Some Recent Results on Paramagnetic Spin-Lattice Relaxation in Hydrated Paramagnetic Salts of the Iron Group

Abstract

The study of relaxation phenomena near the magnetic phase transition has been the aim of many experiments since 1958. In this paper we will first review some early results which have been obtained on manganese chloride and bromide samples. The possibility of using very low frequencies (v min = 0.2 Hz) at strong magnetic fields (H max = 50 kOe) gave us an opportunity to study the behavior of the relaxation times near the magnetic phase transition. Experiments on manganese chloride and bromide with the external magnetic field parallel to the crystal c axis, showed a maximum in the relaxation time as a function of temperature or external magnetic field. This behavior can be ascribed to the anomalies of the specific heat at the transition temperature. Another interesting sample in this study is COCI2 • 2H2O (external magnetic field parallel to the crystal b axis). This crystal becomes antiferromagnetic at 17.2°K3 while below 9.3°K also a ferrimagnetic spin arrangement occurs in magnetic fields between 32.0 and 45.5 kOe. Our susceptibility data did confirm the suggested phase diagram. At the antiferromagnetic-ferrimagnetic transition field of 32.0 kOe a relaxation mechanism can be detected. The relaxation times are remarkably long (1 s at 4.2°K) and can only be measured with a so-called step-field method, in which T is found from the time dependence of the susceptibility after a sudden change of the external magnetic field. The relaxation times vary strongly with temperature; in the range between 3.2° and 4.2°K an increase of a factor 100 is observed. Though the field dependence of the relaxation time cannot be detected accurately because of the lack of sensitivity, the experiments do show a maximum at the transition field.