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Recently 1:12 magnets of Sm-(Fe,V) have shown promising coercivities and the potential to be alternative rare-earth-lean permanent magnets. In this work, we investigated the effects of partial substitution of Cu, Mo and Ti for V in the magnets prepared by hot compaction and hot deformation of mechanically milled powders. The microstructure of the Sm-Fe-(V,Cu) and Sm-Fe-(V,Ti) hot-deformed magnets consisted in fine grains with sizes between 50 and 150 nm. The Sm-Fe-(V,Cu) magnet showed the best performance with μ0H c = 0.96 T, μ0M r = 0.49 T, (BH) max = 42 kJ m − 3 and T C = 362 ∘ C. Atom probe tomography of this magnet revealed the presence of a thin Sm17.5Fe71.5V8Cu3intergranular phase of 3-6 nm surrounding the 1:12 nanograins. The addition of a small amount of Cu, not only improved the magnetic properties but also hindered the grain growth during hot deformation. Micromagnetic simulations of the magnetization reversal agreed with the experimental values of coercivity. The presence of the intergranular phase reduces the number of grains that switch simultaneously.

Iron-rich compounds with the tetragonal ThMn12-type structure have the potential to meet current demands for rare-earth-lean permanent magnets with high energy density and operating temperatures of 150–200 °C. However, while it is normal for magnet technology to lag behind the development of underlying magnetic material, this gap has always been unusually large for ThMn12-type magnets. The gap has widened further in recent years, as excellent combinations of intrinsic magnetic properties have been obtained in compounds synthesized with a smaller amount of structure-stabilizing elements (e.g., SmFe11V or Sm0.8Zr0.2Fe9.2Co2.3Ti0.5) or with no such elements (i.e., SmFe9.6Co2.4 thin films). The search for promising compounds continues—with increasing help coming from theoretical calculations. Unfortunately, progress in the development of magnets beyond polymer-bonded interstitially modified powders remains marginal. The introduction of lanthanum (La) was found to stabilize low-melting-temperature minority phases in Sm(Fe,Ti)12 alloys, thus allowing for liquid-phase sintering for the first time. The high reactivity of La, however, has apparently undermined the development of coercivity (Hc). A controlled crystallization of the initially suppressed ThMn12-type phase makes “bulk” magnetic hardening possible, not only in Sm–Fe–V alloys (in which it has been known since the 1990s), but also is in La-added (Ce,Sm)(Fe,Ti)12 alloys. The properties of the bulk-hardened alloys, however, remain unsatisfactory. Mechanochemically synthesized (Sm,Zr)(Fe,Si)12 and (Sm,Zr)(Fe,Co,Ti)12 powders may become suitable for sintering into powerful fully dense magnets, although not before a higher degree of anisotropy in both alloys and a higher Hc in the latter alloy have been developed. Keywords: Permanent magnets, Rare earths permanent magnets, ThMn12 structure

Cobalt nanowires have been synthesized by electrochemical deposition using track-etched anodized aluminum oxide (AAO) templates. Nanowires with varying spacing-to-diameter ratios were prepared, and their magnetic properties were investigated. It is found that the nanowires’ easy magnetization direction switches from parallel to perpendicular to the nanowire growth direction when the nanowire’s spacing-to-diameter ratio is reduced below 0.7, or when the nanowires’ packing density is increased above 5%. Upon further reduction in the spacing-to-diameter ratio, nanowires’ magnetic properties exhibit an isotropic behavior. Apart from shape anisotropy, strong dipolar interactions among nanowires facilitate additional uniaxial anisotropy, favoring an easy magnetization direction perpendicular to their growth direction. The magnetic interactions among the nanowires were studied using the standard method of remanence curves. The demagnetization curves and Delta m (Δm) plots showed that the nanowires interact via dipolar interactions that act as an additional uniaxial anisotropy favoring an easy magnetization direction perpendicular to the nanowire growth direction. The broadening of the dipolar component of Δm plots indicate an increase in the switching field distribution with the increase in the nanowires’ diameter. Our findings provide an important insight into the magnetic behavior of cobalt nanowires, meaning that it is crucial to design them according to the specific requirements for the application purposes.

In this paper, we report a one-pot chemical synthesis technique for the preparation of iron and iron-carbide nanoparticles. Mössbauer spectroscopy, X-ray diffraction and magnetometry were used as the main tools to identify the different phases of Fe-C present. The influence of experimental parameters on the structural and compositional properties of nanoparticles was investigated in detail. These particles show ferromagnetic behavior with room temperature coercivity higher than 300 Oe. The X-ray diffraction was complemented by Mössbauer spectroscopy and thermo-magnetic analysis. Remarkably, the carbon content in iron-carbide nanoparticles (carbon rich or carbon poor iron-carbides) can be modulated simply by varying the experimental conditions, like the reaction time, temperature and iron precursor concentration. Magnetic properties can be tailored based upon crystallographic structure and particles composition.

