Matahiro Komuro

Giant magnetic moment and other magnetic properties of epitaxially grown Fe16N2 single-crystal films (invited)

Single‐crystal Fe16N2 films have been grown epitaxially on Fe(001)/InGaAs(001) and InGaAs(001) substrates by molecular beam epitaxy (MBE). Saturation flux density Bs of Fe16N2 films has been demonstrated to be 2.8–3.0 T at room temperature, which is very close to the value obtained by Kim and Takahashi using polycrystalline evaporated Fe–N films. Temperature dependence of Bs has been measured. Bs changed with temperature reversibly up to 400 °C, while beyond 400 °C, Bs decreased irreversibly. X‐ray diffraction showed that Fe16N2 crystal is stable up to 400 °C, while beyond 400 °C, Fe16N2 dissolves into Fe and Fe4N, and also some chemical reactions between Fe16N2 and the substrate occurs. This caused the temperature dependence of Bs mentioned above. From the temperature dependence of Bs up to 400 °C, the Curie temperature of Fe16N2 is estimated to be around 540 °C by using the Langevin function. The above mentioned Bs of 2.9 T at room temperature and 3.2 T at −268 °C corresponded to an average magnetic mom...

Magnetic and Mössbauer studies of single‐crystal Fe16N2 and Fe‐N martensite films epitaxially grown by molecular beam epitaxy (invited)

Single‐phase, single‐crystal Fe16N2(001) films and Fe‐11 at. %N martensite films of 200–900 A thickness have been epitaxially grown on In0.2Ga0.8As(001) substrates by evaporating Fe in an atmosphere of mixed gas of N2 and NH3, followed by annealing. The saturation magnetizations 4πMs’s for Fe16N2 and Fe‐N martensite films have been measured to be around 29 and 24 kG at room temperature, respectively, and almost constant in the above thickness range by using a vibrating sample magnetometer. 4πMs for Fe‐N martensite films has been increased with ordering of N atoms caused by annealing and finally reached around 29 kG for Fe16N2. Mossbauer spectra have been measured for those films. The spectrum for Fe‐N martensite films was a superposed one with hyperfine fields of 360, 310, and 250 kOe, similar to those previously reported for martensite. While the spectrum became simpler with ordering, finally reaching a single hyperfine field of 330 kOe for Fe16N2. 4πMs of 29 kG for Fe16N2 (3.2 μB/Fe atom) and 4πMs of 24...

Ferromagnetic resonance studies of Fe16N2 films with a giant magnetic moment

The g factor and 4π Ms for epitaxially grown Fe16N2(001)/In0.2Ga0.8As(001) films have been investigated by ferromagnetic resonance along with Fe films for comparison. Angular dependence of the resonance fields in the film plane of Fe16N2 films had four‐fold symmetry, which was attributed to the in‐plane anisotropy. The g factor for Fe16N2 films was about 2.0, which means that the magnetic moment originates mainly from spin. Thus, nothing unusual is seen about the g factor. The g factor for Fe films was about 2.1, which is very similar to the value reported previously. 4πMs values for Fe16N2 and Fe films were 2.8×104 and 2.1×104 G, respectively, which agree well with the previous data obtained by a vibrating sample magnetometer. This confirmed that Fe16N2 has a giant magnetic moment. Torque magnetometer measurements showed that Fe16N2 films have a larger perpendicular anisotropy of 7.8×106 erg/cm3, which can originate from its bct structure.

Magnetic and electrical properties of single‐phase, single‐crystal Fe16N2 films epitaxially grown by molecular beam epitaxy (invited)

The average magnetic moment per Fe atom for a single‐phase, single‐crystal Fe16N2(001) film epitaxially grown on a GaAs(001) substrate by molecular beam epitaxy has been confirmed to be 3.5μB at room temperature by using a vibrating sample magnetometer (VSM) and Rutherford backscattering. The value was in good agreement with that obtained by using a VSM and by measuring the film thickness (3.3μB per Fe atom). The saturation magnetization 4πMs has been found to increase with decreasing temperature, obeying T3/2 law at lower temperatures. The slope was steeper than that of a pure Fe film, suggesting a lower exchange constant for Fe16N2. The g factor for Fe16N2 has been accurately measured to be 2.17 by using ferromagnetic resonance with changing frequencies of 35.5–115 GHz, which is not unusual compared with the g factor of 2.16 for pure Fe. The resistivity for Fe16N2 has been measured to be around 30 μΩ cm at room temperature compared with 10 μΩ cm for pure Fe and decreases linearly with decreasing tempera...

