Spin-flop transition in small nanoparticles: Internal magnetic study of surface effects

Abstract

In this work we study the surface effects on small antiferromagnetic (AF) nanoparticles with diameters ϕ ≤ 11 nm and Néel temperature TN ∼ 12 K under spin-flop conditions by means of Montecarlo simulations. Our model consists of spherical nanoparticles with internal AF ordering and a surface with radial anisotropy. We have considered different values ​​of surface anisotropy (SA) and studied the effect it produces as a function of the particle size. Our results show that by decreasing the size of the nanoparticle, the effective anisotropy of the system increases and with it the spin-flop field. Likewise, the influence of SA induces magnetic disorder that is manifested in the magnetization curves as a function of applied field and temperature. These effects are studied by means of some antiferromagnetic order indicators that provide information about the degree of ordering of the system as a function of SA, particle size and temperature. The spin cluster structures play crucial role on the magnetic behavior of the particle as the size is diminished. We develop a very simple model that reproduce the observed behaviors as function of the SA and particle size for low temperatures.