Mn–Bi alloys modified with Mg and other elements were recently developed into promising rare-earth-free permanent magnets through successive melt spinning, warm compaction, and short magnetic field annealing. This letter reports on the crystal structures, texture, and magnetic properties observed in Mn 50 Bi 49- $x$ Mg $x$ Sb 0.5 In 0.5 alloys ( $x$ = 0, 1.5, 3.0, 4.5) throughout these processing steps. The Mg substitution favors MnBi-based intermetallic phases over the Bi- and Mn-based solid solutions and the stable α intermetallic phase over the metastable β ′ intermetallic phase. Magnesium increases the $c/a$ ratio of the α phase lattice parameters for all studied $x$ values, and it increases the degree of the field-annealing-induced texture for $x$ ≤ 3. The effect of Mg on coercivity is explained as a combination of a positive one via the α phase magnetocrystalline anisotropy and a negative one via the development of texture. The effect of the substitution on the maximum energy product additionally involves the decreasing density of the alloys; the optimum $x$ is found to be ≈3. Also reported are the thermal properties of selected alloys. The mechanism of field annealing effect on the coercivity is briefly discussed.

This study reports the stabilization of the RFe12-type based compounds where part of R and Fe are substituted with Zr and Co and Si, respectively, in order to examine whether these rare-earth-lean materials are suitable for applications as permanent magnets. Structural and magnetic characterization of the family of alloys with the general formula Nd0.4Zr0.6Fe10–xCoxSi2 (x = 0–3) and their melt-spun ribbons were carried out using X-ray diffraction and Mossbauer spectroscopy. The ThMn12-type structure is obtained for all samples as the majority phase with a minority α-Fe(CoSi) phase (less than 5 wt%) as it was estimated by XRD for x = 1 and 2. The Curie temperature increases linearly with Co substitution from 561 K for x = 0 to 712 K for x = 3. The saturation magnetization decreases slightly from 130.5 (x = 1) to 129.1 A·m2/kg (x = 3), while the anisotropy field is following the same trend.

In this paper, a general method for making nanoscale Au–Fe3O4 clusters in an oil-in-water emulsion is reported, which involves thermolytic decomposition of Fe(acac)3 and reduction of AuCl3 in a hot oleyl amine–oleic acid mixture. A detailed investigation by transmission electron microscopy (TEM), X-ray diffraction, UV-vis, and vibrating-sample magnetometry was done in order to study the morphology and properties of the hybrid nanoparticulate clusters (NPCs). The NPCs are all spherical and their building blocks are individual Au and Fe3O4 nanoparticles (NPs) with an average diameter of 9.6 and 7.1 nm, respectively. TEM images show that these clusters have a flower-like morphology with controllable size. It is found that the cetyltrimethylammonium bromide concentration, the time, and the temperature of reaction are three important parameters that significantly affect the shape, size, and magnetic properties of these clusters. Magnetic measurements reveal the superparamagnetic behavior of individual NPs of Fe3O4 with a blocking $T_{\mathrm {B}}= 80.2$ K. The blocking temperature is significantly shifted to higher temperatures in the case of the clusters. The Au–Fe3O4 NPCs show a plasmonic peak at 530 nm and have a much higher magnetization than the individual NPs, making them better suited for biomedical applications.

Melt-spun Mn50Bi50-xSbx alloys with x ≤ 5 were prepared at different solidification rates and characterized both in the as-spun state and after annealing. In the as-spun alloys, the Sb substitution leads to the formation of a metastable phase, similar to the binary “quenched high-temperature phase” but without the superstructure of the latter and exhibiting a different – increasing – temperature dependence of coercivity. The new nanocrystalline metastable phase is not only itself characterized by a high room-temperature coercivity (in excess of 20 kOe), but upon annealing it transforms into a high-coercivity (up to 13.5 kOe) “low-temperature” α phase. However, the advantage of obtaining a high coercivity without the otherwise required milling made possible by the Sb substitution is undermined by the absence of a local crystallographic texture in the melt-spun alloys. The best combination of the isotropic hard magnetic properties realized for the Mn55Bi43.5Sb1.5 composition is a remanence of 35.5 emu/g and a coercivity of 8.1 kOe. The annealed Sb-free Mn50Bi50 alloys did possess a local crystallographic alignment and, for high solidification rates, a moderate coercivity up to 5.6 kOe; however, they had an inadequate “rectangularity” in the demagnetization curve. The orthorhombic compound known as MnBi0.9Sb0.1 and described earlier as antiferromagnetic was obtained in the annealed ribbons with x ≈ 5. The compound was found to order ferromagnetically between 200 K and ≈250 K and to exhibit a significant coercivity, 14.3 kOe at 50 K. It is suggested that the MnBi0.9Sb0.1 may actually be a Sb-stabilized “new phase” observed earlier as metastable in the Bi–MnBi eutectic alloys.

