NMR Shifts in 3He in Aerogel Induced by Demagnetizing Fields

Аннотация

Magnetic materials generate demagnetizing field that depends on geometry of the sample and results in a shift of magnetic resonance frequency. This phenomenon should occur in porous nanostructures as well, e.g., in globally anisotropic aerogels. Here, we report results of nuclear magnetic resonance experiments with liquid ³He confined in anisotropic aerogels with different types of anisotropy (nematic and planar aerogels). Strands of aerogels in pure ³He are covered by a few atomic layers of paramagnetic solid ³He which magnetization follows the Curie–Weiss law. We have found that in our samples the nuclear magnetic resonance shift in solid ³He is clearly seen at ultralow temperatures and depends on value and orientation of the magnetic field. The obtained results are well described by a model of a system of non-interacting paramagnetic cylinders. The shift is proportional to the magnetization of solid ³He and may complicate nuclear magnetic resonance experiments with superfluid ³He in aerogel.