Research

New paper The reprint of our Journal of Applied Physics (JAP) paper “Grain-size dependent demagnetizing factors in permanent magnets” has been updated since the old version was not being discovered by the Google Scholar crawler. It is also available on arXiv.org. Full reference S. Bance, B. Seebacher, T. Schrefl, L. Exl, M. Winklhofer, G. Hrkac, G. Zimanyi, T. Shoji, M. Yano, N. Sakuma, M. Ito, A. Kato and A. Manabe, “Grain-size dependent demagnetizing factors in permanent magnets”, J. Appl. Phys. 116, 233903 (2014)

New paper Our latest paper has been published in JOM (Springer). It is a summary of the achievements (so far) of the ROMEO fp7 project. ROMEO is a project that aims to research and develop novel microstructural-engineering strategies that will dramatically improve the properties of magnets based purely on light rare earth elements, especially the coercivity, which will enable them to be used for applications above 100 degrees C. ...

This week our new paper titled “Thermal Activation in Permanent Magnets” has been published in JOM (Springer). The invited paper is under a special topic, “Permanent Magnets beyond Nd-Dy-Fe-B”. An author manuscript (reprint) is available here. Fig 1: Explanation of method for calculating the thermally activated coercivity of using micromagnetics. In the paper we provide a more detailed overview of the micromagnetic methods we have developed to model the thermal activation of permanent magnets. These methods allow us to simulate and understand the behaviour of permanent magnets at finite temperatures, which is important since the generators in wind turbines and electric motors in green cars operate at higher temperatures. For example, in electric cars the typical operation temperature of the motors can be around 450ºK (177º C). ...

Our EU-funded FP7 project “Replacement and Original Magnet Engineering Options” (ROMEO) has recently been covered in the news in Slovenia, to promote the participation of our Slovenian collaborators. Video »Video file« Newsletter There is also now a project newsletter highlighting the aims and progress of the project. Please click here to download it.

Our new paper “Thermally activated coercivity in core-shell permanent magnets” has been published today as J. Appl. Phys. 117, 17A733 (2015); http://dx.doi.org/10.1063/1.4916542 . In the paper we use numerical miromagnetics to calculate the performance of nanostructured core-shell-like permanent magnets, like the type that can now be produced by grain boundary diffusion of granular hot-deformed or sintered rare earth permanent magnets. FIG. 3. Reversal processes in the sin- gle grain models with (i) a pure NdFeB grain, (ii) a NdFeB grain with a soft outer defect, and (iii) NdFeB core, (Dy, Nd)FeB shell and an outer soft defect. Thermally activated coercive field values are indicated with the field direction (red arrows). The saddle point image is the configuration with the highest total energy, forming the peak of the energy barrier. ...

Our paper titled “Micromagnetics for the coercivity of nanocomposite permanent magnets” has been published in the proceedings of the 23rd International Workshop on Rare Earth and Future Permanent Magnets and Their Applications (REPM2014). The proceedings were not made available to the public but we are providing a PDF reprint here. Fig. 1. Switching field of Nd 2 Fe 14 B cubes and spheres with volume V The work was presented by Johann Fischbacher on 19th August 2014 in Annapolis, Maryland. ...

Our paper titled “Enhanced Nucleation Fields due to Dipolar Interactions in Nanocomposite Magnets” was presented by first author, Johann Fischbacher, at the JEMS 2012 conference and subsequently published in the The European Physical Journal B. Figure: Magnetic reversal process: The pictures show the magnetic flux lines. The color denotes the magnetization direction (red: magnetization up, blue magnetization down). The gap between the soft magnetic spheres (d incl = 8 nm) is 1 nm in the first two columns and 4 nm in the third column. The external field is applied in z-direction and its value is written next to each picture. In the first column the soft magnetic inclusions are aligned perpendicular to the applied external field. The interaction with the outside inclusions is weakening the central sphere and forces it to switch first. In the second and third column the soft magnetic spheres are aligned in a parallel manner to the applied external field. The two outside spheres reinforce the central one and therefore nucleation should not start in the center. But for gaps smaller than 4 nm a strong demagnetizing field in the location of the central sphere caused by the shell diminishes the strengthening effect due to dipolar interaction. ...

Our new paper “Grain-size dependent demagnetizing factors in permanent magnets” has been published in Journal of Applied Physics (JAP). Abstract: The coercive field of permanent magnets decreases with increasing grain size. The grain size dependence of coercivity is explained by a size dependent demagnetizing factor. In Dy free Nd2Fe14B magnets, the size dependent demagnetizing factor ranges from 0.2 for a grain size of 55 nm to 1.22 for a grain size of 8300 nm. The comparison of experimental data with micromagnetic simulations suggests that the grain size dependence of the coercive field in hard magnets is due to the non-uniform magnetostatic field in polyhedral grains. ...

