High and Ultra-High Coercive Materials in Spring-Exchange Systems—Review, Simulations and Perspective

Liu X., Zuo S., Wang H., Zhang T., Dong Y., Jiang C.

Exchange-coupled core-shell nanoparticles are expected to be the new generation of permanent magnets, where the orientation of the hard magnetic phase is supposed to play a key role in improving their magnetic performance. In this work, L10-FePt/Co core-shell nanoparticles with Co thickness ranging from 0.6 to 2.2 nm have been synthesized by a seed-mediated growth method. The exchange coupling effect between the hard core and soft shell led to a 60% improvement of the maximum magnetic energy product ((BH)max), compared with the pure L10-FePt core. By tuning the amount of precursor, nanoparticles with different Co shell thicknesses were synthesized. Furthermore, the L10-FePt/Co core-shell nanoparticles were dispersed in epoxy resin and oriented under an external magnetic field. The (BH)max of the anisotropic nanocomposite magnet with a Co thickness of 1 nm is 7.1 MGOe, enhanced by 117% compared with the isotropic L10-FePt magnet, which paves the way for the development of high-performance permanent magnets for energy conversion applications.

Ziółkowski G., Chrobak D., Chełkowska G., Zivotsky O., Chrobak A.

The paper refers to Monte Carlo magnetic simulations for fractal-like nano and mesoscopic grains. The analyzed objects differed in the size, surface development, magnetic anisotropy and the spin values attributed to the system nodes inside the fractal. Such an approach allowed us to determine their magnetization processes as well as optimization characteristics in the direction to enhancement of hard magnetic properties. As it was shown, the size effects depend on the chosen value of magnetic anisotropy. In the case of fractals with ultra-high coercivity, the decreasing of their size leads to deterioration of coercivity, especially for the high surface to volume ratio. Opposite effects were observed for soft magnetic fractals when the nanostructure caused an appearance of the coercive field, and the maximum of energy product was predictably significantly higher than for conventional rare earths’ free permanent magnets.

Ma Q., Jia M., Hu Z., Yue M., Liu Y., Zhao T., Shen B.

Bulk anisotropic Sm2Co7 nanocrystalline magnets were successfully prepared by hot deformation process using spark plasma sintering technology. The coercivity of the isotropic Sm2Co7 nanocrystalline magnet is 34.76 kOe, further, the ultra-high coercivity of 50.68 kOe is obtained in the anisotropic hot deformed Sm2Co7 magnet when the height reduction is 70%, which is much higher than those of the ordinarily produced hot deformed Sm2Co7 magnet. X-ray diffraction (XRD) analysis shows that all the samples are Sm2Co7 single phase. The investigation by electron backscatter diffraction indicates that increasing the amount of deformation is beneficial to the improvement of the (00l) texture of Sm2Co7 magnets. The Sm2Co7 nanocrystalline magnet generates a strong c-axis crystallographic texture during large deformation process.

Zhuge Y., Li Y., Xu X., Zhang D., Zhang H., Liu W., Yue M.

In this article, the Sm2Co7/α-Fe nanocomposite magnets were prepared by high energy ball milling and spark plasma sintering method. The effect of soft phase content on the magnetic properties was studied. Up to 30 wt% α-Fe was added into Sm2Co7 matrix without the decrease of remanence. Optimal energy product (BH)max of 9.2MGOe was obtained with 20 wt% α-Fe. TEM observation shows that the grain size of α-Fe is 20–50 nm which ensures a good coupling effect between soft and hard phase. One more thing needs to be mentioned is that there exists inter-diffusion between Sm–Co phase and α-Fe phase. Moreover, our results can also illustrate that the Sm2Co7/α-Fe nanocomposite magnets are able to acquire better magnetic properties than the SmCo5/α-Fe magnets prepared by the same process due to the large domain width of Sm2Co7 phase.

Hasegawa T.

Chrobak A., Ziółkowski G., Chrobak D., Chełkowska G.

