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Scripta Materialia 67 (2012) 521–523

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Preface

Preface to the viewpoint set on: Magnetic materials for energy

A greater reliance on renewable energy sources and mately to Dy-less magnets. State-of-the-art magnetoca-

increased concern for energy efficiency in the total en-ergy lifecycle has accelerated research in energy-relatedtechnologies. Due to their ubiquity, magnetic materialsplay an important role in improving the efficiency andperformance of devices in electric power generation,conditioning, conversion, transportation, and other en-ergy-use sectors of the economy [1]. This viewpoint seton Magnetic Materials for Energy provides opinionatedarticles by well-known experts in the field on the per-spectives of hard and soft magnets and magnetocaloricmaterials for energy applications. We have organizedduring the recent years various symposia on the topicof Magnetic Materials for Energy at Intermag, MMM,TMS, JEMS international conferences. It deems timelynow to summarize some of the current discussions inthe field by the present viewpoint set. It comprises 13articles on the topics of hard (articles 1–4) and soft (arti-cles 5–7) magnets as well magnetocaloric materials (arti-cles 8–13).

At the beginning we want to put briefly this viewpointset into a broader context. Hard magnets are key com-ponents in many household and industrial devices, andthey are especially critical within the renewable energysector. Motors and generators for electric vehicles andwind turbines demand temperature stabilities of themagnets well-exceeding 120 �C. These magnets are cur-rently based on rare earth elements such as neodymiumand dysprosium, and to less extent terbium, which areright now almost exclusively mined in China (�97%).The heavy rare earths provide the temperature stability,especially in the named climate relevant applications.Over the last couple of years China has been restrictingexport of these materials and this monopolistic situationtriggered a price escalation approaching 3000% at somestage. Once global rare earth production will diversify(again) the availability of light rare earths will be se-cured, however, it is likely that bottlenecks for the heavyrare earths will remain. With this perspective, there is anobvious and urgent need for a more efficient usage ofrare earth elements in the production of various func-tional magnetic materials. In fact, the challenge goes fur-ther: there is the necessity to greatly reduce, and finallysubstitute altogether the (heavy) rare earth elements inpermanent magnets. In other words, we need to movefirst to less Dy by microstructural engineering, i.e. grainboundary phase control on the nanoscale, and ulti-

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loric and permanent magnet materials both currentlycontain rare earth metals. Magnetic cooling relies onthe reversible magnetization and demagnetization of amagnetic material by an external magnetic field, result-ing in a change of temperature. The latter phenomenonis known as the magnetocaloric effect (MCE) and ismaximal at temperatures near to a magnetic phase tran-sition. So for magnetocaloric materials we also have toconsider the criticality of raw materials and Gd-basedand to less extent La-based compounds need to be re-placed. Furthermore, improvements in the performanceof magnetocaloric materials would drastically reduce theconsumption of rare earth permanent magnets which areneeded to drive the effect.

Complex shapes of magnetic materials lead to signif-icant wastage of critical materials during production andthis needs to be overcome through more efficient manu-facturing of large volumes of magnetic material parts,e.g. by advanced net-shape and polymer bonding tech-nologies. Recycling options for end-of-life applicationsand the magnets within need to be explored and ad-vance-device-design for recycling will have to be imple-mented to make disassembly economic (“urbanmining”), with this avoiding the dissipation of criticalmaterials. Finally, looking at rare earth free materialalternatives, we have to develop resource efficient newmagnets which are not necessarily the best in absoluteperformance, but which would assure a secure and inex-pensive raw materials supply. However, it is still a longway to move from compounds such as iron nitrides orMn-based alloys with some exciting intrinsic magneticproperties to a highly textured, fully-dense, thermallystable magnet material with satisfactory extrinsic mag-netic properties. In any case, a route worthwhile pursu-ing provided the targets are set realistically.

The viewpoint set starts with a paper by Coey provid-ing the basics of permanent magnetism and stating thefact that bulk permanent magnets are indispensablecomponents of numerous consumer and industrial prod-ucts for energy conversion. It provides numbers for cur-rent production rates and raw materials prices and fromthere derives opportunities for the development of newmagnets with an energy product intermediate betweenferrite and NdFeB magnets. It then looks more closelyat a few interesting uniaxial ferromagnetic compoundswith potential to “plug” this gap.

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522 Preface / Scripta Materialia 67 (2012) 521–523

The paper by Hono and Sepehri-Amin discusses op-tions of how to improve coercivity in NdFeB magnetswith less or none of the named critical elements. Its cen-tral argument is around the statement that the forma-tion of continuous thin layers of a Nd-rich amorphousphase surrounding Nd2Fe14B grains is the key micro-structural feature for high coercivity. They also chal-lenge common assumptions – based on their advancedand quantitative 3D atom probe investigation – by stat-ing that the thin grain boundary phase is actually ferro-magnetic rather than paramagnetic.

Woodcock et al. continue on the same theme by scru-tinizing the subtle link between coercivity and micro-structure which is essential for the development ofhigh performance less Dy-containing magnets. They em-ploy a combination of high resolution characterization,molecular dynamics and micromagnetic simulations andmodel thick film systems to gain new insight into coer-civity mechanism in NdFeB magnets and to identifythe weak link in the microstructure in terms of magneti-zation reversal.

The last paper on permanent magnets by Balamur-ugan et al. discusses how magnetic nanoparticles withhigh magnetocrystalline anisotropy can be used asbuilding blocks for next-generation permanent magnets.Advances in processing such as self-assembly, alignmentof easy axes, and nanostructuring are described, whichpotentially will permit the fabrication of densely-packednanoparticle assemblies with enhanced energy products.They also propose an idealized nanocomposite structurefor future development.

