Magnetic dipole interactions in dysprosium ethyl sulphate - I. Susceptibility and specific heat between 20 and 1°K

Measurements have been made of the magnetic susceptibility and the magnetic contributions to the specific heat of dysprosium ethyl sulphate, at temperatures between 20 and 1 degrees K. It is shown that below about 3 degrees K these properties, and also the earlier optical rotation measurements of Becquerel et al., can be accounted for satisfactorily in terms of a doubly degenerate ground state of the magnetic ions, with spectroscopic splitting factors parallel and perpendicular to the crystal axis g$_{\|}$ = 10$\cdot $8 and g$_{\perp}$= 0, together with an unusually strong coupling between the ions arising from magnetic dipole-dipole interaction. The effect of other interactions is shown to be small. Because of this, and the extreme anisotropy of the g-tensor, the properties of this substance at very low temperatures ($\sim $ 0$\cdot $1 degrees K) should closely resemble those of a classical Ising model with known, dipolar forces between the ions. At temperatures above 3 degrees K other states of the ions become populated and it is shown that the first of these lies at an energy of (23 $\pm $ 3) k, in good agreement with other experiments.