Fumihisa Ono

Magnetic Field Dependence of the Magnetocrystalline Anisotropy Energy in hcp Co

Magnetic field dependences of the first and the second uniaxial magnetocrystalline anisotropy constants K u1 and K u2 for hcp Co were obtained at various temperatures between 4.2 and 616 K by using the 90-points least mean square routine. It was found that the contribution of the Fermi level-dependent term to d K u1 /d H is as large as that of the magnetization dependent one, while in the temperature variation of K u1 the Fermi level-dependent term is negligibly small. The temperature variation of K u2 was explained reasonably by a relation derived by considering a changes in the lattice parameter ratio and in the magnetization.

High Field Susceptibilities of Ni-Base Face Centered Cubic Alloys up to the Magnetic Field of 300 kOe

Magnetic susceptibility measurements have been made by using a pulse magnet wound with Cu-Be wire, which is connected to a condenser bank of the capacity 3000µF. Susceptibilities observed for Fe-Ni, Fe-Ni-Cr alloys show anomalous increase near the Invar region. For Ni-Co alloys observed values are small (2 ×10 -5 emu/cc), and for Ni-Cu alloys a maximum has been observed at the Ni concentration of 80 at %. These experiments are well explained in terms of the rigid band model.

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.

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.

Relation between the Thermal Expansion Coefficient and the Magnetization in Fe-Ni Invar Alloys

Measurements of magnetization curves for Fe-Ni Invar alloys have been made at various temperatures between room temperatures and about 450°C. Specimens used were in a sphere shape, which were obtained from the same ingots that the specimens used for the thermal expansion measurements had been taken from. By comparing the results of the magnetization measurements with the observed temperature dependence of the thermal expansion coefficients, it was found that the anomalous part of the thermal expansion vs temperature curve can be well expressed with the quantity, \(\frac{2}{3}CM_{s}\text{d}M_{s}/\text{d}T\), where C is a magneto-volume coupling constant and M s is the spontaneous magnetization.

Magnetovolume Effect in Fe-Ni Invar Alloys below and above the Curie Temperature

A detailed comparison of the thermal expansion coefficient in Fe-Ni Invar alloys with the results of magnetization measurements has been made in a wide temperature range both below and above the Curie temperatures. A new method of determining the lattice term from the observed thermal expansion coefficient has been proposed and applied to the estimation of the magnetic contribution to the thermal expansion coefficient. It has been found that the magnetic contribution below the Curie temperature is well expressed with the factor M d M /d T and that it is proportional to dχ -1 /d T above the Curie temperature. These results can well be explained by the spin fluctuation theory developed by Moriya and Usami.

Determination of Magnetic Anisotropy Constants from Unsaturated Torque Curves in Nd2Fe14B

A simple method has been established for a determination of the magnetic anisotropy constants from an unsaturated torque curve. In this method a least mean square routine was adopted instead of the usual Fourier analysis which has been widely used by many authors. The method has been applied to the recently developed high energy permanent magnet, Nd 2 Fe 14 B, of which torque curves were not saturated by using an ordinary electromagnet. The validity of the present method has been checked by applying it to the case of hcp Co for which the saturated values were known.

The Relation between the Magneto-Crystalline Anisotropy and the Magnetization of a Fe–Ni Invar Alloy

Magnetic field and the temperature dependences of the magnetization of a single crystal disk of Fe–Ni Invar alloy have been measured, and compared with those of the magneto-crystalline anisotropy constant, K 1 , obtained from the torque measurements. A close relationship was found between K 1 and the magnetization. The values of (d K 1 /d M ) T , obtained from the field dependence of K 1 and M , is found to be equal to (d K 1 /d M ) H , obtained from the temperature dependence of K 1 and M , even in a low temperature region. The tenth power law, proposed by Zener is only satisfied at the temperatures around 200 K.