A bulk magnet based on Sm-Fe-V with the ThMn12 crystal structure has been fabricated for the first time by hot-compaction of mechanically milled powders with a density of 92% of the theoretical density. The isotropic magnet exhibits a maximum coercivity of 1.06 T with a magnetization of 0.59 T, a remanent magnetization of 0.42 T and a (BH)max of 28 kJ m−3 at 3 T applied field. The Curie temperature is found to be 330 °C and the temperature coefficients of remanent magnetization and coercivity are 0.14% C−1 and 0.39% C−1, respectively. Minor hysteresis loops indicate a coercivity mechanism similar to that of the nanocrystalline Nd-Fe-B magnets. The isotropic magnet was hot-deformed up to 75% of its height, and the best magnetic properties obtained were μ0M3T = 0.63 T, μ0Mr = 0.45 T, μ0Hc = 0.88 T and (BH)max = 33 kJ m−3. A small texture perpendicular to compaction direction was detected when the amount of vanadium was reduced, and the deformation temperature was increased from 800 to 1000 °C.

In this work, we present experimental and theoretical results on SmFe12-xVx (x = 0.5 - 2.0) alloys with the ThMn12 (1:12) structure as possible candidates for rare earth-lean permanent magnets. The ...

International audience; A series of (Nd1-xSmx)Fe11Ti alloys with the ThMn12 crystal structure have been fabricated and characterized in order to promote a strong uniaxial anisotropy in NdFe11Ti without the need of nitrogenation. The compounds show small changes in the lattice parameters and cell volume, as well as Curie temperature and spontaneous magnetization. The anisotropy field, however, rapidly increases with the incorporation of Sm and overcomes the effect of nitrogenation, reaching values > 4 T for a 30% Sm content. Mössbauer spectroscopy indicates that the quadrupole splitting is correlated with the magnetic anisotropy. These alloys are good candidates for permanent magnets, provided the correct microstructure is developed to increase coercivity.

Fe-based colloids with a core/shell structure consisting of metallic iron and iron oxide were synthesized by a facile hot injection reaction of iron pentacarbonyl in a multi-surfactant mixture. The size of the colloidal particles was affected by the reaction temperature and the results demonstrated that their stability against complete oxidation related to their size. The crystal structure and the morphology were identified by powder X-ray diffraction and transmission electron microscopy, while the magnetic properties were studied at room temperature with a vibrating sample magnetometer. The injection temperature plays a very crucial role and higher temperatures enhance the stability and the resistance against oxidation. For the case of injection at 315 °C, the nanoparticles had around a 10 nm mean diameter and revealed 132 emu/g. Remarkably, a stable dispersion was created due to the colloids’ surface functionalization in a nonpolar solvent.

ThMn12 nitrides are good candidates for high performance permanent magnets. However, one of the remaining challenges is to transfer the good properties of the powder into a useful bulk magnet. Thus, understanding the denitrogenation process of this phase is of key importance. In this study, we investigate the magnetic and structural stability of the (Nd0.75,Pr0.25)1.2Fe10.5Mo1.5Nx compound (x=0 and 0.85) as function of temperature by means of neutron powder diffraction. Thermal dependence of the lattice parameters, formation of α-(Fe,Mo), as well as the nitrogen content in the nitrides are investigated by heating the compounds up to 1010 K. The decomposition takes place mainly via the formation of the α-(Fe,Mo) phase, which starts at around 900 K, whereas the nitrogen remains stable in the lattice. Additionally, we show that the magnetic properties of the nitrides [M(4T)=90Am2/kg and Hc=0.55 T] are maintained after the thermal treatments up to 900 K. This study demonstrates that the ThMn12 nitrides with the Mo stabilizing element offer good prospects for a bulk magnet provided an adequate processing route is found., The authors acknowledge the project “SpINS: Spanish Initiatives on Neutron Scattering,” funded by the Spanish Ministry of Science and Innovation, and the CSIC for the neutron beam time granted on the “CRG-D1B.” Institut Laue-Langevin (ILL) is also acknowledged. We are also thankful for technical and human support provided by SGIker (UPV/EHU) and particularly the services of X-Ray Molecules and Materials (Dr. Aitor Larrañaga Varga) and Magnetic Measurements (Dr. Iñaki Orue). This work has received funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 686056 (NOVAMAG).

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.

The microstructure, precipitation hardening and magnetic properties of melt-spun Fe78Co11Mo11 and Fe74Co11Mo11B4 alloys were studied. It is shown that B addition increases the precipitation reaction temperature of the supersaturated matrix from 605 °C for the Fe78Co11Mo11 alloy to 640 °C for the Fe74Co11Mo11B4 alloy. In the Fe78Co11Mo11 alloy, a cotton-shaped (Fe,Co)7Mo6 precipitate is observed inside the grains of the matrix phase. In the Fe74Co11Mo11B4 alloy, in addition to the (Fe,Co)7Mo6 phase, spherical MoB2 precipitates are also observed. Compared to the Fe78Co11Mo11 sample, the Fe74Co11Mo11B4 sample exhibits a finer and more homogeneous microstructure with more equiaxed grains. This finer microstructure explains the enhanced hard magnetic properties in the Fe74Co11Mo11B4 sample (Mr = 60.4 A m2/kg, iHc = 20.8 kA/m) compared to those of the Fe78Co11Mo11 sample (Mr = 56.5 A m2/kg, iHc = 17.3 kA/m). Furthermore, the Fe74Co11Mo11B4 sample shows better antioxidant ability than the Fe78Co11Mo11 sample, especially when the temperature exceeds 400 °C.

