Magnetic excitations in the XY-pyrochlore antiferromagnet Er2Ti2O7

The XY-pyrochlore antiferromagnet Er2Ti2O7 is studied by heat-capacity measurements and electron-spin resonance spectroscopy performed on single-crystal samples. The magnetic phase diagrams are established for two directions of applied field, H parallel to[100] and H parallel to[111]. In the magnetically ordered phase observed below T-N = 1.2 K, the magnetic excitation spectrum consists of a Goldstone mode acquiring an isotropic gap in an applied field, and another mode with a gap softening in the vicinity of a field-induced phase transition. This second-order transition takes place at a critical field H-c above which the magnetization process is accompanied by a canting of the magnetic moments off their local easy planes. The specific-heat curves for H parallel to[100] (H >> H-c) are well described by a model presuming a single dispersionless excitation mode with the energy gap obtained from the spectroscopic measurements.