Highly spin-polarized materials and devices for spintronics∗

Koichiro Inomata; Naomichi Ikeda; Nobuki Tezuka; Ryogo Goto; Satoshi Sugimoto; Marek Wojcik; Eva Jedryka

doi:10.1088/1468-6996/9/1/014101

Highly spin-polarized materials and devices for spintronics^∗

Sci Technol Adv Mater. 2008 Mar 13;9(1):014101. doi: 10.1088/1468-6996/9/1/014101. eCollection 2008 Jan.

Authors

Koichiro Inomata¹, Naomichi Ikeda², Nobuki Tezuka², Ryogo Goto², Satoshi Sugimoto², Marek Wojcik³, Eva Jedryka³

Affiliations

¹ National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan.
² Department of Material Science, Graduate School of Engineering, Tohoku University, Aobayama, Sendai 980-8579, Japan.
³ Institute of Physics, Polish Academy of Sciences, Warszawa 02-668, Poland.

Abstract

The performance of spintronics depends on the spin polarization of the current. In this study half-metallic Co-based full-Heusler alloys and a spin filtering device (SFD) using a ferromagnetic barrier have been investigated as highly spin-polarized current sources. The multilayers were prepared by magnetron sputtering in an ultrahigh vacuum and microfabricated using photolithography and Ar ion etching. We investigated two systems of Co-based full-Heusler alloys, Co₂Cr_{1 - x} Fe _x Al (CCFA(x)) and Co₂FeSi_{1 - x} Al _x (CFSA(x)) and revealed the structure and magnetic and transport properties. We demonstrated giant tunnel magnetoresistance (TMR) of up to 220% at room temperature and 390% at 5 K for the magnetic tunnel junctions (MTJs) using Co₂FeSi_0.5Al_0.5 (CFSA(0.5)) Heusler alloy electrodes. The 390% TMR corresponds to 0.81 spin polarization for CFSA(0.5) at 5 K. We also investigated the crystalline structure and local structure around Co atoms by x-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analyses, respectively, for CFSA films sputtered on a Cr-buffered MgO (001) substrate followed by post-annealing at various temperatures in an ultrahigh vacuum. The disordered structures in CFSA films were clarified by NMR measurements and the relationship between TMR and the disordered structure was discussed. We clarified that the TMR of the MTJs with CFSA(0.5) electrodes depends on the structure, and is significantly higher for L2₁ than B2 in the crystalline structure. The second part of this paper is devoted to a SFD using a ferromagnetic barrier. The Co ferrite is investigated as a ferromagnetic barrier because of its high Curie temperature and high resistivity. We demonstrate the strong spin filtering effect through an ultrathin insulating ferrimagnetic Co-ferrite barrier at a low temperature. The barrier was prepared by the surface plasma oxidization of a CoFe₂ film deposited on a MgO (001) single crystal substrate, wherein the spinel structure of CoFe₂O₄ (CFO) and an epitaxial relationship of MgO(001)[100]/CoFe₂ (001)]110]/CFO(001)[100] were induced. A SFD consisting of CoFe₂ /CFO/Ta on a MgO (001) substrate exhibits the inverse TMR of - 124% at 10 K when the configuration of the magnetizations of CFO and CoFe₂ changes from parallel to antiparallel. The inverse TMR suggests the negative spin polarization of CFO, which is consistent with the band structure of CFO obtained by first principle calculation. The - 124% TMR corresponds to the spin filtering efficiency of 77% by the CFO barrier.

Keywords: Co-ferrites; Heusler alloys; magnetic tunnel junctions; nuclear magnetic resonance; spin filters; spin polarization.

Publication types

Review