2015 Publications

Needle-type GMR sensor to estimate the magnetic properties of diluted ferrofluid for biomedicine application
H. Shirzadfar, R. Haraszczuk, M. Nadi, D. Kourtiche, S. Yamada, T. Hauet (2015)
Innovation and Research in BioMedical Engineering 36, 178

This paper proposes a needle-type giant magnetoresistance (GMR) sensor for very low magnetic fluid weight density of ferrofluid estimation. The magnetic fluids or ferrofluids due to their biocompatibility are used for various applications in medicine such as cancer treatment, magnetic resonance imagining (MRI), cell separation and also to detect the bacteria. The high sensitivity of GMR sensor gives us to accurately measure the magnetic properties of ferrofluid in a weak magnetic field. The results were obtained by GMR sensor are compared by theoretical results and show that the GMR sensor has a good ability for biofluid magnetic characterization.

A comparative study of triaxial and uniaxial magnetic shields made out of YBCO coated conductors
Wera, L., Fagnard, J.-F., Levin, G., Vanderheyden, B., & Vanderbemden, P. (2015)
Superconductor Science and Technology, 28, 074001

Persistent current loops of arbitrary size can be made from currently manufactured RE123 coated conductors. Our previous work has shown that an assembly of such loops is able to shield effectively quasi static axial magnetic field due to the absence of resistive joint. The shielding effectiveness depends on the aspect ratio and the number of layers. In the present work we study experimentally the detailed magnetic response of two different configurations of the magnetic shields for various orientations of the applied field. Using a 3-axis Hall probe we determine the amplitude of magnetic field generated by the induced persistent currents and its direction with respect to the applied field. The effectiveness of the uniaxial shield decreases strongly when the applied field is not collinear with its axis. In the triaxial (Polywell type) structure comprised of three pairs of coils whose axes are mutually orthogonal, the field attenuation is shown to be only weakly dependent on the direction of the magnetic field. We discuss the properties of the triaxial shield and the ways to improve its screening performance.

Measurements on magnetized GdBCO pellets subjected to small transverse ac magnetic fields at very low frequency: Evidence for a slowdown of the magnetization decay
Fagnard, J.-F., Kirsch, S., Morita, M., Teshima, H., Vanderheyden, B., & Vanderbemden, P. (2015)
Physica C: Superconductivity, 512, 42-53.

Due to their ability to trap large magnetic inductions, superconducting bulk materials can be used as powerful permanent magnets. The permanent magnetization of such materials, however, can be significantly affected by the application of several cycles of a transverse variable magnetic field. In this work, we study, at T = 77 K, the long term influence of transverse ac magnetic fields of small amplitudes (i.e. much smaller than the full penetration field) on the axial magnetization of a bulk single grain superconducting GdBCO pellet over a wide range of low frequencies (1 mHz–20 Hz). Thermocouples are placed against the pellet surface to probe possible self-heating of the material during the experiments. A high sensitivity cryogenic Hall probe is placed close to the surface to record the local magnetic induction normal to the surface. The results show first that, for a given number of applied triangular transverse cycles, higher values of dBapp/dt induce smaller magnetization decays. An important feature of practical interest is that, after a very large number of cycles which cause the loss of a substantial amount of magnetization (depending on the amplitude and the frequency of the field), the rate of the magnetization decay goes back to its initial value, corresponding to the relaxation of the superconducting currents due to flux creep only. In the amplitude and frequency range investigated, the thermocouples measurements and a 2D magneto-thermal modelling show no evidence of sufficient self-heating to affect the magnetization so that the effect of the transverse magnetic field cycles on the trapped magnetic moment is only attributed to a redistribution of superconducting currents in the volume of the sample and not to a thermal effect.

A flux extraction device to measure the magnetic moment of large samples; application to bulk superconductors
Egan, R., Philippe, M., Wera, L., Fagnard, J.-F., Vanderheyden, B., Dennis, A., Shi, Y., Cardwell, D. A., & Vanderbemden, P. (2015)
Review of Scientific Instruments, 86(2), 025107.

