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Neutron Scattering from Magnetic Materials part 14

Tham khảo tài liệu 'neutron scattering from magnetic materials part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Inelastic neutron polarization analysis 389 Energy transfer meV Fig. 16. Energy dependence of AI a I a I a and I a I a I a in CsMnBr3 close to the antiferromagnetic point 1 3 1 3 1 at a temperature of 9.2 K and afield of 4 T. The difference A.I a I a I a represents the polarization dependent chiral part of the scattering AI a Mch . As previously discussed see Section 2.2 the chiral contribution should be an odd function of a at a kBT 1 corresponding to the relation Mch A Ệ a rc 0 56 ch 1 a Fch 2 2 where rch characterizes the fluctuation rate of chiral fluctuations. Examples of spectra I a closed symbols and AI a open symbols measured at 9.2 K TN are given at Figure 16. According to the theory A a T TN C P0 where ỘC is the chiral crossover exponent. In CsMnBr3 the polarized INS measurements 67 gave ỘC 1.28 and pC 0.44 implying YC 0.84. These values are found in good quantitative agreement with the Monte Carlo calculations of Kawamura 62 thus confirming the chiral universality class. Chiralfluctuations in the itinerant helimagnet MnSi. MnSi is a very interesting itinerantelectron weak ferromagnet with a Curie temperature TC 29 K 68 69 . The magnetic structure in zero field is a long-period spiral described by the propagation vector k t t t with T 0.017 and equivalents deduced by symmetry 70 . Theoretical work by Bak and Jensen 71 have shown that the spiral order in MnSi was due to the existence of a sizable Dzyaloshinskii-Moriya vectorial interaction term resulting from the noncen-trosymmetric arrangement of Mn atoms in the cubic unit cell. Consequently the spiral arrangement in MnSi must be very different from that arising in more conventional helimagnets with competing exchange interactions because in MnSi it should be single handed right or left depending on the sign of the Dzyaloshinskii-Moriya vector Ddm DDMk. In MnSi LPA measurements 70 have clearly demonstrated that the spiral is right handed even in zero field. The chiral nature of magnetic fluctuations above