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Post-sinter annealing process plays an important role in the microstructures and magnetic properties of the Nd-Fe-B sintered magnets. In this paper, systematically investigated are the magnetic properties and microstructures of the as-sintered and post-sinter annealed Nd-Fe-B magnets with Pr-Fe-Ga boundary addition. Two choice consecutive annealing methods are adopted at high and low temperatures, namely the 1st annealing at 880 oC for 2 h and then the 2nd annealing at 440 oC for 3 h. It is exceptional to find out that both the remanence and coercivity are improved after 2nd annealing process for this type of magnet. The coercivity is hugely increased from 10.09 kOe for the as-sintered sample to 17.19 kOe for the 2nd annealed magnet, with a significant increment of 70.37% in coercivity. The extraordinary magnetic properties of Br=14.44 kGs, Hcj=17.19 kOe and (BH)max=51.08 MGOe are obtained for the designated Nd-Fe-B sintered magnets without heavy rare earth (HRE) elements manufactured by dual alloy method. The Curie temperature is monotonically decreased from 634 K to 602 K while the c-axis alignment degree is optimized after annealing. Microstructural observation and analysis indicate that the elemental distribution patterns are altered after the 2nd annealing. The diffusion of the aggregate (Pr,Nd,Cu,Ga)-rich phase from triple junctions into the grain boundary regions is ascribed to the formation of thin and continuous grain boundary layer, which is critical to improve the microstructures and magnetic properties.

The magnetic properties, magnetocaloric effect and magnetoresistance in ErNi single crystal have been investigated in detail. With decreasing temperature, ErNi single crystal undergoes two successive magnetic transitions: a paramagnetic to ferromagnetic transition at TC =11 K and a spin-reorientation transition at TSR = 5 K. Meanwhile, a sharp field-induced metamagnetic transition is observed below the TC along the a axis. ErNi single crystal possesses a giant magnetocaloric effect around TC. The maximum magnetic entropy change is -36.1 J (kg K)−1 along the a axis under the field change of 0−50 kOe. In particular, the rotating magnetocaloric effect in ErNi single crystal reaches its maximum under a relatively low field, and the maximum rotating entropy change with a value of 9.3 J (kg K)−1 is obtained by rotating the applied field from the [011] to [100] directions under 13 kOe. These results suggest that ErNi could be a promising candidate for magnetic refrigeration working at liquid-helium temperature region. Moreover, a complicated transport behavior is uncovered in ErNi single crystal, which is attributed to the complex magnetic states and magnetic polaronic effect. Both positive and negative magnetoresistance are observed. A considerable large magnetoresistance with the value of -34.5 % is acquired at 8 K under 50 kOe when the field is along the [100] direction.

Solid-state refrigeration based on the magneto- or mechano-caloric effect, including elasto- and baro-caloric in ferroic phase transition materials is promising to replace the current vapor compression refrigeration in consideration of environmental-friendliness and energy-saving. However, both high driven field and small thermal changes in all of these caloric materials hinder the development of solid-state refrigeration. Here we report a giant baro-caloric effect near room temperature induced by a low hydrostatic pressure in Co-based Co51Fe1V33Ga15 Heusler alloy. The maximum adiabatic temperature change under the applied pressure change of Δp = 0.1–100 MPa can be as high as Δ T ad M a x = 7.7 K ( Δ T ad M a x / Δ p reaches up to ∼7.7 K kbar−1), surpassing the Δ T ad M a x / Δ p value reported hitherto in baro-caloric alloys. In addition, the microstructure is also studied by using the electron microscopes. Along with the austenite and martensite, the submicron V-rich particles are precipitated in this alloy, which are believed to account for enhancing mechanical properties.

Nowadays, searching for the materials with multiple magneto-functional properties and good mechanical properties is vital in various fields, such as solid-state refrigeration, magnetic actuators, magnetic sensors and intelligent/smart devices. In this work, the magnetic-field-induced metamagnetic reverse martensitic transformation (MFIRMT) from paramagnetic martensite to ferromagnetic austenite with multiple magneto-responsive effects is realized in Fe-doped Co-V-Ga Heusler alloys by manipulating the magnetic ordering. The martensitic transformation temperature Tm reduces quasi-linearly with increasing Fe-content. In strikingly contrast with the Fe-free alloys, the magnetization difference (ΔM') across martensitic transformation increases by three orders of magnitude for Fe-doped alloys. The increased ΔM' should be ascribed to the reduction of Tm, almost unchanged Curie temperature of austenite and the increased magnetic moment in the samples with higher Fe-content. The large ΔM' provides strong driving force to realize the MFIRMT and accordingly multiple magneto-responsive effects, such as magnetocaloric, magnetoresistance and magnetostriction effects. Meanwhile, giant Vickers hardness of 518 HV and compressive strength of 1423 MPa are achieved. Multiple magneto-responsive effects with exceptional mechanical properties make these alloys great potential candidates for applications in many fields.

