Magnetization process in thin Ni sheets: Effect of cold-rolling and recrystallization annealing

Reversible and irreversible magnetization processes have been investigated in thin Ni sheets by means of flux-metric and alternating-gradient-force ma gnetometer techniques. Cold-rolled samples exhibit a rotation-dominated pro cess at low fields, and the value of the associated susceptibility, related to the stress-induced and the magnetocrystalline anisotropies, can provide an estimate of the residual stress in the material. Reversible displacemen ts of the domain walls come into play and combine with rotations in the ann ealed samples. A method is devised, by which domain-wall and rotational con tributions to the reversible susceptibility can be singled out. The Rayleig h law always accounts for the behavior of hysteresis loops and losses at lo w fields, but rotations engender some peculiar evolution of the related par ameters and their relationship with the coercive field. Magnetic softening by stress relief is assessed, as a whole, through Preisach analysis. It is found that the local stochastic coercive fields can be described by means o f a Lorentzian distribution function, whose relative width is minimum in th e cold-rolled material. By removing the internal stresses through recrystal lization annealing, the contribution of grain boundaries and surface effect s to coercivity is put in evidence, with the latter apparently posing the m ajor restraints to domain-wall motion for grain sizes larger than the sheet thickness. (C) 2001 American Institute of Physics.