It is commonly understood that among the intermetallic phases used for permanent magnets, practically none can fully realize its potential based on the intrinsic magnetic properties. We discuss ...different reasons leading to this limitation, known as the Brown paradox, and propose some possible ways of overcoming it. We compare the intrinsic magnetic properties of (Nd1−xCex)2(Fe1−yCoy)14B single crystals with the extrinsic characteristics of sintered and hot compacted magnets made from the very same alloys. In addition, looking at RE-free materials, our results obtained on Mn- and Co-based RE-free single crystals are compared with the hard magnetic properties of Mn-based permanent magnets.
Mastering hysteresis in magnetocaloric materials Gutfleisch, O.; Gottschall, T.; Fries, M. ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
08/2016, Letnik:
374, Številka:
2074
Journal Article
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Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratory-scale prototypes of magnetic ...refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and, therefore, it is necessary to understand the mechanisms causing hysteresis and to find solutions to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss the fundamental aspects that can contribute to thermal hysteresis and the strategies that we are developing to at least partially overcome the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. In doing so, we refer to the most relevant classes of magnetic refrigerants La-Fe-Si-, Heusler- and Fe2P-type compounds.
This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.
We report on (1) direct measurements of ΔTad for binary Fe49Rh51 during field cycling and (2) maximum possible ΔTad measured under discontinuous protocol. Our results show that the ΔTad is 9.2 K on ...the first application of magnetic field of Δμ0H = 1.9 T and it remains as high as 6.2 K during the cycling in alternated field of the same magnitude. In addition, the adiabatic temperature change and magnetic entropy change under the first application of magnetic field and under cyclic conditions were determined indirectly using three different approaches: (1) from magnetic measurements (M(T)H dependences, Maxwell relations), (2) from calorimetry (C(T)p,H, S-T diagram) and (3) from H-T diagram. While the indirectly measured maximum possible ΔTad lies in the range of 10.5–12 K, the reduced value of ΔTad measured directly under cycling (6.2 K) is still extraordinarily high and is 15% higher than in Gd in similar magnetic field. This demonstrates the potential of materials with a first order metamagnetic transition for magnetocaloric applications despite the presence of hysteresis.
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A higher saturation magnetization obtained by an increased iron content is essential for yielding larger energy products in rare-earth Sm
Co
-type pinning-controlled permanent magnets. These are of ...importance for high-temperature industrial applications due to their intrinsic corrosion resistance and temperature stability. Here we present model magnets with an increased iron content based on a unique nanostructure and -chemical modification route using Fe, Cu, and Zr as dopants. The iron content controls the formation of a diamond-shaped cellular structure that dominates the density and strength of the domain wall pinning sites and thus the coercivity. Using ultra-high-resolution experimental and theoretical methods, we revealed the atomic structure of the single phases present and established a direct correlation to the macroscopic magnetic properties. With further development, this knowledge can be applied to produce samarium cobalt permanent magnets with improved magnetic performance.Understanding the factors that determine the properties of permanent magnets, which play a central role in many industrial applications, can help in improving their performance. Here, the authors study how changes in the iron content affect the microstructure of samarium cobalt magnets.
Nd–Fe–B permanent magnets have been coated with 0.6wt.% dysprosium and annealed at various temperatures to study the impact of the temperature-dependent Dy diffusion processes on both the magnetic ...properties and the microstructure. When optimum annealing conditions are applied the Dy processed magnets with initial coercivity of ∼1100kAm−1 yield coercivity increases which can exceed 400kAm−1 without a significant reduction of the remanent magnetic polarization. The improved stability against opposing magnetic fields can be observed up to a depth of ∼3mm along the diffusion direction, restricting the application of the Dy diffusion process to either thin magnets or magnets with tailored coercivity gradients. While in the proximity of the Dy-coated surface, each grain has a Dy-enriched shell with a Dy content of ∼6at.%; the Dy concentration decreases exponentially to ∼1.8at.% after a diffusion depth of 400μm and to ∼1at.% after a diffusion depth of 1500μm, as was found with wavelength dispersive X-ray spectroscopy and scanning transmission electron microscopy–energy dispersive X-ray spectroscopy, respectively. In the vicinity of the Dy-coated surface, the mechanism of the Dy-shell formation is attributed to the melting/solidification of a heavy-rare-earth-rich intermediate phase during high-temperature annealing. This is based on the observation that a constant Dy concentration over the width of the shells was found. Also an epitaxial relation between the Dy-poor core and the Dy-rich shell was observed by electron backscattered diffraction, which is supported by results obtained with Kerr microscopy.
Achieving a very strong magnetic anisotropy in a 3d material is a difficult, but not an impossible task. It is difficult because there is no general recipe (necessary condition) for a strong ...anisotropy in a band magnet. Several strategies can be pursued in this situation. One of them is to re-examine the less studied 3d compounds, somewhat neglected since the discovery of the Nd-Fe-B magnets 30 years ago. As an example, a single crystal of (Fe0.7Co0.3)2B has been investigated in this work.
