SPATIAL DISTRIBUTION OF MAGNETIC MOMENT IN MNSI

Recent measurements1 of the magnetic properties of the monosilicides of some 3d transition metals have shown that MnSi has a small ferromagnetic moment (≈0.4 μB/formula unit) at temperatures below 30°K. Neutron-diffraction measurements have been made on both powder and single-crystal specimens. The powder data show that there is no ordered antiferromagnetic component to the moment, so that the total moment is that derived from the saturation magnetization. This result may be compared with the value of 1.4 μB/Mn obtained1 from the Curie-Weiss dependence of the susceptibility above 30°K. The integrated intensities of the (hk0) reflections from a single crystal at room temperature were measured and the corresponding nuclear scattering amplitudes derived. A comparison of these values with those calculated from Pauling's2 structure (B20) confirmed the two positional parameters and indicated a 5% deficiency of manganese in the sample. This latter observation can be correlated with the lower saturation magnetisation of this sample (0.34 μB/Mn atom). Furthermore, the measurements showed the presence of a small amount of secondary extinction in the strongest reflections. Using the same crystal in a magnetic field at 4.2°K and the polarized neutron technique, measurements of the polarization ratios were made for the same set of reflections; the secondary extinction parameter derived from the room-temperature measurements enabled corrected magnetic scattering amplitudes to be deduced. The results indicate that the spatial distribution of magnetic moment around the Mn atom deviates significantly from spherical symmetry and that a significant negatively polarized moment is associated with the silicon atom. © 1968 The American Institute of Physics.