The development of new high-performance refrigerants has been highly desired from the viewpoints of energy efficiency and environmental safety. Recently, we developed promising magnetic refrigerant compounds La(Fe1-xSix)13 (0.86≤x≤0.90), that show the first-order of itinerant-electron metamagnetic transition from the paramagnetic state to the ferromagnetic state in applied magnetic fields. By adjusting the hydrogen concentration in La(Fe1-xSix)13Hy, the Curie temperature TC and the working temperature for cooling are controlled covering room temperature. Furthermore, the thermal conductivity of the present compounds is excellent. It should also be emphasized that the elements of the present compounds are very cheap economically and completely harmless to humans.
A low-noise, pulse-tube cryocooler has been developed. It is suitable for cooling variable semiconductor or superconducting devices. The cryocooler's electro-magnetic noise and inclination characteristics are improved, and the cooling capacity is 4.9 W at 77 K with an input power of 800 W. The minimum temperature was less than 49 K. The cooling performance at 77 K was within 5% degradation for all orientations. The low-noise pulse-tube cryocooler was installed in a cryostat designed for a high-Tc SQUID sensor. The electro-magnetic noise was measured by the magnetometer-type SQUID sensor cooled in the cryostat. The noise level was influenced by the direction of the sensor. When the sensor was parallel to the cryocooler cylinder, the white noise was about 300 fT/√Hz and the cyclic noise produced by the cryocooler's magnetic material or mechanical vibration was not observed. When the sensor was vertical to the cylinder, the white noise was about 2 pT/√Hz and the cyclic noise was about 10 pT rms. A real-time magnetocardiography was successfully measured using a high-Tc SQUID magnetometer cooled by the pulse-tube cryocooler. The signal-to-noise ratio was better than the conventional cryocooling system.