H. Kubota

Huge Spin-Polarization of L21-Ordered Co2MnSi Epitaxial Heusler Alloy Film

Magnetic tunnel junctions (MTJs) with a stacking structure of epitaxial Co2MnSi/Al–O barrier/poly-crystalline Co75Fe25 were fabricated using an ultrahigh vacuum sputtering system. The epitaxial Co2MnSi bottom electrode exhibited highly ordered L21 structure and very smooth surface morphology. Observed magnetoresistance (MR) ratios of 70% at room temperature (RT) and 159% at 2 K are the highest values to date for MTJs using a Heusler alloy electrode. A high spin-polarization of 0.89 at 2 K for Co2MnSi obtained from Jullieres model coincided with the half-metallic band structure that was predicted by theoretical calculations.

Fabrication of high-magnetoresistance tunnel junctions using Co75Fe25 ferromagnetic electrodes

Temperature dependence of tunnel magnetoresistance (TMR) ratio, resistance, and coercivity from 4.2 K to room temperature and applied voltage dependence of the TMR ratio and resistance at room temperature for a tunnel junction, Ta (5 nm)/Ni79Fe21 (3 nm)/Cu (20 nm)/Ni79Fe21 (3 nm)/Ir22Mn78 (10 nm)/Co75Fe25 (4 nm)/Al (0.8 nm)-oxide/Co75Fe25 (4 nm)/Ni79Fe21 (20 nm)/Ta(5 nm), were investigated. TMR ratio, effective barrier height and width, and breakdown voltage of the junction can be remarkably enhanced after annealing at 300 °C for an hour. High TMR ratio of 49.7% at room temperature and 69.1% at 4.2 K were observed. The value of spin polarization of Co75Fe25, P=50.7%, deduced from the TMR ratio at 4.2 K was corresponding well to the experimental data measured at 0.2 K in a spin polarized tunneling experiment using a superconductor/insulator/ferromagnet tunneling junction.

Large tunnel magnetoresistance in magnetic tunnel junctions using Co2MnX (X = Al,Si) Heusler alloys

We fabricated B2-ordered Co2MnAl and L21-ordered Co2MnSi Heusler alloy films by optimizing various fabrication conditions (substrate, composition of sputtering target, substrate and post-annealing temperature, etc) and applied these films to bottom electrodes of magnetic tunnel junctions (MTJs). We used Al-oxide insulating tunnel barriers for our MTJs and varied oxidation times of Al films to control qualities of the Al-oxide insulating layer and Heusler-alloy/Al-oxide interface. Observed tunnel magnetoresistance (TMR) ratios were extremely sensitive to the structure and surface morphology of the prepared Heusler alloy films. Epitaxially grown Heusler alloy films showed good structural quality, very flat surfaces and enhanced TMR ratios. The behaviour of the TMR ratios towards oxidation time for the preparation of the Al-oxide barriers and the measurement temperature dependence of the TMR ratios were quite different between the MTJs with Co2MnAl and Co2MnSi electrodes. The obtained TMR ratio of 83% at 2 K in the MTJ with epitaxially grown B2-ordered Co2MnAl was large among the MTJs with an amorphous Al-oxide tunnel barrier. This result suggests that B2-ordered Co2MnAl is a highly spin-polarized material, as predicted by our theoretical calculation. Moreover, we observed a very large TMR ratio of 159% at 2 K in the MTJ with a high-quality epitaxially grown L21-ordered Co2MnSi electrode. This TMR ratio is the highest value to date in MTJs using an amorphous Al-oxide tunnel barrier. Spin-polarization of the Co2MnSi bottom electrode obtained from Jullieres formula was about 0.89. This value is also the largest achieved to date for a Heusler material and is much larger than those of conventional ferromagnetic materials such as Co–Fe. This large spin-polarization is attributed to a half-metallic band structure, as predicted by theoretical calculations.

Large Magnetoresistance in Magnetic Tunnel Junctions Using Co-Mn-Al Full Heusler Alloy

Magnetic tunnel junctions with a stacking structure of Cr/Co-Mn-Al/Al-O/Co-Fe/Ir-Mn were fabricated using a UHV magnetron-sputtering machine. Co-Mn-Al films showed B2 structure, which involves partial disorder between Mn and Al sites. Tunnel magnetoresistance ratios at room temperature were 27% and 40% before and after annealing at 250°C, respectively. Those values are much higher than the ones obtained in previous experiments using half Heusler alloys.

