Exchange-coupled Nanocomposites Research Articles

Mechanochemical synthesis of MnBi/Fe3C@C exchange coupled hard magnetic nanocomposites

Despite the better suitability of rare earth permanent magnets for various applications, the scarcity of rare earths limits their application range. The, low temperature phase of MnBi is sought as a probable gap magnet without any rare earth elements. Further, the nanocomposites (soft/hard magnets) are known to provide better properties. In this study, Mn55Bi45/Fe3C@C nanocomposites with enhanced magnetic properties were prepared using cryogenic ball milling. The feasibility of utilizing Fe3C@C as a soft phase in hard-soft composites was explored. Fe3C@C was synthesized through the solvothermal technique, followed by calcination, resulting in a remarkable saturation magnetization (MS) of 150 emu/g. Different amounts of Fe3C@C (0, 5, 10, 15, and 20 wt.%) were added to Mn55Bi45 powder, and after cryo-milling, it was observed that the addition of 10 wt. % yielded the optimum results, with a 37% enhancement in MS and 58% increase in Mr values. The presence of exchange-coupled nanocomposites was confirmed through x-ray diffraction, transmission electron microscopy, and magnetic studies. This study not only paves the way for increasing the energy product of the MnBi system but also explores the potential of utilizing Fe3C@C as a soft phase—a relatively oxidation-resistant phase—in other hard magnetic systems.

Enhanced magnetic properties of SrFe12O19 through exchange-coupled nanocomposite

This work presents the synthesis and characterization of hard/soft (1-x)SrFe12O19/xCoFe2O4 exchange-coupled nanocomposites with x = 0.0, 0.05, 0.1, and 0.2 using the conventional coprecipitation and ball milling preparation methods followed by heat treatment. CoFe2O4 material was used as a soft magnetic material when the effects of its concentration on the structural and magnetic properties of the obtained nanocomposites were investigated using different characterization techniques. Structural and microstructural analyses showed the presence of both spinel ferrite and the M-type hexaferrite phase. Single-phase magnetic loops were observed for the prepared nanocomposites, indicating an exchange coupling interaction between CoFe2O4 and SrFe12O19 phases. To support this finding, a Henkel plot was performed, which revealed the presence of strong exchange coupling interaction in CS5 compared to CS10, unlike CS20, where the dipolar interaction is dominant. The obtained results demonstrate the possibility of enhancing the (BH)max of the M-type strontium hexaferrite through an exchange-coupled nanocomposite, using a simple and reproducible synthesis process with a small content of the soft magnetic phase.

Synthesis of Mesoscopic Particles of Multi-Component Rare Earth Permanent Magnet Compounds

Multielement rare earth (R)–transition metal (T) intermetallics are arguably the next generation of high-performance permanent magnetic materials for future applications in energy-saving and renewable energy technologies. Pseudobinary Sm2Fe17N3 and (R,Zr)(Fe,Co,Ti)12 (R = Nd, Sm) compounds have the highest potential to meet current demands for rare-earth-element-lean permanent magnets (PMs) with ultra-large energy product and operating temperatures up to 200°C. However, the synthesis of these materials, especially in the mesoscopic scale for maximizing the maximum energy product ((BH)max), remains a great challenge. Nonequilibrium processes are apparently used to overcome the phase-stabilization challenge in preparing the R–T intermetallics but have limited control of the material’s microstructure. More radical bottom-up nanoparticle approaches based on chemical synthesis have also been explored, owing to their potential to achieve the desired composition, structure, size, and shape. While a great achievement has been made for the Sm2Fe17N3, progress in the synthesis of (R,Zr)(Fe,Co,Ti)12 magnetic mesoscopic particles (MMPs) and R–T/T exchange-coupled nanocomposites (NCMs) with substantial coercivity (Hc) and remanence (Mr), respectively, remains marginal.

Tip Interface Exchange-Coupling Based on "Bi-Anisotropic" Nanocomposites with Low Rare-Earth Content.

Specially designed SmCo5/Co magnetic nanocomposites have been fabricated by a "bottom up" process. SmCo5 nanochips were first prepared by solution-phase chemical synthesis combined with reductive annealing and then coated by chemical deposition of Co nanorods. Both the SmCo5 nanochips and Co nanorods are anisotropic and could be simultaneously aligned under the external magnetic field. Magnetic measurements applied on these "bi-anisotropic" SmCo5/Co composites show high magnetic performance with the Co phase content in a wide range from 10 to 80 wt %. For the first time ever, the applicable exchange-coupled nanocomposites with a rare-earth content lower than 7 wt % was realized, which exhibits the coercivity close to 10 kOe and remanence 31% larger than that of single phase SmCo5. 3-D micromagnetic simulations were performed to reveal that the reversal mechanism in the Co phase was transferred from the incoherent mode to the coherent mode under a tip interface exchange-coupling with a SmCo5 surface.