We report the design and construction of a flux extraction device to measure the DC magnetic moment of large samples (i.e., several cm3) at cryogenic temperature. The signal is constructed by integrating the electromotive force generated by two coils wound in series-opposition that move around the sample. We show that an octupole expansion of the magnetic vector potential can be used conveniently to treat near-field effects for this geometrical configuration. The resulting expansion is tested for the case of a large, permanently magnetized, type-II superconducting sample. The dimensions of the sensing coils are determined in such a way that the measurement is influenced by the dipole magnetic moment of the sample and not by moments of higher order, within user-determined upper bounds. The device, which is able to measure magnetic moments in excess of 1 Am2 (1000 emu), is validated by (i) a direct calibration experiment using a small coil driven by a known current and (ii) by comparison with the results of numerical calculations obtained previously using a flux measurement technique. The sensitivity of the device is demonstrated by the measurement of flux-creep relaxation of the magnetization in a large bulk superconductor sample at liquid nitrogen temperature (77 K).

Magnetic shielding of an inhomogenous magnetic field source by a bulk superconducting tube
Hogan, K., Fagnard, J.-F., Wera, L., Vanderheyden, B., & Vanderbemden, P. (2015)
Superconductor Science and Technology, 28(3), 035011.

Bulk type-II irreversible superconductors can act as excellent passive magnetic shields, with a strong attenuation of low frequency magnetic fields. Up to now, the performances of superconducting magnetic shields have mainly been studied in a homogenous magnetic field, considering only immunity problems, i.e. when the field is applied outside the tube and the inner field should ideally be zero. In this paper, we aim to investigate experimentally and numerically the magnetic response of a high-Tc bulk superconducting hollow cylinder at 77 K in an emission problem, i.e. when subjected to the non-uniform magnetic field generated by a source coil placed inside the tube. A bespoke 3-D mapping system coupled with a 3-axis Hall probe is used to measure the magnetic flux density distribution outside the superconducting magnetic shield. A finite element model is developed to understand how the magnetic field penetrates into the superconductor and how the induced superconducting shielding currents flow inside the shield in the case where the emitting coil is placed coaxially inside the tube. The finite element modelling is found to be in excellent agreement with the experimental data. Results show that a concentration of the magnetic flux lines occurs between the emitting coil and the superconducting screen. This effect is observed both with the modelling and the experiment. In the case of a long tube, we show that the main features of the field penetration in the superconducting walls can be reproduced with a simple analytical 1D model. This model is used to estimate the maximum flux density of the emitting coil that can be shielded by the superconductor.

Lanthanide Complexes with Multidentate Oxime Ligands as Single-Molecule Magnets and Atmospheric Carbon Dioxide Fixation Systems
M. Hołyńska, R. Clérac and M. Rouzières (2015)
Chemistry-A European Journal, 21(38), 13321-13329

The synthesis, structure, and magnetic properties of five lanthanide complexes with multidentate oxime ligands are described. Complexes 1 and 2 (1: [La2(pop)2(acac)4(CH3OH)], 2: [Dy2(pop)(acac)5]) are synthesized from the 2‐hydroxyimino‐N‐[1‐(2‐pyridyl)ethylidene]propanohydrazone (Hpop) ligand, while 3, 4, and 5 (3: [Dy2(naphthsaoH)2(acac)4H(OH)]⋅0.85 CH3CN⋅1.58 H2O; 4: [Tb2(naphthsaoH)2(acac)4H(OH)]⋅0.52 CH3CN⋅1.71 H2O; 5: [La6(CO3)2(naphthsao)5 (naphthsaoH)0.5(acac)8(CO3)0.5(CH3OH)2.76H5.5(H2O)1.24]⋅2.39 CH3CN⋅0.12 H2O) contain 1‐(1‐hydroxynaphthalen‐2‐yl)‐ethanone oxime (naphthsaoH2). In 1–4, dinuclear [Ln2] complexes crystallize, whereas hexanuclear LaIII complex 5 is formed after fixation of atmospheric carbon dioxide. DyIII‐based complexes 2 and 3 display single‐molecule‐magnet properties with energy barriers of 27 and 98 K, respectively. The presence of a broad and unsymmetrical relaxation mode observed in the ac susceptibility data for 3 suggest two different dynamics of the magnetization which might be a consequence of independent relaxation processes of the two different Dy3+ ions.

