SCIENCE SPIN-OFFS

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High-throughput micromagnetic simulations of Nd-Fe-B nanocomposites

Sergey Erokhin, Dmitry Berkov, Masaaki Ito, Akira Kato, Masao Yano, Andreas Michels
(Submitted on 1 Mar 2017)

We report on high-throughput micromagnetic simulations of the magnetic microstructure and magnetization reversal of nanocrystalline Nd-Fe-B based materials, which are potential candidates for future permanent magnets. In order to establish the model which is most suitable for the description of this material class, we compare experimental data with simulation results obtained for the following three models: (i) a model based on Stoner-Wohlfarth particles with and without the magnetodipolar interaction, (ii) a model based on core-shell particles, and (iii) the latter model including the contribution of superparamagnetic clusters probably present in these materials. The parameter space of these models is systematically scanned revealing the complex interrelationship between magnetic interactions and macroscopic magnetic properties. It is demonstrated that only the model which is based on core-shell particles and includes the superparamagnetic contribution is able to adequately fit the experimental hysteresis loops

https://arxiv.org/abs/1703.00288

Methods
Sample under study The Nd2Fe14B/ a -Fe nanocomposite (containing 5wt. % of
a -Fe) was prepared by means of the melt-spinning technique. Sample characterization was carried out using transmission electron microscopy and synchrotron x-ray scattering (see Ref.24 for details). The melt-spun sample had an average Nd2Fe14B grain size of about 20nm; it represents a nanocomposite material with Nd2Fe14B as the hard magnetic phase and a -Fe as the soft phase. Magnetization data (up to µ 0Hmax = 14T) were recorded at 300K using a Cryogenics vibrating sample magnetometer.
Micromagnetic simulation methodology Our micromagnetic algorithm was originally developed for the simulation of the magnetization distribution of magnetic nanocomposites and the computation of the related magnetic-?eld-dependent neutron scattering cross sections (see Refs. 20–23 for details). The four standard contributions to the total magnetic energy (external ?eld, magnetic anisotropy, exchange, and dipolar interaction) are taken into account. In the present study, we have employed two different micromagnetic models, as will be motivated below: the ?rst model was based on the assumption that Nd-Fe-B grains inside the sample can be represented as uniformly-magnetized Stoner-Wohlfarth (SW) particles; the second model was based on a core-shell-type description of the Nd-Fe-B grains. Further, the second model has been extended by taking into account the possibility of a superparamagnetic contribution (arising from small magnetic clusters inside the sample) to the measured hysteresis loop