Demagnetization Rate Research Articles

Optimization of an Asymmetric-Rotor Permanent Magnet-Assisted Synchronous Reluctance Motor for Improved Anti-Demagnetization Performance

Permanent magnet-assisted synchronous reluctance motors (PMA-SynRMs) are widely used in various fields due to their significant advantages, including strong torque output, high efficiency, excellent speed regulation, and low cost. The PMA-SynRM with asymmetric-rotor structure has a weaker anti-demagnetization performance than the conventional PMA-SynRM due to its multi-layer and thin permanent magnets construction. According to the finite element (FEM) simulation analysis, the anti-demagnetization performance of the asymmetric-rotor PMA-SynRM can be improved by adding bypass magnetic bridges on the ribs of the flux barriers and by changing the positions of the permanent magnets. The rotor structure of the proposed model is globally optimized by combining the two methods. Anti-demagnetization performance is improved as much as possible under the premise of ensuring the torque performance of the basic model. After multi-objective optimization, there is almost no difference between the optimized model and the basic model in terms of no-load air-gap flux density, no-load Back-electromotive force (EMF), and average torque. The maximum demagnetization rate of the optimized model is reduced by 81.44% compared with the basic model, and the anti-demagnetization performance is significantly improved. At the same time, the torque ripple is also reduced by 44.14%, which is obviously reduced. Compared with the basic model, the optimized model has better stability and reliability.

3D printed suspension and its evaluation in an ADR refrigerator

The salt pill suspensions play a critical role in determining the performance of adiabatic demagnetization refrigerators (ADR). Kevlar suspensions are commonly used to hold the salt pill in place and to minimize the thermal leak. However, traditional solutions such as tensioning the Kevlar and fixing it on mechanical frame with the help of epoxy or running it around a pulley are quite challenging. This paper presents an innovative suspension using 3D printing technology and a polyether ether ketone (PEEK) multi-ring suspension has been designed and tested. Numerical analysis shows that a single piece of the suspension may contribute about 1.10 μW thermal leak between 4 K and 0.3 K. To test its performance, a two-stage ADR is built with Gadolinium Gallium Garnet (GGG) as the first stage and chromium potassium alum (CPA) salt pill as the second stage which is supported by the PEEK suspension. The GGG stage is mainly used to provide an initial temperature for CPA stage. Typically, starting from 1 K@4 T, a no-load lowest temperature of 66.1 mK is achieved at a demagnetization rate of 0.005 T/s in the experiments. The thermal leak of the PEEK suspension with and without 1 K heat sink has been analyzed and compared, with the former one showing a big reduction of the thermal leak. This paper confirms the feasibility of PEEK as an ADR suspension material and provides a potential 3D printing method for complex suspension designs.

Study of the demagnetization behavior of no-insulation persistent-current mode HTS coils under external AC fields by 3D FEM simulation

The no-insulation (NI) winding technique is promising for applications in the persistent-current mode (PCM) operation of high-temperature superconducting (HTS) coils. To produce an NI PCM coil, it is essential to understand its demagnetization behavior (i.e. decay of persistent DC current) under an external AC field, which occurs in maglev trains, electric machines and other dynamic magnet systems. For this purpose, a 3D finite-element method (FEM) model, capturing the full electromagnetic properties of NI HTS coils is established. This work studied three kinds of AC fields, observing the impact of turn-to-turn contact resistivity on demagnetization rates, which is attributed to current distribution modulations. Under a transverse AC field, the lower contact resistivity attracts more transport current to flow in the radial pathway to bypass the ‘dynamic resistance’ generated in the superconductor, leading to slower demagnetization. Under an axial AC field, the demagnetization rate exhibits a non-monotonic relation with the contact resistivity: (1) the initial decrease in contact resistivity leads to a concentration of induced AC current on the outer turns, which accelerates the demagnetization; (2) the further decrease in contact resistivity makes the current smartly redistribute to avoid flowing through the loss-concentrated outer turns, thus slowing down the demagnetization. Under a rotating DC field, a hybrid of transverse and axial fields, the impact of contact resistivity on the demagnetization rate exhibits combined characteristics of the transverse and axial components. Additionally, quantitative prediction of the demagnetization rate of NI PCM coil under external AC field is instructive for practical designs and operations, which is tested by this 3D FEM model, and a comparison with experimental results is conducted.

