscopy (HAXPES) under an applied magnetic field of 1 kOe to study the electronic and magnetic states related to the MgO/Fe interface-induced perpendicular magnetic anisotropy (PMA). In this work, we used MgO (2 nm)/Fe (1.5 and 20 nm)/MgO(001) structures to reveal the interface-induced electronic states of the Fe film. Perpendicular magnetization of the 1.5-nm-thick Fe film without extrinsic oxidation of the Fe film was detected by the Fe 2p core-level magnetic circular dichroism (MCD) in HAXPES under a magnetic field, and easy magnetization axis perpendicular to the film plane was confirmed by ex situ magnetic hysteresis measurements. The valence-band HAXPES spectrum of the 1.5-nm-thick Fe film revealed that the Fe 3d electronic states were strongly modified from the thick Fe film and a reference bulk Fe sample due to the lifting of degeneracy in the Fe 3d states near the MgO/Fe interface. We found that the tetragonal distortion of the Fe film by the MgO substrate also contributes to the lifting of degeneracy in the Fe 3d states and PMA, as well as the Fe 3d-O 2p hybridization at the MgO/Fe interface, by comparing the valence-band spectrum with density functional theory calculations for MgO/Fe multilayer structures. Thus, we can conclude that the Fe 3d-O 2p hybridization and tetragonal distortion of the Fe film play important roles in PMA at the MgO/Fe interface. HAXPES with in situ magnetization thus represents a powerful new method for studying spintronic structures.
Highlights
Ferromagnetic metal (FM) thin films having a magnetization easy axis perpendicular to the film surfaces due to a strong magneto-crystalline anisotropy (MCA) have attracted much attention in spintronic applications, since thermally stable perpendicular magnetization sufficiently above room temperature (RT) is required to realize non-volatile ultrahigh-density magnetic storage devices
A direct observation of the electronic structure for a MgO/Fe interface with perpendicular magnetization will provide new information on the interface perpendicular magnetic anisotropy (PMA), because the MgO/Fe interface is a prototypical example of the interface PMA and the materials used in the MgO/Fe interface are of low cost, earth abundant, and environmentally friendly, which are demands for sustainable device applications
Since the magnetic circular dichroism (MCD) signal is proportional to the magnetization projected to the direction of X-rays, the geometry shown in Figure 2(b) is sensitive to the in-plane magnetization, while that shown in Figure 2(a) is sensitive to both the in-plane and perpendicular magnetizations
Summary
Ferromagnetic metal (FM) thin films having a magnetization easy axis perpendicular to the film surfaces due to a strong magneto-crystalline anisotropy (MCA) have attracted much attention in spintronic applications, since thermally stable perpendicular magnetization sufficiently above room temperature (RT) is required to realize non-volatile ultrahigh-density magnetic storage devices. A direct observation of the electronic structure for a MgO/Fe interface with perpendicular magnetization will provide new information on the interface PMA, because the MgO/Fe interface is a prototypical example of the interface PMA and the materials used in the MgO/Fe interface are of low cost, earth abundant, and environmentally friendly, which are demands for sustainable device applications. Trajectories of photoelectrons moving along magnetic field lines are less affected by the magnetic field owing to the fact that F(r) is almost zero, that is, B(r) is nearly parallel to v(r) We have assessed this assumption experimentally, and that photoelectrons at around 6 keV emitted within ±30° with respect to the direction of the dipole moment appears to satisfy the condition of B(r) nearly parallel to v(r), which allows us to perform HAXPES under a magnetic field in the experimental geometries shown in Figure 2(a) and 2(b). The steep decrease of the magnetic field, which is proportional to ~1/r3, is crucial for several keV photoelectrons to reduce the changes of photoelectron trajectories
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