Effects of the Addition of Co or Ni Atoms on Structure and Magnetism of FeB Amorphous Alloy: Ab Initio Molecular Dynamics Simulation

The effects of the substitution of Fe by Co or Ni on both the structure and the magnetic properties of FeB amorphous alloy were investigated using first-principle molecular dynamics. The pair distribution function, Voronoi polyhedra, and density of states of Fe_80−xTM_xB₂₀ (x = 0, 10, 20, 30, and 40 at.%, TM(Transition Metal): Co, Ni) amorphous alloys were calculated. The results show that with the increase in Co content, the saturation magnetization of Fe_80−xCo_xB₂₀ (x = 0, 10, 20, 30, and 40 at.%) amorphous alloys initially increases and then decreases upon reaching the maximum at x = 10 at.%, while for Fe_80−xNi_xB₂₀ (x = 0, 10, 20, 30, and 40 at.%), the saturation magnetization decreases monotonously with the increase in Ni content. Accordingly, for the two kinds of amorphous alloys, the obtained simulation results on the variation trends of the saturation magnetization with the change in alloy composition are in good agreement with the experimental observation. Furthermore, the relative maximum magnetic moment was recorded for Fe₇₀Co₁₀B₂₀ amorphous alloy, due to the induced increased magnetic moments of the Fe atoms surrounding the Co atom in the case of low Co dopant, as well as the increase in the exchange splitting energy caused by the enhancement of local atomic symmetry.     

Investigation of Mechanical and Magnetic Properties of Co-Based Amorphous Powders Obtained by Atomization

Properties of Co-based alloys with high Glass Forming Ability (GFA) in the form of powder are still not widely known. However, powders of high GFA alloys are often used for the development of bulk metallic glasses by additive manufacturing. In this work Co_47.6B_21.9Fe_20.4Si_5.1Nb₅% at. and Co₄₂B_26.5Fe₂₀Ta_5.5Si₅Cu₁% at. were developed by gas-atomization. Obtained powders in size 50–80 µm were annealed at T_g and T_x of each alloy. Then SEM observation, EDS analyses, differential thermal analysis, X-ray diffraction, nanoindentation, Mössbauer, and magnetic properties research was carried out for as-atomized and annealed states. The gas atomization method proved to be an efficient method for manufacturing Co-based metallic glasses. The obtained powder particles were spherical and chemically homogeneous. Annealing resulted in an increase of mechanical properties such as hardness and the elastic module of Co_47.6B_21.9Fe_20.4Si_5.1Nb₅% at and Co₄₂B_26.5Fe₂₀Ta_5.5Si₅Cu₁%, which was caused by crystallization. The magnetic study shows that Co_47.6B_21.9Fe_20.4Si_5.1Nb₅ and Co₄₂B_26.5Fe₂₀Ta_5.5Si₅Cu₁ are soft magnetic and semi-hard magnetic materials, respectively.     

Phase Formation,Microstructure, and Magnetic Properties of Nd14.5Fe79.3B6.2 Melt-Spun Ribbons with Different Ce and Y Substitutions

Phase formation and microstructure of (Nd_1-2xCe_xY_x)_14.5Fe_79.3B_6.2 (x = 0.05, 0.10, 0.15, 0.20, 0.25) alloys were studied experimentally. The results reveal that (Nd_1-2xCe_xY_x)_14.5Fe_79.3B_6.2 annealed alloys show (NdCeY)₂Fe₁₄B phase with the tetragonal Nd₂Fe₁₄B-typed structure (space group P4₂/mnm) and rich-RE (α-Nd) phase, while (Nd_1-2xCe_xY_x)_14.5Fe_79.3B_6.2 ribbons prepared by melt-spun technology are composed of (NdCeY)₂Fe₁₄B phase, α-Nd phase and α-Fe phase, except for the ribbon with x = 0.25, which consists of additional CeFe₂ phase. On the other hand, magnetic properties of (Nd_1-2xCe_xY_x)_14.5Fe_79.3B_6.2 melt-spun ribbons were measured by a vibrating sample magnetometer (VSM). The measured results show that the remanence (B_r) and the coercivity (H_cj) of the melt-spun ribbons decrease with the increase of Ce and Y substitutions, while the maximum magnetic energy product ((BH)_max) of the ribbons decreases and then increases. The tendency of magnetic properties of the ribbons could result from the co-substitution of Ce and Y for Nd in Nd₂Fe₁₄B phase and different phase constitutions. It was found that the H_cj of the ribbon with x = 0.20 is relatively high to be 9.01 kOe, while the (BH)_max of the ribbon with x = 0.25 still reaches to be 9.06 MGOe. It suggests that magnetic properties of Nd-Fe-B ribbons with Ce and Y co-substitution could be tunable through alloy composition and phase formation to fabricate novel Nd-Fe-B magnets with low costs and high performance.     

Effect of Co Substitution and Thermo-Magnetic Treatment on the Structure and Induced Magnetic Anisotropy of Fe84.5−xCoxNb5B8.5P2 Nanocrystalline Alloys

In the present work, we investigated in detail the thermal/crystallization behavior and magnetic properties of materials with Fe_84.5-xCo_xNb₅B_8.5P₂ (x = 0, 5, 10, 15 and 20 at.%) composition. The amorphous ribbons were manufactured on a semi-industrial scale by the melt-spinning technique. The subsequent nanocrystallization processes were carried out under different conditions (with/without magnetic field). The comprehensive studies have been carried out using differential scanning calorimetry, X-ray diffractometry, transmission electron microscopy, hysteresis loop analyses, vibrating sample magnetometry and Mössbauer spectroscopy. Moreover, the frequency (up to 300 kHz) dependence of power losses and permeability at a magnetic induction up to 0.9 T was investigated. On the basis of some of the results obtained, we calculated the values of the activation energies and the induced magnetic anisotropies. The X-ray diffraction results confirm the surface crystallization effect previously observed for phosphorous-containing alloys. The in situ microscopic observations of crystallization describe this process in detail in accordance with the calorimetry results. Furthermore, the effect of Co content on the phase composition and the influence of annealing in an external magnetic field on magnetic properties, including the orientation of the magnetic spins, have been studied using various magnetic techniques. Finally, nanocrystalline Fe_64.5Co₂₀Nb₅B_8.5P₂ cores were prepared after transverse thermo-magnetic heat treatment and installed in industrially available portable heating equipment.     

