Abstract
Polarized neutron diffraction allows to determine the local susceptibility tensor on the magnetic site both in single crystals and powders. It is widely used in the studies of single crystals, but it is still hardly applicable to a number of highly interesting powder materials, like molecular magnets or nanoscale systems because of the low luminosity of existing instruments and the absence of an appropriate data analysis software. We show that these difficulties can be overcome by using a large area detector in combination with the two-dimensional Rietveld method and powder samples with magnetically induced preferred crystallite orientation. This is demonstrated by revisiting two test powder compounds, namely, low anisotropy (soft) ferrimagnetic compound Fe3O4 and spin-ice compound Ho2Ti2O7 with high local anisotropy. The values of magnetic moments in Fe3O4 and the susceptibility tensors of Ho2Ti2O7 at various temperatures and fields were found in perfect agreement with these found earlier in single crystal experiments. The magnetically induced preferred crystallite orientation was used to study the local susceptibility of a single-molecule magnet Co([(CH3)2N]2CS)2Cl2. Hence, the studies of local magnetic anisotropy in powder systems might now become accessible.
Highlights
Polarized neutron diffraction (PND), called the “flipping ratio method,” is a powerful tool to investigate intraor intermolecular magnetic interactions
Can use these “magnetically textured” samples in polarized neutron powder diffraction (PNPD). We found that such an approach in combination with the 2D Rietveld method has a number of advantages and we applied it to the studies of local susceptibility in the cobalt(II) complex with a molecular formula Co(L1)2Cl2, where L1 is tetramethylthiourea [(CH3)2N]2CS [35]
Our results suggest that the combination of an area detector, 2D Rietveld analysis, and the technique of magnetically induced preferred crystallite orientation opens direct route to the studies of local magnetic susceptibility in polycrystalline materials by polarized neutron diffraction
Summary
Polarized neutron diffraction (PND), called the “flipping ratio method,” is a powerful tool to investigate intraor intermolecular magnetic interactions. Most recent powder diffractometers at advanced neutron spallation sources (WISH, POWGEN) use very large area detectors and operate in the time-of-flight (TOF) mode This generates rather complex three-dimensional (3D) angular- and wavelength-dispersive data which are eventually transformed into a one-dimensional diffraction pattern I (2θ ) [or I (λ)] [18] to allow standard Rietveld refinement. We show that the combination of a large area detector with 2D Rietveld analysis and magnetically induced preferred crystallite orientation enables PNPD in systems not available as single crystals We illustrate this by the results of two test cases of magnetic materials: the low anisotropy (soft) ferrimagnetic compound Fe3O4 and the spin-ice compound Ho2Ti2O7 with high local anisotropy. We present the results of the local susceptibility studies on the single-molecule magnet Co([(CH3)2N]2CS)2Cl2 with a magnetically induced preferred orientation of the crystallites, which shows that the PNPD opens large opportunities in the local anisotropy quantification of complex structures
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