This article addresses the most recent developments in the processing of high-performance magnets based on rare earth-transition metal (R-T) compounds. It gives an overview of the relevant classes of ...R-T compounds together with a description of the appropriate manufacturing route and an assessment of their potential for application. The paper pays particular attention to state-of-the-art maximum energy density and high-temperature magnets as well as to the synthesis of nanostructured materials using non-equilibrium techniques and reversible hydrogen-induced phase transformations. The possibilities of increasing the energy density of the nanocrystalline magnets by remanence enhancement utilizing intergranular exchange coupling, by texturation via hot deformation or by hydrogenation disproportionation desorption and recombination to produce anisotropic powders are discussed. Focus is on Nd-Fe-B-based magnets.
The 2017 Magnetism Roadmap Sander, D; Valenzuela, S O; Makarov, D ...
Journal of physics. D, Applied physics,
09/2017, Letnik:
50, Številka:
36
Journal Article
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Building upon the success and relevance of the 2014 Magnetism Roadmap, this 2017 Magnetism Roadmap edition follows a similar general layout, even if its focus is naturally shifted, and a different ...group of experts and, thus, viewpoints are being collected and presented. More importantly, key developments have changed the research landscape in very relevant ways, so that a novel view onto some of the most crucial developments is warranted, and thus, this 2017 Magnetism Roadmap article is a timely endeavour. The change in landscape is hereby not exclusively scientific, but also reflects the magnetism related industrial application portfolio. Specifically, Hard Disk Drive technology, which still dominates digital storage and will continue to do so for many years, if not decades, has now limited its footprint in the scientific and research community, whereas significantly growing interest in magnetism and magnetic materials in relation to energy applications is noticeable, and other technological fields are emerging as well. Also, more and more work is occurring in which complex topologies of magnetically ordered states are being explored, hereby aiming at a technological utilization of the very theoretical concepts that were recognised by the 2016 Nobel Prize in Physics. Given this somewhat shifted scenario, it seemed appropriate to select topics for this Roadmap article that represent the three core pillars of magnetism, namely magnetic materials, magnetic phenomena and associated characterization techniques, as well as applications of magnetism. While many of the contributions in this Roadmap have clearly overlapping relevance in all three fields, their relative focus is mostly associated to one of the three pillars. In this way, the interconnecting roles of having suitable magnetic materials, understanding (and being able to characterize) the underlying physics of their behaviour and utilizing them for applications and devices is well illustrated, thus giving an accurate snapshot of the world of magnetism in 2017. The article consists of 14 sections, each written by an expert in the field and addressing a specific subject on two pages. Evidently, the depth at which each contribution can describe the subject matter is limited and a full review of their statuses, advances, challenges and perspectives cannot be fully accomplished. Also, magnetism, as a vibrant research field, is too diverse, so that a number of areas will not be adequately represented here, leaving space for further Roadmap editions in the future. However, this 2017 Magnetism Roadmap article can provide a frame that will enable the reader to judge where each subject and magnetism research field stands overall today and which directions it might take in the foreseeable future. The first material focused pillar of the 2017 Magnetism Roadmap contains five articles, which address the questions of atomic scale confinement, 2D, curved and topological magnetic materials, as well as materials exhibiting unconventional magnetic phase transitions. The second pillar also has five contributions, which are devoted to advances in magnetic characterization, magneto-optics and magneto-plasmonics, ultrafast magnetization dynamics and magnonic transport. The final and application focused pillar has four contributions, which present non-volatile memory technology, antiferromagnetic spintronics, as well as magnet technology for energy and bio-related applications. As a whole, the 2017 Magnetism Roadmap article, just as with its 2014 predecessor, is intended to act as a reference point and guideline for emerging research directions in modern magnetism.
•Different WEEE were analyzed and the rare-earth permanent magnets were collected.•Chemical composition evolution of Nd-Fe-B alloys over the years is illustrated.•Loudspeakers from laptops and ...loudspeakers from flat TV contained Nd-Fe-B magnets.•The amount of Nd-Fe-B could double if the loudspeakers streams are included.
Nd-Fe-B permanent magnets are a strategic material for a number of emerging technologies. They are a key component in the most energy efficient electric motors and generators, thus, they are vital for energy technologies, industrial applications and automation, and future forms of mobility. Rare earth elements (REEs) such as neodymium, dysprosium and praseodymium are also found in waste electrical and electronic equipment (WEEE) in volumes that grow with the technological evolution, and are marked as critical elements by the European Commission due to their high economic importance combined with significant supply risks. Recycling could be a good approach to compensate for the lack of rare earths (REs) on the market. However, less than 1% of REs are currently being recycled, mainly because of non-existing collection logistics, lack of information about the quantity of RE materials available for recycling and recycling-unfriendly product designs. To improve these lack of information, different waste streams of electrical and electronic equipment from an industrial recycling plant were analyzed in order to localize, identify and collect RE permanent magnets of the Nd-Fe-B type. This particular type of magnets were mainly found in hard disk drives (HDDs) from laptops and desktop computers, as well as in loudspeakers from compact products such as flat screen TVs, PC screens, and laptops. Since HDDs have been investigated thoroughly by many authors, this study focusses on other potential Nd-Fe-B resources in electronic waste. The study includes a systematic survey of the chemical composition of the Nd-Fe-B magnets found in the selected waste streams, which illustrates the evolution of the Nd-Fe-B alloys over the years. The study also provides an overview over the types of magnets integrated in different waste electric and electronic equipment.
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.
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