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In the present work, we reveal a novel finding, that is, self-assembled grain boundary precipitates for realizing the magnetic-field-induced metamagnetic transition by employing low melting point metal in materials. This strategy has been experimentally verified in a well-established ferrimagnetic compound Mn2Sb by doping the bismuth (Bi) element with a low melting point of just 271.3°C. Bi solidifies later than the Mn2(Sb,Bi) main phase in (Mn2Sb)1-xBix system (the melting point of ~948°C for Mn2Sb), and aggregates spontaneously along the grain boundaries, forming Bi-rich grain boundary precipitates to coat main phase grains. This is very similar to the Nd-rich grain boundary phase in Nd-Fe-B permanent magnets. The fraction of Bi-rich grain boundary phase can be controlled by Bi-content. As a result, the magnetic field induced steep magnetoelastic transition from antiferromagnetic to ferrimagnetic is achieved in (Mn2Sb)0.89Bi0.11 alloy with giant multiple functional properties in Bi-doped Mn2Sb. Especially, the magnetic entropy change maximum nearly quadruples when Bi-doping increases from 0.03 to 0.11. Giant negative magnetoresistance of more than 65% under μ0∆H = 0–5 T, and magnetostrain of ~1802 ppm under μ0H = 8.5 T are obtained in (Mn2Sb)0.89Bi0.11. It should pave a way to achieve the magnetic transition and enhance the magnetoresponsive effects in designing similar coherent materials by employing low melting point metal doping to form the dual-phase heterogeneous structure.

The purpose of this paper is to study the influence of Mo addition on the phase morphologies, microstructures and magnetic properties of the designated alloys. It is found out that the coercivity Hcj increases unevenly from 12.2 kOe for (Nd0.8Ce0.2)13Fe82B5 to the maximum value of 13.3 kOe for (Nd0.8Ce0.2)13Fe80B5Mo2. The transmission electron microscopy images demonstrate that the grain size decreases with the addition of Mo, which indicates that Mo has grain refinement effect. The correlative analysis gives rise to the conclusion that the coercivity mechanism of the investigated alloys is dominated by pinning type. All in all, the enhancement of the magnetic properties is mainly attributed to the synergistic impact of grain refinement, pinning effects and the microstructural homogenization. The research may shed light on the potential development and application of rare earth-based counterpart magnets.

The effects of Ta doping on the magnetic properties, temperature stabilities, phase constituents, intergranular exchange interactions, and microstructures of (Nd0.8Ce0.2)13.5Fe80.5−xTaxB6 (x = 0–1.5) nanoribbons are investigated in this paper. It is found that Ta doping increased the coercivity of the alloys, while the remanence remained nearly unchanged. The temperature stability of the alloys improved significantly as the absolute value of temperature coefficient of coercivity β enhanced from 0.48%/K for Ta-free alloy to 0.38%/K for the alloys with 0.3–1.5 at.% Ta addition. The lattice constants (a, c), unit cell volume, and phase transition temperatures (Tsr, TC) of the alloys changed by adding Ta into the alloys. Comprehensive best magnetic properties of Hcj = 16.0 kOe, Br = 7.65 kG, and (BH)max = 12.60 MGOe are obtained in (Nd0.8Ce0.2)13.5Fe79.6Ta0.9B6 ribbon. The improvement in magnetic properties is mainly attributed to refined and uniform microstructure of the Ta-doped alloys.

Nanocrystalline Ce–Fe–B based rare earth permanent magnets are prepared by spark plasma sintering (SPS) method using grounded ribbons as raw material. The phase compositions, microstructure, magnetic properties, intergranular magnetic interactions, recoil loops, and corrosion resistance of the magnets are investigated systemically. The relationship among sintering processes, microstructures, and magnetic characteristics are discussed in depth by mainly using transmission electron microscope (TEM) and Physical Property Measurement System (PPMS) techniques. It is demonstrated that the sintering temperature has a significant influence on the average grain size and width of coarse grain areas. The optimized magnetic properties of remanent magnetization $J_{r}= 0.37$ T, coercivity $H_{\mathrm {ci}} = 227$ kA/m, and maximum energy product $(BH)_{\mathrm {max}}= 16$ kJ/m3 are obtained at 650 °C under 50 MPa for 2 min. Their exchanging coupling interactions are quite weak compared with those of the counterpart alloys. The exchanged decoupled of soft magnetic materials and inhomogeneous magnetic anisotropy result in open and steep recoil loops. The electrochemical experiment results display that the corrosion resistance of the magnets is improved by sintering at high temperature. This paper plays a role in guiding the future research and development of nanocrystalline Ce–Fe–B based permanent magnetic materials.

In this work, Ni-Co-Mn-Ti ferromagnetic shape memory alloy thin films, which consists of all 3d metals, are well prepared by direct-current (DC) double targets magnetron co-sputtering method. The deposition temperature (DT) dependence of microstructure and magnetic properties is studied in these thin films. The elemental compositions of these thin films are almost invariable, while the average grain size increases with the increasing DT. Micro-morphology shows that the crystal grains of films are spherical and homogeneous. For the samples deposited at 523 K and 573 K, an unusual magnetization reversal behavior is observed at ∼342 K and ∼225 K, respectively, in the thermomagnetic curves at the applied field μ0H = 1 T. Meanwhile, the hysteresis loops at ambient temperature demonstrate that in Ni-Co-Mn-Ti thin films the ferromagnetic contribution increases at the expense of diamagnetic one as DT increases. The reasons for these results are discussed.

The magnetic properties, thermal stabilities and microstructures of misch-metal (MM) alloys with nominal composition of MM2.0+xFe14B (x = 0–0.6) prepared using melt-spun technique are investigated. The XRD pattern showed that the ribbons consist of the MM2Fe14B main phase, α-Fe, Fe3B and ReFe7 phases. The coercivity of all ribbons enhances with increasing the content of MM, while the remanence gradually decreases. Interestingly, there is an increase in remanence of MM2.2Fe14B ribbon, which is attributed to the strong intergranular exchange coupling effect. The Curie temperature of all the ribbons is around 546 K, which does not change with the content of MM. By the EDS analysis, we have clarified that the changes of the temperature stability are mainly related to the distribution of rare earth content in the main phase. This study shows that the magnetic properties of the alloys can be improved by optimizing the content of rare earth elements. The ribbons in the optimum condition exhibit a coercivity of 899 kA/m and a (BH)max of 116 kJ/m3, which can be used to produce high cost performance MM-Fe-B magnets.

