CRITICAL BEHAVIOR OF THE MAGNETIC-SUSCEPTIBILITY OF THE UNIAXIAL FERROMAGNET LIHOF4

The magnetic susceptibility of two LiHoF4 single crystals has been measured in the range 1.2-4.2 K. Ferromagnetic order occurs at Tc=1.527 K. Above 2.5 K, the susceptibilities parallel and perpendicular to the fourfold c axis are well interpreted by the molecular-field approximation, taking into account the ground state and the first excited state of Ho3+ in the crystal field of S4 symmetry. The experimental results are consistent with g=13.95 and g=0 for the ground state. The dipolar contribution to the magnetic interaction is about three times larger than the exchange one. Near Tc, the parallel susceptibility is well described by the classical law with logarithmic corrections theoretically predicted by Larkin and Khmel'mitskii for the uniaxial dipolar ferromagnet or by a power law with a critical-exponent value =1.05 rather close to 1. The upper limit of the critical region is (Tmax-Tc)Tc=1.1×10-2. © 1978 The American Physical Society.