In this work, we report the effects of partial substitution of Ce by Sm in CeFe9Co2Ti alloys on their magnetic and structural properties. The alloys were prepared in bulk and melt-spun ribbons. The alloys present the ThMn12 phase in the full range of substitutions, and reach anisotropy fields above 8 T for the best alloy. After heat treatment, melt-spun ribbons show high values of magnetization (>112 A m2/kg), Curie temperatures >370 °C and enhanced coercivity. Although the coercivity values obtained are already suitable for permanent magnet applications, they are still low as compared to the anisotropy field (less than 6%). Coercivity is expected to improve by optimizing the microstructure of the best samples.

Experimental investigations of the magnetocaloric response of the intermetallic layered AlFe2B2 compound along the principle axes of the orthorhombic cell were carried out using aligned plate-like crystallites with an anisotropic [101] growth habit. Results were confirmed to be consistent with density functional theory calculations. Field-dependent magnetization data confirm that the a-axis is the easy direction of magnetization within the (ac) plane. The magnetocrystalline anisotropy energy required to rotate the spin quantization vector from the c-to the a-axis direction is determined as K∼0.9 MJ/m3 at 50 K. Magnetic entropy change curves measured near the Curie transition temperature of 285 K reveal a large rotating magnetic entropy change of 1.3 J kg−1K−1 at μ0Happ = 2 T, consistent with large differences in magnetic entropy change ΔSmag measured along the a- and c-axes. Overall, this study provides insight of both fundamental and applied relevance concerning pathways for maximizing the magnetocaloric potential of AlFe2B2 for thermal management applications.

Powders of Sm(Fe,Co)11.2Ti0.8 alloys modified with Sc, Nb and Zr, as well as with additional Ti were prepared by reducing mechanically activated raw oxides with Ca metal in the furnace preheated to 990–1250 °C. Expansion of the crystal lattice upon introduction of Nb or additional Ti implies that atoms of these elements replace the smaller Fe atoms in the tetragonal ThMn12-type structure. On the other hand, contraction of the lattice upon introduction of Sc or Zr was smaller than what was expected for replacement of the Sm atoms, which suggests that the Sc and Zr atoms replace both the Sm and Fe atoms. Washing away the reduction byproducts expands the crystal lattice of the 1:12 particles and increases their coercivity. The lattice expansion associated with the washing is believed to be caused by interstitial H atoms; more research, however, is needed to establish the mechanism(s) of the washing effect on the coercivity. The earlier reported development of a high coercivity in zirconium-modified monocrystalline particles achieved by increasing the reduction annealing temperature to ≈1200 °C was similarly characteristic of the particles modified with Sc (the coercivity reaches 11.5 kOe) and Nb (8.1 kOe), but not for the particles prepared with additional Ti where the maximum coercivity of 8.3 kOe develops for a lower annealing temperature. It is concluded that Sc, Nb and Zr modify the high-temperature phase equilibria of the Sm(Fe,Co)11.2Ti0.8 alloys allowing for an effective high-temperature processing, whereas the alloy coercivity increases with the synthesis temperature through a different, still unknown mechanism which may involve suppression of the defects specific to the 1:12 crystals.

The effects of Ge substitution and synthesis via rapid solidification on the structure and magnetic character of the intermetallic Fe 5 SiB 2 (5-1-2) system were determined, with the objective to investigate the potential of Fe 5 (Si 0 . 75 Ge 0.25 )B 2 as a new permanent magnet material. Samples were made by arc-melting/annealing as well as by melt-spinning/annealing and were evaluated by probing the structure, composition, and magnetic response. Results indicate that the tetragonal Cr 5 B 3 crystal structure, large saturation magnetization (Ms = 153.8 emu/g) and Curie temperature (Tc = 784 K) of the parent Fe 5 SiB 2 phase may be preserved by substituting up to 25 at % of Si in the lattice with Ge. The room-temperature magnetocrystalline anisotropy constant of a Ge-substituted alloy of nominal composition Fe 5 (Si 0 . 75 Ge 0.25 )B 2 was evaluated as K 1 = 5 × 10 6 ergs/cc using the law of approach to saturation and is the largest reported to date within the 5-1-2 materials system. As a result of compositional modification, the spin reorientation observed in Fe 5 SiB 2 is greatly suppressed from 172 K to 60 K. A comparison of the 5-1-2 phase evolution when synthesized by standard casting and by rapid solidification indicates that melt spinning is a simpler and more expeditious synthesis method relative to previously reported solid-state reaction methods.