Ce-Fe-B spark plasma sintered (SPSed) magnets prepared with different ratio of alloys of Ce-Fe-B and Pr-Nd-Fe-B by dual alloy method are investigated in this paper. As expected, the remanent magnetization Jr, coercivity Hci and maximum energy product (BH)max of SPSed magnets increase obviously with increasing weight percentage of Pr-Nd-Fe-B alloy. The magnetic properties of Jr = 0.71 T, Hci = 915 kA/m, (BH)max = 72 kJ/m3 are obtained for the SPSed magnets with 80 wt% Pr-Nd-Fe-B alloy. There are three Curie temperatures in this type magnet, which implies the coexistence of three hard magnetic phases. The second Curie temperature depends on the Pr-Nd-Fe-B content. With increasing Pr-Nd-Fe-B alloy content, the volume fraction and width of coarse grain zone decrease. It is shown by the microstructure analysis that the rare earth elements diffuse during sintering process resulting in the formation of Pr-Nd-Ce-Fe-B hard magnetic phase. The intergranular exchange coupling strength is enhanced with increasing Pr-Nd-Fe-B content. The present research may be a potential reference for further research and development of this type Ce-containing nanocrystalline permanent magnetic materials.

Al film, which was coated in sintered Nd-Fe-B magnets prepared by magnetron sputtering, was employed for grain boundary diffusion source. Effects of the grain boundary diffusion processes (GBDP) on the microstructure evolution and properties were investigated in detail. Through grain boundary diffusion processes, the highest coercivity of 1184 kA/m and maximum energy product of 238 kJ/m3 could be obtained, increasing by 21.8% and 3.9%, respectively, compared with the initial magnet. Meanwhile, our results showed that fine, uniform and continuous intergranular phase induced by Al diffusion, was the main reason for properties improvement, while a weak ferromagnetic phase and the vague interface between main phase and RE-rich phase should be responsible to the deterioration of coercivity when the diffused temperature exceed 600 °C. Besides, the corrosion resistance of Al-diffused magnets was also greatly improved, owing to the fact that the Al element could promote the electrochemical potential of RE-rich phase and the more thin, continuous intergranular phase would also narrow the corrosion channel.

In this paper Hf doped (Nd0.8Ce0.2)13Fe82−xB5Hfx (x = 0, 0.5, 1.0, 2.0, 3.0) ribbons were fabricated by melt-spinning technique, and their microstructure, magnetic properties and coercivity mechanism have been investigated methodically. It is found that the coercivity of (Nd0.8Ce0.2)13Fe82−xB5Hfx ribbons is enhanced for 0 ≤ x ≤ 2.0 at.% while the remanence decreases slightly. The optimal comprehensive magnetic properties Jr = 7.5 kG, Hcj = 12.7 kOe, and (BH)max = 12.3 MGOe, are obtained at x = 1.0 at.%. The grain size decreased with the addition of Hf and more uniform microstructure evolved. The coercivity mechanism of the (Nd0.8Ce0.2)13Fe82−xB5Hfx (x = 0–3.0) ribbons is found to be the pinning mechanism based on the systematic investigations and analysis. The synchronization of the grain refinement and the pinning effect lead to the improvement of magnetic properties.

There is a close relationship between the magnetic properties of alnico permanent magnetic alloys and the nano-scaled spinodally decomposed structure which results from various heat treatment cycles. In this paper, different heat treatment cycles are employed to cast alnico 8 alloys with composition 32.3Fe-37.5Co-13.7Ni-6.2Al-5.8Ti-3.4Cu-0.2Zr-0.1S-0.8Nb. It is observed that magnetic field treatment at high temperature for specific time is the most effective method to obtain better microstructure and magnetic properties. The mosaic structures consisting of Fe-Co phase and Al-Ni rich phase are best separated during thermomagnetic treatment for 4–5 min and refined during low temperature treatments. The bias growth of the ferromagnetic phase does not develop in absence of magnetic field, and hence the phases are not refined and separated completely. This produces isotropic alnico alloys with low magnetic properties. However, continuous cooling of the alloys in magnetic field followed by isothermal treatment without magnetic field provides moderate magnetic properties. Treatment at high temperatures in field and without field for longer time leads to the coarsening of spinodal phases and poor magnetic properties. The optimum magnetic properties of Hcj = 1.7 kOe, Br = 8.0 kGs and (BH)max = 5.02 MGOe are attributed to the refined microstructure of the thermomagnetically treated alloys.

Reducing the spatial dimension of ferromagnetic (α1) phase is the most promising route for coercivity enhancement of alnico alloys. It is an experimental fact that the spatial dimension of α1 phase cannot be reduced below a certain limit (∼25 nm) using conventional processing methods. To obtain finer α1 phase we have fabricated alnico ribbons with nominal composition of 32.2Fe-36Co-13.5Ni-7.6Al-6.2Ti-3.8Cu-0.5Zr-0.2B by melt spinning and subsequent heat treatments. Very fine Fe-Co rich (α1) rods of the order of 5 nm diameter and 100 nm length embedded in Al-Ni rich matrix are obtained. After simplified heat treatment the properties of the isotropic ribbons are Hcj = 770 Oe, Br = 6.5 kGs and (BH)max = 1.86 MGOe. It is shown that alnico ribbons have unprecedented thermal stability described in terms of temperature coefficient of remanence (α) and temperature coefficient of coercivity (β) by measuring magnetic properties at high temperatures (∼800 K). The microstructures of the alloys have been analyzed by Optical Microscope, Scanning Electron Microscope and Transmission Electron Microscope. The phase transition temperatures have been observed by DTA and magnetic properties are measured by PPMS.

