Abstract
Synopsis The magnetic susceptibility of a MnCl2.4H2O single crystal has been measured parallel to its preferred (c-axis) and perpendicular (b-axis) directions of magnetization in the temperature regions of liquid hydrogen and liquid helium. It was found to follow a Curie-Weiss law χ = C/(T + Δ). The susceptibility has the same value in the two directions at higher temperatures down to about 1.7°K where it becomes anisotropic. It passes through a maximum in the b-axis direction and decreases suddenly in the c-axis direction at the same temperature 1.62°K, the Neel temperature, which is the same as that found by Friedberg and Wasscher in the specific heat determination. Its behaviour in the antiferromagnetic state is in general agreement with the molecular field theory. The dependence of the susceptibility on the magnetic field was studied and compared with the extension of the Mel molecular field theory by Gorter and Mrs. Van Peski-Tinbergen. Relaxation measurements have been carried out on the crystals with each of the two axes parallel to the field at liquid helium temperatures and in alternating fields at frequencies below 1135 Hz. In the liquid helium region at temperatures above the lambda point of liquid helium the experimental dispersion and absorption curves deviate from the Casimir and Du Pre theoretical curves due to the limited heat conduction in the crystal. The agreement between the experimental and the Casimir-Du Pre theoretical curves below that temperature is good. The relaxation parameters are independent of the field strength and vary,with the inverse 4th power of temperature for each of the two axes. Values of the specific heat of the spin system are inversely proportional to T2 above 2°K. Below that temperature the specific heat is increasing more rapidly as the temperature approaches the Noel temperature. This is in agreement with the direct specific heat determination.
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