Magnetic Materials

The focus of our work includes fundamental and applied aspects of novel materials used in energy applications. We synthesize nanocrystalline and microcrystalline materials using a variety of melting, powder metallurgy and deformation techniques. We carry out materials characterisation on length scales from pm to cm using electrons, x-rays and a variety of physical properties measurements.

  • Our main Research topics are rare earth transition metal and novel rare earth free permanent magnets, and the physics and microstructural aspects of metamagnetic transitions in FeRh.

Recent Highlights

Preferential Grain Boundary Nucleation of Equilibrium Phases During the Decomposition of MnAl-C

As tau-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 tau 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.

F. Bittner et al., Journal of Alloys and Compounds 727 (2017) 1095-1099 doi

Microstructure Affects the Magnetic Transition in FeRh Alloys

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.

A. Chirkova et al., Acta Materialia 131 (2017) 31-38 doi

A Thermodynamically Stable L10 Phase in MnAl Alloyed with a few Atomic Percent of Ga

Two different L10 phases can be made to coexist in alloys of the form Mn55Al45-xGax with 5 < x ≤ 9. One is 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.

T. Mix et al., Acta Materialia 128 (2017) 160-165 doi

The Impact of Dislocations on Coercivity in MnAl

Swaged bars of tau-MnAl show a highly twinned microstructure and a high coercivity of about 0.3 T. On annealing this material, the microstructure does not visibly change but the coercivity decreases dramatically. The sharpness of EBSD patterns is affected by the local density of dislocations. EBSD pattern quality results implied that the dislocation density was much higher in the as-swaged state. It was concluded that dislocations can act as pinning centres for domain walls in MnAl and this was supported by the magnetic new curve.

F. Bittner et al., Journal of Alloys and Compounds 704 (2017) 528-536 doi

Grain Growth in Fine-Grained Nd-Fe-B Sintered Magnets Produced from He Jet Milled Powders

Dy-free, fine-grained Nd-Fe-B sintered magnets were produced from He jet milled powders (particle size = 1.5 µm) by sintering at T<1000°C. Surprisingly, abnormal grain growth occured at this low sintering temperature. EBSD results showed that the abnormally large grains were better aligned than the surrounding fine grains. The high coercivity of > 1600 kA/m resulted from the fine starting materials but the presence of abnormally large grains reduced the rectangularity of the demagnetisation curve.

F. Bittner et al., Journal of Magnetism and Magnetic Materials 426 (2017) 698-707 doi

Magnetization Reversal Processes in Hot Extruded MnAl-C

Kerr microscopy was used to image the magnetic domains in hot extruded MnAl-C samples and a magnetic field was applied in situ in order to observe reversal processes. Digital image analysis was used to extract quantitative information from the micrographs. This is the first time that these techinques has been applied to MnAl and the results showed that the fine, recrystallised grains have a higher resistance to demagnetisation than do the larger, polytwinned grains.

J. Thielsch et al., Journal of Magnetism and Magnetic Materials 426 (2017) 25-31 doi

Cyclic Measurements of Adiabatic Delta T in FeRh

In order to avoid overestimation of the magnetocaloric effect in materials with a first order magnetic transition it is vital to carry out the measurements under cyclic conditions, thus accouting for the effect of hysteresis. Direct measurements of the adiabatic temperature change for Fe49Rh51 show values of 9.2 K on the first application of the field and values of 6.2 K during repeated cycling. The value under cycling is 15% higher than that of the benchmark material, Gd, which shows the potential of first order materials for application.

A. Chirkova et al., Acta Materialia 106 (2016) 15-21 doi

Analysis of Twin-like Defects in MnAl-C using EBSD

Electron backscattered diffraction was used to identify three distinct interfaces which are present in L1_0 ordered MnAl-C. They are denoted as true twins, order twins and pseudo twins and can be described by different rotations about the normal to {111}. True twins were often observed in this material but the other two interfaces are heretofore unknown in MnAl-C. The frequency of the different interfaces was very sensitive to the sample state. Hot extrusion led to a clear preference of true twins while order twins and pseudo twins tend to disappear. A mechanism for the formation of the two yet unknown interfaces is proposed.