Towards bi-magnetic nanocomposites as permanent magnets through the optimization of the synthesis and magnetic properties of SrFe12O19 nanocrystallites

Sol–gel synthesis was used in order to obtain nanocrystallites of the SrFe12O19 (SFO) hexaferrite in an efficient and reliable way. By optimizing the initial synthetic conditions, we were able to control the size of the nanoparticles (NPs), at lower annealing temperature. The x-ray powder diffraction, transmission electron microscopy (TEM), and magnetic measurements have demonstrated a significant relation between the morphology, size, and magnetic properties of the nanoscale SFO, revealing a definite dependence on the crystallite size along the c-axis. The obtained NPs appear almost isotropic, in the form of platelets and exhibit similar magnetic performance, in terms of the energy product (BH)MAX, thus, demonstrating the suitability of reducing the annealing temperature without any deterioration in the magnetic properties. Additionally, this work illustrates the feasibility of the sol–gel bottom-up approach to employ magnetic NPs as building-blocks for designing hard/soft exchange-coupled bi-magnetic nanocomposites, combining the high coercivity of a hard phase (SFO) and the high saturation magnetization of a soft phase (CoFe2O4); in this regard, we discuss the tunability of the magnetic anisotropy by symbiotically restricting the growth of both phases.

Exchange-coupled nanocomposites with novel microstructure and enhanced remanence by a new approach

Exchange-coupled nanocomposites with hard/soft magnetic phases are promising for the next generation of permanent magnets. Chemical methods have an advantage in controlling the nanoscale size of both phases. The nanocomposites obtained by the chemical method generally consist of a hard phase core and a soft phase shell. However, the soft-phase shell is easily oxidized leading to small enhancement of remanence. Here, a novel microstructure of Fe@FePt nanocomposites with Fe soft phase core and FePt hard phase shell has been synthesized by replacement reaction, in which the size of core and shell can be controlled below 10 nm by adjusting the ratio of Fe nanoparticles to PtCl4. Excellent exchange-coupling (single-phase-like demagnetization curves) between soft-hard phases was observed due to the precise size control of both phases, and substantial enhancements both in remanence (32 %) and saturation magnetization (81 %) were obtained in optimal nanocompistes. This work provides an alternative routine to prepare heterostructure materials with various applications.

Bonded ferrite-based exchange-coupled nanocomposite magnet produced by Warm compaction

Many modern technologies require permanent magnets with combinations of properties that cannot be met by conventional metallic or ceramic magnets. Ferrite/polymer composite magnets are a type of rare-earth free magnet with a wide range of magnetic and material property combinations. The uncertainty surrounding the supply and pricing of rare-earth elements, along with environmental issues of using these elements, have motivated many researchers to develop high-performance ferrite-based magnets via an exchange spring method. The present study explores magnetite coated M-type ferrite nanocomposites synthesised via a hydrothermal and coprecipitation method, and investigates the mechanical and magnetic properties of warm compressed high-performance exchange-coupled nanocomposites in an epoxy matrix. We show how the powder-to-resin ratio and preparation conditions lead to optimised mechanical properties, and enhancement in the maximum energy product of the composite magnet by up to 120% compared to a commercial SrM bonded plasto-ferrite magnet. These high performance composite magnets can lower the final cost of ferrite based bonded magnets without reducing the permanent magnetic properties or can be used in applications that a ferrite magnet has inadequate performances.

