Quantum critical behavior for a model magnet

The classical, thermally driven transition in the dipolar-coupled Ising ferromagnet LiHoF_4 (T_c=1.53K) can be converted into a quantum transition driven by a transverse magnetic field H_t at T=0. The transverse field, applied perpendicular to the Ising axis, introduces channels for quantum relaxation, thereby depressing T_c. We have determined the phase diagram in the H_t−T plane via magnetic susceptibility measurements. The critical exponent, γ=1, has a mean-field value in both the classical and quantum limits. A solution of the full mean-field Hamiltonian using the known LiHoF_4 crystal-field wave functions, including nuclear hyperfine terms, accurately matches experiment