Three-dimensional frustrated systems

Apart from spin ice our attention has been devoted to other representatives of rare-earth pyrochlores. Specifically, Er2Ti2O7 represents three-dimensional (3d) S=1/2 antiferromagnetic system on a pyrochlore lattice, where crystal field introduces planar anisotropy [1]. The spatial arrangement of exchange paths causes a high degree of frustration in this system [2]. The combined effect of the frustration and quantum fluctuations induces symmetry breaking in the ground state manifold. The suppression of the degeneracy should lead to magnetic ordering with a novel nontrivial ordered state.  The neutron scattering and specific heat data are consistent with the proposed scenario. More specifically, specific heat data revealed ?-like anomaly at 1.2 K suggesting  onset of long-range  ordering (see Fig. 1).
















Fig. 1 Temperature dependence of specific heat of Er2Ti2O7. Inset : the same quantity in reduced coordinates.

The  results of  elastic scattering revealing non-zero local magnetization below 1.2 K are consistent with specific heat [3]. However, the ordered moment was found to be massively suppressed to about a third of its classical value suggesting strong fluctuations even below critical temperature. The specific heat shows T3  dependence up to nominally 1 K  requiring q2 density of states with energy up to at least 1 K and supporting the presence  of spin waves in this 3d antiferromagnet. On the other hand, inelastic neutron scattering reveals more complicated excitation spectrum. More specifically, no dispersion was observed in that energy scale. Possibly the existence of a hidden branch of excitations that do not couple to neutrons directly may indicate different kind of excitations.
Current interest is focused on the behavior of Er2Ti2O7 in magnetic field to clarify the nature of the excited states and potential anomalous magnetocaloric behavior of this material.

[1] H. W. J. Blöte et al., Physica 43 (1969) 549.
[2] R. Siddhartan et al., Phys. Rev. Lett. 83, (1999) 1854.
[3] J. D. M. Champion et al., Phys. Rev. B 68 (2003) 020401.