Composite Permanent Magnets Research Articles

Study on magnetic and structural properties of Fe65Co35 soft magnetic alloy prepared by arc melting and subsequent annealing

Fe65Co35 based soft magnetic alloys (SMAs) have been gaining attention for their superior magnetic features and playing significant roles in composite permanent magnets, magnetic storage and automotive electrification technologies. In the present work, the Fe65Co35 alloy were produced by arc melting with subsequent annealing. The effects of arc melting and annealing upon the structural and magnetic properties of the soft magnetic phase of the alloy Fe65Co35 have been investigated. The variation in magnetic and microstructural characteristics with melting and annealing have been observed using the X-ray diffractometer (XRD), scanning electron microscope (SEM), and the vibrating sample magnetometer (VSM). The XRD patterns verified the existence of the Fe65Co35 alloy. After melting and annealing, the alloy surface morphologies were examined using SEM. In this approach, the Fe65Co35 alloy was annealed at 600 °C for 4 h in an argon environment which resulted in the highest saturation magnetization of 237 emu/g and the lowest coercivity of 31 Oe. The rise in saturation magnetization was attributed to the removal of impurities, during the melting and annealing process.

Dense strontium hexaferrite-based permanent magnet composites assisted by cold sintering process

The use of rare-earth-based permanent magnets is one of the critical points for the development of the current technology. On the one hand, industry of the rare-earths is highly polluting due to the negative environmental impact of their extraction and, on the other hand, the sector is potentially dependent on China. Therefore, investigation is required both in the development of rare-earth-free permanent magnets and in sintering processes that enable their greener fabrication with attractive magnetic properties at a more competitive price. This work presents the use of a cold sintering process (CSP) followed by a post-annealing at 1100 °C as a new way to sinter composite permanent magnets based on strontium ferrite (SFO). Composites that incorporate a percentage ≤ 10% of an additional magnetic phase have been prepared and the morphological, structural and magnetic properties have been evaluated after each stage of the process. CSP induces a phase transformation of SFO in the composites, which is partially recovered by the post-thermal treatment improving the relative density to 92% and the magnetic response of the final magnets with a coercivity of up to 3.0 kOe. Control of the magnetic properties is possible through the composition and the grain size in the sintered magnets. These attractive results show the potential of the sintering approach as an alternative to develop modern rare-earth-free composite permanent magnets. • First dense rare-earth-free composite permanent magnets assisted by cold sintering process. • Control of the grain size during the manufacturing route of the final magnets improving their magnetic response. • Highly competitive magnetic properties reached at lower sintering temperatures with an energy efficiency of around 29%.

Exchange coupling and improved properties of the multilayer CoFe2O4/La0.7Sr0.3MnO3 thin films

Hard/soft interface plays a role in the properties of the composite metal-based magnets, while it is less talked in the composite permanent oxide magnets on the magnetic interaction and properties due to easy interdiffusion. Here, multilayer thin films composed with hard CoFe2O4 phase and soft La0.7Sr0.3MnO3 layers were choose and synthesized without interdiffusion by a chemical solution method. Furthermore, the effects of the annealing temperature and the number of soft layer on the magnetic interaction and properties are investigated. All composite films behavior as one rigidly coupled composite magnet at room temperature, and the magnetic properties increase with increasing annealing temperature. The coercivity and maximum magnetic energy product can respectively reach 8.27 kOe and 2.4 MG Oe at the temperature of 300 K. Exchange coupling was effectively enhanced and even dominate the magnetic interaction with less soft layers for the in-plane direction. Conversely, dipolar interaction predominates, but becomes weak in the composite films with more soft layers and attenuates with increasing annealing temperature for the out-of-plane direction. All of the results suggest that a suitable grain size of the hard phase and an improved interface may be more important for a rigidly coupled composite oxide magnet with good performance.

