The Memory in Magnetic Systems

Most of the features of ferromagnetic materials are embedded in their hysteresis loop [1]: a matter compelling one to focus the attention on the memory properties of magnetic systems. Starting from the landmark work of Preisach, where the memory effects of scalar hysteresis are suitably accounted for by means of the so called Preisach plane, a vector generalization to ensembles of Stoner-Wohlfarth particles has been recently proposed [2]. In this approach the stability properties of particles are described by adopting the conventional astroid representation, a strategy also exploited to discuss incoherent particle rotations [3]. On the other hand, in order to include the possibility of reversal by domain wall motion, a modification of the coherent rotation mechanism, graphically represented by a “truncated astroid”, has been proposed as well [4]. Within this framework, we present a general mathematical tool able to track the history of any magnetic system, with reversal of magnetization driven by reversible and irreversible rotations and domain wall displacements. Coupled to a vector hysteresis model, the procedure outlined allows one to decrease to a large extent the computational time needed to reproduce the hysteresis loop. Eventually, this instrument supplies a set of thermodynamic variables providing an estimate of the magnetic energy loss entailed by irreversible processes, and the consequent entropy production. [1] “The Science of Hysteresis”, G Bertotti and I. Mayergoyz Editors, Academic Press (Elsevier), 2006 [2] Carlo Appino, “Representation of Memory in Particle Assembly Hysteresis Models”, IEEE Trans. on Mag., VOL. 45, NO. 11, Nov. 2009 [3] A. Stancu, C.Papusoi, L. Spinu, “Mixed –type model of hysteresis”, J. Magn. Magn. Mater. 150 (1995) 124-130. [4] C. Appino, M. Valsania, V. Basso, “A vector hysteresis model including domain wall motion and coherent rotation”, Physica B 275 (2000) 103-106