The geometrically frustrated magnet gadolinium gallium garnet (GGG) has a unique low temperature (<0.38 K.) antiferromagnetic phase which exists only in finite fields ($H\sim 1$ 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 $\alpha <0\mathrm{}$ at the ordering transition, consistent with a transition in a random magnetic environment.

• Received 20 May 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.2500