Magnetooptical Visualization and Mapping of Magnetic Stray Fields over the Surface of Composite Permanent Magnets Using Gadolinium–Cobalt Amorphous Films

Stray fields of magnetic systems with a known degree of structural and magnetic nonuniformities have been mapped using indicator amorphous films with a perpendicular anisotropy. The characteristic magnetooptical patterns have been determined, which correspond to the distribution of the magnetic-field component normal to the plane of the indicator films. The theoretical patterns of model magnetic systems that are similar to the studied systems in the degree of the structural and magnetic nonuniformity have been calculated. The validity of the magnetooptical visualization has been confirmed by comparison of the experimental and theoretical patterns. The possibility of the express monitoring of the parameters of the magnetic system has been shown using a laboratory model of a magnetooptical visualizer by the observation of the magnetooptical contrast of gadolinium–cobalt indicator films under the action of an ac magnetic field.

Deterioration of the coercivity due to the diffusion induced interface layer in hard/soft multilayers.

Hard/soft permanent magnets have aroused many interests in the past two decades because of their potential in achieving giant energy products as well as their rich variety of magnetic behaviors. Nevertheless, the experimental energy products are much smaller than the theoretical ones due to the much smaller coercivity measured in the experiments. In this paper, the deterioration of the coercivity due to the interface atomic diffusion is demonstrated based on a three dimensional (3D) micromagnetic software (OOMMF) and a formula derived for the pinning field in a hard/soft multilayer, which can be applied to both permanent magnets and exchange-coupled-composite (ECC) media. It is found that the formation of the interface layer can decrease the coercivity by roughly 50%, which is responsible for the observed smaller coercivity in both composite and single-phased permanent magnets. A method to enhance the coercivity in these systems is proposed based on the discussions, consistent with recent experiments where excellent magnetic properties are achieved.

Stacked pulse-electroplated CoNiMnP–AAO nanocomposite permanent magnets for MEMS

The paper presents a CMOS compatible pulse-electroplating technique combined with a low temperature bonding process for the synthesis of CoNiMnP–AAO (anodic alumina oxide) nanocomposite films and the fabrication of stacked composite permanent magnets (PMs). The magnetic nanocomposite film exhibits the best characteristics of the coercivity of 2472 Oe, remanence of 4000 G, and of 16.13 kJ m−3, in the existing CoNiMnP systems. Meanwhile, a surface magnetic flux density of 9.2 mT generated by a 15-layer-stacked composite PM with a volume of 9 mm3 has shown the potential for various magnetic microelectromechanical systems (MEMS) fabrication using the nanocomposite material.

Status and prospect for multi-(hard magnetic) main-phase permanent magnet and coercivity mechanism

The mechanisms of magnetization and magnetic reversal have been reviewed. This review focuses on the design, the basic theory of controllable preparation and the mechanism of coercivity in novel composite rare earth permanent magnet of “multi-main-phase, multiscale” (including R-Fe-B/Nd-Fe-B, R-Fe-B/Sm-Co, and multi-main-phase). The novel two-main-phase (or multi-main-phase) composite rare earth permanent magnet has the advantages of different single phase permanent magnet and the preparation technology has been investigated. This paper discusses the mechanism of two-main-phase composite permanent magnet. Both the interaction between different hard permanent phases with different magneto-crystalline anisotropy constant, and the volume fractions and the distributions of different phases should be taken into account. Finally, the perspectives of permanent materials have been emphasized.

Development of a low-cost double rotor axial flux motor with soft magnetic composite and ferrite permanent magnet materials

This paper proposes a low-cost double rotor axial flux motor (DRAFM) with low cost soft magnetic composite (SMC) core and ferrite permanent magnets (PMs). The topology and operating principle of DRAFM and design considerations for best use of magnetic materials are presented. A 905 W 4800 rpm DRAFM is designed for replacing the high cost NdFeB permanent magnet synchronous motor (PMSM) in a refrigerator compressor. By using the finite element method, the electromagnetic parameters and performance of the DRAFM operated under the field oriented control scheme are calculated. Through the analysis, it is shown that that the SMC and ferrite PM materials can be good candidates for low-cost electric motor applications.

