Symposium FG
Magnetic Materials for Energy

Session FG-1 - Hard Magnetic Materials

FG-1:IL01  New Research Strategies in RE-based Magnets
A.M. GABAY, G.C. HADJIPANAYIS, University of Delaware, Newark, DE, USA

Two powerful factors shape the current research activity related to rare-earth (RE) magnets: the rapidly growing demand for high-energy-density magnets operating at 150-200 °C and the RE supply crisis of 2010-11. The incentive for increasing the operating temperature of the Nd-Fe-B magnets using smaller amounts of the heavy REs has led to considerable efforts been dedicated to a localized alloying, usually through a post-sintering infiltration. A wider use of the Sm-Co magnets is also being considered; with this in mind, the maximum energy product of these magnets has recently been increased to 35 MGOe. For less demanding applications, substitutions with the relatively abundant and inexpensive Ce are being attempted. The ThMn12-type compounds see a renewed interest; they are being intensively studied both for a superior hard magnetic performance at elevated temperatures and for smaller RE concentrations. The search for new hard magnetic compounds is increasingly being assisted by first-principle calculations and the combinatorial approach. Regarding the manufacturing methods, chemical synthesis of anisotropic powders such as Sm2Fe17Nx has been greatly advanced, and it is likely to be extended to the other hard magnetic materials.
Work supported by DOE, Grant DE-FG02-90ER45413.


FG-1:IL02  High-performance Permanent Magnets without Rare Earths: Challenges and Perspectives
K.P. SKOKOV, O. GUTFLEISCH, Technische Universität Darmstadt, Institut für Materialwissenschaft, Darmstadt, Germany

Together with the activity aimed at reduction of heavy rare-earth content in high performance permanent magnets, we can observe a steadily growing interest towards the new magnetic materials containing no rare-earth elements at all, but nonetheless exhibiting a remanent magnetization Br and the coercivity Hc better than commercially available hard ferrites. The main obstacle in the production of a 3d-only high-performance permanent magnet is insufficient anisotropy (and hence, coercivity). In this talk we will outline the combinatorial material science as an experimental technique allowing drastically accelerate the discovery of new phases with high anisotropy constant K1 and spontaneous magnetization Ms. In addition to that, this talk also deals with metastable phases and their magnetic properties. We will also discuss the use of additional degrees of freedom, for example field-assisted processing in developing of rare earth-free permanent magnets. Finally, we would like to re-examine the relationship between intrinsic magnetic properties of the material (K1 and Ms) and extrinsic magnetic properties of the magnet itself (Br and Hc). We will consider different reasons leading to the ‘Braun paradox’ and we will suggest some possible ways to overcome this situation.


FG-1:IL03  Multidriver Processing Routes to Chemical Order in FeNi
L.H. LEWIS, Northeastern University, Boston, MA, USA

Global supply and demand uncertainties continue to motivate the development of new types of sustainable, ecologically friendly and accessible advanced permanent magnets. As chemically ordered L10-type FeNi (aka tetrataenite) has been confirmed in meteoritic materials to exhibit a theoretical magnetic energy product exceeding (BH)max = 335 kJ/m3 (42 MGOe)(1), protocols to synthesize this phase in industrially relevant timescales remain of high interest. To this end, guidance from literature reports concerning ordered phase formation in the analogous FePd system(2,3) has been applied to FeNi-based bulk materials. In this work FeNiX materials were subjected to high-strain processing and then annealed in excess of 40 days in a specially designed furnace that simultaneously applies a static strain and a saturating magnetic field to drive the ordering transformation. Preliminary results obtained by magnetic force microscopy confirm a large change in the nature of the magnetic domains as a function of processing in these alloys, consistent with the development of elevated magnetic anisotropy. These results augment the case that attainment of L10 FeNi is indeed possible on earthly timescales.
This work is supported by the NSF under Grant Number Grant # 1601895.


FG-1:IL04  Heusler Compounds: Towards Rare-Earth-Free Permanent Magnets
C. FELSER, A. MARKOU, Max Planck Institute for Chemical Physics of Solids, Dresden, Germany

The increasing global demand for permanent magnets (PMs) has been driven by the development of high-efficiency motors and generators for various clean energy commercial applications. However, the rare earth replacement in PMs is desirable, due to the volatility of their prices and the strategic issues associated with them. Apart from the high-end applications, new magnets with mid-range performance are required as well, where novel hard magnetic materials are predicted to bridge the gap between low-cost hard ferrites and expensive rare-earth-based magnets. The development of simple rules to understand the structure to function relation in magnetic Heusler compounds allowed to discover many unique properties for different applications [1]. Mn-based Heusler compounds with structural distortion are promising hard magnetic materials, showing high magnetocrystalline anisotropy, large coercivity and high Curie temperature. Here, we present our results in novel Heusler compounds with strong uniaxial anisotropy towards PMs applications [2].
[1] T. Graf, S.S.P. Parkin, and C. Felser, Prog. Solid State Chem. 39, 1-50 (2011). [2] Adel Kalache, et al., APL Materials 5, 096112 (2017).


