A theoretical study of magnetic phase transitions in ultra-thin films: Application to NiO

Phase diagrams and critical temperatures of {HIsing=-1/2Jd∑i,jnnSiSj+1/2Jse∑i,jnnnSiSj} projected onto fcc (1 0 0) layers, which is believed to be applicable to first-row transition metal oxides such as VO, MnO and NiO, are obtained from mean field theory and Monte Carlo simulations. Within the regime, Jse > 0, which includes MnO and NiO, both approaches predict bicritical behaviour of the AF2 and AF3 antiferromagnetic spin alignments for odd numbers of layers greater than one and monocritical behaviour for even numbers of layers, even when films are described by single values of Jd and Jse. The ferromagnetic alignment, on the other hand, exhibits monocritical behaviour for all thicknesses from the monolayer through to the bulk. For values of (x = Jd/Jse) which are close to those obtained from first principles calculations for NiO and also those derived from measured magnon spectra, estimates of the thickness dependence of the critical temperature from Monte Carlo simulations are similar to that derived from linear polarised X-ray absorption spectra of NiO(1 0 0) ultra-thin films grown epitaxially on MgO(1 0 0) [D. Alders, L.H. Tjeng, F.C. Voogt, T. Hibma, G.A. Sawatzky, J. Vogel, M. Sacchi, S. Iacobucci, Phys. Rev. B 57 (1998) 11623].