Thermal studies of the spin liquid state and analog to the He-4 melting curve in a geometrically frustrated magnet

Gadolinium gallium garnet, Gd3Ga5O12 (GGG) has an extraordinary low-tempera ture phase diagram. Although the Curie-Weiss temperature of GGG is similar to - 2 K, GGG shows no long-range order down to T similar to 0.4 K. At low temperatures GGG has a spin glass phase at low fields (less than or similar to 0.1 T), a field-induced long-range ordered antiferromagnetic state at f ields of between 0.7 and 1.3 TI and, at intermediate fields, an apparent sp in-liquid state without long-range order. We have characterized the interme diate held (IF) state through heat capacity, thermal conductivity, and magn etocaloric measurements. Our results show a sharp high-field phase boundary of the thermal irreversibility of the spin glass phase of GGG implying tha t the intermediate held phase is distinct from the spin glass. The lower fi eld boundary of the AFM phase is shown to have distinct minimum at T simila r to 0.2 K, in analogy to the minimum in the melting curve of He-4. The exi stence of such a minimum is confirmed by measurements of the latent heat of the transition below that temperature. (C) 2000 Elsevier Science B.V. All rights reserved.