Nuclear Magnetic Orderings and Frustration in Bcc 3He in High Magnetic Fields

We studied phase transitions and thermodynamic properties of the field-induced antiferromagnetic nuclear-spin ordered phase in bcc solid 3He by measuring the melting pressure at temperatures down to 0.4 mK in magnetic fields up to 15 T. The transition temperature from the paramagnetic phase is reentrant with increasing field with a maximum at 10 T. This indicates that the system is highly frustrated by the competing multiple-spin exchanges and excludes other mechanisms related to the zero-point vacancies. The upper critical field was estimated as 19.7 T, which reveals non-negligible contributions from higher order exchanges beyond six spins. A considerable softening of the spin waves in the ordered phase in low fields also suggests the strong frustration.