Effective high-energy ball milling in air of Fe65Co35 alloys

Fe65Co35 alloys are technologically relevant, especially in magnetic storage and composite permanent magnets, due to the fact that they present higher saturation magnetization per volume than any other material. Out of the various approaches undertaken for its production, mechanical ball milling remains the most common and efficient method, especially considering the large industrial scale of the applications. With the development of cost-efficient processing in mind, the influence of performing the synthesis of the FeCo alloys in air instead of the standard argon atmosphere is studied. The structural and magnetic characterization, along with the study of the oxygen content of the samples, proves that synthesizing FeCo alloys in air produce materials with nearly identical magnetic performance as their argon-milled counterpart, with the oxidation extent of the materials consisting almost exclusively of the oxide passivating layer located at the surface. In addition, no aging effect was observed in the saturation magnetization up to 6 months. It is concluded that the use of argon atmospheres, desiccators and/or glove boxes may be entirely removed from the process without affecting the magnetic properties.

High Electrical Resistivity Sm(Co,Fe,Cu,Zr)$_{\rm z}$/Sm$_{2}$S$_{3}$ Permanent Magnets With Layered Structure for Advanced Motors

High electrical resistivity, sintered Sm(Co,Fe,Cu,Zr)z/Sm2S3 layered composite permanent magnets were fabricated by a one-step sintering process. The thickness of the Sm2S3 layers was uniform in various laminated magnet architectures and controlled between 50 and 200 μm. The dielectric Sm2S3 layer provides an enhanced electrical resistivity while the existence of an interface diffusion reaction layer between the magnet matrix and dielectric component may help to improve the mechanical properties of the laminated magnet. The Sm(Co,Fe,Cu,Zr)z/Sm2S3 magnets exhibited nearly the same magnetic properties and demagnetization curve shape as the standard non-laminated counterparts, with only a small dilution of the magnetization. These sequentially laminated Sm(Co,Fe,Cu,Zr)z/Sm2S3 magnets showed about 30 times higher electrical resistivity and good mechanical strength combined with excellent magnetic properties. The laminated Sm(Co,Fe,Cu,Zr)z magnet with 190 μ m Sm2 S3 layer thickness exhibited a remanent magnetization (Br) of 10.51 kG, an intrinsic coercivity Hci of >; 24.5 kOe and a maximum energy product of 25 MGOe.

Random magnetic anisotropy in isotropic nanocrystalline composite permanent magnets

In this study, the random magnetic anisotropy in isotropic nanocrystalline composite permanent magnets was investigated by means of numerical simulations. The magnetization reversal of randomly oriented hard-soft exchange-coupled grains was simulated using the Landau–Lifshitz–Gilbert equation in which the magnetization in a particular grain is assumed to align in the same direction (single-spin model). The calculations show that the energy product (BH)max of nanocomposite magnets has a peak value 50 MGOe at 6 nm. It is about 80% of that of single-crystal Nd2Fe14B. The coercivity HC exhibits a steep decrease toward smaller grain size, which is the result of the suppression of the random magnetic anisotropy by exchange interaction. Therefore, in isotropic nanocomposite magnets, the enhancement of energy product is limited by the suppression of the random magnetic anisotropy by exchange interaction.

A MEMS AC current sensor for residential and commercial electricity end-use monitoring

This paper presents a novel prototype MEMS sensor for alternating current designed for monitoring electricity end-use in residential and commercial environments. This new current sensor design is comprised of a piezoelectric MEMS cantilever with a permanent magnet mounted on the cantilever's free end. When placed near a wire carrying AC current, the magnet is driven sinusoidally, producing a voltage in the cantilever proportional to the current being measured. Analytical models were developed to predict the applicable magnetic forces and piezoelectric voltage output in order to guide the design of a sensor prototype. This paper also details the fabrication process for this sensor design. Released piezoelectric MEMS cantilevers have been fabricated using a four-mask process and aluminum nitride as the active piezoelectric material. Dispenser-printed microscale composite permanent magnets have been integrated, resulting in the first MEMS-scale prototypes of this current sensor design.