The microstructure and magnetic properties of alnico 8 alloys have been investigated in this paper. The alloys with nominal composition of 32.5-xFe-7.5Al-1.0Nb-35.0Co-4.0Cu-14.0Ni-6.0Ti-xB ( $x=0-0.40$ ) are prepared by arc melting and subsequent heat treatment cycles. The alloys are characterized by X-ray diffractometer, optical microscopy, scanning electron microscopy, and physical property measurement system. The results indicate that magnetic properties which originate from spinodal decomposition strongly depend on alloy chemistry, size of the spinodal phases, and processing parameters. Purifying $\alpha $ -phase and reducing the spatial dimensions of $\alpha _{1}$ - and $\alpha _{2}$ -phases are effective means to increase the coercivity $H_{\text {cj}}$ of the alloys. Small concentration of B is beneficial to refine the microstructure and enhance the magnetic properties of alnico magnetic alloys. The alloys with 32.48Fe-7.5Al-1.0Nb-35.0Co-4.0Cu-14.0Ni-6.0Ti-0.02B exhibit best magnetic properties of $H_{\text {cj}}=1.35$ kOe, $B_{r}=5.6$ kGs, and (BH) $_{\text {max}} = 2.35$ MGOe. The optimized alloy is further treated thermo-magnetically, and the properties are enhanced to $H_{\text {cj}} =1.62$ , $B_{r} =7$ kGs, and (BH) $_{\text {max}}= 4.3$ MGOe.

The Mn-Fe-Ni-Ge-Si alloy ribbons are prepared by the melt spinning technique with different wheel speeds and the microstructure, magnetic and magnetocaloric properties are systematically investigated. All the samples reveal the coincident magnetostructural transformation from the ferromagnetic TiNiSi-type martensite to paramagnetic Ni2In-type austenite on heating. The magnetostructural transformation temperature decreases remarkably from 380 K to 320 K with the increase in wheel speed from 10 m/s to 30 m/s. Giant magnetocaloric effect with excellent magnetic reversibility is obtained and the effective refrigeration capacity shows an evident increase, namely 103.9, 125.3, and 185.9 J/kg for three ribbons (under a field change of 0–5 T), though the maximal magnetic entropy change peak value of 27.4 J/kg K is achieved for the sample at 20 m/s. All these features suggest that the wheel speed is an effective way to manipulate the magnetostructural transformation temperature and magnetocaloric properties for MnNiGe-based alloys. The underlying mechanisms are discussed.

Alnico alloys are prepared with nominal composition of 31.4-xFe-7.0Al-36.0Co-4.0Cu-1.0Nb-14.0Ni-6.0Ti-0.6Zr-xB (x = 0.02, 0.04, 0.06, 0.08, in wt%) by arc melting and casting techniques and subsequent heat treatment. The alloys are characterized by X-ray diffraction method, optical microscope, scanning electron microscope and pulse field magnetometer by plotting magnetic hysteresis demagnetization curve. The results of HRSEM show at least two new phases at α-grain boundaries and triple junctions. These phases, when retained at low concentration, help in enhancing magnetic properties of alnico alloys by purifying spinodal phases and reducing the adverse effects of impurity elements. Two different heat treatment cycles are employed. In the first phase, the alloys are processed by using heat treatment cycles without magnetic field; and Hc of 1.35 kOe, Br of 4.87 kGs and (BH)max of 1.96 MGOe are obtained by furnace cooling below TC and subsequent tempering at 680 °C and 550 °C. In the second phase, the alloy with best magnetic properties is treated thermo-magnetically; and Hc of 1.68 kOe, Br of 7.1 kG and (BH)max of 4.45 MGOe are obtained.

The rod-shaped Mn-Ni-Fe-Ge compounds are prepared by the copper mould spraying casting. These alloys reveal the coincident martensitic magnetostructural transformation with giant room temperature magnetocaloric effect from the high-temperature paramagnetic Ni2In-type austenite to low-temperature ferromagnetic TiNiSi-type martensite. Interestingly, when the rod increases in diameter, the magnetostructural transformation temperature decreases slightly, whereas the magnetization jump and magnetocaloric properties improve. In contrast, the time of heat treatment has almost no effect on crystallographic, magnetic and magnetocaloric properties, even with fourfold elongation of annealing time. This suggests that the synthesis of rod samples by spraying casting pave a pathway for the development of new rod-shaped magnetic refrigerant with desirable magnetocaloric properties and low preparation cost due to time- and energy-saving.

Excellent microwave absorption performance was obtained from NdFe magnetic powders. A suitable oxidation heat treatment temperature can optimize the impedance matching effectively and result in a good absorption performance. By varying the coating thickness, the reflection loss values of oxidized NdFe magnetic powders are less than −10 dB nearly in the whole range of 2–18 GHz. The maximum reflection loss value of the oxidized NdFe magnetic powders at 373 K reaches up to −55.9 dB at 3.6 GHz with a thickness of 3.0 mm. In addition, the microwave absorption peak frequency can also be tuned by the oxidation heat treatment temperature. The obtained results and systematic analysis suggest that NdFe magnetic powders would be attractive candidates for microwave absorbing materials and the oxidation heat treatment could be an effective way to optimize the impedance matching of some ferromagnetic alloy absorption materials.

By combining rare earth-based Ce-Fe-B and rare earth free Alnico powders, composite Ce-Fe-B/Alnico spark plasma sintered (SPS) magnets have been fabricated and reported for the first time. It is observed that the two phases exit in the alloys interacting magnetostatically. The Alnico phase constitutes α1 and α2 phases as a consequence of spinodal decomposition process, while the Ce-Fe-B phase contained Ce2Fe14B and CeFe2 phases. By increasing the content of Alnico phase, the remanence magnetization and the maximum energy density of the composite alloys increased owing to the higher saturation magnetization of Alnico phase. Magnetic properties, such as remanence magnetization Js = 0.93 T, intrinsic coercivity Hcj = 202 kA/m and maximum energy density (BH)max = 19.8 kJ/m3 were obtained in spark plasma sintered composite magnets containing 15 wt% Alnico. The recoil loops demonstrated the presence of anisotropy variations in the alloys specifically at high applied magnetic field. Analysis of the Curie temperature and elemental analysis validated that the constituent phases of SPS magnets exist in the alloys. This work may shed light on the development of composite magnets with distinct phases and enhanced magnetic properties.

