ThMn12-type Structure Research Articles

Explanation of structural differences and similarities between the AT2Al10 phases (where A=actinide, lanthanide or rare earth element and T=transition metal)

Abstract In the current work we have studied the crystallographic relationship between the AT2Al10 phases (where A = actinide, lanthanide and rare earth element and T = transition metal). It is known that with this stoichiometry two structure types exist: tetragonal CaCr2Al10 and orthorhombic YbFe2Al10. It was found that both CaCr2Al10 and YbFe2Al10 types are structural derivatives of the ThMn12 type structure (which has more general formula of AT x Al12− x , with x > 2). CaCr2Al10 structure has a group-subgroup relationship with the ThMn12 structure, while the relationship of the YbFe2Al10 to the ThMn12 was proved applying the strong reflection approach, suggested initially for approximants of quasi-crystals. Proposed here relationship between the studied structures explains the small difference in total energies, calculated using Density Functional Theory. Understanding the connection between these phases allows regarding AT2Al10 composition as somewhat extension of the AT x Al12− x compositional range. Due to the unique magnetic properties of the AT x Al12− x phases, tunable as a function of crystallographic structure, study of structural stability and crystallographic relationships of related phases are of outmost importance.

Effect of Melt Spinning and Nitriding on the Structure and Magnetic Hysteresis Properties of (Nd1 −xCex)Fe11Ti Alloys (0 ≤ x ≤ 0.3)

The effect melt spinning, nitriding, and cerium doping have on the structural and magnetic hysteresis properties of alloys based on NdFe11Ti compound with ThMn12-type structure is studied at ambient temperature. It is established that the best combination of magnetic hysteresis properties is observed for (Nd1 – xCex)Fe11Ti alloys at x = 0.2 and 0.3 after melt spinning and nitriding.

Effect of cobalt substitution on structure and magnetic properties of Nd0.4Zr0.6Fe10–xCoxSi2 (x = 0–3) alloys and their ribbons

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 Mössbauer 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.

ThMn12-Type Alloys for Permanent Magnets

Abstract 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.

Structure and magnetic properties of (Sm0.9Zr0.1)Fe11Ti alloys with ThMn12-type structure

Abstract Rapidly increasing demand for high-energy permanent magnets and volatility of the rear-earth market encourage a search of hard magnetic materials that can compete with those based on the Nd2Fe14B phase. Sm-Fe compounds with the ThMn12-type crystal structure are considered as promising candidates for it. However, their synthesis and achievement of theoretically predicted magnetic properties are still challenging tasks that have not been solved yet. This paper addresses this problem. Its aim was to synthesize and to study magnetic properties of the (Sm0.9Zr0.1)Fe11Ti compound with ThMn12-type structure. Initially amorphous alloy was obtained by melt spinning and its subsequent heat treatments were performed. Structural and phase transformations of the (Sm0.9Zr0.1)Fe11Ti alloy with annealing temperatures were studied along with magnetic properties. For the optimally annealed alloy features of magnetization reversal were discussed and temperature dependences of coercivity and maximum energy product were obtained.

An investigation of the Gd-Fe-Cr phase diagram: Phase equilibria at 800 °C

The phase equilibria of Gd-Fe-Cr system were investigated at 800 °C by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). The isothermal section of the phase diagram consisted of eight single-phase regions, eight two-phase regions and seven three-phase regions. The binary compounds GdFe2, GdFe3, Gd2Fe17, σ(FeCr) and the ternary compound GdFe10Cr2 were confirmed. Conversely, the binary compound Gd6Fe23 was unstable at the level of 800 °C. The maximum solid solubilities of Cr in GdFe2 and Gd2Fe17 were about 11.7 and 13.2 at. % Cr respectively. The compound GdFe12−xCrx with the ThMn12-type structure (space group I4/mmm) has a broad solubility ranging from 15.4 to 23.1 at. % Cr.

Nitrogenation and sintering of (Nd-Zr)Fe10Si2 tetragonal compounds for permanent magnets applications

Nd-xZrxFe10Si2 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 increases from 280 to 300 °C to about 390 °C and from 2.8 to 3 Tesla to 4.5–5 Tesla 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 °C, but segregated Fe-Si at elevated temperatures. Nitrogenation and SPS processing are, therefore, appropriate for sintering metastable materials such as (NdZr)Fe10Si2 into compact material without loosing functional properties. This opens a way towards a new family of permanent magnets, lean of critical raw materials.