Fe-nanoparticle coated anisotropic magnet powders for composite permanent magnets with enhanced properties

Utilizing the chemical reduction of FeCl2 with NaBH4 in the presence of 2:17 Sm–Co powders, we synthesized composite Sm(Co0.699Fe0.213Cu0.064Zr0.024)7.4/nano-Fe anisotropic hard magnetic powders. The average particle size of the hard magnetic core powder was 21μm while the soft magnetic Fe nanoparticles deposited uniformly on the core powder had a particle size smaller than 100nm. Different reaction protocols, such as immersion of the hard magnetic core powder in each reagent, the use of microemulsion (micelle) technique, or doubling the weight ratio of FeCl2 to core powder, led to different degrees of magnetic coupling of the hard and soft magnetic components of the composite powder. A reaction time of 180s led to deposition of 3.5wt% Fe nanoparticles and improved magnetic properties of the composite powder compared to the uncoated Sm(Co0.699Fe0.213Cu0.064Zr0.024)7.4 powder. The respective magnetic hysteresis parameters were 4πM18kOe=11.3kG, 4πMr=11kG, and Hci>20kOe with a smooth demagnetization curve.

Current Status of Rare-Earth Permanent Magnet Research in USA

In the paper, we summarize the USA research activities in the past two years on high performance rare earth ( R)-based (nano) composite, nanostructured and hybrid permanent magnets. The work discussed is organized in three major sections (i) isotropic magnets based on the R 2Fe 14B hard phase, (ii) anisotropic magnets based on the R 2Fe 14B hard phase and (iii) magnets with a R-Co hard phase (1:5 and/or 2:17), including ultrahigh temperature Sm-Co magnets, Sm-Co permanent magnets with almost zero reversible temperature coefficient of residual induction for critical applications and ultrahigh temperature Sm-Co magnets with small reversible temperature coefficient of residual induction.

Micro-structure and magnetic properties of Fe–Cu nanocomposites for anisotropic permanent magnets

Melt spun ribbons of Fe x Cu 100− x were obtained by different procedures, in order to maximize the dispersion of the Fe atoms in the Cu matrix. Subsequent thermal and mechanical treatments were used for a controlled crystallization process of the soft magnetic phase. A detailed analysis of structural aspects and crystallization dynamics and their relation with the magnetic behavior of the Fe–Cu ribbons has been obtained via Mössbauer spectroscopy, X-ray diffraction (XRD) and magnetic measurements. In order to obtain composite permanent magnets with shape anisotropy, the optimal conditions and the suitable processing were established.

Composite elastic magnet films with hard magnetic feature

Hard magnetic materials with high remnant magnetic moment, Mr, have unique advantages that can achieve bi-directional (push–pull) movement in an external magnetic field. This paper presents the results on the fabrication and testing of novel composite elastic permanent magnet films. The microsize hard barium ferrite powder, NdFeB powder, and different silicone elastomers have been used to fabricate various large elongation hard magnetic films. Three different fabrication methods, screen-coating processing, moulding processing and squeegee-coating processing, have been investigated, and the squeegee-coating process was proven to be the most successful method. The uniform composite elastic permanent magnet films range from 40 µm to 216 µm in thickness have been successfully fabricated. These films were then magnetized in the thickness direction after fabrication. They exhibited permanent magnet behaviour; for instance, the film (0.640 mm3 in volume) made of polydimethyl siloxane (PDMS) and hard barium ferrite powders is measured to give a coercive force, Hc, of 3.24 × 105 A m−1 and Mr of 1.023 × 10−5 A m2, and the film (0.504 mm3 in volume) made of PDMS and NdFeB powders gives 1.55 × 105 A m−1 Hc and 8.081 × 10−5 A m2 Mr. These composite elastic permanent magnet films' mechanical properties, like Young's modulus and deflection force, have been evaluated. To validate the films' Young's modulus, a finite-element computer simulation (ANSYS®) is used and one film is chosen whose Young's modulus (16.60 MPa) is confirmed by the simulation results with ANSYS®. The large elongation composite elastic permanent magnet film provides an excellent diaphragm material, which plays an important role in the micropump or valve. The movement of the 126 µm thick film with 4.5 mm diameter made of PDMS and NdFeB powders has been tested in a 0.21 Tesla external magnetic field. It was proven to have large deflection of 125 µm.