Study on the Magnetocaloric Effect of Room Temperature Magnetic Refrigerant Material La0.5Pr0.5(Fe1−xCox)11.4Si1.6 and the Effect Arising from Co Doping on Its Curie Temperature

The arc-melting method was adopted to prepare the compound La_0.5Pr_0.5(Fe_1−xCo_x)_11.4Si_1.6 (x = 0, 0.02, 0.04, 0.06, 0.08), and the magnetocaloric effect of the compound was investigated. As indicated by the powder X-ray diffraction (XRD) results, after receiving 7-day high temperature annealing at 1373 K, all the compounds formed a single-phase cubic NaZn₁₃ crystal structure. As indicated by the magnetic measurement, the most significant magnetic entropy change |∆S_M(T)| of the sample decreased from 28.92 J/kg·K to 4.22 J/kg·K with the increase of the Co content under the 0–1.5 T magnetic field, while the Curie temperature T_C increased from 185 K to the room temperature 296 K, which indicated that this series of alloys are the room temperature magnetic refrigerant material with practical value. By using the ferromagnetic Curie temperature theory and analyzing the effect of Co doping on the exchange integral of these alloys, the mechanism that the Curie temperature of La_0.5Pr_0.5(Fe_1−xCo_x)_11.4Si_1.6 and La_0.8Ce_0.2(Fe_1−xCo_x)_11.4Si_1.6 increased with the increase in the Co content was reasonably explained. Accordingly, this paper can provide a theoretical reference for subsequent studies.     

Structural,Magnetic, Dielectric and Piezoelectric Properties of Multiferroic PbTi1−xFexO3−δ Ceramics

PbTi_1−xFe_xO_3−δ (x = 0, 0.3, 0.5, and 0.7) ceramics were prepared using the classical solid-state reaction method. The investigated system presented properties that were derived from composition, microstructure, and oxygen deficiency. The phase investigations indicated that all of the samples were well crystallized, and the formation of a cubic structure with small traces of impurities was promoted, in addition to a tetragonal structure, as Fe³⁺ concentration increased. The scanning electron microscopy (SEM) images for PbTi_1−xFe_xO_3−δ ceramics revealed microstructures that were inhomogeneous with an intergranular porosity. The dielectric permittivity increased systematically with Fe³⁺ concentration, increasing up to x = 0.7. A complex impedance analysis revealed the presence of multiple semicircles in the spectra, demonstrating a local electrical inhomogeneity due the different microstructures and amounts of oxygen vacancies distributed within the sample. The increase of the substitution with Fe³⁺ ions onto Ti⁴⁺ sites led to the improvement of the magnetic properties due to the gradual increase in the interactions between Fe³⁺ ions, which were mediated by the presence of oxygen vacancies. The PbTi_1−xFe_xO_3−δ became a multifunctional system with reasonable dielectric, piezoelectric, and magnetic characteristics, making it suitable for application in magnetoelectric devices.     

Structure and Properties of Heat-Resistant Alloys NiAl–Cr–Co–X (X = La,Mo, Zr,Ta, Re) and Fabrication of Powders for Additive Manufacturing

The NiAl–Cr–Co–X alloys were produced by centrifugal self-propagating high-temperature synthesis (SHS) casting. The effects of dopants X = La, Mo, Zr, Ta, and Re on combustion, as well as the phase composition, structure, and properties of the resulting cast alloys, have been studied. The greatest improvement in overall properties was achieved when the alloys were co-doped with 15% Mo and 1.5% Re. By forming a ductile matrix, molybdenum enhanced strength characteristics up to the values σ_ucs = 1604 ± 80 MPa, σ_ys = 1520 ± 80 MPa, and ε_pd = 0.79%, while annealing at T = 1250 ℃ and t = 180 min improved strength characteristics to the following level: σ_ucs = 1800 ± 80 MPa, σ_ys = 1670 ± 80 MPa, and ε_pd = 1.58%. Rhenium modified the structure of the alloy and further improved its properties. The mechanical properties of the NiAl, ZrNi₅, Ni_0.92Ta_0.08, (Al,Ta)Ni₃, and Al(Re,Ni)₃ phases were determined by nanoindentation. The three-level hierarchical structure of the NiAl–Cr–Co+15%Mo alloy was identified. The optimal plasma treatment regime was identified, and narrow-fraction powders (fraction 8–27 µm) characterized by 95% degree of spheroidization and the content of nanosized fraction <5% were obtained.     

Influence of Cu Content on Structure,Thermal Stability and Magnetic Properties in Fe72−xNi8Nb4CuxSi2B14 Alloys

The effect of substitution of Fe by Cu on the crystal structure and magnetic properties of Fe_72−xNi₈Nb₄Cu_xSi₂B₁₄ alloys (x = 0.6, 1.1, 1.6 at.%) in the form of ribbons was investigated. The chemical composition of the materials was established on the basis of the calculated minima of thermodynamic parameters: Gibbs free energy of amorphous phase formation ΔG^amorph (minimum at 0.6 at.% of Cu) and Gibbs free energy of mixing ΔG^mix (minimum at 1.6 at.% of Cu). The characteristic crystallization temperatures T_x1onset and T_x1 of the alpha-iron phase together with the activation energy E_a for the as-spun samples were determined by differential scanning calorimetry (DSC) with a heating rate of 10–100 °C/min. In order to determine the optimal soft magnetic properties, the wound cores were subjected to a controlled isothermal annealing process in the temperature range of 340–640 °C for 20 min. Coercivity Hc, saturation induction Bs and core power losses at B = 1 T and frequency f = 50 Hz P_10/50 were determined for all samples. Moreover, for the samples with the lowest Hc and P_10/50, the magnetic losses were determined in a wider frequency range 50 Hz–400 kHz. The real and imaginary parts of the magnetic permeability µ′, µ″ along with the cut-off frequency were determined for the samples annealed at 360, 460, and 560 °C. The best soft magnetic properties (i.e., the lowest value of Hc and P_10/50) were observed for samples annealed at 460 °C, with Hc = 4.88–5.69 A/m, Bs = 1.18–1.24 T, P_10/50 = 0.072–0.084 W/kg, µ′ = 8350–10,630 and cutoff frequency at 8–9.3 × 10⁴ Hz. The structural study of as-spun and annealed ribbons was carried out using X-ray diffraction (XRD) and a transmission electron microscope (TEM).     

