The spin-polarized superfluid $^3$He A$_1$ phase, which forms below 3 mK inexternal magnetic field, serves as a material in which theoretical notions offundamental magnetic processes and macroscopic quantum spin phenomena may betested. Conventionally, the superfluid component of the A$_1$ phase isunderstood to contain only the majority spin condensate havingenergetically-favoured paired spins directed along the external field and nominority spin condensate having paired spins in the opposite direction. We havedeveloped a novel mechanical spin density detector to measure the spinrelaxation in the A$_1$ phase as functions of temperature, pressure andmagnetic field. The mechanical spin detector is based in principle on theunique magnetic fountain effect occurring only in the A$_1$ phase (occurringbetween two transition temperatures, T$_{c1}$ and T$_{c2}$). Unexpectedly, thespin relaxation rate increases rapidly as the temperature is decreased towardsT$_{c2}$. Our measurements, together with Leggett-Takagi and Monien-Tewordttheories, demonstrate that a minute presence of minority spin pairs isresponsible for the unexpected spin relaxation phenomena in A$_1$ phase.
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