Fabrication of Co2MnAl heusler alloy epitaxial film using Cr buffer layer

Thin films of the full-Heusler compound, Co2MnAl, were grown on Cr-buffered MgO(001) substrates at different growth temperature and post-annealing temperature by magnetron sputtering. Atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements show that Cr-buffer layer can largely improve the surface morphology and structural quality of Co2MnAl film. The (001)-oriented epitaxial growth and B2 structure were confirmed by XRD measurements for all prepared samples. The smallest surface roughness (~2 A) and an identical Ms value to the bulk value were obtained in the sample which was deposited at ambient temperature and post-annealed at 300°C.

Spin-dependent tunneling spectroscopy in single-crystal Fe∕MgO∕Fe tunnel junctions

We report a detailed spin-dependent tunneling spectroscopy in single-crystal Fe(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions (MTJs) that show a giant tunnel magnetoresistance effect. Spectra for antiparallel magnetic configurations show asymmetry because of extrinsic electron scatterings caused by structural defects at the barrier/electrode interfaces. Surprisingly, spectra for parallel magnetic configurations exhibit a complex oscillatory structure that has never been observed in conventional MTJs with an aluminum-oxide tunnel barrier. The complex spectra reflect the tunneling process via interface resonant states. These results provide some information that helps to elucidate the physics of spin-dependent electron tunneling and to further enhance magnetoresistance.

Giant magnetoresistance and interlayer exchange coupling in Ni-Co/Cu multilayer films

Abstract The oscillation of giant magnetoresistance and interlayer exchange coupling as a function of Cu thickness was studied for Ni-Co/Cu multilayer films prepared by magnetron sputtering onto a glass substrate. They had a fcc (111) preferred orientation parallel to the growth direction. Both the value of magnetoresistance ratio and interlayer exchange coupling strength increased with increasing Co content, which were in qualitative agreement with theoretical calculations. The strength of exchange coupling was proportional to the square of the saturation magnetization.

Size dependence of switching field of magnetic tunnel junctions down to 50 nm scale

Tunnel magnetoresistance curves were measured in very small tunnel junctions from scales of 1 μm to 50 nm using conductive atomic force microscopy. The junction arrays were prepared by a simple fabrication process using electron beam lithography. In large size junctions, the minor loops shifted in the negative field direction corresponding to ferromagnetic coupling between free and pinned layers. With decreasing size, the shift changed to the positive field direction corresponding to antiparallel coupling. The dependence of the shift was quantitatively explained by a model taking account of both Neel-type and dipole coupling. The minor loops showed asymmetric shape depending on field sweep directions.

Local current distribution in a ferromagnetic tunnel junction measured using conducting atomic force microscopy

The local electrical properties were measured simultaneously with the topography for a Ta(50 A)/Fe20Ni80(50 A)/IrMn(150 A)/Co(50 A)/Al(13 A)-oxide junction. The electrical image showed the contrast with around a few nm lateral size and a strong correlation with the topographical image was not observed. In the local current–voltage characteristics, data within the bias voltage of ±1.5 V were fitted well to Simmon’s equation and we obtained the barrier height Φ=1.9 eV and the thickness d=12 A. On the other hand, data with the bias voltages higher than 3 V were fitted well to Fowler–Nordheim equation. The histogram of current density was calculated by taking into consideration a Gaussian distribution of the barrier thickness and the height. The distribution of the barrier height can explain the experimental result realistically.

Magnon-assisted inelastic excitation spectra of a ferromagnetic tunnel junction

Inelastic-electron-tunneling spectroscopy (IETS) has been applied to investigate the spin dependent tunneling process for a Ta(50 A)/Fe20Ni80(60 A)/IrMn(300 A)/Co(60 A)/Al(13 A)-oxide/ Co(40 A)/Fe20Ni80(200 A) spin-valve-type tunnel junction. IET spectra for both parallel and antiparallel magnetization configurations of ferromagnetic electrodes showed a strong peak at 12 mV. The subtraction spectrum defined by the difference between the spectra of both the configurations was calculated. Spin-independent inelastic excitation processes are not affected by an external magnetic field, and thus, the subtraction spectrum indicates the inelastic modes induced only by the magnetic origin. It showed a strong peak at 12 and 20 mV for the positively and negatively biased direction of the bottom electrode, respectively. The tendency of the tunneling magnetoresistance ratio to decrease with bias voltage agreed with the shape of the subtraction spectrum. By assuming the surface magnon excitation, we obtained the distri...