Plenary evening talk
Thirty years of material science applied to magnetism: discoveries and new applications
In 1980, soft magnetic materials serving as guides or concentrators of magnetic flux in electromagnetic machines dominated the field of technical magnetism; permanent magnets were essentially hard ferrites, used in loudspeakers and simple motors; Fe2O3 magnetic particles were used in data recording tapes; hard disk drives were an expensive optional component of a new device, the personal computer; spin-dependent transport was a physical phenomenon of secondary importance.
The discovery of NdFeB hard magnets in 1983 led to a transformation in the design of electrical motors and generators and the discovery of Giant Magnetoresistance in 1988 marked the birth of spintronics, one of today’s most active domains of condensed matter physics research. During this same period, society was transformed by the development of information technology while the challenges posed by global warming and resource limitations were progressively recognised.
The advent of high-performance magnets and spintronics led to the extraordinary development in magnetic studies needed to unravel the mechanisms governing magnetization processes and spin-dependent transport phenomena. Magnetism had essentially been understood in terms of exchange interactions and anisotropy; other effects resulting from spin-orbit interactions, the role of disorder, or the possible existence of competing magnetic interactions, were essentially unrecognized. The fact that magnetization reversal may be driven by a (spin) current, was yet to be discovered.
Today, further progress in the understanding of magnetism and magnetic materials is crucial for solving new challenges, from achieving incremental improvements in existing materials to discovering fundamentally new types of magnetic materials. The study of magnetic materials is greatly benefiting from impressive developments in characterisation techniques (e.g. aberration corrected TEM, 3D-atom probe tomography, X-ray magnetic circular dichroism, near-field microscopies) as well as ever more powerful computers, needed for DFT calculations and micromagnetic modelling. Electronic structure calculations combined with data mining are being used to successfully guide the development of new magnetic materials such as magnetic seminconductors, half-metals or magnetocaloric materials. Advanced micro and nanofabrication techniques serve to fabricate new nanoobjects, often based on multilayer stacks of different materials. Magnetic nanoparticles have a growing number of bio-medical applications.
In this talk I will give an overview of recent and on-going developments, focussing on some examples, which illustrate the impressive vitality of the field of material science applied to magnetism.
Magnequench Technology Center
Development of RE–Fe–B melt-spun magnetic powders for high-temperature applications
Melt-spun Nd–Fe–B-based magnetic powders have been successfully applied in bonded magnets for a wide variety of modern applications from office automation, power tools to automotive devices and computer components. In this paper, we will review our R&D efforts in developing high-temperature melt-spun RE-Fe-B powders for automotive industry and/or power tool applications, where good thermal stability is the main requirement. We will report our studies on effects of various substitutions (Dy for Nd and Ce for NdPr, refractory metals Cr, V, Ti, Mo, Nb, Zr and Co for Fe) on thermal stabilities of Nd-Fe-B melt-spun powders and on flux aging losses in their bonded magnets. Our research has found that Nb and Zr provide the best overall properties among all substitutions studied, making Dy additions unnecessary to produce high-temperature powders for applications up to 180°C. We have also achieved good thermal stability in (NdPrCe)-Fe-Zr-B melt-spun powders. Bonded magnets suitable up to 125°C can be produced with Ce up to 70% of the RE concentration and suitable to 150°C up to 50% Ce substitution. Good thermal stability using low cost Ce may lead to wide spread acceptance of bonded magnets in automotive and HVAC applications.
CNRS and Univ. Grenoble Alpes, Institut Néel
RE-TM micro-magnets for MEMS and bio-medical applications
For the same reasons that RE-TM magnets are attractive for use at the macro-scale (high energy density, contactless actuation, stability…), they are also very appealing for use at the micro-scale. Furthermore, the magnetic field gradients produced by a permanent magnet scales up as the size of the magnet scales down, resulting in an increased force per unit volume. There is thus great potential for using high performance RE-TM based micro-magnets in a wide range of applications.
In this talk I will briefly review our work on the development and micro-patterning of hard magnetic films and the low-cost fabrication of micro-magnet arrays based on hard magnetic powders. I will present a Scanning Magneto-Optic-Kerr-Effect system that incorporates an original pulsed magnetic field source, developed for the high throughput non-destructive characterisation of coercivity across compositionally graded hard magnetic films. I will then describe recent advances made in the quantitative characterisation of stray fields and forces produced by micro-magnets using Scanning Hall Probe Microscopy and Magnetic Force Microscopy with specially designed probes. Finally I will give examples of bio-medical applications of the micro-magnets we have developed.
