Size-dependent exchange bias in ferromagnetic (core)/antiferromagnetic (shell) nanoparticles

We report a numerical study of the dependence of exchange bias on ferromagnetic core/antiferromagnetic shell dimension in an isolated nanoparticle, on the basis of a modified Metropolis Monte Carlo simulation. A pronounced exchange bias field is obtained at a finite size of ferromagnetic core radius, while the value of exchange bias field decreases monotonously up to a lower stable value with further increasing core sizes. Due to the complex core/shell structure and the mode conversion of magnetization reversal for various core sizes, the dependence of exchange bias field on the ferromagnetic component size is different from that in planar systems. Moreover the exchange bias field can be stabilized above a certain antiferromagnetic shell thickness, where the effective anisotropy of the whole antiferromagnetic shell becomes strong enough. It has been demonstrated unambiguously how the core and shell dimensions optimize the exchange bias