Rare-earth Free Permanent Magnets Research Articles

Switching of Coercivity Process in MnBi Alloys

MnBi-based alloys represent an interesting choice for developing rare earth-free permanent magnets due to the high magnetocrystalline anisotropy of their characteristic low-temperature intermetallic phase (LTIP) with hexagonal structure. In this work, we discuss the switching of coercivity mechanism in MnBi alloys by modulation of their phase distribution and microstructure. As-cast MnBi alloys obtained by suction-casting technique exhibited LTIP interspersed within Bi- and Mn-rich areas. A noticeable coercivity field of 282 kA/m was observed. The coercivity mechanism for this alloy was explained in terms of the nucleation of reverse domains after saturation, by means of the Kronmuller equation, which incorporates the detrimental effect of microstructure defects through fitting parameters associated to reduced intrinsic magnetic properties at grain size boundaries, interfaces, and local demagnetizing fields. Subsequent annealing at 583 K for 24 h produced a marked reduction of coercivity (down to 16 kA/m), reflecting a switching of coercivity process from nucleation to pinning of domain walls. The key microstructural feature determining this variation is the formation/suppression of Bi-rich areas, which promotes the nucleation and growth of the initial MnBi intermetallic phase.

Development of rare earth free permanent magnet generator using Halbach cylinder rotor design

Direct-drive permanent magnet generators (DDPMGs) offer increased reliability and efficiency for wind turbine application over the more commonly used geared doubly-fed induction generators. However, deployment of DDPMGs is limited in the U.S. wind industry due to reliance on NdFeB permanent magnets, which contain critical rare earth elements Nd and Dy. To allow for the use of lower energy density, rare earth free permanent magnets, Halbach cylinders are employed as the rotor in a 3.5 kW PMG to concentrate magnetic flux over the rotor surface and increase magnetic loading. By varying the slot-to-pole ratio in Halbach PMGs (HPMGs), designs are developed, which allow for the use of ceramic, or hard ferrite, strontium iron oxide permanent magnets. At the 3.5 kW scale, the ceramic HPMGs are able to achieve rated performance at reduced average efficiency of between 82 and 87%, which is due to the difference in permanent magnet material properties. For scaling of the ceramic HPMGs to 3 MW, rated performance and high efficiency were achieved on average at rated speed, demonstrating the potential for a rare earth free PMG in commercial scale wind turbines.

Stacking faults density driven collapse of magnetic energy in hcp-cobalt nano-magnets**Oral talk at 8th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2016), 8–12 November 2016, Ha Long City, Vietnam.

Cobalt nanowires with different shape parameters were synthesized via the polyol process. By calculating the magnetic energy product (BHmax) both for dried nano-powder and for nanowires in their synthesis solution, we observed unexpected independent BHmax values from the nanowires shape. A good alignment of the nanowires leads to a higher BHmax value. Our results show that the key parameter driving the magnetic energy product of the cobalt nanowires is the stacking fault density. An exponential collapse of the magnetic energy is observed at very low percentage of structural faults. Cobalt nanowires with almost perfect hcp crystalline structures should present high magnetic energy, which is promising for application in rare earth-free permanent magnets.

Tuning magnetocrystalline anisotropy of α″-Fe16N2 by interstitial impurity doping: A first principles study

The electronic structure and the magnetocrystalline anisotropy of impurity (Li, Be, B, C, and N) doped α″-Fe16N2 were studied by using full potential linearized augmented plane wave method (FLAPW). From the formation energy calculations, it was evident that the 2s and 2p elements except Li could be easily doped into pure α″-Fe16N2. The impurity doping induced local lattice distortions near the impurity site and the magnetic moment near the impurity atom was substantially suppressed. However, the volume of the cell and the total magnetic moment of the doped systems were weakly suppressed compared to the pure α″-Fe16N2. Despite this relatively weak suppression in magnetization, the local distortion contributed to enhancing the magnetocrystalline anisotropy. Due to this enhancement in the anisotropy, the coercive field was also enhanced about 12–16% compared with that in pure structure. Along with the enlarged coercive field, the energy product was also increased. These results may suggest that impurity doped α″-Fe16N2 can be used as potential rare earth free permanent magnet.