This paper refers to Monte Carlo magnetic simulations for large-scale systems. We propose scaling rules to facilitate analysis of mesoscopic objects using a relatively small amount of system nodes. In our model, each node represents a volume defined by an enlargement factor. As a consequence of this approach, the parameters describing magnetic interactions on the atomic level should also be re-scaled, taking into account the detailed thermodynamic balance as well as energetic equivalence between the real and re-scaled systems. Accuracy and efficiency of the model have been depicted through analysis of the size effects of magnetic moment configuration for various characteristic objects. As shown, the proposed scaling rules, applied to the disorder-based cluster Monte Carlo algorithm, can be considered suitable tools for designing new magnetic materials and a way to include low-level or first principle calculations in finite element Monte Carlo magnetic simulations.

Shao Z., Ren S.

Iron-based rare-earth-free hard magnets achieved by the combination of iron and another element.

Algarou N.A., Slimani Y., Almessiere M.A., Baykal A., Guner S., Manikandan A., Ercan I.

Hard/soft SrCo0.02Zr0.02Fe11.96O19/MFe2O4 (M = Ni, Co, Cu, Mn and Zn) nanocomposites have been fabricated efficiently via one-pot sol-gel combustion route. The influence of different type of spinel were examined by XRD (X-ray diffraction), SEM – TEM (scanning and transmission electron microscopies) systems and VSM (vibrating sample magnetometer). The XRD investigation of Hard/soft nanocomposites revealed the tailoring between hexaferrite and spinel ferrite phases. Magnetization measurements (M vs. H) were performed at room (T = 300 K) and low temperature (T = 10 K) and diverse magnetic parameters comprising Ms (saturation magnetization), Mr (remanence), coercivity (Hc), and so on were determined. Smooth M vs. H curves and single peaks in dM/dH vs. H plots were observed for various prepared nanocomposites. The Henkel plots and maximum energy products were also determined and analyzed. Various prepared hard/soft nanocomposites indicated relatively high values of diverse magnetic parameters. Diverse findings disclosed the incidence of exchange-coupling spring behavior among various soft and hard magnetic phases. Measurements of ZFC-FC magnetizations showed the existence of a peak temperature values, which are largely ascribed to competition between the motion of magnetic domain walls and thermal activations. It is revealed that one-pot sol-gel combustion route is valuable to attain strong exchange coupling among hard and soft nanoparticle phases grown near to each other. The obtained findings indicated that these nanocomposites are promising candidates for numerous practical applications.

Coey J.M.

Rare earth permanent magnets constitute a mature technology, but the shock of the 2011 rare earth crisis led to the re-evaluation of many ideas from the 1980s and 1990s about possible new hard magnets containing little or no rare earth (or heavy rare earth). Nd–Fe–B magnets have been painstakingly and skillfully optimized for a wide range of applications in which high performance is required at reasonable cost. Sm–Co is the material of choice when high-temperature stability is required, and Sm–Fe–N magnets are making their way into some niche applications. The scope for improvement in these basic materials by substitution has been rather thoroughly explored, and the effects of processing techniques on the microstructure and hysteresis are largely understood. A big idea from a generation ago—which held real potential to raise the record energy product significantly—was the oriented exchange-spring hard/soft nanocomposite magnet; however, it has proved very difficult to realize. Nevertheless, the field has evolved, and innovation has flourished in other areas. For example, electrical personal transport has progressed from millions of electric bicycles to the point where cars and trucks with electrical drives are becoming mainstream, and looks ready to bring the dominance of the internal combustion engine to an end. As the limitations of particular permanent magnets become clearer, ingenuity and imagination are being used to design around them, and to exploit the available mix of rare earth resources most efficiently. Huge new markets in robotics beckon, and the opportunities offered by additive manufacturing are just beginning to be explored. New methods of increasing magnet stability at elevated temperature are being developed, and integrated multifunctionality of hard magnets with other useful properties is now envisaged. These themes are elaborated here, with various examples.

Ziółkowski G., Chrobak A., Klimontko J., Granek K., Chrobak D.

