Superflow-stabilized nonlinear NMR in rotating He-3-B

Nonlinear spin precession has been observed in He-3-B in large counterflow of the normal and superfluid fractions. The new precessing state is stabili zed at a high rf excitation level and displays frequency-locked precession over a large range of frequency shifts, with the magnitude of magnetization at its equilibrium value. Comparison to analytical and numerical calculati on indicates that in this state the orbital angular momentum L of the Coope r pairs is oriented transverse to the external magnetic field in a ''non-Le ggett'' configuration with broken spin-orbit coupling. The resonance shift depends on the tipping angle theta of the magnetization as omega - omega(L) = (Omega(B)(2)/2 omega(L))(cos theta-1/5). The phase diagram of the preces sing modes with arbitrary orientation of L is constructed. [S0163-1829(99)0 0501-9].