Minority Spin Condensate in Spin-Polarized Superfluid ³He-A₁ Phase

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The spin-polarized superfluid ³He A₁ phase, which forms below 3 mK in external magnetic field, serves as a material in which theoretical notions of fundamental magnetic processes and macroscopic quantum spin phenomena may be tested. Conventionally, the superfluid component of the A₁ phase is understood 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. We have developed a novel mechanical spin density detector to measure the spin relaxation in the A₁ phase as functions of temperature, pressure and magnetic field. The mechanical spin detector is based in principle on the unique magnetic fountain effect occurring only in the A₁ phase (occurring between two transition temperatures, Tc₁ and Tc₂). Unexpectedly, the spin relaxation rate increases rapidly as the temperature is decreased towards Tc₂. Our measurements, together with Leggett-Takagi and Monien-Tewordt theories, demonstrate that a minute presence of minority spin pairs is responsible for the unexpected spin relaxation phenomena in A₁ phase.