Nobuki Tezuka

Spin Injection, Transport, and Detection at Room Temperature in a Lateral Spin Transport Device with Co2FeAl0.5Si0.5/n-GaAs Schottky Tunnel Junctions

We observed spin-valve signals and Hanle signals in four-terminal nonlocal measurements on a lateral spin transport device with Co2FeAl0.5Si0.5(CFAS)/n-GaAs Schottky tunnel junctions. The estimated spin injection/detection efficiency was 0.06 at 4.2 K, which is larger than those of the devices with Fe and CoFe electrodes [Nature Physics 3 (2007) 197 and Appl. Phys. Lett. 99 (2011) 082108]. The spin diffusion length estimated from Hanle signals was consistent with the gap length dependency of the spin-valve signals. Furthermore, the spin-valve signals were observed at up to 290 K. This is the first demonstration of detecting spin accumulation in semiconductor with full-Heusler alloys electrodes at room temperature.

Preparation of ultrafine jet-milled powders for Nd-Fe-B sintered magnets using hydrogenation–disproportionation–desorption–recombination and hydrogen decrepitation processes

Dy addition is used to increase the coercivity of Nd-Fe-B sintered magnets. Given that Dy is rare and expensive, a method is needed for reducing the Dy content in such magnets without decreasing their coercivity. Refining Nd2Fe14B grains is a prospective method for increasing the coercivity of Nd-Fe-B magnets. Conventional jet milling, however, cannot crush strip-casted Nd-Fe-B alloys into powders less than 1u2009μm in size. We report a process for preparing ultrafine jet-milled powders with an average size of 0.33u2009μm for Nd-Fe-B sintered magnets: a combination of hydrogenation–disproportionation–desorption–recombination, hydrogen decrepitation, and He jet milling.

Microstructure evaluation for Dy-free Nd-Fe-B sintered magnets with high coercivity

Nd-Fe-B sintered magnets are used for motors of hybrid or electric vehicles due to their high energy products. Dy is added to Nd-Fe-B sintered magnets to work in a high temperature environment. Although the addition of Dy decreases the magnetization of Nd-Fe-B magnets, it increases coercivity; a decrease in the amount of Dy is strongly required. Recently, Nd-Fe-B sintered magnets with a grain size of 1u2009μm achieved high coercivity of ∼20 kOe without the addition of Dy or other heavy rare earth elements. In this paper, the microstructure of their magnets was observed and compared to magnets with a grain size of ∼3u2009μm. The coercivity of magnets consisting of larger particles was 17 kOe. Microstructures were observed by the scanning electron microscope and the shapes of grains and the distribution of the Nd-rich phase were evaluated. The observation was promoted in two directions. One direction is the plane perpendicular to the magnetically aligned direction (c plane side) and the other is the side parallel t...

Maximum magnitude in bias-dependent spin accumulation signals of CoFe/MgO/Si on insulator devices

We study in detail how the bias voltage (Vbias) and interface resistance (RA) depend on the magnitude of spin accumulation signals (|ΔV| or |ΔV|/I, where I is current) as detected by three-terminal Hanle measurements in CoFe/MgO/Si on insulator (SOI) devices with various MgO layer thicknesses and SOI carrier densities. We find the apparent maximum magnitude of spin polarization as a function of Vbias and the correlation between the magnitude of spin accumulation signals and the shape of differential conductance (dI/dV) curves within the framework of the standard spin diffusion model. All of the experimental results can be explained by taking into account the density of states (DOS) in CoFe under the influence of the applied Vbias and the quality of MgO tunnel barrier. These results indicate that it is important to consider the DOS of the ferromagnetic materials under the influence of an applied Vbias and the quality of tunnel barrier when observing large spin accumulation signals in Si.

Three-Terminal Hanle Signals in Schottky Tunnel Junctions with Co2FeAl0.5Si0.5 Full-Heusler Alloy Electrodes Deposited at Various Temperatures

We investigated the crystal structures, electrical transport properties, and three-terminal (3T) Hanle signals of Co2FeAl0.5Si0.5 (CFAS)/n-GaAs Schottky tunnel junctions fabricated with different deposition temperature of the CFAS (TCFAS). CFAS thin films were deposited on the n-GaAs at TCFAS ranging from room temperature to 300 °C, and 3T-Hanle signals were observed for all CFAS/n-GaAs junctions. Although the degree of structural ordering in the CFAS electrodes decreased and the rectifying characteristic disappeared as TCFAS decreased, the spin resistance area products (ΔRA) increased and the estimated spin relaxation time (τ) decreased monotonically with decreasing TCFAS. Moreover, the bias voltage dependence of ΔRA and τ became larger and smaller with decreasing TCFAS, respectively.

