Critical scattering of polarized neutrons in the invarFe65Ni35alloy

Abstract

The magnetic phase transition in the invar Fe 65 Ni 35 alloy has been investigated by small-angle polarizedneutron scattering ͑SAPNS͒ 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 ⌬T C and by the characteristic length of the T C variations, R 0 .The deduced parameters are ͗T C ͘ϭ485Ϯ0.5 K, ⌬T C ϭ12.5Ϯ0.2 K, and R 0 Ϸ300 nm. The critical SAPNS experiment was performed in a special inclined geometry (H ជ is inclined to the wave vector k ជ ). Two different contributions to the critical scattering were analyzed in the magnetic-field range ͑1-1000 G͒ and the temperature range (T C Ϯ0.1T 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-spincorrelation 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.

High-temperature induced ferromagnetism onγ−Feprecipitates in FeCu solid solutions

“…1 Nevertheless, some issues, especially those related with iron in a face centered cubic ͑fcc͒ lattice, the so-called ␥-Fe, are still a hot topic of discussion at the present. 2,3 Total-energy band calculations predict a very rich and complex magnetic phase diagram, 4,5 including the presence of large moment-volume instabilities, [6][7][8] responsible for the existence of Invar and anti-Invar behaviors. Contrary to the Invar effect, where a nearly zero thermal expansion is observed in some magnetically ordered materials, other systems exhibits, in the paramagnetic state, an anomalous thermal expansion larger than that described by a Grüneisen lattice.…”

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

“…Grigoriev et al studied the magnetic phase transition in disordered Fe-Ni alloys doped with carbon. [4][5][6] The size of LOR depends on the doping concentration p in the following way R 0 ϳ͓p͑1− p͔͒ −1 . 22 However the present experimental data is not enough to test this prediction.…”

“…For example, LOR is needed to explain experiments in doped ferromagnets. [4][5][6] In addition the inhomogeneous phase has been extensively studied in various quantum phase transition in disordered systems. For example, it is found that there are two kinds of Mott transition: homogeneous and inhomogeneous.…”

“…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.…”

Experimental investigation of inhomogeneities, nanoscopic phase separation, and magnetism in arc melted Fe-Cu metals with equal atomic ratio of the constituents

“…7,18,19 Metastable fcc Fe-Cu phases are found to decompose into fcc Cu and bcc Fe phases at 1000 K. 19 Nevertheless, issues like Fe in Fe-Cu system exhibiting fcc structure, the so called c-Fe, is still a hot topic of discussion. 20,21 Negligible mutual solubility of both the constituent elements in equilibrium state also requires a detailed investigation of presence of small fraction of intermetallic compounds. Commercially, bcc Fe having nanosized Cu precipitates exhibits an enhanced thermal stability and an increased hardness.…”