Susceptibility of dilutely doped CrFe alloys

For low dopant concentrations, Fe is the only impurity atom which exhibits Pauli paramagnetism below the Néel temperature T_N of a Cr host. A series of measurements on Cr_1-xFe_x and (Cr-2.7%Fe)_1-xV_x alloys reveal that the magnetization M is sensitive to the amplitude of the spin-density wave (SDW) below T_N. The Fe moment also depends on the period of the SDW and is about 6% larger in the incommensurate than in the commensurate SDW state. Surprisingly, the differential susceptibility dM/dH peaks at the field H_p, which is about 5 kOe for low temperatures and small V concentrations. While H_p increases with the temperature, it decreases with the V and Fe concentrations. These observations may be explained by making the radical assumption that the rigidity of the Fe moment is broken by its interaction with the SDW. Whereas part of the Fe moment is bound to the SDW by the nesting free energy, the remainder experiences a weak effective field exerted by the surrounding SDW and by the ferromagnetically coupled, nearest-neighbour pairs of Fe atoms. The peak in dM/dH occurs when the external field H overcomes the antiferromagnetic field experienced by the single Fe atoms. This model explains the temperature and doping dependence of H_p, as well as the difference between the Fe moments in the commensurate and incommensurate SDW hosts.