SPring-8, the large synchrotron radiation facility

Speaker : Dr. Masafumi Shirai

Language : JAPANESE

Affiliate : RIEC, Tohoku University

Title : Theoretical design of perpendicularly magnetized materials by ab initio calculations

Abstract :
 Perpendicularly magnetized materials with strong magneto-crystalline anisotropy (MCA) attract much attention due to their application to high density magnetic recording media and nonvolatile spin-memories. In order to ensure enough endurance against thermal fluctuations of the magnetization direction in downsizing a recording bit or a memory cell, it is inevitable to adopt perpendicularly magnetized materials possessing higher MCA energy.
 Recently, a L1₀-ordered alloy FeNi, which is free from rare-earth and/or noble metal elements, has been successfully fabricated as thin films by using alternative monatomic layer deposition technique [1-3]. We investigate the MCA energy of the L1₀-FeNi theoretically on the basis of first-principles density-functional calculations in order to elucidate the physical origin of the perpendicular MCA found in L1₀-FeNi. We also seek for short-period Fe/Ni(001) multilayers showing higher MCA energy than L1₀-FeNi. We focus on the dependence of the MCA energy on the in-plane lattice constant of the materials. The perpendicular MCA energy of L1₀-FeNi arises predominantly from the Fe atoms and it increases with decreasing the in-plane lattice constant. While the Fe(2ML)/Ni(2ML) multilayer shows weak MCA, the Fe(3ML)/Ni(3ML) multilayer possesses strong in-plane MCA at the equilibrium lattice constants. These multilayers show remarkable dependences of the MCA energy on the in-plane lattice constants and become perpendicularly magnetized materials under in-plane tensile strains.
 Heusler alloys are suitable for applications to electrodes of magnetic tunnel junctions due to their excellent properties such as high spin polarization and low magnetic damping. Recently, perpendicularly magnetized thin films of a Heusler alloy Co₂FeAl (CFA) have been successfully achieved in CFA/MgO junctions [4]. We evaluate the MCA energy of CFA/MgO junctions theoretically in order to elucidate the correlation between the MCA and the interfacial structure of the junctions, i.e. Co- and FeAl-terminated interfaces. We found that the Co-terminated CFA/MgO junctions show perpendicular MCA, however, the FeAl-terminated junctions in-plane MCA. Comparing formation energies between both the interfaces, the FeAl-terminated junction is more favorable energetically. Thus, the perpendicular MCA found in the CFA/MgO junctions experimentally cannot be attributed to the Co-terminated junctions. A possible explanation is the formation of Fe-rich layers at the CFA/MgO interfaces. Indeed, we confirmed that a strong perpendicular MCA energy is achieved by inserting a monatomic Fe layer into the CFA/MgO interface.

Acknowledgements
This work was partly supported by a Grant-in-Aid for Scientific Research (No. 22246087 and No. 22360014) from JSPS/MEXT, by Sangaku-Kyoso Basic Research Program from JST, and by FIRST Program form JSPS/CSTP.

References
[1] T. Shima, M. Okamura, S. Mitani, and K. Takanashi, J. Magn. Magn. Mater. 310, 2213 (2007).
[2] M. Mizuguchi, S. Sekiya, and K. Takanashi, J. Appl. Phys. 107, 09A716 (2010).
[3] T. Kojima, et al., Jpn. J. Appl. Phys. 51, 010204 (2012).
[4] Z. Wen, H. Sukegawa, S. Mitani, and K. Inomata, Appl. Phys. Lett. 98, 242507 (2011).

Organizer : Masato Kotsugi
Mail:kotsugi@spring8.or.jp
PHS:3488