We report the study of Zr 1-x Ce x Fe 10 Si 2 (x = 0.0, 0.3 and 0.6) compounds, with tetragonal ThMn 12 structure, by means of neutron diffraction and Mossbauer spectroscopy to understand the role of Ce in the increasing magneto-crystalline anisotropy. Fitting of the 57 Fe Mossbauer spectra to the three Fe atomic positions, namely 8i, 8j and 8f, indicate that Ce displaces Fe from the 8i to the 8j positions and in parallel increases the quadrupole splitting (QS) in position 8f, correlating with the increase of the magneto-crystalline anisotropy. However, the hyperfine field B hf remains constant for all Ce substitutions. The full occupancy of the 8j positions by Fe at Ce = 0.6 can explain the instability of the ThMn 12 structure for similar and higher Ce concentrations. Neutron diffraction experiments show the evolution of the magnetic moment and crystal structure as a function of temperature, showing Ce to cause an increase of the lattice parameters and tetragonal distortion. A large anomaly of the expansion coefficient, due to the spontaneous magnetostriction, is also disclosed. These results support a pure geometric influence of Ce on the magneto-crystalline anisotropy through a selective distortion of the lattice.

International audience; The influence of quenching rate and nitrogenation in melt-spun Nd 1.2 Fe 10.6 Mo 1.4 has been investigated in terms of microstructure, phase formation and magnetic properties. Increasing the quenching rate leads to smaller grain size. However, it also implies a change in the crystallized phase structure. We obtained a pure ThMn 12 (1:12) structure at quenching rates up to 30 m/s, leading to an average grain size of 220 nm. Magnetic measurements of the as-spun ribbons revealed a reduction of the saturation magnetization for samples quenched above 30 m/s. This is attributed to the formation of a paramagnetic phase and/or magnetic phase with a Curie temperature (Tc) close to room temperature which is confirmed by 57 Fe Mössbauer spectroscopy. The analysis of the spectra rules out the presence of a ferromagnetic TbCu7 (1:7) phase, which is usually reported in such system. The ribbons were nitrogenated in order to form the harder magnetic phase Nd 1.2 Fe 10.6 Mo 1.4 N x. The ribbon quenched at 30 m/s with the pure ThMn 12 nitride structure is the optimum sample for getting hard magnetic properties, with a coercivity of 0.6 T, saturation magnetization of 1.15 T and Curie temperature of 350 °C. Finally, we show the good stability of the later phase structure at elevated temperatures (≤ T C), making this compound a good candidate for permanent magnet applications.

Iron carbide nanoplatelets with an orthorhombic Fe3C structure were synthesized following a simple liquid chemical approach. The formation of the carbide phases was shown to depend on the presence of a long chain diol and the reaction temperature. Confirmation of the iron carbide phases and structural characterization was made by X-ray diffraction (XRD) and Mossbauer spectroscopy. Particle morphology was characterized by transmission electron microscopy (TEM) and HR-TEM and the magnetic properties were measured with magnetometry (VSM). The sample with the Fe3C phase shows a ferromagnetic behavior with a magnetization of 139 emu g−1 under a 30 kOe applied field. The simple methodology presented here for producing iron carbide nanoplatelets has promising application in the biomedical and catalyst industries.

Magnets with chiral crystal structures and helical spin structures have recently attracted much attention as potential spin-electronics materials, but their relatively low magnetic-ordering temperatures are a disadvantage. While cobalt has long been recognized as an element that promotes high-temperature magnetic ordering, most Co-rich alloys are achiral and exhibit collinear rather than helimagnetic order. Crystallographically, the B20-ordered compound CoSi is an exception due to its chiral structure, but it does not exhibit any kind of magnetic order. Here, we use nonequilibrium processing to produce B20-ordered ${\mathrm{Co}}_{1+x}{\mathrm{Si}}_{1\ensuremath{-}x}$ with a maximum Co solubility of $x=0.043$. Above a critical excess-Co content (${x}_{c}=0.028$), the alloys are magnetically ordered, and for $x=0.043$, a critical temperature ${T}_{c}=328\text{ }\text{ }\mathrm{K}$ is obtained, the highest among all B20-type magnets. The crystal structure of the alloy supports spin spirals caused by Dzyaloshinskii-Moriya interactions, and from magnetic measurements we estimate that the spirals have a periodicity of about 17 nm. Our density-functional calculations explain the combination of high magnetic-ordering temperature and short periodicity in terms of a quantum phase transition where excess-cobalt spins are coupled through the host matrix.

The onset of ferromagnetism in cobalt-tin alloys is investigated experimentally and theoretically. The $\mathrm{C}{\mathrm{o}}_{1+x}\mathrm{Sn}$ alloys were prepared by rapid quenching from the melt and form a modified hexagonal NiAs-type crystal structure for $0.45\ensuremath{\le}x\ensuremath{\le}1$. The magnetic behavior is described analytically and by density-functional theory using supercells and the coherent-potential approximation. The excess of Co concentration $x$, which enters the interstitial $2d$ sites in the hypothetical NiAs-ordered parent alloy CoSn, yields a Griffiths-like phase and, above a quantum critical point $({x}_{c}\ensuremath{\approx}0.65)$, a quantum phase transition to ferromagnetic order. Quantum critical exponents are determined on the paramagnetic and ferromagnetic sides of the transition and related to the nature of the magnetism in itinerant systems with different types of chemical disorder.