Performance of a foam copper-based chromium potassium alum salt pill in an adiabatic demagnetization refrigerator

Hydrated paramagnetic salt pills are important components in an adiabatic demagnetization refrigerator (ADR) for temperatures below 500 mK. To improve the heat transfer performance between salt crystals and external thermal links, thermal buses are commonly employed inside the salt pill. Common forms of thermal buses are either thin copper pillars made from electric discharge machining (EDM) or wire bundles, both of which generally involve considerable machining or labor. This paper introduces a novel design and fabrication of a simple foam copper-based paramagnetic salt pill. The foam copper has 30 pores per inch (PPI), a porosity of 98 %, 0.005 m2/g specific surface area, and an estimated pore diameter of 0.82 mm. The complete design, assembling process, and growth of the foam copper-based chromium potassium alum (CPA) salt pill (40 mm in length and 23 mm in diameter) are introduced. A single-stage ADR is built based on this salt pill to test its performance, while another salt pill with the wire thermal bus has also been tested for comparison. A typical cool-down from 4.3 K @ 4 T with a demagnetization rate of 0.0025 T/s led to a no-load temperature of 176.5 mK, slightly better than that using a wire thermal bus. In terms of cooling power per gram of salt, the cooling capacities at 600 mK of both salt pills have also been measured and both have shown similar performance. This preliminary study has shown the potential of the foam copper-based salt pill though more research is required to optimize its performance.

Preparation of Bose-Einstein condensate of dysprosium atoms based on demagnetization cooling

In magnetic atomic gases, the dipolar relaxation process couples the system spin and kinetic degrees of freedom. When the average kinetic energy is significantly lower than the Zeeman splitting, the atoms predominantly occupy the lowest Zeeman state. As the Zeeman splitting approaches the average kinetic energy, some atoms transfer to adjacent Zeeman states through dipolar relaxation, converting kinetic energy into Zeeman energy. By utilizing optical pumping, atoms transferred to higher spin states can be repumped to the ground state, thereby achieving a continuous cooling cycle and effectively lowering the system’s temperature. As the energy removed in a single cooling cycle is much larger than the energy of scattered photons, this demagnetization cooling scheme significantly enhances cooling effciency and reduces atomic loss. In this work, we establish state-coupled equations that incorporate dipolar relaxation and optical pumping to analyze the demagnetization cooling process, modeling the evolution of atom number and temperature during the cooling of <inline-formula><tex-math id="M1">\begin{document}$^{164}{\text{Dy}}$\end{document}</tex-math></inline-formula> atoms. We develop a strategy to generate an optimal magnetic field waveform by maximizing the demagnetization rate. Based on this strategy, we investigate the influence of crucial experimental parameters on demagnetization cooling and determine their specific ranges for producing large atom number of BEC, including the optical dipole trap frequency, as well as the intensity and polarization purity of the optical pumping light. The results indicate that demagnetization cooling enables the direct preparation of a large number of dysprosium BEC with sub-second timescales, reducing the cooling time by an order of magnitude compared to conventional methods for dysprosium atoms. Furthermore, it could achieve a cooling effciency of <inline-formula><tex-math id="M2">\begin{document}$\chi \approx 44.92$\end{document}</tex-math></inline-formula>, an order of magnitude higher than that of traditional evaporative cooling.

Demagnetization effects of surface-mounted permanent magnet synchronous wind generator

To prevent fossil resources from being depleted and protect the natural balance, renewable resources come to the forefront as an alternative to fossil resources. Wind energy resources, among the renewable energy resources, are important in terms of ensuring the reliability of energy and the use of their resources. Generators are the most important components for the conversion of wind power. Permanent magnet synchronous generators (PMSGs) are preferred in wind turbines since they have high efficiency and high volume/torque densities, thereby optimization of the PMSGs is an important topic for the wind energy community. On the one hand, these machines can cause problems due to overheating and mechanical friction during their long-time operation. To identify the performance lack of the machines due to the de-magnetization faults, systematic work has been performed. When a magnet of a PMSG is de-magnetized at different rates (i.e. 33%, 50%, and 100%), we have explored the artifacts in the electric generation. Besides, the torque performances of the generator at rated load are examined and the flux density distributions are revealed. The rated torque decreased substantially when the demagnetization rate of the magnet increased.

