Hard Magnetic Phase Research Articles

Microstructure and magnetic properties of multi-main-phase Ce-Fe-B spark plasma sintered magnets by dual alloy method

Abstract 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.

Impact of interface structure on magnetic exchange coupling in MnBi/FexCo1−x bilayers

In this study we investigate the exchange coupling between the hard magnetic compound MnBi and the soft magnetic alloy FeCo including the interface structure between the two phases. Exchange spring MnBi-Fe$_x$Co$_{1-x}$ (x = 0.65 and 0.35) bilayers with various thicknesses of the soft magnetic layer were deposited onto quartz glass substrates in a DC magnetron sputtering unit from alloy targets. Magnetic measurements and density functional theory (DFT) calculations reveal that a Co-rich FeCo layer leads to more coherent exchange coupling. The optimum soft layer thickness is about 1 nm. In order to take into account the effect of incoherent interfaces with finite roughness, we have combined a cross-sectional High Resolution Transmission Electron Microscopy (HR-TEM) analysis with DFT calculations and micromagnetic simulations. The experimental results can be consistently described by modeling assuming a polycrystalline FeCo layer consisting of crystalline (110) and amorphous grains as confirmed by HR-TEM. The micromagnetic simulations show in general how the thickness of the FeCo layer and the interface roughness between the hard and soft magnetic phases both control the effectiveness of exchange coupling in an exchange spring system.

Exchange-coupling behavior in soft/hard Li0.3Co0.5Zn0.2Fe2O4/SrFe12O19 core/shell composite synthesized by the two-step ball-milling-assisted ceramic process

Soft/hard xLi0.3Co0.5Zn0.2Fe2O4@(1 − x)SrFe12O19 (x = 0.1, 0.2, and 0.3) core/shell magnetic composites have been synthesized by the two-step ball-milling-assisted ceramic process. X-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometer are employed to analysis of structural and magnetic features of synthesized samples. The analysis of XRD indicates the formation of hard/soft ferrite composites in combination with a small amount of the α-Fe2O3 phase. FT-IR analysis confirms the formation of soft/hard Li0.3Co0.5Zn0.2Fe2O4@SrFe12O19 ferrite composite with core/shell structure. Single smooth hysteresis loops of composites show the presence of exchange-coupling between hard and soft magnetic phases. Exchange-coupling behavior between hard and soft magnetic phases is also confirmed by the switching field distribution curves. Ferrite composite with 3.0:7.0 mass ratio of soft-to-hard magnetic phase has the highest specific saturation magnetization value (74.08 emu/g). The specific saturation magnetization (Ms) of ferrite composites is markedly increased compared with Li0.3Co0.5Zn0.2Fe2O4 and/or SrFe12O19. This is attributed to the exchange-coupling interaction between the hard and soft phases due to formation of core/shell structure. Two-step ball-milling-assisted ceramic process could be considered as a suitable method for synthesizing exchange-coupled magnet.

Evolution of Magnetic Anisotropy With Sm Contents in Sm–Co Thin Films

In this paper, the evolution of magnetic anisotropy as a function of Sm contents in buffer-free Sm-Co thin films grown by molecular beam epitaxy is investigated. From the soft magnetic Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sub> phase, the anisotropy evolves to the hard magnetic SmCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> phase with a distinct perpendicular anisotropy and a maximum magnetic anisotropy energy of 1.67 MJ/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> as measured by torque magnetometry at 300 K. For SmCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> , a saturation magnetization of 845 emu/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , close to the bulk single crystal value is achieved, while a squareness ratio close to unity is measured in the hysteresis curve perpendicular to the film plane. Previously grown YCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> and CeCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> thin films with perpendicular anisotropy show a considerably reduced uniaxial anisotropy as well as a reduced squareness ratio as compared to SmCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> . These results demonstrate that the magnetic parameters of rare earth-cobalt thin films can be adapted according to the requirements of applications such as magnetic recording.

