Kurima Kobayashi

High‐resolution neutron powder diffraction study on nitrogenated Nd2Fe17

High‐resolution neutron powder diffraction measurements with λ=1.8232 A and collimation of 6’‐20’‐6’ on Nd2Fe17Nx with x=0, 2.85, and 2.91 were carried out at room temperature. Structural parameters and magnetic moments were determined by the Rietveld profile‐fitting calculation. The magnetic moments of iron atoms increased with increasing nitrogen concentration; e.g., the magnetic moments of iron located at 6c, 9d, 18f, and 18h sites increase from less than 0.7μB in the x=0 sample to about 2.1μB in the x=2.91 sample. It is noticed that a local atomic group composed of two Fe(1) atoms at the 6c site plus six Fe(3) atoms at the 18f site keeps its shape against the nitrogen uptake.

Magnetic properties of the single magnetic domain particles of Sm2Fe17Nx compounds

The magnetic material Sm2Fe17Nx (0 < x < 6) can be prepared by reacting Sm2Fe17 in an NH3-H2 mixed gas atmosphere. As a fine powder, Sm2Fe17Nx (x = 3.0–3.2) exhibits the characteristics of single magnetic domain particles. The oxygen content of the particle increases to about 5 at.% with increasing specific surface area, but such an oxidized region is largely confined to an outer layer of the particle. The thickness of the O-rich layer is about 10 nm, as observed by transmission electron microscopy and Auger electron spectroscopy.

Dependence of coercivity on particle size in Sm2Fe17N3 powders

Abstract The coercivity of Sm 2 Fe 17 N 3 particles varies as the inverse of the average particle size D , when 1 μm D H c = 2 H a L D / D , where H a is the anisotropy field of Sm 2 Fe 17 N 3 , is explained in a model where there is a random spatial distribution of nucleation centers whose average separation L 0 is equal to 120 nm and whose size follows a 1/ r 4 power law. Hysteresis has been measured as a function of the angle θ between the alignment direction and the applied field for aligned powder samples with D = 3,0, 4.4, 5.8, 13 and 52 μm. It has a 1/cos θ dependence.

The origin of coercivity enhancement in newly prepared high coercivity Dy-free Nd-Fe-B sintered magnets

Coercivity enhancement in Nd-Fe-B sintered magnets without substitution of Dy for Nd is an important research target in the field of permanent magnets. Our newly prepared Dy-free Nd-Fe-B sintered magnets with fine crystal grain sizes of approximately 1.9 μm showed high coercivities near 1.6 MA/m. The characteristic magnetization behavior of the magnets that we have not observed previously in other Nd-Fe-B sintered magnets, under applied fields of 1.4–2.4 MA/m, appears to be responsible for the appearance of the high coercivity. In this study, we examine the origin of the characteristic magnetization mechanism and conclude that it was caused by the strong interaction between the domain walls (DWs) and the grain boundaries (GBs). We introduce our novel step method and investigate the characteristic mechanism using the method. This method can assist in clarifying whether the characteristic magnetization mechanism results from the hopping of DWs from one GB to another, and/or by magnetic reversal appearing to...

High‐field magnetization process in Sm2Fe17N3.0: Importance of exchange‐enhanced mixing of excited J multiplets

The high field magnetization process in Sm2Fe17N3.0 has been investigated by using dc magnetic fields of up to 260 kOe generated by a hybrid magnet. It was found that the hard‐axis magnetization at 4.2 K is only 78% of the easy‐axis value in the field of 260 kOe, while at 296 K, it almost saturates at the same field, the saturation value being about 3% smaller than that along the easy axis. Experimental results have been analyzed on the basis of a model calculation taking the excited J multiplets of the Sm ion into account. It has been found that, when the field is applied along the hard axis at 296 K, the Sm magnetic moment rotates away from the field direction and finally, at the saturation field, it becomes antiparallel to the field so that the Sm and Fe moments are arranged ferrimagnetically. The origin of such a magnetization process was clearly explained as a result of the mixing of the excited J multiplets caused by the Sm–Fe exchange interaction. Observed anisotropy of saturation magnetization at ...

