Helimagnetism and competition of exchange interactions in bulk giant magnetoresistance alloys based on MnAu2

By using the generalized perturbation method within the screened Korringa-Kohn-Rostoker formalism we calculate magnetic exchange interactions in the helimagnetic bulk MnAu2 compound upon alloying Mn with Fe and Cr. The obtained interactions reproduce well the results of neutron diffraction experiments concerning the periodicity of the helix in MnAu2 and its weakening with Fe substitution thus supporting the conventional interpretation of a metamagnet-ferromagnet transitions due to external fields. In the case of Mn1-xCrxAu2 alloys our results reveal that the experimentally observed magnetization process follows a completely different scenario, predicted theoretically a few decades ago for nearly orthogonal helimagnetics.