Grain boundary diffusion of Tb can improve the microstructural and magnetic properties of NdFeB magnets. In this paper, we investigated diffusion behavior and coercivity enhancement by dip-coating TbH3 powders in sintered NdFeB magnets. The results show that the coercivity increased rapidly from 14.02 kOe to 23.72 kOe after diffusion. Tb atoms diffused to the surface of the magnet and then into the interior, forming a network Tb-rich shell around (PrNd)2Fe14B grains. The higher magnetocrystalline anisotropy (PrNd,Tb)2Fe14B phases determine the coercivity enhancement. The Tb diffusion model was build up from coating layer to the magnet. This work may shed light on developing sintered NdFeB magnets with coercivity enhancement by diffusion technology.

Ce 17 Fe 78−x B 6 Hf x (x = 0–1.0) alloys were fabricated by a melt-spinning technique in order to study their magnetic properties and microstructure. Magnetic investigations of Ce 17 Fe 78−x B 6 Hf x (x = 0–1.0) alloys show that the room-temperature coercivity increases linearly from 352 kA/m at x = 0 to 420 kA/m at x = 1.0. The Curie temperature ( T c ) decreases monotonically from 424.5 K to 409.1 K. The Ce L3-edge X-ray absorption near edge structure (XANES) spectrums reveal that there is more Ce 4+ in ribbons under total electron yield (TEY) than fluorescence yield (FY). Hf addition has no effect on the weight of Ce 3+ and Ce 4+ in CeFeB-based alloys. The grain refinement and microstructure uniformity are essential for improving the magnetic properties of Hf-doped alloys. This paper may shed light on the further development of the Ce-based magnets and offer a feasible way for using the rare earth resources effectively.

The rapidly solidified Fe-free and 15 at. % Fe-substituted Mn-Ni-Ge-based samples in the rod shape were fabricated with different diameters by the copper mould spraying casting method. Firstly, the lath martensite-like variants was strikingly observed in these compounds by the field emission scanning electron microscope (FESEM), especially in the annealed Mn-Ni-Fe-Ge samples after the mechanical and chemical treatments. It is also found that the martensite (TiNiSi-type with orthorhombic structure) coexists with the austenite (Ni 2 In-type with hexagonal structure) around room temperature in the Mn-Ni-Fe-Ge compounds by the FESEM and transmission electron microscope. Resultantly, the thermomagnetic measurements shows that the single, more intimate magnetostructural transformation is achieved around room temperature for the annealed samples compared to the as-cast rods.

MnNiGe:Fe ribbon samples are prepared. Partial Ni- and Mn-substitution of Fe element can both induce the antiferromagnetic-ferromagnetic conversion in the TiNiSi-type state of these MnNiGe:Fe ribbon systems. It is found out, however, that some factors such as annealing, temperature variation process, field-cycling, substituted site and magnetic field can affect the conversion and competition between the antiferromagnetic and ferromagnetic states in these ribbons. Therefore, in this paper these major influencing factors are studied systematically and further discussed are the related magnetic and magnetocaloric properties in MnNiGe:Fe ribbon systems.

With the intention to reduce the Nd content in Nd 2 Fe 14 B-type alloys, 20 at.% Ce and 0.5 at.% Nb substituting Nd and Fe in the Nd 13 Fe 82 B 5 alloys were previously employed to improve successfully the coercivity and the thermal stability without the energy product reduction. In this study, a light increase of the remnant polarization J r was observed in (Nd 0.8 Ce 0.2 ) 13 Fe 82−x Nb x B 5 alloy at x = 0.5 and x = 1.0, resulting from the increasing amount of α-Fe phase. The optimum magnetic properties obtained with 0.5 at.% Nb doping are H c j = 13.1 kOe, J r = 0.79 T, ( BH ) max = 13.3 MGOe, respectively. Besides, the coercivity H cj and maximum energy product ( BH ) max for the melt-spun ribbons with 0.5 at.% Nb addition are higher than those of the Nb-free ribbons in the temperature range of 300–450 K. Both the variations of Curie temperature T c and a increase of lattice constants a and c of the hard magnetic phase with Nb addition imply that some of Nb atoms may directly enter into the hard magnetic phase, occupying the Fe sites. With the analysis on the demagnetization curve, Henkel curve and the observation of transmission electron microscope (TEM), the results indicate that a small amount of Nb can enhance the coercivity and exchange coupling though improving the microstructure of alloys.

Much demanded and overused are the critical rare-earth elements such as Pr, Nd, Dy, and Tb with increasing need of NdFeB-type rare-earth permanent magnets in the enlarging application areas, developing new high-tech industries, and emerging cutting-age frontiers. The balance and efficient use of rare-earth resources comes into being the national strategy, national defense, and border safety for many major countries and regions in the world. (Nd,Ce)FeB-based permanent magnetic materials, which can not only reduce cost but also offer a feasible way for integrated and effective utilization of rare earth resources, have received much attention in recent years. The existence of CeFe 2 and the mixed valence state of Ce in CeFeB compound, the different metallurgy behavior and the particular processing as well as potential various magnetic-hardening mechanisms, however, make it quite different from Nd-based alloys. For instance, the coercivity of Ce-containing magnets in some certain composition range, is even higher than that of the counterpart pure Nd-based magnets though the Ce-containing magnets possess inferior intrinsic properties. Consequently, it is very important to design proper composition and structure, optimize processing, and analyze the mechanisms in depth for this kind of magnet. High performance and cost-effective magnets can be fabricated if we can make full use of the composition’s inhomogeneous and abnormal coercivity variation of the Ce-containing permanent magnets. In this paper, we have summarized the phase structures, magnetic properties and microstructures of (Nd,Ce)FeB-based permanent magnetic materials to shed light on further research and development of this type of so-called “gap magnet”.