Partial Nitrogen Atom Transfer: A New Synthetic Tool to Design Single-Molecule Magnets
M. Ding, M. Rouzières, Y. Losovyj, M. Pink, R. Clérac and J.M. Smith (2015)
Inorg. Chem., 2015, 54 (18), pp 9075–9080

Incomplete nitrogen atom transfer from the iron(IV) nitride complex PhB(MesIm)3Fe≡N to the vanadium(III) complex V(Mes)3(THF) quantitatively provides the bimetallic complex PhB(MesIm)3Fe-N═V(Mes)3. Structural and spectroscopic characterizations reveal that the nitride ligand forms a linear bridge between V(V) and high-spin Fe(II) metal ions, confirming that atom transfer is accompanied by electron transfer. In the presence of an applied dc field, the complex displays slow relaxation of the magnetization, revealing its single-molecule magnet properties with an estimation of the energy barrier at about 10 K. This complex establishes a synthetic principle for the assembly of paramagnetic complexes bridged by nitride ligands.

Coordination Complexes of a Neutral 1,2,4-Benzotriazinyl Radical Ligand: Synthesis, Molecular and Electronic Structures, and Magnetic Properties
I.S. Morgan, A. Mansikkamäki, G.A. Zissimou, P.A. Koutentis, M. Rouzières, R. Clérac and H.M. Tuononen (2015)
Chemistry-A European Journal, 21(44), 15843-15853

A series of d‐block metal complexes of the recently reported coordinating neutral radical ligand 1‐phenyl‐3‐(pyrid‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl (1) was synthesized. The investigated systems contain the benzotriazinyl radical 1 coordinated to a divalent metal cation, MnII, FeII, CoII, or NiII, with 1,1,1,5,5,5‐hexafluoroacetylacetonato (hfac) as the auxiliary ligand of choice. The synthesized complexes were fully characterized by single‐crystal X‐ray diffraction, magnetic susceptibility measurements, and electronic structure calculations. The complexes [Mn(1)(hfac)2] and [Fe(1)(hfac)2] displayed antiferromagnetic coupling between the unpaired electrons of the ligand and the metal cation, whereas the interaction was found to be ferromagnetic in the analogous NiII complex [Ni(1)(hfac)2]. The magnetic properties of the complex [Co(1)(hfac)2] were difficult to interpret owing to significant spin–orbit coupling inherent to octahedral high‐spin CoII metal ion. As a whole, the reported data clearly demonstrated the favorable coordinating properties of the radical 1, which, together with its stability and structural tunability, make it an excellent new building block for establishing more complex metal–radical architectures with interesting magnetic properties.

A green approach to magnetically-hard electrically-conducting polyaniline/CoFe2O4 nanocomposites
C. Della Pina, A. M. Ferretti, A. Ponti, E. Falletta (2015)
Composites Science and Technology, 110, 138-144

Magnetically-hard, electrically-conducting polyaniline/CoFe2O4 nanocomposites were prepared by oxidative polymerization of N-(4-aminophenyl)aniline using molecular oxygen or hydrogen peroxide as the oxidants and magnetic CoFe2O4 nanoparticles, both uncoated and oleic acid-coated, with the double role of polymerization catalyst and magnetic filler. Oleic acid-coated nanoparticles showed higher catalytic activity than uncoated ones, especially under aerobic conditions. The size of the nanoparticles did not undergo significant changes during the polymerization process. The nanocomposites are magnetically hard with large remanence/saturation ratio, very large coercivity (8–15 kOe at 5 K) and do not display superparamagnetic effects even at RT. The addition of Fe3+ as a further oxidant allowed to tune the electroconductive properties of the materials, with conductivity ranging from 7.3 ⋅ 10−5 S/cm to 5.5 ⋅ 10−3 S/cm.

Nanoferrites as catalysts and fillers for polyaniline/nanoparticle composites preparation
E. Falletta, A. Ponti, A. Sironi, A. M. Ferretti, C. Della Pina (2015)
Journal of Advanced Catalysis Science and Technology, 2(2), 8-16

In this paper, we report on the catalytic activity of MFe2O4 nanoparticles (M = Mn, Ni, Cu, Zn, Mg) in the oxidative polymerization of N-(4-aminophenyl)aniline yielding polyaniline/nanoparticle composites. Hydrogen peroxide is used as the oxidizing agent and ferrite nanoparticles play the dual role of reaction catalysts and fillers. The obtained polyaniline/MFe2O4 composites maintained the pristine size of the ferrite crystallites and showed a modest conductive behavior. Polyaniline/MFe2O4 (M = Mn, Ni) composites displayed good dispersion of the nanoparticles in the polyaniline matrix and magnetic hardness intermediate between those of soft M = Fe and hard M = Co composites, evidencing the tunability of the magnetic properties.