Effect Analysis of Demagnetization of Permanent Magnet Synchronous Motor for Electric Drilling Tool on Air-gap Flux Density

Electric drilling tools have been developed in the field of drilling because of their high efficiency. As the core component of electric drilling tool, permanent magnet synchronous motor(PMSM) provides power for drilling tool. However, in the process of drilling, the permanent magnet of PMSM will appear demagnetization.The value of air-gap flux density affects the performance and efficiency of the motor directly. Demagnetization faults are divided into uniform demagnetization and local demagnetization.In order to study the influence of demagnetization of PMSM on the air-gap flux density, mechanism of permanent magnet is analyzed Firstly, and then establishing numerical model with analytical method. Finally, simulation motor demagnetization using finite element method(FEM) based on Maxwell. No-load and load operating state are analyzed under the two fault modes are considered in this paper, and compared with the healthy state of the motor. The analytical results show that when the demagnetization fault occurs in the motor permanent magnet, the air-gap flux density will decrease accordingly.The FEM method results show that with the increase of uniform demagnetization rate, the flux density of the air-gap almost decreases in proportion to the increase of demagnetization rate. The demagnetization of single pole permanent magnet has little effect on the magnetic density of the nearby permanent magnet, but it will lead to the imbalance of the whole electromagnetic field of the motor. The air-gap flux density can be used as a characteristic index to judge the fault of demagnetization.

Effect of NdCoGa alloy addition to waste wind turbine magnets to enhance the characteristics of recycled sintered NdFeB permanent magnets

Neodymium (NdFeB) permanent magnets play a key role in motors and other devices, and are important to both defense and civilian industrial uses; however, due to the limited rare earth resources, the issue of recycling waste NdFeB magnets to achieve sustainable development has attracted much attention. An innovative and valid method for the recycling of waste NdFeB magnets is demonstrated in this study. Recycled sintered NdFeB permanent magnets were prepared on a large scale up to 100 kg per batch by adding NdCoGa ternary rare earth alloy to waste wind turbine NdFeB magnets, and their magnetic properties, corrosion resistance and high temperature aging characteristics were evaluated. The recycled magnets were manufactured by mixing waste magnets A, B, and NdCoGa alloy according to weight percentage ratio of 90:7:3 respectively. Through BH Curve measurement and FE-EPMA qualitative element analysis, the differences in the magnetic properties and microstructure before and after adding the NdCoGa elements were observed. The results show that the magnetic properties of waste magnet A are Br = 12.60 kG, iHc = 18.92 kOe and (BH)max = 39.19 MGOe; the magnetic properties of waste magnet B are Br = 12.77 kG, iHc = 18.15 kOe and (BH)max = 40.25 MGOe. In contrast, the magnetic properties of the recycled magnets prepared in this study are Br = 12.63 kG, iHc = 18.72 kOe, and (BH)max = 39.95 MGOe, indicating that the proposed process restores the magnetic properties of the recycled magnets to the level of the original waste magnet. Also, a brand new commercial N40H magnet is further used as a control group, where the corrosion resistance and high temperature aging characteristics of bare materials were evaluated by Pressure Cooker Test (PCT), and oven/flux meter respectively. The results show that the average corrosion weights per unit area are 0.89 mg/cm2 and 1.23 mg/cm2, as well as the demagnetization rate of the high temperature aging is less than 0.4%. Compared with the new magnet, the recycled magnet has the same level of high temperature aging resistance, and 28% higher corrosion resistance. As a result, adding only 3% NdCoGa alloy to the waste magnet is beneficial to restore the magnetic properties and high temperature aging resistance, up to the level of new magnets, and also greatly enhances its corrosion resistance and reduces the cost of magnet remanufacturing. This result indicates that the recycled NdFeB permanent magnets developed in this research are more suitable for offshore wind turbine generators and electric vehicle motors operating at high temperatures and high humidity for a long period of time.

