High-temperature Magnetic Properties Research Articles

Research and Application Progress of High-Entropy Alloys

With the continuous improvement of global technological levels and the increasing demand for high-performance alloy materials in national economic construction, the traditional single principal component alloy is increasingly unable to meet people’s increasing service needs. High-entropy alloys play an important role in aerospace, mechanical manufacturing, biomedicine, energy development and other engineering fields because of their unique physical, chemical and mechanical properties. Based on the concept of high-entropy alloys, the high-entropy effect, lattice distortion effect, sluggish diffusion effect and cocktail effect of high-entropy alloys are represented in this paper. The common preparation methods of high-entropy alloys are summarized according to the classification of melting-casting method, mechanical alloying method and coating method. The strength and toughness, wear resistance, corrosion resistance, high temperature resistance, fatigue resistance, radiation resistance and magnetic properties of the high-entropy alloys are discussed. The application prospect of high-entropy alloys is summarized, and the future research and development direction of high-entropy alloys are prospected on this basis.

Tracing titanomagnetite alteration with magnetic measurements at cryogenic temperatures

SUMMARY Titanomagnetite containing up to 0.6–0.7 Ti atoms per formula unit is a primary magnetic mineral phase in submarine basalts and in some terrestrial volcanic rocks. On a geological timescale, it often undergoes alteration, forming new magnetic phases that may acquire (thermo)chemical remanent magnetization. The initial stage of this natural process can be modelled by prolonged laboratory annealing at moderately elevated temperatures. In this study, our goal is to characterize the alteration products resulting from annealing a submarine basalt containing homogeneous titanomagnetite Fe3−xTixO4 (x ≈ 0.46) at temperatures of 355, 500 and 550 °C for up to 375 hr, by examining their magnetic properties over a wide range of temperatures. The effect of extended annealing is most apparent in the low-temperature magnetic properties. In the fresh sample, a magnetic transition is observed at 58 K. Below the transition temperature, the field-cooled (FC) and zero-field-cooled (ZFC) saturation isothermal remanent magnetization (SIRM) curves are separated by a tell-tale triangular-shaped area, characteristic for titanomagnetites of intermediate composition. The room-temperature SIRM (RT-SIRM) cycle to 1.8 K in zero field has a characteristic concave-up shape and is nearly reversible. For the annealed samples, the magnetic transition temperature shifts to lower temperatures, and the shape of the curves above the transition changes from concave-up to concave-down. The shape of the RT-SIRM cycles also progressively changes with increasing annealing time. The SIRM loss after the cycle increases up to ∼30 per cent for the samples annealed for 375 hr at 355 °C, and for 110 hr at 500 and 550 °C. The Curie temperatures of the newly formed magnetic phases exceed the Curie temperature of the fresh sample (205 °C) by up to 350 °C. While this effect is most commonly attributed to extensive single-phase oxidation (maghemitization), the behaviour observed at cryogenic temperatures appears incompatible with the known properties of highly oxidized titanomaghemites. Therefore, we propose that, at least in the initial stage of the ‘dry’, that is, not involving hydrothermalism, alteration of titanomagnetite, temperature- and time-controlled cation reordering is the primary mechanism driving changes in both low- and high-temperature magnetic properties.

High Temperature Magnetic Properties of Selected Steel Grades

In order to effectively control the microstructure of steel strip during manufacture it is vital that techniques be developed to monitor microstructural changes in the steel at high temperatures. Electromagnetic non-destructive testing is increasingly being used in these applications as it is non-contact, robust, relatively inexpensive and has the potential to provide real-time feedback to control the manufacturing process. However, if the data produced by these systems is to be fully exploited it is necessary to accurately measure the magnetic properties of steel grades of interest at temperatures up to the Curie point. This work details measurements carried out to assess the magnetic properties of selected steels grades, supplied by Tata Steel, up to the Curie temperature using a novel test rig based on a modified Epstein frame designed by the University of Manchester with high temperature trials at the University of Warwick. Several steel grades are studied with the full magnetic hysteresis (BH) loops reported during heating and cooling, along with extracted magnetic parameters including coercivity, magnetic saturation and magnetic permeability. The magnetic parameters of the selected steel grades are compared and implications of the results on electromagnetic monitoring of steel strip during manufacture considered. Results show the developed system has the potential to provide valuable data to inform online electromagnetic monitoring systems.