International audience; Nd(1-x)Zr(x)Fe10Si2 alloys have been prepared in the tetragonal ThMn12-type structure by arc-melting and melt-spinning and then nitrogenated to improve their magnetic properties. For x = 0.4 and 0.6 the Curie temperature and magnetic anisotropy fields increase from 280-300 ºC to about 390 ºC and from 2.8-3 T to 4.5-5 T respectively. The saturation magnetization remains almost unchanged. The nitrogenated powders were processed by spark plasma sintering (SPS) leading to compact pellets, which retain the full nitrogen content and magnetic properties up to 600 ºC, but segregated Fe-Si at elevated temperatures. Nitrogenation and SPS processing are, therefore, appropriate for sintering metastable materials such as (Nd,Zr)Fe10Si2 into compact material without loosing functional properties. This opens a way towards a new family of permanent magnets, lean of critical raw materials.

Rare-earth-lean Sm(Fe,Co,Ti)12 alloys with the ThMn12 crystal structure and less than one Ti atom per formula unit have the potential of exceptionally powerful permanent magnets, but all prior attempts to develop high coercivity in bulk alloys, especially coercivity combined with crystallographic texture, have fallen short of the expectations. This study was aimed at improvement of the currently best Sm(Fe,Co,Ti)12 magnets prepared through melt-spinning which are inherently isotropic. Modifications of the alloys with B and Zr, already demonstrated in earlier studies to be effective separately, have been implemented simultaneously. A systematic study of Sm1.1-x(Fe,Co)11.3-yTi0.7By alloys melt-spun at a tangential speed of 50 m/s and annealed at 600–950 °C allowed for monitoring the continuous evolution of the two consecutive crystal structures, those of the TbCu7 and ThMn12 types. Zirconium was found to facilitate the formation of the 1:12 structure at the expense of the 1:7, whereas boron has the opposite effect, at certain concentrations completely suppressing the 1:12. When the two alloying elements are introduced simultaneously, they inhibit growth of the 1:12 crystallites at annealing temperatures higher than 800 °C, thus allowing for the development of a higher coercivity. Because of instrumental limitations, bulk magnets were prepared through a two-step process – compaction of the melt-spun ribbons at 650 °C and additional treatment at a higher temperature – and they were characterized by a reduced, 90–93%, density. Nevertheless, an isotropic Sm0.9Zr0.2(Fe,Co)10.8Ti0.7B0.5 magnet exhibited fair values of the remanence (7.4 kG), maximum energy product (8.5 MGOe) and coercivity (5.4 kOe), as well as high Curie temperature of 525 °C and remarkably small temperature coefficient of the coercivity, −0.25%/°C.

In this work, the mechanochemical synthesis method was used for the first time to produce powders of the nanocrystalline Nd1.1Fe10CoTi compound from Nd2O3, Fe2O3, Co and TiO2. High-energy-milled powders were heat treated at 1000 °C for 10 min to obtain the ThMn12-type structure. Volume fraction of the 1:12 phase was found to be as high as 95.7% with 4.3% of a bcc phase also present. The nitrogenation process of the sample was carried out at 350 °C during 3, 6, 9 and 12 h using a static pressure of 80 kPa of N2. The magnetic properties Mr, µ0Hc, and (BH)max were enhanced after nitrogenation, despite finding some residual nitrogen-free 1:12 phase. The magnetic values of a nitrogenated sample after 3 h were Mr = 75 Am2 kg–1, µ0Hc = 0.500 T and (BH)max = 58 kJ·m–3. Samples were aligned under an applied field of 2 T after washing and were measured in a direction parallel to the applied field. The best value of (BH)max ~ 114 kJ·m–3 was obtained for 3 h and the highest µ0Hc = 0.518 T for 6 h nitrogenation. SEM characterization revealed that the particles have a mean particle size around 360 nm and a rounded shape.

The ThMn12-type (Sm,Zr)1(Fe,Co,Ti)12 compounds have the potential of powerful permanent magnets. Magnetically hard and anisotropic powders of such compound have been prepared by subjecting elemental oxides and Co mixed with Ca and CaO dispersant to a succession of high-energy ball-milling, reduction diffusion at 990–1220°C and repeated washing. The size of the resulting ThMn12-type crystallites, their coercivity and fraction of monocrystalline particles were all found to increase with the reduction-diffusion temperature. Particles synthesized at 1220°C were highly monocrystalline with a mean size of 0.54 μm and, after a magnetic-field alignment, exhibited a coercivity of 1.26 T and a full-density-projected maximum energy product of at least 209 kJ/m3 (26.3 MGOe). The strong positive effect of the reduction-diffusion temperature on the coercivity has been attributed to separation of the crystallites and to decrease in the incidence of structural defects.

