Koki Takanashi

Large tunnel magnetoresistance in magnetic tunnel junctions using a Co2MnSi Heusler alloy electrode and a MgO barrier

A large tunnel magnetoresistance (TMR) ratio of 753% has been observed at 2 K in a magnetic tunnel junction (MTJ) using a Co2MnSi Heusler alloy electrode and a crystalline MgO tunnel barrier. This TMR ratio is the largest reported to date in MTJs using a Heusler alloy electrode. Moreover, we have observed a large TMR ratio of 217% at room temperature (RT). This TMR at RT is much larger than that of MTJs using an amorphous Al-oxide tunnel barrier. However, the temperature dependence of the TMR ratio is still large because of inelastic tunneling in the antiparallel magnetic configuration.

Future perspectives for spintronic devices

Spintronics is one of the emerging research fields in nanotechnology and has been growing very rapidly. Studies of spintronics were started after the discovery of giant magnetoresistance in 1988, which utilized spin-polarized electron transport across a non-magnetic metallic layer. Within 10 years, this discovery had been implemented into hard disk drives, the most common storage media, followed by recognition through the award of the Nobel Prize for Physics 19 years later. We have never experienced such fast development in any scientific field. Spintronics research is now moving into second-generation spin dynamics and beyond. In this review, we first examine the historical advances in spintronics together with the background physics, and then describe major device applications.

Tunnel magnetoresistance in Co nanoparticle/Co–C60 compound hybrid system

A nanometer-scale hybrid film of Co particle/Co–C60 compound was prepared by alternate deposition of Co and C60 under UHV condition. All of Raman spectra, magnetization curves, and tunnel conductivity concluded that the hybrid system has a granular structure consisting of Co nanoparticles embedded in a Co–C60 compound matrix. The magnetoresistance ratio of 26% was obtained at 2K and 10kOe for the electron tunneling across the Co–C60 compound barrier. In addition, anomalously large bias voltage dependence was found in the magnetotransport properties.

Magnetooptical Kerr Spectrometer for 1.2-5.9 eV Region and its Application to FePt/Pt Multilayers

The apparatus for measurement of magnetooptical spectra using a polarization modulation technique has been improved to extend the short-wavelength limit to about 200 nm in the ultraviolet range. Short-wavelength characteristics of the light source, the focusing system including mirrors and lenses, the monochromator, the polarizer and the analyzer were improved. The techniques of calibration for Kerr rotation and ellipticity were also refined. With the use of the newly designed system, the magnetooptical Kerr rotation and Kerr ellipticity of FePt/Pt multilayers, as well as of FePt alloys of different compositions were evaluated for photon energies between 1.2 and 5.9 eV.

Fe?Ni composition dependence of magnetic anisotropy in artificially fabricated L1 0-ordered FeNi films

We prepared L10-ordered FeNi alloy films by alternate deposition of Fe and Ni monatomic layers, and investigated their magnetic anisotropy. We employed a non-ferromagnetic Au-Cu-Ni buffer layer with a flat surface and good lattice matching to L10-FeNi. An L10-FeNi film grown on Au6Cu51Ni43 showed a large uniaxial magnetic anisotropy energy (Ku = 7.0 × 10(6) erg cm(-)3). Ku monotonically increased with the long-range order parameter (S) of the L10 phase. We investigated the Fe-Ni composition dependence by alternating the deposition of Fe 1 − x and Ni 1 + x monatomic layers (− 0.4 < x < 0.4). Saturation magnetization (Ms) and Ku showed maxima (Ms = 1470 emu cm(-3), Ku = 9.3 × 10(6) erg cm(-3)) for Fe60Ni40 (x = -0.2) while S showed a maximum at the stoichiometric composition (x = 0). The change in the ratio of lattice parameters (c/a) was small for all compositions. We found that enrichment of Fe is very effective to enhance Ku. The large Ms and Ku of Fe60Ni40 indicate that Fe-rich L10-FeNi is promising as a rare-earth-free permanent magnet.

