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2021-09-21
Azhari, Budi, Yazid, Edwar, Devi, Merry Indahsari.  2020.  Dynamic Inductance Simulation of a Linear Permanent Magnet Generator Under Different Magnet Configurations. 2020 International Conference on Sustainable Energy Engineering and Application (ICSEEA). :1–8.
Recently, some innovations have been applied to the linear permanent magnet generator (LPMG). They are including the introduction of high-remanence rare-earth magnets and the use of different magnet configurations. However, these actions also affect the flow and distribution of the magnetic flux. Under the load condition, the load current will also generate reverse flux. The flux resultant then affects the coil parameters; the significant one is the coil inductance. Since it is influential to the output voltage and output power profiles, the impact study of the permanent magnet settings under load condition is essential. Hence this paper presents the inductance profile study of the LMPG with different magnet configurations. After presenting the initial designs, several magnet settings including the material and configuration were varied. Finite element magnetic simulation and analytical calculations were then performed to obtain the inductance profile of the LPMG. The results show that the inductance value varies with change in load current and magnet position. The different magnet materials (SmCo 30 and N35) do not significantly affect the inductance. Meanwhile, different magnet configuration (radial, axial, halbach) results in different inductance trends.
Wang, Yuzheng, Jimenez, Beatriz Y., Arnold, David P..  2020.  \$100-\textbackslashtextbackslashmu\textbackslashtextbackslashmathrmm\$-Thick High-Energy-Density Electroplated CoPt Permanent Magnets. 2020 IEEE 33rd International Conference on Micro Electro Mechanical Systems (MEMS). :558–561.
This paper reports electroplated CoPt permanent magnets samples yielding thicknesses up to 100 μm, deposition rates up to 35 μm/h, coercivities up to 1000 kA/m (1.25 T), remanences up to 0.8 T, and energy products up to 77 kJ/m3. The impact of electroplating bath temperature and glycine additives are systematically studied. Compared to prior work, these microfabricated magnets not only exhibit up to 10X increase in thickness without sacrificing magnetic performance, but also improve the areal magnetic energy density by 2X. Using a thick removeable SU-8 mold, these high-performing thick-film magnets are intended for magnetic microactuators, magnetic field sensors, energy conversion devices, and more.
Wang, Meng, Zhao, Shengsheng, Zhang, Xiaolong, Huang, Changwei, Zhu, Yi.  2020.  Effect of La addition on structural, magnetic and optical properties of multiferroic YFeO3 nanopowders fabricated by low-temperature solid-state reaction method. 2020 6th International Conference on Mechanical Engineering and Automation Science (ICMEAS). :242–246.
Nanosize multiferroic La-doped YFeO3 powders are harvested via a low-temperature solid-state reaction method. X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectra analysis reveal that with La addition, YFeO3 powders are successfully fabricated at a lower temperature with the size below 60 nm, and a refined structure is obtained. Magnetic hysteresis loop illustrates ferromagnetic behavior of YFeO3 nano particles can be enhanced with La addition. The maximum and remnant magnetization of the powders are about 4.03 and 1.22 emu/g, respectively. It is shown that the optical band gap is around 2.25 eV, proving that La doped YFeO3 nano particles can strongly absorb visible light. Both magnetic and optical properties are greatly enhanced with La addition, proving its potential application in magnetic and optical field.
2021-08-31
Won, Hoyun, Hong, Yang-Ki, Choi, Minyeong, Yoon, Hwan-sik, Li, Shuhui, Haskew, Tim.  2020.  Novel Efficiency-shifting Radial-Axial Hybrid Interior Permanent Magnet Sychronous Motor for Electric Vehicle. 2020 IEEE Energy Conversion Congress and Exposition (ECCE). :47–52.
A novel efficiency-shifting radial-axial hybrid permanent magnet synchronous motor that can realize two high-efficiency regions at low and high speeds is developed to extend the maximum driving distance and track the reference speed more accurately for electric vehicle application. The motor has two stators, which are radial and axial, to rotate one shared rotor. The rotor employs two combined topologies, i.e., inner surface-inset-mounted and outer V-shaped interior-mounted. For both outer and inner permanent magnets, Nd-Fe-B, having the remanent flux density of 1.23 T and coercivity of 890 kA/m, is used. The simulation result shows that the designed motor exhibits not only high maximum torque of 400 Nm and the maximum speed of 18,000 rpm but also two high-efficiency regions of 97.6 % and 92.0 % at low and high speed, respectively. Lastly, the developed motor shows better performance than corresponding separated radial and axial permanent magnet motor.