Analysis and Suppression of Cross-Coupling Demagnetization in Dual Permanent Magnet Machine for Direct-Drive Application

Dual permanent magnet (PM) machines synthesize two PM excitations together and potentially exhibit higher torque density than conventional single-PM excited machines. In dual-PM excited machines, the existence of one PM excitation tends to cause interior flux density reduction and irreversible demagnetization of the other PM set, and this feature is recognized as cross-coupling demagnetization. Therefore, this article systematically analyzed the mechanism and influence of this unique phenomenon in a dual-PM prototype machine. The results reveal that the cross-coupling demagnetization can result in up to 18% torque reduction under a severe working conditions if the machine is not properly designed. Increasing one set of PM thickness along the magnetization direction is beneficial to enhance the self-demagnetization withstanding ability, whereas the cross-coupling demagnetization on the other PM set has deteriorated. In order to eliminate such a side effect, some techniques, e.g., removing the demagnetization region and alleviating the mutual flux path, are presented to suppress the demagnetization. It is revealed that the demagnetization rate can be reduced to less than 3% with proper design. Finally, a dual-PM prototype is manufactured and tested to validate the improvement of demagnetization withstanding capability by using the presented suppression technique.

Demagnetization Fault Diagnosis of the Permanent Magnet Motor for Electric Vehicles Based on Temperature Characteristic Quantity

Permanent magnet motors are widely used in the driving system of electric vehicles because of their high power density and small size. However, the permanent magnet motor is prone to demagnetization due to the complex operating environment of electric vehicles and characteristics of permanent magnet motors. To ensure the normal operation of vehicles, the demagnetization fault diagnosis of permanent magnets should develop. In this study, the thermal behavior of a permanent magnet motor under local demagnetization fault is discussed. Results show that the demagnetization of permanent magnets has a significant effect on the temperature of the motor; thus, the temperature is added into input signals of the permanent magnet demagnetization fault diagnosis. The healthy state of the permanent magnet is predicted by the back propagation (BP) neural network to realize the demagnetization fault diagnosis. First, the demagnetization test platform is built to test the performance of the motor, and the current, torque, and temperature of the motor are measured according to different demagnetization degrees. Second, the demagnetization simulation model of the motor is established by the finite-element method, and the air gap magnetic field, loss, and temperature of the motor are analyzed. Lastly, the temperature, current, speed, and torque signals are selected as input signals of the neural network model, and the demagnetization rate of the rotor is taking as the output signal. The neural network prediction model is established and trained, and the good generalization ability is obtained.

Electromagnetic alternators with composite amorphous alloy shell for improving characteristics

In this study, a magnetic casing encapsulating a generator is used to improve the electromagnetic and power characteristics of the generator. Two types of two-pole AC generators are proposed: one is a bare-bonded (NC) type and the other is an iron-based amorphous alloy material covering the generator casing. In this study, Fe-Si-B amorphous thin strips were annealed between 300 and 380 °C and soaked for 60 min to observe and analyze the structural changes and the evolution of properties under different temperature conditions. After the thin strip is annealed at 400 °C, α-Fe dendrites begin to appear, and at 450 °C, a large number of α-Fe dendrites are formed. The demagnetization rate of the permanent magnet area near the bottom of the pole piece is the highest for the generator with an amorphous alloy shell because the magnetic flux leakage from the side frame of the pole piece changes the magnetic flux path, thereby affecting the demagnetization performance. In this study, three amorphous alloys, HB1-M, HB1, and SA1, were used as the cladding permanent magnet generator shell, and annealing and non-annealing procedures were conducted to observe the influence on the magnetic power of the generator. The magnetic flux on the surface of the amorphous shell was measured by a magnetic fluxmeter. For experiments with an amorphous shell, the results show that the annealed center point magnetic flux densities were approximately HB1-M (15 mT), HB1 (12 mT), and SA1 (10 mT), and the marginal non-annealed amorphous thin strip could reach ∼7–5 mT. The measured generator harmonic component of SA1 is higher than the voltage and current harmonics, reflecting that the magnetization direction of the magnetic shell in the demagnetization region is covered by the magnetic flux density. Because of central flame annealing, the magnetic shell is paramagnetic, and it helps to reduce the noise by at least 5 dB. The generator current harmonic characteristic of the HB1-M material is smaller than that of the HB1 and SA1 materials. In this study, a composite amorphous material was used to cover the generator casing; it was verified that HB1-M + HB1 + SA1 has the lowest noise characteristics for the generator. The generator noise of the composited material (HB1-M + HB1 + SA1) can be reduced to below 69 dB, which is ∼7–9 dB lower than that of a single amorphous material.