Lorentz microscopy sheds light on the role of dipolar interactions in magnetic hyperthermia
M. Campanini, R. Ciprian, E. Bedogni, A. Mega, V. Chiesi, F. Casoli, C. de Julian Fernandez, E. Rotunno, F. Rossi, A. Secchi, F. Bigi, G. Salviati, C. Magen, V. Grillo, F.Albertini (2015)
Nanoscale, 7(17), 7717-7725

Monodispersed Fe3O4 nanoparticles with comparable size distributions have been synthesized by two different synthesis routes, co-precipitation and thermal decomposition. Thanks to the different steric stabilizations, the described samples can be considered as a model system to investigate the effects of magnetic dipolar interactions on the aggregation states of the nanoparticles. Moreover, the presence of magnetic dipolar interactions can strongly affect the nanoparticle efficiency as a hyperthermic mediator. In this paper, we present a novel way to visualize and map the magnetic dipolar interactions in different kinds of nanoparticle aggregates by the use of Lorentz microscopy, an easy and reliable in-line electron holographic technique. By exploiting Lorentz microscopy, which is complementary to the magnetic measurements, it is possible to correlate the interaction degrees of magnetic nanoparticles with their magnetic behaviors. In particular, we demonstrate that Lorentz microscopy is successful in visualizing the magnetic configurations stabilized by dipolar interactions, thus paving the way to the comprehension of the power loss mechanisms for different nanoparticle aggregates.

Achieving Giant Magnetically Induced Reorientation of Martensitic Variants in Magnetic Shape-Memory Ni–Mn–Ga Films by Microstructure Engineering
P. Ranzieri, M. Campanini, S. Fabbrici, L. Nasi, F. Casoli, R. Cabassi, E. Buffagni, V. Grillo, C. Magén, F. Celegato, G. Barrera, P. Tiberto, and F. Albertini (2015)
Advanced Materials, 27(32), 4760-4766

Giant magnetically induced twin variant reorientation, comparable in intensity with bulk single crystals, is obtained in epitaxial magnetic shape‐memory thin films. It is found to be tunable in intensity and spatial response by the fine control of microstructural patterns at the nanoscopic and microscopic scales. A thorough experimental study (including electron holography) allows a multiscale comprehension of the phenomenon.

Synthesis of magnetic zeolite at low temperature using a waste material mixture: Fly ash and red mud
C. Belviso, E. Agostinelli, S. Belviso, F. Cavalcante, S. Pascucci, D. Peddis, G. Varvaro, S. Fiore (2015)
Microporous and Mesoporous Materials, 202, 208-216

A low temperature environmentally friendly synthesis of magnetic zeolites by hydrothermal activation is reported. The novelty of the process is related to the use of a mixture of waste materials (fly ash (FA) and red mud (RM)) as precursors for the one-step synthesis (without passing through the additional synthesis of magnetic nanoparticles) of zeolites with good magnetic properties. The structural and magnetic investigation indicated that different types of zeolites were obtained for different FA/RM percentages and incubation temperatures, and all of these zeolites possess sufficiently high magnetic moments to enable their easy separation from the solution using an external magnet. Therefore, the time consuming and expensive high performance centrifugation processes, which are typically employed to recover zeolites, can be eliminated. In detail, sodalite and mixed Ti–Fe oxides formed using the 80% RM mixture, and a higher amount of A-type zeolites was observed for the 50% RM mixture. However, a mixture of A-, X- and ZK-5-type zeolites was obtained using 20% RM. The global magnetic properties of the newly formed minerals are discussed based on the magnetic properties of the precursors in which different magnetic behaviours were observed. A preliminary characterisation of the synthetic products was performed.

