O. Yamada

Magnetization and torque measurements on Nd2Fe14B single crystals

Abstract Magnetization and torque measurements on single crystal specimens of Nd2Fe14B have been carried out. The magnetization values measured always in the direction of easy magnetization and those in the [001] direction have been precisely determined at temperatures from 4.2K to 600K with a superconducting magnet up to 52.65 kOe. Below the spin reorientation temperature 135K, the magnetization value of the direction of easy magnetization increases anomalously with decreasing temperature. The direction of easy magnetization tilts from the [001] axis to the [110] axis and this tilt angle has been also precisely determined by torque measurement in the temperature range below the spin reorientation temperature. The four-fold symmetry in torque curve for the (001) plane is continously observed at even up to near room temperature and the [110] direction of easy magnetization and the [100] direction of hard magnetization do not change below and above the spin reorientation temperature.

Analysis of high‐field magnetization measurements on R2Fe14B single crystals (R=Tb, Dy, Ho, Er, and Tm)

Single crystals of the compounds R2Fe14B some 1–4 mm in size have been grown for a study of the anisotropy of the magnetization curves. These curves for crystals with R=Tb, Dy, Ho, Er, and Tm were measured at the Service National des Champs Intenses, Grenoble, between 4.2 and 275 K, with fields of 0–18 T being applied along the [100], [110], and [001] directions. Magnetization curves for all five compounds are analyzed in terms of the exchange and crystal field interactions (including terms up to sixth order, which may differ at 4 f and 4g sites) following the analysis previously developed for Nd2Fe14B. Molecular field coefficients representing the exchange interactions between R and Fe spins decrease from light to heavy R compounds as previously deduced from analysis of Curie temperatures. The CEF parameters are approximately the same across the series. In particular, the A20 terms are constant to within 10%.

The ultrastructure of pinealomas.

SummaryFour pinealomas of the two-cell type (atypical teratomas) were investigated by electron microscopy. They all showed the same unique histological ultrastructure. The lymphocyte-like cells did not differ ultrastructurally from lymphocytes. Many plasma cells with well-developed rough surfaced endoplasmic reticulum were also seen. The small cells and the plasma cells were considered to be derived from blood cells. A variety of the large cells in the process of differentiation were observed. They had a large and ovoid nucleus with uniform granular chromatin and contained one or more prominent nucleoli. The large cells had many dilated cytoplasmic cisternae, numerous glycogen granules of about 250–300 Å in diameter and annulate lamellae. Ultrastructurally, the pinealoma of the two-cell type resembled the seminoma of testis which is of germ cell origin. This suggests that the pinealoma may be of germ cell origin.

The Magnetovolume Effect due to the Decrease of Local Magnetic Moment in Fe-Ni Invar Alloys

The local magnetic moment at the Curie temperature in the 35 at% Ni-Fe alloy retains still a half value of the magnetic moment at 0 K. Taking into consideration this fact, new formulas are proposed, which describe reasonably the phenomena of spontaneous volume magnetostriction and forced volume magnetostriction. The magnetovolume coupling constant determined by the new formulas is approximately 1.2 ×10 -8 cm 6 /emu 2 for Fe-Ni Invar alloys, which is independent of the Ni concentration.

Contribution of T2 Term to the Magnetization of a 35.4 at% Ni–Fe Invar Alloy

The magnetization of a single crystal of the 35.4 at% Ni–Fe Invar alloy has been investigated between 4.2 K and 280 K in external fields up to 16.9 kOe to an accuracy of about 5 parts in 10 5 . The spin-wave stiffness constant including its temperature variation and the higher-order term than the T 3/2 term due to the spin-wave excitations are determined from only the data of magnetization measurements. The observed decrease in magnetization excluding the spin-wave term depends on T 2 and seems to be associated with the Stoner-type excitations. The contribution of T 2 term is found to be as large as the spin-wave contribution.

Role of T2 Term in Temperature Dependence of the Magnetization for Fe-Ni Invar Alloys

Precise measurements of magnetizations for single crystals of FCC Fe–Ni alloys in the Invar region have been made at temperatures from 4.2 K to 300 K in external applied fields up to 16.9 kOe to a relative accuracy of 5×10 -5 . In an analysis of these observed results with iterative least-squares method, it is found that there exists a large Stoner-type contribution varying as T 2.00±0.05 besides the spin-wave one to the temperature change in magnetization and that the coefficient of the Stoner-type contribution increases rapidly with decreasing Ni concentration, showing a maximum at about 35 at%Ni, and decreases for the alloys containing less than 35 at%Ni. It becomes evident that ferromagnetism in FCC Fe–Ni alloys disappears at a critical concentration of about 31 at%Ni. An anomalous behavior of the magnetization at low temperatures is also shown.

Anomalous Temperature and Field Dependences of Magnetocrystalline Anisotropy in Disordered Fe–Pt and Fe–Pd Invar Alloys

The first magnetocrystalline anisotropy constant K 1 was determined by means of torque method for 26.5, 28.3, 29.9, 30.9, 33.2 at%Pt–Fe and 31.9, 34.2 at%Pd–Fe disordered alloys and the temperature and field dependences of K 1 of these alloys were compared with those of Fe–Ni Invar alloys. Anomalous temperature and field dependences of K 1 were found below room temperature for all Fe–Pt disordered alloys and for 31.9 at%Pd–Fe alloy in the low temperature region. These anomalous behaviors of K 1 seems to be caused by the contribution of uniaxial anisotropy of f.c.t. clusters.

High-field hysteresis suggesting the coexistence of antiferromagnetism and ferromagnetism in Fe―Ni invar alloys at low temperatures

In order to study the magnetic state of Fe–Ni Invar alloys at low temperatures in detail, magnetization measurements have been carried out on Fe–Ni alloys with the compositions from 34.2 to 45.0 at% Ni. A high-field hysteresis suggesting the coexistence of antiferromagnetism and ferromagnetism has been observed. The Neel temperature, T N , below which the antiferromagnetic phase is superposed on the ferromagnetic matrix, has been obtained as a function of Ni concentration. The high-field susceptibility has been determined to be independent of the crystallographic direction. A temperature hysteresis of magnetization under a fixed magnetic field has also been observed. These characteristic features are understandable by the idea that clusters of the antiferromagnetic phase are formed randomly in the ferromagnetic matrix.

Temperature Dependence of the Uniaxial Magneto-Crystalline Anisotropy Energy of Co

Torque measurements have been made for a thin disc specimen of Co of which the disc plane was parallel to the (10\bar10) plane at various temperatures between 4.2 K and the room temperature in an external magnetic field of 16.9 kOe which was strong enough to saturate the magnetization. The obtained values of K u 1 and K u 2 were in good agreement with those obtained by Rebouillat and Sucksmith and Thompson. The temperature dependence of K u 1 below about 200 K could well be explained with a model based on the itinerant electron theory by considering the thermal expansion of the lattice parameter ratio.

Comparison of magnetic properties of Fe-Pt and Fe-Pd invar alloys with those of Fe-Ni invar alloys

Abstract Temperature dependence of the high-field susceptibility above 300 K for Fe-Pd and Fe-Pt Invar alloys has a strong resemblance to that of Fe-Ni Invar alloys. There exists also a large T 2 term in temperature decrease in magnetization for Fe-Pd and ordered Fe-Pt Invar alloys, similarly for Fe-Ni Invar alloys.