Castro-Coronado, Habib, Antonino-Daviu, Jose, Quijano-López, Alfredo, Fuster-Roig, Vicente, Llovera-Segovia, Pedro.  2020.  Evaluation of the Detectability of Damper Cage Damages in Synchronous Motors through the Advanced Analysis of the Stray Flux. 2020 IEEE Energy Conversion Congress and Exposition (ECCE). :2058–2063.
The determination of the damper cage health is a matter of great importance in those industries that use large synchronous motors in their processes. In the past, unexpected damages of that element implied economic losses amounting up to several million \$. The problem is that, in the technical literature, there is a lack of non-invasive techniques enabling the reliable condition monitoring of this element. This explains the fact that, in industry, rudimentary methods are still employed to determine its condition. This paper proposes the analysis of the stray flux as a way to determine the condition of the damper cage. The paper shows that the analysis of the stray flux under starting yields characteristic time-frequency signatures of the fault components that can be used to reliably determine the condition of the damper. Moreover, the analysis of the stray flux at steady-state operation under asynchronous mode could give useful information to this end. The paper also analyses the influence of the remanent magnetism in the rotor of some synchronous motors, which can make the damper cage diagnosis more difficult; some solutions to this problem are also suggested in the paper.
Nonprivun, Choktawee, Plangklang, Boonyang.  2020.  Study and Analysis of Flux Linkage on 12/8 pole Doubly Salient Permanent Magnet Machine in Square Envelope. 2020 International Conference on Power, Energy and Innovations (ICPEI). :141–144.
This paper presents a study and analysis of flux linkage performance on 12/8 pole doubly salient permanent magnet machine in square envelope conventional. Analyzed model was using a finite element method. The investigated model was constructed by changing the size of the structure as the main parameters of the speed 500 rpm, PM coercivity 910 kA/m, PM remanence 1.2 T, copper loss 30 W, turns per coil 45, and stator side length 100 mm. The study and analysis of flux linkage, induced voltage, and torque are also included in this paper.
Yang, Jiahui, Yuan, Yao, Wang, Shuaibing, Bao, Lianwei, Wang, Ren.  2020.  No-load Switch-in Transient Process Simulation of 500kV Interface Transformer Used in HVDC Flexible. 2020 IEEE International Conference on High Voltage Engineering and Application (ICHVE). :1–4.
Interface transformer used in asynchronous networking was a kind of special transformer which's different from normal power transformer. During no-load switch-in, the magnitude of inrush current will be high, and the waveform distortion also be severity. Maybe the protections will be activated, even worse may lead the lockdown of the DC system. In this paper, field-circuit coupled finite element method was used for the study of transient characteristic of no-load switch-in, remanence simulation methods were presented. Quantitative analysis of the effect of closing making angle and core remanence on inrush current peak value, meanwhile, the distribution of magnetic field inside the tank during the transient process. The result indicated that the closing making angle and core remanence have obvious effect on inrush current peak value. The research results of this paper can be used to guide the formulation of no-load switch-in strategy of interface transformer, which was of great significance to ensure the smooth operation of HVDC Flexible system.
Murai, Toshiya, Shoji, Yuya, Nishiyama, Nobuhiko, Mizumoto, Tetsuya.  2020.  Magneto-Optical Isolator and Self-Holding Optical Switch Integrated with Thin-Film Magnet. 2020 Conference on Lasers and Electro-Optics (CLEO). :1–2.
Novel magneto-optical isolator and self-holding optical switch with an a-Si:H microring resonator are demonstrated. The devices are driven by the remanence of integrated thin-film magnet and, therefore, maintain their state without any power supply.