Silicon carbide capped magnetite nanoparticles: Role of capping on high-temperature stability, magnetic properties, and water dispersibility

The functionalization of nanoparticles is important to improve their oxidation resistance, prevent agglomeration and improve stability of colloids. However, capping magnetic nanoparticles with a stable ultrathin layer of organic or inorganic materials, without compromising magnetic properties, is still a challenge. In this paper, we report for the first time a novel approach to prepare magnetite (Fe3O4) nanoparticles with superior thermal stability by capping them with a thin shell of silicon carbide (SiC), using sonochemical degradation of dimethyldichloro silane (DMDCSi). The synthesized silicon carbide capped Fe3O4 nanoparticles are characterized using Fourier transform infrared (FT-IR) spectroscopy, and thermogravimetric analyzer. FTIR analysis confirms the hydrophilic SiC capping over the Fe3O4 nanoparticles. The influence of silicon carbide capping over magnetic property and crystal structure was analyzed by vibrating sample magnetometer (VSM) and X-ray diffractometer (XRD) measurements, respectively. The possible mechanism of SiC capping over the particles is explained. SiC capped particles are hydrophilic in nature, where water molecules dissociate spontaneously on the surface of the SiC layer and form Si–OH and electropositive Si atom stabilizes the surface dangling bonds and reduces surface energy. The SiC-capped particles showed better thermal stability with an increased magnetic γ-Fe2O3 to the non-magnetic α-Fe2O3 phase transition temperature of >167 °C as compared to bare particles. The enhancement of phase transition temperature in SiC capped Fe3O4 is due to the increased activation energy by effective binding of SiC with the surface oxidized γ-Fe2O3 layer and the formation of an oxidized passive layer that acts as a thermal barrier. The results suggest that the surface oxidized maghemite (γ-Fe2O3) content over the Fe3O4 particles surface can improve SiC adsorption and enhance their phase stability significantly.

Experimental and computational study on the influence of cobalt substitution on the structural, impedance, electronic, magnetic, and optical properties of pseudobrookite-structured Fe2TiO5.

We report on how Co substitution of the Fe sites of pseudobrookite (Fe2TiO5) influences the crystal structure, high-temperature electric permittivity, impedance, electronic structure, magnetic, and optical properties via experimental and theoretical investigations. The pseudobrookite phase contains two types of octahedral sites, Fe atoms reside on type of the sites while Ti on the others and replacing Fe with Co can have a huge influence on one or more physical properties that can render the material more useful for solar energy applications. X-ray diffraction and high-temperature electric permittivity/impedance were the experimental tools used. A temperature range of 20-300 °C and a frequency range of 100 Hz to 1 MHz were used for studying various types of relaxation mechanism via impedance analysis, including grains, grain boundaries, and interfacial effects. To explore the electronic structure, magnetic, and optical properties from first principles, dispersion-corrected density functional theory (PBE-D2/U) was employed. The structure as well as the electric impedance properties are impacted slightly by the Co substitution of Fe in Fe2TiO5 whereas the electronic structure and magnetic properties are influenced significantly. The bandgap is reduced slightly and the average magnetic moment per Fe ion is reduced upon Co substitution of Fe in Fe2TiO5.

The evolution of microstructure, micromechanical and magnetic properties of FeCoNiAlSi alloys with peritectic structure processed by high-pressure solidification

In this work, we examined the microstructure evolution, as well as the micromechanical and magnetic properties of FeCoNiAlSi high-entropy alloys (HEAs) under high pressure solidification (HPS). According to the results, FeCoNiAlSi alloy solidified at 0.1 MPa (atmospheric pressure) contains three kinds of phases: grain boundary (GB)phases, peritectic phases, and primary phases. As the solidification pressure increases to 7 GPa, the volume fraction of GB phases and peritectic phases decreases, while the volume fraction of primary phase increases by 82.8%. In addition, the Fe of the primary phase increases by 63.6 %, which enhances the ferromagnetism and exchange coupling properties of the alloy. Furthermore, HPS also reduces the magneto-crystal anisotropy of the GB phases and the pinning effect of boundaries on domain movement. Additionally, with the increasing solidification pressure, the FeCoNiAlSi HEAs shows stronger ferromagnetic. When solidified at 7Gpa, a reduction of 51.5 % in intrinsic coercivity(H MC ) was observed, as was an increase of 106 % in magnetic polarization strength (Js). As well, HPS significantly increased the high temperature magnetic properties of FeCoNiAlSi alloy as well as the microhardness(i.e. from 8.25 GPa to 10.15 GPa). According to these results, HPS is an effective method for developing new functional applications for FeCoNiAlSi HEAs. • The Js (HMC)of the FeCoNiAlSi HEAs increased (decreased) by 106% (51.5%) after HPS at 7Gpa, The Tc are also improved. • The pinning effect of boundary on magnetic domain movement were reduced after HPS and exchange coupling was enhanced. • The volume fraction of primary phase and the content of Fe in primary phase increased when solidified under 7GPa. • The promoted solution strengthening after HPS significantly improved the microhardness (i.e. from 8.25GPa to 10.15GPa).