Large Low-Field Reversible Magnetocaloric Effect in Itinerant-Electron Hf1−xTaxFe2 Alloys

First-order isostructural magnetoelastic transition with large magnetization difference and controllable thermal hysteresis are highly desirable in the development of high-performance magnetocaloric materials used for energy-efficient and environmental-friendly magnetic refrigeration. Here, we demonstrate large magnetocaloric effect covering the temperature range from 325 K to 245 K in Laves phase Hf_1−xTa_xFe₂ (x = 0.13, 0.14, 0.15, 0.16) alloys undergoing the magnetoelastic transition from antiferromagnetic (AFM) state to ferromagnetic (FM) state on decreasing the temperature. It is shown that with the increase of Ta content, the nature of AFM to FM transition is gradually changed from second-order to first-order. Based on the direct measurements, large reversible adiabatic temperature change (ΔT_ad) values of 2.7 K and 3.4 K have been achieved under a low magnetic field change of 1.5 T in the Hf_0.85Ta_0.15Fe₂ and Hf_0.84Ta_0.16Fe₂ alloys with the first-order magnetoelastic transition, respectively. Such remarkable magnetocaloric response is attributed to the rather low thermal hysteresis upon the transition as these two alloys are close to intermediate composition point of second-order transition converting to first-order transition.     

Magnetic Properties of the Ferromagnetic Shape Memory Alloys Ni50+xMn27?xGa23 in Magnetic Fields

Thermal strain, permeability, and magnetization measurements of the ferromagnetic shape memory alloys Ni_50+xMn_27−xGa₂₃ (x = 2.0, 2.5, 2.7) were performed. For x = 2.7, in which the martensite transition and the ferromagnetic transition occur at the same temperature, the martensite transition starting temperature T_Ms shift in magnetic fields around a zero magnetic field was estimated to be dT_Ms/dB = 1.1 ± 0.2 K/T, thus indicating that magnetic fields influences martensite transition. We discussed the itinerant electron magnetism of x = 2.0 and 2.5. As for x = 2.5, the M⁴ vs. B/M plot crosses the origin of the coordinate axis at the Curie temperature, and the plot indicates a good linear relation behavior around the Curie temperature. The result is in agreement with the theory by Takahashi, concerning itinerant electron ferromagnets.     

Magnetic and Magneto-Caloric Properties of the Amorphous Fe92−xZr8Bx Ribbons

Magnetic and magnetocaloric properties of the amorphous Fe_92−xZr₈B_x ribbons were studied in this work. Fully amorphous Fe₈₉Zr₈B₃, Fe₈₈Zr₈B₄, and Fe₈₇Zr₈B₅ ribbons were fabricated. The Curie temperature (T_c), saturation magnetization (M_s), and the maximum entropy change with the variation of a magnetic field (−ΔS_m^peak) of the glassy ribbons were significantly improved by the boron addition. The mechanism for the enhanced T_c and −ΔS_m^peak by boron addition was studied.     

Biological Safety Evaluation and Surface Modification of Biocompatible Ti–15Zr–4Nb Alloy

We performed biological safety evaluation tests of three Ti–Zr alloys under accelerated extraction condition. We also conducted histopathological analysis of long-term implantation of pure V, Al, Ni, Zr, Nb, and Ta metals as well as Ni–Ti and high-V-containing Ti–15V–3Al–3Sn alloys in rats. The effect of the dental implant (screw) shape on morphometrical parameters was investigated using rabbits. Moreover, we examined the maximum pullout properties of grit-blasted Ti–Zr alloys after their implantation in rabbits. The biological safety evaluation tests of three Ti–Zr alloys (Ti–15Zr–4Nb, Ti–15Zr–4Nb–1Ta, and Ti–15Zr–4Nb–4Ta) showed no adverse (negative) effects of either normal or accelerated extraction. No bone was formed around the pure V and Ni implants. The Al, Zr, Nb, and Ni–Ti implants were surrounded by new bone. The new bone formed around Ti–Ni and high-V-containing Ti alloys tended to be thinner than that formed around Ti–Zr and Ti–6Al–4V alloys. The rate of bone formation on the threaded portion in the Ti–15Zr–4Nb–4Ta dental implant was the same as that on a smooth surface. The maximum pullout loads of the grit- and shot-blasted Ti–Zr alloys increased linearly with implantation period in rabbits. The pullout load of grit-blasted Ti–Zr alloy rods was higher than that of shot-blasted ones. The surface roughness (Ra) and area ratio of residual Al₂O₃ particles of the Ti–15Zr–4Nb alloy surface grit-blasted with Al₂O₃ particles were the same as those of the grit-blasted Alloclassic stem surface. It was clarified that the grit-blasted Ti–15Zr–4Nb alloy could be used for artificial hip joint stems.     

Optimization of Single α-Phase for Promoting Ferromagnetic Properties of 44Fe–28Cr–22Co–3Mo–1Ti–2V Permanent Magnet with Varying Co Concentration for Energy Storage

The thermal stability and structural, microstructural and magnetic properties of (40 + x) Fe–28Cr–(26 − x) Co–3Mo–1Ti–2V magnets with x = 0, 2, 4 addition in cobalt content were investigated and presented. The magnetic alloys were synthesized by vacuum arc melting and casting technique in a controlled argon atmosphere. Magnetic properties in the alloys were convinced by single-step isothermal field treatment and subsequent aging. The alloys were investigated for thermal stability, structural, microstructural and magnetic properties via differential thermal analysis (DTA), X-ray diffractometery (XRD), optical microscopy (OM), field emission scanning electron microscope (FESEM) and DC magnetometer. Metallurgical grains of size 300 ± 10 μm were produced in the specimens by casting and refined by subsequent thermal treatments. The magnetic properties of the alloys were achieved by refining the microstructure, the optimization of conventional thermomagnetic treatment to modified single-step isothermal field treatment and subsequent aging. The best magnetic properties achieved for the alloy 44Fe–28Cr–22Co–3Mo–0.9Ti–2V was coercivity H_c = 890 Oe (71 kA/m), B_r = 8.43 kG (843 mT) and maximum energy product (BH)_max = 3 MGOe (24 kJ/m³). The enhancement of remanence and coercivity enabled by the isothermal field treatment was due to the elongation of the ferromagnetic phase and step aging treatment due to the increase in the volume fraction. This work is interesting for spin-based electronics to be used for energy storage devices.     