In this work, Sm24.6Co50Fe17·4Cu5·7Zr2.3-xHfx (x = 0–2.0) ribbons and magnets were prepared by melt-spinning method and spark plasma sintering (SPS) technique. The magnetic properties of the melt-spun ribbons with Hf addition are much higher than those of the Hf-free ribbons in the temperature range of 300–400 K. The optimal comprehensive magnetic properties of ribbons and isotropic magnets were obtained with 1.0 wt% Hf substitution. Microstructural results showed that Hf elements do not enter into the hard magnetic 1:7H and 2:17R phase in SPSed magnets and Zr–Hf rich phases are formed around grain boundaries as particulates. The enhancement of magnetic properties of the ribbons and SPSed magnets can be attributed to grain refinement and more uniform microstructure as a result of Hf addition. The micro-twin structure of 2:17R phase also have been discovered in the spark plasma sintered magnets. This work may provide a rapid preparation of Sm2Co17 magnets.

A hexagonal Ni2In-type single phase is obtained in the (Mn0.7Cu0.3)66Ga34 sample. The superspin glass (SSG) behavior and exchange bias effect are investigated in depth. The DC thermomagnetic M(T) measurements show a remarkable bifurcation between the zero-field cooling and field-cooling patterns below 100 K, suggesting a SSG behavior, which is verified by the simulation of de Almeida–Thouless relation from M(T) curves under different fields. Furthermore, the SSG behavior is justified by the AC susceptibility, temporal magnetic relaxation, and memory effect measurements. The asymmetric response of memory effect clearly indicates that it supports the hierarchical model. More importantly, the fitted parameters including Mydosh parameter Φ = 0.0255, dynamical critical exponent zv = 10.02, and τ0 ~ 5.5 × 10−9 s give the convincing evidences for the SSG behavior at low temperatures for (Mn0.7Cu0.3)66Ga34. Above all, a giant exchange bias field of μ0HEB = 308.4 mT is obtained at T = 2 K under the cooling field μ0HCF = 0.5 T and maximum applied magnetic field |μ0Hmax| = | +μ0H | = | −μ0H | = 8 T. The origin for giant exchange bias effect is discussed.

We report the observation of the giant topological Hall effect near room temperature in a complex noncollinear ferromagnet NdMn2Ge2 single crystal. Three successive magnetic transitions are observed below 400 K, including a spin reorientation transition at TSR = 215 K. The complex noncollinear magnetic structures give rise to anomalous transport behaviors. When the current flows along the a axis and the magnetic field is applied along the c axis, the anomalous Hall effect is observed, which is found to be dominated by the skew scattering mechanism. Strikingly, a giant topological Hall effect appears in a wide temperature range, which stems from the noncollinear spin configuration with finite scalar spin chirality. The topological Hall resistivity reaches the maximum of −1.35 μΩ cm at 300 K and drops slightly with temperature until below TSR. These results suggest that the NdMn2Ge2 single crystal would be a promising topological material for spintronic applications at room temperature.

In this paper, ZnFe2O4-coated FeSiCr (FeSiCr@ZnFe2O4) nanocomposites were fabricated by plasma arc discharging and hydrothermal methods. The polarization strength of composite materials is weakened and the complex permittivity and conductivity are reduced by ZnFe2O4 coating on the FeSiCr surface. The impedance matching of the microwave absorbing materials is optimized. The reflection loss of FeSiCr@ZnFe2O4 nanocomposites is calculated to be −44.2 dB at 15.9 GHz (d = 1.5 mm), and the effective bandwidth (

The Mn1-xCrxCoGe (x = 0.01, 0.015, 0.02, 0.03) alloy ribbons are prepared. The microstructure, structure, magnetic and magnetocaloric properties were studied in these ribbons. The substitution of Cr for Mn effectively reduces the structural transformation temperature of MnCoGe alloy. In annealed Mn0.98Cr0.02CoGe ribbons, a coupled magnetostructural transformation from the ferromagnetic TiNiSi-type to paramagnetic Ni2In-type state with the increasing temperature is observed. As a result, large magnetocaloric effect near room temperature and simultaneously almost zero magnetic hysteresis losses presented in these ribbons would be favorable to room-temperature magnetic refrigeration. In measuring the isothermal magnetization curves, both the loop and standard methods are adopted to compare and make sure the reliability of our results.

Alumina ceramic coatings were prepared on the sintered NdFeB magnets by micro-arc oxidation (MAO) in aluminate solution. The effects of anodic voltages on the microstructure and corrosion resistance of the coatings were investigated and the formation mechanism of the coating was also detailedly discussed. The microstructure and composition of the MAO coatings were characterized by SEM, XRD, EDS, XPS and surface roughness, respectively. The MAO coatings on NdFeB magnets were mainly composed of Al 2 O 3 crystal phase, and Fe 2 O 3 and Nd 2 O 3 amorphous phase with some absorbed H 2 O and –OH. With increasing the voltages, the crystallinity of Al 2 O 3 phase enhanced, while the surface roughness of the coatings also increased. At the same time, the pore sizes of MAO coatings increased, while the amount of the pores decreased. The corrosion resistance of NdFeB samples was improved due to the existence of the MAO coatings. With increasing the voltages, the corrosion resistance of the coated NdFeB samples increased first, reaching the optimum at 420 V, and then decreased. The corrosion protection efficiency of the coatings could be up to 94.3%. At the initial stage of MAO process, the compact barrier layer of NdFeB magnets was formed through the deposition of the electrolyte, which was the critical procedure for the MAO treatment of NdFeB magnets and the biggest difference from the valve metals through the conventional anodizing.

In order to improve the thermal stability of Nd 2 Fe 14 B-type rare earth permanent magnets, Nd 13 Fe 81−x B 6 Hf x (x = 0–1.0) alloys are fabricated by a melt spinning technique. The magnetic properties, microstructure and thermal stability of Nd 13 Fe 81−x B 6 Hf x (x = 0–1.0) alloys are investigated. When x = 0.5, a coercivity of 505 kA/m is obtained at 450 K, which is much higher than that of 290 kA/m for the sample without Hf. Curie temperature ( T c ) decreases slightly from 588 K to 579 K. The temperature coefficients of remanence ( α ) and coercivity ( β ) of the ribbons are improved from −0.20 %/K, −0.48 %/K for x = 0 to −0.17 %/K, −0.34 %/K for x = 0.5 in the temperature range of 300–450 K, respectively. The betterment of the magnetic properties and thermal stability with Hf doping is discussed in terms of the grain refinement and a more uniform microstructure. Furthermore, it is found out that the domain wall pinning mechanism is responsible for enhancing the coercivity of Nd 13 Fe 81−x B 6 Hf x alloys.

