Isotropic Magnets Research Articles

Inner loop model predictive control and outer loop PI reference governor for PMSMs with input and state saturation for torque control

This contribution considers a torque control scheme consisting of model predictive control (MPC) in the inner control loop together with PI reference governor in the outer control loop and a decoupling feedforward control for an isotropic permanent magnet synchronous machine (PMSM). This innovative approach is known in literature as PI-MPC dual loop control. A particular emphasis is given to the control governor strategy which is the outer loop PI reference governor and allows to regulate the machine in the flux weakening region and is therefore only active for field weakening. In this context the analysis of the stability based on Lyapunov’ approach of the control loop in flux weakening region is shown. The desired currents represent the reference currents for the MPC, which forms the inner control loop. The MPC is adapted using an extended Kalman filter (EKF), which estimates inductance of the electrical system in dq coordinates by using a bivariate polynomial. Compared measurements with a hardware-in-the-loop (HIL) system show the effectiveness of the proposed control scheme with respect to a standard PI controller in inner loop (PI-PI scheme) in the presence of saturated inputs and state of a PMSM. The proposed MPC uses just an optimal, proportional control and thus avoids windup effects. Measurement results in the presence of input and state saturations show that MPC is working without overshoot in the currents which leads to less needed power in input.

The Role of Interfacial Interactions and Oxygen Vacancies in Tuning Magnetic Anisotropy in LaCrO3/LaMnO3 Heterostructures

AbstractThe interplay of lattice, electronic, and spin degrees of freedom at epitaxial complex oxide interfaces provides a route to tune their magnetic ground states. Unraveling the competing contributions is critical for tuning their functional properties. The relationship between magnetic ordering and magnetic anisotropy and the lattice symmetry, oxygen content, and film thickness in compressively strained LaMnO3 (LMO)/LaCrO3 (LCO) superlattices is investigated. Mn–O–Cr antiferromagnetic superexchange interactions across the heterointerface result in a net ferrimagnetic magnetic structure. Bulk magnetometry measurements reveal isotropic in‐plane magnetism for as‐grown oxygen‐deficient thin samples due to equal fractions of orthorhombic a+a‐c‐, and a‐a+c‐ twin domains. As the superlattice thickness is increased, in‐plane magnetic anisotropy emerges as the fraction of the a+a‐c‐ domain increases. On annealing in oxygen, the suppression of oxygen vacancies results in a contraction of the lattice volume, and an orthorhombic to rhombohedral transition leads to isotropic magnetism independent of the film thickness. The complex interactions are investigated using high‐resolution synchrotron diffraction and X‐ray absorption spectroscopy. These results highlight the role of the evolution of structural domains with film thickness, interfacial spin interactions, and oxygen‐vacancy‐induced structural phase transitions in tuning the magnetic properties of complex oxide heterostructures.

A Multiphysics Thermoelastoviscoplastic Damage Internal State Variable Constitutive Model including Magnetism.

We present a macroscale constitutive model that couples magnetism with thermal, elastic, plastic, and damage effects in an Internal State Variable (ISV) theory. Previous constitutive models did not include an interdependence between the internal magnetic (magnetostriction and magnetic flux) and mechanical fields. Although constitutive models explaining the mechanisms behind mechanical deformations caused by magnetization changes have been presented in the literature, they mainly focus on nanoscale structure-property relations. A fully coupled multiphysics macroscale ISV model presented herein admits lower length scale information from the nanoscale and microscale descriptions of the multiphysics behavior, thus capturing the effects of magnetic field forces with isotropic and anisotropic magnetization terms and moments under thermomechanical deformations. For the first time, this ISV modeling framework internally coheres to the kinematic, thermodynamic, and kinetic relationships of deformation using the evolving ISV histories. For the kinematics, a multiplicative decomposition of deformation gradient is employed including a magnetization term; hence, the Jacobian represents the conservation of mass and conservation of momentum including magnetism. The first and second laws of thermodynamics are used to constrain the appropriate constitutive relations through the Clausius-Duhem inequality. The kinetic framework employs a stress-strain relationship with a flow rule that couples the thermal, mechanical, and magnetic terms. Experimental data from the literature for three different materials (iron, nickel, and cobalt) are used to compare with the model's results showing good correlations.