Magnetic field-induced self-assembly of iron oxide nanocubes
G. Singh, H. Chan, T. Udayabhaskararao, E. Gelman, D. Peddis, A. Baskin, G. Leitus, P. Kral, R. Klajn (2015)
Faraday discussions, 181, 403-421

Self-assembly of inorganic nanoparticles has been studied extensively for particles having different sizes and compositions. However, relatively little attention has been devoted to how the shape and surface chemistry of magnetic nanoparticles affects their self-assembly properties. Here, we undertook a combined experiment–theory study aimed at better understanding of the self-assembly of cubic magnetite (Fe3O4) particles. We demonstrated that, depending on the experimental parameters, such as the direction of the magnetic field and nanoparticle density, a variety of superstructures can be obtained, including one-dimensional filaments and helices, as well as C-shaped assemblies described here for the first time. Furthermore, we functionalized the surfaces of the magnetic nanocubes with light-sensitive ligands. Using these modified nanoparticles, we were able to achieve orthogonal control of self-assembly using a magnetic field and light.

Coexistence of superconductivity and ferromagnetism in cluster-assembled Sn–Co nanocomposites
K. Houben, E. Menéndez, C. P. Romero, M. Trekels, T. Picot, A. Vantomme, K. Temst, M. J. Van Bael (2015)
Journal of Alloys and Compounds, 637, 509-516

The coexistence of superconductivity and ferromagnetism is investigated in granular Sn–Co nanocomposites. The nanocomposites have been prepared by co-deposition of Sn atoms and Co clusters, the morphology and composition of which can be tuned by varying the deposition rate of Co clusters relative to Sn atoms. Flat isolated Sn islands are obtained at zero or low Co cluster flux, while granular nanocomposites are formed with increasing Co cluster flux, reaching Co concentrations up to 44 vol.%. Interfaces with a low electronic transparency between superconductor and ferromagnet are obtained by a combination of the granular nature of the nanocomposites and the formation of Sn–Co alloys at the Sn/Co interfaces. The structure and composition of the nanocomposites have been thoroughly characterized by atomic force microscopy, X-ray diffraction and conversion electron Mössbauer spectroscopy. Over the entire Co concentration range, the hybrids show a ferromagnetic response. The superconducting phase boundary and the Meissner response depend on the morphology and composition of the nanocomposites. In particular, the superconducting critical temperature decreases with increasing Co concentration, while the Meissner response varies from a reversible to a strongly hysteretic behaviour depending on the morphology of the samples with different Co content. The persistence of superconductivity at high Co concentrations is attributed to a suppression of the superconducting proximity effect in these nanocomposites, which is ascribed to the low interface transparency between the Sn and Co components that make up these hybrid systems.

Increased magnetic moment induced by lattice expansion from α-Fe to α′-Fe8N
I. Dirba, P. Komissinskiy, O. Gutfleisch, and L. Alff (2015)
Journal of Applied Physics 117, 173911

Buffer-free and epitaxial α-Fe and α′-Fe8Nx thin films have been grown by RF magnetron sputtering onto MgO (100) substrates. The film thicknesses were determined with high accuracy by evaluating the Kiessig fringes of X-ray reflectometry measurements allowing a precise volume estimation. A gradual increase of the nitrogen content in the plasma led to an expansion of the iron bcc unit cell along the [001] direction resulting finally in a tetragonal distortion of about 10% corresponding to the formation of α′-Fe8N. The α-Fe lattice expansion was accompanied by an increase in magnetic moment to 2.61 ± 0.06μB per Fe atom and a considerable increase in anisotropy. These experiments show that—without requiring any additional ordering of the nitrogen atoms—the lattice expansion of α-Fe itself is the origin of the increased magnetic moment in α′-Fe8N.

Growth, structure, and magnetic properties of γ′-Fe4N thin films
I. Dirba, M. Baghaie Yazdi, A. Radetinac, P. Komissinskiy, S. Flege, O. Gutfleisch, and L. Alff (2015)
Journal of Magnetism and Magnetic Materials, 379, 151-155

We have grown phase-pure, buffer-free, and epitaxial α-Fe and γ′-Fe4N thin films by RF magnetron sputtering onto MgO (100) substrates. The film thicknesses and densities have been determined with high accuracy by evaluating the Kiessig fringes of the X-ray reflectometry measurements. We have determined the volume saturation magnetization of γ′-Fe4N with corresponding accuracy to be Ms = 1556±62 emu/cm3 at 10 K or 2.30±0.09 μB per Fe atom. The Curie-temperature, TC, of the γ′-Fe4N thin films was 716 K. The slightly increased average magnetic moment per Fe atom of γ′-Fe4N as compared to α-Fe is in agreement with literature values and attributed to the volume expansion of the unit cell.