Nd-rich phase plays a critical role in wetting grain boundary and facilitating texture formation for hot deformed (HD) Nd-Fe-B magnets. In this study, a non-uniform distribution of Nd-rich phase with dimension up to a few micrometers was observed in nanocrystalline HD magnets. The aggregation of the Nd-rich phase is confirmed to result from the low density precursor prepared by spark plasma sintering (SPS). The large local demagnetizing fields induced by Nd-rich phase aggregation led to the open recoil loops and reduced coercivity. Upon reducing recoil loop openness by eliminating Nd-rich phase aggregation, the coercivity of the HD magnet was significantly improved from 226 kA/m to 995 kA/m, and a high maximum energy product of 293 kJ/m3 was obtained. The dependences of microstructure and coercivity on the recoil loop characteristics suggest an essential approach for improving the magnetic properties of nanocrystalline HD Nd-Fe-B magnets.

Co7Hf ribbons have been prepared by melt spinning technique. The effects of wheel speeds on the Structures and magnetic properties were investigated. At wheel speed of 35 m/s, the Co7Hf ribbons show the best intrinsic coercivity, remanence and maximum energy density, 2.0 kOe, 53 emu/g and 4.7 MGOe, respectively. XRD and TEM results show that Co7Hf ribbons are mainly composed of Co7Hf phase and Co phase. DSC and TEM results show that amorphous phase appeared at high wheel speed. The grain size refinement and the formation of amorphous phase were the main reasons for the changes of magnetic properties. Meanwhile, the excellent high temperature stability of Co7Hf ribbons was observed. In range of 27–500 °C, the temperature coefficients of remanence (α) and coercivity (β) of Co7Hf ribbons are −0.11%/°C, −0.19%/°C, respectively.

The effects of annealing at different temperatures (1053, 1103, 1153 K) on the martensitic transformation (MT) and magnetocaloric effect have been investigated in all-3d-metal Mn50Ni32Co8Ti10 alloy ribbons. In melt-spun ribbons, a coexistence of predominant 5-layer modulated martensite (5 M, monoclinic, space group P2/m) and a small amount of B2-type austenite with a cubic structure is revealed at room temperature (RT), demonstrating that the forward MT temperature ( T t FC ) lies just above the RT. After annealing, the coexistence of three different phases, the non-modulated L10 martensite (NM, tetragonal, space group P4/mmm), 5 M martensite, and B2 austenite, and the enhanced intensity of both B2 (400) and L10 (220) characteristic peaks exhibited in X-ray diffraction patterns are suggestive of the fact that T t FC is reducing. The ( T t FC ) -reduction induced by heat treatment is also confirmed by thermomagnetic measurements. T O D C : T t Z F C i n t h i s s e n t e n c e s h o u l d b e a l s o d e l e t e d The magnetic entropy change maximum also decreases after annealing because the MT slows down. However, it is worth noting that after annealing the effective refrigeration capacity increases more than twice of the melt-spun sample. The origin of ( T t FC ) -reduction induced by annealing is discussed.

The magnetic glass state has been proposed and studied in the ferromagnetic (FM) compounds undergoing the first-order FM-antiferromagnetic (AFM) transition. In this work, we have unequivocally demonstrated the occurrence of magnetic-glass-like transition during the field cooling process in the ferrimagnetic (FIM) Mn1.975Cr0.025Sb alloy showing a first-order FIM-AFM transition. The magnetic, resistance and relaxation studies characterize the formation of a nonergodic magnetic glass-like state at low temperature arising from the coexistence of AFM and kinetically arrested FIM phases above a critical applied field. The magnetic field-temperature (μ0H-T) phase diagram based on magnetic and resistance measurements further evidences the generality of onset of glassy behavior in the first-order magnetic transition materials, in which the transition temperature shifts strongly with increasing applied field.

To improve the coercivity (Hcj) and thermal stability of hot-deformed (HD) Nd-Fe-B magnets, Pr50Dy20Cu15Ga15 (at.%) alloy powders with minor heavy rare earth (HRE) are introduced. The magnetic properties, thermal stability and microstructure of the HD magnets with different amounts of Pr50Dy20Cu15Ga15 alloy powders are investigated. The Hcj of the HD magnets is enhanced from 1242 kA/m to 1595 kA/m by doping 3 wt.% Pr50Dy20Cu15Ga15 alloy powders. The thermal stability of the doped HD magnet is also improved at high temperatures. The better isolation of Nd2Fe14B grains by rare earth-rich (RE-rich) phases and the formation of local high-anisotropy (Nd, Pr, Dy)2Fe14B hard magnetic phases are the main reasons for the increase of Hcj. Notably, the non-uniform aggregation of RE-rich phases results in the inhomogeneity of magnetic anisotropy, which eventually leads to the large openness of the recoil loops. This study gives an intuitive view on the relationship between microstructure evolution and magnetic properties of the HD magnets.

We report studies of perovskite La0.64Sr0.36FeO3−δ ceramics through measurements of magnetization and mechanical spectroscopy. We find an unusual magnetic anomaly around 225 K, R 3 ¯ c ↔ P n m a octahedral tilting transition where a step-like change of magnetization was observed. The step-like change depends on the external magnetic field. The magnetic anomaly indicates a change of the magnetocrystalline anisotropy axis and it suggests a substantial role of spin-lattice coupling in La0.64Sr0.36FeO3−δ. Below the tilting transition, a broad peak in zero-field-cooling dc magnetization was observed around TP = 125 K and exchange bias was observed below TP. The exchange bias field shows a strong dependence on temperature and cooling field. It suggests that the exchange bias is resulting from the coupling at cluster glass or ferromagnetic and antiferromagnetic interface where ferromagnetic and antiferromagnetic domains coexist below TP in La0.64Sr0.36FeO3−δ.

