A search for non-collinear ferromagnetism in INVAR

Measurements have been made to test the proposal that the INVAR effect is associated with a non-collinear ferromagnetic state. Neutron scattering experiments with polarization analysis of the incident and scattered beams have been made to obtain the absolute spin-dependent scattering cross-sections from the archetypal INVAR alloy Fe65Ni35. The measured average spin-flip cross-section for this alloy has been found to be close to zero, independent of the scattering vector Q and effectively constant as the sample temperature is reduced from 300 to 4.2 K and the magnetic field increased from 1.4 to 2.0 T. All this suggests that the INVAR sample is a collinear ferromagnet. In addition, the measured spin-flip cross-section is also in poor agreement with calculated curves based on models of the proposed non-collinear state. The magnitudes of the magnetic moments on the iron and nickel atoms in the collinear state have been obtained from the intensities of the Bragg peaks in the non-spin-flip cross-sections. The diffuse scattering between the Bragg peaks has also been analysed to determine the magnetic disorder present. These data have been discussed in the context of different models of the magnetic structure and the results also support the conclusion that Fe65Ni35 is a collinear ferromagnet.

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