Experimental SANS studies and micromagnetic simulations of neutron scattering on polycrystalline magnetic materials
Final Report Abstract
Magnetic field-dependent SANS data of iron-based Fe89Zr7B3Cu soft magnetic nanocomposite are analyzed by means of micromagnetic simulations. In particular, the observed angular anisotropy of the SANS cross-section (clover leaf) can be reproduced in close agreement with experiment. This is the evidence that the magnetic microstructure near saturation is dominated by magnetization jumps at internal interfaces. Using newly developed algorithm we have obtained the magnetization distribution in ferromagnets (Fe and Co) containing nanopores. These results were used for the computation of (i) the correlation function of the spin-misalignment SANS cross-section, which is affected in a twofold manner by the magnetodipolar interaction, and (ii) the autocorrelation function of the spin misalignment itself, which depends only on the magnetization distribution. Our approach was validated by comparing simulation results with experimental neutron scattering data. As a consequence of the long-range and anisotropic character of the internal magnetodipolar field, we find strongly anisotropic magnetic correlations. For a polycrystalline magnet with an anisotropic distribution of magnetocrystalline anisotropy axes of individual grains, the existence of nontrivial 3D correlations of the magnetization distribution was demonstrated. For Sr-ferrite based nanocomposite materials, a detailed numerical study of the dependence of magnetic properties on exchange coupling between various crystallites (including the coupling between soft and hard grains) is presented. In contrast to the common paradigm, we have shown that the maximal energy product (BH)max is a non-monotonous function of the intergrain exchange coupling κ and that the optimal κ-value is far below the perfect coupling. We have shown that for composite materials SrFe12O19/Fe and SrFe12O19/Ni the maximal value of (BH)max as a function of the shape of the hard grains was obtained for the oblate hard grains (also in contrast with common expectations). Physical reasons for this behavior are revealed. The spin structure of spherical iron nanoparticles obtained by means of large-scale micromagnetic simulations reveals that a three-dimensional vortex is formed for particle sizes larger than a critical diameter. We predict a characteristic angular anisotropy of the neutronintensity pattern as the fingerprint of this vortex structure. Core-shell particle model of grains forming a Nd − Fe − B polycrystalline material along with a considerable amount of superparamagnetic inclusions allowed us to explain the unexpected ’hard-soft’ hysteresis loop of the experimentally obtained nanocomposite sample.
Publications
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(2013). Analysis of magnetic neutron-scattering data of two-phase ferromagnets. Physical Review B - Condensed Matter and Materials Physics, 88(9)
Honecker, D., Dewhurst, C., Suzuki, K., Erokhin, S., and Michels, A.
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(2015). Comment on ”Origin of surface canting within Fe3 O4 nanoparticles”. Physical Review Letters, 114(14):9–10
Michels, Andreas; Honecker, Dirk; Erokhin, Sergey & Berkov, Dmitry
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(2015). Dipolar spin- misalignment correlations in inhomogeneous magnets: Comparison between neutron scattering and micromagnetic approaches. Physical Review B - Condensed Matter and Materials Physics, 92(1)
Erokhin, Sergey; Berkov, Dmitry & Michels, Andreas
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ICM 2015 Barcelona: International Conference on Magnetism. Micromagnetic simulations of nanocomposites and their neutron scattering cross-section
Erokhin, S., Michels, A., and Berkov, D.
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Nanopyme Workshop 2015 Madrid. Prediction of optimal composition and structure of nanocomposites for permanent magnets using micromagnetic simulations
Berkov, D., Erokhin, S., Gorn, N., and Semenova, E.
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REPM 2015 Darmstadt: International Workshop on Rare-Earth and Future Permanent Magnets and Their Applications. Micromagnetic modelling as an efficient tool for optimization of permanent magnets [poster award]
Erokhin, S., Berkov, D., and Michels, A.
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Optimization of nanocomposite materials for permanent magnets: Micromagnetic simulations of the effects of intergrain exchange and the shapes of hard grains. Physical Review Applied 7, 014011, Jan 2017
Erokhin, Sergey & Berkov, Dmitry
