Isentropic Pressure of Magnetically Ordered bcc Solid 3He Near the Triple Point

We have studied the thermodynamics of magnetically ordered bcc solid ³He near the triple point, where the high-field phase (HFP), low-field phase (LFP), and paramagnetic phase (PP) coexist, by using isochoric pressure measurements along isentropes. The experiment gives the first unambiguous proof that the transition from the paramagnetic phase to the high-field phase is first order. It also shows that Δv _h/Δv _l>Δs _h/Δs _l at the triple point, where Δv _{h, l} is the molar-volume difference between the HFP (LFP) and the PP, and Δs _{h, l} the corresponding molar entropy difference.