Nuclear spin ordering of solid³He in applied magnetic fields

Abstract

The effects of a magnetic field on nuclear spin ordering in solid³He have been observed through the influence of the field on the solid entropy. By making use of the Clausius-Clapeyron equation, the³He melting temperature as a function of pressure was determined in several magnetic fields from 0 to 1.2 T from measurements of the latent heat of solidification in a Pomeranchuk cell. The solid entropy as a function of temperature was then calculated from the melting curve slope in each field by again utilizing the Clausius-Clapeyron equation. The solid ordering manifested itself as a substantial reduction in entropy occurring over a narrow temperature interval. A portion of the magnetic phase diagram of solid³He was established by plotting the ordering temperature versus field. In low fields the reduction in entropy occurs over a very narrow temperature interval, and the ordering temperature is depressed by the field. Above about 0.41 T, however, the ordering region is broadened and moves to significantly higher temperatures with increasing fields. The behavior in low fields is tentatively interpreted as a magnetic phase transition from the paramagnetic to an ordered phase, while the effect in higher fields is believed to represent paramagnetic ordering of the nuclear spins by the applied field.