Predictions for the classical Heisenberg model. The figure compares the Callen and Callen results (dashed) with the corresponding exact solutions (solid). Note that the deviations from the power-law behavior are unrelated to critical fluctuations.
Spin structure of transition-metal-rich RE-TM intermetallics: (a) zero temperature and (b) room temperature. The rare-earth atoms (spheres) are embedded in a transition-metal environment. The anisotropy, indicated by the thickness of the arrows, is dominated by the rare-earth atoms. Due to the relatively weak interatomic RE-TM exchange, the anisotropy breaks down well below .
Violation of the Callen and Callen law in rare-earth transition metallics. The solid line describes, for example, materials such as . This illustrates the complete failure of the Callen and Callen theory.
Spin structures (schematic): (a) ferromagnetism and (b) paramagnetism. This figure illustrates the physical origin of the temperature dependence of the anisotropy of magnets.
Temperature dependence of second-order anisotropy constants of some magnetic compounds. Cubic and noncubic actinide magnets exhibit (Ref. 15 ).
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