Phase diagrams and magnetic properties of diluted Ising and Heisenberg magnets with competing interactions

Abstract

Ising and Heisenberg magnets with nearest-neighbor ferromagnetic exchangeJ 1 and next-nearest antiferromagnetic exchangeJ 2 and randomly distributed frozen-in nonmagnetic impurities of arbitrary concentration 1−x are studied by several methods: systematic series expansions inx, 1−x and inverse temperature (1/T) as well as Monte Carlo simulation. Depending onR≡J 2/J 1,T andx the model is in paramagnetic, ferromagnetic, antiferromagnetic or spin glass phases. The microscopic magnetic structures of all these phases are investigated and found to be more complicated than usually (e.g., the ferromagnetic state contains spins and clusters either aligned antiparallel or not aligned at all, when “frustration” effects make bonds ineffective). We suggest that the concentrationx c of magnetic ions below which no (anti-)ferromagnetic long range order occurs depends onR continuously, andx c →1 at the multicritical point (R m ,T=0) where the order changes from ferromagnetic to antiferromagnetic. Our results for phase diagram, susceptibility etc. are compared to recent data on the Eu x Sr1−x S system and very good agreement is found.