Yamaguchi A, Kobayashi S, Ishimoto H, Kojima H
Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
Nature. 2006 Dec 14;444(7121):909-12. doi: 10.1038/nature05391.
The magnetic properties of (3)He in its various phases originate from the interactions among the nuclear spins. The spin-polarized 'ferromagnetic' superfluid (3)He A(1) phase (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 A(1) 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. 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 A(1) 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 A(1) phase as a function of temperature, pressure and magnetic field. Our mechanical spin detector is based in principle on the magnetic fountain effect; 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 A(1) phase near T(c1), the measured spin relaxation time is long, as expected. Unexpectedly, the spin relaxation rate increases rapidly as the temperature is decreased towards T(c2). Our measurements, together with Leggett-Takagi theory, demonstrate that a minute presence of minority spin pairs is responsible for this unexpected spin relaxation behaviour. Thus, the long-held conventional view that the A(1) phase contains only the majority spin condensate is inadequate.
³He在其不同相态下的磁性源于核自旋之间的相互作用。自旋极化的“铁磁”超流³He A₁相（在外部磁场中，于两个转变温度Tₑ₁和Tₑ₂之间，低于3 mK时形成）可作为一种材料，用于检验基本磁过程和宏观量子自旋现象的理论。传统上，A₁相的超流成分被认为仅包含多数自旋凝聚体，其具有沿外场方向能量上更有利的配对自旋，而不存在相反方向配对自旋的少数自旋凝聚体。由于难以同时满足产生可观相空间所需的超低温和强磁场条件，很少有关于自旋动力学现象的研究可用于检验A₁相的传统观点。在此，我们开发了一种能在所需条件下工作的机械自旋密度探测器，使我们能够测量A₁相中的自旋弛豫随温度、压力和磁场的变化。我们的机械自旋探测器原则上基于磁喷泉效应；自旋极化的超流运动既能通过磁场也能通过机械方式诱导产生，并且我们证明了通过机械自旋过滤过程增加自旋极化的可行性。在靠近Tₑ₁的A₁相高温范围内，测得的自旋弛豫时间很长，正如预期的那样。出乎意料的是，随着温度向Tₑ₂降低，自旋弛豫率迅速增加。我们的测量结果与莱格特 - 高木理论一起表明，少数自旋对的微量存在是导致这种意外自旋弛豫行为的原因。因此，长期以来认为A₁相仅包含多数自旋凝聚体的传统观点是不充分的。
