Volume dependence of the lower critical field in nuclear-spin ordered bccHe3

The zero-temperature lower critical field ${\mathit{B}}_{\mathit{c}1}$(0) between the two nuclear antiferromagntic phases of bcc ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ has been determined as a function of molar volume from pressure measurements down to 120 μK. In a volume range between 22.45 and 24.06 ${\mathrm{cm}}^3$/mol, the Grüneisen parameter for ${\mathit{B}}_{\mathit{c}1}$(0) is anomalously small compared to typical values for other physical quantities associated with the exchange interactions. The result suggests that the two-spin exchange frequency decreases more rapidly with decreasing volume than three- and four-spin exchanges in the terms of the multiple-spin exchange model.