An A- and B-Site Substitutional Study of SrFeO3−δ Perovskites for Solar Thermochemical Air Separation

An A‑ and B‑site substitutional study of SrFeO_3−δ perovskites (A’_xA_1−xB’_yB_1−yO_3−δ, where A = Sr and B = Fe) was performed for a two‑step solar thermochemical air separation cycle. The cycle steps encompass (1) the thermal reduction of A’_xSr_1−xB’_yFe_1−yO_3−δ driven by concentrated solar irradiation and (2) the oxidation of A’_xSr_1−xB’_yFe_1−yO_3−δ in air to remove O₂, leaving N₂. The oxidized A’_xSr_1−xB’_yFe_1−yO_3−δ is recycled back to the first step to complete the cycle, resulting in the separation of N₂ from air and concentrated solar irradiation. A-site substitution fractions between 0 ≤ x ≤ 0.2 were examined for A’ = Ba, Ca, and La. B-site substitution fractions between 0 ≤ y ≤ 0.2 were examined for B’ = Cr, Cu, Co, and Mn. Samples were prepared with a modified Pechini method and characterized with X-ray diffractometry. The mass changes and deviations from stoichiometry were evaluated with thermogravimetry in three screenings with temperature- and O₂ pressure-swings between 573 and 1473 K and 20% O₂/Ar and 100% Ar at 1 bar, respectively. A’ = Ba or La and B’ = Co resulted in the most improved redox capacities amongst temperature- and O₂ pressure-swing experiments.     

Theoretical Predictions of the Structural and Mechanical Properties of Tungsten–Rare Earth Element Alloys

Tungsten (W) is considered as the potential plasma facing material of the divertor and the first wall material in fusion. To further improve the ductility of W, the structural and mechanical properties of W–M (M = rare earth element Y, La, Ce and Lu) alloys are systematically investigated by first-principles calculations. Our results reveal that all the W_1−xM_x (x = 0.0625, 0.125, 0.1875, 0.25) alloys can form binary solid solution at the atomic level, and the alloys keep bcc lattice structures until the concentration of M increases to a certain value. Although the moduli of the alloys are reduced compared to that of pure W metal, the characteristic B/G ratio and Poisson’s ratio significantly increase, implying all the four rare earth elements can efficiently improve the ductility of W metal. Considering both factors of mechanical strength and ductility, La and Ce are better alloying elements than Y and Lu.     