Chemically synthesizing exchange-coupled SmCo5/Sm2Co17 nanocomposites

A new strategy to chemically synthesize exchange-coupled SmCo5/Sm2Co17 nanocomposites by in situ decomposition of SmCox (5 < x < 8.5) is reported in this work. Our synthesis starts with the fabrication of Co/Sm2O3 (Sm to Co atomic ratio of Sm/Co = 1:4.2), which can be reduced into 40-nm SmCo5 single crystal nanoparticles by Ca under the protection of CaO, showing a high coercivity of 2.85 T and saturation magnetization (Ms) of 0.0671 A·m2·g−1. By changing the Sm/Co to 1:4.5, 1:4.8 and 1:5.2, SmCo5/Sm2Co17 nanocomposites with different proportions were acquired using the same process. Owing to the in situ decomposition of SmCox intermediate, the small size (both of their size less than 10 nm) and uniform phase distribution were achieved in our nanocomposites. Thus, the as-prepared nanocomposites display a strong exchange-coupling interaction. As a consequence, SmCo5/Sm2Co17 (Sm/Co = 1:5.2) exhibits a coercivity of 1.23 T and enhanced M7T (magnetization at 7 T) of 0.0812 A·m2·g−1, increasing by 21% than pure SmCo5. Our synthesis provides a new protocol to prepare exchange-coupled high-performance nanocomposites. The exchange-coupled SmCo5/Sm2Co17 nanocomposites with small size (both of their size less than 10 nm) have been chemically synthesized by in situ decomposition of SmCox (5 < x<8.5), which exhibits a coercivity of 1.23 T and enhanced Ms, increasing by 21% than pure SmCo5 .

Micromagnetic modelling of nanorods array-based L10-FeNi/SmCo5 exchange-coupled composites

Exchange-coupled nanocomposites are considered as the most promising materials for production of high-energy performance permanent magnets, which can exceed neodymium ones in terms of energy product. In this work, micromagnetic simulations of L10-FeNi/SmCo5 composites based on the initially anisotropic structure of nanorods array were performed. Texturing effect on magnetic properties was investigated. It was revealed that even 30% of anisotropy axes misalignment of grains in L10-FeNi phase would lead to only ≈10% drop of coercivity. To maximize magnetic properties of the composites, parameters of microstructure were optimized for 120 × 120 array of interacting nanorods and were found to be 40 nm nanorod diameter and 12–20 nm interrod distance. The estimated diameter of nanorods and the packing density of the array provide energy product values of 149 kJ m−3. Influence of interrod distance on energy product values was explored. Approaches for production of exchange-coupled composites based on anisotropic nanostructures were proposed.

Magnetic Properties of SmCo5 + 10 wt% Fe Exchange-Coupled Nanocomposites Produced from Recycled SmCo5.

Nanostructured alloy powders of SmCo5 + 10 wt% Fe obtained using recycled material were studied for the first time. The SmCo5 precursor was obtained from commercial magnets recycled by hydrogen decrepitation. The results were compared with identically processed samples obtained using virgin SmCo5 raw material. The samples were synthesized by dry high-energy ball-milling and subsequent heat treatment. Robust soft/hard exchange coupling was observed—with large coercivity, which is essential for commercial permanent magnets. The obtained energy products for the recycled material fall between 80% and 95% of those obtained when using virgin SmCo5, depending on milling and annealing times. These results further offer viability of recycling and sustainability in production. These powders and processes are therefore candidates for the next generation of specialized and nanostructured exchange-coupled bulk industrial magnets.

Exchange-coupling effect in hard/soft SrTb0.01Tm0.01Fe11.98O19/AFe2O4 (where A = Co, Ni, Zn, Cu and Mn) composites

In this study, series of hard/soft SrTb0.01Tm0.01Fe11.98O19/AFe2O4 (where A = Co, Ni, Zn, Cu and Mn) composites were fabricated via a single-pot citrate sol-gel approach. The structure, morphology and magnetic properties of prepared composite samples were investigated via X-ray diffraction (XRD), scanning and transmission electron microscopes (SEM - TEM) and vibrating sample magnetometer (VSM). The XRD analysis of all composite samples showed the co-existence of both hard (Sr hexaferrite) and soft (spinel ferrites) ferrite phases with minor impurity. TEM micrographs displayed well-distinguished particles of SrM and AFe2O4 with different symmetry. The magnetic M − H hysteresis loops were performed at room temperature (RT; T = 300 K) and low temperature (T = 10 K) using VSM instrument. The magnitudes of various magnetic parameters including saturation magnetization (Ms), squareness ratio (SQR = Mr/Ms), remanence (Mr) and coercivity (Hc) were determined. M − H loops revealed smoothed curves and the dM/dH versus H curves exposed only a single peak, indicating that the exchange-coupling effect was accomplished in one-step. Moreover, the various composites showed relatively high Ms, Mr, and Hc values. The obtained results revealed the occurrence of exchange-coupling effect among soft and hard magnetic phases. The magnetic properties of various hard/soft SrTb0.01Tm0.01Fe11.98O19/AFe2O4 composites (where A = Co, Ni, Zn, Cu and Mn) were evaluated also by ZFC-FC magnetization measurements with respect to different soft phases. A peak temperature in ZFC curves occurred for various prepared composites. This peak is attributed to competition of the movement of magnetic domain walls and thermal activation. The present study offers a simple but efficient route for the fabrication of exchange-coupled nanocomposites with the chemical formula SrFe11.98Tb0.01Tm0.01O19/AFe2O4 (where A = Co, Ni, Zn, Cu and Mn) having controllable magnetic properties. It was found that the SrTb0.01Tm0.01Fe11.98O19/CoFe2O4 composite sample displayed the strongest exchange-coupling behavior among the different prepared composite products.