Loreto, Jayson, Gerasta, Olga Joy L., Gumera, Aileen C..  2020.  Residual Current Circuit Implemented in Complementary Metal Oxide Semiconductor for Remanence Correction. 2020 IEEE 12th International Conference on Humanoid, Nanotechnology, Information Technology, Communication and Control, Environment, and Management (HNICEM). :1–6.
This research paper presented a design that will address the challenges brought by remanence in ground-fault current interrupter devices (gfci). Remanence or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Remanence will make the gfci devices less accurate and less reliable in tripping the current above threshold in just five (5) years. It affects the performance of the device in terms of efficiency, accuracy, and response time. In this research, the problems caused by remanence were alleviated by using two identical transformers in detecting residual current both for hot and neutral wires. The difference of the current detected by the two transformers will be the basis of the signal threshold in tripping the device. By doing so, the problems caused by remanence phenomenon will be solved without compromising the response time of the circuit which is around 16 mS. The design will extend the life span of GFCI devices up to 15 years.
2020-11-30
Procházka, P..  2019.  Impulse Exciter of Rotating Blades With an Increased Excitation Force. IEEE Transactions on Instrumentation and Measurement. 68:300–302.
This paper deals with electromagnetic vibration excitation of rotating machine blades in a purpose of dynamic measurement and testing. A novel method for increasing the excitation force is presented. At the same time, the suggested method allows to reduce blade remanent induction. Examples of measurements are shown, and obtained results are discussed.
Machida, H., Fujiwara, T., Fujimoto, C., Kanamori, Y., Tanaka, J., Takezawa, M..  2019.  Magnetic Domain Structures and Magnetic Properties of Lightly Nd-Doped Sm–Co Magnets With High Squareness and High Heat Resistance. IEEE Transactions on Magnetics. 55:1–4.
The relationship between magnetic domain structures and magnetic properties of Nd-doped Sm(Fe, Cu, Zr, Co)7.5 was investigated. In the preparation process, slow cooling between sintering and solution treatment was employed to promote homogenization of microstructures. The developed magnet achieved a maximum energy product, [BH]m, of 33.8 MGOe and coercivity, Hcb, of 11.2 kOe at 25 °C, respectively. Moreover, B-H line at 150 °C was linear, which means that irreversible demagnetization does not occur even at 150 °C. Temperature coefficients of remanent magnetic flux density, Br, and intrinsic coercivity, Hcj, were 0.035%/K and 0.24%/K, respectively, as usual the conventional Sm-Co magnet. Magnetic domain structures were observed with a Kerr effect microscope with a magnetic field applied from 0 to -20 kOe, and then reverse magnetic domains were generated evenly from grain boundaries. Microstructures referred to as “cell structures” were observed with a scanning transmission electron microscope. Fe and Cu were separated to 2-17 and 1-5 phases, respectively. Moreover, without producing impurity phases, Nd showed the same composition behavior with Sm in a cell structure.
Li, X., Deng, M., Wang, X., Li, H., Yu, M..  2019.  Synthesis and magnetic properties of Fe-doped CdS nanorods. Micro Nano Letters. 14:275–279.
Hexagonal CdS and Fe-doped CdS nanorods were synthesised by a facile hydrothermal method and characterised by X-ray diffraction, energy dispersive X-ray spectroscopy, UV-vis absorption, photoluminescence, and X-ray photoelectron spectroscopy. The magnetic properties of undoped and Fe-doped CdS nanorods were investigated at room temperature. The experimental results demonstrate that the ferromagnetism of the Fe-doped CdS nanorods differs from that of the undoped CdS nanorods. The remanence magnetisation (Mr) and the coercive field (Hc) of the Fe-doped CdS nanorods were 4.9 × 10-3 emu/g and 270.6 Oe, respectively, while photoluminescence properties were not influenced by doping. First-principle calculations show that the ferromagnetism in Fe-doped CdS nanocrystal arose not only from the Fe dopants but also from the Cd vacancies, although the main contribution was due to the Fe dopants.
Anyfantis, D. I., Sarigiannidou, E., Rapenne, L., Stamatelatos, A., Ntemogiannis, D., Kapaklis, V., Poulopoulos, P..  2019.  Unexpected Development of Perpendicular Magnetic Anisotropy in Ni/NiO Multilayers After Mild Thermal Annealing. IEEE Magnetics Letters. 10:1–5.