Irreversible Demagnetization Improvement Process of Hybrid Traction Motors with Dy-Free Magnets

Permanent magnet synchronous motors (PMSMs), with their high power density, high efficiency, and thermal stability, are widely used nowadays. PMSM magnets are composed of rare earth elements. However, rare earth elements are subject to severe price fluctuations because of their limited availability and the monopoly of some countries. Therefore, extensive research on magnets devoid of rare earth elements has been conducted recently. Although a magnet devoid of rare earth elements has a high irreversible demagnetization rate at high temperatures owing to its low coercive force, an irreversible demagnetization improvement process is proposed in this paper to compensate for this disadvantage. This process analyzes the contribution of the magnet’s back EMF (electromotive force) using the flux linkage equation, which does not change with time. Next, the location of irreversible demagnetization is moved to a position with a low contribution to the back electromotive force. Consequently, even if irreversible demagnetization occurs at the same size, the irreversible demagnetization ratio is reduced. The proposed process can minimize irreversible demagnetization while maintaining performance.

Role of Spin Transport through the β-Ta/Co20Fe60B20 Interface on its Ultrafast Demagnetization: Implications for Ultra-High-Speed Spin-Orbitronic Devices

Femtosecond laser-induced spin manipulation has fueled intense interest but understanding its underlying microscopic mechanisms in complex systems such as nonmagnet (NM)/ferromagnet (FM) heterostructures having strong spin-orbit effect is still at its infancy. Here, we report on the ultrafast demagnetization, remagnetization, and damping study in β-Ta(t nm)/Co20Fe60B20(d nm)/SiO2(2 nm) thin-film heterostructures by systematically varying both the Ta and Co20Fe60B20 thicknesses. Based on experimental results, we have established a direct relationship between the variation of the demagnetization rate and the Gilbert damping constant, indicating interface spin transport as the prevailing mechanism for both the ultrafast demagnetization and the damping. At higher thicknesses (t ≥ 7 nm) of β-Ta, the spin accumulation coefficient is found to be ∼0.24 eV, which is about 1.8 times less than its value in the lower thickness regime (t < 7 nm). These results will have important implications toward the evolution of ultra-high-speed spin-orbitronic devices.

Effect of Local Annealing on Magnetic Flux Distribution and Noise in a Micro-Generator with Amorphous Shell.

This study examined micro-alternators with two different housing structures--an uncoated shell and a shell coated with an iron-based amorphous-alloy soft magnetic material. The electromagnetic power and noise characteristics of generators with these shell structures were measured and analyzed. The material used for the shell coating was the SA1 amorphous alloy. The magnetic property of the SA1 material was evaluated, including its hysteresis expansion, hysteresis-loop parameters, α-Fe crystal formation, thermogravimetric transfer, and Curie temperature. The center point of the casing was subjected to flame local-heating annealing to attain ferromagnetism and paramagnetism material characteristics. The experimental shell was between these magnetic-phase-transition properties and was used to observe the magnetic power and noise characteristics of the microgenerator. The measured magnetic flux at the center of the amorphous shell was 1.2-2.4 mT, and the magnetic flux distributed around the shell was 0.6-1.0 mT. The generator with the amorphous-alloy shell had the lowest demagnetization rate in the permanent magnet region, which was close to the bottom of the pole piece, and the magnetic flux leakage of the pole-piece side frame changed the magnetic flux path, thus affecting the demagnetization performance. For the noise experiment, the flame-annealing temperature of the local center point of the amorphous casing reached the Curie temperature, and the noise characteristics of the casing can be reduced by 15 dB compared to those of the generator without the casing. However, the overall performance of generator harmonics and power were not fully improved.

