Volume Dependence of H _c1, H _c2, and ΔP(H _c1) in Magnetically Ordered bcc ³He

Abstract

We have used constant-volume pressure measurements over the entire range of molar volumes of bcc solid ³He, 20>v>24 cm³/mole, to determine the zero-temperature critical field H _c1 and the pressure discontinuity ΔP(H _c1) between the two antiferromagnetically ordered phases. The upper critical field H _c2 has been determined for 20>v>21.75 cm³/mole. The ³He was cooled by direct (two-stage) adiabatic demagnetization from T≈100 µK and H=2.5 T where its entropy S≈0. We find the Gruneisen parameters for H _c2 and ΔP to be larger than in previous work, but still smaller than for H _c2 and other physical quantities such as the Neel temperature T _N and e ^1/2 ₂, where e ₂ is the coefficient of the second-order term in the free energy of the paramagnetic phase. These resugts are significant for testing the mugtiple-exchange model for solid ³He.