Reentrant behaviour in spin glasses

We present the phase diagram of thed-dimensional random bond Ising model as a representative system for spin glasses. We consider nearest neighbour ferromagnetic couplingsJ with concentration 1-p and impurity couplingsaJ (|a|1) with concentrationp. It is shown that for antiferromagnetic couplings, –1<a<0, the system quite generally exhibits reentrant behaviour, i.e. two phase transitions at finite temperatures, in certain ranges of the concentrationp. It is further argued that this behaviour is a quite common feature for spin glass systems characterized by competing interactions.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday