Dispersion of spin waves in the amorphous ferromagnetic alloy Fe 48 Ni 34 P 18 can be described within the model of a ferromagnet with random anisotropy: ϵ ( q ) = Aq 2 + g μ B H + δω( q ), where δω( q ) is an additional term linear in | q |. The method of small-angle scattering of polarized neutrons is used to prove the importance of the additional term δω( q ) in dispersion. The measurements are carried out for different values of the external magnetic field H and neutron wavelength λ. The scattering map of neutrons represents a circle centered at the point q = 0. The stiffness A of spin waves is derived directly from the λ-dependence of the radius of this circle. The spin-wave stiffness A of the amorphous alloy weakly decreases from 140 to 110 meV Å 2 as temperature increases from 50 to 300 K. The field dependence of the radius demonstrates the presence of an additional term δω( q ) in the form of an energy gap that is almost independent of field and temperature. The value of the additional term is Δ = 0.015 ± 0.002 meV.

Aspects of the experimental implementation of the small-angle neutron scattering (SANS) method on a compact neutron source are considered. A scientific justification of the demand for this type of installation is formulated. A scheme for implementing the SANS method on a pulsed neutron source is proposed, which ensures the high efficiency of using a neutron beam by limiting the operating wavelength range, and, as a result, maximizing the frequency and time-averaged power/intensity of the source. The physical parameters of the key elements of the installation, such as the cold neutron moderator, the beam-chopper cascade, the collimation system, the sample unit and the wide-aperture position-sensitive detector, are described. It is shown that the small-angle scattering method can be implemented on a university-type pulsed neutron source.

Dispersion of spin waves in the amorphous ferromagnetic alloy Fe48Ni34P18 can be described within the model of a ferromagnet with random anisotropy: @(q) = Aq2 + gμBH + δω(q), where δω(q) is an additional term linear in |q|. The method of small-angle scattering of polarized neutrons is used to prove the importance of the additional term δω(q) in dispersion. The measurements are carried out for different values of the external magnetic field H and neutron wavelength λ. The scattering map of neutrons represents a circle centered at the point q = 0. The stiffness A of spin waves is derived directly from the λ-dependence of the radius of this circle. The spin-wave stiffness A of the amorphous alloy weakly decreases from 140 to 110 meV Å2 as temperature increases from 50 to 300 K. The field dependence of the radius demonstrates the presence of an additional term δω(q) in the form of an energy gap that is almost independent of field and temperature. The value of the additional term is Δ = 0.015 ± 0.002 meV.

The cubic noncentrosymmetric structure of the B20 compounds produces the spin helix with the wave vector ks = D/J balanced by the competition of two interactions: the large ferromagnetic exchange interaction J and small antisymmetric Dzyaloshinskii- Moriya (DM) interaction D. The mixed Fe1-xCoxSi compounds demonstrate a switch of the sign of magnetic chirality in depen-dence of concentration x. The sign of magnetic chirality is dic-tated by the structural chirality and chemical elements (Fe, Co) as well. The switch of chirality is accompanied by transformation of the helix structure to the ferromagnet and is observed at xc = 0.65. Moreover, this transformation (helimagnet-ferromagnet) occurs as a function of temperature. We observe the decrease of the helical wave vector ks with lowering temperature from Tc = 17 K and its abrupt zeroing at Tf = 7 K. The magnetic field applied not along the easy but along the hard anisotropic axis is able to restore the helical structure in the temperature range below Tf. The mechanism of the transformation is theoretically described by a competition between the cubic anisotropy and the DM interaction. We show that anisotropy-induced ferromagnet has nonreciprocal magnon spectrum due to DM interaction even in the absence of external magnetic field.(c) 2022 Elsevier Inc. All rights reserved.

This work studies the magnetic excitations of amorphous ferromagnetic alloys (Fe 40 Ni 40 P 14 11 B 6 and Fe 48 Ni 34 P 18 ) by small-angle polarized neutron scattering in the geometry when the magnetic field is inclined towards the neutron beam direction. Polarized neutrons are used in order to extract the scattering arising from the spin waves only. In this case, the energy-integrated neutron cross section contains a component which depends on neutron polarization and has a left–right asymmetry in the plane determined by the directions of the field and the neutron beam. Small-angle polarized neutron scattering measurements on spin waves in amorphous iron–nickel alloys were performed at different values of the external magnetic field H and neutron wavelength λ. The spin-wave spectrum is quadratic in terms of the momentum transfer and contains both a field gap and an inherent gap Δ of a non-field nature: E q = Aq 2 + g μ B H + Δ. Only by measuring simultaneously two dependencies of the cut-off angle θ c – on the applied magnetic field and on the neutron wavelength – is it possible to obtain reliable information about the spin-wave stiffness and the spin-wave gap in these amorphous ferromagnetic systems.

The parameters of the mesostructure of amorphous zirconium dioxide and their evolution at different stages of heat treatment are determined by small-angle neutron scattering. Particles of amorphous zirconium dioxide, which form mass fractals with the dimension Dv = 2.21, are rearranged into surface fractals with a surface dimension of Ds = 2.52 upon annealing at a temperature of 400°C or higher. In the resulting system, a shell with a fractal structure is formed over a dense core (a cluster of nanoparticles of zirconium dioxide with a constant density). Transformation of the fractal system from a mass fractal into a surface one is characterized by the appearance of a core, and its growth is due to the crystallization of hydrated zirconia particles at high temperatures. A model for the formation of a fractal particle, implying the existence of a core–shell surface fractal system, is proposed. The characteristic radius of ZrO2 nanoparticles increases from 14 to 200 Å with an increase in the annealing temperature from 400 to 600°C.