FG-1:IL05  Intrinsic Magnetic Properties of RFe12 Based Hard Magnetic Phase
YUSUKE HIRAYAMA, Magnetic Powder Metallurgy Research Center, National Institute of Advanced Industrial Science and Technology, Nagoya, Japan

In order to develop a permanent magnet superior to Nd-Fe-B magnets, a ferromagnetic compound with higher saturation magnetization and higher anisotropy field compared to those of the Nd2Fe14B compound must be found. Fe-rich rare earth compunds such as SmFe10N with the TbCu7 structure [1] and RFe12(N) with the ThMn12 structure [2,3] receive much interest as new hard magnetic compounds and there are several publications on the hard magnetic properties of polycrystalline isotropic samples. However, the intrinsic properties of these phases have not been well established. In this work, we show that Sm(Fe0.8Co0.2)12 with the ThMn12 structure has excellent hard magnetic properties with μ0MS ~1.78 T, μ0HA ~12 T at room temperature. Also the Curie temperature of 859 K is much higher than that of Nd2Fe14B, and the temperature dependence of both μ0MS and μ0HA are superior to those of Nd2Fe14B [3]. However, this phase is stabilized only in a film form, limiting the real application as permanent magnets. We discuss possible ways to extend this compound to bulk magnets by reviewing relevant literature.
[1] S. Sakurada et.al., Journal of Applied Physics 79, 4611 (1996). [2] Y. Hirayama et al., Scripta Materialia, 95 (2015) 70. [3] Y. Hirayama et al., Scripta Materialia, 138 (2017) 62.


FG-1:IL07  Hard Magnetic Materials based on Nanowires
J. PING LIU, Department of Physics, University of Texas at Arlington, Arlington, TX, USA

It is of a grand challenge to develop strong magnetic anisotropy in nanostructured bulk permanent magnets. Recently, we have worked on bottom-up approaches to produce hard magnetic materials based on magnetic nanoparticles with high aspect ratio. The approaches start from synthesis of ferromagnetic nanowires and nanorods with modified solvothermal synthesis techniques. We have successfully synthesized monodisperse nanorods, nanowires and submicron chips of ferromagnetic FePt, FeCo, CoNi, CoCx and Co based materials. The aligned Co nanowires have their coercivity values exceeding 10 kOe and energy product exceeding 40 MGOe. The ferromagnetic nanoparticles are used as building blocks for advanced bulk and thin film permanent magnets, and can be also applied in biomedicine and ferrofluid technologies.


FG-1:IL08  Recent Advances in Micro-magnetic Modelling of Permanent Magnets
T. SCHREFL, A. KOVACS, J. FISCHBACHER, M. GUSENBAUER, Danube University Krems, Wiener Neustadt, Austria

Micromagnetic modelling is a key research tool for the quest for new permanent magnets. With the advance massively parallel computing on graphics processors and the development of new numerical algorithms fast simulations for realistic structures become possible. In the talk we will describe the methodology for computing coercivity, remanence and energy density product as function of typical microstructural features. We computed structure property relations for several candidate phases for new permanent magnets. Whenever possible we synthetically generated a set of suitable microstructures, computed the hysteresis properties for the selected structures using a finite element method, and applied a numerical optimization tool to maximize coercivity or energy density product. The free parameters for optimization are typical microstructure features such as the grain shape or the magnetization of the grain boundary phase. The following (coercive field (T), energy density product (kJ/m³)) pairs were computed for Fe3Sn0.75Sb0.25: (0.49, 290), L10 FeNi: (1, 400), CoFe6Ta: (0.87, 425), and MnAl: (0.53, 80).
Work supported by the EU H2020 project Novamag (686056).


FG-1:IL09  Novel Developments in Hybrid Ferrite-based Hard Nano-materials
C. DE JULIAN FERNANDEZ, IMEM - CNR, Parma, Italy  

A study on exchange effects in hybrid cobalt ferrite-based nanomaterials to obtain new types of magnets is presented. Nowadays, alternative materials to substitute Rare Earth (RE) compounds in magnets are being researched. Cobalt ferrite (CFO) is between the spinels the one that exhibits the largest anisotropy, even if it cannot employed in magnets due to the cubic symmetry of its magnetocrystalline anisotropy. We have investigated the exchange coupling mechanism between the hard CFO with a second magnetic phase to get novel type of nanomagnets. I will illustrate several cases of hybrid core@shell (CS) nanoparticles (NPs) obtained by colloidal thermal decomposition synthesis. First, CFO and soft MnZn ferrite CS NPs have been synthesized with a morphology of spring magnet model. Second exchange-bias effects are investigated in hybrid antiferromagnetic FeCo monoxide core @ CFO shell NPs. The exchange bias can be induced by interface or intergranular exchange effects leading to the increase of the energy product. The magnetic properties of these nanostructures will be correlated with their interfacial structure and compositional properties.
This research was supported by EU-FP7 NANOPYME (n. 310516) and EU- H2020 AMPHIBIAN Projects (n. 720853).


FG-1:IL10  New Processing of RE-based Magnetic Materials
S. KOBE, Jožef Stefan Institute, Ljubljana, Slovenia

Rare-Earth Transition Metals permanent magnets are vital components in the rapidly-developing renewable energy sector, where the motors require strong magnets with the ability to operate at temperatures well over 100°C. To achieve high coercivity, remanence and consequently high energy product at elevated temperatures the addition of heavy rare earth (HRE) to the basic Nd-Fe-B composition is needed. On the list of Critical Raw Materials published by the EC in 2014, HRE is on the very top of it. To drastically reduce the use of HRE we focused on developing a new method, which should enable us to achieve the properties needed for high-temperature application with the lowest amount of scarce elements. By our new inventive technique further transferred to a pilot production, we could minimize the amount of HRE used, down to 0.2 at %, the improvement of coercivity was 30 % with minimal loss in remanence. The total saving of the HRE is 16-times less need for the same performance, which is a significant contribution to the world economy and clean environment. In studying the mechanism for such an improvement in coercivity without significantly decreasing the remanence, a detailed microstructure investigation was performed by using high-resolution transmission electron microscopy.


