Detailed measurements of the low-field antiferromagnetic resonance spectrum of spin-ordered bcc ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ exhibit large shifts from the Larmor frequency, with a zero-field resonant frequency near zero temperature of $\frac{{\Omega }_0}{2\pi }\simeq 825$ kHz. Analysis of the spectrum leads to stringent constraints on possible sublattice structures. The temperature dependence of ${\Omega }_0$ shows low-temperature behavior expected from spin-wave theory, and indicates a first-order transition at 1.03 mK.

• Received 1 February 1980

DOI:https://doi.org/10.1103/PhysRevLett.44.792