Interplay between superconductivity and magnetism in Fe1?xPdxTe

The attractive/repulsive relationship between superconductivity and magnetic ordering has fascinated the condensed matter physics community for a century. In the early days, magnetic impurities doped into a superconductor were found to quickly suppress superconductivity. Later, a variety of systems, such as cuprates, heavy fermions, and Fe pnictides, showed superconductivity in a narrow region near the border to antiferromagnetism (AFM) as a function of pressure or doping. However, the coexistence of superconductivity and ferromagnetic (FM) or AFM ordering is found in a few compounds [RRh₄B₄ (R = Nd, Sm, Tm, Er), R′Mo₆X₈ (R′ = Tb, Dy, Er, Ho, and X = S, Se), UMGe (M = Ge, Rh, Co), CeCoIn₅, EuFe₂(As_1−xP_x)₂, etc.], providing evidence for their compatibility. Here, we present a third situation, where superconductivity coexists with FM and near the border of AFM in Fe_1−xPd_xTe. The doping of Pd for Fe gradually suppresses the first-order AFM ordering at temperature T_N/S, and turns into short-range AFM correlation with a characteristic peak in magnetic susceptibility at T′_N. Superconductivity sets in when T′_N reaches zero. However, there is a gigantic ferromagnetic dome imposed in the superconducting-AFM (short-range) cross-over regime. Such a system is ideal for studying the interplay between superconductivity and two types of magnetic (FM and AFM) interactions.Since the first discovery of superconductivity (SC) a century ago, the effects of magnetic impurities and the possibility of magnetic ordering in superconductors has been a central topic of condensed matter physics. Due to strong spin scattering (1, 2), it has generally been believed that the conduction electrons cannot be both magnetically ordered and superconducting. Even though it is thought that Cooper pairs in cuprates, heavy fermions, and Fe-based compounds are mediated by spin fluctuations (3–5), SC generally occurs after suppressing the magnetic ordering either through chemical doping or the application of hydrostatic pressure (6–10). However, there is growing evidence for the coexistence of superconductivity with either ferromagnetic (FM) (11–20) or antiferromagnetic (AFM) ordering (21–24). With the decrease of temperature (T), some of these systems show magnetic ordering before the superconducting transition (T_c) (14–17, 20), some are ordered in a reversed sequence (11–13, 18, 19, 22), some have the two orderings occur concomitantly (22, 25), and some show reentrant superconductivity (partially) overlapping with a magnetically ordered phase (11–13, 26). Despite extensive investigations of interaction between SC and magnetic moments, there is so far no unified theory for the coexistence of SC and magnetism. With the lack of theoretic guidance, the existing experimental findings lead to two schools of thought: one is that both orders result from the same conduction electrons as evidenced by their synchronized magnetic and superconducting orders (22), and the other is that there are two separate sets of electrons responsible for magnetic ordering and superconductivity, respectively (19, 21, 25). What remains incomprehensible is the case where superconductivity and magnetic ordering coexist but are in competition with each other, as seen in Fe-based systems (23, 24, 27).The discovery of superconductivity in Fe-based compounds has sparked enormous interest in the scientific community. Although Fe is the most well known ferromagnet, all parent compounds of Fe-based superconductors exhibit AFM ordering. Though superconductivity is induced after suppressing the AFM ordering, it can coexist with either remaining AFM ordering (23, 24, 27) or new FM ordering (18, 19), and this provides an ideal platform for studying the interplay between superconductivity and magnetism. Among the Fe-based superconductors, the chalcogenide FeTe_1−xSe_x is unique in several aspects: (i) it is the only compound composed of slabs of Fe(Te/Se)₄ stacked together without an interlayer spacer; (ii) it becomes superconducting via isovalent doping of Se for Te, with the highest T_c occurring at the 50% doping level; and (iii) the parent compound Fe_1+yTe shows nonmetallic electrical conduction and forms (π, π)-type AFM ordering with a large magnetic moment (28, 29), in contrast to the (π, 0)-type ordering with a small magnetic moment seen in Fe pnictides. The unusual AFM order in Fe_1+yTe cannot be explained by a simple Fermi-surface nesting picture, thus leading to arguments for a correlated local-moment scenario (27). Because of these differences, the application of hydrostatic pressure or partial doping on the Fe site, such as with Co or Ni, does not generate the generic phase diagram as seen for other Fe-based superconductors (30–33).To gain insight into the relationship between magnetism and superconductivity, we choose to substitute Pd for Fe in FeTe. This decision is motivated by the following: (i) Pd is known to exhibit FM instability (34, 35) even though it is paramagnetic in bulk, and (ii) PdTe is a superconductor with T_c ∼4.5 K (36–42). The partial replacement of Fe by Pd will allow us to understand the roles of Fe, Pd, and Te in both magnetism and superconductivity. Based on electrical transport, and magnetic and thermodynamic property measurements, we show that the ground state of Fe_1−xPd_xTe varies from AFM to FM ordering, to a superconducting state; this is one of rare cases where superconductivity flirts with both AFM and FM.Single crystals of Fe_1−xPd_xTe were grown via high-temperature melting, with the procedure described in SI Text. The crystal structure and the phase purity were measured by both powder and single-crystal X-ray diffraction. Fig. 1 shows powder X-ray diffraction patterns (Fig. 1A) and lattice parameters obtained from single-crystal X-ray refinement (Fig. 1B) for different doping levels; it confirms that the undoped FeTe forms a tetragonal structure, belonging to the P4/nmm space group. At room temperature, the lattice parameters are a = 3.8202 Å and c = 6.2686 Å. Similar to previous observations (43), our single-crystal refinement result indicates that there are ∼9% extra Fe atoms (T2 site of Fig. 1B) incorporated at interstitial sites of the Te layers. As soon as Pd is introduced into the system, the interstitial sites of Fe_1−xPd_xTe are occupied by Pd atoms (i.e., T2 = Pd when x > 0); though it remains in the same tetragonal space group, its lattice parameters a and c both increase with increasing Pd doping concentration for x ≤ x_s ∼0.6 (Fig. 1B). This finding indicates that Pd doping creates negative pressure, which is surprising because Pd²⁺ (∼0.80 Å) has a nearly identical ionic radius as that of Fe²⁺ (∼0.77 Å). The crystal structure of Fe_1−xPd_xTe remains tetragonal up to x_s (∼0.6). Above x_s, Fe_1−xPd_xTe crystallizes in a hexagonal structure (space group P6₃/mmc), and its lattice parameters also increase with increasing x (Fig. 1B); this results in an increase of the unit cell volume with increasing x in the entire doping range (Fig. 1B). In the hexagonal structure, the system no longer incorporates any interstitial sites (Tables S1–S4). Though the hexagonal structure at x > x_s can be regarded as a deformed FeTe structure, the local environment of Fe/Pd is transformed from a tetrahedron (x < x_s) to an octahedron (x > x_s) (44).Open in a separate windowFig. 