Plane-polarized light was passed through a single crystal of terbium ethylsulphate, parallel to the $c$ axis, at temperatures between 77 mK and 1.5 K, in magnetic fields up to 3 kG. The Faraday rotation was measured, and above the transition temperature (240 mK) was fitted to theoretical magnetization curves. Below the transition temperature, hysteresis was observed, and also depolarization of the light. The observations are consistent with a ferromagnetic state in domains parallel to the $c$ axis below the transition. Molecular-field calculations also indicate ferromagnetism. The value of the splitting between the two ground-state singlets was determined.

• Received 23 December 1974

DOI:https://doi.org/10.1103/PhysRevB.11.4409