Abstract
Measurements have been made to test the proposal that the INVAR effect isassociated with a non-collinear ferromagnetic state. Neutron scattering experimentswith polarization analysis of the incident and scattered beams have been made toobtain the absolute spin-dependent scattering cross-sections from the archetypalINVAR alloy Fe65Ni35. The measured average spin-flip cross-section for this alloyhas been found to be close to zero, independent of the scattering vectorQ andeffectively constant as the sample temperature is reduced from 300 to 4.2 K andthe magnetic field increased from 1.4 to 2.0 T. All this suggests that the INVARsample is a collinear ferromagnet. In addition, the measured spin-flip cross-sectionis also in poor agreement with calculated curves based on models of the proposednon-collinear state. The magnitudes of the magnetic moments on theiron and nickel atoms in the collinear state have been obtained from theintensities of the Bragg peaks in the non-spin-flip cross-sections. Thediffuse scattering between the Bragg peaks has also been analysed todetermine the magnetic disorder present. These data have been discussed inthe context of different models of the magnetic structure and the resultsalso support the conclusion that Fe65Ni35 is a collinear ferromagnet.
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