Magnetization reversal mechanisms in small antiferromagnetic particles

We discuss the magnetization reversal mechanisms in small antiferromagnetic particles at temperatures below the blocking temperature. Our calculations have been done with the Monte Carlo simulation technique. We consider clas sical Heisenberg exchange interactions between the spins and distinct aniso tropy for the core and the surface of the particles. For the surface anisot ropy we consider two cases for the easy axis orientation: (a) along a fixed direction, which is the same in the core of the particles and (b) radial a t each surface site. We found two magnetization reversal mechanisms in both cases of surface anisotropy. In the first mechanism, the switching field r everses all spins in the particle and the magnetization relaxes to the oppo site remanence after removing the applied field. A phenomenological model i s also developed and the magnetization reversal is explained in terms of th e surface anisotropy and the surface exchange field. In the second mechanis m, the particle magnetization vanishes at a switching field which distorts the surface spin alignment and induces an opposite moment in the antiferrom agnetic core of the particle. The main feature of this mechanism is that it causes a reversible effect, i.e., the remanence is restored as soon as the applied field is removed. In this case the switching field exhibits strong temperature dependence. (C) 1999 American Institute of Physics. [S0021-897 9(99)01702-8].