Uniaxial Anisotropy in Permalloy Films at Temperatures Below 4.2°K

Abstract

Permalloy films (80%-84% Ni; 20%-16% Fe) of the order of 1000 Å thickness were prepared by vacuum deposition from a tungsten filament onto heated glass substrates at a temperature of 600°K in the presence of a 200-Oe magnetic field. The anisotropy field (Hk) of these films was measured at temperatures ranging from 295° to 1.46°K. It was possible to measure induced voltages proportional to M with at least 4% precision at drive levels of 0.01 Oe at all temperatures. In general, the shapes of the difficult-direction magnetization curves changed with temperature between 295° and 4.2°K especially at drive levels of about 1.0 Oe, but shape changes were not observed below 4.2°K and changes below 77°K were reversible. These changes were presumably due to strain effects. The changes in the loops with temperature at 0.1 Oe drive levels were small enough to allow meaningful measurement of Hk throughout the temperature range. While Hk (77)/Hk (295) was found to be about 1.5, Hk(1.46)/Hk(4.2) was found to be about 1.9, implying that a new source of anisotropy is observed at temperatures below 4.2°K. This added anisotropy can be represented analytically as Hk(T) = A exp (−T/T0), with A about 50 Oe and T0 about 2.6°K. It is perhaps significant that kT0 corresponds to the energy of about four Bohr magnetons in the internal field of the film.