We report on the significant enhancement of perpendicular magnetic anisotropy of Ni/NiO multilayers after mild annealing up to 90 min at 250 °C. Transmission electron microscopy shows that after annealing, a partial crystallization of the initially amorphous NiO layers occurs. This turns out to be the source of the anisotropy enhancement. Magnetic measurements reveal that even multilayers with Ni layers as thick as 7 nm, which in the as-deposited state showed inplane anisotropy with square hysteresis loops, show reduced in-plane remanence after thermal treatment. Hysteresis loops recorded with the field in the normal-to-film-plane direction provide evidence for perpendicular magnetic anisotropy with up and down magnetic domains at remanence. A plot of effective uniaxial magnetic anisotropy constant times individual Ni layer thickness as a function of individual Ni layer thickness shows a large change in the slope of the data attributed to a drastic change of volume anisotropy. Surface anisotropy showed a small decrease because of some layer roughening introduced by annealing.
Beran, P., Klöhn, M., Hohe, H..  2019.  Measurement Characteristics of Different Integrated Three-Dimensional Magnetic Field Sensors. IEEE Magnetics Letters. 10:1–5.
Datasheets of different commercially available integrated sensors for vector measurements of magnetic fields provide typical specifications, such as measurement range, sampling rate, resolution, and noise. Other characteristics of interest, such as linearity, cross-sensitivity, remanent magnetization, and drifts over temperature, are mostly missing. This letter presents testing results of those characteristics of integrated three-dimensional (3-D) sensors working with different sensor principles and technologies in a reproducible measuring process. The sensors are exposed to temperatures from -20 °C to 80 °C and are cycled in hysteresis loops in fields up to 2.5 mT. For applying high-accuracy magnetic fields, a calibrated 3-D Helmholtz coil setup is used. Commercially available integrated 3-D magnetic field sensors are put in operation on a printed circuit board using nonmagnetic passive components. All sensors are configured for best measurement accuracy according to their data-sheets. The results show that sensors based on anisotropic magnetoresistance have high accuracy and low offsets yet also a high degree of nonlinearity. Hall-based sensors show good linearity but also high cross-sensitivity. A magnetic remanence appears for Hall-based sensors with integrated magnetic concentrators as well as for sensors using anisotropic magnetoresistance. Nearly all sensors show remaining drifts over temperature regarding offset and sensitivity up to several percentages.
Hsu, W., Victora, R. H..  2019.  Micromagnetic Study of Media Noise Plateau in Heat-Assisted Magnetic Recording. IEEE Transactions on Magnetics. 55:1–4.
The relationship between integrated media noise power and linear density in heat-assisted magnetic recording (HAMR) is discussed. A noise plateau for intermediate recording density has been observed in HAMR, similar to that found in perpendicular magnetic recording (PMR). Here, we show, by changing the temperature profile of the heat spot in HAMR, that we can tune the noise plateau regions to different recording densities. The heat spot with sharp temperature gradient favors a plateau at high recording density, while the heat spot with gradual temperature gradient favors a plateau at low recording density. This effect is argued to be a consequence of the competition between transition noise and remanence noise in HAMR.
Guerra, Y., Peña-Garcia, R., Padrón-Hernández, E..  2019.  Remanence State and Coercivity in 1-D Chain of Polycrystalline Hollow Cobalt Nanospheres. IEEE Transactions on Magnetics. 55:1–5.
In this paper, we present a study about the remanence state and coercivity in 1-D chain of cobalt hollow nanospheres, by using micromagnetic simulation. The high coercivity values (Hc is determined in the range of 600-1800 Oe) and the monotonic decrease of remanence are attributed to the shape anisotropy effect due to an increase in the aspect ratio value. The configuration of magnetization in remanence showed the onion state for hollow spheres (HSs) with Re = 15 nm, whereas for Re = 30 nm, appear the curling-vortex (CV) state. Finally for a cluster of chains, constituted by cobalt HSs, with random orientations the CV state is preserved.
Guan, L., Lin, J., Ma, Z., Luo, B., Xia, L., Jing, J..  2018.  Copker: A Cryptographic Engine Against Cold-Boot Attacks. IEEE Transactions on Dependable and Secure Computing. 15:742–754.