Recently it is a hot topic to make full use of high abundant Y element in Nd2Fe14B-type permanent magnets. In contrast to Pr and Nd elements, Y shows different metallurgical behaviors during preparation process. In this paper, we have explored the magnetic properties, microstructures and corrosion performance of Pr-Nd-Y-Fe-B magnets fabricated by spark plasma sintering (SPS) technique from the ribbons of nanocrystalline and amorphous precursors, respectively. The coercivity and maximum energy product were improved for the magnets prepared from amorphous precursor materials (denoted as SPS-A hereafter) compared with the magnets prepared from crystalline precursor materials (denoted as SPS-C hereafter). Magnetic properties of Jr = 0.79 T, Hci = 864 kA/m, and (BH)max = 102 kJ/m3 were obtained for SPS-A magnets. In contrast with SPS-C magnets, the magnetic properties of SPS-A magnets are not so sensitive to the preparation conditions, which is quite beneficial to the homogeneity of microstructure and enhancement of coercivity for large-scale production of the designated magnets. Aggregated (Pr,Nd,Y)-rich phase was found out in SPS-C magnets. Pr and Nd elements are rich at grain boundary while Y is distributed uniformly at main phase and grain boundary phase. The strip grains and equiaxed grains exist in SPS-C and SPS-A magnets, respectively. The enhanced magnetic properties for SPS-A magnets are accredited to the uniform distribution of rare-earth-rich phase and low demagnetization factor. It is revealed by electrochemical test and dipping test that the corrosion potential is more positive and the corrosion rate is slower for the SPS-A magnets in 3.5 wt.% NaCl solution. The work is also expected to shed light on developing the nanocrystalline Pr-Nd-Y-Fe-B SPSed high-performance magnets in industry.

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.

Grain boundary diffusion process (GBDP) serves as a promising approach in improving magnetic properties and thermal stability of NdFeB permanent magnets. Herein, non-heavy rare earth Pr-Zn films deposited on the magnet surface using DC-magnetron sputtering system are reported. The thermal stability and coercivity enhancement mechanism of Pr-Zn GBDP magnets were investigated. Results show that the coercivity of Pr-Zn GBDP magnet increases from 963.96 kA m−1 to 1317.14 kA m−1 without any remanence reduction. Notably, the demagnetization curve of Pr-Zn GBDP magnet still remains a high squareness ratio. The temperature coefficient of coercivity and anti-demagnetization ability of Pr-Zn GBDP magnet under high temperatures are improved after GBDP treatment. The well-optimized rare earth-rich (RE-rich) grain boundary phases and high effective anisotropy field of (Nd,RE)2Fe14B magnetic hardening layers surrounding main grains are the key factors to impact the magnetic properties and thermal stability of NdFeB permanent magnets via GBDP treatment.

The formation of the paramagnetic CeFe2 phase has deleterious effects on the magnetic properties of Ce Fe B type alloys. In order to reduce the volume fraction of the CeFe2 phase and improve the magnetic properties of cost efficient Ce Fe B magnetic alloys, Ce17Fe77-xGexB6 (x = 0.0–1.5) alloys were fabricated by melt spinning technique. The crystal structures, magnetic properties, intergranular exchange interactions and phase transition temperatures of CeFe2 and Ce2Fe14B phases were investigated. The critical amount of Ge doping enhanced the magnetic properties of the alloys by partially decreasing the formation of paramagnetic CeFe2 phase and refining the nano-grains. The optimized magnetic properties of coercivity Hcj = 467 kA/m, remanence Br = 0.51 T and energy density (BH)max = 40.6 kJ/m3 were obtained for Ce17Fe76.4Ge0.6B6 alloy. The analysis of Henkel plots confirmed that the exchange interaction among nano-grains was greatly improved for the optimized doping of Ge. The improved magnetic properties of the investigated alloys are attributed to the refined grains, enhanced intergranular exchange interactions and the decrease of paramagnetic phase for the critical amount of Ge.

