Co2FeSi Films Research Articles

Structural dependence of the tunnel magnetoresistance for magnetic tunnel junctions with a full-Heusler Co2Fe(Al,Si) electrode

We have investigated the crystal structure and magnetic moment of the Co2FeAl and Co2FeSi films deposited onto thermally oxidized Si and MgO(100) single-crystal substrates, and the structural effect on the tunnel magnetoresistance (TMR) of the magnetic tunnel junctions (MTJs) using a Co2FeZ (Z=Al or Si). The structure was changed by the substrate and postannealing temperatures, in which the fully epitaxial and polycrystalline Co2FeAl and Co2FeSi films were obtained with the different disorder structure. The magnetic moment of Co2FeAl films was found to be uninfluenced by the crystal structure. Spin-valve-type MTJs consisting of Co2FeZ(100nm)∕Al(1.2nm)-oxide∕Co75Fe25(2nm)∕IrMn(10nm)∕Ta(60nm) were fabricated (Z=Al or Si) on thermally oxidized Si and MgO(100) single-crystal substrates. The maximum TMR obtained is about 50% at room temperature for MTJs with Co2FeAl films, regardless of the crystal structure of Co2FeAl.

Structural and magnetic properties and tunnel magnetoresistance for Co2(Cr,Fe)Al and Co2FeSi full-Heusler alloys

We have investigated the structure and magnetization of Co2(Cr1−xFex)Al (0 ⩽ x ⩽ 1) and Co2FeSi full-Heusler alloy films deposited on thermally oxidized Si (SiO2) and MgO (001) single crystal substrates by ultra-high vacuum sputtering at various temperatures. The films were also post-annealed after deposition at room temperature (RT). Magnetic tunnel junctions with a full-Huesler alloy electrode were fabricated with a stacking structure of Co2YZ (20 nm)/Al (1.2 nm)-oxide/Co75Fe25 (3 nm)/IrMn (15 nm)/Ta (60 nm) and microfabricated using electron beam lithography and Ar ion etching with a 102 µm2 junction area, where Co2YZ stands for Co2(Cr1−xFex)Al or Co2FeSi. The tunnel barriers were formed by the deposition of 1.2 nm Al, followed by plasma oxidization in the chamber. The x-ray diffraction revealed the A2 or B2 structure depending on heat treatment conditions and the substrate, but not L21 structure for the Co2(Cr1−xFex)Al (0 ⩽ x ⩽ 1) films. The L21 structure, however, was obtained for the Co2FeSi films when deposited on a MgO (001) substrate at elevated temperatures above 473 K. The maximum tunnelling magnetoresistance (TMR) was obtained with 52% at RT and 83% at 5 K for a junction using a Co2(Cr0.4Fe0.6)Al electrode. While the junction using a Co2FeSi electrode with the L21 structure exhibited the TMR of 41% at RT and 60% at 5 K, which may be improved by using a buffer layer for reducing the lattice misfit between the Co2FeSi and MgO (001) substrate.