Transformations ordre-désorde et propriétés magnétiques des alliages fer-nickel

The magnetic anisotropy of a FeNi 50/50 single crystal changes considerably when the sample is ordered by irradiation in a magnetic field, showing either cubic or quadratic symmetry in the ordered state, according to the direction of the field. The experimental values of the anisotropy constants are interpreted in terms of the coupled-crystallite model proposed by Néel and confirmed by X-ray measurements. The order-disorder transformation diagrams for FeNi- and FeNi3-type order are studied in the composition range of 37 to 70 percent of nickel. The simultaneous existence of the two types of order is remarked. In the composition range from 50 to 75 percent of nickel, a detailed study of the observed magnetecrystalline symmetry shows up the different contributions of FeNi, FeNi2, and directional order. The Curie temperature in the ordered state is higher than that in the disordered state, the difference between the two values reaches 300° for same composition.