The origin of high magnetic properties in (R,Zr)(Fe,Co)11.0–11.5Ti1.0–0.5Ny (y=1.0–1.4 for R=Nd, y=0 for R=Sm) compounds

Ten alloys and nitrogenated compounds of (R,Zr)(Fe,Co)11.0–11.5Ti1.0–0.5Ny (y=1.0–1.4 for R=Nd, y=0 for R=Sm) with a ThMn12-type structure were prepared. The average Fe–Fe interatomic distances, d(Fe–Fe), for Fe sites were calculated based on the reported atomic parameters. The hyperfine splittings (inner field (IF), in teslas) were measured by Mössbauer spectroscopy, and the IF increased with increasing d(Fe–Fe) for Fe sites, indicating a magneto-volume effect. The order of IF magnitude in Fe sites was Fe(8i)>Fe(8j)>Fe(8f) in all alloys. Co substitution for Fe sites, (Fe0.75Co0.25), increased the IF by 25% for the R=Nd alloy and 15% for the R=Sm alloy. Decreasing Ti content from −Ti1.0 to −Ti0.5, which increased the Fe and Co content, preserved the ThMn12 structure with Zr substitution for R(2a) sites, and caused a slight increase in the IF of 2% for the R=Nd alloy and 7% for the R=Sm alloy. Nitrogenation, where N was introduced into the 2b sites, also increased IF in R=Nd alloys, by 23% for the Co- and Zr-free alloys, NdFe11Ti1.0N1.5, and by 7% for the Co-containing, (Nd0.7Zr0.3) (Fe0.75Co0.25) 11.5Ti0.5N1.3 alloy. The IF values of the R=Nd alloys were slightly larger than those of the R=Sm alloys. In conclusion, the magneto-volume effect was clearly observed at the Fe sites, and Co substitution into Fe sites and nitrogenation (R=Nd alloys) compensated for the increased IF. Increasing the Fe and Co fractions also increased IF slightly.

Effect of Zr substitution on the structural and magnetic properties of the series Nd1−xZrxFe10Si2 with the ThMn12 type structure

Alloys of the ThMn12-type structure with the general formula Nd1−xZrxFe10Si2 (0.4 ≤ x ≤ 0.8) have been prepared by arc-melting the constituent elements. X-ray diffraction and Mössbauer spectroscopy measurements indicate the formation of the tetragonal ThMn12-type structure with substitution of Nd with Zr, without further annealing. The lattice parameters a and c and the unit cell volume follow Vegard’s law the metallic radius of the Zr (1.602 Å) substitution being smaller than that of the Nd (1.821 Å) metallic radius. The Curie temperature decreases linearly with Zr substitution from 301 °C for x = 0 to 285 °C for x = 0.8. The saturation magnetization and magnetic anisotropy fields were also measured as a function of Zr substitution with the room temperature values ranging from 12.5 kG and 30 kOe for x = 0.4 to 11.2 kG and 26 kOe for x = 0.8. The success of this synthesis contributes to the search for novel magnetic phases, lean or free of critical raw materials, towards a new generation of permanent magnets.

Structure and permanent magnet properties of Zr1-xRxFe10Si2 alloys with R = Y, La, Ce, Pr and Sm

Arc-melted (Zr0.7R0.3)1.1Fe10Si2 alloys with R = Y, Ce, Pr and Sm crystallize into the ThMn12-type structure; La substitution leads to the formation of a separate NaZn13-type phase. Compared to the rare-earth-free parent compound, Y, Ce and Sm strengthen the magnetocrystalline anisotropy in this order; whereas Pr weakens it. The effect of Sm is by far the strongest; an anisotropy field of 40.7 kOe determined for the Zr0.7Sm0.3Fe10Si2 by the singular point detection technique is high enough to make this compound a good candidate for a very rare-earth-lean, cobalt-free permanent magnet material. The effect of Sm has been demonstrated in isotropic nanocrystalline alloys prepared via melt-spinning followed by vacuum annealing. The optimally processed alloys with R represented by Zr and Sm exhibited coercivity values of 0.7 and 2.4 kOe, respectively. The analysis suggests that the development of the coercivity is controlled by decomposition of the 1:12 structure in the former sample and by the crystallite growth in the latter. Calculated remanence and maximum energy product of the Sm-substituted nanocrystalline alloy were 6.3 kG and 4.6 MGOe. The cubic Zr6Fe16Si7 compound, which is the frequent minority phase in the studied alloys, was also synthesized and found to be paramagnetic at the room temperature.