Ab initio study of ultrafast spin dynamics in Gdx(FeCo)1−x alloys

Using an ultrashort laser pulse, we explore ab initio the spin dynamics of Gdx(FeCo)1−x at femtosecond time scales. Optical excitations are found to drive charges from Fe majority d-states to unoccupied Gd f-minority states with f-electron character excited occupation lagging behind that of the d-electron character, leading to substantial demagnetization of both species while leaving the global moment almost unchanged. For x &amp;gt; 0.33, this results in the creation of an ultrafast ferromagnetic (FM) transient by the end of the laser pulse with the Gd demagnetization rate slower than that of Fe. For all concentrations, the Gd moments begin to rotate from their ground state orientations developing in-plane moments of between 0.2 and 0.5 μB. Thus, the ultrafast spin dynamics of the material captures three important ingredients of all optical switching that occurs at much later (picosecond) times: (i) the development of a FM transient, (ii) the different rates of demagnetization of Fe and Gd, and (iii) the breaking of the collinear symmetry of the ground state. Furthermore, several predictions are made about the behavior of Fe–Gd alloys that can be experimentally tested and can lead to a spin-filtering device.

Design and Analysis of Interior Permanent Magnet Motor for Electric Vehicle Application Considering Irreversible Demagnetization

The design of permanent magnet motors for the battery-powered electric vehicle has different aspects. In this study, the conceptual design process of an electric machine for Sedan Low car is presented. Based on the vehicle requirements, an interior permanent magnet motor with a maximum power of 120 kW with different slot&#x002F;pole combinations is designed. To obtain the best slot&#x002F;pole combination, The New European Driving Cycle in terms of crucial performance characteristics is compared. Given the importance of irreversible demagnetization phenomenon to an electric vehicle application, an accurate approach to determine the irreversible demagnetization rate is first introduced. Design tips are then proposed to reduce the probability of the irreversible demagnetization. Finally, fault-tolerant design in case of irreversible demagnetization withstand capability is carried out using a two-level Taguchi optimization approach. The results obtained from this study indicated that the proposed approach is effective in the fault-tolerant design of the permanent magnet machine for electric vehicle applications.

Multi-Objective Optimization of Permanent Magnet Synchronous Motor for Electric Vehicle Considering Demagnetization

The irreversible demagnetization of permanent magnets causes the deterioration of the performance in permanent magnet synchronous motors (PMSMs), which are used for electric vehicles. NdFeB, which is the permanent magnet most commonly used in PMSMs for electric vehicles, is easily demagnetized at high temperatures. Because traction motors for electric vehicles reach high temperatures, and a high current can be instantaneously applied, permanent magnets of PMSM can be easily demagnetized. Therefore, it is important to study the demagnetization phenomenon of PMSMs for electric vehicles. However, since the demagnetization analysis procedure is complicated, previous studies have not been able to perform optimization considering demagnetization characteristics. In this study, we optimized the shape of a PMSM for electric vehicles by considering the demagnetization characteristics of permanent magnets using an automated design of experiments procedure. Using this procedure, a finite element analysis for each experimental point determined by a sampling method can be performed quickly and easily. The multi-objective function minimizes the demagnetization rate and maximizes the average torque, and the constraints are the efficiency and torque ripple. Various metamodels were generated for each of the multi-objective functions and constraints, and the metamodels with the best prediction performance were selected. By applying a multi-objective genetic algorithm, 1902 various optimal solutions were obtained. When the weight rate of the demagnetization rate to the torque was set to 0.1:0.9, the demagnetization rate and average torque were improved by 4.45% and 2.7%, respectively, compared to those of the initial model. The proposed multi-objective optimization method can guide the design of PMSMs for electric vehicles with high reliability and strong demagnetization characteristics.