Magnetic ordering in magnetic shape memory alloy Ni-Mn-In-Co
K. Ollefs, Ch. Schöppner, I. Titov, R. Meckenstock, F. Wilhelm, A. Rogalev, J. Liu, O. Gutfleisch, M. Farle, H. Wende, and M. Acet (2015)
Phys. Rev. B 92, 224429

Structural and magnetic properties across the martensite-austenite phase transitions in the shape memory alloy Ni-Mn-In-Co are studied using complementary experimental techniques: ferromagnetic resonance, macroscopic magnetization measurements, and x-ray magnetic circular dichroism in the temperature range from 5 to 450 K. Ferromagnetic resonance experiments show coexisting antiferromagnetic and ferromagnetic correlations for the martensite phase and ferromagnetic and paramagnetic correlations in the austenite phase. Magnetization measurements reveal spin-glass-like behavior for T<30 K and Ni and Co K-edge x-ray magnetic circular dichroism measurements confirm an assignment of a ferromagnetic resonance line purely to Ni (and Co) for a wide temperature range from 125 to 225 K. Hence a combined analysis of ferromagnetic resonance and x-ray magnetic circular dichroism allows us to attribute particular magnetic resonance signals to individual elemental species in the alloy. Magnet properties of Mn70Ga30 prepared by cold rolling and magnetic field annealing
S. Ener, K. P. Skokov, D. Y. Karpenkov, M. D. Kuz’min, and O. Gutfleisch (2015)
Journal of Magnetism and Magnetic Materials, 382, 265-270

The remanence and coercivity of arc melted Mn70Ga30 can be substantially improved by cold rolling. For best performance the rolled material should be annealed at T=730 K in the presence of a magnetic field of 1 T. The so-obtained magnet has a remanence of 0.239 T and a coercivity of 1.24 T at room temperature. The underlying reason for the high coercivity and remanence is the increase of the content of a metastable ferrimagnetic D022 phase at the expense of the normally stable anti-ferromagnetic D019. Magnetic field significantly increases the nucleation rate of the ferromagnetic D022 phase that leads to grain size refinement and as a consequence of improving remanence and coercive field.

Preparation, Characterization, and Modeling of Ultrahigh Coercivity Sm–Co Thin Films
O. Akdogan, H. Sepehri‐Amin, N. M. Dempsey, T. Ohkubo, K. Hono, O. Gutfleisch, T. Schrefl, and D. Givord (2015)
Advanced Electronic Materials, 1(5), 1500009

Isotropic Sm–Co thin films with different SmxCoy phases (1:7, 1:5, and 2:7) are prepared by triode sputtering of targets of variable composition. A room‐temperature coercivity value of 6.8 T is achieved in the film with the SmCo5 phase. Transmission electron microscopy (TEM) and 3D atom probe analyses of films that comprise this compound reveal the presence of Sm‐rich 4‐nm‐sized precipitates within grains and along grain boundaries. Atomic‐resolution scanning transmission electron microscopy/high‐resolution high‐angle annular dark‐field (STEM/HAADF) imaging show that stacking faults occur within SmCo5 grains, which correspond to local phase variants including Sm2Co7, Sm5Co19 and SmCo3. The contribution to domain wall pinning of precipitates and stacking faults, as well as grain boundaries between misaligned grains, is discussed semi‐quantitatively. Micromagnetic simulations are carried out to evaluate the influence of stacking faults and grain boundaries on magnetization reversal. The results indicate that the high coercivity values achieved can mainly be attributed to the strong pinning of magnetic domains at the grain boundaries of randomly oriented SmCo5 nanograins.

