Magnetic phase diagram of NiCl₂·6H₂O: The low-temperature canted-paramagnetic boundary and the bicritical point

The magnetic phase diagram of the antiferromagnetic NiCl₂·6H₂O (T_N=5.34 K) was determined from dM / dH measurements down to temperatures T as low as 0.35 K, and in magnetic fields H up to 150 kG. The T=0 spin-flop field and canted-paramagnetic critical field were determined as H_SF(0)=39.45±0.30 kG and H_c(0)=143.9±1.5 kG, respectively. From these values, the exchange field H_E=77.4±0.9 kG and the uniaxial anisotropy field H_A=10.8±0.3 kG were obtained. The corresponding single-ion parameters are z|J| / k_B=11.5±0.3 K and |D| / k_B=1.61±0.07 K. These values are compared with the ones obtained previously from other experiments. A careful analysis of the temperature dependence of the canted-paramagnetic critical field H_c is presented. For T≲0.25T_N, the T^{3 / 2} dependence predicted by Green's-function and spin-wave calculations is observed. For 0.3T_N≲T≲0.6T_N, the observed T dependence is approximately T^{5 / 2}, similar to the results of Rives et al. in MnCl₂·4H₂O and CoCl₂·6H₂O for comparable reduced temperatures. Our results suggest that temperatures above 0.3T_N are too high for comparing the T dependence of H_c with spin-wave theory. Near the bicritical point (H_b=44.22 kG and T_b=3.940 K) the boundaries were determined with higher precision, and compared with the recent predictions based on scaling and renormalization-group theories. Colinearity between H→ and the easy axis was achieved to about 0.1 degrees. Least-squares fits indicated good overall agreement with the theoretical predictions for the case where the number of critical spin components is n=2.