NMR shifts in $^3$He in aerogel induced by demagnetizing fields

Abstract: 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 (NMR) experiments with liquid $^3$He confined in anisotropic aerogels with different types of anisotropy (nematic and planar aerogels). Strands of aerogels in pure $^3$He are covered by a few atomic layers of paramagnetic solid $^3$He which magnetization follows the Curie-Weiss law. We have found that in our samples the NMR shift in solid $^3$He 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 $^3$He and may complicate NMR experiments with superfluid $^3$He in aerogel.