A neutron scattering study of the dilute dipolar-coupled ferromagnets LiTb0.3Y0.7F4and LiHo0.3Y0.7F4structure, magnetisation and critical scattering

By neutron scattering the authors have studied the ferromagnetic phase transition in the dilute dipolar-coupled Ising magnets LiTb 0.3 Y 0.7 F 4 and LiHo 0.3 Y 0.7 F 4 . The Curie temperatures were 0.49 K and 0.36 K, respectively. At low temperatures, the saturation magnetic moment per rare earth ion is only 60% of that observed in the undiluted materials. Just below T c the singular variation of the magnetisation is consistent with mean-field theory as well as with renormalisation group theories. Above the phase transition, the critical scattering from LiHo 0.3 Y 0.7 F 4 is strongly asymmetric in Q-space, in agreement with renormalisation group theory. In contrast, the scattering from LiTb 0.3 Y 0.7 F 4 , although also strongly asymmetric, contains an anomalous component at small angles. The saturation magnetisation data are in good agreement with a Monte Carlo simulation study.