Giant intrinsic magnetic hardness

Crystallographically partly randomized materials with high magnetic anisotropy exhibit anisotropy and exchange fluctuations. Possibly the most dramatic consequence of these interaction fluctuations is the presence of giant intrinsic magnetic hardness observed, especially at cryogenic temperatures. This magnetic hardness (resistance to demagnetization) is anintrinsic solid state property, quite in contrast with hardness in technologically important materials based on the presence of fine particles or precipitate phases. It introduces rather a further variety in the phenomenology of interaction fluctuation materials such as spin glasses, representing the extreme case of high anisotropy and high moment concentrations. In this paper the phenomenon of strong intrinsic magnetic hardness is reviewed, and some new experimental data are presented for homogeneous pseudobinary regions of composition on the basis of such compounds as SmNi₅, SmCu₅, SmCo₅, TbFe₂ and others. Magnetic hardness in some of these cases reaches the highest values found so far for any class of materials. For instance, maximum values of coercive force of aboutH _c=230 kOe are observed for SmCo_5−xNi_x. Even higher values are extrapolated in other cases. Maximum values ofH _c are observed in materials with ordering temperatures of order 60–200 K. A strong temperature and a weaker time dependence ofH _c is observed and discussed on the basis of theories involving thermal activation of domain propagation. Comparisons are drawn with effects generally observed in magnetically hard materials, and the relationship of intrinsic magnetic hardness to technologically interesting materials is discussed. Tentative conclusions as to the details of the origins of giant intrinsic magnetic hardness are drawn and areas of future interest are indicated. This study was supported by a grant from the National Science Foundation.