Cooperative magnetism and the Preisach model

Abstract

Cooperative and noncooperative magnetization processes in magnetic nanostructures are investigated. Using model calculations it is shown that the Preisach model and related approaches, such as Henkel, ⌬M , and ⌬H plots, describe magnetism on a mean-field level and cannot account for intra-and inter-granular cooperative effects. For example, the ⌬M plot of a nucleation-controlled two-domain particle gives the false impression of a positive intergranular interaction. A simple but nontrivial cooperative model, consisting of two interacting but nonequivalent particles, is used to show that cooperative effects are most pronounced for narrow switching-field distributions, i.e., for nearly rectangular loops. This is unfavorable from the point of magnetic recording, where one aims at combining narrow loops with a noncooperative local switching behavior. A general rule is that the neglect of cooperative effects leads to an overestimation of the coercivity.

Interparticle interactions in annealed FePt nanoparticle assemblies

“…This rounded loop shape with a broad SFD results from nanocrystallites with a random distribution of the magnetic easy axis, which no longer couple tightly, and switch more or less independently. This is because the increase of the magnetocrystalline anisotropy at low temperatures tends to break the interparticle exchange coupling, making them switch relatively independently rather than cooperatively [16]. As can be seen from Fig.…”

Hysteresis of Granular FePt:Ag Films With Perpendicular Anisotropy

“…The relation between the correlation length and the coercivity is less straightforward. In nearly perfect magnets, the coercivity is nucleation-controlled and decreases with decreasing correlation length [7]. However, it can be shown that the opposite is true for strongly disordered magnets, whose coercivity is determined by domain-wall pinning.…”

“…Strong exchange leads to an unphysical overskewing of the loop, whereas strong demagnetizing fields lead to vortex and other flux-closure effects going beyond a simple demagnetizing field [5]- [7].…”

“…As shown for a two-particle model [7], cooperative effects amount to a narrowing of the switching-field distribution: The switching fields of individual particles are random quantities, and exchange interactions lead to an averaging of the switching fields over the localization length L. In the numerical nucleation analysis of Fig. 5, this effect is automatically considered, but it can also be included in the model of (1) and (2).…”

“…5 illustrates this point by showing nucleation modes for a square lattice of 100 100 particles with per-pendicular anisotropy. (For simplicity, the particle positions are not shown in the figure.) The particles exhibit some nonequivalence, modeled as anisotropy disorder , so that the nucleation of reverse domain is localized [7]. The localization length, which roughly corresponds to correlation length visible in Fig.…”