In this work, bidirectionally controlling the valence electron concentration (e/a) is proposed to tune and realize the coincident magneto-structural transformation in MnCoGe alloy. With the common effect of e/a and ‘chemical pressure’ produced by the change of chemical composition, the martensitically magneto-structural transformation is achieved from paramagnetic Ni2In-type austenite to ferromagnetic TiNiSi-type martensite in both Mn1−xCoGe1+x and Mn1+yCoGe1−y systems under the same experimental capabilities. It is interesting that a magnetic-field-induced metamagnetic martensitic transformation is observed in Mn1−xCoGe1+x system. Accordingly, large magnetic entropy change peak values of ~31.6 and ~20.1 J kg−1 K−1 are obtained under the applied field change of μ0ΔH = 0–5 T at 276 and 294 K for Mn0.98CoGe1.02 and Mn0.985CoGe1.015 alloys, respectively. Likewise, a moderate magnetic entropy change ~9.13 J kg−1 K−1 is acquired under μ0ΔH = 0–5 T around 320 K for Mn1.03CoGe0.97 alloy. Bidirectionally controlling a physical variable may pave a new and effective way in designing and optimizing compositions to simultaneously realize the magneto-structural transformation in both directions in MnCoGe and other systems.

In this work, a giant exchange bias field of over 1 kOe, which is uncommon in film systems, is obtained in all-3d-metal Heusler-type Ni38.8Co2.9Mn37.9Ti20.4 thin films. The thermomagnetic curves show an unambiguous noncoincidence between zero-field-cooled and field-cooled protocols, indicating the presence of magnetically inhomogeneous phases in the film in the low temperature range. Magnetic measurements, including magnetic relaxation and magnetic memory effects, demonstrate that the exchange bias should be attributed to the exchange interaction between ferromagnetic clusters and the antiferromagnetic host in the Ni38.8Co2.9Mn37.9Ti20.4 thin film at low temperature. The local Mn-Co-Mn configuration provides the ferromagnetic contribution, which is formed in the antiferromagnetic Heusler main phase due to the small amount of Co-substitution for Ni. The achievement of the large exchange bias effect in the Ni38.8Co2.9Mn37.9Ti20.4 thin film is a significant addition to the existing multiple magneto-responsive effects in all-3d-metal Ni-Mn-based Heusler systems.

Discussed in this work are the microstructure, magnetic properties, and exchange bias in (Mn0.7Co0.3)65Sn35 alloy ribbons. After annealing, the crystalline grains grow distinctly from ~0.93 μm to ~8.41 μm. The surface of ribbon samples becomes more regular and dense. The ribbons reveal a ferromagnetism with the Curie temperature TC ~ 169 K, which is determined from the thermomagnetic M(T) measurements. The maximum magnetic entropy change ~4.5 J kg−1 K−1 is obtained around ferromagnetic-paramagnetic transition under the field change of μ0ΔH = 0–7 T. Large exchange bias field of μ0HEB = 80 mT is achieved at T = 2 K when the cooling field is μ0HCF = 0.2 T. The reasons for the appearance of ferromagnetism and large exchange bias effect are discussed.

Two-step grain boundary diffusion processes (GBDP) were carried out to obtain the diffused magnets with excellent magnetic properties and high thermal stability, using Dy and Al coated films as diffused sources in turn, prepared by magnetron sputtering. It is found that through the two-step diffusion with Dy/Al films, the highest coercivity, up to 1382 kA/m is achieved, increasing by 42.3% compared to as-prepared magnet, moreover, the maximum energy product is improved significantly as well. Besides, whether for as-prepared magnets or one-step diffused magnets with Al film and Dy film, respectively, the thermal stability and magnetic properties of two-step diffused magnet with Dy/Al film, are the highest among magnets. Microstructural investigation reveals that Dy is preferred to diffuse along the RE-rich phase and forming the “shell” of (Nd, Pr, Dy)2Fe14B phase around the outer region of main phase, restraining the nucleation of anti-magnetization domain, while the Al tends to locate in the RE-rich region, playing the role of modifying the interface between RE-rich phase and 2:14:1 main phase. Eventually, the temperature coefficient of coercivity (|β|) and remanence (|α|) with 0.65%/K and 0.12%/K, can be obtained, respectively.

Alnico 8 elements are added into Ce-Fe-B ribbons and the effects of the multi elements on the magnetic properties, phase transition temperatures and microstructure are investigated. It is found that the addition of alnico elements is helpful for enhancing the magnetic properties of the Ce-Fe-B alloys. The Curie temperature of Ce2Fe14B (2:14:1) main phase increased from 422 K for the standard alloy to 490 K by 20 wt% addition of alnico, while the Curie temperature of CeFe2 (1:2) phase decreased from 223 K to 122 K. The transmission electron microscopy (TEM) analysis showed that the nano grains of the alloys are refined which is attributed to the refractory character of Ti. The intergranular exchange interactions measured by applying Henkel plots also enhanced by adding alnico elements into Ce-Fe-B ribbons. The magnetic properties Hcj, Br and (BH)max improved significantly for the optimized addition of alnico. Magnetic properties of Hcj = 5.16 kOe, Br = 4.59 kG and (BH)max = 4.14 MGOe are obtained in Ce35Fe56.5(alnico)7.5B ribbon. The improved magnetic properties in alnico-doped Ce-Fe-B alloys are attributed to the refined microstructure and enhanced intergranular exchange interactions.

The microstructure, crystalline structure, martensitic transformation, and magnetocaloric effect are systematically studied in all-d-metal Ni36.0Co14.0Mn35.7Ti14.3 alloy ribbons fabricated by melt-spinning technique. Five-layer modulated martensite (5 M) and B2 austenite coexist in both melt-spun and annealed ribbons. Interestingly, all ribbons show a staircase-like magnetic-field-induced metamagnetic reverse martensitic transformation. Accordingly, large magnetocaloric effect near room temperature is obtained, including large magnetic entropy changes of 10 and 13 Jkg−1K−1 and large effective refrigerant capacity of 62 and 63 Jkg-1 under the field change of Δμ0H = 0–2 T, for melt-spun and annealed ribbons, respectively, making the all-d-metal Ni-Mn-based alloy ribbons promising for room temperature magnetic refrigeration.