Modelling the magnetic-mechanical coupled viscoelastic behaviour of transversely isotropic soft magnetorheological elastomers

Soft magnetorheological elastomers (s-MRE) is a kind of smart material mainly fabricated by embedding soft magnetic particles into an elastomer matrix. It can be categorized as isotropic and transversely isotropic, depending on the arrangement of internal magnetic particles. In isotropic s-MRE, magnetic particles are randomly distributed, while transversely isotropic s-MRE forms chain structures for the magnetic particles. Compared with isotropic s-MRE, transversely isotropic s-MRE exhibits more significant magnetically-enhanced mechanical properties, making it highly applicable in the vibration control area. While past theoretical work has mainly focused on the magneto-mechanical coupling behaviour of isotropic s-MRE, less attention has been given to modelling the magneto-mechanical coupling behaviour of transversely isotropic s-MRE. Specifically, understanding the impact of different particle chain-magnetic field spatial locations on the magnetization and magnetic-enhanced viscoelastic behaviour of transversely isotropic s-MRE remains an open topic. To address this research gap, we characterize the quasi-static, viscoelastic and magnetization performance of transversely isotropic s-MRE under different particle chain-magnetic field spatial locations. Subsequently, we develop a novel constitutive model for transversely isotropic s-MRE, integrating magneto-hyperelasticity, magneto-viscoelasticity and magnetization. We then implement the magneto-mechanical coupled model for transversely isotropic s-MRE at a finite element level. Following model calibration and validation, we conduct a case study to demonstrate the model’s ability to predict the magneto-mechanical coupling performance of a transversely isotropic s-MRE-based laminated isolator. This model is a valuable tool for predicting the magneto-mechanical performance of transversely isotropic s-MRE-based smart devices, thereby facilitating the design and advancement of transversely isotropic s-MRE in vibration control.

Production of Permanent Magnets from Recycled NdFeB Powder with Powder Extrusion Moulding

In the last fifteen years, several groups have investigated metal injection moulding (MIM) of NdFeB powder to produce isotropic or anisotropic rare earth magnets of greater geometric complexity than that achieved by the conventional pressing and sintering approach. However, due to the powder’s high affinity for oxygen and carbon uptake, sufficient remanence and coercivity remains difficult. This article presents a novel approach to producing NdFeB magnets from recycled material using Powder Extrusion Moulding (PEM) in a continuous process. The process route uses powder obtained from recycling rare earth magnets through Hydrogen Processing of Magnetic Scrap (HPMS). This article presents the results of tailored powder processing, the production of mouldable feedstock based on a special binder system, and moulding with PEM to produce green and sintered parts. The magnetic properties and microstructures of debinded and sintered samples are presented and discussed, focusing on the influence of filling ratio and challenging processing conditions on interstitial content as well as density and magnetic properties.

The effect of powder shape on the magnetic anisotropy in NdFeB bonded magnets

The advent of NdFeB bonded magnets with freedom of shape design is effective in achieving motor size and weight reductions. In this paper, the NdFeB bonded magnets were produced through calendaring molding, and the effect of powder shape on degree of alignment (DOA) of NdFeB bonded magnets was investigated. The magnetic measurement results demonstrate that platelet-shaped anisotropic Master Quality Authenticated (MQA) powders exhibit a significantly higher DOA compared to sphere-shaped anisotropic HDDR powders. Microstructural analysis reveals the presence of an oriented structure induced by mechanical stress in MQA bonded magnets, resulting in magnetic anisotropy. This observation is consistent with the difference in X-ray diffraction (XRD) patterns between the cross-section and surface of MQA bonded magnets. Conversely, spherical HDDR particles display minimal orientation and maintain a random distribution, resulting in magnetic isotropy. The XRD pattern of the cross-section of HDDR bonded magnets closely resembles that of its surface. In summary, our findings highlight the superior potential of platelet-shaped anisotropic MQA powders for achieving enhancing magnetic properties during the calendaring molding process, in contrast to sphere-shaped anisotropic HDDR powders. This study provides valuable insights into the determinants of mechanical particle orientation during the fabrication of anisotropic NdFeB bonded magnets, with implications for the development of high-performance bonded magnets.