FeSiCr nanoparticles were fabricated by plasma arc-discharging method. The particle size is controlled by regulating the ratio of Ar and H 2 . The proper particle size improves the microwave attenuation capability and optimizes the impedance matching of the FeSiCr nanoparticles. When the ratio of Ar:H 2 = 80:20, the calculated reflectivity of the FeSiCr nanoparticle reaches −50.7 dB at 17.3 GHz ( d = 1.5 mm). By regulating the proportion of heat source gas H 2 and changing the coating thickness from 1.0 to 4.0 mm, the effective absorbing bandwidth ( 2 is a feasible way to control the nanoparticle size.

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.

The all-d-metal Ni50-xCoxMn35Ti15 (x = 13,13.5) ferromagnetic shape memory alloy ribbons are successfully fabricated. The intermartensitic transformation and martensitic transformation arrest phenomenon are systematically investigated. In contrast to melt-spun ribbons, the annealed ribbons reveal an intermartensitic transformation from five-fold (5 M) modulated to non-modulated (L10) martensite below B2→5 M martensitic transformation. The XRD, TEM, DSC and temperature dependence of electrical resistivity measurements validate the occurrence of intermartensitic transformation. On the other hand, the magnetic-field-induced martensitic transformation arrest is achieved in these all-d-metal ribbons like other normal Ni-Mn-based Heusler alloys consisting of p-group and d-group elements. Strikingly, a kinetic de-arrest phenomenon is brought about in all-d-metal ribbons with the increasing heat treatment temperatures. The associated origins are discussed in these ribbons, which is very significant for deeply understanding the arrest and de-arrest phenomenon in ferromagnetic shape memory alloys. What's important, excellent magneto-functional and mechanical properties are obtained in these ribbons. Ni36.5Co13.5Mn35Ti15 annealed ribbon displays the best integrated properties, such as large magnetoresistance (∼34.9%), magnetic entropy change peak value (∼24.9 Jkg−1K−1) and effective refrigerant capacity (∼122.5 Jkg−1) under the field change of Δμ0H = 0–5 T, and large Vickers hardness [2.75 GPa (280.7HV)]. These make the all-d-metal Ni-Mn-based Heusler alloy ribbons attractive in the practical applications.

In this work, the exchange bias behavior and magnetocaloric effect have been studied in Mn7Sn4 alloy. The X-ray powder diffraction pattern recorded at room temperature indicates that the sample crystallizes in a single phase with Ni2In-type hexagonal structure (space group P 6 3 / mmc ). The maximum magnetic entropy change value across paramagnetic/ferrimagnetic transition is about 3.3 J kg−1 K−1 under the magnetic field change of μ 0 Δ H = 0 - 5 T . With further cooling, the reentrant spin-glass-like state is obtained below 150 K, for which the exchange bias effect has been observed. The exchange bias field is ∼7.8 mT and ∼6.7 mT at T = 10 K when the cooling field is μ 0 H CF = 0.1 T and 0.5 T, respectively. The magnetic behavior and the origin of exchange bias in Mn7Sn4 are discussed.

The carbon coated core-shell C@Fe3C/Fe nanocomposites were prepared by plasma arc-discharging method. The core-shell and graphite network structures were obtained, which could improve the absorbing properties by optimizing the impedance matching and increase the internal reflections of the propagated microwave. By varying the coating thickness from 1.0 to 5.0 mm, the reflectivity of C@Fe3C/Fe nanocomposites prepared in 30% CH4 is less than −20 dB nearly in the whole range of 2–18 GHz. The minimum reflection loss value reaches −32.9 dB at 14.2 GHz and the effective bandwidth (

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.

MnBi alloys with high content of low temperature phase (LTP) MnBi and excellent magnetic properties were prepared by surfactant assistant ball milling (SABM), using annealed ribbons as precursors. Effects of the SABM processes on the phase constituent, microstructure, and magnetic properties were investigated in detail. It is found that the reduction of saturation magnetization is mainly attribute to the decomposition of LTP MnBi into Mn phase, Bi phase and MnO phase, with increasing the ball milling time. Meanwhile, due to the fine grain size and increased magnetic isolation effects between LTP MnBi grains, the coercivity increases monotonously with prolonging the ball milling time to 10 h for Mn55Bi45 powders. Through compression molding under the magnetic field, based on the SABM powders, the optimum anisotropic bonded magnet displays the maximum energy product (BH)max of 9.06 MGOe at room temperature, and a value of 7.05 MGOe at 380 K can be still achieved. In addition, the magnetic hardening mechanism of bonded magnets can be well explained by the strong pinning model. Strongly favorable magnetic properties make bonded MnBi magnets an attractive candidate material for small permanent magnets used for temperature applications up to 380 K.