FG-1:IL11  Study of Anisotropic Bonded Permanent Magnetic Materials
JINBO YANG, JINGZHI HAN, School of Physics, Peking University, Beijing, China

Permanent magnetic materials currently play essential role in the modern life, which are widely used in information electronics, electric motors and generators, and transportation fields. In this talk, we summarized the research results of development on anisotropic hard magnetic materials: (1) Through the study of the relationship between the structural and magnetic properties, the high performance anisotropic interstitial atoms modified rare earth-iron based magnetic materials was developed. Pseudo-single crystal-like powders were obtained, thereby realizing high coercivity and high energy product. The maximum energy product over 40 MGOe was achieved for Sm-Fe-N compounds. (2) The HDDR process was investigated to prepare the anisotropic R2Fe14B-type materials. It is found that the key mechanism of HDDR anisotropy is related to the columnar microstructure with "dissipative structure" formed in the disproportionation stage. Maximum energy product of 41 MGOe could be achieved for R2Fe14B powders. (3) In the case of non-rare earth permanent magnetic materials, the anisotropic MnBi nanostructured powders with anomalous temperature coefficient of the coercivity were developed. The fabrication of the hybrid magnets based on these kinds of magnetic powders is discussed.


FG-1:IL12  Towards the Optimized Use of Permanent Magnets: Development of 3d Printed Magnets
D. SUESS¹, C. HUBER¹, S. SCHUSCHNIGG³, M. GRÖNEFELD³, ¹Christian Doppler Laboratory for Advanced Magnetic Sensing and Materials, Faculty of Physics, University of Vienna, Vienna, Austria; ²Magnetfabrik Bonn GmbH, Bonn, Germany; ³Department of Polymer Engineering and Science, Montanuniversitaet Leoben, Leoben, Austria

3D printing is a fast growing technology for single-unit production and allows producing structures that have been historically difficult or impossible to build, like holes that change direction, unrealistic overhangs, or square interior cavities. Within this talk we will give a review about our activity in additive manufacturing of polymer bonded NdFeB magnets using a commercial 3D printers. The used 3D printing system is also capable of extrude two different filaments, which allows a gradual change in magnetic properties. Hence, a continuous change from a magnetic material to a non-magnetic can be realized as function of space. In order to fully make use of these new production flexibility advanced algorithm are required to determine the shape of magnet as well as the local magnetic properties to obtain the required and predefined magnetic field. An outlook will be given how the printed magnet can be locally magnetized, which will allow to realize magnets with locally varying magnetization directions such as Halbach arrays. The capability of printing magnets with complex shapes and internal magnetization profiles will open the path for new magnetic designs and applications which are not existing yet, since they are impossible to fabricate with state of the art methods.

 
Session FG-2 - Soft Magnetic Materials

FG-2:IL02  Magnetic Sensors and Actuators Based on Soft Magnetic Materials
C. GOMEZ-POLO, I. ROYO-SILVESTRE, J.M. JIMENEZ-RUIZ, J.J. BEATO-LOPEZ, Physics Departament & Institute for Advanced Materials (INAMAT), Universidad Pública de Navarra, Pamplona, Spain

In this work the application of soft magnetic materials for sensing and actuator purposes is presented. Two main applications are described: (i) vibrational devices for energy harvesting systems and (ii) non-contact position sensors based on the Giant MagnetoImpedance (GMI) Effect. Related to energy harvesting two electromagnetic inductive vibrational prototypes are analyzed: a first one based on high permeability Co-based amorphous ribbons and a moving NdFeB permanent magnet (inertial mass) attached to a mechanical spring; the second, based on a magnetically levitated permanent magnet, where the restoring force appears due to the effective magnetic forces acting on the inertial mass. Finite element analysis (FEA), followed by analytical calculations are employed to optimized the output voltage and the characteristic resonant frequency. Regarding the non-contact position sensor, the sensing device is based on the changes of the high frequency electric impedance of a soft magnetic element as a function of the relative position of a moving permanent magnet. Optimum micrometric sensitivities are achieved and controlled through the magnetic field gradient generated by the permanent magnet and the magnetoimpedance sensor response.


FG-2:IL03  Engineering of Soft Magnetic Properties of Amorphous and Nanocrystalline Magnetic Microwires for Sensor Applications
A. ZHUKOV^1,2,3, M. IPATOV^1,2, A. TALAAT^1,2, J.M. BLANCO², M. CHURYUKANOVA⁴, V. ZHUKOVA^1,2, ¹Dept. Phys. Mater., University of Basque Country, UPV/EHU San Sebastián, Spain; ²Dpto. de Física Aplicada, EUPDS, UPV/EHU, San Sebastian, Spain; ³IKERBASQUE, Basque Foundation for Science, Bilbao, Spain; ⁴National University of Science and Technology «MISIS», Moscow, Russia

Recently studies of magnetic microwires attract great attention owing to excellent magnetic properties such as magnetic bistability, excellent magnetic softness and Giant Magnetoimpedance (GMI) effect [1]. We present an overview of the factors affecting soft magnetic properties, fast domain wall propagation and giant magnetoimpedance (GMI) effect in thin amorphous wires. The magnetoelastic anisotropy is one of the most important parameters that determine the magnetic properties of glass-coated microwires and therefore annealing can be very effective for manipulation the magnetic properties of amorphous ferromagnetic glass-coated microwires. Increasing of DW velocity in Fe-rich and Fe-Ni based (low Ni content) microwires is achieved after annealing. After heat treatment of Co-rich microwires we can observe transformation of inclined hysteresis loops to rectangular and coexistence of fast magnetization switching and GMI effect in the same sample. On the other hand stress annealing of Fe- and Co-rich microwires allows achievement of considerable magnetic softening and GMI effect enhancement.
[1] A. Zhukov, M. Ipatov and V. Zhukova, Advances in Giant Magnetoimpedance of Materials, Handbook of Magnetic Materials, ed. K.H.J. Buschow, 24: chapter 2 (2015) 139-236.


