Takao Furubayashi

Structural and Magnetic Studies of Metal-Insulator Transition in Thiospinel CuIr2S4.

Crystal structure and magnetic susceptibility were investigated for a thiospinel compound CuIr 2 S 4 , which exhibits a transition from metallic to insulating on cooling at the transition temperature T c of 230 K. It was found that the transition is accompanied by a first-order structural transition. As the compound is cooled through T c , the structure transforms from cubic to tetragonal with a volume contraction of 0.7%. The tetragonal phase has an axial ratio of about 1.03. Magnetic susceptibility, which is paramagnetic in the metallic region, decreases sharply and becomes diamagnetic at T c on cooling. The susceptibility is mostly temperature independent in each region above and below T c . Magnetic measurements also revealed that high pressure stabilizes the insulating phase, in contrast with most of the compounds that exhibit metal-insulator transitions. Possible mechanisms of the metal-insulator transition are discussed.

Large magnetoresistance in current-perpendicular-to-plane pseudospin valve using a Co2Fe"Ge0.5Ga0.5… Heusler alloy

Using a newly developed highly spin-polarized Heusler alloy, Co2Fe(Ga0.5Ge0.5) (CFGG), as ferromagnetic layers, we have fabricated a current-perpendicular-to-plane pseudospin valve with large resistance change-area product (ΔRA) of 9.5 mΩ μm2 and magnetoresistance (MR) ratio (100×ΔR/R) of 41.7% at 300 K. These values were further enhanced to ΔRA=26.4 mΩ μm2 and MR=129.1% at 10 K. The large MR values are attributed to the high spin polarization of the CFGG alloy confirmed by point contact Andreev reflection measurements.

Preparation and magnetic properties of colloidal ferromagnetic metals

Abstract Stable magnetic fluids containing fine particles of Fe, Co or Ni have been prepared by evaporating the metals in vacuum onto the surface of the oil containing an oil-soluble surfactant. At room temperature, these magnetic fluids are superparamagnetic and have saturation magnetizations of 165, 270 and 84 G for the Fe, Co and Ni magnetic fluids, respectively.

Enhancement of giant magnetoresistance by L21 ordering in Co2Fe(Ge0.5Ga0.5) Heusler alloy current-perpendicular-to-plane pseudo spin valves

We report large magnetoresistance (MR) output in fully epitaxial Co2Fe(Ge0.5Ga0.5)/Ag/Co2Fe(Ge0.5Ga0.5) current-perpendicular-to-plane pseudo spin valves. The resistance-area product change (ΔRA) of 12 mΩμm2 at room temperature (RT), equivalent to MR ratio of 57%, and ΔRA = 33 mΩμm2 at 10 K, equivalent to MR ratio of 183%, were obtained by using L21-ordered Co2Fe(Ge0.5Ga0.5) ferromagnetic electrodes. The bulk spin scattering asymmetry (β) were estimated to be ∼0.83 at RT and ∼0.93 at 10 K for the L21-ordered Co2Fe(Ge0.5Ga0.5) films by the Valet-Fert model, indicating that the L21-ordered Co2FeGe0.5Ga0.5 Heusler alloy is virtually half-metal at 10 K, but its half-metallicity is degraded at RT.

Tunnel magnetoresistance with improved bias voltage dependence in lattice-matched Fe/spinel MgAl2O4/Fe(001) junctions

We fabricated fully epitaxial Fe/MgAl2O4/Fe(001) magnetic tunnel junctions using plasma oxidation of an Mg/Al bilayer. The MgAl2O4 showed a (001)-oriented spinel-type structure, and there were few misfit dislocations at the interfaces between the MgAl2O4 and the two Fe layers due to a small lattice mismatch (∼1%). Tunnel magnetoresistance (TMR) ratios up to 117% (165%) were obtained at room temperature (15 K). The bias voltage for one-half of the zero-bias TMR ratio (Vhalf) was relatively large, ranging from 1.0 to 1.3 V at room temperature, which is attributed to the small misfit dislocation density.

High spin-filter efficiency in a Co ferrite fabricated by a thermal oxidation

Co ferrites fabricated by a thermal oxidation have been tested as a spin filter. Spin-filter efficiencies of 44% and 4.3% were confirmed at 10 K and RT, respectively, using a magnetic tunnel junction of Pt/CoFe2O4/MgO/Co. By increasing the bias voltage, the tunneling magnetoresistance (TMR) at 10 K increases then decreases. This is interpreted as a tunneling via spin-down conduction band in the Co ferrite. Since the electron at RT is attributed to a hopping conduction on the defect states in the Co ferrite, the reduction of the defects will be the key to achieving a higher spin-filter effect.

A 4‐Fold‐Symmetry Hexagonal Ruthenium for Magnetic Heterostructures Exhibiting Enhanced Perpendicular Magnetic Anisotropy and Tunnel Magnetoresistance

A 4-fold-symmetry hexagonal Ru emerging in epitaxial MgO/Ru/Co2 FeAl/MgO heterostructures is reported, in which an approximately Ru(022¯3) growth attributes to the lattice matching between MgO, Ru, and Co2 FeAl. Perpendicular magnetic anisotropy of the Co2 FeAl/MgO interface is substantially enhanced. The magnetic tunnel junctions (MTJs) incorporating this structure give rise to the largest tunnel magnetoresistance for perpendicular MTJs using low damping Heusler alloys.

Indium‐Based Perovskites: A New Class of Near‐Room‐Temperature Multiferroics

These systems may have wide technological applicationsbecause they allow control of electric properties by magneticfield and control of magnetic properties by electric field.Applications might include multiple-state memory elementsand will allow exploitation of the advantages of both ferro-electric and magnetic random-access memories.

Exchange bias of spin valve structure with a top-pinned Co40Fe40B20∕IrMn

We have investigated the exchange bias of a directly top-pinned Co40Fe40B20∕IrMn structure. An exchange bias was realized on the as-deposited samples, in which Co40Fe40B20 exhibits a fully amorphous structure. A current-in-plane giant magnetoresistance effect was demonstrated on simple Ru∕CoFeB∕Cu∕CoFeB∕IrMn∕Ru stacks prior to and after annealing. The amorphous CoFeB layer partially crystallized from the interface with a Cu spacer layer after annealed at 280°C.

Spin-transfer switching in an epitaxial spin-valve nanopillar with a full-Heusler Co2FeAl0.5Si0.5 alloy

We report magnetization switching by spin-transfer torque in an epitaxial spin-valve nanopillar made with a half-metallic full-Heusler Co2FeAl0.5Si0.5 (CFAS) alloy. The CFAS/Ag/CFAS spin valves showed a magnetoresistance ratio of 7%–9%, and spin-transfer switching was clearly observed in the nanopillar by applying a relatively small dc current (∼106 A/cm2 in current density). Statistical analysis based on a thermal activation model revealed an averaged critical current density (Jc0) of 9.3×106 A/cm2 with a thermal stability factor (KV/kBT) of ∼40.