Effect of severe plastic deformation on the specific heat and magnetic properties of cold rolled Gd sheets
S. Taskaev, K. Skokov, V. Khovaylo, V. Buchelnikov, A. Pellenen, D. Karpenkov, M. Ulyanov, D. Bataev, A. Usenko, M. Lyange, and O. Gutfleisch (2015)
Journal of Applied Physics, 117(12), 123914

We report on specific heat and magnetic properties of thin Gd sheets obtained by means of a cold rolling technique. At temperatures well below Curie temperature TC, the cold rolling has a minor impact on the specific heat Cp. However, a well defined λ-type anomaly of Cp seen in the vicinity of TC in a polycrystalline Gd sample is markedly suppressed in the severely deformed samples. Depression of the λ peak is due to a large decrease of magnetization that presumably originates in a local magnetic anisotropy induced by the severe plastic deformation. Results of calculation of magnetocaloric effect from the Cp and magnetization data indicate that the magnetocaloric effect gradually decreases as the degree of plastic deformation increases. This trend is further confirmed by the direct measurements of the adiabatic temperature change ΔTad.

Mechanism of the texture development in hydrogen-disproportionation–desorption-recombination (HDDR) processed Nd–Fe–B powders
H. Sepehri-Amin, T. Ohkubo, K. Hono, K. Güth, O. Gutfleisch (2015)
Acta Materialia, 85, 42-52

Microstructure evolution in Nd12.8Fe80.1B6.6Ga0.3Nb0.2 alloy powders at different hydrogen pressures during the hydrogen-disproportionation process has been studied in order to understand the underlying mechanism of the texture development during the dynamic hydrogen-disproportionation–desorption-recombination (d-HDDR) process. Transmission electron microscopy showed that Fe2B grains “memorize” the crystallographic orientation of the initial Nd2Fe14B phase and transfer it to the recombined Nd2Fe14B grains in the highly textured sample. 3-D tomography of backscattered electron SEM images showed that recombined Nd2Fe14B grains nucleate at the interfaces of Fe2B/NdH2 phases, which grow through the interfaces of NdH2/α-Fe phases during the DR process. Boron segregation was found at the NdH2/α-Fe interfaces, which serve as a boron source for the growth of the recombined Nd2Fe14B grains. The mechanism of the texture development is discussed based on the microstructure observations and the corresponding evolution of magnetic properties.

P. Mcguiness, O. Akdogan, A. Asali, S. Bance, F. Bittner, J. M. D. Coey, N. M. Dempsey, J. Fidler, D. Givord, O. Gutfleisch, M. Katter, D. Le Roy, S. Sanvito, T. Schrefl, L. Schultz, C. Schwöbl, M. Soderžnik, S. Šturm, P. Tozman, K. Üstüner, M. Venkatesan, T. G. Woodcock, K. Žagar, S. Kobe (2015)
Acta Materialia, 85, 42-52

The rare-earth crisis, which peaked in the summer of 2011 with the prices of both light and heavy rare earths soaring to unprecedented levels, brought about the widespread realization that the long-term availability and price stability of rare earths could not be guaranteed. This triggered a rapid response from manufacturers involved in rare earths, as well as governments and national and international funding agencies. In the case of rare-earth-containing permanent magnets, three possibilities were given quick and serious consideration: (I) increased recycling of devices containing rare earths; (II) the search for new, mineable, rare-earth resources beyond those in China; and (III) the development of high-energy-product permanent magnets with little or no rare-earth content used in their manufacture. The Replacement and Original Magnet Engineering Options (ROMEO) project addresses the latter challenge using a two-pronged approach. With its basis on work packages that include materials modeling and advanced characterization, the ROMEO project is an attempt to develop a new class of novel permanent magnets that are free of rare earths. Furthermore, the project aims to minimize rare-earth content, particularly heavy-rare-earth (HRE) content, as much as possible in Nd-Fe-B-type magnets. Success has been achieved on both fronts. In terms of new, rare-earth-free magnets, a Heusler alloy database of 236,945 compounds has been narrowed down to approximately 20 new compounds. Of these compounds, Co2MnTi is expected to be a ferromagnet with a high Curie temperature and a high magnetic moment. Regarding the reduction in the amount of rare earths, and more specifically HREs, major progress is seen in electrophoretic deposition as a method for accurately positioning the HRE on the surface prior to its diffusion into the microstructure. This locally increases the coercivity of the rather small Nd-Fe-B-type magnet, thereby substantially reducing the dependence on the HREs Dy and Tb, two of the most critical raw materials identified by the European Commission. Overall, the ROMEO project has demonstrated that rapid progress can be achieved when experts in a specific area are brought together to focus on a particular challenge. With more than half a year of the ROMEO project remaining, further progress and additional breakthroughs can be expected.