Optimization of Ca–Al–Mn–Si Substitution Level for Enhanced Magnetic Properties of M-Type Sr-Hexaferrites for Permanent Magnet Application

Enhanced hard magnetic properties were achieved in M-type hexaferrite by optimizing the substitution levels of Mn, Al, and Si for Fe, and Ca for Sr within SrFe12O19. The addition of Al–Si–Mn effectively controlled crystallite growth, resulting in an increased coercivity (HC), while causing a decrease in the remanent magnetization (4πMr). A higher Ca content exhibited a trend of increasing the sintered density but decreasing the 4πMr and HC. The optimized composition, considering both the 4πMr and HC, was determined to be Sr0.8Ca0.2Fe10.2Mn0.1Al0.2Si0.1O19−d, with a sintered density of 4.84 g/cm3, 4πMr = 2.22 kG, and HC = 5.10 kOe. This result demonstrates the achievement of isotropic magnets with controlled crystal growth and densification without additional sintering additives. This development is promising, as this enhancement could be achieved without the use of cobalt, an expensive but essential ingredient in high-performance permanent magnets.

Singular points of the isotropic Non-Heisenberg magnet

The symmetry properties of an isotropic non-Heisenberg magnet with the values of the magnetic ion spin S = 1 or S = 3/2 are studied in this work. It is shown that the systems are invariant under the transformation group SO(3) or SU(2). The conditions under which magnets with spin S = 1 or S = 3/2 are invariant with respect to the transformation groups SU(2S + 1) are determined. The study of the dynamic properties of the magnets under consideration has shown that at SU(2S + 1) points the systems become two-level, similarly to systems with magnetic ion spin S = 1/2.

Quantitative comparison of low-cost electrical machines with soft magnetic composite and ferrite magnet materials

Because of its special structure and manufacturing process, the soft magnetic composite (SMC) material has shown its advantages over silicon sheets, including magnetic and thermal isotropy, low high-frequency core loss, low material wastes during its manufacturing process, etc.. Considering the price of SMC raw material can be very cheap in the future, it is an ideal material for developing the low-cost electrical machine. Combined with SMC cores and low-cost ferrite magnets, some electrical machines with SMC and ferrite magnets for low-cost applications were developed in the past decades, which include the transverse flux flux switched permanent magnet motor (TFFSPMM), axial flux claw pole permanent magnet motor (AFCPM), axial flux vernier motor (AFVM), dual rotor axial flux permanent magnet motor (DRAFM) and axial radial flux permanent magnet motor (RAFM). To better utilize SMC material, all these machines are designed following the design guidelines of SMC machines. Though both of them with good performance for low-cost applications, it is necessary to know the main advantages of each machine. The main target of this paper is to compare these machines quantitatively. For obtaining the comparison results fairly all these machines are optimized by using the sequential robust Taguchi method. Finally, it can be seen that the AFVM and TFFSPMM are with relatively high torque ability and the DRAFM is with high efficiency and power factor ability.

Breaking through the Mermin-Wagner limit in 2D van der Waals magnets

The Mermin-Wagner theorem states that long-range magnetic order does not exist in one- (1D) or two-dimensional (2D) isotropic magnets with short-ranged interactions. Here we show that in finite-size 2D van der Waals magnets typically found in lab setups (within millimetres), short-range interactions can be large enough to allow the stabilisation of magnetic order at finite temperatures without any magnetic anisotropy. We demonstrate that magnetic ordering can be created in 2D flakes independent of the lattice symmetry due to the intrinsic nature of the spin exchange interactions and finite-size effects. Surprisingly we find that the crossover temperature, where the intrinsic magnetisation changes from superparamagnetic to a completely disordered paramagnetic regime, is weakly dependent on the system length, requiring giant sizes (e.g., of the order of the observable universe ~ 1026 m) to observe the vanishing of the magnetic order as expected from the Mermin-Wagner theorem. Our findings indicate exchange interactions as the main ingredient for 2D magnetism.

