Effect of surface pinning on magnetic nanostuctures

We show that pinning of surface spins affects the hysteresis properties of the core-shell magnetic nanostuctures of different shapes, sizes, and different spin interactions, namely, Ising, XY, and Heisenberg models. The asymmetry in hysteresis loops occuring due to pinning turns out to be more prominent in an inverse core-shell structure where spin interaction in the core is antiferromagnetic and that in the shell is ferromagnetic. Monte Carlo simulations of the inverse core-shell nanostructures show that the exchange bias, even under zero-field-cooled conditions, increases with increase of both the pinning density and the fraction of up spins among the pinned ones. The exchange bias also exhibits a switch---from negative to positive---depending on the fraction of up spins pinned. These results are remarkably well reproduced by a simple model of the outermost surface layer. The surface spin pinning appears to affect the magnetic properties of heterostructures as well, besides nanostructures.