Boundary magnetism in liquid 3He at very low temperatures

The low-temperature magnetic susceptibility of normal and superfluid 3He confined between mylar foils spaced 40 p apart shows a large and field-dependent enhancement over the bulk Fermi liquid magnitude. The excess susceptibility disappears when the liquid is doped with 0.5 7; 4He. The reason for the excess is unknown. We also studied the proton resonance of the mylar substrate and obtained spin polarizations large enough to permit the direct observation of double spin-flip RF absorption. We have observed that the nuclear magnetic susceptibility of liquid 3He confined in a large stack of mylar plates increases to values much larger than that of bulk liquid when cooled to very low temperatures. An initial report of this effect was made by Ahonen et a1 (1975b) in a post deadline paper of LT 14. At the time the susceptibility increase was attributed to a possible condition of the superfluid B phase for liquid contained in very small channels. Our more recent measurements, reported in this work, show that the effect exists even in the normal liquid and that the effect is unambigously a boundary phenomenon. The physical cause of the effect remains a puzzle. We also made a brief investigation of the proton resonance of the plastic environment of the chamber. The measurements were conducted in a nuclear demagnetization cryostat which could cool the liquid 3He to 0.7 mK. The experimental arrangement is illustrated in figure 1. The NMR pick-up coil was wound around a cylindrical form containing a stack of 500 mylar plates, or foils, with an average spacing of 4.0 ym. Each plate was 3.6 ym thick. The entire assembly of mylar plates, the NMR coil and a graphite support structure was immersed within a cylindrical cavity of the nuclear cooling apparatus and the temperatures monitored with a platinum NMR thermometer. A general discussion of the techniques for measuring and maintaining low temperatures with this apparatus is to be published elsewhere (Ahonen et a1 1976). $ Presently at University of California (San Diego), La Jolla, California, USA. 8 Guggenheim Fellow on leave from Cornell University, Ithaca, New York, USA. 7 On leave from Regensburg University, Regensburg, West Germany, supported by Deutsche Forschungsgemeinschaft. On leave from Faculty of Science. Osaka City University, Osaka, Japan.