The isothermal section phase diagram of the Sm-Fe-Ni ternary system at 800 °C

Isothermal section at 800 °C of the ternary Sm-Fe-Ni system was established in the whole concentration range, by means of powder X-ray diffraction and scanning electron microscopyenergy dispersive X-ray spectroscopy (SEM/EDS). All measured compositions and unit-cell refinements were performed at room temperature from quenched samples annealed at 800 °C for one week. Ten binary compounds and α(Fe1−x Nix) (Im−3m-W type structure) have been confirmed.The SmNi5−xFex (P6/mmm-CaCu5 type structure) was found to have a solubility range from 0 to 50 at% Fe. The maximum solubility at 800 °C of Fe in Sm2Ni7−xFex (P63/mmc-Ce2Ni7 type structure), SmNi3−xFex (R3¯m-PuNi3 type structure) and SmNi2−xFex (Fd−3m-MgCu2 type structure) are about 2.7 at% Fe, 40 at% Fe and 6.66 at% Fe, respectively. The homogeneity domain of Sm2Fe17−xNix (R3¯m-Th2 Zn17 type structure) ranges from 0 to 21.05 at% Ni. The XRD and the SEM/EDS analysis show that there are no solubility of Fe in the Sm2 Ni17, Sm5Ni19 and SmNi. The same analysis show that there are no solubility of Ni in the SmFe2 and SmFe3 and no ternary compound was found in all ternary alloy samples. The addition of Ni in the Sm-Fe system did not stabilize the ThMn12 type structure.

ThMn12-type structure and uniaxial magnetic anisotropy in ZrFe10Si2 and Zr1−Ce Fe10Si2 alloys

Abstract 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.

High-field magnetization study of a HoFe6Al6 single crystal

Magnetization measurements have been performed in static (up to 14 T) and pulsed (up to 60 T) magnetic fields on a HoFe6Al6 single crystal with a tetragonal crystal structure of ThMn12 type. HoFe6Al6 is a ferrimagnet with exact compensation of the Ho and Fe sublattices at low temperatures, the spontaneous magnetic moment is Ms = 0 at 4.2 K. At elevated temperatures, Ms passes through a wide maximum (4.2 μB at 200 K) and vanishes at the Curie temperature, TC = 315 K. The compound displays a high magnetic anisotropy of the easy-plane type, a noticeable anisotropy exists also within the easy plane with the [110] axis as the easy magnetization direction. HoFe6Al6 exhibits two field-induced magnetic phase transitions along the easy [110] axis and two more along the [100] axis, the hard direction in the basal plane. The critical fields of the lower- and higher-field transitions display qualitatively different temperature dependencies. A T–H magnetic phase diagram has been constructed for the [100] and [110] directions. The Ho–Fe inter-sublattice exchange constant has been determined to be nHoFe = 3.8 T/μB.

Magnetic properties of Ce–Nd–Fe–Mo alloys and their nitrides

New quaternary alloys of Ce1−xNdxFe12−yMoy with x=0, 0.2, 0.4, 0.6, 0.8, 1 and y=0, 1.5, 2 have been prepared and magnetically hardened by melt spinning. X-ray diffraction indicates that the as-spun materials exhibit the tetragonal ThMn12-type structure. Prior to nitriding, the coercivity Hci is less than 0.6kOe in all alloys and is independent of Nd content, while the magnetization 4πM19 (measured in an applied field of 19kOe) and Curie temperature Tc increase with added Nd content x. The effects of nitriding pressure P, time t, and temperature T on magnetic properties have been carefully evaluated on NdFe10Mo2 in order to identify the optimal nitriding parameters. The optimized nitriding profile was subsequently adopted to nitride the remaining samples. After nitrogenation, Tc and 4πM19 have been substantially enhanced primarily due to the increased Fe–Fe exchange from nitrogen induced lattice dilation. Benefitting from the positive contribution from Nd, Hci has been greatly improved in the Nd containing samples. As a result, Ce0.2Nd0.8Fe10Mo2 nitride features Hci=2.9kOe and (BH)max=1.6MGOe and Ce0.2Nd0.8Fe10.5Mo1.5 nitride demonstrates Hci=2.5kOe, (BH)max=1.5MGOe at room temperature, and Tc=337°C, which are substantial advancements compared to the pure Ce based ThMn12-type materials previously reported.