Cellular microstructure modification and high temperature performance enhancement for Sm2Co17-based magnets with different Zr contents

• Sm(Co bal Fe 0.09 Cu 0.09 Zr x ) 7.2 magnets were prepared with x ranging from 0.025 to 0.04. • It was investigated that the high temperature magnetic properties changing regulation as Zr content changes, and the intrinsic coercivity at 773 K up to 667 kA . m −1 was achieved. • The Cu-lean characteristic at cell boundary phase is the reason that gives rise to higher magnetic properties at elevated temperatures for magnets with higher Zr content. Influence of Zr contents on high-temperature magnetic performance of Sm(Co bal Fe 0.09 Cu 0.09 Zr x ) 7.2 ( x = 0.025, 0.03, 0.035, 0.04) magnets were investigated. As x increases from 0.025 to 0.04, the temperature coefficient of intrinsic coercivity ( H cj ) is optimized from -0.1673% K −1 to -0.1382% K −1 and the H cj at 773 K gradually increases from 556.32 kA m −1 to 667 kA m −1 . The microstructure and microchemistry of different Zr-content magnets were revealed by a transmission electron microscope equipped with EDS. The increasing Zr content induces that the average size of cells decreases from ∼ 76 nm to ∼ 56 nm and the weight fraction of 1:5H cell boundary phase increases from ∼ 25% to ∼ 37% as well, resulting decreasing of the average Cu content at cell boundaries from 13.59 at% to ∼ 8.52 at%. It is found that the Cu-lean characteristic at cell boundary phase is the reason that gives rise to higher magnetic properties at elevated temperatures for x = 0.04 magnet.

Enhancement of Domain Wall Pinning in High-Temperature Resistant Sm2Co17 Type Magnets by Addition of Y2O3.

In this study, the effects of Y2O3 addition on the magnetic properties, microstructure and magnetization reversal behavior of Sm(Co0.79Fe0.09Cu0.09Zr0.03)7.68 magnet were investigated. By addition of Y2O3, the coercivity was increased from 21.34 kOe to 27.42 kOe at 300 K and from 5.14 kOe to 6.27 kOe at 823 K. A magnet with a maximum magnetic energy product of 9.86 MGOe at 823 K was obtained. With the interdiffusion of Y and Sm after appropriate addition, the Cu content within the cell boundary phase close to the oxide was detected to be nearly twice as high as that away from the oxide. We report for the first time that a collection of lamellar phases were formed on both sides of the inserted oxide, providing a strong pinning field against magnetic domain wall motion based on in-situ Lorentz TEM observation. Furthermore, the ordering process of the original magnet was delayed after Y2O3 addition, resulting in the refinement of cellular structure, which can also enhance the domain wall pinning ability of cellular structures based on micromagnetic simulation. However, excessive addition of Y2O3 led to large Cu-rich phase and Zr-rich impurity phase precipitated at the edge of the oxide, resulting in the destruction of cellular structures and a significant reduction in coercivity. This study provides a new technical approach to regulate the microstructure of Sm2Co17 type magnets. Addition of Y2O3 is expected to play a significant role in improvement of high temperature magnetic properties.