L10-FeNi films on Au-Cu-Ni buffer-layer: a high-throughput combinatorial study

The fct L10-FeNi alloy is a promising candidate for the development of high performance critical-elements-free magnetic materials. Among the different materials, the Au-Cu-Ni alloy has resulted very promising; however, a detailed investigation of the effect of the buffer-layer composition on the formation of the hard FeNi phase is still missing. To accelerate the search of the best Au-Cu-Ni composition, a combinatorial approach based on High-Throughput (HT) experimental methods has been exploited in this paper. HT magnetic characterization methods revealed the presence of a hard magnetic phase with an out-of-plane easy-axis, whose coercivity increases from 0.49 kOe up to 1.30 kOe as the Au content of the Cu-Au-Ni buffer-layer decreases. Similarly, the out-of-plane magneto-crystalline anisotropy energy density increases from 0.12 to 0.35 MJ/m3. This anisotropy is attributed to the partial formation of the L10 FeNi phase induced by the buffer-layer. In the range of compositions we investigated, the buffer-layer structure does not change significantly and the modulation of the magnetic properties with the Au content in the combinatorial layer is mainly related to the different nature and extent of interlayer diffusion processes, which have a great impact on the formation and order degree of the L10 FeNi phase.

Multiphase microstructure and magnetic properties of high performance Nd-Fe-B ribbons with multielement addition

Abstract In this paper, a new series of multielement multiphase alloys were designed by combining Nd2Fe14B with spinodal decomposition type of Alnico alloy. The Nd11.4Fe81.2−xB5.7+xAl0.5Ni0.4Co0.7Cu0.1 (x = 0, 2, 4) ribbons were prepared by melt-spinning at a wheel speed of 40 m/s, and the effect of B content on the structure and magnetic properties of ribbons was investigated. The results show that three ribbons all formed a multielement, multiphase and fully enclosed microstructure with soft magnetic, weak magnetic and amorphous phases surrounding the hard magnetic phase. Phase contents and grain size changed with an increase in B content. Although the content of hard magnetic Nd2(Fe,M)14B phase was only about 56.1% in x = 2 ribbons, their magnetic properties reached Hc = 10.8 ± 0.2 kOe, Mr = 60.6 ± 0.9 emu/g and M2T = 90.4 ± 2.8 emu/g. Finally, the novel microstructure model was established and its magnetic performance was analyzed.

Influence of high anisotropy phase on the properties of hard–soft magnetic nanocomposite powders obtained by mechanical milling

ABSTRACTIn materials with a good degree of magnetic interphase exchange coupling one has to consider both the extrinsic (microstructure) and intrinsic parameters (magnetisation, anisotropy, etc.) of the composite. This study investigates the feasibility of tuning the interphase exchange coupling condition by varying the anisotropy through the choice of the hard magnetic phase. The studied samples were produced by milling the hard magnetic phase along with 20 wt-% α-Fe powder for 6 h and subsequent annealing. The structure of the annealed powders was studied through X-ray diffraction and the magnetic properties of the nanocomposites were studied from demagnetisation curves and vs H plots in applied fields up to ±10 T.

Plasma arc discharge synthesis of multicomponent Co-Cr-Cu-Fe-Ni nanoparticles

The controllable alloying multiple metallic elements into an individual nanoparticle offers the promise material properties. Here a series of spherical Co-Cr-Cu-Fe-Ni nanoparticles (NPs) with an average size of around 80–120 nm can be easily synthesized with a controllable microstructure and composition by modified plasma arc-discharge method. The crystal structure of the as-prepared NPs is largely composed of Cu-rich FCC phase with a small amount of BCC phase, and the content of BCC phase increases with the decrease of Cu content in the NPs. The differences in components between the raw materials and as-prepared NPs are ascribed to the evaporating pressure and the molar mass of raw materials, and by designing the target to CoaCrbCucFedNie, the atomic composition of CoaCr3bCu12cFe1.5dNie NPs can be obtained. Moreover, the as-prepared nanoparticles exhibit comparable soft magnetic features, and the asymmetric characteristic is probably due to the magnetic coupling of Cu-rich FCC phase with a relatively hard magnetic BCC crystalline phase. The case of Co-Cr-Cu-Fe-Ni NPs is a good example, which opens new ways to synthesize other multicomponent metal NPs with controllable structure and composition by controlling the molar mass of the raw material.