Assembly-induced spin transfer and distance-dependent spin coupling in atomically precise AgCu nanoclusters

Nanoparticle assembly paves the way for unanticipated properties and applications from the nanoscale to the macroscopic world. However, the study of such material systems is greatly inhibited due to the obscure compositions and structures of nanoparticles (especially the surface structures). The assembly of atomically precise nanoparticles is challenging, and such an assembly of nanoparticles with metal core sizes strictly larger than 1 nm has not been achieved yet. Here, we introduced an on-site synthesis-and-assembly strategy, and successfully obtained a straight-chain assembly structure consisting of Ag77Cu22(CHT)48 (CHT: cyclohexanethiolate) nanoparticles with two nanoparticles separated by one S atom, as revealed by mass spectrometry and single crystal X-ray crystallography. Although Ag77Cu22(CHT)48 bears one unpaired shell-closing electron, the magnetic moment is found to be mainly localized at the S linker with magnetic isotropy, and the sulfur radicals were experimentally verified and found to be unstable after disassembly, demonstrating assembly-induced spin transfer. Besides, spin nanoparticles are found to couple and lose their paramagnetism at sufficiently short inter-nanoparticle distance, namely, the spin coupling depends on the inter-nanoparticle distance. However, it is not found that the spin coupling leads to the nanoparticle growth.

Calculation of Three-dimensional Energy Product for Isotropic Nd2Fe14B Magnet

A conventional energy product calculated by the product of the B-field and the H-field is not sufficient for representing the performance of a magnet because it considers the homogeneous and only the uniaxial magnetic properties of the magnet. The conventional energy product has been compared with another energy product obtained by integrating the scalar product of the B-field and the H-field of each cell composed of the three-dimensional components. We investigated a model system by micromagnetic simulation using finite differential method (FDM) and calculated the full hysteresis of the magnet. The model system of a Nd2Fe14B magnet composed of grains with a diameter of about 100 nm was assumed. In the case of the isotropic multi-grain magnet, the energy product calculated by the integration method was 28% larger than the energy product obtained by the conventional way, although a discrepancy between the distribution of the magnetizations and the demagnetizing fields at the reversal process resulted in the decrease of the energy product.

Homotopy transitions and 3D magnetic solitons

This work provides a concept for three-dimensional magnetic solitons based on mapping the homotopy path between various two-dimensional solutions onto the third spatial axis. The representative examples of statically stable configurations of that type in the model of an isotropic chiral magnet are provided. Various static and dynamic properties of such three-dimensional magnetic solitons are discussed in detail.

Chiral standing spin waves in skyrmion lattice

This work studies the resonance excitations of the three-dimensional skyrmions lattice in the finite thickness plate of an isotropic chiral magnet using spin dynamics simulations. We found that the absorption spectra and resonance modes differ from those predicted by the two-dimensional model and the model of the unconfined bulk crystal. The features observed on the spectra can be explained by the formation of chiral standing spin waves, which, contrary to conventional standing spin waves, are characterized by the helical profile of dynamic magnetization of fixed chirality that is defined by the Dzyaloshinskii–Moriya interaction. In this case, the dynamic susceptibility becomes a function of the plate thickness, which gives rise to an interesting effect that manifests itself in periodical fading of the intensity of corresponding modes and makes excitation of these modes impossible at specific thicknesses.

Skyrmion–antiskyrmion pair creation and annihilation in a cubic chiral magnet

A fundamental property of particles and antiparticles (such as electrons and positrons, respectively) is their ability to annihilate one another. A similar behaviour is predicted for magnetic solitons1—localized spin textures that can be distinguished by their topological index Q. Theoretically, magnetic topological solitons with opposite values of Q, such as skyrmions2 and their antiparticles (namely, antiskyrmions), are expected to be able to continuously merge and annihilate3. However, experimental verification of such particle–antiparticle pair production and annihilation processes has been lacking. Here we report the creation and annihilation of skyrmion–antiskyrmion pairs in an exceptionally thin film of the cubic chiral magnet of B20-type FeGe observed using transmission electron microscopy. Our observations are highly reproducible and are fully consistent with micromagnetic simulations. Our findings provide a new platform for the fundamental studies of particles and antiparticles based on magnetic solids and open new perspectives for practical applications of thin films of isotropic chiral magnets.