The evolution of microstructure, micromechanical and magnetic properties of FeCoNiSi alloys solidified under high pressure

A comprehensive study has been conducted on the effects of high-pressure solidification (HPS) on the evolution of microstructures, micromechanical properties, and magnetic properties of FeCoNiSi alloys. As solidification pressure increases(up to 7GPa), FeCoNiSi alloy's columnar grain changes into an equiaxed grain, and the grain size and Ni-rich precipitates are significantly refined. Furthermore, HPS facilitates the conversion of in-coherent matrix/precipitates interface of alloy solidified under atmospheric pressure into semi-coherent, which reduces the pinning effect of phase boundaries on the motion of the magnetic domains and enhances the exchange coupling between matrix and precipitates. As FeCoNiSi alloy solidifying under 7GPa, the maximum magnetic permeability (μ m ) is significantly increased (from 52,600 to 113,000), the inherent coercivity is decreased by 29.5%, and the saturation magnetic polarization( Js ) is increased by 21%. In addition, the high temperature magnetic properties of FeCoNiSi alloys are improved by HPS. Furthermore, HPS produces compressive stress in both the precipitates and matrix, which significantly improved the modulus of precipitates and matrix (i.e. from 171GPa to 215 GPa in matrix),as well as microhardness (i.e. from 5.46 GPa to 10.65 GPa in matrix). Based on the results of these studies, HPS is capable to improve the comprehensive properties of magnetic alloys significantly. • The J s and μ max (H MC ) of FeCoNiSi alloy are increased (decreased) by 21% and 114% (29.5%) after solidified under 7GPa. • HPS produces compressive stress in the precipitates and matrix and significantly improved the modulus and microhardness. • HPS refine the microstructure of FeCoNSi alloy and result in the semi-coherent interface. • The semi-coherent interface enhances the exchange coupling effect between precipitates and matrix.

Tailoring the magnetic properties and microstructure of NdFeB ribbon alloys by Hf addition

In this paper, the effects of Hf addition on phase composition, Curie temperature, exchange coupling effect, coercivity mechanism, room temperature magnetic properties, high temperature magnetic properties and thermal stability of NdFeB alloys with different Nd content (27–31 wt%) are systematically studied. Hf addition showed different effects on the magnetic properties of NdFeB alloys at different Nd contents and different test temperatures. At 300 K, the intrinsic coercivity is increased with low Nd content (27 wt%), while the remanence and the maximum magnetic energy product are significantly improved with higher Nd content (29 wt%, 31 wt%). At 398 K, the intrinsic coercivity, the remanence, the maximum magnetic energy product and the temperature stability improved in NdFeB alloys with high Nd content (29 wt%, 31 wt%).

Quantitative analysis of pinning-hardened intrinsic coercivity of Sm(CoFeCuZr)z (z = 7.0–7.8) high-temperature permanent magnets

SmCo-based permanent magnets possessing high Curie temperature and outstanding magnetocrystalline anisotropy are of great scientific and technological value on high temperature applications. It is widely accepted the coercivity mechanism as well as its temperature dependence is closely related to the microstructure and microchemistry. However, direct domain wall pinning observation with Lorentz microscopy is extremely difficult to explore the strength of domain wall pinning under high temperature and strong magnetic field. The quantitatively understanding of coercivity mechanism such as micromagnetic simulations cannot be carried out without the intrinsic magnetism of 1:5H and 2:17R inside the cellular microstructure of SmCo sintered magnets. Up to nowadays, there are no methods to directly measure the intrinsic magnetism of nanometer scale 1:5H and 2:17R, which retards the exploring of coercivity mechanism especially at high temperature. Herein, we prepared Sm(CoCuFeZr)z high-temperature permanent magnets with the ratio z values ranging from 7.0 to 7.8 and measured the microchemistry of 1:5H and 2:17R phases with transmission electron microscopy. Then single-phase solid solution samples with the same composition as 1:5H and 2:17R phases were prepared and the high temperature magnetic properties are measured directly. The coercivity mechanism can be well explained based on the difference of domain wall energy density qualitatively, including the abnormal temperature dependence of coercivity. More importantly, the quantitatively calculated coercivity according to the pinning-hardened coercivity theory with the obtained intrinsic parameters were found to agree well with the experiment results. Our work on the intrinsic magnetism of 1:5H and 2:17R phases at varying temperatures may offer important guidance for compositions design of the magnets with higher performance.