1.Fe_1−xPd_xTe: (A) Powder X-ray diffraction patterns and (B) its unit cell parameters at room temperature obtained from single-crystal X-ray refinement.To correlate the structural information with physical properties, we show, in Fig. 2, the doping dependence of structural, electrical, and magnetic properties of Fe_1−xPd_xTe, with specifics presented later. The undoped (x = 0) Fe_1.09Te undergoes both an AFM ordering and a structural transition from paramagnetic (PM) tetragonal (high temperatures) to an AFM-ordered monoclinic (low temperatures) phase at T_N/S ∼70 K, with the first-order characteristic. Upon Pd doping, T_N/S is suppressed (solid circles), reaching zero at x = x_N/S ∼0.15 (dash line). In this doping region, the electrical resistivity (ρ) changes from nonmetallic (NM) character (dρ/dT < 0) above T_N/S to metallic (M) behavior (dρ/dT > 0) below T_N/S. When the characteristic feature for the first-order transition is absent at x_N/S, the magnetic susceptibility of Fe_1−xPd_xTe exhibits a peak at T^′_N. The value of T^′_N decreases with increasing x (Fig. 2, open circles), which eventually reaches zero at x_c ∼0.88. The magnetic susceptibility peak is a signature for short-range (SR) AFM correlation developed below T^′_N, with no obvious fingerprint in the electrical resistivity. Above x_c, superconductivity appears (Fig. 2, solid diamonds). The transition temperature T_c increases with increasing x. Remarkably, the system shows ferromagnetism (Fig. 2, solid squares) in the doping range of 0.78 ≤ x ≤ 0.98 with the maximum T_FM (∼190 K) at x_c, where AFM and SC vanish. In addition to the variable magnetic phases, there is doping-induced structural phase transition, being tetragonal at x < x_s ∼0.6 (Fig. 2, dark blue) and hexagonal at x > x_s (light blue). Such a structural transition is responsible for a change of electronic structure, manifested by NM behavior at x < x_s and a metallic character at x > x_s.Open in a separate windowFig. 2.Phase diagram of Fe_1−xPd_xTe. AFM, antiferromagnetic; AFM-M, antiferromagnetically ordered metallic state; FM, ferromagnetically ordered state; M, metallic; NM, nonmetal; PM, paramagnetic; PM-M, paramagnetic metal; SC, superconductivity; SR, short-range; SR-AFM, short-range AFM correlation.We now present the detailed experimental results in support of the phase diagram. Fig. 3A shows the temperature dependence of ρ of Fe_1−xPd_xTe for 0 ≤ x ≤ 0.5 (the experimental technique is described in SI Text). Similar to previous observations (45), the electrical resistivity of undoped (x = 0) Fe_1.09Te increases with decreasing temperature at high temperatures, but sharply decreases below T_N/S ∼70 K; this is caused by the first-order coupled structural and AFM phase transition (45, 46). Below T_N/S, the structure of Fe_1.09Te becomes monoclinic. Upon Pd doping, T_N/S is suppressed, and the first-order transition is no longer observed at x = 0.2. The smooth and monotonic temperature dependence of ρ indicates the absence of both structural and AFM transitions at 0.1 < x < 0.2. Though it sustains a nonmetallic temperature dependence (dρ/dT < 0) down to 2 K, the magnitude of the electrical resistivity deceases with increasing x in the entire doping range above T_N/S (Fig. 3 A and C). For 0.5 < x ≤ 1.0, the electrical resistivity of Fe_1−xPd_xTe continues to decrease with increasing x, as shown in Fig. 3B. More interesting is that ρ shows metallic behavior (dρ/dT > 0) in the entire temperature range measured for 0.5 < x ≤ 1.0. We believe that the cross-over from nonmetallic to metallic character is associated with the structural change at x_s. Furthermore, there is a sharp drop of resistivity at low temperatures for 0.88 < x ≤ 1.0, as shown in Fig. 3D. This drop is due to the emergence of superconductivity because the magnetic susceptibility shows diamagnetism as well in the temperature range (Fig. 3H). Note that the superconducting transition temperature T_c increases with increasing x.Open in a separate windowFig. 3.(A and B) Temperature dependence of the electrical resistivity (ρ) of Pd_1−xFe_xTe at the indicated compositions. (C and D) Temperature dependence of ρ near the first-order AFM/structural transition in the low-doping region, and the superconducting transition in the high-doping region plotted as ρ/ρ(5 K) vs. T, respectively. (E–G) Temperature dependence of the magnetic susceptibility (χ) of Fe_1−xPd_xTe measured at a magnetic field of 1 T. G Inset shows χ vs. x = 0.99 and 1.0; H is the χ(T) measured at 20 oersted (Oe) for 0.97 ≤ x ≤ 1.0.With Pd doping, the magnetic properties of Fe_1−xPd_xTe also change. Fig. 3 E–H shows the temperature dependence of the magnetic susceptibility, χ, at indicated values x. For 0 ≤ x < 0.2, χ initially increases with decreasing temperature, and then drops steeply at T_S/N (Fig. 3E); this confirms the first-order nature of the structural/AFM transition at 0 ≤ x < 0.2. Such a feature is absent for x ≥ x_N/S ∼0.15. Instead, there is a characteristic peak in magnetic susceptibility at T^′_N (Fig. 3F). Because there is no anomaly in electrical resistivity (Fig. 3 A–C) or specific heat, the susceptibility peak cannot be due to a true phase transition, but rather marks the onset of SR AFM correlation as seen in other materials (47). With increasing x, T^′_N moves to lower temperatures, and the magnitude of χ decreases as well (Fig. 3F). Though the characteristic peak is expected to completely vanish around x = x_c ∼0.88, the magnetic susceptibility reveals a dramatic increase below another characteristic temperature T_FM at x ≥ 0.78, as shown in Fig. 3G, which indicates the onset of ferromagnetic ordering at T_FM. With increasing x, T_FM initially increases then decreases after reaching the maximum (∼190 K) at x = x_c ∼0.88. Interestingly, the low-temperature susceptibility peak is still present for samples with x = 0.78, 0.80, 0.83, and 0.88, which suggests the coexistence of FM ordering and AFM interactions in this doping region. Above x_c, χ(T) continues to show FM behavior at high temperatures, but becomes negative at low temperatures (below T_c) under low magnetic fields, as depicted in Fig. 3H. The negative χ below T_c indicates the emergence of superconductivity, because their electrical resistivities also drop sharply (Fig. 3D). Whereas T_FM decreases, T_c increases with increasing x (above x_c). As shown Fig. 3G Inset, χ(T) exhibits paramagnetic behavior above T_c for x = 0.99 and 1.0, indicating the complete suppression of FM. However, T_c continues to increase with x (Fig. 3H).Clearly, there is a coexistence of AFM and FM ordering at 0.78 ≤ x ≤ 0.88, and of FM ordering and SC at 0.88 ≤ x ≤ 0.98 at low temperatures for Fe_1−xPd_xTe, which is further supported by the following results. First, the high-temperature magnetic susceptibility data allows us to extract both Curie–Weiss temperature (θ_CW) and effective magnetic moment (μ_eff) by fitting our experimental data to the Curie–Weiss law , where N_A is the Avogadro’s constant and k_B is Boltzmann’s constant. The x dependences of θ_CW and μ_eff are shown in Fig. 4 A and B, respectively. Note that θ_CW < 0 for x ≤ 0.8, indicating that the dominant magnetic interaction is AFM in this doping region. The fact that | θ_CW | >> T_N/S implies 2D AFM ordering for x < 0.2, whereas a small θ_CW value at 0.2 ≤ x ≤ 0.8 is consistent with our interpretation that there is short-range AFM correlation. And θ_CW > 0 for 0.8 ≤ x < 0.98, which confirms the ferromagnetic interaction in this doping range. Although θ_CW is comparable to the corresponding transition temperature T_FM, the effective magnetic moment is small, as shown in Fig. 4B, which implies that the FM ordering results mainly from Fe, even though Pd has to be the vehicle of coupling between Fe atoms (see discussion below). According to the single-crystal X-ray refinement results (Tables S1–S3), the