104 SUS304鋼の疲労き裂近傍における塑性誘起マルテンサイト変態 : 漏洩磁束密度分布の磁場解析(OS06-1 メゾ組織)(OS06 材料の微視組織とその形成過程のモデル化とシミュレーション)

The present study has attempted to simulate the distribution of α'martensite and vertical magnetic flux density B_z around the plastic wake of a growing crack in an austenitic stainless steel SUS304 in which stress-induced martensitic phase transformation is to occur at room temperature. FEM calculations of martensitic phase transformation have been made on a center-cracked plate specimen of SUS 304 stainless steel subjected to monotonically increasing tension load at room temperature. The resultant contour maps of α'martensite around fatigue cracks agree well with those measured by ferrite scope, showing that the distribution of α'martensite changes according to the applied maximum stress intensity factor K_<max> value. FEM calculations also have been made on the vertical component of leakage magnetic flux density above fatigue cracks by using a composite permanent magnet model. The α'transformation region around a fatigue crack is modeled as a composite of permanent magnets having different B-H properties varying with to α'content. The resultant distributions of leakage magnetic flux density above fatigue cracks obtained by FEM agree well with those obtained experimentally.

Importance of ideal grain boundaries of high remanent composite permanent magnets

The excellent magnetic properties of nanoscaled composite magnets can only be obtained by imposing strong requirements on the grain structure. The soft magnetic phase with a maximum extension smaller than 20 nm should be embedded into a hard magnetic environment. Furthermore the grain boundaries should be as ideal as possible. These requirements for the grain structure guarantee an optimal exchange hardening of the soft magnetic phase and therefore a high remanence and a sufficiently high coercivity. In the framework of computational micromagnetism it is possible to investigate the effect of nonideal grain boundaries on the magnetization reversal process of composite magnets in detail. Nonideal grain boundaries can be simulated by intergranular layers with material parameters deviating from their bulk values. It turns out that the remanence and the coercivity decrease approximately linearly with the simultaneous reduction of the exchange and the anisotropy constants near the grain boundaries. For a reduction of these intergranular material parameters smaller than 10% of their initial values, the remanence and coercivity strongly decrease by a factor of about 2.

Unusual magnetic behavior in Nd–Fe–B alloy powder compacts

We report an unusual magnetic behavior in powders prepared by spark-eroding (in liquid Ar) alloy electrodes containing approximately equal weights of Fe and Nd2Fe14B in an effort to prepare composite permanent magnets. Magnetization exhibits reproducible thermal hysteresis, peaking in all applied fields near 520 °C when warming, but increasing monotonically when cooling to room temperature from 700 °C and above. Mössbauer spectroscopy was used to show that the behavior is due to the metastability of Fe1−xO produced in the powders by partial oxidation in Ar gas flow. This compound, paramagnetic at room temperature, decomposes only slowly below 570 °C into ferromagnetic Fe and ferrimagnetic Fe3O4. The reverse reaction occurs readily at higher temperatures.

Grain-size dependence of remanence and coercive field of isotropic nanocrystalline composite permanent magnets

Micromagnetic calculations using a finite element technique are useful to investigate magnetization processes in nanocrystalline ferromagnetic materials. In particular numerical calculations reveal the microstructural conditions required for high remanence and high coercivity isotropic permanent magnets. composite materials of Nd 2Fe 14B and α-Fe are excellent candidates for such high performance permanent magnets. The soft magnetic α-Fe grains cause a large spontaneous magnetization and the hard magnetic grains induce a large coercive field, provided that both phases are strongly exchange coupled. The numerical investigations on realistic three-dimensional grain arrangements suggest an optimal microstructure consisting of small soft magnetic grains ( D ≈ 10 nm, V soft ≈ 40%) embedded between hard magnetic grains with a mean grain diameter of about D ≈ 20 nm. Additionally, a microstructure with regular shaped grains improves the magnetic properties.