VOLUME DEPENDENCE OF THE LOWER CRITICAL-FIELD IN NUCLEAR-SPIN ORDERED BCC HE-3

The zero-temperature lower critical field B(c1) (0) between the two nuclear antiferromagnetic phases of bcc He-3 has been determined as a function of molar volume from pressure measurements down to 120-mu-K. In a volume range between 22.45 and 24.06 cm3/mol, the Gruneisen parameter for B(c1) (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.