The origin of the INVAR effect, whereby certain ferromagnetic alloys exhibit near zero thermal expansion around room temperature, is still not understood. Various models have been put forward, including two-state models, canted moments and longitudinal spin-fluctuations, but experimental evidence is lacking, if not opposed to the theories. Some time ago, the group of Menshikov and co-workers  used polarized neutrons (D5, ILL) to measure powder samples of INVAR Fe0.65Ni0.35 and non-INVAR Fe0.5Ni0.5 and found considerable small-angle magnetic neutron scattering for the INVAR alloy – and none for the non-INVAR alloy – indicating the presence of transverse magnetization components which were inferred to come from low-spin Fe clusters.
We have used the D33 polarized SANS diffractometer with 3He spin-analysers to measure single-crystal FeNi alloys with the same concentrations as the study of Menshikov. With polarization analysis we are able to distinguish transverse and longitudinal spin fluctuations from one another. We confirm the presence of strong spin-fluctuations deep into the ordered state of Fe0.65Ni0.35 – both longitudinal and transverse – in a single magnetic domain, while the non-INVAR concentration shows no evidence of spin-fluctuations. While the transverse magnetization in INVAR does indeed appear to be associated with finite clusters, the longitudinal fluctuations appear only weakly correlated with a flat dependence on momentum transfer.
 Menshikov, A. Z. & Schwinger, J. The transverse magnetization components in the ground state of invar gamma-Ni1−cFec alloys. Solid State Communications, 100, 251–255 (1996).
Ross Stewart (ISIS Neutron and Muon Facility, Science and Technology Facilities Council, Rutherford Appleton Laboratory, UK)