Suzuki and Herzer start the section on soft magnetsby stating that advanced Fe-rich nanocrystalline softmagnetic alloys – an efficient core material alternativeto Fe-Si steel – also play a vital role in the reductionof greenhouse gasses emissions. They discuss strategiesfor the suppression of unexpectedly large values of fieldinduced magnetic anisotropy in these systems, especiallyat the Fe-rich end, which are detrimental to the ex-change softening process. The application of a rotatingmagnetic field during nano-crystallisation is argued tobe effective in reducing the induced anisotropy.

The paper by Willard et al. elucidates the challengeswhich must be overcome when nanocomposite soft mag-netic materials are used at elevated temperatures. Fea-tures of alloy design are discussed which enable thestability of the microstructure and the maintenance ofexchange coupling. The authors describe fundamentaland engineering approaches for tuning the two magneticphases typically present in order to improve the hightemperature performance of nanocomposite soft mag-netic alloys.

Moses concludes the part on soft magnets by describ-ing the state-of-the-art in electric steels, which still ac-count for 97% of all soft magnetic materials producedtoday. He states that magnetic losses in electrical steelcores account for 5–10% of all electrical power gener-ated and points out that a combination of texture, com-position, grain size and sheet thickness can still lead tosignificant improvements of magnetic properties ofnon-oriented and grain oriented electrical steel. Furtheradvances in the characterization and modeling of thelosses in electrical steels are needed to achieve this goal.

The last part on magnetocaloric materials and solidstate refrigeration at room temperature begins with acontribution by Sandeman who explores the potentialfor new magnetocaloric material systems. He derivesguidelines for the search of novel materials and puts for-ward an Ashby-style map which compares popularmaterials with their respective limits derived from anidealized first-order transition. The key challengesencompassing various length scales are summarized ina very instructive manner.

Gschneidner et al. focus on the nature of the magnet-ocaloric effect in first-order type materials and elucidatevery clearly to what extent the structural entropy changecontributes to the total isothermal entropy change ofmaterials which undergo a magnetostructural transition.They go on by stating that the structural entropychanges are proportional to the volume differences ofthe phases involved in the transition and propose thatthis correlation permits the search for new materialsbased on crystallographic data.

Fujita and Yako carefully examine the fundamentalproperties of NaZn13-type La(FeSi)13 in order to estab-lish even better magnetocaloric performance in this sys-tem. The stability of metallic, magnetic and electronicstates in these compounds is discussed and it is pointedout that all these parameters are extremely sensitive tosmall inhomogeneities within the sample and even moreimportantly to substitutional and interstitial modifica-tions. They conclude by pointing out the necessity ofan understanding of the time-dependency of thetransitions.

The next paper by Liu et al. continues with the samesystem but deals with the application challenges whenemploying La(FeSi)13 alloys. Systematic magnetic entro-py and adiabatic temperature change data of first- andsecond-order type materials are discussed. Processingroutes suitable for up-scaling are compared and thermaltransport properties, corrosion protection and mechani-cal stability are assessed with the idea to move this mate-rial class closer to application.

Another exciting system is discussed next by Brucket al. who demonstrate that substitutional control cangreatly enhance the magnetocaloric properties ofMnFe(P,Ge) compounds which crystallize in the hexag-onal Fe2P-type structure. Co and Ni substitutions for Fehelp to control the magnetic transition to be close to theborder separating the first- and second-order transitionregimes, minimizing in this way the thermal hysteresispaired with high magnetic entropy changes.

Finally, Franco and Conde review amorphous andnanostructured materials which undergo second-orderphase transitions and which represent a different conceptto the material classes discussed above. They examinethe challenges which need to be overcome in order tomake these materials applicable for magnetic refrigera-tion. The potential of composites is assessed and the cur-rent status of modeling this particular subset ofmagnetocaloric materials is summarized.

This viewpoint set is meant to raise awareness in thecommunity for the topic of Magnetic Materials for En-ergy but also within the much broader readership ofScripta Materialia and ideally also within the respectivefunding agencies in the different countries. It is hoped

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that it catalyzes further high quality research work butalso further funding schemes at various places and trig-gers international cooperation which will help to meetthe upcoming challenges in magnetic materialsdevelopment.

We, as the guest editors, wish to thank all authors fortheir reflective thoughts and excellent contributions. Wewould feel that the main goal of this viewpoint set is ful-filled if it serves to stimulate further exciting develop-ments in the field of magnetic materials and theirapplications in clean energy technology.

Reference

[1] Gutfleisch O, Liu JP, Willard M, Bruck E, Chen C, Shankar SG.Magnetic materials and devices for the 21st century: stronger,lighter, and more energy efficient. Advanced Materials2011;23:821–42.

Oliver Gutfleisch⇑

Institut fur Materialwissenschaft, FG FunktionaleMaterialien, Technische Universitat Darmstadt,

Petersenstr. 23, 64287 Darmstadt, GermanyFraunhofer Project Group Materials Recycling and

Resource Strategies, Rodenbacher Chaussee 4, 63457Hanau-Wolfgang, Germany

⇑ Tel.: +49 6151 16 75559; fax: +49 6151 16 72559.E-mail address: gutfleisch@fm.tu-darmstadt.de

Victorino FrancoDpto. Fisica de la Materia Condensada, ICMSE-CSIC,

Universidad de Sevilla,P.O. Box 1065, 41080 Sevilla, Spain

E-mail address: vfranco@us.es

Available online 13 July 2012