Effect of Cu substitution on structural and hard magnetic properties of rapidly solidified Zr18Co82-xCux melt spun ribbons

The necessity to have rare earth free permanent magnets has driven the research efforts world over to explore the rare earth free compositions to make permanent magnets. The composition of Zr18Co82-xCux (x = 0–3) was chosen, melt spun followed by a detailed investigation on structural and magnetic properties. The melt spun ribbons obtained from these nominal compositions contained rhombohedral (R) Zr2Co11, orthorhombic (O) Zr2Co11 and fcc Co phases. The substitution of near atomic radius Cu for Co results in increase of O-Zr2Co11 phase. As a result, for an increase of Cu content from x = 0 to 3, the saturation magnetization (Ms) increases from 56 to 101emu/g, while the coercivity (Hc) decreases monotonously from 2.2 to 1.5 kOe. The easy magnetization direction lies along the plane of the ribbon. There is a significant change in the solidification morphology of the ribbons as a function of Cu content. It changes from dendritic to equiaxed and again to dendritic with increase in Cu addition.

Enhanced saturation magnetization in perpendicular L1–MnAl films upon low substitution of Mn by 3d transition metals

In order to enhance the saturation magnetization (Ms) of a L10−MnAl alloy, which is a promising candidate for use in rare-earth free permanent magnets, this work assesses perpendicular L10−MnAl films in which Mn is substituted by the 3d transition metals (TMs) Fe, Co, Ni, or Cu. In the films for which the TM is Fe or Ni, the lattice constant, a, of the L10−(Mn, TM)Al phase increases from 0.390 to 0.397 nm with a substitution amount, x, of 5 at. %. Simultaneously, the lattice constant, c, decreases from 0.355 to 0.350 nm, such that the lattice ratio, c/a, changes from 0.91 to 0.88. Upon increasing x to more than 5 at. %, an unknown phase (either Mn or another Mn−Al phase) forms together with the L10−(Mn, TM)Al phase. The Ms of the MnAl film increases from 360 to more than 400 emu/cm3 with an increasing x when substituting Fe or Ni for Mn. In addition, although a MnAl film without substitution shows a Ms of 360 emu/cm3 and a Hc of 4.1 kOe, a MnAl film substituted with Fe at x = 2.5 at. % exhibits a Ms of 400 emu/cm3 and a Hc of 3.8 kOe. These results suggest that the lattice changes induced in the L10−MnAl alloy upon substituting Fe or Ni for Mn, corresponding to an expansion of the interatomic distance between (Mn, TM) atoms, are an effective means of enhancing the Ms value of the alloy.

Properties of magnetically semi-hard (FexCo1−x)3B compounds

The (FexCo1−x)3B (x = 0.0, 0.1, 0.2, 0.3) compounds have been prepared by induction melting with subsequent annealing. All of the compounds form in the orthorhombic Fe3C-type structure and exhibit ferromagnetic behaviour: the Curie temperature increases from 750 K for x = 0.0 to 923 K for x = 0.3. Magnetic domain images obtained by Kerr microscopy reveal the presence of uniaxial magnetocrystalline anisotropy. The magnetocrystalline anisotropy energy and anisotropy field of magnetically oriented fine particles have been determined. An anisotropy energy of 651 kJm−3 was obtained for Co3B (x = 0.0), the anisotropy energy was found to decrease with increasing Fe content reaching a value of 507 kJm−3 for x = 0.3. These compounds with uniaxial magnetocrystalline anisotropy are potential candidates to be used as semi-hard magnetic phase for the preparation of rare-earth free nanocomposite permanent magnets.

Fascinating Magnetic Energy Storage Nanomaterials: A Brief Review.

In this brief review, the importance of nanotechnology in developing novel magnetic energy storage materials is discussed. The discussion covers recent patents on permanent magnetic materials and especially covers processing of permanent magnets (rare-earth and rare-earth free magnets), importance of rare-earth permanent magnets and necessity of rare-earth free permanent magnets. Magnetic energy storage materials are those magnetic materials which exhibit very high energy product (BH)max (where B is the magnetic induction in Gauss (G) whereas H is the applied magnetic field in Oersted (Oe)). (BH)max is the direct measure of the ability of a magnetic material to store energy. In this context, processing of magnetic energy storage composite materials constituted by soft and hard magnetic materials played a predominant role in achieving high (BH)max values due to the exchange coupling phenomenon between the soft and hard magnetic phases within the composite. Magnetic energy storage composites are normally composed of rare-earth magnetic materials as well as rare-earth free magnetic materials. Nanotechnology's influence on the enhancement of energy product due to the exchange coupling phenomenon is of great prominence and therefore discussed in this review.