Chemical Synthesis of Magnetic Nanoparticles for Permanent Magnet Applications.

Permanent magnets are a class of critical materials for information storage, energy storage, and other magneto-electronic applications. Compared with conventional bulk magnets, magnetic nanoparticles (MNPs) show unique size-dependent magnetic properties, which make it possible to control and optimize their magnetic performance for specific applications. The synthesis of MNPs has been intensively explored in recent years. Among different methods developed thus far, chemical synthesis based on solution-phase reactions has attracted much attention owing to its potential to achieve the desired size, morphology, structure, and magnetic controls. This Minireview focuses on the recent chemical syntheses of strongly ferromagnetic MNPs (Hc >10 kOe) of rare-earth metals and FePt intermetallic alloys. It further discusses the potential of enhancing the magnetic performance of MNP composites by assembly of hard and soft MNPs into exchange-coupled nanocomposites. High-performance nanocomposites are key to fabricating super-strong permanent magnets for magnetic, electronic, and energy applications.

A hybrid coercivity mechanism for exchange-coupled nanocomposite permanent magnets

We propose a hybrid coercivity mechanism for exchange-coupled hard/soft multilayers, which incorporates elements of both the traditional nucleation and pinning mechanisms based on both three-dimensional (3D) and one-dimensional (1D) micromagnetic calculations. The magnetic reversal starts with the nucleation of the domain wall near the defects or soft phases, which ends by the pinning usually in the same place. Therefore, pinning near the nucleation centers are the dominant coercivity mechanism for both exchange-coupled nanocomposites and so-called single-phased permanent magnets. Our proposed coercivity mechanism and calculated results agree very well with available experimental data, especially the recently reported high energy products achieved in NdFeB and SmCo based hard/soft multilayers. The hybrid coercivity mechanism can be readily extended to single-phased permanent magnets with defects and other magnetic systems.

Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets.

During the past decade, CoFe2O4 (hard)/Co–Fe alloy (soft) magnetic nanocomposites have been routinely prepared by partial reduction of CoFe2O4 nanoparticles. Monoxide (i.e., FeO or CoO) has often been detected as a byproduct of the reduction, although it remains unclear whether the formation of this phase occurs during the reduction itself or at a later stage. Here, a novel reaction cell was designed to monitor the reduction in situ using synchrotron powder X-ray diffraction (PXRD). Sequential Rietveld refinements of the in situ data yielded time-resolved information on the sample composition and confirmed that the monoxide is generated as an intermediate phase. The macroscopic magnetic properties of samples at different reduction stages were measured by means of vibrating sample magnetometry (VSM), revealing a magnetic softening with increasing soft phase content, which was too pronounced to be exclusively explained by the introduction of soft material in the system. The elemental compositions of the constituent phases were obtained from joint Rietveld refinements of ex situ high-resolution PXRD and neutron powder diffraction (NPD) data. It was found that the alloy has a tendency to emerge in a Co-rich form, inducing a Co deficiency on the remaining spinel phase, which can explain the early softening of the magnetic material.

Exchange coupling effects in Co-Pt nanochessboards

To complement recent experimental findings [Vetter et al., APL Mater. 4, 096103 (2016)] on strong exchange coupling effects in Co-Pt nanochessboards featuring a single peak in the first-order reversal curve density plot and associated magnetic hardening as the characteristic structural dimension is reduced below the critical correlation length, the effects of the strong exchange coupling on the underlying magnetic domains and their evolutions are investigated using micromagnetic simulations. The nanochessboard consists of regularly distributed aligned nanorods of a magnetically hard tetragonal L10 phase and a magnetically soft cubic L12 phase, representing a unique type of exchange-coupled magnetic nanocomposites. The simulations reveal detailed magnetic domain processes specific to the nanochessboard structure and uncover an unusual dependence of the coercivity on the length scale, where an expected magnetic hardening is followed by an unexpected magnetic softening with decreasing characteristic dimension. The length scale-dependent exchange coupling effects, effective magnetic anisotropy, and magnetic domain structures and evolution pathways are analyzed to provide insights into the peculiar magnetic behaviors of Co-Pt nanochessboards.