Increase of Coercivity and Composition Distribution in Fluoride-Diffused NdFeB Sintered Magnets Treated by Fluoride Solutions

Rare-earth and fluorine were diffused along grain boundaries in NdFeB sintered magnets during heat treatment after rare-earth fluoride liquid coating. In the case of Dy fluoride coating, the coercivity for NdFeB sintered magnets with a thickness of 1 mm after the diffusion was increased from 0.80 MA/m to 1.13 MA/m (41%) with remanence reduction of 0.6%. Segregations for Dy atoms near the grain boundaries and fluorine atoms at the grain boundary were found by TEM observations. The increase of coercivity per the weight of the rare-earth fluorides has a correlation with the anisotropy field of the RE2Fe14B (RE is rare-earth elements). Furthermore, the Dy quantity required for the observed increase of coercivity can be reduced to 1/3 compared to the previous NdFeB sintered magnets using the fluoride coating and diffusion process.

Magnetic properties of a Nd–Fe–B sintered magnet with Dy segregation

The magnetic properties and microstructures of a Nd–Fe–B sintered magnet with Dy segregation obtained by applying a fluoride-solution technique were studied using transmission electron microscopy and micromagnetic analysis. By applying 0.1–0.2 wt % fluorides in comparison with weight of the magnet and heating the magnet, coercivity increased by about 1.3 times from 1.1 to 1.4 T at 298 K while retaining remanence. Dy segregated within about 250 nm from the grain surfaces and coercivity was controlled by the area near the grain boundaries. Furthermore, we estimated that the range for reducing the magnetic anisotropy field near the grain boundaries was one or two unit cells by applying micromagnetic analysis to the magnet, which may indicate larger coercivity with less Dy.

Structures and magnetic properties of Fe/Cu multilayered films fabricated by a magnetron sputtering method

Fe and Cu single‐layered films and Fe/Cu multilayered films were prepared by an rf‐magnetron sputtering method, and the film structures and magnetic properties were investigated. Fe and Cu single‐layered films become continuous when their thickness is more than 4 nm for Fe and 10 nm for Cu. The crystal structure of Fe/Cu multilayered films depends on the thickness or continuity of Fe and Cu layers. A Fe 1.6 nm/Cu 2 nm multilayered film has a periodic structure. As the thickness of the Fe layer decreases, the magnetic flux density and coercivity decrease and the magnetostriction constant changes from negative to positive. Uniaxial magnetic anisotropy in the plane of the multilayered Fe/Cu films is observed when the Fe layer is discontinuous, but it is not clearly observed for a continuous Fe film.

High electrical resistance hot-pressed NdFeB magnet for low loss motors

Fluoride-coated hot-pressed NdFeB magnets with electrical resistivity of 1.4mΩcm, which is ten times larger than that of a noncoated sintered NdFeB magnet, were prepared using fluoride coating powders. The high resistive NdFeB magnets consist of coated powders and are sintered with a neodymium fluoride layer, which was grown using a fluoride solution. No degradation of coercivity was observed in the fluoride coated NdFeB powders. The increase in magnet temperature caused by an alternating magnetic field (Eddy currents) was eightfold with the use of a fluoride-treated magnet. Furthermore, the increase in temperature at the magnet’s rotor was reduced by 50% when the high resistive hot-pressed magnet was substituted for the conventional commercial magnet.

Cogging torque due to roundness errors of the inner stator core surface

Cogging torque of brushless permanent magnet (PM) motors employing segmented stator cores with narrow slot openings has been investigated. Two types of test motors were examined: one with a segmented Halbach cylinder rotor and the other with a radially oriented ring PM rotor. The measured cogging torque was predicted from the measured dimensional errors for the inner radius of the stator core in both types.

Giant Magnetoresistance of Spin Valve Films with NiO Antiferromagnetic Films

The magnetic and magnetoresistive properties of films having spin valve structures were investigated. The basic structure of such films consists of two ferromagnetic layers separated by a Cu layer. One of the ferromagnetic layers is strongly biased by an NiO antiferromagnetic film in direct exchange contact with it. Spin valve films having an NiO/NiFe/Cu/NiFe structure exhibit good sensitivity, and the MR ratio (¿¿/¿) is 4% in an applied field of 10 Oe. Films with a layered NiO/NiFe/Cu/NiFe/Cu/NiFe/NiO structure have an MR ratio of 7% in a field of 10 Oe. On the other hand, NiO/NiFe/Co/Cu/Co/NiFe films exhibit an MR ratio of 6% to 7.5%. The thermal stability of spin valve films was also studied.