Cryptosystems are essential for computer and communication security, e.g., RSA or ECDSA in PGP Email clients and AES in full disk encryption. In practice, the cryptographic keys are loaded and stored in RAM as plain-text, and therefore vulnerable to cold-boot attacks exploiting the remanence effect of RAM chips to directly read memory data. To tackle this problem, we propose Copker, a cryptographic engine that implements asymmetric cryptosystems entirely within the CPU, without storing any plain-text sensitive data in RAM. Copker supports the popular asymmetric cryptosystems (i.e., RSA and ECDSA), and deterministic random bit generators (DRBGs) used in ECDSA signing. In its active mode, Copker stores kilobytes of sensitive data, including the private key, the DRBG seed and intermediate states, only in on-chip CPU caches (and registers). Decryption/signing operations are performed without storing any sensitive information in RAM. In the suspend mode, Copker stores symmetrically-encrypted private keys and DRBG seeds in memory, while employs existing solutions to keep the key-encryption key securely in CPU registers. Hence, Copker releases the system resources in the suspend mode. We implement Copker with the support of multiple private keys. With security analyses and intensive experiments, we demonstrate that Copker provides cryptographic services that are secure against cold-boot attacks and introduce reasonable overhead.
Gerdroodbari, Y. Z., Davarpanah, M., Farhangi, S..  2018.  Remanent Flux Negative Effects on Transformer Diagnostic Test Results and a Novel Approach for Its Elimination. IEEE Transactions on Power Delivery. 33:2938–2945.
Influence of remanent flux on hysteresis curve of the transformer core is addressed in this paper. In addition, its significant negative effect on transformer diagnostic tests is quantified based on experimental studies. Furthermore, a novel approach is proposed to efficiently and quickly eliminate the remanent flux. This approach is evaluated based on simulation studies on a 230/63-kV power transformer. Meanwhile, experimental studies are performed on both 0.2/0.2 and 20/0.4 kV transformers. These studies reveal that the approach not only is well able to eliminate the remanent flux, but also it has various advantages over the commonly used method. In addition, this approach is equally applicable for various power, distribution, and instrument transformer types.
Chen, Z., Bai, B., Chen, D., Chai, W..  2018.  Design of Distribution Devices for Smart Grid Based on Magnetically Tunable Nanocomposite. IEEE Transactions on Power Electronics. 33:2083–2099.
This paper designs three distribution devices for the smart grid, which are, respectively, novel transformer with dc bias restraining ability, energy-saving contactor, and controllable reactor with adjustable intrinsic magnetic state based on the magnetically tunable nanocomposite material core. First, the magnetic performance of this magnetic material was analyzed and the magnetic properties processing method was put forward. One kind of nanocomposite which is close to the semihard magnetic state with low coercivity and high remanence was attained. Nanocomposite with four magnetic properties was processed and prepared using the distribution devices design. Second, in order to adjust the magnetic state better, the magnetization and demagnetization control circuit based on the single-phase supply power of rectification and inverter for the nanocomposite magnetic performance adjustment has been designed, which can mutual transform the material's soft and hard magnetic phases. Finally, based on the nanocomposite and the control circuit, a novel power transformer, an energy-saving contactor, and a magnetically controllable reactor were manufactured for the smart grid. The maintained remanence of the nanocomposite core after the magnetization could neutralize the dc bias magnetic flux in the transformer main core without changing the transformer neutral point connection mode, could pull in the contactor movable core instead of the traditional electromagnetic-type fixed core, and could adjust the reactor core saturation degree instead of the traditional electromagnetic coil. The simulation and experimental results verify the correctness of the design, which provides reliable, intelligent, interactive, and energy-saving power equipment for the smart power grids safe operation.
Wang, Y., Huang, F., Hu, Y., Cao, R., Shi, T., Liu, Q., Bi, L., Liu, M..  2018.  Proton Radiation Effects on Y-Doped HfO2-Based Ferroelectric Memory. IEEE Electron Device Letters. 39:823–826.
