Helimagnetism in MnP-type compounds: MnP,FeP, CrAs and CrAs1?xSbx mixed crystals

The solid solution CrAs_1?xSb_x has been investigated by neutron diffraction at various temperatures. The phase boundary between mixed crystals with MnP-structure and those with NiAs-structure moves from 59 mol% CrAs at room temperature to 48 mol% CrAs at liquid helium temperature. The mixed crystals with NiAs-type structure at 4.2K have the collinear magnetic structure of CrSb. Samples having the MnP-type structure exhibit a “double spiral” spin structure like CrAs. The variation of the magnetic moment and the spiral structure have been studied. Using neutron diffraction data for the magnetic propagation vector and the phase difference of the two helices, the signs and the ratios of the exchange integrals are discussed in a Heisenberg model. The stability regions in parameter space are determined by a matrix method for the ferromagnetic mode observed in MnP above 50 K and for the helical modes observed in MnP below 50 K, and in FeP, CrAs and CrAs_1?xSb_x. We have also considered the possibility of antisymmetric and symmetric exchange between nearest neighbours only, using a group theoretical approach.The high ratios of antisymmetric to symmetric exchange would be necessary to explain the observed helical structures preclude however antisymmetric exchange as a dominant factor.