title: Highly spin-polarized materials and devices for spintronics∗.
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, Co2Cr1 -xFexAl (CCFA(x)) and Co2FeSi1 -xAlx(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 Co2FeSi0.5Al0.5(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 L21than 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 CoFe2film 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 CoFe2changes 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.
authors.affiliation: National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan.
authors.forename: Koichiro
authors.initials: K
authors.lastname: Inomata
authors.affiliation: Department of Material Science, Graduate School of Engineering, Tohoku University, Aobayama, Sendai 980-8579, Japan.
authors.forename: Naomichi
authors.initials: N
authors.lastname: Ikeda
authors.affiliation: Department of Material Science, Graduate School of Engineering, Tohoku University, Aobayama, Sendai 980-8579, Japan.
authors.forename: Nobuki
authors.initials: N
authors.lastname: Tezuka
authors.affiliation: Department of Material Science, Graduate School of Engineering, Tohoku University, Aobayama, Sendai 980-8579, Japan.
authors.forename: Ryogo
authors.initials: R
authors.lastname: Goto
authors.affiliation: Department of Material Science, Graduate School of Engineering, Tohoku University, Aobayama, Sendai 980-8579, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Sugimoto
authors.affiliation: Institute of Physics, Polish Academy of Sciences, Warszawa 02-668, Poland.
authors.forename: Marek
authors.initials: M
authors.lastname: Wojcik
authors.affiliation: Institute of Physics, Polish Academy of Sciences, Warszawa 02-668, Poland.
authors.forename: Eva
authors.initials: E
authors.lastname: Jedryka
keywords: Co-ferrites; Heusler alloys; magnetic tunnel junctions; nuclear magnetic resonance; spin filters; spin polarization