FG-2:IL04  Effect of Stress Components on Magnetostatic and Magnetostrictive Properties of Amorphous Microwires
V. RODIONOVA^1,2, K. CHICHAY¹, I. BARABAN¹, A. LITVINOVA¹, ¹STP ”Fabrika” and Center for Functionalized Magnetic Materials (FunMagMa), Immanuel Kant Baltic Federal University, Kaliningrad, Russia; ²National University of Science and Technology «MISIS», Moscow, Russia

Interest in study of amorphous ferromagnetic microwires in glass coating is caused by the opportunity to easily tune their magnetic properties by changing both the microwire parameters and external conditions. This tunability provides a lot of practical applications of microwires. There are the internal mechanical stress in metallic core of microwire formed during the production process. The stress components have radial distribution and together with a sign of magnetostrictive coefficient defines the micromagnetic structure of microwire. The stress also influences on the magnetostrictive properties themselves. Therefore, formation of micromagnetic structure and, as a consequence, magnetic properties of microwires is a complex process, which is very sensitive to any changes of the microwire parameters. We studied the properties of Fe77.5Si7.5B15 microwires with the metallic core diameter of 2.5–20 mkm and the metallic core diameter/total microwire diameter ratio of 0.1-0.8 in 2 K-1250 K temperature range. Effect of stress components on magnetostatic and magnetostrictive properties of amorphous microwires is studied. The role of partially stress removal with help of glass removal and re-distribution of stress components on magnetic characteristics is analyzed.


FG-2:IL05  Magnetic Properties of Electrocatalytically Active 3D Nanoporous Fe-containing Metallic Films Prepared by Micelle-assisted Electrodeposition
E. PELLICER¹, E. ISARAIN-CHÁVEZ¹, M.D. BARÓ¹, J. SORT^1,2, ¹Departament de Física, Universitat Autònoma de Barcelona, Bellaterra, Spain; ²Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain

Nanoporous iron-platinum (Fe-Pt) films showing either smooth or nodular surfaces have been fabricated by micelle-assisted aqueous electrodeposition from metal chloride salts in the presence of an amphiphilic triblock copolymer (Pluronic P123). The copolymer serves as a structure-directing agent for the formation of the mesopores. As a result, films with regularly spaced sub-15 nm pores are obtained on a variety of surfaces (e.g. Au, Cu and Al). By fine-tuning the electrochemical parameters and the substrate type, large surface area-to-volume ratio Fe-Pt films with spheroidal or very flat morphologies can be obtained. In both cases, soft-magnetic mesoporous films with tuneable saturation magnetization and coercivity values are achieved thanks to their dissimilar chemical composition and crystallographic structure. For example, the nodular mesoporous films possess Fe/Pt weight ratios, disregarding oxygen, varying from 4/96 to 52/48. Even for Pt-rich compositions, the partial alloying of Pt and Fe enables the occurrence of ferromagnetism. Besides their magnetic character, the films are electrochemically active towards hydrogen evolution reaction in both alkaline and acidic media.


FG-2:L06  Multi-parameter Search of Optimal Properties of Soft Magnetic Microwires
A. CHIZHIK¹, J. GONZALEZ¹, A. ZHUKOV^1,2, A. STUPAKIEWICZ³, ¹Universidad del País Vasco, UPV/EHU, San Sebastián, Spain; ²IKERBASQUE, Basque Foundation for Science, Bilbao, Spain; ³Laboratory of Magnetism, University of Bialystok, Bialystok, Poland

Following the modern trend of extended study of cylindrical magnetic objects of micro-scale we direct our efforts to the investigation of soft magnetic microwires. In particular we study the influence of different external parameters on the magnetic, magneto-optic and magneto-electric properties of Co- and Fe- rich microwires. Now we present the results of the study of the magnetic structures transformation in the presence of such external parameters as torsion stress, temperature, DC, pulsed and high frequency electric current. Kerr effect is the main used method. We have established the technological chain which permits to select the optimal properties for sensor application of microwires. Applying heating-cooling cycles we have chosen the basic surface magnetic states. Different torsion stresses lead to stability or meta-stability of different selected states. Additionally we have stabilized the predicted types of surface domain structures. At the finish stage, using DC, pulsed and high frequency electric current we established the sequence of the reversible switching between stable and meta-stable helical states. As we have found, this switching is the key element in control of giant magneto-impedance effect – one of the basic effects in magnetic sensor application.