Evidence of exchange coupling in τ-MnAlC/FeCo system

Abstract The τ -MnAlC/FeCo system has potential to be applied as rare earth-free permanent magnet, due to exchange coupled between hard and soft magnetic phases as alternative for the development of high energy product permanent magnets. In this work the hard-magnetic phase Mn53.3Al45C1.7 ( μ 0Hc = 0.37 T of coercive force, Mr = 23.52 Am2/kg of remanent magnetization, Ms = 52.94 Am2/kg of saturation magnetization and (BH)max = 3.89 kJ/m3 of maximum energy product) was mixed with the soft magnetic phase Fe65Co35 ( μ 0Hc = 0.007 T and Ms = 221.33 Am2/kg) using a solid-state procedure. The magnetic materials were sintered using different ratios of hard and soft magnetic phases: 95:5, 90:10, 85:15 and 80:20; and different sintering temperatures (300, 400, 500 and 600 °C) in order to improve the magnetic properties. The magnetic exchange coupling between MnAlC and FeCo was proved by the hysteresis loop and its corresponding Thamm-Hesse analysis. The best result was obtained for MnAlC/FeCo (95/5 wt%) sintered at 500 °C during 30 min, for which the magnetic properties were μ 0Hc = 0.277 T, Ms = 76.43 Am2/kg and (BH)max = 5.57 kJ/m3.

Special microstructure evolution and enhanced magnetic properties of Ce-Fe-B-based spark plasma sintered magnets with core-shell structure by NdCu addition

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.

Progress with insulating nanocomposites based on ferrite plating of Sm2Fe17N3 micropowders

Abstract Typical exchange spring magnets are composed of two phases, a rare-earth hard magnetic material and a metallic soft magnetic material, whose magnetization value is higher than those of hard magnetic phases, such as α-Fe, Fe-B, or Fe-Co. A novel high-electrical-resistance composite magnet was fabricated by consolidating micron-sized ferromagnetic nitride Sm2Fe17N3 powder particles coated with a continuous nano-sized soft magnetic ferrite oxide layer, which suppressed intergrain conductivity but sustained the magnetic exchange interactions between grains [21] . A non-magnetic resin or ceramic insulator may suffice for the coated layer but gives rise to high electrical resistance of the magnet with large deterioration of magnetization. The soft magnetic ferrite oxide layer not only suppressed intergrain conductivity, but also only slightly reduced the magnetization of the magnet. At present, the only exchange spring magnet having a combination of a soft magnetic oxide and hard magnetic nitride is the ferrite/Sm2Fe17N3 composite magnet. Sm2Fe17N3 powder particles with a size of 2 μm were coated with an “iron ferrite” layer with a grain size of about 10 nm by ferrite plating, which is an aqueous process, following which the samples were consolidated by the explosive consolidation (EC) technique. The coercivity and rectangularity of the ferrite/Sm2Fe17N3 composite magnet decreased slightly compared to those of a Sm2Fe17N3 magnet. The fully dense ferrite/Sm2Fe17N3 magnet exhibited a resistivity of about 4000 μΩ cm, which is ten times larger than that of a fully dense Sm2Fe17N3 magnet. Thus, the soft magnetic ferrite layer in the composite magnet maintained the magnetic exchange coupling between ferromagnetic Sm2Fe17N3 grains and simultaneously suppressed intergrain electrical coupling to increase the resistivity. This decreased the eddy current loss and improved the high-frequency characteristics of the composite magnets. Therefore, ferrite/Sm2Fe17N3 composites are promising materials for magnets that are operated at high frequencies in advanced applications, such as electric vehicle magnets.

