Three-dimensional neutron depolarization study of the ferromagnetic-phase transition in a disordered system

The ferromagnetic-phase transition in the invar alloy system Fe-65 Ni-35 wa s investigated. Neutron depolarization data along with SANS data indicate a broadening of the phase transition. To interpret the data, we introduce a position-dependent Curie temperature T-C, which fluctuates around an averag e value <T-C>. The local T-C-variations are described by the standard devia tion of the transition temperature, DeltaT,, and by the characteristic corr elation length R,. The deduced parameters for Fe65Ni35 are: <T-C> = 485 +/- 0.5 K, DeltaT(C) = 12.5 +/- 0.2 K and R-0 approximate to 6 x 10(3) Angstro m. The magnetic transition can be described by a percolative scenario, as t he criterium R0/a much greater than (DeltaT(C)/<T-C>)(-2/3) is fulfilled, w here a is the lattice constant. (C) 2001 Elsevier Science B.V. Ah rights re served.