From the electronic structure theory of magnetocrystalline anisotropy to novel rare-earth free permanent magnets
We discuss the electronic structure theory of magnetocrystalline anisotropy in the context of finding novel rare-earth free permanent magnets. In particular we use density functional theory (DFT) based calculations to show how uniaxial strain and alloying can be used to tune the properties of 3d-based magnets. Additionally we provide recent examples of how to further improve such materials by adding, for example, 5d-elements.
Ningbo Institute of Material Technology and Engineering
The effect of Ce distribution on the magnetic properties of Nd-Ce-Fe-B sintered magnets
Recently, the efficient usage of rare earth element Ce, which possesses the merits of the low cost and high abundance, has attracted more attentions in the permanent magnetic society. Many efforts have been made to investigate the Ce substitution for Nd in Nd-Fe-B based sintered magnets. It was found that the coercivity of magnets could be drastically deteriorated after replacing Nd with Ce due to the poor magnetic hardness of Ce2Fe14B compared with Nd2Fe14B. On the other hand, the Nd-Fe-B sintered magnets mainly consist of the Nd2Fe14B matrix phases and rare earth element-rich boundary phases. Therefore, the high performance could be expected if Ce diffusion into the matrix phases Nd2Fe14B would be hindered or weakened for Nd-Ce-Fe-B sintered magnets. In this work, Ce distribution was tuned through the dual alloy method. It was found that the magnets with the dual alloys exhibit much better properties than the counterpart with the single alloy. Dual alloy powders were prepared by blending the Ce-free and Ce-containing single alloy powders, then the mixed powders were pressed in a DC magnetic filed to fabricate green body. The green body was subsequently sintered and annealed in vacuum furnace. Regarding the magnet with the dual alloys, the coercivity was enhanced from 10.3 kOe to 12.1 kOe and the remanence was increased from 13.1 kG to 13.3 kG in comparison to the magnet with the single alloy. Additionally, the remanence temperature coefficient α and coercivity temperature coefficient β was also slightly improved for the magnet with the dual alloys. Figure 1 shows SEM images and the elemental mapping of two magnets. It is very clear that the two magnets exhibit a different Ce distribution. Ce shows the uniform distribution for the magnet with a single alloy. For the magnet with the dual alloys, the Ce-rich and Ce-lean areas are coexisted as indicated with circles.Through the δm-H plots, the different coupling appears owing to different elemental distributions.
Max Planck Institute Chemical Physics of Solids
Heusler compounds for permanent magnets
Heusler compounds are a remarkable class of materials with more than 1,000 members and a wide range of extraordinary multifunctionalities . The tunabilty of this class of materials is exceptional and nearly every functionality can be designed . The magnetic coupling in Cobalt, Iron, and Manganese X2YZ Heusler compounds is strong and high Curie temperature far above room temperature up to 1200 K can be observed. In 2007 Mn3-xGa was identified as a material with a high magnetocrystalline anisotropy , the recondition for a large coercitive field. In general Manganese-rich Heusler compounds are attracting much interest in the context of materials with large spin transfer torque, spin Hall effect, non collinear magnetism and rare-earth free hard magnets. Here we give a comprehensive overview of the magnetic properties of Heusler materials with a large magneto crystalline anisotropy, precondition for a large figure of merit (BH product) [4,5]. Tetragonal Heusler compounds with large magneto crystalline anisotropy can be easily designed by positioning the Fermi energy at the van Hove singularity in one of the spin channels. The Mn3+ ions in Mn2YZ cause a Jahn Teller distortion [4,5]. The second condition for a large BH product is a large moment, but most of the materials tend to order ferrimagnetic or non collinear [6,7]. Design principles and new directions will be discussed.  C. Felser, et al, Angewandte Chemie, Interna. Ed. 46 668 (2007).
 Tanja Graf, et al, Progress in Solid State Chemistry 39 1 (2011).
 Benjamin Balke, et al., Appl. Phys. Lett. 90 152504 (2007).
 Jürgen Winterlik, et. al., Adv. Mat. 24 6283 (2012).
