Spin Dynamics Of The Quantum Dipolar Magnet Yb3ga5o12 In An External Field

We investigate ytterbium gallium garnet Yb3Ga5O12 in the paramagnetic phase above the supposed magnetic transition at T-lambda approximate to 54 mK. Our study combines susceptibility and specific heat measurements with neutron scattering experiments and theoretical calculations. Below 500 mK, the elastic neutron response is strongly peaked in the momentum space. Along with that, the inelastic spectrum develops flat excitation modes. In magnetic field, the lowest energy branch follows a Zeeman shift in agreement with the field-dependent specific heat data. An intermediate state with spin canting away from the field direction is evidenced in small magnetic fields. In the field of 2 T, the total magnetization almost saturates and the measured excitation spectrum is well reproduced by the spin-wave calculations taking into account solely the dipole-dipole interactions. The small positive Curie-Weiss temperature derived from the susceptibility measurements is also accounted for by the dipolar spin model. Altogether, our results suggest that Yb3Ga5O12 is a quantum dipolar magnet.