Effect of Magnetic Annealing on the Magnetostriction and Magnetization of Single Crystals of Ni-25%Co Alloy at High Temperatures

Abstract

The time-variations of magnetostriction and magnetization of rod-shaped single crystals of Ni-25%Co alloy have been observed in an external magnetic field of 6.4 kA/m at high temperatures. The magnetostriction and magnetization at room temperature were also measured in an external field of 82 kA/m when the specimens were slowly cooled in the furnace without magnetic field after their time-variations had been terminated.
The absolute value of magnetostriction decreased with time showing a relaxation phenomenon in a temperature range from 350 to 500°C. At the same time, the magnetization increased with decreasing magnetostriction and finally both values became gradually constant. These variations in the direction of easy magnetization at high temperatures were very remarkable and most of the values remained unchanged when cooled down to room temperature. But in the direction of hard magnetization, their variations were small and the values at room temperature after magnetic annealing returned to those prior to magnetic annealing. However, activation energies calculated from the relaxation curves of magnetostriction and magnetization in both directions showed a same value of about 2.7 eV . In this case, the activation energy was a similar kind of energy required for the diffusion of atoms in a solid.
It seems most probable that the magnetic annealing effect arises from the rearrangement and fixing of domains due to the relaxation of internal stress caused by magnetostriction at high temperatures. In this process, it is supposed that the constituent atoms of the crystals and impurities, and structural defects such as dislocations, vacancies, etc. are moving and diffusing.