Swiveling Magnetization for Anisotropic Magnets for Variable Flux Spoke-Type Permanent Magnet Motor Applied to Electric Vehicles

This paper investigates the application of anisotropic low-coercive force (LCF) magnets to a novel variable-flux spoke-type permanent magnet synchronous motor (VFS-PMSM) for electrical vehicles with a wide speed range. In the VFS-PMSM, flux is regulated by swiveling the magnetization of the anisotropic LCF magnets instead of directly magnetizing or demagnetizing them. The previously proposed VFS-PMSM uses only isotropic LCF magnets for easily swiveling the magnetic pole direction, resulting in lower torque density. The challenge thus lies in the feasibility to swivel the magnetic pole direction of the anisotropic LCF magnet, and the impact of the different magnetization strengths of the anisotropic magnets on the motor performance. This paper first studies the feasibility to swivel the magnetization direction of anisotropic LCF magnets through experiments. It is confirmed that the magnetization direction can be successfully swiveled by 90 degrees with a reduced external magnetizing field. Then, two VFS-PMSM topologies and various rotor configurations are compared in terms of key performance indices to determine critical sizing factors for performance enhancement. Finite element analysis is used for simulations. In comparison with the VFS-PMSM equipped with isotropic LCF magnets, the maximum torque of the proposed topology can be improved for the same flux adjustment ability. Alternatively, the flux adjustment ability can also be enhanced by 37.43% for the same maximum torque.

An Investigation Into Magnetic Properties, Sintering Shrinkage Characteristics, and Formability of Isotropic Barium Ferrite Permanent Magnets

In this study, traditional ceramic process was used to prepare isotropic barium-based ferrite permanent magnets. The lower grade iron oxide powder (Fe₂O₃) manufactured by China Steel Corporation (CSC) was used as the main raw material to prepare calcined magnetic powders with different Fe₂O₃/BaCO₃ mole ratios (5.3&#x2013;5.65). It can be seen from the experimental results that the remanence (<inline-formula> <tex-math notation="LaTeX">$B_{\mathrm {r}}$ </tex-math></inline-formula>) decreases with the increase of mole ratio, but the intrinsic coercivity (<inline-formula> <tex-math notation="LaTeX">$_{\mathrm {i}}H_{\mathrm {c}}$ </tex-math></inline-formula>) shows the opposite trend. On the other hand, the sintering radial shrinkage rate (Sh-D) and sintering height shrinkage rate (Sh-H) both decrease as mole ratio increases, but the required molding pressure is higher. On condition of Fe₂O₃/BaCO₃ mole ratio &#x003D; 5.4, Fe₂O₃ &#x003D; 81.38 wt&#x0025;, BaCO₃ &#x003D; 18.62 wt&#x0025;, the magnetic properties of the final sintered magnet can reach: <inline-formula> <tex-math notation="LaTeX">$B_{\mathrm {r}}$ </tex-math></inline-formula> &#x003E; 2250 G, <inline-formula> <tex-math notation="LaTeX">$_{\mathrm {b}}H_{\mathrm {c}}$ </tex-math></inline-formula> &#x003E; 1950 Oe, <inline-formula> <tex-math notation="LaTeX">$_{\mathrm {i}}H_{\mathrm {c}}$ </tex-math></inline-formula> &#x003E; 3300 Oe, (<i>BH</i>)_max &#x003E; 1.15 MGOe, which are obtained on the basis without expensive lanthanum and cobalt. In terms of the operability, the sintering shrinkage rate Sh-D &#x003C; 15.1&#x0025; and required molding pressure of magnetic powder &#x003C; 2.5 ton/cm² both meet the specifications of the permanent magnet industry. Above results successfully overcome high-cost technical bottlenecks in the past in order to achieve high characteristics and formability, and comprehensively improve magnetic properties, operability, and production efficiency. The low-cost isotropic barium-based ferrite permanent magnets with good magnetic properties and operability realized in this work have been practically used in high-efficiency small automobiles motors, such as electric rearview mirror motor.