FG-2:IL07  Soft Magnetic Ferrites for Biomedical Applications
P. TIBERTO, G. BARRERA, F. CELEGATO, M. COISSON, Nanoscience and Materials Division, INRIM, Torino, Italy

Ferrites have been extensively studied in the last decades because of their high versatility, low cost, and ease of tailoring their magnetic properties. Therefore, they have found a variety of applications, as in power and electronics. The oxide-based spinel ferrite nanoparticles are also considered as promising materials for hyperthermia due to their chemical stability and proper self-heating temperature-rising. Ni1 −xZnxFe2O4 ferrites with 0 ≤ x ≤ 0.8 have been prepared by a sol-gel autocombustion method. The specific absorption rate (SAR) has been measured using ad-hoc developed set-ups: static hysteresis loops areas, dynamic hysteresis loops areas and hyperthermia of a water solution. The latter has been fully modelled to provide a direct measurement of SAR of the magnetic nanoparticles by taking into account the heat exchange with the surrounding environment in non-adiabatic conditions and the parasitic heating of the water due to ionic currents. Static hysteresis loops consistently underestimate the specific absorption rate but can be used to select the most promising samples. A means of reliably measure SAR of magnetic nanoparticles by different approaches for hyperthermia applications is presented and its validity discussed by comparing different methods. .


FG-2:IL08  Magnetite Nanoparticles Biosynthesized by Magnetotactic Bacteria for Biomedical Application
M.L. FERNANDEZ-GUBIEDA, Departamento de Electricidad y Electrónica, Universidad del País Vasco UPV/EHU, Leioa, Spain

Magnetotactic bacteria biosynthesize magnetic nanoparticles. These microorganisms are able to mineralize under precise biological control high quality magnetic nanocrystals in terms of chemical purity, morphology, and size. Most of the species of magnetotactic bacteria synthesize magnetite, Fe3O4, and some of them greigite, Fe3S4. A lipid bilayer membrane, around 3 - 4 nm thick, with embedded proteins, surrounds the magnetic nanoparticle. The nanoparticle and its enveloping membrane comprise a magnetosome. Magnetosomes present characteristics of interest for biomedical applications that define a new type of theranostic agent: i) perfectly crystalline and regular nanocrystal of magnetite; ii) high reproducibility; iii) magnetic properties that allow easy separation or guidance; iv) a natural lipid bilayer membrane coating the nanoparticles, conferring electrostatic stability avoiding aggregation; v) an easy functionalization of the lipid bilayer surface with biological functions for cellular targeting or in situ enzymatic catalysis; vi) therapeutic action due to the ferric composition that can generate a localized hyperthermia under focused magnetic excitation. In the talk, I will present a review of the present use of magnetosomes and bacteria themselves as theranostic agent.


Session FG-3 - Magnetocaloric and Multifunctional Magnetic Materials

FG-3:IL02  Thermodynamics of Multicaloric Materials
T. CASTAN, LL. MAÑOSA, A. PLANES, Departament de Física de la Matèria Condensada, Facultat de Física, Universitat de Barcelona, Spain; A. SAXENA, Los Alamos National Laboratory, NM, USA; E. STERN, Department of Material Sciences, University of Cambridge, UK

The talk is aimed at presenting a general thermodynamic framework to deal with caloric effects in multicaloric materials. Caloric effects are quantified by the isothermal entropy change and the adiabatic temperature change induced by the application (or removal) of an external field. Multicaloric materials have the interesting property of responding to the application of more than a single external field and the corresponding caloric effects are interdependent. This is because of the underlying coupling existing between the thermodynamic variables conjugated to the fields. Such coupling is the responsible of the cross-contribution in the total multicaloric effect. The general thermodynamic theory will be particularized to FeRh metamagnetic alloys and magnetic Heusler alloys. Finally, the results will be compared with available experimental data.


FG-3:IL03  Tuning Magnetocaloric Materials with Stress
X. MOYA, Department of Materials Science, University of Cambridge, Cambridge, UK

Mechanical stress can modify the magnitude of a magnetocaloric effect, the operating temperature, and any hysteretic losses. Mechanical stress can also create new phase transitions, permitting large magnetocaloric effects to be explored in an expanded range of materials. Here I will present recent advances on magnetocaloric materials tuned with stress.


FG-3:IL04  Materials with Giant Mechanocaloric Effects: Cooling by Strength
L. MANOSA, A. PLANES, Departament de Física de la Matèria Condensad, Universitat de Barcelona, Barcelona, Spain

The search for caloric materials with large caloric effects has become a major challenge in material science due to their potential in developing near room-temperature solid-state cooling devices which are both efficient and clean and that can succesfully replace present refrigeration technologies . While magnetocaloric and electrocaloric materials have been studied intensively in the last few decades, mechanocaloric materials are only very recently receiving a great deal of attention. Mechanocalorics encompass elastocaloric and barocaloric effects which refer to the materials thermal response to the application of a uniaxial stress and hydrostatic pressure, respectively. Typically, giant caloric effects are observed near ferroic phase transitions, and for the particular case of mechanocaloric effects, the ferroic property is a symmetry adapted combination of components of the strain tensor. In my talk I will present key results for a selection of representative giant mechanocaloric materials [1].
[1] L. Mañosa, A. Planes, Adv. Mater. 2017, 29, 1603607.


FG-3:IL05  Structural Instabilities of Heusler Alloys
P. ENTEL, University Duisburg-Essen, Duisburg, Germany; V. SOKOLOVSKIY, Chelyabinsk State University, Chelyabinsk, Russia

Martensitic transformations of rapidly quenched and less rapidly  cooled Heusler alloys of type Ni-Mn-X with X = Ga, In and Sn are investigated  by ab initiio calculatioms.  For the rapidly cooled alloys, we obtain the well known magnetocaloric properties near the magnetocaloric transition. For the less rapidly quenched  alloys these magnetocaloric properties start to change considerably, This shows that none of the Heusler  alloys is in thermal equilibrium:  Each alloy transforms during  temper-annealing into a dual-phase composite alloy. The two phases are  dentified to be cubic Ni-Mn-X and tetragonal NiMn. We highlight this decomposing phenomenon.
 

