Tham khảo tài liệu 'neutron scattering from magnetic materials part 17', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Small angle neutron scattering investigations of magnetic nanostructures 481 identify magnetic contributions from other scattering contributions. This commonly used techniques has two limitations. First when the magnetic contrast is weak compared to the nuclear one or vice versa . 10 the anisotropy of the 2D scattering signal becomes very weak 1 due to the squaring of the amplitudes. In practice such weak effects are very hard to detect precisely. Second compositions densities and magnetization of matrix and particles cannot be obtained on absolute scale because the sign of the contrast is lost in the squaring of the individual nuclear and magnetic amplitudes. . SANS with polarized neutrons When the neutron spins are aligned antiparallel denoted by or parallel - to a preferred orientation z II H four types of scattering process are to be distinguished two for conserving the neutron spin F 2 and F 2 non-spin-flip scattering . and two with reversal of the spin by the scattering F - 2 and F- spin-flip scattering . 11 12 . Whereas coherent nuclear scattering is . differences in the magnitude of magnetic moments give rise to magnetic . and fluctuations of the magnetization away from perfect alignment yields . scattering. When the spin state of the scattered neutrons is not analyzed the intensity collected in the detector contains both . and . contributions and depends on the polarization state of the incident neutrons. We denote this technique as SANSPOL. For simplicity we treat first the case of a monodisperse system with noninteracting particles where the magnetic moment is fully aligned along an external magnetic field 13 19 . When neutron polarization is antiparallel to the magnetic field the intensity is given by I Q F 2 F - 2 F2 fMM - 2FnFm sin2a 14a while for neutron polarization parallel to the field it reads I- Q F 2 F- 2 F2 FM 2FnFm sin2 a. 14b The anisotropy of both scattering signals is again described by a relation similar