 Lukas Wollmann, et al. Phys. Rev. B 92 (2015) 064417
 A. K. Nayak, et al., Phys. Rev. Lett. 110 127204 (2013)
 A. K. Nayak, et al., Nat. Mater. 14 (2015) 679
University of Delaware
Development of ThMn12-type compounds for permanent magnets
Interest on the ThMn12-type hard magnetic materials has increased markedly in recent years motivated in part by the fact that the Nd-Fe-B energy product limit has been reached and in part by the desire to decrease the reliance of permanent magnets on critical elements. New approaches, such as high-throughput screening and mechanochemical synthesis enriched the arsenal of techniques that are currently being utilized in the development of the ThMn12-type functional materials. The maximized concentration of Fe in NdFe12Nx films, which was stabilized by epitaxy, resulted in fundamental hard magnetic characteristics superior to those of the Nd2Fe14B. Other-than-epitaxy approaches which are more appropriate for bulk materials are also being sought. The relatively abundant cerium has been considered as the principal rare earth element in lower-cost medium-grade ThMn12-type magnets; the Si-stabilized compounds with Ce were found to be of particular interest because of their anomalously high Curie temperatures. A new rare-earth-free uniaxially anisotropic ZrFe10Si2 compound has been discovered, and R1-xZrxFe10Si2 compounds with R = Ce, Sm were proposed for the development of magnets that are minimally reliant on the rare earth elements. The newly employed mechanochemistry proved to be an attractive synthesis technique having produced in particular a submicron Sm0.7Zr0.3Fe10Si2 powder with a coercivity of 10.8 kOe and a calculated maximum energy product of 13.8 MGOe. At the same time, little progress has been made in the fabrication of fully dense ThMn12-type magnets because of the inherent instability of the nitrides and the unfavourable phase equilibria involving the more stable compounds. The work was supported by U.S. Department of Energy and University of Delaware Energy Institute.
High-coercivity Dy-free Nd-Fe-B permanent magnets
Due to the recent concern about the stable supply of heavy rare earth elements, attaining high coercivity in Nd-Fe-B magnets without using Dy has received intense research interest. In this talk, we will overview our recent progresses at NIMS toward the development of high coercivity Dy-free Nd-Fe-B permanent magnets. To obtain better understandings of the microstructure-coercivity relationships, we revisited the microstructures of Nd-Fe-B sintered and hot-deformed magnets using aberration-corrected STEM complemented by atom probe tomography (APT), magneto-optical Kerr microscopy and finite element micromagnetic simulations. We found that the intergranular phase parallel to the c-planes are mostly crystalline with a higher Nd concentration in contrast to that lying parallel to the c-axis that contains higher Fe content with an amorphous structure. Micromagnetic simulations suggest the reduction of the magnetization in the latter is critical to enhance the coercivity. Based on these new experimental findings together with our recent detailed characterization results of the intergranular phases in Ga-doped Nd-Fe-B magnets, we developed a method to increase the coercivity of Nd-Fe-B hot-deformed magnets while keeping relatively high remanence.
North Eastern University Boston
Towards New FeNi-Based Permanent Magnets of High Energy Density
Motivation persists to diversify the supply of rare-earth-based supermagnets in response to ever-growing demand in energy, defense, transportation and consumer product sectors. In contrast to many rare earth elements that are currently utilized in ultra-strong permanent magnets, the elements Fe and Ni are among the most abundant, most readily processed and most studied magnetic elements in the Earth’s crust. An intriguing proposition is to develop the equiatomic FeNi compound with the tetragonal L10 structure, a meteoritic mineral known as tetrataenite, into a magnet. Tetrataenite exhibits confirmed impressive permanent magnetic properties that are derived from, in part, its lower-symmetry tetragonal crystal structure (1). This tetragonal anisotropy leads to a large magnetic remanence which significantly amplifies the amount of magnetic energy that may stored in this compound.
While tetrataenite has been confirmed to exhibit excellent permanent magnetic properties, it is only found naturally and in appreciable volumes in selected meteorites subjected to extraordinarily slow cooling rates, as low as 0.3 K per million years. In this presentation new results concerning recent progress towards laboratory-based synthesis of tetrataenite will be presented, with the goal to foster discussion of its potential as a rare-earth-free permanent magnetic material. In particular, effects on phase formation accelerated by severe plastic deformation and post-processing treatments that allow access to kinetically hindered but thermodynamically accessible states will be described. In this manner tailored synthesis and processing protocols for the realization of a new type of magnetic material are identified and advanced, with high relevance for the creation of next-generation permanent magnets comprised entirely of easily accessible, earth-abundant elements that may be utilized for energy-relevant applications and beyond.
