Minority spin condensate in the spin-polarized superfluid ³He A₁ phase

Abstract

The magnetic properties of ³He in its various phases originate from the interactions among the nuclear spins¹. The spin-polarized ‘ferromagnetic’ superfluid ³He A₁ phase² (which forms below 3 mK between two transition temperatures, T_c1 and T_c2, in an external magnetic field) serves as a material in which theories of fundamental magnetic processes and macroscopic quantum spin phenomena may be tested. Conventionally, the superfluid component of the A₁ phase is understood^3,4,5,6 to contain only the majority spin condensate, having energetically favoured paired spins directed along the external field and no minority spin condensate having paired spins in the opposite direction. Because of difficulties in satisfying both the ultralow temperature and high magnetic field required to produce a substantial phase space, there exist few studies of spin dynamics phenomena that could be used to test the conventional view of the A₁ phase. Here we develop a mechanical spin density detector that operates in the required regime, enabling us to perform measurements of spin relaxation in the A₁ phase as a function of temperature, pressure and magnetic field. Our mechanical spin detector is based in principle on the magnetic fountain effect⁷; spin-polarized superfluid motion can be induced both magnetically and mechanically, and we demonstrate the feasibility of increasing spin polarization by a mechanical spin filtering process. In the high temperature range of the A₁ phase near T_c1, the measured spin relaxation time is long, as expected^2,8,9. Unexpectedly, the spin relaxation rate increases rapidly as the temperature is decreased towards T_c2. Our measurements, together with Leggett–Takagi theory⁵, demonstrate that a minute presence of minority spin pairs is responsible for this unexpected spin relaxation behaviour. Thus, the long-held conventional view² that the A₁ phase contains only the majority spin condensate is inadequate.