Neutron Diffraction Magnetic and Mossbauer Spectroscopic Studies of Pb0.8Bi0.2Fe0.728W0.264O3 and Pb0.7Bi0.3Fe0.762W0.231O3 Ceramics

We report on high-temperature crystallographic structure, magnetic and physical properties of chemically B-site disordered lead-bismuth iron tungstate, (PFW)1−x(BFO)x (which can be written as Pb0.8Bi0.2Fe0.728W0.264O3 (0.8PFW–0.2BFO) for x = 0.2 and Pb0.7Bi0.3Fe0.762W0.231O3 (0.7PFW–0.3BFO) for x = 0.3) or PBFW), solid solutions through neutron diffraction (ND), magnetization, electron paramagnetic resonance, and Mössbauer spectroscopic studies. From the high temperature magnetic susceptibility measurement, it is observed that increase antiferromagnetic to paramagnetic phase transition around TN = 435 K (Pb0.8Bi0.2Fe0.728W0.264O3) and 504 K (Pb0.7Bi0.3Fe0.762W0.231O3), compared to pure PFW. Room-temperature crystallographic study confirms the formation of pseudo cubic structure with Pm-3m space group, whereas the magnetic structure is commensurate G-type antiferromagnetic ordering. The obtained temperature dependent structural parameters from the ND, evidenced to existence of strong spin-lattice coupling around TN for both the compounds. The discontinuity in the Pb/Bi–O bond length around ferroelectric transition (TC) indicates the presence of magnetoelectric coupling. Interestingly, microscopic 1:1 B-site ordered nanoclusters of PBFW exhibits the ferrimagnetic clusters along with antiferromagnetic order and it observed through the opening of M vs H hysteresis curves in the lower field regime. The EPR and Mössbauer spectroscopic studies well support the magnetic property and also reveal the Fe+3 state, and the weak signal in EPR and broader linewidth in the Mössbauer spectra exhibit the B-site disorderliness.

Effect of annealing temperature on structure and high-temperature soft magnetic properties of (Fe0.9Co0.1)72.7Al0.8Si13.5Cu1Nb3B8V1 nanocrystalline alloy

The structure and soft magnetic properties of nanocrystalline (Fe0.9Co0.1)72.7Al0.8Si13.5Cu1Nb3B8V1 alloy annealed at 510−690 °C were investigated by XRD patterns and initial permeability. It was observed that annealing within the ranges of 510−630 °C, only soft magnetic crystalline phase precipitated from the amorphous matrix. With the increase of annealing temperature, the grain size (D) and volume fraction of the crystalline phase (Vcry) both gradually increase from 11.9 nm to 13.7 nm and from 58 % to 89 % respectively. In the meanwhile, the thickness of the amorphous layer (Λ) gradually decreases from 2.37 nm to 0.54 nm. The change of initial permeability with temperature (μi -T curves) showed that the μi declines with increasing temperature. However, the rate of descent is different and the higher annealing temperature (Ta) corresponds to the slower attenuation of μi. The high-temperature μi gradually improves with annealing temperature (Ta) increasing up to 630 °C, and the higher μi of 1500 can be held up to 600 °C. However, the high-temperature μi dramatically deteriorates after annealing at 690 °C due to the precipitating hard magnetic phase from the residual amorphous matrix.

Mechanical, optical and high-temperature magnetic properties of Sn-substituted Mg–Zn ferrites

ABSTRACT Sn-substituted Mg–Zn ferrites (0 ≤ x ≤ 0.10) have been synthesized using conventional ceramic technique and the mechanical, optical and high-temperature magnetic properties were studied. The formalism of elastic constants was assessed based on FTIR data. The mechanical stability was confirmed from the obtained values of stiffness constants. The bulk modulus, shear modulus and Young’s modulus have been estimated for different Sn concentrations. The Poisson’s ratio and Pugh ratio confirmed the brittleness character of the studied compounds. The Debye temperature was also calculated. The optical band gap has been calculated from diffuse reflectance spectra. The magnetization curves measured at different temperatures revealed the significant effect of temperature on the magnetic properties of the considered compositions. A second-order phase transition of ferrimagnetic to paramagnetic phase was observed in temperature-dependent magnetization vs. applied field and magnetic moment curves. The Curie temperature was observed to decrease owing to Sn substitution.

