Magnetic and Thermal Properties of Nickel-and Copper Fluosilicates below 1°K

Abstract

The adiabatic magnetization curves have been measured on single crystals of NiSiF 6 ·6H 2 O and CuSiF 6 ·6H 2 O using a Cioffi-type recording flux-meter. The specific heats have also been determined. Nickel fluosilicate was found to be ferromagnetic with the easy axis parallel to the c -axis. The magnetic Curie temperature is about 0.15°K while the specific heat maximum is at 0.13 5 °K. The magnetic behaviour of copper fluosilicate is ferromagnetic and nearly isotropic. The magnetic Curie temperature is about 0.07°K. The entropy versus temperature curve has a kink at 0.10°K (at an entropy value of 0.46 R) showing the ordering in two steps. Weiss constants were determined at helium temperatures; θ // =+0.15°K, θ ⊥ =+0.08°K for nickel salt; and θ(powder)=+0.07°±0.02°K, θ // c =+0.08°±0.02°K, θ ⊥ c =0.05°±0.02°K, and g (powder)=2.2 5 , g // c =2.2 5 , g ⊥ c =2.2 6 (mean values) for copper salt.