Alloy synthesis consisting of mechanical activation followed by annealing was explored as a method of manufacturing medium-grade permanent magnet materials with a reduced content of the critical rare earth elements. Four RxFe10Si2 alloys with R=Sm, Sm0.7Zr0.3, Sm0.3Ce0.3Zr0.4 and Ce0.6Zr0.4 (nominal compositions) were prepared from mixtures of Sm2O3, CeO2, ZrO2, Fe2O3 and Si powders in the presence of a reducing agent Ca and a CaO dispersant. The collected alloy particles typically consisted of few joined submicron crystals. For R=Sm, X-ray diffraction analysis reveals a significant amount of the unwanted Th2Zn17-type compound forming alongside the desired ThMn12-type 1:12 compound. A more pure 1:12 phase could be obtained for R=Ce0.6Zr0.4, but it exhibited a room-temperature coercivity of less than 1 kOe. The most pure 1:12 phase and the highest values of the coercivity (10.8 kOe) and calculated maximum energy product (13.8 MGOe) were obtained for R=Sm0.7Zr0.3 processed at 1150 °C. The calculated maximum energy products of the Sm0.3Ce0.3Zr0.4Fe10Si2 particles, with half of their rare earths constituents represented by the relatively abundant Ce, was 10.1 MGOe.

The aim of this work is to investigate the effect of very fine grain sizes on the spinodal decomposition in the Alnico system. Commercial Alnico 5 was melted and melt-spun with varying copper wheel speeds, which led to a grain size of 1–2 µm. This value was further reduced to sub-micrometer size by a small addition of Boron (1 at%). The spinodal decomposition was induced through a two-step annealing treatment under magnetic field in the range of 600–900 °C. It was found that the size of the spinodal structures is not influenced much by increased wheel speeds but becomes smaller with the addition of Boron. However, the difference in coercivity between the samples with and without Boron is only 50 Oe (4 kA/m). To study the influence of the annealing treatment two sets of samples are compared, one with the highest coercivity (366 Oe/29 kA/m) and the other one with lower coercivity (180 Oe/14.5 kA/m). We found with Scanning transmission electron microscopy Energy-dispersive X-ray spectroscopy (STEM EDX) a much sharper chemical interface between the α1 and α2 precipitates in the former sample, which we attribute to be the main reason for the higher coercivity.

A room temperature magnetocaloric material with composition Mn 1.25 Fe 1.75 Ga is reported in this paper. The as-synthesized melt-spun ribbons have the pure L2 1 cubic structure and a homogeneous microstructure with an average grain size of 6 μm and show a ferromagnetic to paramagnetic transition at 296 K. The change in entropy determined from the M versus H curves was found to be 3 J kg −1 K −1 at 296 K and 3 T, which is good in comparison with the values of commonly used materials for magnetic refrigeration. However, in comparison with other materials, the room temperature magnetocaloric effect is obtained by tuning the composition of the parent alloy without the need of another heat treatment. This important result opens opportunities for magnetic refrigeration near room temperature.

Arc-melted (Zr1−xCex)1.1Fe10Si2 alloys were found to crystallize into a pure or nearly pure ThMn12 structure for 0 ≤ x ≤ 0.6. At room temperature, the alloys exhibit ferromagnetism with an uniaxial magnetocrystalline anisotropy. Metastable ZrFe10Si2 compound possesses room-temperature saturation magnetization of at least 11 kG and Curie temperature of 325 °C; both properties slightly decrease when Ce is being substituted for Zr. The anisotropy field, on the other hand, increases with the Ce from 16.9 to 24 kOe at x = 0.6. These intrinsic magnetic characteristics as well as the absence of expensive rare-earths and Co make the compounds interesting for development of low-cost permanent magnets. At 0.7 ≤ x ≤ 0.8, the ThMn12 structure was found to co-exist with the Th2Ni17-type structure, whereas the equilibrium Th2Zn17-type structure was observed only at x = 1. Prepared under similar conditions Hf1.1Fe10Si2 alloy does not crystallize into the ThMn12-type structure.

In this paper, we explore the possibility of producing hexagonal Cobalt nanoparticles, with high saturation magnetization by direct chemical synthesis. The nanoparticles were synthesized by reduction of anhydrous cobalt (II) chloride by NaBH 4 in tetraglyme at temperatures in the range of 200–270 °C under a nitrogen–hydrogen atmosphere. The reactions were done at high temperatures to allow for the formation of as-made hexagonal cobalt. The size of the particles was controlled by the addition of different surfactants. The best magnetic properties so far were obtained on spherical hexagonal Co nanoparticles with an average size of 45 nm, a saturation magnetization of 143 emu/g and coercivity of 500 Oe. the saturation magnetization and coercivity were further improved by annealing the Co nanoparticles leading to saturation magnetization of 160 emu/g and coercivity of 540 Oe.

