Enhancing Ferromagnetism and Tuning Magnetic Anisotropy Utilizing Spin-Orbit Interaction
21 Pages Posted: 16 Jan 2020
Date Written: November 8, 2019
Magnetic materials demonstrating a specific property known as perpendicular magnetic anisotropy (PMA) are currently of immense interest for nonvolatile magnetic data storage as they require less energy to switch their magnetic direction (changing states ‘0’ to ‘1’ or the other way) than with current and allow for much higher data bit density. The spin-orbit (S-O) interaction, a relativistic interaction occurring when ferromagnetic elements are combined with high atomic number elements, renders an efficient atomic level method to tune the magnetic anisotropy — direction of preferred magnetization — of thin films. Because the magnetic moments in ferromagnetic materials, such as cobalt (Co), nickel (Ni), and permalloy (80% Ni-20% Fe alloy), tend to lie in-plane, placing an ultra-thin S-O causing layer such as platinum (Pt), gold (Au), or iridium (Ir) on top of a thin film of FM material would enable PMA to be achieved. Due to various interface effects, including what are known as Dzyaloshinskii-Moriya (DM) interaction and surface anisotropy, the stable magnetic configuration tends to be in the out-of-plane direction (magnetic moment lying perpendicular to the film plane) when a small perpendicular magnetic field is applied. The project goal was to explore this phenomenon in some select layer combinations. Using several magnetic characterization techniques, it was possible to generate the magnetic hysteresis loops for various layer combinations to investigate the direction of the magnetization axis. We found that the thickness of the layers, the number of layers, and the type of metals determined the strength of the out of plane moment. Moreover, interestingly, it was also observed that the Co-Pt multi-layer structures yielded a higher net magnetic moment than the moment of bulk Co. These observations and results help to advance our knowledge on interface effects, allowing the optimization of material combinations that possess strong PMA as well as contributing to the creation of highly efficient, enhanced memory storage elements and higher density magnetic recording hard drives. Another important recent phenomenon in such bilayer material combinations is the occurrence of magnetic skyrmions, topologically protected particle-like spin textures. Research on this new area is rapidly progressing because of the interesting underlying physics and also because magnetic skyrmions’ nanometer-scale size and thermal stability make them highly useful in magnetic information storage and processing.
Keywords: perpendicular magnetic anisotropy, magnetic anisotropy, magnetic skyrmions
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