In the present article the recent developing process on the new application of high entropy magnetic materials in the He temperature range will be shown. First, the theoretical basis of these applications will be explained briefly and the necessary condition for selection of the magnetic materials will be discussed. Then, the several experimental results, such as the temperature dependences of the specific heat, in some promising Er compounds, Er3Ni, ErNi, ErNi2-DyNi2 etc., will be shown. In order to show clearly the present stage of the magnetic regenerator material investigation, we will show several results which verify the effectiveness of the magnetic regenerator materials obtained from refrigeration experiment using the GM refrigerator. Moreover, it has been made clear that the regenerator characteristics depends largely on the structure of the regenerator which contains several kinds of magnetic regenerator materials. Therefore, as an example, the experimental result of regenerator characteristics in the double layer type magnetic regenerator, in which Er3Ni powders are piled up on the ErNi powders, will be shown. Lastly, on the basis of the above experimental results, the development of the magnetic material application to regenerator in future will be discussed.
Procedures to measure critical parameters such as the critical temperature, Tc, the upper critical field, Hc2, the critical current, Ic, and the AC loss in practical superconductors are described. The criteria and factors which influence the evaluation of the parameters are briefly discussed. The importance of the holder in the 4 probe method which supports the sample against the Lorentz force is emphasized from the view point of strain effect. The magnetization method to measure the AC loss of a multifilamentary wire is described in detail.