paper

Transition to Long Range Magnetic Order in the Highly Frustrated Insulating Pyrochlore Antiferromagnet Gd_2Ti_2O_7

arXiv:cond-mat/9906043 · doi:10.1103/PhysRevB.59.14489

Abstract

Experimental evidence from measurements of the a.c. and d.c. susceptibility, and heat capacity data show that the pyrochlore structure oxide, Gd_2Ti_2O_7, exhibits short range order that starts developing at 30K, as well as long range magnetic order at K. The Curie-Weiss temperature, = -9.6K, is largely due to exchange interactions. Deviations from the Curie-Weiss law occur below 10K while magnetic heat capacity contributions are found at temperatures above 20K. A sharp maximum in the heat capacity at K signals a transition to a long range ordered state, with the magnetic specific accounting for only 50% of the magnetic entropy. The heat capacity above the phase transition can be modeled by assuming that a distribution of random fields acts on the ground state for Gd. There is no frequency dependence to the a.c. susceptibility in either the short range or long range ordered regimes, hence suggesting the absence of any spin-glassy behavior. Mean field theoretical calculations show that no long range ordered ground state exists for the conditions of nearest-neighbor antiferromagnetic exchange and long range dipolar couplings. At the mean-field level, long range order at various commensurate or incommensurate wave vectors is found only upon inclusion of exchange interactions beyond nearest-neighbor exchange and dipolar coupling. The properties of Gd$_2Ti_2O_7 are compared with other geometrically frustrated antiferromagnets such as the Gd_3Ga_5O_{12} gadolinium gallium garnet, RE_2Ti_2O_7 pyrochlores where RE = Tb, Ho and Tm, and Heisenberg-type pyrochlore such as Y_2Mo_2O_7, Tb_2Mo_2O_7, and spinels such as ZnFe_2O_4

arXiv commentLetter, 6 POSTSCRIPT figures included. (NOTE: Figure 5 is not included --) To appear in Physical Review B. Contact: [email protected]

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