The classical, thermally driven transition in the dipolar-coupled Ising ferromagnet LiHoF 4 ͑T c 1.53 K͒ 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, g 1, has a mean-field value in both the

more »

... l 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. [S0031-9007(96)