Effect of Ti addition on microstructure and high-temperature soft magnetic properties of nanocrystalline (Fe0.9Co0.1)72.7Al0.8Si13.5B9Cu1Nb3 alloy

The crystallization behavior and microstructure of (Fe0.9Co0.1)72.7Al0.8Si13.5B9-xCu1Nb3Tix(x = 0,1,2) alloys were analyzed by the differential scanning calorimetry (DSC) and X-ray diffractometry (XRD). The soft magnetic properties of alloys after thermal treatment were studied by the temperature dependence of initial permeability (μi-T curves). Experimental results show that the proper addition of Ti can decrease the onset temperature of primary crystallization Tx1 for precipitating soft magnetic phase from 506 °C for x = 0–487 °C for x = 1 and increase the onset temperature of second crystallization Tx2 from 669 °C for x = 0–703 °C for x = 2. The extended onset temperature of second crystallization Tx2 of 703 °C and crystallization temperature interval ΔTx of 199 °C make the 650oC-annealed (Fe0.9Co0.1)72.7Al0.8Si13.5B7Cu1Nb3Ti2 alloy exhibit superior magnetic softness at room- and high-temperatures. And the 610oC-annealed (Fe0.9Co0.1)72.7Al0.8Si13.5B8Cu1Nb3Ti1 alloy shows improved high-temperature magnetic softness at 450–650 °C with respect to Ti-free (Fe0.9Co0.1)72.7Al0.8Si13.5B9Cu1Nb3 alloy.

Thermal stability of high-temperature compound La2Fe14B and magnetic properties of Nd-La-Fe-B alloys

The thermal stability and refined crystal structure of ternary La2Fe14B compound were investigated by integrating X-ray powder diffraction, electron probe microanalyzer and differential scanning calorimetry techniques. It was found that La2Fe14B is a high-temperature stable ternary compound, which decomposes with a eutectoid reaction: La2Fe14B→La + Fe + LaFe4B4 at 794 °C and melts via a peritectic reaction: Fe + Fe2B + Liquid→La2Fe14 B at 926 °C. By investigating the effects of La substitution on the magnetic properties of Nd2-xLaxFe14B alloys, it was revealed that the Nd1.3La0·7Fe14B alloy still keeps excellent maximum energy product (113 kJ/m3), intrinsic coercivity (523 kA/m) and high remanent magnetic polarization (0.83 T). Moreover, the magnetic improvement mechanisms of Nd2-xLaxFe14B alloys were also discussed in detail. The present work is beneficial to design high-performance 2:14:1-type magnets with low-cost and abundant La addition.

Overview of composition and technique process study on 2:17-type Sm–Co high-temperature permanent magnet

Sm–Co permanent magnets are widely used in aerospace equipments, tubes, sensors and hybrid electric vehicles due to their excellent magnetic properties at both room and high temperatures. Compared to that of Nd–Fe–B magnets, the capability of reliably operating at temperatures up to 500 °C is the biggest preponderance of Sm–Co permanent magnets. However, the reliable high-temperature magnetic properties are susceptible to composition and technique processes. So, appropriate composition and matched technique processes are prerequisites for good high-temperature magnetic properties. This review mainly emphasizes the influences of Sm, Fe, Cu and Zr contents and technique processes on development of high-temperature performance and discusses the controversial high-temperature coercivity mechanism of 2:17-type high-temperature Sm–Co magnets.

Elevated temperature behavior of rapidly quenched La/Ce substituted nanocrystalline NdFeB alloys with various compositions

Here we report the elevated temperature behaviors of rapidly quenched La/Ce substituted nanocrystalline NdFeB alloys with a wide range of compositions, including nano-composites of (Nd0.8Ce0.2)10Fe84B6, (Nd0.9La0.1)10Fe84B6, [Nd0.7(La0.5Ce0.5)0.3]10Fe84B6 and [Dy0.7(La0.5Ce0.5)0.3]10Fe84B6, single phase alloys of (Nd0.95La0.05)12Fe82B6 and (Nd0.8Ce0.2)12Fe82B6, as well as Co substituted single phase [Nd0.7(La0.5Ce0.5)0.3]12Fe72Co10B6 and RE-rich phase [Nd0.7(La0.5Ce0.5)0.3]14Fe70Co10B6 alloys. All compositions were fabricated by melt spinning technique. For nanocomposite alloys, the optimum high temperature magnetic properties at 400 K including remanent magnetization (Mr), maximum energy product (BH)max and coercivity (Hc) up to 89 emu/g, 65 kJ/m3, 226 kA/m, respectively, for Ce-substituted alloy and 70 emu/g, 46 kJ/m3 and 246 kA/m, respectively, for La-substituted alloys have been obtained. The decrease of Hc is ascribed to the microstructure sensitivity of La/Ce substituted NdFeB nanocomposite alloys at higher temperature. For single phase alloys, the optimum magnetic properties such as Mr, (BH)max and Hc up to 81 emu/g, 67 kJ/m3, 412 kA/m and 77 emu/g, 53 kJ/m3 and 349 kA/m have been obtained for La-substituted and Ce-substituted alloys at 400 K, respectively. The Ce-substituted alloys are relatively more sensitive to the elevated temperature compared to La-substituted alloys for both composites and single phase alloys. Substitution of 10% Co for Fe in both single phase and RE-rich phase alloys leads to the excellent elevated temperature magnetic properties at 400 K. The existing results state that La, Ce or LaCe substitutions for Nd and partial substitution of Co for Fe maintain a rather good combination of improved Curie temperature and elevated temperature magnetic properties at or above 400 K in all alloys with various phase constitutions.