In this letter, ferroelectric memory performance of TiN/Y-doped-HfO2 (HYO)/TiN capacitors is investigated under proton radiation with 3-MeV energy and different fluence (5e13, 1e14, 5e14, and 1e15 ions/cm2). X-ray diffraction patterns confirm that the orthorhombic phase Pbc21 of HYOfilm has no obvious change after proton radiation. Electrical characterization results demonstrate slight variations of the permittivity and ferroelectric hysteresis loop after proton radiation. The remanent polarization (2Pr) of the capacitor decreases with increasing proton fluence. But the decreasing trend of 2Pr is suppressed under high electric fields. Furthermore, the 2Pr degradation with cycling is abated by proton radiation. These results show that the HYO-based ferroelectric memory is highly resistive to proton radiation, which is potentially useful for space applications.
Peng, Y., Yue, M., Li, H., Li, Y., Li, C., Xu, H., Wu, Q., Xi, W..  2018.  The Effect of Easy Axis Deviations on the Magnetization Reversal of Co Nanowire. IEEE Transactions on Magnetics. 54:1–5.
Macroscopic hysteresis loops and microscopic magnetic moment distributions have been determined by 3-D model for Co nanowire with various easy axis deviations from applied field. It is found that both the coercivity and the remanence decrease monotonously with the increase of easy axis deviation as well as the maximum magnetic product, indicating the large impact of the easy axis orientation on the magnetic performance. Moreover, the calculated angular distributions and the evolution of magnetic moments have been shown to explain the magnetic reversal process. It is demonstrated that the large demagnetization field in the two ends of the nanowire makes the occurrence of reversal domain nucleation easier, hence the magnetic reversal. In addition, the magnetic reversal was illustrated in terms of the analysis of the energy evolution.
Song, W., Li, X., Lou, L., Hua, Y., Zhang, Q., Huang, G., Hou, F., Zhang, X..  2018.  High-Temperature Magnetic Properties of Anisotropic SmCo7/Fe(Co) Bulk Nanocomposite Magnets. IEEE Transactions on Magnetics. 54:1–5.
High-temperature magnetic properties of the anisotropic bulk SmCo7/Fe(Co) nanocomposite magnets prepared by multistep deformation have been investigated and compared with the corresponding isotropic nanocomposites. The anisotropic SmCo7/Fe(Co) nanocomposites with a Fe(Co) fraction of 28% exhibit much higher energy products than the corresponding isotropic nanocomposites at both room and high temperatures. These magnets show a small remanence (α = -0.022%/K) and a coercivity (β = -0.25%/K) temperature coefficient which can be comparable to those of the conventional SmCo5 and Sm2Co17 high-temperature magnets. The magnetic properties of these nanocomposites at high temperatures are sensitive to the weight fractions of the Fe(Co) phase. This paper demonstrates that the anisotropic bulk SmCo7/Fe(Co) nanocomposites have better high-temperature magnetic properties than the corresponding isotropic ones.
Chen, Z., Bai, B., Chen, D., Chai, W..  2018.  Direct-Current and Alternate-Decay-Current Hybrid Integrative Power Supplies Design Applied to DC Bias Treatment. IEEE Transactions on Power Electronics. 33:10251–10264.
This paper proposes a novel kind of direct-current and alternate-decay-current hybrid integrative magnetization and demagnetization power supplies applied to transformer dc bias treatment based on a nanocomposite magnetic material. First, according to the single-phase transformer structure, one dc bias magnetic compensation mechanism was provided. The dc bias flux in the transformer main core could be eliminated directionally by utilizing the material remanence. Second, for the rapid response characteristic of the magnetic material to an external magnetic field, one positive and negative dc magnetization superimposed decaying ac demagnetization hybrid integrative power supplies based on single-phase rectifier circuit and inverter circuit was designed. In order to accurately control the magnetic field strength by which a good de/-magnetization effect could be achieved, this paper adopts the double-loop control technology of the magnetic field strength and magnetizing current for the nanocomposite magnetic state adjustment. Finally, two 10 kVA transformers and the experiment module of the hybrid integrative power supplies were manufactured and built. Experimental results showed that the integrated power supplies have good de/-magnetization effect and practicability, proving the validity and feasibility of the proposed scheme.
