Shizuo Kadowaki

Magnetocrystalline Anisotropy of Co and Co-Ni Alloys

The magnetocrystalline anisotropy constants K u1 and K u2 of Co–Ni alloys containing Ni up to 30% were measured at temperatures between 77 K and 700 K by means of a torque magnetometer. For the alloys containing Ni less than about 7%, the sign of K u1 was found to change from positive to negative in the h.c.p. phase. The temperature at which K u1 becomes zero was found to increase from 510 K for Co to 620 K for 5% Ni–Co alloy. The temperature dependences of K u1 and K u2 are discussed on the basis of the theory developed by Zener and Carr. It is clarified that the temperature dependences of K u1 and K u2 cannot be explained by the power law of the reduced magnetization. The contributions of the magnetoelastic coupling energy to K u1 and K u2 are discussed. The obtained value of Δ K u2 for Co is one-tenth or one-hundredth as small as the value of K u2 .

Anomalous Temperature Dependence of the Magnetocrystalline Anisotropy in Dilute Cobalt-Iron Alloys

A study on the temperature dependence of the uniaxial magnetocrystalline anisotropy constant K u1 of Co–Fe alloys containing less than 4.21 at% Fe has been carried out. A strange behavior of K u1 with temperature was found in 0.84∼1.27 at%Fe alloys. On heating, K u1 decreases and changes its sign, followed by an anomalously drastic decrease, and then increases up to the h.c.p.→f.c.c. phase transition temperature. On cooling, K u1 decreases exhibiting a large temperature hysteresis and increases abruptly, becoming positive. This strange phenomenon is cyclic with temperature.

Induced uniaxial magnetic anisotropy and preferred orientation in Co and Co–Ni alloy by magnetic annealing

The preferred orientation of hexagonal close packed hcp grains in magnetically annealed Co and Co–20%Ni alloy was studied by both Schulz’s x-ray transmission and reflection methods to clarify the origin of the magnetic anisotropy induced by the magnetic annealing effect. The preferred orientation was first found at a low polar angle α<50° area for both Co and Co–20%Ni alloy. The mechanism of the magnetic anisotropy induced by the field cooling effect was successfully explained qualitatively and quantitatively by this preferred orientation of the grains. The formation of this preferred orientation was discussed in terms of field-induced martensitic transformation. The uniaxial magnetocrystalline anisotropy energy could be the motive energy of the field-induced martensitic transformation.

Field-induced dhcp → hcp transformation and anisotropy change in Co-Fe alloys

Abstract The easy axis for the Co-1.2 at.% Fe alloy was found to change from the c-plane to the c-axis by an applied magnetic field at room temperature. Electron diffraction study revealed that this was associated with the dhcp → hcp transformation. The results are discussed using thermodynamic considerations.

Magnetic anisotropy induced by cold rolling in Co and Co‐Fe alloys

The roll reduction and concentration dependences of the uniaxial magnetic anisotropy induced by cold‐rolling, Kur, were investigated for hexagonal Co and Co‐Fe alloys. The maximum value of Kur was 3×105 erg/cm3 at 10%–20% roll reduction for Co and Co‐1.5% Fe. The easy direction was perpendicular and parallel to the rolling direction for the alloys containing less than 1.2% Fe and more than 1.2% Fe at room temperature, respectively. The easy direction of Kur changed at 260 °C for Co. The texture produced by cold‐rolling was detected by Schulz’s method. The induced anisotropies were evaluated by using the Ku1, Ku2, and the x‐ray reflected intensities. The calculated values and easy direction agreed well with the experimental results. The origin of Kur for hexagonal Co and Co‐Fe alloys is clearly explained by formation of rolling texture.

Magnetostriction Constants of Nickel-Cobalt Alloys

The magnetostriction constants, λ 100 and λ 111 , of f.c.c. Ni-Co alloys have been measured in the temperature range between 77 and 450 K using the strain gauge technique. The sign of λ 100 changes from negative to positive with increasing Co content in the temperature range between 77 and 450 K, while the values of λ 111 are negative. The magnitude of both constants decreases monotonically with increasing temperature. The present data were analyzed by Neels theory and the coefficient of pseudo-dipole interaction was estimated. The temperature dependence or the magnetoelastic coupling constants does not obey the third power law of the reduced magnetization. The contribution of the magnetoelastic coupling effect to the anisotropy constant, K 1 , were evaluated. The contribution is too small to explain the change of sign in K 1 with increasing temperature and/or Co concentration.

Magnetic Anisotropy Induced by Magnetic Annealing and Cold Rolling for Co and Co-Ni Alloys. I. Experimental

The uniaxial magnetic anisotropies, K uf , induced by magnetic annealing and K ur by cold rolling were investigated for Co and Co-Ni alloys containing Ni up to 30%. The values of K uf obtained for Co-Ni alloys ranged from -4 to 8 ×10 5 erg/cm 3 in fairly good agreement with the earlier results. The obtained values of K ur are about -2 ×10 5 erg/cm 3 for any composition of alloys. The pole figure analysis by the Schulzs reflection method with X-ray was carried out.The results revealed no texture for the magnetically annealed specimens. Contrary, for the cold rolled specimens, the crystallographic texture was detected, even though the magnitude of K ur is smaller than that of K uf . Influence of annealing on K uf and K ur was examined in the range of temperature between 300 and 1000°C. By the annealing above the phase transformation temperature the value of K ur is reduced suddenly, while the value of K ur remains unchanged.

Magnetic Anisotropy Induced by Magnetic Annealing and Cold Rolling for Co and Co-Ni Alloys. II. Analysis by a Statistical Model

A statistical model developed by Trout and Graham is used to describe the induced magnetic anisotropies, K uf , by magnetic annealing and K ur by cold rolling, taking into account the orientation of the easy axes of magnetization and the slip system for h.c.p. crystal. In the case of cold rolling, the calculated absolute value of K ur is nearly the same as the experimental one, however the sign of them disagrees each other. In the case of magnetic annealing, since the crystallographic texture is not present, it is attempted to estimate the volume fraction of the grains having preferred orientation to the whole volume of the specimen. The estimated volume fraction is more than 20% and is enough to detect the crystal-lographic texture within the accuracy of the X-ray measurement carried out in a preceding paper. It is concluded that the origin of the magnetic annealing effect for Co and Co-Ni alloys is not attributed to the formation of crystallographic texture.

Field cooled induced magnetic anisotropy in pure Co metal

Abstract A development of crystalline texture in Co metal cooled in a magnetic field is detected by applying a statistical method on Schulzs method of pole figure analysis. The field cooled induced uniaxial magnetic anisotropy originates from the texture.