F. Bittner et al., Acta Materialia 101 (2015) 48-54 doi

Phase Stability and Magnetic Properties of L10 Mn-Ga Alloys

Binary Mn-Ga alloys have been prepared in the composition range 50-70 at.% Mn, in steps of 5 at.%. After suitable heat treatment, those alloys in the range 55-65 at.% Mn exhibited a single-phase L1_0 structure. The Mn 55 at.% alloy had a saturation polarisation of J_sat = 0.807 T. The coercivity of all the alloys was dramatically improved by milling. The highest coercivity for Mn 55 at.% powder was Hc = 0.393 T. Partial alignment of the easy axes of the powder particles was demonstrated after the application of a magnetic field. Hot compaction of the powders resulted in a maximum packing density of 99% with a small loss in coercivity.

T. Mix et al., Journal of Magnetism and Magnetic Materials 391 (2015) 89-95 doi

Atomic Scale Features of Phase Boundaries

Aberration-corrected scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) has been used to investigate phase boundaries in hot deformed Nd-Fe-Co-B-Ga magnets which had been infiltrated by a Nd-Cu eutectic liquid. Atomic resolution "Z-contrast" images (STEM-HAADF) were correlated with EELS data, which resolved the chemical composition of individual lattice planes. Enrichment of Co in the outermost unit cell of the 2:14:1 phase was observed at several different surfaces. Interfacial Cu segregation and step defects a half or a whole unit cell in size were also observed.

T.G. Woodcock et al., Acta Materialia 77 (2014) 111-127 doi

Focus on Modeling of Magnetic Materials in JOM Vol. 66, July 2014

A set of papers focussing on the modeling of magnetic materials has been published in the July 2014 issue of JOM, the member's journal of the TMS. The papers cover first principles calulculations to investigate the formation of Nd-oxide phases in Nd-Fe-B magnets (Chen et al. JOM 66 2014 1133), advanced atomistic and micromagnetic simulations of interfaces based on results from electron microscopy (Hrkac et al. JOM 66 2014 1138), and use solutions of the nucleation field equation to find optimum geometries for the next generation of exchange coupled nanocomposite magnets (Skomski et al. JOM 66 2014 1144). An introduction to the set of papers was written by T.G. Woodcock.

T.G. Woodcock, JOM 66 (2014) 1132 doi

Reversible and Repeatable Increase in Coercivity Following Post Sinter Annealing of Nd-Fe-B Magnets

The effect of different heat treatments on the coercivity (Hc) of three different Nd-Fe-B sintered magnets has been investigated. Annealing the as-sintered magnets at 500°C led to an approx. 30% increase in Hc. Heat treating these fully annealed materials at 1050°C led to a reduction in Hc of a similar magnitude. This process could be repeated several times. A simple mechanism to explain the observed results was proposed and supported by results from scanning and transmission electron microscopy.

T.G. Woodcock et al., J. Magn. Magn. Mater. 360 (2014) 157-164 doi

The Impact of Different Nd-rich Phases on the Coercivity of Grain Ensembles in Nd-Fe-B Magnets

Results from electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) have been used as input for atomistic and finite element micromagnetic simulations of grain junctions in Nd-Fe-B magnets. In the centre of the grain junction is a Nd-rich phase and this is surrounded by several grains of Nd2Fe14B. Simulations were carried out with various different Nd-rich phases in the centre of the grain junction in order to clarify the effect of the phases on the coercivity. Some Nd-rich phases were found to be more beneficial to coercivity than others.

G. Hrkac, T.G. Woodcock et al., Scripta Mater. 70 (2014) 35-38 doi

2.7 T Coercivity in an Anisotropic Nd-Fe-B Thick Film without Heavy Rare Earth Elements.

Nd-Fe-B thick films have been deposited by triode sputtering. The films have a strong <001> out of plane texture and are therefore magnetically anisotropic. Films with two different Nd contents have been compared. The film with a higher Nd content had a room temperature coercivity of 2.7 T. No heavy rare earth elements were used in the preparation of the films. Magnetic measurements, transmission electron microscopy and 3D atom probe tomography have been used to characterise the films.

