Magnetic phase transition in disordered Fe-Ni alloys studied by means of small-angle neutron scattering and three-dimensional analysis of the neutron depolarization

Abstract

The magnetic phase transition in iron-nickel fcc alloys Fe 70 Ni 30 doped by carbon ͑0.7% at.͒ is investigated using the three-dimensional analysis of the neutron depolarization ͑ND͒ and small-angle neutron scattering ͑SANS͒. The coexistence and the growth of two different scale magnetic correlations are observed in a paramagnetic phase as temperature closes to T C . The usual critical fluctuations of a Lorentzian shape with size R c Ͻ200 Å are found by the analysis of SANS intensity I(q). The ND analysis shows also presence of the large scale correlations ͑about 10 3 Ϫ10 4 Å) with the ''squared'' Lorentzian shape. We attribute these large scale correlations to local variations of the Curie temperature T C . The local T C variations are described by the disorder parameters of the system: ͗T C ͘, a spread of T C variations ⌬T C , and characteristic size of the local areas R 0 , where T C variations occur. The ratio between the depth of the T C variations (⌬T C /͗T C ͘) and its characteristic correlation length R 0 /a (a is a lattice constant͒ determines a scenario of the transition: ''percolative'' or ''homogeneous'' ones. At R 0 /aӷ(⌬T C /͗T C ͘) Ϫ2/3 locally ordered ferromagnetic regions appear in paramagnetic phase and form a large percolative cluster. In this case the connectivity length of the ordered regions dominates at the correlation length of thermal fluctuations. Therefore this transition goes in accordance with the ''percolative'' scenario. All three parameters of the transition are obtained from the data of the three dimension analysis of the ND. From the temperature dependence of the magnetization, we found ͗T C ͘ϭ397 Ϯ0.5 K and ⌬T C ϭ4.55Ϯ0.05 K. From ND data we estimated the temperature independent characteristic size R 0 of the T C variations. It is equal to 10 4 Å.

Critical scattering of polarized neutrons in the invarFe65Ni35alloy

“…4,5,8,9 As shown below, this assumption is confirmed by experimental data. Let us consider the presence of T C variations with a Gaussian distribution:…”

“…In previous works 8, 9 we concluded from neutron depolarization ͑ND͒ and small-angle neutron-scattering ͑SANS͒ measurements that two different length scales for the magnetic correlations coexist in invar FeNi alloys around the mean critical temperature ͗T C ͘. Using SANS it was shown that the smaller length scale is attributed to the critical fluctuations and that their shape is well described by a Lorentzian in reciprocal space.…”

“…% carbon. 9 We interpreted the data of the ND experiment in terms of an appearance of large-scale magnetic inhomogeneities above T C . The characteristic parameters derived for the ferromagnetic phase transition of Fe 70 Ni 30 are ͗T C ͘ϭ397 K, ⌬T C ϭ4.5 K, and R 0 Ϸ10 3 nm.…”

Evidence for complex multistability in photomagnetic cobalt hexacyanoferrates from combined magnetic and synchrotron x-ray diffraction measurements

“…which convolutes a scaling law for 3D Ising-like systems with a Gaussian distribution of local critical temperatures, T c s. 37 The dependence of T c with the applied magnetic field was neglected for simplicity and the critical exponent ␤ was taken as 0.33. 38 The experimental data were fairly well modeled using a mean critical temperature, ͗T c ͘, of 21.2 K ͑16.7 K͒ and a spread, ⌬T c , of 2.7 K ͑2.2 K͒ for the photoexcited ͑quenched͒ phase.…”

Excited states of the saddle point equation of the Landau-Ginzburg-Wilson Hamiltonian with random temperature

“…LOR is discovered in the magnetic phase transition in disordered systems. [4][5][6] The experiments on the superfluid transition of 4 He in porous media also revealed the localized Bose condensation above the global superfluid transition temperature. 7,8 In order to explain recent scanning tunnel microscope STM experiments, an inhomogeneous gapped superconductor with superconducting islands and metallic regions is proposed.…”