Local magnetic anisotropy by polarized neutron powder diffraction: Application of magnetically induced preferred crystallite orientation

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.

Quantifying magnetic anisotropy using X-ray and neutron diffraction

“…Recently, some of us have shown how polarized neutron diffraction (PND) from single crystals can provide accurate magnetic susceptibility tensors, clearly showing the direction and size of the easy axis of magnetization (Klahn et al, 2018;Ridier et al, 2016;Gukasov & Brown, 2002;Tripathi et al, 2021). Now, a potentially disruptive innovation allows similar information to be obtained from PND on powder (pPND) samples (Kibalin & Gukasov, 2019) and we present the first thorough analysis using this approach. In a parallel research path, we have recently shown how the d-orbital populations obtained from multipole modeling of single-crystal X-ray diffraction (Holladay et al, 1983) provide significant insight into the magnetic properties of transition-metal based SMMs (Craven et al, 2018;Thomsen et al, 2019;Bunting et al, 2018;Damgaard-Møller et al, 2020a,b,c).…”

Competing interactions in dysprosium garnets and generalized magnetic phase diagram of S=12 spins on a hyperkagome network

“…Detailed description of the instrument scattering geometry and typical measurement conditions can be found in Ref. [32]. Initially the powder was introduced in a vanadium container of 6 mm diameter without compressing, and a powder diffraction diagram was collected at 5 K in zero field.…”

“…The tensor, which describes the anisotropy on the rare-earth ion, can be determined by 2D Rietveld refinement of the diffraction patterns. Refinements were performed with the newly developed RHOHI program [32] . More experimental details can be found in Appendix A.…”

“…Application of a magnetic field H on powder samples can induce crystallite orientation, as the net moment of the crystallites tends to align in the field direction. Since the resultant preferred orientation can be determined from the 2D patterns, one can use these "magnetically textured" samples in PNPD [32]. Flipping difference (y + − y − ) diffraction patterns, collected on DyAG and on DyGG in the paramagnetic state (5 K) in a field of 5 T are shown in Figs.…”

“…To tackle this issue, this study first focuses on the anisotropy and magnetic ordering of DyAG and DyGG. We use to this end a combination of different neutron scattering techniques, including neutron inelastic scattering, powder diffraction and polarized neutron powder diffraction (PNPD) [31,32], a method which has been used in the past successfully to quantify axial and planar anisotropies in R 2 Ti 2 O 7 pyrochlores [33,34]. To provide a new understanding of the role of dipolar and exchange interactions in the model Hamil- tonians, magnetic phase diagrams are presented for DyAG and DyGG, based on mean-field calculations using the crystal electric field (CEF) parameters estimated from experimental data.…”