1. Lewis, L. H., et al. “Inspired by nature: investigating tetrataenite for permanent magnet applications.” Journal of Physics: Condensed Matter 26.6 (2014): 064213.
University of Luxembourg
Magnetic Neutron Scattering Studies on Nd-Fe-B Magnets
Nd-Fe-B-based permanent magnets have been continuously investigated for the last three decades due to their technological relevance as materials used in energy-related applications (e.g., motors and wind turbines) . A crucial issue is the understanding of the magnetization-reversal process, which eventually may result in the preparation of dysprosium and terbium-free Nd-Fe-B alloys with characteristic magnetic parameters (coercivity, remanence, maximum energy product) that guarantee their performance also at the high operating temperatures of motors (up to 200 °C).
In order to achieve this goal, the combination of advanced characterization techniques such as aberration-corrected high-resolution transmission electron microscopy and three-dimensional atom-probe tomography with ab initio calculations and numerical micromagnetic modeling is required. Indeed, recent studies along these lines (e.g., [2-4]) have provided important information regarding the nature (chemical composition, crystalline structure, ferro- or paramagnetic) of the intergranular Nd-rich phases in Nd-Fe-B magnets (including nanocomposites), which decisively determine the coercivity mechanism of these materials.
Magnetic neutron scattering, in particular, small-angle neutron scattering (SANS) is another important technique for characterizing the bulk magnetic microstructure of engineering permanent magnets. Magnetic SANS provides information on variations of both the magnitude and orientation of the magnetization on a nanometer length scale ( 1-300 nm). However, this method has only recently been employed for studying the spin microstructure of this class of materials (e.g., [5-10]). In this talk, we will discuss SANS data on both isotropic and textured sintered and nanocrystalline Nd-Fe-B magnets; specifically, the field dependence of characteristic magnetic length scales during the magnetization-reversal process is addressed, the exchange-stiffness constant has been determined, the observation of the so-called spike anisotropy in the magnetic SANS cross section (see figure below) has been explained with the formation of flux-closure patterns, and the effect of grain-boundary diffusion in isotropic Nd-Fe-B magnets has been studied.
Beijing University of Technology
Recent progress in recycling of Nd-Fe-B permanent magnets wastes
Nd-Fe-B permanent magnets have been widely used in both high-tech devices and conventional appliances. Moreover, with the fast development of some green-energy technologies, the consumption of Nd-Fe-B magnets is expected to double or even triple in the near future . As a result, the global proved reserves of some key rare earth elements exhibit an annual decrease, which in turn results in price hikes for these elements and concerns about the shortage of resource supplies. It is worth to mention that large amount of waste materials were generated through the electronic wastes and fabrication process of Nd-Fe-B. The waste materials, which were composed of sintered Nd-Fe-B magnet bulks and severely contaminated sludge, as well as bonded magnets, are valuable secondary resources of rare earth. In current papers, we report the progress in recycling of Nd-Fe-B permanent magnets wastes with the emphasis of our recent studies on large batch (up to 800 kg/batch) Nd-Fe-B sintered magnet wastes recycling, severely contaminated Nd-Fe-B sludge recycling, and Nd-Fe-B bonded magnet wastes recycling. Upon these studies and their final applications in industry, the consumption of valuable natural rare earth resources was expected to reduced and preserved in the future.
