A study of Barkhausen avalanche statistics through the critical disorder in a ferromagnetic thin film: Experimental investigation and theoretical modeling

摘要

It is well known from theoretical models that a ferromagnetic system should undergo a disorder induced phase transition in its magnetization reversal mode at a critical disorder level R-C, below which, the reversal is characterized by the appearance of the so-called spanning avalanche. The critical regime above R-C is characterized by power-law distributed avalanche sizes and durations, whose exponents determine the universality class to which the magnetic system belongs. The most commonly studied among these is the size-exponent tau, which most theoretical models estimate to be in the range 1.3-1.5. However, experimental determinations of this exponent fall in a much wider interval, between 0.98 and 1.8, well outside the domain of the models. This discrepancy, which is far more common for 2D systems, has not been effectively addressed for more than a decade. In this article, we have determined experimentally the size-exponent 'tau' in 2D Permalloy(Ni80Fe20) thin films by measuring Barkhausen noise through the disorder-induced transition using planar Hall effect. This was compared with our simulations of the random field Ising model, an established nucleation model, with a modification that better represented the experimental condition. Our findings indicated that large 'tau'-values would arise in samples with very low disorder levels well outside the critical regime, where the statistics were non-universal. In such samples, our measured 'tau' increased monotonically with temperature, which indicated a decrease in the effective disorder with temperature according to the nucleation models. This temperature dependence could be understood in the light of an extended Neel-Brown model which we had studied earlier, that had predicted a decrease in the width of switching field distribution with increasing temperature. In this context, we also used micromagnetic simulations, which helped to understand the dependence of coercivity on disorder, a property not predicted correctly by the Ising model.