The temperature of the annealing treatment applied to the MnBi alloys with the intention to stabilize the α-MnBi “low-temperature” phase – a promising material for the rare-earth-free permanent magnets – is usually kept below the range of the β-Mn1.08Bi “high-temperature” phase. This work is aimed at the investigation of the effects of partial and complete α → β phase transformation in bulk magnetic-field-annealed magnets on their structure and magnetic properties. MnBi alloys modified with In, Mg and Sb were melt-spun, compacted to their full density and then heated in a magnetic field of 30 kOe to temperatures sufficiently high for the beginning of the α → β transformation. A heating of 15 °C/min was used which was immediately followed by cooling at the rate of 7.5 °C/min; the degree of the phase transformation was measured by in situ monitoring of changes in the sample magnetization. In all the studied alloys, the α → β transformation was found to improve the [0 0 1] fiber texture induced by the applied magnetic field, although it simultaneously decreased the coercivity. A partial transformation has resulted in a 11% increase of the maximum energy product for the Mn50Bi49Sb0.5In0.5 magnet, to 8.3 MGOe, and a 3% increase for the Mn50Bi46Mg3Sb0.5In0.5 magnet, to 12.8 MGOe. In the latter alloy, the α → β and reverse transformations were found to initially occur in well-defined pockets leading to clusters of large, highly textured α crystallites. In the Mn50Bi48.5Sb1.5 magnet, only part of the α phase could be recovered after the transformation, which instead promoted the paramagnetic MnBi0.9Sb0.1 phase.

A treatment consisting of annealing in a moderately strong, ≤30 kOe, magnetic field was applied to arc-melted ingots and compacted melt-spun alloys with composition Mn50Bi50-x-y-zMgxInySbz. The high degree of texture of the magnetic α phase induced with annealing of the ingot samples (except the Mn–Bi–Sb one) allowed for accurate determination of the phase anisotropy field. Alloying was found to increase the anisotropy field from 45 kOe in the Mn50Bi50 alloy to 65 kOe in the Mn50Bi45Mg3In0.5Sb1.5 alloy. In the compacted melt-spun alloys subjected to field annealing, a higher degree of texture could be achieved after compaction which had involved less heating, when a smaller fraction of the metastable β′ phase had been converted into the α phase; compaction at room temperature allowed for the highest degree of texture in the Mn50Bi45Mg3In0.5Sb1.5 magnet. The addition of indium was found to decrease the solidus temperatures of the alloys, facilitating the development of the α phase during the field annealing and modifying the properties of the resulting magnets. Introduction of indium resulted in a higher coercivity Hc of the Mn–Bi–Mg–In magnets and in a better texture of the Mn–Bi–Mg–Sn–In magnets. The former effect allowed, in particular, for a viable maximum energy product (BH)max of 10 MGOe to be developed in the Mn50Bi46.5Mg3In0.5 magnet through annealing in a relatively low field of 15 kOe, whereas the latter effect allowed for an excellent combination of (BH)max = 11.6 MGOe and Hc = 8.5 kOe in the Mn50Bi45Mg3In0.5Sb1.5 magnet. The highest (BH)max of 12 MGOe was realized in the Mn50Bi46Mg3In0.5Sb0.5 magnet.

The structural and magnetic properties of melt-spun Co-Ge intermetallic alloys have been investigated over a wide range of compositions Co62+xGe38−x (x = 0, 1, 2, 3, 4, 4.7, 8 and 18). The research is practically motivated by inconsistencies in the literature data regarding phase equilibria in the Co-rich part of the Co-Ge phase diagram. We resolve these contradictions by showing that the structural and magnetic properties of Co62+xGe38−x melt-spun alloys depend on the cooling rate, the composition, and, as a result, on the lattice parameters of the phases present. A new orthorhombic phase with a high Curie temperature of 805 K has been observed and reported for the first time.

A method has been developed for manufacturing rare-earth-free MnBi-based magnets capable of filling the existing “gap” between the ferrite and rare-earth permanent magnets. Whereas the earlier approaches relied on sintering of fine and easily degrading single-crystal MnBi particles, the new method achieves refinement of the key α-MnBi phase through melt-spinning combined with appropriate alloying. Modification of the MnBi alloys with Mg and Sb generates a high-coercivity nanostructure of the metastable β' phase. A subsequent compaction at 150 °C produces fully dense materials while converting the β' phase into the stable α phase. Finally, a short annealing at 265–300 °C in a magnetic field of 3 T increases the fraction of the α phase to 97–98% and aligns the c axes of the α crystallites. A maximum energy product (BH)max of 11.5 MGOe and an intrinsic coercivity Hc of 5.6 kOe have been obtained in a magnet with the nominal composition Mn50Bi46.5Mg3Sb0.5. Because the coercivity increases with temperature, the maximum energy product of this magnet is still as high as 8.9 MGOe at 175 °C. Increasing the Sb content to 1.5 at.% increases the Hc to 9.3 kOe, but at the same time inhibits the development of the texture thus decreasing the (BH)max. The addition of Mg was found to increase the c lattice parameter of the α phase resulting in an unusually large ratio c/a ≈ 1.432.