In the present work, we are focused on the influence of Tb/Y substitution on structural and magnetic properties of (Fe80Nb6B14)0.88Tb0.12-xYx (x = 0.02, 0.04, 0.06, 0.08, 0.1 and 0.12) nanocrystalline bulk alloys. The samples were prepared in the form of rods using the vacuum suction technique. The base alloy (x = 0) is characterized by an ultra-high coercivity and the Tb/Y substitution leads to an increase in magnetic saturation from 38 emu/g to 112 emu/g and the decrease in coercivity from 5.88 T to 0.05 T. However, optima of magnetic remanence and |BH|max parameter were observed. It was shown that the changes in magnetic properties are related to a partial replacement of Tb by Y in the unit cell of the Tb/Y2Fe14B phase. In the paper, the optimization of hard magnetic properties as well as its relation to phase composition and microstructure are widely discussed.

Chrobak A., Ziółkowski G., Granek K., Chrobak D.

The paper presents a modification of the so-called adding probability used in the Monte Carlo cluster algorithms . Certainly, the probability formula was supplemented by an entropy factor, depending on the local disorder of some selected properties of the studied system. The idea bases on the fact that disorder of some physical properties may affect clusterization of the system. The proposed modification enhances effectiveness of the simulations that is especially important in analysis of magnetization processes of irregular multiphase systems. In order to prove correctness of our approach a set of reverse magnetization curves was simulated for the systems composed of magnetically hard and soft phases.

Liu S.

Torkian S., Ghasemi A.

Ferrite based hard-soft magnetic nanocomposites with the composition of (100 − x) SrFe 10 Al 2 O 19 /(x) Co 0.8 Ni 0.2 Fe 2 O 4 , where x = 10, 15, 20 and 30 wt%, were prepared by one-pot sol-gel auto-combustion method. The nanocomposites prepared by this method showed crystallographically two phase behavior, but magnetically a good single phase exchange spring-coupled behavior with high saturation as well as high coercivity. The nature of magnetic interactions in these nanocomposites and their magnetic properties were evaluated in detail using various characteristic methods including vibrating sample magnetometer measurements, Henkel plots measurements, magnetic force microscopy and magneto-optical Kerr effect microscopy. Microstructural evolution showed formation of soft phase below the critical size of exchange length in the vicinity of hard phase. The nanocomposite with 15 wt% of soft phase showed a 10.5% increase in the maximum energy product compared to pure hard phase and reaches as high as 29.5 kJ/m 3 that is a high value of ( BH ) max for this type of materials.

Cui J., Kramer M., Zhou L., Liu F., Gabay A., Hadjipanayis G., Balasubramanian B., Sellmyer D.

Permanent magnets (PM) are critical components for electric motors and power generators. Key properties of permanent magnets, especially coercivity and remanent magnetization, are strongly dependent on microstructure. Understanding metallurgical processing, phase stability and microstructural changes are essential for designing and improving permanent magnets. The widely used PM for the traction motor in electric vehicles and for the power generator in wind turbines contain rare earth elements Nd and Dy due to their high maximum energy product. Dy is used to sustain NdFeB's coercivity at higher temperature. Due to the high supply risk of rare earth elements (REE) such as Dy and Nd, these elements are listed as critical materials by the U.S. Department of Energy and other international institutes. Other than Dy, finer grain size is also found to have effect on sustaining coercivity at higher temperature. A proper control of phase stability and microstructures has direct impact on mitigating REE supply risk. Compared to rare earth PMs, non-rare earth (non-RE) PMs typically have lower maximum energy products, however, given their small supply risks and low cost, they are being intensively investigated for less-demanding applications. The general goal for the development of non-RE PMs is to fill in the gap between the most cost-effective but low performing hard ferrite magnet and the most expensive but high performing RE PMs. In the past five years great progress has been made toward improving the microstructure and physical properties of non-RE PMs. Several new candidate materials systems were investigated, and some have showed realistic potential for replacing RE PMs for some applications. In this article, we review the science and technology of various types of non-RE materials for PM applications. These materials systems include Mn based, high magnetocrystalline anisotropy alloys (MnBi and MnAl compounds), spinodally decomposing alloys (Alnico), high-coercivity tetrataenite L10 phase (FeNi and FeCo), and nitride/carbide systems (such as α based, high saturation magnetization Fe16N2 type phase and Co2C/Co3C acicular particle phase). The current status, challenges, potentials as well as the future directions for these candidates non-RE magnet materials are discussed.