Correlation between the nanocrystalline Sm(Fe,Mo)12 and its out of equilibrium phase Sm(Fe,Mo)10

Nanostructured Sm–Fe–Mo semi-hard magnetic material exhibiting enhanced magnetic properties can be produced by ball milling followed by recrystallization. Milled samples were annealed for 30min in a vacuum at different temperatures (TA) between 700 and 1190°C. The effects of heat treatment and Mo content on structural and magnetic property changes have been investigated by means of X-ray diffraction using the Rietveld method, transmission electron microscopy and magnetic measurements. For samples annealed at TA>900°C the tetragonal ThMn12-type structure is identified, while for 700<TA<900°C a new out of equilibrium P6/mmm type structure was found as the major phase. This novel nanocrystalline phase has never been synthesized before. The correspondent stoichiometry is determined on the basis of the vacancy model. The Rietveld analysis gives a stoichiometry ratio equal to 1:10, for the out of equilibrium hexagonal phase, which is described with three crystallographic transition metal sites: 3g is fully occupied, 6l occupation is limited to hexagons surrounding the Fe dumbbell pairs 2e. We have performed a magnetic and structural study of nanocrystalline metastable P6/mmm Sm(Fe1−xMox)10, correlated with structural transformation towards its equilibrium derivative I4/mmm Sm(Fe1−xMox)12. A maximum of the coercive field HC (HC>5kOe) has been observed for the new hexagonal P6/mmm phase suggesting that nanocrystalline Sm(Fe,Mo)10, is a semi-hard material, and is potential candidate for magnetic recording.

On the YFe11Mo intermetallic characterization

Rare-earth intermetallic compounds adopting the tetragonal ThMn12–type structure and containing high Fe concentrations have attracted considerable attention in the field of permanent magnets. Among them, the Y-Fe-Mo series has been extensively investigated, especially by X-ray diffraction (XRD), but the microstructural characterization was very limited.In the present work, Y:11Fe:Mo has been prepared by melting Y, Fe and Mo in an arc furnace followed by splat-quenching and/or annealing treatments. The structure, chemistry and magnetic domain configurations of the resulting polycrystalline aggregates have been investigated by XRD, scanning and transmission electron microscopies (SEM and TEM, respectively), energy dispersive X-ray spectroscopy (EDS) and Lorentz microscopy.A special emphasis was given to Lorentz microscopy, where contrast is based on the Lorentz deflection imposed on electrons by the passage through a magnetic specimen. The magnetic domains have been imaged by the intermediate lens in Fresnel mode, with the objective lens switched off. In these conditions the intermediate lens is defocused so that out-of-focus images of the specimen are formed: the magnetic domain walls are imaged as alternate bright (convergent) and dark (divergent) lines. For the overfocused image, bright lines occur at the position of domain walls for which the magnetisation on either side is deflecting the electrons towards the wall, whereas dark lines are observed at the walls for which the magnetisation on either side is deflecting the electrons away from the wall. The opposite contrast is observed at the underfocused image.The results have shown that the tetragonal YFe11-xMoy phase is predominant, with a cellular dendritic morphology (Figure 1), but a considerable presence of -Fe(Mo) could be inferred, forming a coarse intercellular eutectic mixture. Significant Fe segregation occurred during annealing. However, this composition variation corresponded to an extremely limited evolution of the lattice parameters, the Rietveld analysis pointing to Fe vacancies at the 8i sites on the annealed material. XRD and EDS results indicate that the fraction of point defects in the ThMn12-type structure adapts to the processing route and that the stable configuration depends on the temperature. The current study also showed that grain boundaries are usually associated with domain walls and that YFe11Mo grains present internal domain walls forming stripe/maze patterns characteristic of high anisotropy materials (Figure 2), while residual -Fe(Mo) grains exhibit vortex configurations (see arrows in Figures 2 (a-c)).The work was supported by the Portuguese Science Foundation through the CTM/48617/2002 and PEst-OE/CTM-UI0084/2011 grants.

