These experiments were performed to investigate whether half-quantum vortices could be stabilized in a  confined slab geometry where the orientation of the orbital angular momentum vector prefers to be oriented perpendicular to the plates. Half-quantum vortices were not found, but vortex pinning was observed to play a substantial role in these experiments using a stack of parallel plates to confine the helium liquid. Among other things, this confinement led to observable persistent currents in ³He-A which could be seen by the use of novel surface spin waves trapped on the non-uniform texture of the order parameter near a surface [PRL 58, 678 (1987)].

Order parameter textures induced by external magnetic field and superflow in narrow channels display intricate phenomena, including Freedericksz type of transitions. By using transverse cw NMR at 28.4 mT, we have made the exciting observations of a localized spin-wave mode trapped by the textural boundary layer in He-A. This surface mode, similar to a spin wave trapped by a pure l texture soliton, has been studied theoretically by Bruinsma and Maki. Our observations agree well with their calculations. Furthermore, we find that this spin-wave mode, and the interaction between the l texture and the superflow, provide in He3-A a method for studying the creation and pinning of vortices in narrow gaps when the sample is rotated.

FIGURES (Top left) Measured NMR absorption spectra of He3-A as a function of scaled frequency in a stack of Mylar plates with average spacing of about 10 microns. The line shape (a) in the stationary state and (b) at an angular speed of 0.6 rad/s. The Ω-dependent  spin wave resonance peak discovered in this work is indicated by arrows. The spectra were recorded at p =33 bars and at T = 0.73Tc. (Bottom left) Observed hysteresis in the relative NMR absorption of the surface spin-wave mode (SU) vs the angular velocity of the cryostat; open circles and filled circles refer to acceleration and deceleration, respectively. When the speed was varied between 0 and 0.5 rad/s after the hysteresis loop had been traversed, the reversible branch with a clear minimum was obtained as shown in the inset (open and filled triangles). The measuring time at each successive speed was 4 min.