FG-3:IL06  Shell Magnetism in Heusler Compounds
M. ACET¹, A. CAKIR², ¹Faculty of Physics, University of Duisburg-Essen, Duisurg, Germany; ²Department of Metallurgical and Materials Engineering, Mugla Sitki Kocman University, Mugla, Turkey

Ternary Heusler alloys in the form Ni50Mn50-xXx (X: Al, Ga, In, Sn, Sb) with 0 < x < 25 transform during temper-annealing into a dual-phase composite alloy. The phases are made up of 2-5 nm cubic ferromagnetic Ni50Mn25X25 precipitates embedded in a tetragonal L10 antiferromagnetic Ni50Mn50 matrix. In such mixed phases, the magnetic-proximity effect can give rise to shell-ferromagnetism when the annealing is carried out under a magnetic field. The shell of the precipitate is strongly pinned in the direction of the annealing field by the antiferromagnet, whereas the core is a loose ferromagnet. The magnetic anisotropy can be adjusted to be in any direction by the direction of the annealing field with respect to the sample geometry. Coercive fields of the shell in the range 5-20 T are observed. Such alloys are candidates for heat and magnetic-field-resistant magnetic recording media.


FG-3:L07  Ni-Mn-In Heusler Alloys Showing both Direct and Inverse Magnetocaloric Effect for Room Temperature Magnetic Refrigeration
S. FABBRICI, MIST E-R scrl, Bologna, Italy; C. BENNATI, R. CABASSI, D. CALESTANI, F. ALBERTINI, IMEM-CNR, Parma, Italy; F. CUGINI, N. SARZI AMADE, M. SOLZI, SMFI Department, University of Parma, Parma, Italy; A. PEPICIELLO, C. VISONE, Engineering dep., University of Sannio, Benevento, Italy

Room temperature magnetic refrigeration requires materials with large isothermal entropy and adiabatic temperature (ΔT_ad) changes at T≈293 K and negligible thermo-magnetic hysteresis when cycled in magnetic fields below 2 T. Ni-Mn based Heuslers hold a great promise thanks to their huge inverse magnetocaloric effect (MCE) at the martensitic transformation, yet fall short in applicability due to the associated hysteresis, which hampers cyclability. Among different strategies to reduce hysteresis, we focus on samples with converging magnetostructural and Curie transitions, which display a quasi-superposition of direct and inverse MCEs. The chosen system is Ni2MnIn based Heusler alloys, gifted with promising values of ΔT_ad at the second order Curie transition. The starting composition, Ni48Mn36In16, is optimized by substituting Fe and Cu at Mn sites, to enhance magnetization and tailor the Curie point, or by varying the Mn-In ratio to tune the distance between the magnetic and structural transitions. In-field calorimetry and ΔT_ad measurements are performed to reveal the reversible and irreversible contributions to the MCE. By studying the ΔT_ad curves we consider the possibility to exploit the concurrent direct and inverse MCEs in alternative refrigeration cycles.


FG-3:IL09  Kinetics of the Heat Flux Avalanches at the First Order Magnetic Transitions in Magnetocaloric Materials
V. BASSO¹, M. PIAZZI^1,2, C. BENNATI³, ¹Istituto Nazionale di Ricerca Metrologica, Torino, Italy; ²Università degli Studi di Pavia, Pavia, Italy; ³Istituto dei Materiali per l’Elettronica ed il Magnetismo - CNR, Parma, Italy

Magnetic refrigeration around room temperature is an environmentally friendly technology relying on the magnetocaloric effect (MCE) of magnetic materials [1]. While large MCE has been identified in first order magnetostructural transitions, little is known about the velocity of the thermal response of the system to the varying magnetic field [2]. In this paper we approach the problem by describing the velocity of the moving phase boundary interface between the ferromagnetic and the paramagnetic phases as the balance between the out-of-equilibrium thermodynamic forces driving the transition and the intrinsic damping mechanisms. The resulting model is able to describe the time behavior of the heat flux signals measured on La(Fe-Si)13, showing individual avalanches varying in shape and number as the temperature and the magnetic field approach the critical point of the system, and to address the intrinsic damping and the microstructure in the MCE phase transition speed [3].
[1] A. Kitanovski et al. Magnetocaloric Energy Conversion, Springer (2015) [2] M. D. Kuz’min Appl. Phys. Lett. 90, 251916 (2007) [3] C. Bennati et al. Appl. Phys. Lett. 109, 231904(2016); M.Piazzi et al, Phys. Rev. Applied 8 044023 (2017); M.Piazzi et al. J. Phys.: Conf. Ser. 903 012046 (2017)


FG-3:IL10  Efficient Energy-conversion near Room-temperature with Transition Metal Based Magnetic Materials
E. BRUECK, N. VAN DIJK, Fundamental Aspects of Materials and Energy, Faculty of Science, TU Delft, Delft, Netherlands

Magnetocaloric power conversion can be used to convert heat into electricity that up to now was considered as waste. This new technology therefore has the potential to significantly contribute to the energy transition on a global scale. With the advent of giant magnetocaloric effects (MCE) that occur in conjunction with magneto-elastic or magnetostructural phase transition of first order (FOT), room temperature heat-pump applications became feasible. In this context the MnFe(P,X) system is of particular interest as it contains earth abundant ingredients that are not toxic. This material family derives from the Fe2P compound, a prototypical example known since a long time to exhibit a sharp but weak FOT at 210 K (-63°C). Magnetocaloric power-conversion calls for a somewhat different combination of properties, in particular a large latent heat that is favourable for a heat-pump, is detrimental for power conversion. Yet a large change of magnetization is required, which suggests one should either employ materials exibiting exchange inversion or second order materials. Magnetically highly responsive materials in combination with the field generated by a permanent magnet open the way to new technology for magnetic refrigeration, heat pumps and power generation.


