INVESTIGATION OF THE FIELD-INDUCED ANTIFERROMAGNETIC PHASE-TRANSITION IN THE FRUSTRATED MAGNET - GADOLINIUM GALLIUM GARNET

The geometrically frustrated magnet gadolinium gallium garnet (GGG) has a unique low temperature (<0.38 K) antiferromagnetic phase which exists only in finite fields (H similar to 1 T). We have measured the specific heat and magnetic susceptibility of GGG to obtain the first accurate map of the low temperature phase diagram. A ground state magnetic structure in this field regime has been determined by simulations including both the exchange and dipole interactions. We find a critical exponent alpha < 0 at the ordering transition, consistent with a transition in a random magnetic environment.