Critical scattering of polarized neutrons in the invar Fe65Ni35 alloy -: art. no. 134417

The magnetic phase transition in the invar Fe65Ni35 alloy has been investigated by small-angle polarized-neutron scattering (SAPNS) along with both three-dimensional neutron depolarization and thermal-expansion measurements. The data give evidence for a smeared phase transition. A spatial distribution in the Curie temperature T-C is introduced to explain the experimental data. The local T-C variations are described by the standard deviation of the transition temperature DeltaT(C) and by the characteristic length of the T-C variations, R-0. The deduced parameters are <T-C>=485+/-0.5 K, DeltaT(C)=12.5+/-0.2 K, and R(0)approximate to300 nm. The critical SAPNS experiment was performed in a special inclined geometry (H is inclined to the wave vector k). Two different contributions to the critical scattering were analyzed in the magnetic-field range (1-1000 G) and the temperature range (T-C+/-0.1T(C)) of interest. First, the two-spin-correlation function was studied. Second, the dynamical three-spin-correlation function, or the so-called dynamical chirality of the system, was investigated by extracting the asymmetric part of the polarization-dependent scattering. We distinguish two contributions in the chiral scattering, which stem from the spin waves in the large magnetic inhomogeneities and from the three-spin-correlation function of the critical fluctuations. The data are interpreted in terms of the static and dynamic scaling theory accounting for the concept of local T-C variations.