ИСТИНА

NiMn-based Heusler alloys doped with Ga, In and Sn exhibit in the martensitic state a thermally broad, coupled antiferromagnetic-ferromagnetic (AF-FM) magnetic transition below the Curie temperature TCM giving rise to a splitting between ZFC and FC magnetization curves and exchange bias effects. We explain this by Mn atoms, coupling together both AF and FM depending on the Mn-Mn distance. These are evidenced from FMR and polarized neutron scattering experiments on NiMnZ Heusler alloys with Z as In, Sb and Sn. To study the conditions giving rise to mixed magnetism, we consider the magnetic interactions in over and under Mn-stoichiometry with respect to Ni2MnZ with Z as Ga and Sn. In such stoichiometries, the system can be adjusted so that Mn-Mn neighbors can either only be next nearest or both nearest and next nearest. FMR and polarized neutron studies on Ni-Mn-Ga and Ni-Mn-Sn alloys with varying concentrations of Sn and Ga provide complementary information on the nature of the coupling in such alloys. The results show that for understoichiometry predominantly FM interactions are observed, whereas for overstoichiometry additional AF coupling is observed. Work supported by the DFG (SPP 1239).