Novel Mn-doped chalcopyrites

Abstract Heavily Mn-doped II–VI–V 2 semiconductors, such as CdGeP 2 and ZnGeP 2 have been prepared by depositing Mn on single crystalline substrate at nearly 400 °C in an ultra high vacuum chamber. Well-defined ferromagnetic hysteresis with a saturation behavior appears in the magnetization curve up to above room temperature. The chemical states of the ZnGeP 2 :Mn interface has been clarified by a careful in situ photoemission spectroscopy. The as-prepared surface consists of Ge-rich, metallic Mn compound. In and below the sub-surface region, dilute divalent Mn species as precursors of the DMS phase exist. No MnP phase was observed at any stage of the depth profile. Theoretical band-calculation suggests that the system with vacancies (Cd, V c , Mn)GeP 2 or a non-stoichiometric (Cd, Ge, Mn)GeP 2 are ferromagnetic and energetically stable although ferromagnetism is not stable in a stoichiometric compound (Cd, Mn)GeP 2 .

Giant tunnel magnetoresistance in codeposited fullerene-cobalt films in the low bias-voltage regime

Magnetotransport properties in the low bias-voltage regime were investigated for codeposited C60–Co films. A giant tunnel magnetoresistance (MR) ratio (ΔR∕Rmax) of 80%, which is the highest in ferromagnetic metal/organic molecule systems, was found at low temperatures. The observed bias-voltage dependence of the MR ratio is expressed by an unusual exponential form, suggesting that the MR ratio of nearly 100% can be realized in the low bias-voltage limit.

Anomalous Nernst Effect in an L10-Ordered Epitaxial FePt Thin Film

The anomalous Nernst effect in a perpendicularly magnetized L10-ordered epitaxial FePt(001) thin film has been investigated, and the anomalous Nernst coefficient and the anomalous Nernst angle of the FePt thin film were experimentally evaluated. Furthermore, the voltage due to the anomalous Nernst effect in the spin-Hall device was simulated by the finite element method. A good agreement between the experiment and the simulation was found. It was revealed that the anomalous Nernst effect could be quantitatively discussed even in nanoscale devices.

Anomalous Nernst Effect in L10-FePt/MnGa Thermopiles for New Thermoelectric Applications

We propose a new-type of thermopile consisting of two ferromagnetic materials with anomalous Nernst effects (ANEs) of opposite signs. L10-FePt and L10-MnGa have been chosen as the materials because they show large ANEs with opposite signs. The combination of perpendicularly magnetized FePt and MnGa wires enhances the ANE voltage effectively. The ANE in in-plane magnetized FePt films induced by applying a perpendicular temperature difference shows no variation against the film thickness, which is a promising characteristic for thermoelectric applications because the internal resistance of the thermopile, which determines the extractable electric power, can be reduced by increasing the thickness of ferromagnetic wires.

Structural, magnetic and electronic state characterization of L1 0-type ordered FeNi alloy extracted from a natural meteorite

To understand the hard magnetism of L10-type ordered FeNi alloy, we extracted the L10-FeNi phase from a natural meteorite, and evaluated its fundamental solid-state properties: sample composition, magnetic hysteresis, crystal structure and electronic structure. We executed multidirectional analyses using scanning electron microscopy with an electron probe micro-analyzer (SEM-EPMA), a superconducting quantum interference device (SQUID), x-ray diffraction (XRD) and magnetic circular dichroism (MCD). As a result, we found that the composition was Fe: 50.47 ± 1.98 at.%, Ni: 49.60 ± 1.49 at.%, and an obvious superlattice peak is confirmed. The estimated degree of order was 0.608, with lattice constants a = b = 3.582 Å and c = 3.607 Å. The obtained coercivity was more than 500 Oe. MCD analysis using the K absorption edge suggests that the magnetic anisotropy could originate from the orbital magnetic moment of 3d electrons in Fe; this result is consistent with that in a previous report obtained with synthetic L10-FeNi.