The group of Tom Woodcock focusses on fundamental and applied aspects of novel magnetic materials, which are needed for application in highly efficient electric motors and generators. Our current interest is in rare-earth-free permanent magnets based on Mn-Al-C alloys. The performance of magnetic materials depends not only on their intrinsic magnetic properties but also on the microstructure over length scales from pm to cm. Understanding and controlling the complex interactions which arise is highly challenging and requires both high quality materials synthesis and state-of-the-art materials characterisation. We synthesize nano- and microcrystalline materials using a variety of melting, powder metallurgy and deformation techniques. We carry out materials characterisation using scanning and transmission electron microscopy (SEM and TEM), electron backscatter diffraction (EBSD), x-ray diffraction and a variety of physical properties measurement techniques including at applied magnetic fields of up to 14 T.
Head of Research Group "Magnetic Materials"
Room: B 1E.11
Phone: +49 351 4659 221
M. Gusenbauer et al., Journal of Applied Physics 129, 093902 (2021)
MnAl-C is a prominent candidate for the replacement of rare earth magnets with a moderate energy density product. Crystallographic defects have a strong effect on magnetization properties. In this work, we show the influence of twinning defects in the nanometer regime on the quality of the magnet. Standard micromagnetic simulations and computations of the saddle point configuration for magnetization reversal highlight the importance of optimizing the fraction of and reducing the width of crystallographic twin defects. Switching field distributions and the maximum possible coercive field for ideal microstructures without defects are estimated using a reduced order micromagnetic model.
F. Bittner et al., Journal of Alloys and Compounds 727 (2017) 1095-1099
As τ-MnAl is a thermodynamically metastable phase, it tends to decompose into the equilibrium phases at elevated temperatures. This restricts the kind of processing which can be carried out. Preventing the decomposition of τ is therefore a critical factor in developing MnAl magnets. Here, the preferential nucleation of the equilibrium phases at general grain boundaries rather than other interfacial types is shown using electron backscatter diffraction measurements. This explains the higher resistance to decomposition of materials which contain low fractions of general grain boundaries.
A. Chirkova et al., Acta Materialia 131 (2017) 31-38
FeRh alloys undergo a magnetic transition from the antiferromagnetic state to the ferromagnetic state. The transition temperature has been shown to vary with prior heat treatment but the reason for this was unknown. In this paper, microstructural investigations showed that heat treaments led to different number density, size, shape and distribution of the secondary fcc phase. Finite element models indicated that stress fields from the secondary phase grains could overlap, thus influencing the transition temperature of the main phase through the well-known effect of pressure.
T. Mix et al., Acta Materialia 128 (2017) 160-165
Two different L10 phases can be made to coexist in alloys of the form Mn55Al45-xGax with 5 < x < 9. One appears to be thermodynamically stable, like binary MnGa, and the other is metastable, like binary MnAl, but in the ternary alloys, both phases contain only a few atomic percent of Ga. The thermodynamically stable L10 phase does not undergo a phase transformation at temperatures up to at least 700°C. These results enable longer processing times at higher temperatures thus facilitating the development of rare earth free MnAl-based magnets which are capable of providing a sustainable alternative to certain types of Nd-Fe-B.