Coey, M., Stamenov, P. S., Venkatesan, M., Porter, S. B., Iriyama, T..  2018.  Remanence enhancement melt-spun Nitroquench Sm2Fe17N3. M.. 2018 IEEE International Magnetics Conference (INTERMAG). :1–1.
The discovery of the interstitial rare earth nitride Sm2Fe17N3 came about seven years after the discovery of the rare earth iron boride Nd2Fe [1],[2], and the nitride initially seemed to offer intrinsic magnetic properties that were superior (Curie temperature TC, magnetocrystalline anisotropy K1 or comparable (spontaneous magnetization Ms to those of its illustrious predecessor. However, the promise of the new material to seriously challenge Nd2Fe14B was not realized. The 2:17 nitride powder, prepared by a low-temperature gas-phase interstitial modification process proved difficult to orient and worse still, it lost its nitrogen at the temperatures needed to process dense sintered magnets [3]. Attempts at explosive compaction [4] or spark sintering [5] failed to yield material with good enough coercivity. Nevertheless, work continued in Japan and China to develop a coercive powder that could be used for bonded magnets. An early realization was zinc-bonded Sm2Fe17N3 [6] with an energy product of 84 kJm3 but a rather low coercivity of 480 kAm-1, less than 5 % of the anisotropy field (Ha = 2K1/Ms ≈ 11 MAm-1). The anisotropy field of Nd2Fe14B is significantly less (6 MAm-1) yet several decades of intensive development have led to higher values and continuous improvements of the coercivity, even in unsubstituted material. Historical experience with permanent magnets shows that a long period of materials development is needed to arrive at the best composition and processing conditions for a microstructure that allows the hard magnetism to be optimized. Coercivities of about 25% of the anisotropy field are ultimately achieved. Here we compare the magnetic properties of melt-spun material. Our Nitroquench powder, produced by Daido Steel, was in the form of flakes 10 μm thick and up to 100 μm in diameter. A crystal-lite size of approximately 15 nm deduced from Scherrer broadening of the X-ray reflections. Composition was checked by EDX microprobe analysis. Hysteresis loops have been measured in applied fields of up to 14 T, at room temperature and at 4 K.The material exhibits a room-temperature coercivity of 690 kAm-1 after saturation in 14 T, with a remanence of 700 kAm-1 in zero applied field and an extrapolated saturation magnetization of 1230 kAm-1. The remanence ratio Mr/Ms of 63% when the remanence is corrected to zero internal field, is reflected in a preferred orientation seen in the X-ray powder diffraction patterns and in 57Fe Mössbauer spectra of magnetized powder. Spectra obtained after saturation of an immobilized powder absorber either in-plane or perpendicular to the sample plane exhibit distinctly different relative intensities of the ΔM=0 absorption lines. The maximum energy product for the powder, assuming full density, is 162 kJm-3. The remanence enhancement is attributed to fact that the nanocrystallite size is not much greater than the exchange length. Melt-spun Sm-Fe-N powder has superior corrosion resistance and thermal stability compared to melt-spun Nd-Fe-B. The Nitroquench powder may be used to produce polymer-bonded magnets with an energy product in excess of 100 kJm-3.
2019-09-30
Elbidweihy, H., Arrott, A. S., Provenzano, V..  2018.  Modeling the Role of the Buildup of Magnetic Charges in Low Anisotropy Polycrystalline Materials. IEEE Transactions on Magnetics. 54:1–5.

A Stoner-Wohlfarth-type model is used to demonstrate the effect of the buildup of magnetic charges near the grain boundaries of low anisotropy polycrystalline materials, revealed by measuring the magnetization during positive-field warming after negative-field cooling. The remnant magnetization after negative-field cooling has two different contributions. The temperature-dependent component is modeled as an assembly of particles with thermal relaxation. The temperature-independent component is modeled as an assembly of particles overcoming variable phenomenological energy barriers corresponding to the change in susceptibility when the anisotropy constant changes its sign. The model is applicable to soft-magnetic materials where the buildup of the magnetic charges near the grain boundaries creates demagnetizing fields opposing, and comparable in magnitude to, the anisotropy field. The results of the model are in qualitative agreement with published data revealing the magneto-thermal characteristics of polycrystalline gadolinium.