Goto S., Kura H., Watanabe E., Hayashi Y., Yanagihara H., Shimada Y., Mizuguchi M., Takanashi K., Kita E.

Tetrataenite (L10-FeNi) is a promising candidate for use as a permanent magnet free of rare-earth elements because of its favorable properties. In this study, single-phase L10-FeNi powder with a high degree of order was synthesized through a new method, nitrogen insertion and topotactic extraction (NITE). In the method, FeNiN, which has the same ordered arrangement as L10-FeNi, is formed by nitriding A1-FeNi powder with ammonia gas. Subsequently, FeNiN is denitrided by topotactic reaction to derive single-phase L10-FeNi with an order parameter of 0.71. The transformation of disordered-phase FeNi into the L10 phase increased the coercive force from 14.5 kA/m to 142 kA/m. The proposed method not only significantly accelerates the development of magnets using L10-FeNi but also offers a new synthesis route to obtain ordered alloys in non-equilibrium states.

Ziolkowski G., Chrobak A., Chrobak D.

PurposeThe presentation refers to simulations of magnetization processes of the spring-exchange magnetic composites containing magnetically soft and ultra-high coercive phases. In particular, the aim of this study is to investigate the possibility of reducing expensive rare earth (RE) in the so-called neodymium magnets and improving their efficiency.Design/methodology/approachIn order to model hysteresis loops, a special disorder-based Monte Carlo procedure, suitable for irregular geometry of the composites, was applied. The chosen system parameters were defined in order to model Nd2Fe14B/Fe composites.FindingsThe results suggest potential for optimizing hard magnetic composites. Magnetization curve parameters are sensitive to grain coupling and easy magnetization axis ordering. Strong coupling for a single-phase hysteresis loop is unachievable for grains above a certain size, i.e. found to be a few hundred nanometers. Considering these factors and their interdependencies, it’s possible to enhance the |BH|max parameter or reduce the RE content.Research limitations/implicationsThe research was carried out using computer simulations, which by their nature are only approximations of physical processes. The next stage of research is to produce the described composites and test their actual properties.Practical implicationsThe research enhances permanent magnets, boosting efficiency in technologies like wind turbines and electric motors, indirectly benefiting the environment. It also reduces RE elements in magnets for environmental, economic and political gains.Originality/valueThe unique approach is to consider the random orientation of the magnetic anisotropy of the hard magnetic grains, which is close to real powder composites. The results provide valuable guidance for the production process of permanent magnets.

Carvalho T.C., Simão R.A., Archanjo B.S., Araújo J.R., Pereira K.F., Barthem V.M.

The fundamental contribution of the Ta layer to the highly coercive Ta/SmCo / Ta films was investigated. A room temperature coercivity value of 4.5 T was achieved in film with 60 % SmCo5 and 40 % Sm2Co17. In films, in which only the Sm2Co17 phase was observed in XRD (X-ray diffraction), a high coercivity of 2 T was also obtained. All films were obtained by codeposition at temperature environment of Sm and Co atoms on a Ta layer, followed by annealing to promote the crystallization of the Sm-Co film. The following results were observed: (i) the presence of voids in the Sm-Co layer, observed in the cross- section STEM (Scanning Transmission Electron Microscope) images, (ii) the presence of stable Taα phase, observed in the XRD analysis, (iii) Ta diffusion in the Sm-Co layer, identified in the XPS (X-ray Photoelectron Spectroscopy) deep profile and, (iv) the initial magnetization curve, typical of a magnetization reversal controlled by pinning that led us to conclude that the voids produced during annealing by the diffusion of Ta in the Sm-Co layer, and they act as a pinning for the magnetization inversion, resulting in highly coercive films. Ta diffusion promoted the transition from the metastable Taβ phase to the stable Taα phase, at temperatures below 800 ◦C. Another important result was the strong exchange interaction between the grains reflected in the square hysteresis with Mr/Ms≈1.

Paulischin A., Weissitsch L., Wurster S., Fellner S., Krenn H., Bachmaier A.

