Hysteresis loops characterized by multiple frozen-in viscosity jumps

Abstract

The conditions are examined for the formation of hysteresis loops characterized by several magnetization jumps due to frozen-in viscosity steps. A similar loop was previously observed on a Mumetal specimen after neutron irradiation in the demagnetized state. This effect is clearly related to the formation of a directional order of the solute atom pairs. In the present investigation a Fe 40-Ni 60 alloy was therefore used because of its sensitivity to a directional anisotropy energy. This energy was induced by annealing at 440 °C for 15 h on thick (7 mm) or thin (0.25 mm) specimens obtained by lamination or by tool working. It has been found that multiple jumps stable over repeated loops can essentially be created on specimens made up of multiple thin toroids, similar to the ones used in the irradiation experiments. The jump multiplicity is thus basically related to the toroid multiplicity. However, the results also show another feature of particular interest: only on thin specimens it is possible to create a stable magnetization jump, while on the thick ones an induced viscosity step is easily destroyed by the application of relatively small fields. This seems to prove the importance played by the surface domain distributions in controlling the magnetic behaviour of the whole specimen. It is qualitatively suggested that a finer network of domains exists at the specimen surface because of a larger density of nucleation and pinning centres for the Bloch walls. This results in a stronger stabilization at the surface following the formation of an induced directional order, which may affect the whole sample magnetic properties when the surface-to-volume ratio is conveniently large. The problem is also considered of some lack of coincidence between the loop of an assembled specimen made up of many toroids and the one given by the sum of the loops of the same separate toroids. This effect is considered as evidence of the existence of some weak interaction between different parts of the assembled specimen.