FG-3:IL11  Magnetocaloric Performance of La(FeSi)13 Compounds
L.E. COHEN, Imperial College London, London, UK

In this work I will discuss our recent progress understand the dynamics of the magnetic transition in La(FeSi)13 as well as the performance under hydrostatic pressure. Although materials which show a strong first order magnetic transition offer advantages in terms of peak entropy and adiabatic temperature, we have revisited La(FeCoSi)13 compounds exploring the control over the magnetic transition temperature and magnetocaloric performance by hydriding using an effective, low temperature and readily available electrochemical method [1]. Discussion of hydrogen segregation in these compounds will be discussed [2].
[1] J. Lyubina, U. Hannemann, M.P. Ryan and L.F. Cohen Adv. Mater. 24 2042 (2012). [2] O. L. Baumfeld, Z. Gercsi, M. Krautz, O. Gutfleisch, and K. G. Sandeman, J. Appl. Phys. 115, 203905 (2014).


FG-3:IL12  Molecular Spin Crossover Crystals as Barocalorics
S. VALLONE^1,2, A.M. DOS SANTOS³, J. MOLAISON³, M. HALCROW⁴, K. SANDEMAN^1,2, ¹The Graduate Center of The City University of New York, USA; ²Brooklyn College of The City University of New York, USA; ³Oak Ridge National Laboratory, USA; ⁴University of Leeds, USA

A change of state from low spin (LS) to high spin (HS) can occur at a so-called spin crossover temperature, T_SCO in compounds where the crystal field splitting of d-orbitals associated with a magnetic moment is of the order of k_BT. Crucially for caloric applications, the change of state from LS to HS can be either continuous or first order, and it can occur at temperatures up to and beyond room temperature. Since SCO compounds are paramagnets, the largest caloric effects will be barocaloric rather than magnetocaloric. In this presentation, we use neutron scattering as well as SQUID measurements to examine the evolution of the structure and transition hysteresis of a first order SCO compound in moderate applied pressures.


FG-3:IL13  Manipulating Magnetic Frustration for Caloric Effects
J.B. STAUNTON, E. MENDIVE-TAPIA, Department of Physics, University of Warwick, Coventry, UK

We discuss the Disordered, Local Moment (DLM) theory of magnetic materials and its quantitative description of the temperature and field dependence of magnetic phase transitions and magnetocaloric effect. The intricate interplay between itinerant electrons and local moments in the theory also enables the variation of the magnetic transitions on both composition and, crucially for other caloric effects, atomic spacing to be described. To illustrate we model how the application of strain affects the 1st. order FM-AFM transition in Fe-Rh and estimate the magnitude of the associated strain-induced isothermal entropy change. Lastly we present our conclusions for novel elastocaloric effect-driven cooling cycles potentially available from Mn antiperovskite nitride refrigerants [1]. These are based on the large adiabatic temperature and isothermal entropy changes which we predict for Mn3GaN that arise from the geometrically frustrated interactions between the Mn local moments. As a consequence our calculated temperature-strain phase diagram predicts two new phases, shows both 1st. and 2nd. order phase transitions and two tricritical points among paramagnetic, ferrimagnetic, collinear and non-collinear AFM states.
[1] J. Zemen, E. Mendive-Tapia et al.,Phys. Rev. B 95, 184438, (2017).


FG-3:IL14  Tuning Magnetism and Functional Properties in Ferromagnetic Shape Memory Films and Nanodisks
F. CASOLI, M. TAKHSHA GHAHFAROKHI, L. NASI, R. CABASSI, F. ALBERTINI, IMEM - CNR, Parma, Italy; S. FABBRICI, MIST E-R Laboratory, Bologna, Italy; F. CELEGATO, G. BARRERA, P. TIBERTO, INRIM, Torino, Italy; M. CAMPANINI, EMPA, Dübendorf, Switzerland; C. MAGEN, Instituto de Nanociencia de Aragón, Zaragoza, Spain; V. GRILLO, NANO - CNR, Modena, Italy

Magnetic shape memory alloys have a great potential for micro- and nano-actuators and energy harvesters, thanks their outstanding magnetomechanical properties. We will show that the microstructure and magnetic properties of Ni-Mn-Ga thin films can be engineered by properly choosing the substrate and growth conditions. The films were epitaxially grown on Cr/MgO(100) by r.f. sputtering, with thicknesses up to 200 nm. We then realized Ni-Mn-Ga nanodots (d=160, 650 nm) by polystyrene-nanosphere lithography and freestanding nanodisks by subsequently removing the Cr underlayer via chemical etching. The microstructure and magnetic configuration of the nanostructures are influenced by the lateral confinement and release from the substrate. Furthermore, by varying temperature and applying a magnetic field to the free-standing nanodisks, we have obtained important microstructural changes, enabling different actuation modes. We have also examined the relation between microstructure and magnetization process, simulating magnetization processes in systems with different orientation and spatial organization of the martensitic twin variants. The micromagnetic simulations show a good agreement with the experimental results.
 