Absence of Normal Fluctuations in an Integrable Magnet.

We investigate dynamical fluctuations of transferred magnetization in the one-dimensional lattice Landau-Lifshitz magnet with uniaxial anisotropy, representing an emblematic model of interacting spins. We demonstrate that the structure of fluctuations in thermal equilibrium depends radically on the characteristic dynamical scale. In the ballistic regime, typical fluctuations are found to follow a normal distribution and scaled cumulants are finite. In stark contrast, on the diffusive and superdiffusive timescales, relevant, respectively, for the easy-axis and isotropic magnet at vanishing total magnetization, typical fluctuations are no longer Gaussian and, remarkably, scaled cumulants are divergent. The observed anomalous features disappear upon breaking integrability, suggesting that the absence of normal fluctuations is intimately tied to the presence of soliton modes. In a nonequilibrium setting of the isotropic magnet with weakly polarized step-profile initial state we find a slow drift of dynamical exponent from the superdiffusive towards the diffusive value.

Influence of divalent metal ion (Zn2+) on magnetic properties, curie temperature and DC electrical properties in Ce3+ substituted Ni-Zn ferrites

Nanoparticles of Ni1−xZnxCe0.1Fe1.9O4 ferrite substituted with Zn2+ ion concentrations (0.0, 0.2, 0.4, 0.6, 0.8 & 1.0) have been synthesised via aqueous citrate precursor auto combustion method. The obtained Powder x-ray diffraction data suggests a good crystalline phase; the crystallite size exists in the range of 14 ∼ 38 nm. The lattice constant of samples increases with Zn2+ concentration. It validates Vegard’s law and the decreasing trends in porosity of the samples were identified. The inhomogeneity in the grains was confirmed from FE-SEM. The EDS spectrogram attributes the good stoichiometry in the product. The Thermogravimetric analysis reveals that the crystallization occurred within the temperature 800 °C. The high-temperature DC conductivity of the samples shows that NTC behaviour. The Curie temperature and activation energy are estimated and the activation energy of carriers are found to be more at the paramagnetic region. The magnetic saturation (‘Ms*’) has maximum for Zn2+ = 0.2 (57.7 emu g−1) and coercive field (Hc) related with a radius of occupancy of Zn2+ ion validates the relation and also, the value of the remanence ratio suggests that isotropic magnetization took place in the heterogeneous material.

Microstructure and Magnetic Properties Evolution of Isotropic Nanocrystalline NdFeB Hot-Pressed Magnets With CeCu Addition

Bulk dense anisotropic rare earth (RE)-Fe-B magnets synthesized by hot pressing and subsequent hot deforming melt-spun powders have attracted much attention because of their good all-around properties. It is well known that the formation of the desired crystallographic texture is critical to obtain good magnetic properties in the preparation process of anisotropic RE-Fe-B magnets. Melt-spun ribbons with nominal compositions of Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">78</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> and Ce <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> were consolidated into isotropic nanocrystalline NdFeB hot-pressed magnets by the spark plasma sintering technique, and the microstructure and magnetic properties of the magnets were investigated. Results demonstrate that the coercivity of the magnets is enhanced dramatically by the grain boundary diffusion of the low-melting-point CeCu alloy. With 10 wt% CeCu addition, the coercivity increases from 7.5 to 12.44 kOe. The excessive CeCu, however, reduces the remanence of the NdFeB magnets from 7.76 to 5.66 kGs. Moreover, an appropriate holding time effectively improves the magnetic properties of the magnets. The microstructure indicates that the main phase NdFeB with a grain thickness of 20 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> , and the rest are Nd-rich phase and CeCu grain boundary phase. The intergranular phase of CeCu acts as a diffusion channel, which improves the diffusion efficiency and the uniform distribution of elements. The magnets sintered by spark plasma form a special two-zone structure with a coarse grain region and a fine grain region, and the CeCu alloy effectively inhibits the growth of the grains.