Stabilizing Fe Nanoparticles in the SmCo5 Matrix.

We report a new strategy for stabilizing Fe nanoparticles (NPs) in the preparation of SmCo5-Fe nanocomposites. We coat the presynthesized Fe NPs with SiO2 and assemble the Fe/SiO2 NPs with Sm-Co-OH to form a mixture. After reductive annealing at 850 °C in the presence of Ca, we obtain SmCo5-Fe/SiO2 composites. Following aqueous NaOH washing and compaction, we produced exchange-coupled SmCo5-Fe nanocomposites with Fe NPs controlled at 12 nm. Our work demonstrates a successful strategy of stabilizing high moment magnetic NPs in a hard magnetic matrix to produce a nanocomposite with tunable magnetic properties.

Application of Mechanochemical Synthesis to Manufacturing of Permanent Magnets

Synthesis of permanent magnet powders through mechanically and thermally activated reduction of metal oxides with calcium, which allows for the production of fine single crystal particles, may contribute to the ongoing search for alternatives to the Nd-Fe-B magnets. The reviewed efforts demonstrate the possibilities to prepare (1) high-coercivity SmCo5 nanoparticles for anisotropic exchange-coupled nanocomposites, (2) YCo5/LaCo5 powders with the magnetic properties superior to those obtained through other manufacturing methods and (3) submicron Mn-Bi powders with large concentration of the MnBi phase. Also discussed are less successful attempts to prepare Mn-Al, Zr-Co and Hf-Co hard magnetic alloys in which the principal phases are known or likely to be metastable.

ChemInform Abstract: Exchange‐Coupled Nanocomposites: Chemical Synthesis, Characterization and Applications

AbstractReview: 101 refs.

Exchange-coupled fct-FePd/α-Fe nanocomposite magnets converted from Pd/Fe3O4 core/shell nanoparticles.

We report the controlled synthesis of exchange-coupled face-centered tetragonal (fct) FePd/α-Fe nanocomposite magnets with variable Fe concentration. The composite was converted from Pd/Fe3O4 core/shell nanoparticles through a high-temperature annealing process in a reducing atmosphere. The shell thickness of core/shell Pd/Fe3O4 nanoparticles could be readily tuned, and subsequently the concentration of Fe in nanocomposite magnets was controlled. Upon annealing reduction, the hard magnetic fct-FePd phase was formed by the interdiffusion between reduced α-Fe and face-centered cubic (fcc) Pd, whereas the excessive α-Fe remained around the fct-FePd grains, realizing exchange coupling between the soft magnetic α-Fe and hard magnetic fct-FePd phases. Magnetic measurements showed variation in the magnetic properties of the nanocomposite magnets with different compositions, indicating distinct exchange coupling at the interfaces. The coercivity of the exchange-coupled nanocomposites could be tuned from 0.7 to 2.8 kOe and the saturation magnetization could be controlled from 93 to 160 emu g(-1). This work provides a bottom-up approach using exchange-coupled nanocomposites for engineering advanced permanent magnets with controllable magnetic properties.

Synthesis and magnetic properties of hard-soft SRFe10Al2O19/NiZnFe2O4 ferrite nanocomposites.

Pure phase exchange-coupled nanocomposites of hard-soft magnetic oxides, (hard) SrFe10Al2O19(-) (soft) Ni0.75Zn0.25Fe2O4 were prepared via autocombution method. Magnetic properties of the nanocomposites were assessed as a function of soft-phase content in the nanocomposite. A 40% increase in M(s) value was observed for nanocomposite with 30 Wt.% of the soft phase. A linear increase in M(r)/M(s) with soft-phase content indicates the presence of enhanced exchange-coupling between hard and soft phases of the nanocomposite. The highest M(r)/M(s) ratio of 0.68 was obtained for nanocomposite containing 30 Wt.% of the soft-phase. The observed reduction in coercieve field values of the nanocomposite with increase in soft-phase content is explained on the basis of competition between exchange and dipolar interaction between hard-soft and soft-soft phases of the nanocomposite.