Ultra-low temperature adiabatic demagnetization refrigerator for sub-Kelvin region

&lt;sec&gt;With the development of space observation, quantum technology and other frontier scientific research fields, the demand for ultra-low temperature refrigeration in sub-Kelvin region is increasing. Compared with dilution refrigeration and sorption refrigeration, adiabatic demagnetization refrigeration (ADR) has the outstanding advantages of high efficiency, compact, gravity independence and accessibility of working materials, which make ADR a promising technology for sub-Kelvin cooling.&lt;/sec&gt;&lt;sec&gt;A single-stage ultro-low temperature adiabatic demagnetization refrigerator is designed and developed. The thermodynamic principle and quantitative analysis are presented, from the macroscopic and microcosmic view, and operating results show good agreement with the theoretical value.&lt;/sec&gt;&lt;sec&gt;This refrigerator is precooled to 3 K by a GM-type refrigerator, with 252 g gadolinium gallium garnet (monocrystalline) used as a working medium. The maximum magnetic field of 4 T is provided by a superconducting coil. Flexible heat connection is used between the pre-cooler and ADR, so heat generated by vibration decreases. From (3 K, 4 T), the lowest temperature can reach 0.47 K by adiabatic demagnetization, which is consistent with the result drawn from the entropy data. In a constant-temperature-control mode, the demagnetization rate is controlled by a feedback loop, so the temperature can be held in the presence of a load. A cooling capacity of 2.7 J is provided at 1 K, with temperature fluctuation being lower than 0.5 mK, and the second thermodynamic efficiency of adiabatic demagnetization refrigeration is 57%. at 0.8 K, the cooling capacity is 1.2 J.&lt;/sec&gt;&lt;sec&gt;Future work on improving the performance includes the improving of the on-off ratio of the heat switch, so, the irreversible loss caused by the heat transfer temperature difference in conduction state can be reduced. Improving the heat transfer performance of the salt pill, the heat can be ejected in a shorter period.&lt;/sec&gt;&lt;sec&gt;This refrigerating machine is the first Chinese adiabatic demagnetization refrigeration system that can be operated in circulation, which is expected to be the 1&lt;sup&gt;st&lt;/sup&gt; stage of a three-stage adiabatic demagnetization refrigeration system in a 50 mK temperature zone. This study lays a foundation for further developing continuous multistage adiabatic demagnetization refrigeration at ultra-low temperature.&lt;/sec&gt;

Optimal Design of Spoke-Type IPM Motor Allowing Irreversible Demagnetization to Minimize PM Weight

In this study, we introduce an optimal design for a spoke-type interior permanent magnet (IPM) motor that can minimize PM weight by allowing irreversible demagnetization, unlike the conventional motor design method that does not allow irreversible demagnetization. The spoke-type rotor structure has the advantage of concentrating the magnetic flux around the rotor core. However, part of the PM near the surface of the stator tooth becomes sensitive to the armature reaction and external magnetic factors. Therefore, typical spoke-type rotor construction follows a limited design process that does not allow irreversible demagnetization. Although the manufacturing process allows tolerance for the demagnetization rate, motor design without considering the demagnetization rate in all the design stages becomes a design constraint and limits the performance area. In this study, the performance improvement is examined, when irreversible PM demagnetization is allowed up to −3%, and is not zero. For optimal design, Latin hypercube sampling (LHS), the kriging model, and genetic algorithm (GA) are utilized. With the proposed design process, motors are effectively designed to reduce the PM weight by 23.7%, compared to the initial design model while maintaining an efficiency almost similar to that of the initial design model.

S−d model for local and nonlocal spin dynamics in laser-excited magnetic heterostructures

We discuss a joint microscopic theory for the laser-induced magnetization dynamics and spin transport in magnetic heterostructures based on the $s$-$d$ interaction. Angular momentum transfer is mediated by scattering of itinerant $s$ electrons with the localized ($d$ electron) spins. We use the corresponding rate equations and focus on a spin one-half $d$ electron system, leading to a simplified analytical expression for the dynamics of the local magnetization that is coupled to an equation for the non-equilibrium spin accumulation of the $s$ electrons. We show that this description converges to the microscopic three-temperature model in the limit of a strong $s$-$d$ coupling. The equation for the spin accumulation is used to introduce diffusive spin transport. The presented numerical solutions show that during the laser-induced demagnetization in a ferromagnetic metal a short-lived spin accumulation is created that counteracts the demagnetization process. Moreover, the spin accumulation leads to the generation of a spin current at the interface of a ferromagnetic and non-magnetic metal. Depending on the specific magnetic system, both local spin dissipation and interfacial spin transport are able to enhance the demagnetization rate by providing relaxation channels for the spin accumulation that is build up during demagnetization in the ferromagnetic material.