MWCNT reinforced τ-Mn-Al nanocomposite magnets through spark plasma sintering

We herein exploit the inherent benefits of both multi-walled carbon nanotubes (MWCNTs) and spark plasma sintering (SPS) for augmenting the performance of rare-earth free Mn56Al44 permanent magnets through grain refinement and stabilization of τ-phase. A good combination of remanent magnetization, coercivity and energy product values: 42.5 emu/g, 3.22 kOe and 1.92 MGOe; and 45.6 emu/g, 3.64 kOe and 2.26 MGOe were achieved for the SPSed Mn-Al magnets with 1.5 and 2 wt % additions of MWCNTs, respectively. The observed coercivity enhancement in the SPSed Mn-Al/MWCNT nanocomposite magnets is hypothesized to originate from the domain-wall pinning, as a consequence of precipitation of MWCNTs in the form of broken nanotubes and amorphous carbonaceous material at the grain boundaries. Although, a more detailed study is essential to verify the above fact, the present strategy provides a new basis for producing high performance rare earth free Mn-Al permanent magnets in a simpler way using the SPS process.

On the advantages of spring magnets compared to pure FePt: Strategy for rare-earth free permanent magnets following a bottom-up approach

Nanostructured magnets benefiting from efficient exchange-coupling between hard and soft grains represent an appealing approach for integrated miniaturized magnetic power sources. Using a bottom-up approach, nanostructured materials were prepared from binary assemblies of bcc FeCo and fcc FePt nanoparticles and compared with pure L10-FePt materials. The use of a bifunctional mercapto benzoic acid yields homogeneous assemblies of the two types of particles while reducing the organic matter amount. The 650°C thermal annealing, mandatory to allow the L10-FePt phase transition, led to an important interdiffusion and thus decreased drastically the amount of soft phase present in the final composites. The analysis of recoil curves however evidenced the presence of an efficient interphase exchange coupling, which allows obtaining better magnetic performances than pure L10 FePt materials, energy product above 100kJm−3 being estimated for a Pt content of only 33%. These results clearly evidenced the interest of chemically grown nanoparticles for the preparation of performant spring-magnets, opening promising perspective for integrated subcentimetric magnets with optimized properties.

Insights into formation and stability of τ-MnAlZx (Z = C and B)

The τ-phase MnAl alloys are promising candidate for rare earth free permanent magnets. In this study, In order to better understand the MnAl ε→τ phase transition mechanism in a continuous cooling process and metastable MnAl τ-phase high temperature stability, Mn0.54Al0.46, Mn0.55Al0.45C0.02 and Mn0.55Al0.45B0.02 alloys were systematically studied by in situ synchrotron X-ray powder diffraction (SR-XRD). The relationship between τ-phase formation tendency and different cooling rates of Mn0.55Al0.45C0.02 was investigated. Besides, the high temperature stabilities of undoped τ-MnAl and carbon/boron doped τ-MnAl were studied. Differential thermal analysis (DTA) was also employed to study the phase transformation as well. The research results show that a high cooling rate of 600 °C/min leads to a 50/50 wt% mixture of ε- and τ-phase; almost pure τ-phase was obtained when cooled at a moderate cooling rate of 10 °C/min; while for a slow cooling rate of 2 °C/min, the τ-phase partially decomposed into β and γ2 phases. No intermediate ε’-phase was observed during the ε→τ phase transition during the experiments. For the boron and carbon doped τ-MnAl, the 800 °C high temperature stability experiments reveal that C stabilizes the τ-MnAl while doped B destabilises the tetragonal structure and it decomposes into β- and γ2-phases.

Effect of Temperature/Pressure on Lattice Dynamics of FeNi

Tetratenite phase of L10 (CuAu) FeNi is identified as a hard ferromagnet in spite of that common FeNi alloys are classified as a soft magnet. Due to its strong magnetic anisotropy and large coercivity, tetrataenite phase of L10 FeNi is under investigation as a rare earth free advanced permanent magnet. Our computed equilibrium lattice constant and c/a ratio for tetratenite phase of L10 (CuAu) FeNi are in 10 % deviation with the other available results. The vibrational and electronic properties of L10 FeNi at finite temperatures/pressures are studied using the first-principles plane wave self-consistent method under the framework of density functional theory. Conclusions based on the phonon dispersion curves, phonon density of states and electronic band structure along with total and projected density of states at finite temperatures/pressures are outlined.