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.

Ce-Fe-B-based rare earth permanent magnets are prepared by spark plasma sintering (SPS) technique, and low melting NdCu alloy is added to improve its magnetic properties and thermal stabilities. The X-ray diffraction (XRD) result indicates that the lattice constants of the unit cell of 2:14:1 phase increase with NdCu addition. The remanent magnetization, coercivity and maximum energy product are enhanced remarkably from 0.37 T, 227 kA/m, 16 kJ/m3 for the sintered magnets without NdCu addition to 0.47 T, 476 kA/m, 30 kJ/m3 for those with 20 wt% NdCu addition. The temperature coefficients of remanence (α) and coercivity (β) of the spark plasma sintered magnets (denoted as SPSed magnets thereafter) are improved from −0.46%/K, −0.60%/K to −0.34%/K, −0.55%/K in the range of 300–400 K. There are two Curie temperatures in the Ce-based magnets with NdCu addition, which implies the coexistence of two hard magnetic phases. The second Curie temperature depends on the NdCu content. It is shown by the microstructure analysis that a typical core-shell structure is formed with Ce-rich core and Ce-lean shell by NdCu addition. Furthermore, it is displayed that Nd atoms prefer to diffuse into the Ce-lean flakes. The Nd-Ce exchange mechanism and immigration behavior of elements are finally investigated. The intergranular exchange coupling strength is enhanced in NdCu-added magnet. It is found out that the core-shell structure plays an important role in the special microstructure evolution and the enhancement of the magnetic properties for the typical Ce-Fe-B-based SPSed magnets by NdCu addition.

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.

In this work, the Mn1-xNixCoGe (0.01 ≤ x ≤ 0.075) alloys are prepared. By slight substitution of Mn upon Ni element, the magnetic transition and structural transformation of stoichiometric MnCoGe ferromagnet are controlled to coincide. Accordingly, the coupled magnetostructural transformation occurs during the wide temperature window of ∼91.5 K covering room temperature from the ferromagnetic TiNiSi-type to paramagnetic Ni2In-type state. Strikingly, a magnetic-field-driven metamagnetic martensitic transformation is observed in Mn0.975Ni0.025CoGe alloy. As a result, large room temperature magnetocaloric effects are obtained including large magnetic entropy change of ∼30.3 Jkg−1K−1 for sample with x = 0.025 and large refrigeration capacity of ∼276.8 Jkg−1 for x = 0.035 under the applied field change of ΔH = 0–7 T. The influence of slight Ni-substitution for Mn on the magnetic and magnetocaloric properties of MnCoGe alloy is studied and the origins are discussed.

The purpose of this paper is to explore Dy-free Nd-Fe-B magnetic alloys which could be developed into Dy-free permanent magnets operating at high temperatures. Nd13.5Fe80.5-xB6Tax (x = 0.0–1.2) alloys have been fabricated by melt spinning technique. It is found out that the addition of Ta in Nd-Fe-B alloys improves the extrinsic and microstructural properties resulting in better magnetic properties and thermal stability. The thermal stability of the alloys measured in terms of temperature coefficient of coercivity (β) improved from −0.41%/K for Nd13.5Fe80.5B6 to −0.27%/K for Nd13.5Fe79.9B6Ta0.6 in the temperature range 300–400 K. It is demonstrated that Ta acts as refractory element, which refines grains and reduces the volume fraction of the undesirable non-magnetic phases. As a result the magnetic properties (Br, Hcj and (BH)max) of the alloys are enhanced, however the intrinsic properties (Tc, Tsr) remain unchanged. The domain wall pinning, determined to be the dominant coercivity mechanism in the investigated alloys, also strengthened by the addition of Ta. It is concluded that Ta alloying can improve the microstructures, magnetic properties and thermal stability of the designated alloys. This research may shed light on further research and development of Dy-free high temperature resistant rare earth magnets.