actual Fe concentration is very close to the nominal value for 0.8 ≤ x ≤ 0.98. We thus estimate the effective magnetic moment of Fe using the nominal Fe concentrations; this gives μ_eff = 1.60μ_B/Fe (x = 0.80), 2.42μ_B/Fe (x = 0.83), 3.79μ_B/Fe (x = 0.88), 4.21μ_B/Fe (x = 0.92), 3.53μ_B/Fe (x = 0.95), and 2.52μ_B/Fe (x = 0.98). Note that the highest magnetic moment for x = 0.92 is close to the Fe moment in Fe_1.09Te (∼ 4.65μ_B/Fe), as plotted in Fig. 4B. In the latter case, the actual ordered magnetic moment is ∼3.31μ_B/Fe (28), smaller than that obtained from the Curie–Weiss constant; this is explained as due to the itinerant nature of the electrons in FeTe (28). For Fe_1−xPd_xTe, we may estimate the ordered magnetic moment from the magnetization. Fig. 4C shows the field dependence (H) of magnetization (M) at T = 3 K for x between 0.80 and 0.98. Note that M(H) reveals a well-defined hysteresis loop, confirming the nature of FM ordering for 0.8 ≤ x ≤ 0.98. Though M(H) is not saturated up to 6 T, we may estimate the lower bound of the ordered magnetic moment using M(H = 6 T) data shown in Fig. 4B. At T = 3 K, μ ∼0.33μ_B/Fe (x = 0.80), 0.48μ_B/Fe (x = 0.83), 1.49μ_B/Fe (x = 0.88), 1.85μ_B/Fe (x = 0.92), 1.67μ_B/Fe (x = 0.95), and 0.22μ_B/Fe (x = 0.98). In addition to the fact that M (H = 6 T) < M_sat, the small ordered magnetic moment could result from (i) the itinerant nature of the electrons and (ii) the reduced concentration of Fe in the highly Pd doped region. Nevertheless, the specific heat reveals anomalies in the compounds with high T_FM (Fig. 4D), indicating a true phase transition at T_FM.Open in a separate windowFig. 4.Fe1-xPdxTe (A) Doping dependence of Curie–Weiss temperature. (B) Doping dependence of the effective magnetic moment and magnetization at 6 T. (C) Magnetization vs. magnetic field at 3 K for x = 0.8, 0.88, 0.92, 0.95, and 0.98. (D) Specific heat vs. temperature for x = 0.92 and 0.88.From the results presented here, it is most likely that the FM dome is intrinsic, due to the ordering of the Fe magnetic moments. Within the dome, our single-crystal X-ray refinement indicates that the structure remains the same as that of pure PdTe with actual Fe concentration close to the nominal value, and there are no interstitial sites of either Pd or Fe, which strongly suggests that the FM ordering is not due to the formation of Fe clusters. In early studies, FM was detected in alloys of Fe in Pd (i.e., Pd_1−xFe_x) at compositions down to 0.08% Fe (48), because magnetic moments on the Fe sites polarize the surrounding Pd matrix (48–51). It seems that Fe doping in PdTe compound gives rise to a similar effect, i.e., Pd matrix mediates the magnetic interactions between Fe atoms.The central question is, how can both FM ordering and superconductivity coexist in the region of 0.88 ≤ x ≤ 0.98? Having excluded the possibility of structural phase separation, one may consider the scenario of electronic phase separation with either macroscopic coexistence of singlet SC and FM in different regions or microscopic coexistence of triplet SC and FM (11, 21, 52). If Fe_1−xPd_xTe were a triplet superconductor, a small amount of impurity or disorder would completely suppress superconductivity, as seen in Sr₂RuO₄ (53). Though we observe anomalous specific heat below T_c of PdTe (42), the insensitivity to Fe doping does not support triplet SC. For the former case, further experimental investigations are necessary, such as combined scanning electron microscopy and tunneling microscopy to directly probe both SC and FM regions. Theoretically, almost all Pd d orbitals in PdTe are occupied due to the covalency of the Pd–Te bonding, according to recent first-principle calculations (44), and this leads to a strong suppression of the local magnetic moment (44). As shown in Fig. 3G (Inset), the magnetic susceptibility of PdTe (x = 1) indeed exhibits paramagnetic behavior above T_c. Upon substitution of Pd by Fe, T_c decreases nearly linearly with increasing Fe concentration (1 − x; Fig. 2). As Fe concentration reaches ∼3% (1 − x = 0.03), both SC and FM ordering coexist. The fact that FM ordering does not immediately kill superconductivity is likely due to the unique electronic structure of PdTe. First-principles calculations indicate that the states in the proximity of the Fermi level consist mainly of Te p electrons, which are weakly hybridized with Pd e_g orbitals (44); this is in dramatic contrast to the electronic structure of FeTe, in which the states near the Fermi level derived from Fe with direct Fe–Fe interactions, and the Te p states lie well below the Fermi level and hybridized weakly with the Fe d states (54). For PdTe, the partial doping of Fe may lead to an even weaker hybridization between Te p and Pd e_g orbitals, due to the shift of Fermi level. Our experimental observation is in support of this scenario: with increasing Fe concentration, superconductivity is gradually suppressed to zero, at which the FM ordering reaches the maximum (Figs. 2 and ​and4).4). After T_c reaches zero at x_c, the disappearance of the superconducting energy gap allows the rearrangement of electronic structure, which is in favor of antiferromagnetic interaction; the latter apparently competes with the existing FM interaction, thus leading to the complete suppression of FM at x < 0.78.In summary, the substitution of Fe by Pd results in an extremely rich phase diagram for Fe_1−xPd_xTe. Powder and single-crystal X-ray diffraction measurements both indicate that there is doping-induced structural transition from a tetragonal phase at x < x_s ∼0.6 to a hexagonal phase at x > x_s. Correspondingly, the electrical resistivity changes from nonmetallic character at x < x_s to metallic behavior at x > x_s. Magnetically, the system undergoes a first-order AFM transition at T_N/S for x ≤ x_N/S ∼0.15, with the structure transition occurring at the same temperature from tetragonal to monoclinic. When T_N/S approaches zero at x_N/S, there is no longer a temperature-induced structural transition, whereas short-range AFM correlation exists for x_N/S < x ≤ 0.88. Superconductivity sets in at x ≥ x_c ∼0.88. In addition, a gigantic FM dome with 0.78 ≤ x ≤ 0.98 is centered at the critical concentration x_c, where both AFM and SC vanish. The coexistence of FM ordering with either SR AFM or SC results most likely from the unique electronic structure of Fe_1−xPd_xTe. With the low Pd concentration, both electrical conduction and magnetism of Fe_1−xPd_xTe are determined by Fe/Pd d electrons, because they are near the Fermi level. Due to the ferromagnetic instability and the extended orbitals of 4d electrons of Pd, both AFM interactions and electrical resistivity in Fe_1−xPd_xTe decrease with increasing x. Before the complete suppression of SR AFM, FM ordering occurs due to the Fe magnetic moment polarization in the Pd matrix. However, the hexagonal structure at x > x_s with Pd/Fe in an octahedral environment leads to more localized d electrons from Pd/Fe but itinerant p electrons from Te. It seems that the weak hybridization between p–d electrons allows for the magnetic ordering of Fe/Pd moments and superconducting ordering of p conduction electrons. Nevertheless, these two orderings compete with each other. Hence, the physics of Fe_1−xPd_xTe in the hexagonal phase is in contrast to known Fe-based superconductors, in which physical properties are mainly determined by Fe d states. Fe_1−xPd_xTe provides another and rare example for studying the interplay between SC, FM, and AFM ordering, where two separate sets of electrons are responsible for FM ordering and superconductivity, respectively.     

