Nuclear Ordered Phases of Solid 3He in Silver Sinters

To determine the exact spin structure of the nuclear magnetic ordered phases of solid 3He, the U2D2 low field and the high field phases above 0.4 T, a European Research and Training Network for neutron scattering from the ordered solid was established which consisted of a collaboration with the Hahn Meitner Institute, Berlin, and other European and US groups. For this experiment it is crucial to grow a single crystal within the sinter needed for cooling the solid to temperatures of the order of 500 muK and to keep it cold long enough to measure a magnetic neutron diffraction. The sinter is also necessary to absorb the major part ( > 90%) of the heat generated by the neutron capture and decay reaction of the 3He nucleus. In this work we studied the growth of crystals in Ag sinters of different pore sizes and with different growth speeds to find an optimal way to obtain single crystalline samples, or at least samples with only a few grains. We used SQUID magnetometry and NMR to measure the magnetization in the ordered phases. They were indicated by the known drop of the intensity, both in the NMR signal and in the dc magnetization, for the U2D2 phase, and by an increase of about 30% for the high field phase. The best results for cooling were obtained with sinters made from 700 A 'Japanese powder' with a packing fraction of 50% which were annealed at 130 deg C after sintering and then had a calculated particle size of about 4200 A. In the dc magnetization we found a paramagnetic surface contribution from a few monolayers of 3He down to 500 muK in addition to the bulk magnetization.