Influence of sample surface on thermomagnetic effects in Fe40-Ni60

It is shown that the effect of an induced anisotropy energy due to directional order in Fe40-Ni60 is strongly dependent on the sample-to-volume ratio. Torodial samples of various thickness (6,3,1,0.5 mm) were machined from the same ingot, chemically etched, and annealed at 1300°C in H2. All further annealings were performed at 450°C in N2in one of the following magnetic conditions: a) demagnetized state; b) under magnetic field; c) remanent state. After these annealings loops show a constricted shape (a), a closely square shape with small coercive field Hc(b), or a square shape with large Hc(c). However, the width of the constricted region after annealing a), and the magnitude of Hcafter b) or c), are found to increase for decreasing sample thickness; in general, changes of loop shape induced by a given annealing are markedly stronger on thinner specimens. These differences are attributed to some influence of sample surface on the thermomagnetic treatment. This conclusion was further checked by electrochemically etching off a sample layer (0.01-0.1 mm); as a consequence the shapes of the loops obtained by the previous annealing suffered changes which were strongly different depending on the original thickness of the sample. It is qualitatively observed that the induction of a directional anisotropy energy Kuoccurs according to the domain pattern present during annealing, and that surface patterns are certainly characterized by smaller spacings of 180° domains and larger densities of 90° closure domains than are bulk patterns. Therefore the effectiveness on the loop shape of a Kudistribution in a surface layer is stronger than the one of a Kudistribution induced in a bulk layer. By etching off a sample layer, the new surface domain pattern bears little correlation to the old Kudistribution, and there is a memory loss of previous annealing results- - , which is more pronounced on samples with larger surface-to-volume ratios.