M CrS2 compounds (M = Li, Na, K, Cu, Ag, and Au) with triangular Cr layers show large variety of magnetic ground states ranging from 120• antiferromagnetic order of Cr spins in LiCrS2 to double stripes in AgCrS2, helimagnetic order in NaCrS2, and, finally, ferromagnetic Cr layers in KCrS2. On the base of ab-initio band structure calculations and an analysis of various contributions to exchange interactions between Cr spins we explain this tendency as originating from a competition between antiferromagnetic direct nearest-neighbor d-d exchange and ferromagnetic superexchange via S p states which leads to the change of the sign of the nearest neighbor interaction depending on the radius of a M ion. It is shown that other important interactions are the third-neighbor interaction in a layer and interlayer exchange. We suggest that strong magneto-elastic coupling is most probably responsible for multiferroic properties of at least one material of this family, namely, AgCrS2.
Magnetocrystalline anisotropy (MCA) in doped Ce 2 Co 17 and other competing structures was investigated using density functional theory. We confirmed that the MCA contribution from dumbbell Co sites is very negative. Replacing Co dumbbell atoms with a pair of Fe or Mn atoms greatly enhance the uniaxial anisotropy, which agrees quantitatively with experiment, and this enhancement arises from electronic-structure features near the Fermi level, mostly associated with dumbbell sites. With Co dumbbell atoms replaced by other elements, the variation of anisotropy is generally a collective effect and contributions from other sublattices may change significantly. Moreover, we found that Zr doping promotes the formation of 1-5 structure that exhibits a large uniaxial anisotropy, such that Zr is the most effective element to enhance MCA in this system.
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