High pressure torsion (HPT) is presented as a new fabrication route to produce bulk Sm‐Co magnets with a strongly refined microstructure down to the nanometer regime. The initial powders, based on the compositions SmCo5, Sm2Co7 and Sm2Co17, are compacted and subsequently deformed by HPT. The microstructural evolution in dependence on the applied deformation parameters is characterized by electron microscopy and the effect of HPT on the phase stability is monitored by synchrotron X‐ray diffraction. An increasing amount of applied strain leads to a stronger reduction in grain size while strain localization counteracts a homogeneous microstructural refinement. The positive effect of elevated deformation temperatures is demonstrated for Sm2Co17, which promotes homogeneous grain refinement, but causes strain induced phase transformations at the same time, strongly affecting the magnetic behavior. SQUID magnetometry is used to characterize the magnetic properties after HPT‐deformation, which indicates the formation of a magnetic texture depending on the respective phase.This article is protected by copyright. All rights reserved.

Afanasyev A.A., Genin V.S., Vasileva L.N., Ivanova N.N., Vatkin V.A., Tokmakov D.A.

To implement the dynamic properties of the designed valve motors and achieve the proper quality of transient processes in the drives of complex mechanisms, it is necessary to study their electromagnetic and functional characteristics with the calculation of the inductive parameters of the windings. The purpose of the study is to study the electromagnetic and functional characteristics of the magnetoelectric valve motor and determine the inductive parameters of its windings. Materials and methods. The magnetic field of the machine is divided into a set of horizontal stripes (mediums), analyzed independently of each other. It is assumed that the magnetic field in these stripes is plane-parallel, and at the boundaries of the selected stripes the conjugation conditions are met: scalar magnetic potentials and normal components of magnetic induction do not undergo a jump (discontinuity). The magnetomotive force of the stator winding, the coercive force of magnets and the magnetization of soft magnetic ferromagnetic media are considered as sources of the magnetic field. Expressions for describing scalar magnetic potentials and normal components of magnetic induction in selected bands are obtained using the Fourier variable separation method. Research results. A study of the electromagnetic and functional characteristics of the motor was carried out using a mathematical model of the magnetoelectric valve motor based on the Fourier method of separation of variables with the division of the active region into homogeneous sections in the form of magnetic sheets. The model enables to determine magnetic inductions for any coordinate points x, y of the active region of the motor magnetic system. Expressions describing the inductive parameters of the windings are obtained, and the equations of the state of the motor are given for the case when the currents in the phase windings of the stator are in antiphase with their no-load EMF. Conclusions. Mathematical models of valve motors, built on the basis of the Fourier variable separation method, make it possible to determine the electromagnetic characteristics of motors and the inductive parameters of windings necessary for calculating transient processes. The most effective is valve motor control, when the phase currents are in antiphase with the no-load EMF. In this mode, with the absence of longitudinal current and with a power factor close to unity, the equations of state of the motor are of the second order.

Zakharov Y.A., Popova A.N., Pugachev V.M., Zakharov N.S., Tikhonova I.N., Russakov D.M., Dodonov V.G., Yakubik D.G., Ivanova N.V., Sadykova L.R.