The Formation of 14H-LPSO in Mg–9Gd–2Y–2Zn–0.5Zr Alloy during Heat Treatment

There is a new long-period stacking ordered structure in Mg–RE–Zn magnesium alloys, namely the LPSO phase, which can effectively improve the yield strength, elongation, and corrosion resistance of Mg alloys. According to different types of Mg–RE–Zn alloy systems, two transformation modes are involved in the heat treatment transformation process. The first is the alloy without LPSO phase in the as-cast alloy, and the Mg_xRE phase changes to 14H-LPSO phase. The second is the alloy containing LPSO phase in the as-cast state, and the 14H-LPSO phase is obtained by the transformations of 6H, 18R, and 24R. The effects of different solution parameters on the second phase of Mg–9Gd–2Y–2Zn–0.5Zr alloy were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The precipitation mechanism of 14H-LPSO phase during solution treatment was further clarified. At a solution time of 13 h, the grain size increased rapidly initially and then decreased slightly with increasing solution temperature. The analysis of the volume fraction of the second phase and lattice constant showed that Gd and Y elements in the alloy precipitated from the matrix and formed 14H-LPSO phase after solution treatment at 490 °C for 13 h. At this time, the hardness of the alloy reached the maximum of 74.6 HV. After solution treatment at 500 °C for 13 h, the solid solution degree of the alloy increases, and the grain size and hardness of the alloy remain basically unchanged.     

Structure and Magnetic Properties of Thermodynamically Predicted Rapidly Quenched Fe85-xCuxB15 Alloys

In this work, based on the thermodynamic prediction, the comprehensive studies of the influence of Cu for Fe substitution on the crystal structure and magnetic properties of the rapidly quenched Fe₈₅B₁₅ alloy in the ribbon form are performed. Using thermodynamic calculations, the parabolic shape dependence of the ΔG^amoprh with a minimum value at 0.6% of Cu was predicted. The ΔG^amoprh from the Cu content dependence shape is also asymmetric, and, for Cu = 0% and Cu = 1.5%, the same ΔG^amoprh value is observed. The heat treatment optimization process of all alloys showed that the least lossy (with a minimum value of core power losses) is the nanocomposite state of nanocrystals immersed in an amorphous matrix obtained by annealing in the temperature range of 300–330 °C for 20 min. The minimum value of core power losses P_10/50 (core power losses at 1T@50Hz) of optimally annealed Fe_85-xCu_xB₁₅ x = 0,0.6,1.2% alloys come from completely different crystallization states of nanocomposite materials, but it strongly correlates with Cu content and, thus, a number of nucleation sites. The TEM observations showed that, for the Cu-free alloy, the least lossy crystal structure is related to 2–3 nm short-ordered clusters; for the Cu = 0.6% alloy, only the limited value of several α-Fe nanograins are found, while for the Cu-rich alloy with Cu = 1.2%, the average diameter of nanograins is about 26 nm, and they are randomly distributed in the amorphous matrix. The only high number of nucleation sites in the Cu = 1.2% alloy allows for a sufficient level of grains’ coarsening of the α-Fe phase that strongly enhances the ferromagnetic exchange between the α-Fe nanocrystals, which is clearly seen with the increasing value of saturation induction up to 1.7T. The air-annealing process tested on studied alloys for optimal annealing conditions proves the possibility of its use for this type of material.     

Study of the Structure,Magnetic, Thermal and Electrical Characterisation of ZnCr2Se4: Ta Single Crystals Obtained by Chemical Vapour Transport

The new series of single-crystalline chromium selenides, Ta-doped ZnCr₂Se₄, was synthesised by a chemical vapour transport method to determine the impact of a dopant on the structural and thermodynamic properties of the parent compound. We present comprehensive investigations of structural, electrical transport, magnetic, and specific heat properties. It was expected that a partial replacement of Cr ions by a more significant Ta one would lead to a change in direct magnetic interactions between Cr magnetic moments and result in a change in the magnetic ground state and electric transport properties of the ZnCr_2−xTa_xSe₄ (x = 0.05, 0.06, 0.07, 0.08, 0.1, 0.12) system. We found that all the elements of the cubic system had a cubic spinel structure; however, the doping gain linearly increased the ZnCr_2−xTa_xSe₄ unit cell volume. Doping with tantalum did not significantly change the semiconductor and magnetic properties of ZnCr₂Se₄. For all studied samples (0 ≤ x ≤ 0.12), an antiferromagnetic order (AFM) below T_N~22 K was observed. However, a small amount of Ta significantly reduced the second critical field (H_c2) from 65 kOe for x = 0.0 (ZnCr₂Se₄ matrix) up to 42.2 kOe for x = 0.12, above which the spin helical system changed to ferromagnetic (FM). The H_c2 reduction can lead to strong competition among AFM and FM interactions and spin frustration, as the specific heat under magnetic fields H < H_c2 shows a strong field decrease in T_N.     

Heat Treatment of Cast and Cold Rolled Al–Yb and Al–Mn–Yb–Zr Alloys

The microstructure, electrical properties and microhardness of as-cast and cold rolled AlYb and AlMnYbZr alloys were investigated. The addition of Mn, Yb and Zr has a positive influence on grain size. A deformed structure of the grains with no changes of their size was observed after cold rolling. The Al₃Yb particles coherent with the matrix were observed in the AlYb alloys. The size of the particles was about 20 nm in the initial state; after isochronal treatment up to 540 °C the particles coarsen, and their number density was lower. The deformation has a massive effect on the microhardness behavior until treatment at 390 °C, after which the difference in microhardness changes between as-cast and cold rolled alloys disappeared. Relative resistivity changes show a large decrease in the temperature interval of 330–540 °C which is probably caused by a combination of recovery of dislocations and precipitation of the Al₃(Yb,Zr) particles. Precipitation hardening was observed between 100 and 450 °C in the AlYb alloy after ageing at 625 °C/24 h and between 330 and 570 °C in the AlMnYbZr alloy after ageing at 625 °C/24 h.