Nd2Fe14B-type permanent magnets have been widely applied in various fields such as wind power, voice coil motors, and medical instruments. The large temperature dependence of coercivity, however, limits their further applications. We have systematically investigated the magnetic properties, thermal stabilities and coercivity mechanisms of the (Pr0.2Nd0.8)13Fe81-xB6Hfx (x=0, 0.5) nanocrystalline magnets fabricated by a spark plasma sintering (SPS) technique. The results indicate that the influence of Hf addition is significant on magnetic properties and thermal stabilities of the (PrNd)2Fe14B-type sintered magnets. It is shown that the sample with x = 0.5 at 300 K has much higher coercivity and remanent magnetization than those counterparts without Hf. The temperature coefficients of remanence (α) and coercivity (β) of the (Pr0.2Nd0.8)13Fe81-xB6Hfx magnets are improved significantly from -0.23 %/K, -0.57 %/K for the sample at x = 0 to -0.17 %/K, -0.49 %/K for the sample at x = 0.5 in the temperature range of 300-400 K. Furthermore, it is found out that the domain wall pinning mechanism is more likely responsible for enhancing the coercivity of the (Pr0.2Nd0.8)13Fe81-xB6Hfx magnets.

MnBi magnets with a high content of low temperature phase (LTP) and excellent magnetic properties were prepared by spark plasma sintering (SPS) using ball milling powders as precursors without magnetic purification. A complicated intergranular phase, which contains Mn phase, Bi phase, MnO phase, and even amorphous phase in MnBi magnets, was characterized and reported systematically. It was found that the formation of intergranular phase which was contributed by ball milling precursors and sintering mechanism, jointly, had important influence on the magnetic properties. The appropriate content of intergranular phase was beneficial in improving the coercivity due to the strong magnetic isolation effects. The optimum magnetic properties with Mr=26.0 emu/g, Hci= 7.11 kOe and (BH)max=1.53 MGOe at room temperature, and a maximum value Hci= 25.37 kOe at 550 K can be obtained. Strongly favorable magnetic properties make SPSed MnBi magnets an attractive candidate material for small permanent magnets used in high-temperature applications.

Interstitial effects of B and Li on the phase transition and magnetocaloric effect in Gd2In alloys had been studied. The antiferromagnetic (AFM) - ferromagnetic (FM) phase transition was found to be of first-order nature while ferromagnetic - paramagnetic (PM) phase transition was of second-order nature in B- or Li-doped Gd2In alloys. AFM-FM phase transition temperature was increased while FM-PM phase transition was decreased with more doping concentrations. During AFM-FM phase transition, the slope of temperature-dependent critical field (μ0Hcr) was increased by increased doping amounts. The magnetic entropy changes under small field change were enhanced by B and Li addition, which showed the beneficial effects of B and Li additions.

Diffusion process by Nd-Al and Pr-Al alloys was compared and investigated in Nd-Fe-B thin films. Enhanced coercivity 2.06T and good squareness was obtained by using Pr85Al15 and Nd85Al15 alloys as diffusion sources. But the coercivity of diffusion-processed thin films by Pr70Al30 and Pr55Al45 alloys decreased to 2.04T and 1.82T. High ambient coercivity of 2.26T was achieved in diffusion-processed thin film by Nd70Al30 leading to an improved coercivity thermal stability because Nd2Fe14B grains were enveloped by Nd-rich phase as seen by transmission electron microscopy Nd-loss image. Meanwhile, microstructure-dependent parameters α and Neff were improved. However, high content of Al in diffusion-processed thin film by Nd55Al45 lead to degraded texture and coercivity.

The microstructure, magnetic and magnetocaloric properties are investigated in the melt-spun and annealed Ni42.9Co6.9Mn38.3Sn11.9 ribbons. The columnar grains grow perpendicular to ribbon surfaces. After annealing, the grain size increases greatly. Meanwhile, the parent phase is suppressed and therefore L10 martensite predominates, indicating obvious shift of martensitic transformation to high temperature. More interestingly, the martensite variants are distinctly observed on the fractured cross-section of annealed ribbons, not just on the free surface in general. The significant enhancement of magnetic entropy change and effective refrigerant capacities with relatively smaller thermal hysteresis make annealed ribbons potential candidate in magnetic refrigeration around room temperature.

The sharp metamagnetic martensitic transformation (MMT) triggered by a low critical field plays a pivotal role in magnetoresponsive effects for ferromagnetic shape memory alloys (FSMAs). Here, a sharper magnetic-field-induced metamagnetic martensitic transformation (MFIMMT) is realized in Mn1−xCo1+xGe systems with a giant magnetocaloric effect around room temperature, which represents the lowest magnetic driving and completion fields as well as the largest magnetization difference around MFIMMT reported heretofore in MnCoGe-based FSMAs. More interestingly, a reversible MFIMMT with field cycling is observed in the Mn0.965Co0.035Ge compound. These results indicate that the consensus would be broken that the magnetic field is difficult to trigger the MMT for MnCoGe-based systems. The origin of a higher degree of sensitivity of martensitic transformation to the magnetic field is discussed based on the X-ray absorption spectroscopic results.

The Ce17Fe78-xB6Gax (x=0-1.0) ribbons were fabricated by a melt-spinning technique in order to study the mechanism of the valence variation of Ce and their magnetic properties as well as improve the thermal stability of Ce-based rare earth permanent magnets. The systematic investigations of the Ce17Fe78-xB6Gax (x=0-1.0) alloys show that the room-temperature coercivity increases significantly from 352 kA/m at x = 0 to 492 kA/m at x = 1.0. The Curie temperature (Tc) increases monotonically from 424.5 K to 433.6 K, and the temperature coefficients of remanence (α) and coercivity (β) of the ribbons are better off from -0.56 %/K, -0.75 %/K for x = 0 to -0.45 %/K, -0.65 %/K for x = 0.75 in the temperature range of 300–400 K, respectively. The Ce L3-edge X-ray absorption near edge structure (XANES) spectrums reveal that there is more Ce4+ in ribbons under total electron yield than fluorescence yield as Ce has a high affinity with oxygen. The weight of Ce3+ increases while the weight of Ce4+ decreases in Ga-added alloys. The refined grain size and a more uniform microstructure are mainly attributed to the improved magnetic properties and thermal stability with Ga doping. This paper may serve as a reference for further developing the so-called gap magnets and the effective utilization of the rare earth resources.