Spin correlations in Ni-Mn-Ga

Results of measuring small-angle neutron scattering and neutron depolarization in a Ni$_{49.1}$Mn$_{29.4}$Ga$_{21.5}$ single crystal in the temperature range $15<T<400$  K and in the range of magnetic fields $0<H<4.5$  kOe are presented. The characteristic temperatures of the alloy were found to be as follows: $T_C=373.7$  K and the martensite transition temperature $T_m=301-310$  K. The magnetic critical scattering at $T_C$ and the scattering at $T<T_C$ were adequately described by the relationship $I_m=A(q^2+\kappa^2)^{-2}$ ($q$ is the transferred wave vector and $R_c=1/\kappa$ is the correlation radius), and the temperature dependences of the A and R c scattering parameters were determined. Left-right asymmetry was observed at $150<T<T_m$, in the scattering of neutrons polarized along or opposite to the applied field. This asymmetry was due to the inelastic magnetic interaction of neutrons in the sample. The magnetization of the alloy at $T_m$, critical scattering at $T\simeq T_C$, anomalies in scattering, and the softening of magnetic excitations at $150<T<T_m$ are discussed.