 
Session FG-4 - Magnetic Devices and Components for Energy Applications

FG-4:IL01  The Use of Compositionally Graded Films as Model Systems to Study Magnetic Materials for Energy Applications
N.M. DEMPSEY¹, N.B. DOAN¹, Y. HONG^1,2, I. DE MORAES¹, A. DIAS¹, G. GOMEZ¹, V.M.T.S. BARTHEM^1,3, M. BONFIM⁴, L. RANNO¹, D. GIVORD^1,3, ¹Univ. Grenoble Alpes, CNRS/UGA, Grenoble INP, Institut Néel, Grenoble, France; ²School of Materials Science and Engineering, South China University of Technology, Guangzhou, China; ³Instituto de Física, Universidade Federal do Rio de Janeiro, RJ, Brazil; ⁴DELT, Universidade Federal do Parana, Curitiba, Brazil

Combinatorial thin film studies are being used for the screening and optimization of a range of functional materials [1]. The basic idea is to produce compositionally graded films, to allow high throughput screening of materials properties as a function of composition, as well as other processing parameters such as deposition temperature and post-deposition annealing. In this talk I will present studies of compositionally graded films of two types of magnetic materials of interest for energy applications, namely FeRh, a magnetocaloric material and FePt a hard magnetic one. Films were produced by sputtering targets consisting of an Fe base and asymmetrically overlaid pieces of Rh or Pt. Composition variations were characterised using EDX mapping while structural and magnetic characterisation was performed with SEM, XRD and magnetometry. High throughput magnetic characterisation of the FePt films was achieved using a scanning polar MOKE set-up incorporating a bipolar pulsed magnetic field system [2]. The relationship between processing parameters and structural and magnetic properties will be presented.
[1] M.L. Green, I. Takeuchi, and J. Hattrick-Simpers, J. Appl. Phys. 113 (2013) 231101. [2] A. Dias, Gabriel Gomez, Dominique Givord, et al., AIP ADVANCES 7 (2017) 056227.


FG-4:IL02  Thermomagnetic Energy Generation Based on Magnetic Shape Memory Alloy Films
M. KOHL, M. GUELTIG, H. OSSMER, Institute of Microstructure Technology (IMT), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany; H. MIKI, M. OHTSUKA, Tohoku University, Sendai, Japan

We report on the development of magnetic shape memory alloy (MSMA) films and of dedicated mechanisms for their use in thermomagnetic energy generation at small temperature difference on a miniature scale (cm range). In this realm, conventional heat engines (e.g. Stirling, Brayton) are unsuitable due to limited downscaling potential, while thermoelectric generators become inefficient and require large heat sinks. In contrast, thermomagnetic energy generators (TMGs) based on MSMA films have the potential to overcome current limitations. MSMA materials under investigation are metamagnetic Ni-Co-Mn-In films as well as ferromagnetic Ni-(Co-)Mn-Ga films produced by magnetron sputtering showing large temperature-induced changes of magnetization. Corresponding TMGs consist of a MSMA film cantilever with an integrated pick-up coil at the freely movable end, a miniature magnet and a heat source. Due to the abrupt temperature dependence of magnetization, magnetic attraction forces in the vicinity of the magnet also change abruptly causing a large oscillatory motion. In the presentation, the concepts of frequency up-conversion and of resonant self-actuation are discussed allowing for unprecedented power output exceeding 100 mW·cm^-3, which competes with state-of-the art thermoelectrics.


FG-4:IL03  Magnetocaloric Heat Pumps
C.R.H. BAHL, S. DALL’OLIO, D. ERIKSEN, K. ENGELBRECHT, Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde, Denmark

Magnetocaloric devices are increasing in performance and efficiency. These devices rely on the utilisation of the magnetocaloric effect (MCE) which is present in magnetic materials, most often strongest around the transition temperature. In order to utilise the MCE in devices, a choice of materials must be made according to the strength of the MCE and the availability of transition temperatures in the desired range. A number of magnetocaloric prototype devices have been designed and constructed at the Technical University of Denmark. Recently, a large national project, ENOVHEAT, has resulted in a domestic heat pump optimised and dimensioned to the requirement of a modern Northern European single family house, with a heating power of 1500 W and a temperature span of about 20 K. The considerations going into the design of each part of a prototype e.g. magnet, regenerators and flow system will be discussed, and performance results from the devices will be presented. Permanent magnet assemblies are designed in order to maximise the flux in the required volumes and minimise it where it is not desired. Shaping of the regenerators is optimised to maximise the magnetic flux through them while fully utilising the available gap in the magnetic circuit.


FG-4:IL04  Novel Concept for Caloric Cooling - The Magnetocaloric Heat Pipe
L. MAIER, T. HESS, K. BARTHOLOME', Fraunhofer Institute for Physical Measurement Techniques IPM, Freiburg, Germany

Since the beginning of the last decade, several dozens of magnetocaloric heat pump systems have been built. Basically all of these systems are based on the AMR concept, where a heat transfer fluid is actively pumped through a bed of magnetocaloric material in order to transfer thermal energy from cold to hot side heat exchangers. Hereby several powerful systems were built, generating large temperature spans of more than 50 K while others provided large cooling capacities of several kW. However, up to now no system has been built which provides large temperature span and cooling capacity while having a COP better than standard compressor-based cooling systems. In this work, a new concept of a magnetocaloric heat pump and experimental data demonstrating the proof of concept will be presented. Hereby the heat transfer is realized by the combination of magnetocaloric material with thermal diodes which are based on latent heat transfer. Similar to thermosyphons, thermal energy is efficiently transported by condensation and evaporation processes leading to heat transfer rates, which are several orders of magnitude larger than in conventional AMR-systems. At the same time, no additional pumps are required for transporting the heat exchange fluids, enabling systems with high COPs.