Interpretation of the spin glass behaviour of diluted magnetic semiconductors below the nearest-neighbour percolation threshold via realistic Monte Carlo simulations

We have performed Monte Carlo simulations of magnetic semiconductors above and below the nearest-neighbour percolation threshold (NNPT) using a classi cal Heisenberg Hamiltonian with up to third nearest-neighbour (nn) interact ions. Large clusters were created allowing use of realistically low magneti c fields (10 G). Above NNPT our results, apart from confirming the existing picture of this class of materials, also show that the inclusion of the se cond and third (nn) interactions increases the frustration, thus making the transition temperature smaller and closer to experiment than calculated vi a the first nn interactions only. A physically plausible explanation is giv en. Below NNPT our results strongly support the validity of the hypothesis (D. Karaoulanis, J.P. Xanthakis, C. Papatriantafillou, J. Magn. Magn. Mater . 161 (1996) 231), that the experimentally observed susceptibility is the s um of two contributions: a paramagnetic one due to isolated magnetic cluste rs, and a spin-glass contribution due to an 'infinite' percolative cluster formed from exchange interactions of further than first nearest neighbours. Our calculated phase diagram is in very good agreement with the experiment . (C) 2000 Elsevier Science B.V. All rights reserved.