Structural, magnetic, and mechanical hardness characterization of the alloy Nd16(Fe76−xNix)B8 with x = 0, 10, 20, and 25

In this work, samples of the alloy Nd16 (Fe76−x Nix) B8 with x = 0, 10, 20, and 25 were produced by arc melting under an Ar atmosphere and heat treated at 1073 K during 30 min, and then quenched in ice–water mixture. X-ray diffraction (XRD), Mossbauer spectroscopy (MS), and vibrating sample magnetometry (VSM) were used to characterize the properties of the obtained samples. Microhardness tests were performed too. XRD results found Nd2Fe14B and Nd1.1Fe4B4 as main phases. It is found for x = 0, 10, 20, and 25, lattice parameters a and c, and the volume of Nd2Fe14B and Nd1.1Fe4B4 phases, which decrease with the increase of nickel content. The parameter a of the Nd1.1Fe4B4 phase has a tendency to increase in value compared to the parameter c which has a tendency to decrease with the concentration of nickel; this behavior causes the lattice to dilate in a and it is compressed in c, indicating that Ni atoms are preferably entering along c. The two phases parallel to crystallite sizes are greater than the perpendicular ones, indicating that the crystallite shape is not spherical but elongated in that direction. XRD results for all samples doped with Ni indicate that the crystallite sizes are of the nanometer order and range between 5 and 90 nm. It is evident the addition of this element does not damage the structural formation of Nd2Fe14B phase. MS shows seven magnetic sites (16k1, 16k2, 8j1, 8j2, 4c, 4e, and sb d) and a paramagnetic phase (doublet) corresponding to the phases found by XRD. The mean hyperfine magnetic field of the samples decreases with the increase in the concentration of Ni. According to the decrease of the percentage of the area and the hyperfine magnetic field of 16k2, 8j2, and 4c sites’ phase Nd2Fe14B in Ni-doped samples, preference of nickel atoms to replace iron atoms at these sites was established. It can see also the relative area of the site d (Nd1.1Fe4B4) which decreases with Ni concentration. VSM results showed that the major phase Nd2Fe14B is the hard magnetic phase, while the paramagnetic phase Nd1.1Fe4B4 is the minority phase. There is a general trend of microhardness increase with nickel content, attributable to nickel doping which reduces the lattice parameters and porosities in the sample improving its hardness.

Low-Temperature Magnetic Hysteresis in Nd(Pr)-Fe-B Nanostructured Alloys with Nd&lt;sub&gt;2&lt;/sub&gt;Fe&lt;sub&gt;14&lt;/sub&gt;B Type Main Phase Composition

Magnetic hysteresis properties of nanostructured industrially manufactured Nd-Fe-B and Pr-Fe-B alloys on the base of a tetragonal Nd2Fe14B (2-14-1) hard magnetic phase (MQP-B, MQP-B+ and MQP-16-7 brands) have been investigated at 4.2 K in magnetic fields up to 58 T. The chemical composition of the alloys given in the certificates was defined more precisely. The grain sizes of the main 2-14-1 phase were determined. The average grain size is much smaller than a critical single domain diameter. Coercivity, remanence magnetization, saturation magnetization and maximal magnetic energy product were determined at 4.2 K and compared with those obtained at room temperature.

Structural and Magnetic Properties of Soft/Hard Mn0.6Zn0.4Fe2O4@Sr0.85Ba0.15Fe12O19 Core/Shell Composite Synthesized by the Ball-Milling-Assisted Ceramic Process