The article reveals for the first time the features of nanoparticle morphology, phase compositions, and their changes when heating FePt and CoPt nanoalloys. Nanoparticles were obtained by co-reduction of precursor solution mixtures with hydrazine hydrate. The features were found by a complex of methods of X-ray diffraction (in situ XRD and X-ray scattering), TEM HR, and cyclic voltammetry. In addition, adsorbometry results were obtained, and the stability of different nanocluster structures was calculated by the molecular dynamics method. There were only FCC solid solutions in the X-ray patterns of the FePt and CoPt nanoalloys. According to XRD, in the case of nanoparticle synthesis with Fe and Co content less than 10 at. %, the composition of solid solutions was close to or practically equal to the composition of the as-synthesized nanoparticles quantified by inductively coupled plasma optical emission spectrometry. For systems synthesis with Fe and Co content greater than the above, the solubility limits (SLs) of Fe and Co in Pt were set 11.4 ± 0.7 at. % and 17.5 ± 0.6 at. %, respectively. Therefore, there were non-registered XRD extra-phases (XRNDPh-1) in the systems when CFe,Co ≥ SL. This statement was supported by the results of TEM HR and X-ray scattering: the smallest nanocrystals (1–2 nm) and amorphous particles were found, which qualitatively agreed with the sorbometry and SAXS results. Molecular dynamics calculations of stability for FePt and CoPt alloys claimed the structures of the most stable phase corresponded to phase diagrams (A1 and L12). Specific peculiarities of the morphology and compositions of the solid solutions of nanoalloys were established: structural blockiness (domain) and composition heterogeneity, namely, platinum enrichment of internal (deep) layers and homogenization of the nanoalloy compositions at relatively low temperatures (130–200 °C). The suggested model of the formation of nanoalloys during the synthesis, qualitatively, was compliant with the results of electrochemical deposition of FePt films on the surface of various electrodes. When nanocrystals of solid solutions (C(Fe, Co) < SL) were heated above specific temperatures, there were phase transformations with the formation of two-phase regions, with solid solutions enriched with platinum or iron (non-registered XRD phase XRNDPh-2). The newly formed phase was most likely intermetallic compounds, FePt3, CoPt3. As a result of the study, the model was developed, taking into account the nanoscale of the particles: XRDPh (A1, FeaPt1−a) → XRDPh (A1, Fem×a−xPtm−m×a+x) + XRNDPh-2 (Fen×a+yPtn−n×a−y) (here, m + n = 1, m ≤ 1, n ≤ 1).

Hamid M., Rianna M., Vania M.D., Yanti I.D., Manurung F.A., Afriandani R., Daulay A.

Since a composite electrode made of carbon and transition metal oxides has much potential to be the best electrode type for a future energy storage system, the low-temperature solution growth method was used to make a carbon framework from sweet potato with NiCo2O4 nanoparticles attached to it. This method is easy, cheap, and can be used for large-scale commercial production. FTIR spectra a peak band of Ni-O and Co-O and the bending functional group at wave number 857 cm−1. XRD shows the crystal planes (1 1 1), (2 2 0), (3 3 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) at 2θ = 18.97°, 31.97°, 37.51°, 38.10°, 44.55°, 55.51°, 58.65°, and 64.92°, which indicates the NiCo2O4. The typical broad peaks around 23.3° can be linked to (0 0 2) lattice planes of amorphous carbon. The average size of the grains in the NiCo2O4/C samples was found to be 21.5 ± 0.5 nm. VSM shows that NiCo2O4/C has strong magnet properties. Based on the CV curve formed, it can be seen that NiCo2O4/C-2.8 has a balanced cathodic and anodic curve and also a higher current density than the others. It shows that NiCo2O4/C-2.8 has a higher ability to move electrons. The addition of the number of variations in the carbon mixture in NiCo2O4 shows the specific capacitance. It shows that carbon can prevent the movement of electrons in NiCo2O4, causing a decrease in performance. The right amount of carbon can increase the electron transfer ability.

Djellal N., Pęczkowski P., Mekki D.E., Navarro E., Tahraoui T., Piętosa J., Michalik J.M., Marín P., Gondek Ł.

Fe-Co alloys are the most important soft magnetic materials, which are successfully used for a wide range of applications. In this work, the magnetic properties of lanthanide-substituted (Fe0.65Co0.35)0.95(RE2O3)0.05 (RE = La, Nd, and Sm) nanoparticles, prepared by mechanical alloying, are reported. Our comprehensive studies (X-ray diffraction, Mössbauer spectroscopy, scanning electron microscopy with X-ray energy dispersive spectrometry, SQUID magnetometry and differential scanning calorimetry) have revealed different properties, depending on the dopant type. The RE2O3 addition led to a decrease in the crystallite size and to an increase in the internal microstrain. Moreover, because of the high grain fragmentation tendency of RE2O3, the cold welding between Fe–Co ductile particles was minimized, indicating a significant decrease in the average particle size. The parent Fe0.65Co0.35 alloy is known for its soft ferromagnetism. For the La-substituted sample, the magnetic energy product was significantly lower (0.450 MG·Oe) than for the parent alloy (0.608 MG·Oe), and much higher for the Sm-substituted compound (0.710 MG·Oe). The processing route presented here, seems to be cost-effective for the large-scale production of soft magnetic materials.