Magnetic properties ofFe5SiB2and its alloys with P, S, and Co

Fe$_5$SiB$_2$ has been synthesized and magnetic measurements have been carried out, revealing that M$_{\text{sat}}$ = 0.92 MA/m at T = 300 K. The M vs T curve shows a broad peak around T = 160 K. The anisotropy constant, K$_1$, estimated at T = 300 K, is 0.25 MJ/m$^3$. Theoretical analysis of Fe$_5$SiB$_2$ system has been carried out and extended to the full range of Fe$_5$Si$_{1-x}$P$_x$B$_2$, Fe$_5$P$_{1-x}$S$_x$B$_2$, and (Fe$_{1-x}$Co$_x$)$_5$SiB$_2$ compositions. The electronic band structures have been calculated using the Full-Potential Local-Orbital Minimum-Basis Scheme (FPLO-14). The calculated total magnetic moments are 9.20, 9.15, 9.59 and 2.42$\mu_B$ per formula units of Fe$_5$SiB$_2$, Fe$_5$PB$_2$, Fe$_5$SB$_2$, and Co$_5$SiB$_2$, respectively. In agreement with experiment, magnetocrystalline anisotropy energies (MAE's) calculated for T = 0 K changes from a negative (easy-plane) anisotropy -0.28 MJ/m$^3$ for Fe$_5$SiB$_2$ to the positive (easy-axis) anisotropy 0.35 MJ/m$^3$ for Fe$_5$PB$_2$. Further increase of the number of p-electrons in Fe$_5$P$_{1-x}$S$_x$B$_2$ leads to an increase of MAE up to 0.77 MJ/m$^3$ for the hypothetical Fe$_5$P$_{0.4}$S$_{0.6}$B$_2$ composition. Volume variation and fixed spin moment calculations (FSM) performed for Fe$_5$SiB$_2$ show an inverse relation between MAE and magnetic moment in the region down to about 15\% reduction of the spin moment. The alloying of Fe$_5$SiB$_2$ with Co is proposed as a practical realization of magnetic moment reduction, which ought to increase MAE. MAE calculated in virtual crystal approximation (VCA) for a full range of (Fe$_{1-x}$Co$_x$)$_5$SiB$_2$ compositions reaches the maximum value of 1.16 MJ/m$^3$ at Co concentration x = 0.3, with the magnetic moment 7.75$\mu_B$ per formula unit. Thus, (Fe$_{0.7}$Co$_{0.3}$)$_5$SiB$_2$ is suggested as a candidate for a rare-earth free permanent magnet.

Phase Concentration Determination of Fe16N2 Using State of the Art Neutron Scattering Techniques

Due to limitations on the availability of rare earth elements it is imperative that new high energy product rare earth free permanent magnet materials are developed for the next generation of energy systems. One promising low cost permanent magnet candidate for a high energy magnet is α″-Fe16N2, whose giant magnetic moment has been predicted to be well above any other from conventional first principles calculations. Despite its great promise, the α″ phase is metastable; making synthesis of the pure phase difficult, resulting in less than ideal magnetic characteristics. This instability gives way to a slew of possible secondary phases (i.e. α-Fe, Fe2O3, Fe8N, Fe4N, etc.) whose concentrations are difficult to detect by conventional x-ray diffraction. Here we show how high resolution neutron diffraction and polarized neutron reflectometry can be used to extract the phase concentration ratio of the giant magnetic phase from ultra-small powder sample sizes (~0.1 g) and thin films. These studies have led to the discovery of promising fabrication methods for both homogeneous thin films, and nanopowders containing the highest reported to date (>95%) phase concentrations of room temperature stable α″-Fe16N2.

Effects of strain and interface on magnetic anisotropy of FeCo/FePt: A first principles study

Using the full potential linearized augmented plane wave method, we explored the structural and magnetic properties of hybrid bilayer FeCo/FePt. The effect of interface hybridization and strain induced in FeCo by FePt buffer layer was studied. We found a tetragonal distortion in FeCo structure due to the lattice mismatch between FePt buffer layer and ferromagnetic FeCo layer. The magnetic moments of Fe and Co were insensitive to the strain effect, but the suppression in the magnetic moments of Fe and Co at the interface was observed because of the interface hybridization. The magnetocrystalline anisotropy energy of FeCo film was enhanced almost twice due to the strain. In FeCo/FePt structure, the uniaxial anisotropy constants of 9.96 and 8.78MJ/m3 were obtained for 2 and 5ML FeCo/FePt hybrid systems respectively. Additionally, we found that the coercive field is decreased from 146 to 119kOe while maximum energy product increases from 74 to 85MGOe with increase in FeCo soft layer from 2 to 5ML. These high values of coercive field and maximum energy product may insure the basic requirement of designing rare-earth free permanent magnets.

