Fundamental aspects of the interface engineering in the heavy metal/ferromagnet-based perpendicularly magnetized systems

Abstract

The ferromagnetic (FM) material finds its applications due to the property of spontaneous magnetization. An FM object can hold its state of magnetization unless any external stimuli change it. The easy manipulation of its state by an external magnetic field or spin current manifests FMs usability for digital data storage and logic devices. The efficiency and speed of magnetic data storage and logic devices should be optimized to match the existing semiconductor devices. This thesis has mainly proclaimed the fundamental dependence of magnetism on the composition of the layers and geometry, established the nexus to understand the underlying physics to identify the promising candidates for data storage applications in the future.

In the first work, systematically modifying the strength of the induced moment in the bottom Pt layer by the thickness variation of the adjacent Ta buffer layer, the Pt spin depth profiles in Ta/Pt/Co/Pt multilayers induced by the magnetic proximity effect due to the adjacent Co layer have been quantified. The Pt spin depth profiles by hard x-ray resonant magnetic reflectivity measurements have been identified, which have been carried out at the third-generation synchrotron PETRA III at DESY. It has been found that the top Pt layer has a comparable induced magnetic moment with the bottom one. The induced magnetic moment in the bottom Pt layer reduces with increasing Ta thickness. Grazing incidence x-ray diffraction measurements have been carried out to show that the Ta buffer layer induces the growth of Pt(011) rather than Pt(111) which in turn reduces the induced moment as confirmed by detailed density functional theory calculations presented in our manuscript.

In the second work, the tilt in magnetic anisotropy and strength of the interfacial Dzyaloshinskii-Moriya interaction (iDMI), in the Pt/Co interface of the Pt/Co/Pt trilayers simultaneously have been quantified. The differential polar-Kerr microscopy technique in the creep regime of the domain wall motion for these measurements has been used. It has been found that an oblique angle sputter deposition technique results in a slight tilt of magnetic anisotropy from the film normal (quasi-perpendicular magnetic anisotropy). The effective in-plane field at the domain wall due to iDMI has been determined by decomposing the symmetric and asymmetric contributions of the domain wall motion. Furthermore, the asymmetric contribution has been decomposed into two contributions due to the tilted magnetic anisotropy and the exponentially decaying chiral damping. The collective coordinate model for a system with iDMI and quasi-PMA has been developed to study the nature of the asymmetric contribution owing to the tilted anisotropy and a functional form of the total asymmetric contribution has been determined from the simulation result.

In the third work, the effective strength of the iDMI, in the Ta/Pt/Co/Au multilayers has been estimated using the field-induced domain wall motion and spin-orbit torque efficiency estimation studies. The current-induced magnetization switching phase diagram has also been constructed. It has been found that from the phase diagram, the effective strength of the iDMI can be quantified. Multilayers with the heavy metal Pt and the novel metal Au on either side of the ferromagnetic Co layer break the structural inversion symmetry, enhancing the iDMI strength.