The ultimate production technology of Nd-Fe-B sintered magnets
Nd-Fe-B sintered magnets are used in motors and generators as thin plates. Typical process for producing the thin plates of Nd-Fe-B sintered magnets is strip- casting, milling, pressing, sintering, annealing and slicing. In this process, sintered bodies are always large blocks and slicing step is necessary to produce thin plates. A new process has been proposed to produce the thin plates of the Nd-Fe-B sintered magnets directly without slicing the blocks1. In this process, the alloy powder is filled into shallow cavities of graphite molds without pressing and the sintering is performed by transferring the molds filled with the alloy powder into a sintering furnace. This process is called pressless process or PLP for short. Advantages of PLP are firstly, direct production of thin plates of magnets, and secondary, the capability of using fine powder with minimizing oxidation no matter how fine the alloy powder is, which enables production of magnets with higher coercivity than the magnets produced by the process with the pressing step. Disadvantages of PLP are firstly, the necessity of preparing very large number of molds in mass production, and secondary, the number of shallow cavities made in a mold is small. We have developed a new technology in which the alloy powder is filled into a mold with 50 cavities separated by thin spacers, alignment of the powder filled in the molds is made by applying pulsed magnetic fields in a long coil, the mold is removed from the 50 powder compacts separated with spacers, and the 50 powder compacts separated with spacers are transferred into sintering furnace, leaving the mold outside the furnace. We call this technology new PLP because no pressing machine is used like in the conventional PLP explained above. The New PLP is the ultimate production technology because it realizes ultimately high productivity of compaction and alignment of the alloy powder, reduces drastically the number of molds necessary for mass production keeping all the advantages that the conventional PLP has. We have built automated equipment of new PLP and confirmed the performance of it; it enables mass production of the Nd-Fe-B sintered magnets with ultimately high magnetic properties, with ultimately high productivity and with ultimately high homogeneity of the magnetic properties.
1 M. Sagawa, Y. Une, A new process for producing Nd-Fe-B sintered magnets with small grain size, Proc.20th Workshop on REPM, Crete, 2008, p.103
Coercivity in RE and MnAl(C) magnets
Nucleation and pinning determines the coercive field of permanent magnets. Numerical micromagnetics is an excellent tool to visualize the nucleation of reversed domains and the pinning of domain walls in model systems. These model systems are constructed artificially on the computer using detailed input from the experimental characterization of the material. The finite element models are built in a way that accentuate the key microstructural features.
The magnetic state prior to the nucleation of reversed domains shows where reversed domains are likely to form. In figure (a) the dark blue areas indicate the regions where the magnetization rotates reversibly out of the alignment direction by more than 25 degrees. In Nd2Fe14B based permanent magnets, nucleation of reversed domains occurs within the weakly ferromagnetic grain boundary phase or at a thin defect layer at the grain surface. In Dy diffused magnets, where the grains are covered with a thin Dy containing shell, the regions with high reversible rotations of the magnetization are in the core of the grains. Consequently, the nucleation field is higher by more than 30 percent. Once a reversed domain is formed magnetization reversal proceeds by forming columns of reversed grains. The pinning field is lower for domains perpendicular to the c-axis than for domain walls parallel to the c-axis.
A structurally stable L10 phase forms in MnAl with small additions of C. This phase is a candidate for rare-earth free permanent magnets with intermediate properties. However, the coercive field is well below the anisotropy field owing to defects in the crystal structure. Using finite element micromagnetics we computed the nucleation field and pinning field at twin boundaries and at antiphase boundaries. The figure (b) shows the motion of a domain wall in MnAl(C) under increasing applied field. The walls are pinned at twin boundaries. The depinning fields, µ0Hp, are = 1.23 T, 1.5 T and 1.9 T. The nucleation field at anti-phase boundaries is µ0Hn = 1.4 T. These results clearly show that the presence of defects limits the coercive field in MnA(C) magnets.
Delivering an environmentally responsible and traceable supply chain for rare earths
This presentation provides and update of Lynas’ operations at Mt Weld in Western Australia, and the Advanced Materials Plant in Malaysia which is ramping up to the nameplate capacity of 22,000 tpa REO, and serving customers in Japan, North America, Europe and China.
An account of the reasons for selecting Malaysia as a refining location are provided as well as an outline of the Lynas supply chain for PrNd from Mt Weld to magnet makers. Also presented is a description of the environmental considerations for the mining and processing of different rare earth mineral types. Responsible processing requires investment in process technologies specific to the rare earth mineral chemistry and the reagent regimes selected for the rare earth separation and product precipitation. An overview of the Lynas investment in environmental processes is provided.
Leading manufacturers of green technologies require confidence in the environmental performance of rare earth producers supply chain to support the green authenticity of their products, and are moving to validate their rare earth supply chains.
Toyota Motor Corporation, Japan
Advanced Material Engineering Div.
Magnetic Materials Research for Vehicle Motor Application
To be added.