Magnetic Properties and Microstructures of Sintered Sm2Co17 Alloys With High Knee-Point Coercivity H k

Sintered permanent magnets based on domain-wall-pinning-type Sm2Co17 have recently attracted considerable attention due to their low-temperature coefficient, excellent high-temperature magnetic properties, and remarkable oxidation and corrosion resistances. However, the relationship between the knee-point coercivity, $H_{\mathrm {k}}$ , and the microstructures of these Sm2Co17-based bulk magnets has not been widely studied. Therefore, herein, sintered permanent magnets with nominal compositions of Sm(CobalFe0.227Cu0.070Zr0.023) z ( $z =7.40$ , 7.47, 7.54, 7.61, and 7.84) (at.%) were prepared via conventional powder metallurgy, and their magnetic properties were investigated. The results showed that as $z$ increased from 7.40 to 7.84, $H_{\mathrm {k}}$ first increased and then decreased. The maximum optimal $H_{\mathrm {k}}$ of 1717 kA $\cdot \text{m}^{-1}$ (21.58 kOe) corresponded to the magnet with dimensions of ~52.8 mm $\times49.2$ mm $\times36.6$ mm, a maximum magnetic energy product ( BH )max of 232 kJ $\cdot \text{m}^{-3}$ (29.1 MG $\cdot $ Oe), and $z =7.61$ . The transmission electron microscopy revealed that the crystalline structure and chemical composition of the magnet are the two main factors determining $H_{\mathrm {k}}$ .

Mechanical behavior, enhanced dc resistivity, energy band gap and high temperature magnetic properties of Y-substituted Mg–Zn ferrites

We report the synthesis of Y-substituted Mg–Zn [Mg0.5Zn0.5Y x Fe2−x O4 (0 ≤ x ≤ 0.05)] ferrites using conventional standard ceramic technique. The samples were characterized by x-ray diffraction (XRD) analysis, field emission scanning electron microscopy (FESEM), FTIR spectroscopy, UV–Vis spectroscopy and quantum design physical properties measurement system (PPMS). XRD patterns confirm the single phase cubic spinel structure up to x = 0.03 and appearance of a secondary phase of YFeO3 for higher Y contents. FESEM images depict the distribution of grains and EDS spectra confirmed the absence of any unwanted element. Completion of solid state reaction and formation of spinel structure has been revealed from FTIR spectra. The FTIR data along with lattice constant, bulk density and porosity were further used to calculate the stiffness constant (C ij), elastic constant and Debye temperatures. Mechanical stability of all studied compositions is confirmed from C ij using Born stability conditions. Brittleness and isotropic nature are also confirmed using Poisson’s ratio and anisotropy constants, respectively. The enhancement of dc electrical resistivity (105Ω cm to 106Ω cm) with Y content is observed. The energy band gap (increased with Y contents) is found in good agreement with dc electrical resistivity. Ferrimagnetic to paramagnetic phase change has been observed from the field dependent high temperature magnetization curves. The magnetic moments and saturation magnetization were found to be decreased with increasing temperature. The Curie temperature (Tc) has been measured from temperature dependent magnetic moment (M-T) and initial permeability (μ′i-T) measurements and found to be in good agreement with each other. Decrease in Tc with Y content is due to redistribution of cations and weakening of the exchange coupling constant. The magnetic phase transition has been analyzed by Arrott plot and found to have second order phase transition. The dc resistivity endorses the prepared ferrites are suitable for high frequency and high temperature magnetic device applications as well.