Soft/hard xMn0.6Zn0.4Fe2O4@(1−x)Sr0.85Ba0.15Fe12O19 (x = 0.15, 0.30, and 0.45) core/shell magnetic composites have been synthesized by the two-step ball-milling-assisted ceramic process. The products are characterized by x-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), a vibrating sample magnetometer (VSM). The XRD analysis indicates the coexistence of ZnFe2O4 and SrFe12O19 phases. FT-IR results show that an Sr0.85Ba0.15Fe12O19 shell is on the surface of the Mn0.6Zn0.4Fe2O4 core. When Mn0.6Zn0.4Fe2O4@Sr0.85Ba0.15Fe12O19 precursor is calcined at 900°C, the crystallite size of obtained core/shell composites is between 33.50 ± 0.50 nm and 47.91 ± 0.72 nm in core Mn0.6Zn0.4Fe2O4 and between 59.34 ± 0.89 nm and 67.61 ± 1.01 nm in shell Sr0.85Ba0.15Fe12O19. Magnetic characterization indicates that the specific saturation magnetization of xMn0.6Zn0.4Fe2O4@(1−x)Sr0.85Ba0.15Fe12O19 composites decreases with the increase in mass ratio of soft-to-hard magnetic phases. Compared with bare Sr0.85Ba0.15Fe12O19 calcined at 950°C, the specific saturation magnetization of the composites is decreased and coercivity is markedly increased. This is attributed to the exchange-coupling interaction between the hard and soft magnetic phases due to formation of the core/shell structure. The 0.15Mn0.6Zn0.4Fe2O4@0.85Sr0.85Ba0.15Fe12O19 composite calcined at 700°C has higher squareness (Mr/Ms = 0.5401 ± 0.0065) and coercivity (4288.09 ± 42.88 Oe) values compared with the bare Sr0.85Ba0.15Fe12O19, which are useful for high-density magnetic recording and enhanced memory storage.

Phase transformation in anisotropic nanocrystalline SmCo5 magnets

SmCo5 nanoflakes with excellent texture have great potential applications in high performance nanocomposite magnets, in which the phase structure of the hard magnetic phase is crucial for obtaining large coercivity. Phase transformation from 1:5H phase to 2:17 R phase is observed in anisotropic nanocrystalline SmCo5 magnets when sintering the nanoflakes at 600–650 °C. The phase transformation is interesting because the 1:5H phase is relatively stable according to the thermodynamic and kinetic analysis. The phase transformation results in much lower coercivity of the wet-milled anisotropic magnets when compared with dry-milled isotropic magnets with 1:5H single phase. Analyses of differential scanning calorimetry, thermogravity and energy dispersive X-ray validate that the phase transformation originates from the oxidation of the nanoflakes with large specific surface area during sintering. The results suggest that controlling oxygen content strictly during sintering is necessary for maintaining 1:5H single phase in anisotropic nanocrystalline SmCo5 magnets prepared by sintering the nanoflakes.

Magnetic properties and exchange interaction of rapidly quenched La or Ce substituted nanocrystalline NdFeB alloys with various compositions

The effects of La and Ce substitutions for Nd on the magnetic properties and intergranular exchange coupling of nanocrystalline (Nd1-xMx)yFe94-yB6, (M = La and Ce; y = 10 and 12; x = 0–0.4) alloys have been investigated and presented in order to improve room temperature magnetic properties and reduce the rare earth content for NdFeB based alloys. Optimum magnetic properties such as remanent magnetization Mr, maximum energy product (BH)max and coercivity Hc up to 104 emu/g, 151 kJ/m3 and 9.0 kOe have been obtained for as-quenched single phase (Nd0.95La0.05)12Fe82B alloy. The decrease in Mr for La/Ce substituted alloys is attributed to saturation magnetization reduction. An unusual increase in Hc for 5% La substituted single phase alloy is speculated to the change in microstructure and magnetic phase separation. The analysis of remanence ratio, susceptibility curve, recoil loops and Henkel plots reveal that La substituted alloys exhibit strong ferromagnetic exchange coupling between hard magnetic phases or hard and soft phases than Ce substituted alloys due to the increase of exchange length and refine microstructure by La substitution. The present results indicate that partial substitution of Nd by La or Ce leads to various change behaviors for magnetic properties and inter-grain exchange coupling.

Synthesis and magnetic properties of the exchange-coupled SrFe10.7Al1.3O19/Co composite

The magnetic composite SrFe10.7Al1.3O19/Co was synthesized by ethylene glycol reduction of cobalt ions on the surface of hexaferrite particles dispersed in the solvent. The resulting material contained magnetically hard submicron hexaferrite particles covered by soft magnetic cobalt nanoparticles. The composite demonstrated the exchange-coupling effect between hard and soft magnetic phases.