A balanced consumption of the rare earth resources can be realized by increasing the content of Ce in Nd-Fe-B magnetic alloys. However, the substitution of Ce for Nd reduces the magnetic properties and the Curie temperature of the 2:14:1 phase resulting in limited application fields. In this manuscript, targeting to increase the Curie temperature by maintaining the magnetic properties at acceptably high level, Fe in 25% Ce substituted Nd-Ce-Fe-B alloys has been substituted by Alnico constituent elements. The Curie temperature of the main phase for 25% Ce substitution (544 K) is enhanced to 589 K by 20% Alnico substitution which is higher than the Curie temperature of the pure Nd2Fe14B compound (⁓585 K). The magnetic properties initially increased as a result of Alnico element substitution, and then slightly decreased. Maximum energy density of 100 kJ/m3 and Curie temperature of 582 K is obtained by substituting 15% Alnico. These properties are comparable to Ce-free Nd-Fe-B melt spun alloys. The working temperature range increased by the increase of Curie temperature and the decrease of spin reorientation temperature. Due to the accumulation of ferromagnetic element, Ni, at the grain boundary and triple junction phases and the refined grains, the short-range exchange interactions among the nanograins increased which helped to obtain high energy density in the alloys.

Post-sinter annealing is essential to improve the microstructures and magnetic properties in NdFeB-based sintered permanent magnetic materials. In this paper, investigated systemically are the effects of post-sinter annealing on the microstructures and magnetic performance of Ga and Cu co-doping Nd2Fe14B based sintered magnets. It is found out that the intrinsic coercive force of the magnets increased from 9.17 kOe to 18.67 kOe after two stage post-sinter annealing (PSA), which shows an extraordinary increment of 103.5%. The remanence, with a decreasing percentage of 1.95%, slightly decreases from 13.87 kGs to 13.60 kGs. The substantial increase of intrinsic coercivity is mainly attributed to the improvement of clean and thin continuous grain boundary layers, the smoothness of hard magnetic matrix phase and the appearance of the RE6(Fe, M)14 antiferromagnetic phase during annealing. The reduction of the remanence is caused by the deteriorated orientation degree and reduced volume fraction of hard magnetic main phases after post-sinter annealing.

Oxide coated nanocomposite NiO@FeNiMo with core–shell structure were fabricated by plasma arc-discharging and oxidative heat treatment method. By optimizing the impedance matching, the absorbing properties of the oxide coated core–shell NiO@FeNiMo were improved significantly. By varying the coating thickness from 1.0 ∼ 5.0 mm, the reflectivity of less than −10 dB, in the frequency range of 4–17 GHz, was obtained in NiO@FeNiMo nanocomposites oxidized at 473 and 498 K. The minimum reflectivity of −54.0 dB at 12.8 GHz (d = 1.47 mm) was achieved by oxidizing the sample at 473 K. The oxide coated core–shell NiO@FeNiMo absorber reported in this paper have the potential be a powerful candidate for microwave absorbing materials for its excellent absorption, simple preparation process, and high stability in complex environments.

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 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.

Elastocaloric refrigeration based on the elastocaloric effect (eCE) is a promising solid-state replacement to the conventional vapor-compression one due to the environment-friendly and energy-efficient advantages. Here, a Heusler-type 〈111〉-oriented Co49Fe3V33Ga15 superelastic alloy fabricated by the directional solidification method is designed showing large eCE around room temperature. Large adiabatic temperature change of 11.0 K upon fast loading is obtained at 290 K when the uniaxial pressure is perpendicular to the solidification direction. Moreover, the cycle stability of textured alloy is remarkably improved with respect to untextured counterpart. This work may inspire the discovery of newly promising elastocaloric materials for solid-state refrigeration.

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.

We report anisotropic magnetocaloric effect and magnetoresistance in antiferromagnetic HoNiGe3 single crystal grown by Ge flux method. HoNiGe3 single crystal exhibits antiferromagnetic order and large magnetocrystalline anisotropy below the Neel temperature TN = 10.5 K. Meanwhile, with increasing the magnetic field, HoNiGe3 undergoes the spin-flip transition induced by the magnetic field along the a axis, while the spin-flop transition occurs for the field along the other orientations, which gives rise to anisotropic magnetoresistance behavior along three axes. With the magnetic field change of 0–50 kOe, the maximum magnetic entropy changes obtained along the a, b, and c axes are −13.9, 2.5 and −7.7 J kg−1K−1, respectively. The maximum rotating magnetic entropy change is −12.3 J kg−1K−1 under 50 kOe by rotating the magnetic field from the b axis to the a axis, and the corresponding refrigeration capacity is 193 J/kg, which demonstrates HoNiGe3 to be an attractive candidate for novel rotating magnetic refrigeration at low temperature region.

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.

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%).

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.