(Sm,Zr)(Fe,Co)11.0-11.5Ti1.0-0.5 compounds as new permanent magnet materials

We investigated (Sm,Zr)(Fe,Co)11.0-11.5Ti1.0-0.5 compounds as permanent magnet materials. Good magnetic properties were observed in (Sm0.8Zr0.2)(Fe0.75Co0.25)11.5Ti0.5powder containing a limited amount of the α-(Fe, Co) phase, including saturationpolarization (Js) of 1.63 T, an anisotropic field (Ha) of 5.90 MA/m at room temperature, and a Curie temperature (Tc) of about 880 K. Notably, Js and Ha remained above 1.5 T and 3.70 MA/m, respectively, even at 473 K. The high-temperature magnetic properties of (Sm0.8Zr0.2)(Fe0.75Co0.25)11.5Ti0.5 were superior to those of Nd2Fe14B.

SUSCEPTIBILITY AND MAGNETIZATION PROCESSES IN SM2FE17N3 POWDERS

Low field ac susceptibility and magnetization curves of milled Sm2Fe17N3 powders are measured for isotropic and aligned samples with average particle sizes ranging from 3.0 to 52 μm. Most of the particles are in a multidomain state before application of a magnetic field. The transverse ac susceptibility (χ⊥ac∼0.08) due to magnetization rotation against the anisotropy field is independent of particle size. The parallel ac susceptibility χ∥ac and the susceptibility of isotropic powders are dominated by almost free domain wall movement in multidomain particles for which 〈χ〉=1.35. In very low fields (<0.5 mT) χ∥ac is small (∼0.1), but it increases toward a constant value of order 1 when the field exceeds a value of order 1 mT which is inversely proportional to the particle size. There is a corresponding increase in loss angle which is attributed to weak pinning of domain walls at surface defects. The volume fraction of single‐domain and multidomain powder are deduced as a function of average particle size bot...

Magnetic properties of the surface layer and its magnetic interaction with the interior of Nd-Fe-B sintered magnets

The magnetization and demagnetization mechanisms in the mechanically polished surface layers (SL) of the c-plane and a-b plane of Nd-Fe-B sintered magnets were investigated. The magnetic interaction between the SL and the interior of the magnet was clarified by using vibrating sample magnetometer measurements of a whole sample and magneto-force microscopy observations of the domain structure of the SL layer. The polishing eliminated the Nd-rich grain boundary phases, which was only about 2 nm thick, from the SL crystal grains in the sintered magnets. The a-b plane polishing caused the independent magnetic reversal of the a-b plane SL, which was about 5.5 μm thick, as in the platy samples. The coercivities (μ0Hc) of the SL were less than 0.3 T. In contrast, the c-plane polishing did not produce independent magnetic reversal of the SL, although the coercivity of bulk samples was clearly decreased by the formation of the c-plane SL. The grains in the SL should form clusters that alter the demagnetizing facto...

Mössbauer spectrometric study of Sm2Fe17Nx(x=0-8)

Ternary rare-earth-iron-nitrogen compounds of Sm2Fe17Nx with x ranging from 0 up to 8, which have a rhombohedral structure of Th2Zn17 and a high Curie temperature up to 475–480°C depending on the degree of nitrogenation, were prepared by the nitrogenation of arc-melted Sm2Fe17 in mixed ammonia and hydrogen gas atmospheres of various ratios, and Mössbauer spectra of these compounds were observed at room temperature in order to analyse the physicochemical state of the nitrogenated products. The obtained data were analysed by focusing main attention on the effect of degree of nitrogenation on the variation of spectral profiles. Paramagnetic peaks due to an amorphous compound became prevalent as x became higher than ~ 2.3.

Magnetic properties of partially oxidized Sm2Fe17Nx

Abstract Magnetic properties and Mossbauer spectra are measured for the nitride powders heat treated in air at 150°C. Coercivity of Sm2Fe17N3 powder initially decreases with the heat treatment time, but increases again after a certain duration of heating. A paramagnetic and a ferromagnetic secondary phase were distinguished by Mossbauer spectroscopy. It is suggested that there is a correlation between the amount of the latter phase and the coercivity of the oxidized powders.