Structural and magnetic properties of soft/hard NiFe2O4@SrCo0.2Fe11.8O19 core/shell composite prepared by the ball-milling-assisted ceramic process

Soft/hard xNiFe2O4@(1−x)SrCo0.2Fe11.8O19 (x = 0.00, 0.15, 0.30, and 0.45) core/shell magnetic composites have been synthesized by the ball-milling-assisted ceramic process. The analysis of XRD indicates the coexistence of NiFe2O4 and SrCo0.2Fe11.8O19. The core/shell structure of the composite sample has been confirmed by Fourier transform infrared spectra. When xNiFe2O4@(1−x)SrCo0.2Fe11.8O19 precursor is calcined at 900 °C, the crystallite size of obtained core/shell composites is between 28.3 and 46.6 nm in core NiFe2O4 and between 59.2 and 74.5 nm in shell SrCo0.2Fe11.8O19. Magnetic characterization indicates that specific saturation magnetization of xNiFe2O4@(1−x)SrCo0.2Fe11.8O19 composites increases with the increase in calcination temperature, and decreases with the increase in mass ratio of soft to hard magnetic phase. Compared with bare SrCo0.2Fe11.8O19 calcined at 950 °C, the specific saturation magnetization of xNiFe2O4@(1−x)SrCo0.2Fe11.8O19 composites is decreased and coercivity is increased. This is attributed to the exchange–coupling interaction between the hard and soft phases due to formation of core/shell structure. Besides, remanence and magnetic moment of xNiFe2O4@(1−x)SrCo0.2Fe11.8O19 composites decrease with the increase in NiFe2O4 content. The xNiFe2O4@(1−x)SrCo0.2Fe11.8O19 composites have higher squareness (Mr/Ms) value compared with the bare SrCo0.2Fe11.8O19, which is useful for high-density magnetic recording and enhanced memory storage.

Annealing-Induced High Ordering and Coercivity in Novel L10 CoPt-Based Nanocomposite Magnets

A novel class of quaternary intermetallic alloys based on CoPt is investigated in view of their interesting magnetic properties induced by the presence of hard magnetic L10 phase. A Co48Pt28Ag6B18 alloy has been prepared by rapid solidification from the melt and subjected to various isothermal annealing procedures. The structure and magnetism of both as-cast and annealed samples as well as the phase evolution with temperature are investigated by means of thermal analysis, X-ray, and selected area electron diffraction, scanning and high-resolution electron microscopy, and magnetic measurements. The X-ray diffraction (XRD) analysis shows that both the as-cast alloy and the sample annealed at 400 °C (673 K) have a nanocrystalline structure where fcc CoPt phase predominates. Annealing at 473 °C promotes the formation of L10 phase triggered by the disorder-order phase transformation as documented in the differential scanning calorimetry results. The sample annealed at 670 °C (943 K) shows full formation of L10 CoPt as revealed by XRD. Magnetic measurements showed coercivity values ten times increased compared to the as-cast state. This confirms the full formation of L10 CoPt in the annealed samples. Moreover, detailed atomic resolution HREM images and SAED patterns show the occurrence of the rarely seen (003) superlattice peaks, which translated into a high ordering of the L10 CoPt superlattice. Such results spur more interest in finding novel classes of nanocomposite magnets based on L10 phase.

Investigation of fabrication of Mn-Ga-Al/Fe-Co nanocomposite hard magnetic materials

Mn-Ga-Al/Fe-Co nanocomposite hard magnetic materials were combined from hard magnetic and soft magnetic phases. The nanoparticles of hard and soft magnetic phases were fabricated by using high energy ball milling method with milling times of 8 h and 32 h to have size of about 50 nm and 70 nm, respectively. The hard and soft magnetic phases were then mixed together for 1 h with different weight fractions of soft magnetic phase from 5 to 20%. The nanocomposites show quite good exchange-spring coupling of hard and soft magnetic phases. Saturation magnetization and coercivity of the nanocomposites strongly depend on the weight fraction of the soft magnetic phase. The increases rapidly, while the decreases with increasing the fraction of the soft magnetic phase. With appropriate fabrication conditions, the Mn-Ga-Al/Fe-Co nanocomposites can possess emu g−1 and . Maximum energy products, , above 4 MGOe have been achieved for this kind of rare earth-free hard magnetic materials.