Surface-anisotropy and training effects of exchange bias in nanoparticles with inverted ferromagnetic-antiferromagnetic core-shell morphology

Abstract

A modified Monte Carlo method using the Metropolis algorithm is performed to simulate the hysteresis behaviors of the nanoparticles with an inverted antiferromagnetic (core)/ferromagnetic (shell) morphology at low temperature after field cooling. We have examined the dependence of exchange bias on the hard ferromagnetic surface anisotropy and the training effect. Our simulations reveal that, besides the antiferromagnetic core, another pinning source, namely, the hard ferromagnetic surface, can also contribute to the exchange bias in such a special structure. Above a critical surface anisotropy, the exchange bias field has a steep increase by means of the change of the magnetization reversal mechanisms, which are affected by the surface anisotropy. During the consecutive hysteresis loops, the exchange bias field decreases gradually to a constant value. The phenomena have been interpreted well by considering the combination of locking, releasing, and stabilizing of the spins on the antiferromagnetic core surface and the energy competition between Zeeman and antiferromagnetic anisotropy. Our results are in good agreement with the experimental findings.

Spin dynamics and criteria for onset of exchange bias in superspin glass Fe/γ-Fe2O3core-shell nanoparticles

“…Recent experiments have also demonstrated EB in the case of samples having a FM layer in contact with a spin glass layer 12,13 ; layered nanoparticle/ferromagnetic structures where various models have been suggested to explain the origin of EB 14 . Core-shell nanoparticles with a conventional FM (core)/AFM (shell) and more recently "inverted" AFM (core)/FM (shell) have also been studied and the general consensus to explain EB is the freezing of interfacial spins or the growth of droplets with uncompensated spins [15][16][17] .…”

Monte Carlo simulations of core/shell nanoparticles containing interfacial defects: Role of disordered ferromagnetic spins

“…2. Recently, Hu and Du [12] have theoretically predicted that the strong FM surface anisotropy enhances the EB field in inverted Co/CoO core/shell nanoparticle. In our case, the behavior is more complicated because there is the competition between two pining-sources at the interfacial region.…”

“…In this paper, we have adapted the experimental values for Co core and CoO shell [6,12,13] as follow :…”

Relative-thickness dependence of exchange bias in bilayers and trilayers

“…On the contrary, computer simulations based either on Monte Carlo (MC) methods [18,26,27] or on micromagnetic approach [28,29] have proved useful to gain insight into the microscopic origin of EB. Moreover, in our previous work, the dependence of EB on various magnetic parameters, including size and shape in core/shell [30][31][32], core/ matrix [33,34], and random alloy systems [35], has been investigated. In this paper, we continue studying the thickness-dependent EB in the layered systems and resolve a more complex problem in order to understand how EB is jointly controlled by the dual influences of an FM (or an AFM) layer which increases in thickness and the other which decreases in thickness at the same time.…”