Texture transitions in superfluid 3 He-A are produced by a moving bellows system controlling mass flow. These texture changes are caused by the competition of magnetic, flow, and gradient energies. We have located the boundary of a region of stability with v* s II / IIH. This boundary may be related to theoretical predictions of a transition to helical textures, but there is substantial disagreement with the theory over the maximum magnitude of the superfluid velocity in what we interpret as uniform textures. PACS numbers: 67.50.Fi, 43.35. + d, 47.20. + m Textures in superfluid 3 He are a striking manifestation of the symmetry of the order parameter. To date, however, only a few relatively simple textures have been studied. We report here the first measurements on nontrivial dynamic textures in the A phase which are produced in a controlled fashion and which exhibit great sensitivity to the effect of superfluid flow. We have located a boundary between the uniform texture and a complex texture by varying the magnetic field while maintaining a constant mass flow.
A system has been developed to directly observe NMR free induction signals at very low frequencies. We use a nonresonant pickup arrangement with a commercial dc SQUID to monitor the precessing magnetization in a 0.5-50-kHz range of Larmor frequencies. A relocking interval for the SQUID's flux locked loop presently sets our pulse recovery around 20-30 J.ls throughout this frequency range. The noise for this system is less than 1.5 times that recorded for the SQUID with shorted input tenninals.
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