Synthesis of strontium ferrite/iron oxide exchange coupled nano-powders with improved energy product for rare earth free permanent magnet applications

We present a simple, scalable synthesis route for producing exchange coupled soft/hard magnetic composite powder that outperforms pure soft and hard phase constituents.

Dense arrays of cobalt nanorods as rare-earth free permanent magnets.

We demonstrate in this paper the feasibility to elaborate rare-earth free permanent magnets based on cobalt nanorods assemblies with energy product (BH)max exceeding 150 kJ m(-3). The cobalt rods were prepared by the polyol process and assembled from wet suspensions under a magnetic field. Magnetization loops of dense assemblies with remanence to a saturation of 0.99 and squareness of 0.96 were measured. The almost perfect M(H) loop squareness together with electron microscopy and small angle neutron scattering demonstrate the excellent alignment of the rods within the assemblies. The magnetic volume fraction was carefully measured by coupling magnetic and thermogravimetric analysis and found in the range from 45 to 55%, depending on the rod diameter and the alignment procedure. This allowed a quantitative assessment of the (BH)max values. The highest (BH)max of 165 kJ m(-3) was obtained for a sample combining a high magnetic volume fraction and a very large M(H) loop squareness. This study shows that this bottom-up approach is very promising to get new hard magnetic materials that can compete in the permanent magnet panorama and fill the gap between the ferrites and the NdFeB magnets.

Influence of mechanical milling on structure, particle size, morphology and magnetic properties of rare earth free permanent magnetic Zr2Co11 alloy

In an effort to explore finding rare earth free permanent magnet material for device applications, the Co based Zr2Co11 compound has been selected for investigation. A detailed study was carried out on the effect of mechanical milling on structure, particle size, morphology and magnetic properties. The alloy of nominal composition Zr18Co82 exhibited co-existence of minor Zr6Co23 phase along with main phase Zr2Co11. When subjected to long duration mechanical milling, the alloy has undergone structural disorder leading to partial amorphization. The particle size has decreased from 200 μm to 120 μm up on milling for 6 h in a planetary milling process. Further milling caused less influence on particle size reduction. However, it had an effect on particle size distribution. The size reduction is not very high due to ductile nature of Zr2Co11 phase and the morphology of the particles too showed coarse plate shape. The coercivity variation against particle size reduction is less than appreciable, given that the crystal structure being rhombohedral with significant magnetocrystalline anisotropy. The defects induced during milling and the presence of deleterious minor phases are considered the main cause for reduction in the coercivity.

Magnetic Anisotropy of $\tau $ -MnAl Thin Films

Manganese compounds such as MnAl, MnBi, MnGa have received much attention for their high magnetic anisotropy constants, and also for rare-earth free permanent magnet applications. Despite the fact that voluminous work has been conducted regarding the magnetic anisotropy, the origin of this high magnetic anisotropy energy is not well understood. In the case of MnAl, both Mn and Al elements are known to possess low spin-orbit coupling constants, but the τ-phase MnAl is well known to exhibit high magnetic anisotropy constant K in the order of 1×107 erg/cm3 at room temperature [1]. A systematic study of temperature dependence of both the saturation magnetization M S and magnetic anisotropy constant K in τ-MnAl thin films has been carried out in order to elucidate the magnetic anisotropy mechanism.

Structural and magnetic properties of strongly carbon doped Fe–Co thin films

In the framework of the ongoing research for novel rare earth free permanent magnet materials, the alloy Fe–Co–C has attracted interest from theorists, since carbon could induce a magneto-crystalline anisotropy. In this work structural and magnetic properties of strongly doped magnetron sputtered thin films were investigated. Au–Cu buffers on MgO (100) substrates were used in order to promote epitaxial FeCo with 001 orientation. By adding carbon as a third element a tetragonal distortion was observed, according to structural measurements. An anisotropic behavior was induced in the magnetic properties of the system, where the magneto-crystalline anisotropy constant Ku value was estimated in the order of 0.8×106J/m3f or 3nm thick Fe–Co(C) magnetic layer.