Magnetic anneal and ordering in Fe-Ni alloys enhanced by excess of vacancies

Abstract

The possibility of obtaining directional-order effects by annealing in a magnetic field, at temperatures lower than those at which the phenomenon is usually observed, has been examined on several iron-nickel alloys by stimulating the diffusion processes with an excess of vacancies introduced by quenching from high temperatures. The results refer to polycrystalline specimens. Directional order appears, after annealing at temperatures as low as 260°C on specimens quenched from 1350°C as compared to about 420°C for nonquenched specimens. For low iron concentrations, on the alloy Fe 15-Ni 85, the amount of directional order obtained increases almost linearly with T - T_{c} , where T is the annealing temperature and T c the Curie temperature, thus, confirming the Neel theory. On Fe 40-Ni 60 and Fe 50-Ni 50 alloys the values of the induced anisotropy energy, down to temperatures of about 400°C, are close to the values obtained by Marchand et al. [1] by magnetic annealing under neutron irradiation. The results are also in agreement with those of Dekhtyar and Kazantseva [2] and actually explain the observed increase in coercive force when annealing specimens of a Fe 50-Ni 50 alloy quenched from 1200°C. In contrast, when annealing these quenched alloys in a magnetic field at temperatures of 320°C or lower, a sudden large drop of the induced anisotropy energy is observed, probably corresponding to large deviations from ideality of the solid solution and short range ordering. The observed changes of resistivity are, however, small, i.e., three percent. These last results on quenched Fe 40-Ni 60 and Fe 50-Ni 50 alloys are, therefore very different from those obtained on the same alloys by fast neutron bombardment and the reason for the difference is not considered to be fully understood.