N.M. Dempsey, T.G. Woodcock et al., Acta Mater. 61 (2013) 4920-4927 doi

Magnetic Materials for Energy Applications IV at the TMS Annual Meeting in San Diego, USA, 16th-20th February 2014.

Organisers: T.G. Woodcock (IFW Dresden), J. Lyubina (Evonik Industries AG), M.A. Willard (Case Western Reserve University). Coinciding with the 30th anniversary of Nd-Fe-B sintered magnets, almost 3 decades of research on nanocrystalline soft magnets and the current explosion in publications concerning magnetocaloric materials and cooling devices, this symposium is a timely opportunity to review progress and clarify future perspectives. Key topics will include the development of rare earth transition metal magnets and rare earth free alternatives, progress in the optimistation of magnetocaloric materials and devices and the state of the art in soft magnets.


Invited Viewpoint Article in Scripta Materialia: Understanding the Microstructure and Coercivity of High Performance NdFeB-Based Magnets

Understanding the subtle link between coercivity and microstructure is essential for the development of higher performance magnets. In the case of R–Fe–B (R = rare earth) based materials this knowledge will be used to enable the development of high coercivity, Dy-free permanent magnets, which are relevant for clean energy technologies. A combination of high resolution characterization, molecular dynamics and micromagnetic simulations and model thick film systems has been used to gain valuable
new insights into the coercivity mechanisms in R–Fe–B magnets.

T.G. Woodcock et al., Scripta Materialia 67 (2012) 536-541, doi

The texture of Nd oxide grains in Nd-Fe-B sintered magnets studied by synchrotron radiation

Synchrotron pole figures have been recorded from the Nd2Fe14B, Nd2O3, and NdO grains in commercial Dy-free, Nd-Fe-B sintered magnets in the as-sintered and fully annealed states. In contrast to the well known <001> fiber texture in the Nd2Fe14B grains, the Nd-oxide phases showed no preferred orientation in the as-sintered or annealed states. The random orientation of the Nd-oxide grains was confirmed from histograms of the pole density values. These showed only small deviations from the pole density of a non-textured material.

T.G. Woodcock et al., Journal of Applied Physics 110, 026103 (2011), doi

Comparison of local and global texture in HDDR processed Nd–Fe–B magnets

The local texture in polycrystalline Nd–Fe–B powder particles processed by hydrogenation disproportionation desorption and recombination (HDDR) has, for the first time, been observed directly by electron backscatter diffraction (EBSD). The  quality of local texture was found to vary strongly with the hydrogen pressure applied during HDDR processing. The local texture within the powder particles was shown to be critical in determining the magnetic properties of compacts produced by aligning the powder in an external magnetic field.

K. Guth, T.G. Woodcock et al., Acta Mater. 59 (2011) 2029-2034, doi


The role of local anisotropy profiles at grain boundaries on the coercivity of Nd2Fe14B magnets

We present numerical evidence from atomistic calculations that the coercivity of high-performance NdFeB-sintered-magnets (< 20% of the theoretical Stoner–Wolfarth-limit) can be explained by a distorted region of Nd2Fe14B at grain boundaries, which has a reduced local magnetic anisotropy. We show that depending on the boundary composition of fcc-NdO and hcp-Nd2O3, the thickness of this region of reduced anisotropy varies between 0.4 for fcc and 1.6 nm for the hcp phase.

G. Hrkac, T.G. Woodcock et al., Appl. Phys. Lett. 97 (2010) 232511, doi

Multi-phase EBSD mapping and local texture analysis in NdFeB sintered magnets

We have extended the study of NdFeB sintered magnets by EBSD to include all of the phases present in the microstructure for the first time. A combination of chemical analysis by EDX and crystal structure determination by EBSD was used to identify the phases unambiguously. The orientation of the Nd-rich grains was shown to be random and the defect density of all phases was qualitatively determined from analysis of the intragranular misorientation angles.

T.G. Woodcock and O. Gutfleisch, Acta Mater. 59 (2011) 1026-1036, doi