Four-terminal nonlocal signals in lateral spin transport devices with variously ordered Co2FeAl0.5Si0.5 full-Heusler alloy electrodes

The structural ordering of the Co2FeAl0.5Si0.5 (CFAS) full-Heusler alloy, the electrical transport properties, and the four-terminal (4T) nonlocal signals were investigated in lateral spin transport devices with CFAS/n-GaAs Schottky tunnel junctions as a function of the deposition temperature of CFAS (TCFAS). The 4T nonlocal signals increased with increasing TCFAS, in contrast to the trend in three-terminal Hanle measurements [Jpn. J. Appl. Phys., Part 1 52, 063001 (2013)]. No relationship between interface resistance and 4T nonlocal signal was confirmed, indicating that conductance mismatch problems did not affect the tendency of signal values.

Effect of Cu addition on coercivity and interfacial state of Nd-Fe-B/Nd-rich thin films

This study provides the effect of Cu addition on coercivity (HcJ) and interfacial microstructure in Nd-Fe-B/Nd-rich thin films. All films were deposited by using ultra high vacuum (UHV) magnetron sputtering, and the Nd-Fe-B layer was oxidized under several atmospheres with different oxygen content. Then, the films were annealed at 250–550 °C under UHV. The films oxidized in low vacuum (10−2−10−5 Pa) (under low oxygen state) exhibited the recovery of HcJ by the annealing at 450 °C. On the contrary, the HcJ of the films oxidized in Ar (under high oxygen state) decreased with increasing annealing temperature. However, the HcJ increased drastically at the temperatures above 550 °C. In addition, the Cu added films, which were annealed at temperatures above 350 °C, showed higher coercivities than the films without Cu addition. The XRD analysis suggested the existence of C-Nd2O3 phase in the Cu added films annealed at 550 °C. It can be considered that the Cu addition decreases the eutectic temperature of Nd-rich phase and influences the interfacial state between Nd2Fe14B and Nd-rich phase.

Influence of microstructural change of the interface between Nd2Fe14B and Nd-O phases on coercivity of Nd-Fe-B films by oxidation and subsequent low-temperature annealing

This study provides the influence of microstructural change of the interface between Nd2Fe14B and Nd-O phases on coercivity of Nd-Fe-B thin films during annealing at low temperature (~350 °C). All films were prepared by using ultra high vacuum (UHV) magnetron sputtering, and the Nd-Fe-B layer was oxidized under Ar gas atmosphere (O2 content; ~2 Vol.ppm). Then, the films were annealed at 250–350 °C under UHV condition. After oxidation, the coercivity of Nd-Fe-B film decreased to around 40% of the coercivity of as-deposited Nd-Fe-B film. The Nd-rich phase changed from α-Nd to amorphous Nd(-O), and the interface of Nd2Fe14B/Nd(−O) became rough. In the Nd-Fe-B films oxidized and subsequent annealed at 350 °C, the coercivity decreased to around 20%. In the films, poly crystalline hcp Nd2O3 phase crystallized in Nd-rich phase, and there were some steps at the surface of Nd2Fe14B phase contacting with hcp Nd2O3 phase. Regardless of crystal orientation of Nd2Fe14B, the microstructural changes of the interface described above were observed.

Nd-Fe-B sintered magnets fabrication by using atomized powders

Nd-Fe-B sintered magnets are required to achieve high coercivity for improvement of their thermal stability. Dy is added to increase coercivity, however, this element decrease magnetization and energy products. Therefore, Dy-lean Nd-Fe-B sintered magnets with high coercivity are strongly demanded. To increase coercivity, it is necessary that microstructure of sintered magnets is consisted of both fine main phase particles and homogeneously distributed Nd-rich phases around the main phase. To meet those requirements, Nd-Fe-B atomized powders were applied to the fabrication process of sintered magnets. Comparing with the case of using strip casting (SC) alloys, jet-milled powders from atomized powders show homogeneous distribution of Nd-rich phase. After optimized thermal treatment, coercivities of sintered magnets from atomized powders and SC alloys reach 1050 kAm−1 and 1220 kAm−1, respectively. This difference in coercivity was due to initial oxygen concentration of starting materials. Consequently, Nd-rich phases became oxides with high melting points, and did not melt and spread during sintering and annealing.

Preparation and magnetic properties of amorphous Co-Fe-B nanoparticles

Amorphous Co-Fe-B nanoparticles coated with PVP (polyvinyl pyrrolidone) were prepared by reduction of metal ions in aqueous solution by use of NaBH4 at room temperature. The size of nanoparticles was in the range of 10-20 nm and from the XRD patterns, these nanoparticles were amorphous. FE-SEM-WDX analyses showed the B content to be in the range of 38–45 at%. The saturation magnetization and coercivity for the samples was in the range of 45–50 emu·g1 and 31–110 Oe, respectively. The composites of nanoparticles with the minimum coercivity and PVP as the binder (particle weight percent = 75 wt.%) showed permeability of μr max = 1.9 at 0.7 GHz and a good microwave absorption property (reflection loss: RL < –20 dB).