Laboratoire de Physique Théorique
Toulouse - UMR 5152

LiHo_xY_1-xF_4 is an interesting magnetic material as a prototype for quantum disordered magnets. More specifically, the ordered compound LiHoF_4 in a transverse field B_x is usually considered from the theoretical point of view as a transverse field Ising model suitable for studying quantum criticality. When the magnetic ions Holmium are site-diluted by non magnetic ions Yttrium, they interact via dipolar long distance interactions which become totally random. We show that the presence of a transverse field, combined with the random dilution and dipolar couplings, generates random fields in the Ising direction. This identifies LiHo_xY_1-xF_4 in a transverse field as a new random field Ising model. The presence of random fields explains in particular the fast decrease of the critical temperature in the diluted regime and the smearing of the nonlinear susceptibility at the spin-glass transition. We will explain in this seminar how these random fields arise from the microscopic Hamiltonian in the low energy sector via the upper levels of the atomic theory.