Magnetic Resonances Observed in the High-Field Magneto-Optical Absorption of the Quantum Ising Ferromagnet LiHoF4

The magneto-optical absorption of the quantum king ferromagnet LiHoF₄ has been measured in the frequency range 38 to 140 GHz, the temperature range 0.5 to 20 K and magnetic fields of up to 15 T. The sample was studied in two orientations, with the external magnetic field aligned perpendicular or parallel to the uniaxial direction, i.e., along the quantum and classical directions. In each case, several families of resonances are observed, including linear EPR branches and a series of non-linear fast moving resonances. We present this data and propose an interpretation for the origin of these resonances.