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SmCo5 magnets were prepared by spark plasma sintering (SPS) technique. The phase constitutions, magnetic properties, and microstructures of magnets prepared under different sintering temperatures were investigated systematically. It is observed that the 1:5 phase and 2:7 phase coexist in the magnet, and 2:17 phase appears when the temperature rises to 1050 °C. The best magnetic properties were obtained for the magnets prepared at 1000 °C, with remanence Jr = 0.45 T, intrinsic coercivity Hcj = 985 kA/m, and maximum energy density (BH)max = 37 kJ/m3. It is shown that the SPSed magnet prepared at 1000 °C exhibits excellent thermal stability which is described in terms of temperature coefficient of remanence (α) = −0.15%/°C and temperature coefficient of coercivity (β) = −0.25%/°C in the temperature range of 27–400 °C. The microstructure analysis showed that the high density of the magnet and the uniform distribution of the hard magnetic phase are the main reasons for the excellent magnetic properties.

NdFeB hot-pressed and hot-deformed magnets were prepared by hot-pressed (HP) sintering and spark plasma sintering (SPS). The effects of sintering mechanisms of SPS and HP on the microstructure of hot deformation magnets were studied. The temperature of the mold surface is much higher than that of the sample in the cavity in HP, but lower than that of the sample in SPS. The Nd-rich phase is uniformly distributed and the grain size is smaller in the hot-deformed magnets prepared by HP, while the Nd-rich phase is excessively enriched and the grain size in the liquid-enriched region is abnormal in the hot-deformed magnets prepared by SPS. The comprehensive magnetic properties of hot deformed magnets prepared by SPS are significantly lower than those prepared by HP.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

Amorphous SiO 2 coatings were prepared on sintered NdFeB magnets by micro-arc oxidation (MAO) in silicate solution. The surface and cross-sectional morphologies, element and phase composition, corrosion resistance and magnetic properties of the coatings were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), potentiodynamic polarization test and physical properties measurements system (PPMS). The results showed that the surface morphologies of the coatings exhibited the “coral reef” like structure, different from the typical MAO porous structure. With increasing the voltages, the thickness of the coatings increased from 12.72 to 19.90 µm, the content of Si element increased, while the contents of Fe, Nd and P elements decreased. The coatings were mainly composed of amorphous SiO 2 and a few amorphous Fe 2 O 3 and Nd 2 O 3 . The amorphous SiO 2 coatings presented excellent thermal shock resistance, while the thermal shock resistance decreased with increasing the voltages. The corrosion resistance of the coatings increased with increasing the voltages, and it could be enhanced by one order of magnitude compared to the uncoated NdFeB magnets. The MAO coatings slightly decreased the magnetic properties of the NdFeB samples in different degrees.

Nd-Fe-B alloys have high intrinsic coercivity, high magnetic energy density, but low Curie temperature and poor temperature stability. While Alnico alloys have high Curie temperature and high temperature stability, but low intrinsic coercivity and low magnetic energy density. It would be of great scientific, technological and economical interest if these two alloys could be combined together in some proportion to make composite magnets with good comprehensive properties. Here we report nanocrystalline composite magnets containing Nd-Fe-B hard phase and alnico semi-hard phase fabricated by melt-spinning technique. A novel core-shell like structure constituting Nd-Fe-B (2:14:1) phase as core and Alnico rich phase as shell was obtained by optimizing the composition and processing conditions. The maximum energy density was enhanced upto 150 kJ/m3 in nanocomposite magnets. The Curie temperature of the main phase increased to 654 K for Nd-Fe-B/Alnico from 581 K for pure Nd-Fe-B alloy, which is attributed to the molecular field penetration of Alnico phase into the hard 2:14:1 phase, in addition to the substitution of Fe by Co and Ni. No kink or dip was observed in the demagnetization curves, which suggested the presence of strong exchange interactions among the phases of the nanocomposite alloys.

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.

In this work, we have investigated the structural and magnetic properties of hexagonal Lu0.5(Sc1-xInx)0.5FeO3 ceramics synthesized by solid state reaction. All samples were identified to crystalize in the hexagonal structure with polar space group (P63cm). The lattice parameters, both a and c, increase with In-substitution, owing to larger ionic radius of In3+ in comparison with that of Sc. Meanwhile, the c/a ratio increases from 1.9952 for x = 0 to 2.0308 for x = 1.0. All samples undergo an antiferromagnetic transition, accompanied by a dielectric anomaly, which is the fingerprint of magnetoelectric coupling in hexagonal LuFeO3 systems. The Neel temperature (TN) shifts towards lower temperature with the increase of In-substitution, demonstrating the weakening magnetic interaction stemming from smaller structural distortion due to smaller tilting angle of FeO5 bipyramids. It is fascinating that the increase of c/a ratio brings about a decrease of TN after the In-substitution for Sc. Another magnetic anomaly was observed above the TN (around 265 K) in Lu0.5Sc0.5FeO3. The temperature of magnetic anomaly increases towards higher temperature with the increase of In-substitution and rises above room temperature in Lu0.5In0.5FeO3. It can be suggested that the magnetic order is more complex than that obtained in previous works, and the room-temperature multiferroics is attainable for application in hexagonal RFeO3 systems.

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.