Magnetization study of a GdFe5Al7 single crystal

A GdFe5Al7 single crystal with the tetragonal ThMn12-type structure has been studied in magnetic fields up to 14 T. The compound is ordered ferrimagnetically below T C = 262 K. Magnetization isotherms measured along the main crystallographic directions indicate that GdFe5Al7 displays easy-plane anisotropy. Anisotropy is also present within the basal plane between the [100] and [110] axes. The easy magnetization direction is the [110] axis with the spontaneous magnetic moment M s = 0.58 µ B /f.u. at T = 2 K.

X-ray photoelectron spectroscopy and magnetic properties of CeCo7Mn5 and CeCo8Mn4 isostructural ThMn12 type compounds

The magnetic properties of CeCo7Mn5 and CeCo8Mn4 compounds have been investigated by combining X-ray photoelectron spectroscopy (XPS) and magnetic measurements in a wide temperature range (4–550) K and magnetic field up to 12 T. X-ray powder diffraction (XRD) measurements showed that CeCo7Mn5 and CeCo8Mn4 compounds are isostructural and crystallize in the ThMn12 structure type. XPS spectra pointed out the intermediate valence state of Ce atoms and that both Co and Mn atoms carry magnetic moments. The complex magnetic structure of CeCo7Mn5 and CeCo8Mn4 is determined by the competition between the ferromagnetic (Co–Co pairs) and antiferromagnetic (Co–Mn and Mn–Mn pairs) interactions. Two different ordering temperatures TN and TC correlated to antiferromagnetic and ferromagnetic coupling of 3d magnetic moments, respectively, are evidenced. Magnetic moments of about 1.6 μB/Co and 3.2 μB/Mn atoms were determined by correlating the magnetic data of the two compounds, in good agreement with the exchange splitting of XPS Co 3s and Mn 3s core levels.

Synthesis, Structure, and Magnetic and Electrical Properties of Yb(Mn,M)xAl12–x (M = Fe, Ru; x ≤ 2.5) Phases

Single crystals of Yb(Mn,M)xAl12–x (M = Fe, Ru; x ≤ 2.5) were grown utilizing an aluminum self-flux. Products crystallized in three structure types, the ThMn12 structure type, and two structural derivatives, the CaCr2Al10 structure type and the YbFe2Al10 structure type, depending on the starting amount of Mn relative to M. We outline the synthetic parameters used to obtain products in all three structure types and specifically address how stabilizing products in one structure type over another is a function of Mn:M reaction ratios. All compounds are structurally characterized with single crystal X-ray diffraction, and Yb(Mn,Fe)xAl12–x compounds are additionally characterized with temperature-dependent magnetic susceptibility and resistivity measurements. Yb(Mn,Fe)xAl12–x compounds exhibit paramagnetic behavior down to 2 K, as well as metallic resistivity down to ∼50 K. Below 50 K, Yb(Mn,Fe)xAl12–x compounds exhibit upturns in resistivity that indicate the onset of Kondo interactions.

Structure Properties of the ${\rm YFe}_{11}{\rm Mo}$ Intermetallic Compound

The structure, chemistry and magnetic domain configurations of YFe11Mo polycrystalline aggregates have been investigated by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and Lorentz microscopy. Although extensive reports on the magnetic properties of this compound can be found in the literature, its exact equilibrium structure remains elusive. The compound was prepared by arc-melting followed by splat-quenching and subsequent annealing. The quenched material consisted of a ThMn12-type matrix and α-Fe(Mo) impurity phase. Annealing lead to iron segregation from the matrix, as evidenced by a clear shift to higher angles of the α -Fe(Mo) peak observed in the diffraction pattern. Nevertheless, the composition variation in the intermetallic compound was accompanied by an extremely limited evolution of the lattice parameters, suggesting the presence of Fe vacancies after annealing. Rietveld analysis carried out on the annealed material points to Fe vacancies at the 8i sites. The present results indicate that the fraction of point defects in the ThMn12-type structure adapts to the processing route and that the stable configuration depends on the temperature. YFe11Mo grains present internal domain walls forming stripe/maze patterns characteristic of high anisotropy materials, while residual α -Fe(Mo) grains exhibit vortex configurations. Annealing hardly affected the magnetic domain configurations of the YFe11Mo and α-Fe(Mo) aggregates.