N. Tezuka

Giant magnetic tunneling effect in Fe/Al2O3/Fe junction

Abstract A giant magnetoresistance ratio of 30% at 4.2 K and 18% at 300 K was observed for the first time in an Fe/Al2O3/Fe junction. The conductance at room temperature was expressed well by G=96.2 (1 + 0.09 cos θ)(Ω-1), where θ is the angle between the magnetizations of two iron electrodes. The dependence of the magnetoresistance ratio, saturated resistance and also the tunneling current on temperature were measured in the range 4.2–300 K. The results support the claim that the giant magnetoresistance is due to the magnetic tunneling of electrons between the electrodes through the thin Al2O3 insulator.

Large Tunneling Magnetoresistance at Room Temperature Using a Heusler Alloy with the B2 Structure

A Co2Cr0.6Fe0.4Al Heusler alloy film exhibited a B2 structure, which was deposited using a magnetron sputtering system on a thermally oxidized Si substrate at room temperature without any buffer layers. The film exhibited the magnetic moment of 2.04µB per formula unit, nearly the integer number of Bohr magnetons, suggesting a localized nature of ferromagnetism similar to that of many Heusler compounds, which is a necessary condition for half metallicity. A spin- valve-type tunneling junction with a Co2(Cr, Fe)Al Heusler alloy film was fabricated using metal masks, which consists of Co2Cr0.6Fe0.4Al(10 nm)/AlOx (1.8 nm)/CoFe (3 nm)/NiFe (5 nm)/IrMn (15 nm)/Ta (5 nm), deposited on a thermally oxidized Si substrate without a buffer layer. The junction demonstrated large tunneling magnetoresistances of 16% at room temperature and 26.5% at 5 K.

Improved tunnel magnetoresistance of magnetic tunnel junctions with Heusler Co2FeAl0.5Si0.5 electrodes fabricated by molecular beam epitaxy

The authors have developed a magnetic tunnel junction of Co2FeAl0.5Si0.5 electrodes and a MgO barrier fabricated by molecular beam epitaxy and observed that this device had a tunnel magnetoresistance ratio of 386% at approximately 300 K and 832% at 9 K. The lower Co2FeAl0.5Si0.5 electrode was annealed during and after deposition resulting in a highly ordered structure with small roughness. This highly ordered structure could be obtained by annealing treatment even at low temperatures. Furthermore, a weak temperature dependence of the tunnel magnetoresistance ratio was observed for the developed magnetic tunnel junction.

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 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.

Tunnel magnetoresistance for junctions with epitaxial full-Heusler Co2FeAl0.5Si0.5 electrodes with B2 and L21 structures

The tunnel magnetoresistance (TMR) effect has been investigated for magnetic tunnel junctions with epitaxial Co2FeAl0.5Si0.5 Heusler electrodes with B2 and L21 structures on a Cr-bufferd MgO substrate. The epitaxially grown Co2FeAl0.5Si0.5 has B2 structure when annealed below 400°C, and has L21 structure for annealing above 450°C. The TMR ratio of 76% at room temperature and 106% at 5K were obtained for a MgO(001)∕Cr∕B2-type Co2FeAl0.5Si0.5∕Al oxide/Co75Fe25∕IrMn∕Ta. The TMR ratio is larger than that of magnetic tunnel junction with an L21-type electrode, which may be due to the smoother surface of the B2 structure and disordered L21 structure due to the Cr atom interdiffusion.

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.

Spin polarized tunneling in ferromagnet/insulator/ferromagnet junctions

Abstract Experimental data and theoretical explanation reported for spin polarized tunneling in ferromagnet/insulator/ferromagnet are reviewed. The magnetoresistance (MR) ratio due to the spin polarized tunneling is relatively smaller value than that of giant magnetoresistance in artificial superlattices. However, recent results reported by us are an order of magnitude larger than those reported in the past. We demonstrate that the magnetoresistance ratio is roughly proportional to the product of spin polarizations of both ferromagnets. The dependence of MR ratio, saturation resistance and conductance on temperature are also discussed.

175% tunnel magnetoresistance at room temperature and high thermal stability using Co2FeAl0.5Si0.5 full-Heusler alloy electrodes

The authors have fabricated epitaxially grown spin-valve-type magnetic tunnel junctions with L21-Co2FeAl0.5Si0.5 full-Heusler alloys for top and bottom electrodes and a MgO barrier. For MgO thickness tMgO=1.5nm, tunnel magnetoresistance (TMR) ratio and resistance and area product (RA) initially increase up to around 350°C and then decrease by annealing, while for tMgO=2.0 and 2.5nm, the TMR ratio increases with annealing temperature and peaks around 500°C. The TMR ratio up to 175% at RT and thermal stability up to 500°C have been achieved for tMgO=2.0nm, suggesting the large tunneling spin polarization and high thermal stability for Co2FeAl0.5Si0.5 with L21 structure.

Giant Tunnel Magnetoresistance at Room Temperature for Junctions using Full-Heusler Co2FeAl0.5Si0.5 Electrodes

The authors have investigated the tunnel magnetoresistance (TMR) effect of magnetic tunnel junctions (MTJs) with Co2FeAl0.5Si0.5 full-Heusler electrodes and a MgO barrier in the thickness range of 1.5–2.5 nm. A cross-sectional transmission electron micrograph showed the epitaxial growth of both the upper and lower Co2FeAl0.5Si0.5 layers and a MgO barrier and some distortion in the MgO barrier after annealing above 400 °C. An exponential dependence of resistance × area product on MgO barrier thickness was observed. TMR ratio was strongly affected by MgO barrier thickness, exhibiting maxima of 220% at room temperature and 390% at 5 K. The latter corresponds to the spin polarization of 0.81. It was also found that the features of this MTJ include a small asymmetry voltage and weak temperature dependence of its TMR ratio.

Large tunnel magnetoresistance at room temperature with a Co2FeAl full-Heusler alloy electrode

Magnetic tunnel junctions (MTJs) with a Co2FeAl Heusler alloy electrode are fabricated by the deposition of the film using an ultrahigh vacuum sputtering system followed by photolithography and Ar ion etching. A tunnel magnetoresistance (TMR) of 47% at room temperature (RT) are obtained in a stack of Co2FeAl∕Al–Ox∕Co75Fe25 magnetic tunnel junction (MTJ) fabricated on a thermally oxidized Si substrate despite the A2 type atomic site disorder for Co2FeAl. There is no increase of TMR in MTJs with the B2 type Co2FeAl, which is prepared by the deposition on a heated substrate. X-ray photoelectron spectroscopy (XPS) depth profiles in Co2FeAl single layer films reveal that Al atoms in Co2FeAl are oxidized preferentially at the surfaces. On the other hand, at the interfaces in Co2FeAl∕Al–Ox∕Co75Fe25 MTJs, the ferromagnetic layers are hardly oxidized during plasma oxidation for a formation of Al oxide barriers.