Research for Hyperfine Nuclear Magnetism of PrPba in the Antiferro-Quadrupoinr Ordered State

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We have investigated the hyperfine enhanced nuclear magnetic properties of PrPb3 at the antiferro quadrupole (AFQ) ordered state at ultra low temperatures. The static magnetization and the ac susceptibility of PrPb3 single crystals in the magnetic fields up 6.21 mT parallel to [100] and [110] directions were simultaneously measured down to below 1 mK by an ac impedance bridge using a SQUID magnetometer. In the AFQ ordered state, the hyperfine enhanced nuclear magnetic susceptibility increased below 40 mK according to the Curie Weiss law, and showed a peak at 5 mK. The enhancement factor obtained from the Curie constant was 1+K=30. The peak height at 5 mK was only 10^<3> of the expected saturated nuclear magnetization. These results suggest that the hyperfine enhanced nuclear magnetic moments of <141>^Pr antiferromagnetically order below 5 mK. The ac susceptibility in the magnetic field and the static magnetization around the zero magnetic field have a different temperature dependence from the static magnetization in the magnetic field. The ac susceptibility and the static magnetization show the superconductive diamagnetism at 35.7 mK, and the critical magnetic field at zero temperature is 0.568 mT.In order to measure the magnetic susceptibility in higher magnetic fields, we have developed a new ac susceptibility measurement system without a SQUID magnetometer which is assembled from five pick up coils and a modulation coil for mutual inductance measurement, a small superconducting solenoid for applying static magnetic filed, and the magnetic shield of niobium and it-metal cylinders. Furthermore, cooperating with Metoki group in Japan Atomic Energy Agency, we have developed a nuclear adiabatic demagnetization refrigerator for a neutron diffraction measurement at sub milikelvin temperatures for the purpose of direct observation of nuclear ordered spin structures.