This August Prof. Dominique Givord of Institut Néel CNRS presented our paper titled “Hard Magnet Coercivity” during the 23rd International Workshop on Rare earth and Future Permanent Magnets and Their Applications (REPM2014) in Annapolis, Maryland. The manuscript was included in the conference proceedings and we would now like to make the reprint available to the wider public: Please click here for the PDF file. Abstract: Based on a critical analysis of the experimental coercive properties, general considerations on the reversal mechanisms in RFeB magnets are recalled. By plotting together the experimental parameters obtained in various magnets, common features of the reversal processes are demonstrated. Modeling provides an almost quantitative description of coercivity in these materials and permits connecting the defect characteristic properties to reversal mechanisms. ...

Our new, cake-themed paper on nanostructured permanent magnets has now been published in Applied Physics Letters. In the paper we present results from micromagnetic simulations that assess the performance of multi-phase nanostructured permanent magnets, whose cross-section resemble that of a Battenberg cake. By including a super-hard outer shell we are able to counteract the effects of thermal fluctuations and surface defects, both of which are detrimental to the performance of such permanent magnets. Such magnets are important for the motors in electric vehicles and for the generators in wind turbines, and these machines usually operate at elevated temperatures. ...

Our new paper titled “Thermally-activated coercivity in core-shell permanent magnets” has been accepted for publication in Journal of Applied Physics. Click here to download the paper as a PDF file. In the paper we describe recent micromagnetics simulations on NdFeB grains that have undergone a dysprosium (Dy) grain boundary diffusion process (GBDP). The super hard (Dy,Nd)FeB shell that is formed during this process stabilizes the grains against thermal fluctuations that can be detrimental to the coercivity of the magnet in high-temperature situations. NdFeB permanent magnets are usually doped with Dy to increase their performance at high temperatures but the GBDP allows us to target the Dy at the most important locations i.e. the grain surface, thus reducing the overall required amount of Dy, which is expensive and in short supply. ...

Our new paper titled “Influence of defect thickness on the angular dependence of coercivity in rare-earth permanent magnets” has just been published in the journal Applied Physics Letters. It was a long time coming and the result of much collaborative effort but I am really proud that this work is finally out there. Thank you to all of my co-authors for their input, support and advice. The work shows using micromagnetics simulations and experimental data that there is a direct connection between the coercivity in rare earth permanent magnets and the thickness of the surface defect with reduced anisotropy. The applicability of the two main theoretical models for reversal in such magnets is related to the role of such soft defects and their sizes. ...

New paper I am pleased to announce our new paper, which can be downloaded from http://arxiv.org/abs/1312.3791 Abstract In the search for rare-earth free permanent magnets, various ideas related to shape anisotropy are being pursued. In this work we assess the limits of shape contributions to the reversal stability using micromagnetic simulations. In a first series of tests we altered the aspect ratio of single phase prolate spheroids, ranging from 1 (sphere) to 20. We start with a sphere with a radius of 4.3 Lex and kept the total magnetic volume constant as the shape is transformed. For a ferromagnet with zero magnetocrystalline anisotropy the maximum coercive field reached up to 0.5 Ms. Therefore, in materials with moderate uniaxial magnetocrystalline anisotropy, the addition of shape anisotropy could even double the coercive field. Interestingly due to non-uniform magnetization reversal there is no significant increase of the coercive field for an aspect ratio greater than 5. A similar limit of the maximum aspect ratio was observed in cylinders. The coercive field depends on the wire diameter. By decreasing the wire diameter from 8.7 Lex to 2.2 Lex the coercive field increases by 40 percent. In the cylinders nucleation of a reversed domain starts at the corners at the end. Smoothing the edges can improve the coercive field by about 10 percent. In further simulation tests we compacted soft magnetic cylinders into a bulk like arrangement. There are various effects that reduce the coercivity in assemblies of rods: Misalignment, magnetostatic interactions, and direct coupling through exchange interactions. These cause a spread of 0.1 Ms in the switching field of the rods. Comparing the volume averaged hysteresis loops computed for isolated rods and the hysteresis loop computed for interaction rods, we conclude that magnetostatic interactions reduce the coercive field by up to 30 percent.

Photos from our European Researcher’s Night can now be found at http://www.destinationfuture.eu/fotos

In July our paper “System Dynamics models for improved power plant operation” was presented at ICOSC 2013 (The Fourth International Conference on Optimization, Simulation and Control) in Ulaanbaatar by our colleague and friend Bayasgalan Zagdkhorol of Mongolian University of Science & Technology (MUST). The abstract, which shows some of our early results, can be found here. Thank you, Zagdaa, for representing us and our work! Model of power generation grid including naive load model, coal, solar, wind and hydroelectric power sources. Various scenarios can be tested. ...