Demagnetization Process Research Articles

Exchange interaction and demagnetization process of high-abundance rare-earth magnets sintered using dual alloy method

Exchange interaction and demagnetization process of high-abundance rare-earth magnets sintered using dual alloy method

Thermal stability and magnetic anisotropy dispersion in high-density hard-disk media

Abstract A method to correct the anisotropy field distribution for granular recording media, measured by switching methods (isothermal remanent magnetization (IRM) and direct-current demagnetisation (DCD) processes were used as an example) for the effects of thermal relaxation, that supposes an a priori knowledge of the grain size distribution, is presented. For a hard-disk medium, it is shown that the anisotropy field distribution is much narrower (about one order of magnitude) with respect to the grain size distribution. The anisotropy field value estimated from the dependence of the remanence coercivity on the field application time is found in agreement (to within less than 10 %) with the most probable value of the anisotropy field distribution.

Coercivity engineering in Sm(Fe0.8Co0.2)12B0.5 thin films by Si grain boundary diffusion

Achieving a large coercivity in the SmFe12-based compounds with excellent intrinsic magnetic properties is the main challenge toward the development of new high-performance permanent magnets. In this study, we investigated the effect of microstructural factors on coercivity using Sm(Fe0.8Co0.2)12B0.5 thin films as a model system. The films were composed of columnar Sm(Fe0.8Co0.2)12 grains with [001] out-of-plane texture separated by ∼5 nm-thick (Fe,B)-rich amorphous intergranular phase. To decrease the Fe content in the intergranular phase and improve the magnetic isolation of Sm(Fe0.8Co0.2)12 grains, grain boundary diffusion of Si was performed, which led to an increase in coercivity from 1.11 T to a record high value of 1.32 T for the Sm(Fe0.8Co0.2)12 compound. Detailed microstructure characterization using scanning transmission electron microscopy (STEM) and atom probe tomography (APT) confirmed that Si diffused in-part into the intergranular phase which became depleted of Fe and Co. Micromagnetic simulations on a model constructed based on STEM images have shown that triple junctions of the intergranular phase can act as nucleation centers during demagnetization process. This detrimental effect can be suppressed by full-depth diffusion of Si weakening the ferromagnetism of the intergranular phase. However, the presence of α-(Fe,Co) grains at the interface with a V underlayer substantially reduces the benefits of grain boundary diffusion. Thus, high coercivity in the SmFe12-type magnets cannot be obtained unless the soft magnetic α-(Fe,Co) phases are eliminated.

Characterization of magnetic field noise in the ARIADNE source mass rotor

The Axion Resonant Interaction Detection Experiment (ARIADNE) is a nuclear-magnetic-resonance-based experiment that will search for novel axion-induced spin-dependent interactions between an unpolarized source mass rotor and spin-polarized $^{3}\mathrm{He}$ nuclei placed nearby. To detect a feeble axion-mediated signal at the subattotesla level, the experiment relies on ultralow magnetic background and noise. We measure and characterize the magnetic field from a prototype tungsten rotor. We show that the field is dominantly caused by a few discrete magnetic dipoles, likely due to impurities in the rotor. This is done via a numerical optimization pipeline which fits for the locations and magnetic moments of each dipole. We find that under the current demagnetization procedure, the magnetic moment of the impurities is bounded at ${10}^{\ensuremath{-}9}$ A ${\mathrm{m}}^{2}$. We further show that a shielding factor of ${10}^{9}$ will support ARIADNE's design sensitivity with the current level of tungsten purity and demagnetization process.

Development of the integrated sorption cooler for an adiabatic demagnetization refrigerator (ADR)

The integrated refrigerator composed of sorption cooler and adiabatic demagnetization refrigerator (ADR) is proposed for sub-kelvin cooling without vibration. In this configuration, the demagnetization process of the ADR is supposed to start after the termination of the sorption cooling. The liquid helium-4 inside the thermosiphon of the sorption cooler should evaporate as quickly as possible. This paper presents the results of preliminary test of the sorption cooling process. The developed sorption cooler primarily consists of the sorption pump filled with activated charcoal of 5.2 g and the helium thermosiphon. The adsorption capacity of the sorption pump is experimentally confirmed. Furthermore, the empirical correlation of adsorbate concentration which predicts the experimental results within 15% error in the temperature range of 5–70 K is proposed. The sorption pump is thermally connected to the 4.5 K heat sink through a pure tin strap, and regulates its temperature by the heater installed on it. Sorption cooling experiment is conducted under two experimental conditions. The experiment shows that the cooler can refrigerate the thermal capacitor to 2.8 K from 4.5 K heat sink, and provides the cooling capacity of 2.29 J. Moreover, it is confirmed that the pumping speed of the sorption pump is dominantly affected by its transient cool-down process. The new configuration of the thermosiphon condenser is proposed to enhance the evaporation characteristic of liquid helium while the pumping speed of the sorption pump is not changed. Since the new thermosiphon has smaller gas volume, liquid helium evaporates more quickly due to the rapid reduction of the vapor pressure inside the thermosiphon.

Blume–Emery–Griffiths model for a magnetic bilayer. Magnetocaloric properties in the presence of the 1st-order phase transitions

In the paper the Blume–Emery–Griffiths model is studied for a magnetic bilayer with coordination number z=5. The Cluster Variational Method in Pair Approximation is employed. A special emphasis is put on the magnetic and magnetocaloric properties in the cases of the 1st order phase transitions. For such cases the magnetization, quadrupolar moment and magnetic entropy, as well as the isothermal entropy change and the adiabatic temperature change during demagnetization process are calculated. It is shown that in the vicinity of the temperature corresponding to the 1st order phase transitions the adiabatic demagnetization is not possible. Instead, the demagnetization process with the smallest entropy production is proposed.

軟X線磁気円二色性顕微分光によるNd-Fe-B熱間加工磁石の初磁化過程および減磁過程における磁区観察

A large step on the initial magnetization curve has been widely observed in Nd-Fe-B hot-deformed magnets. This behavior has been considered as that the first- and latter-half parts of the initial magnetization curve correspond to the magnetization reversals of multi- and single-domain grains, respectively. In this study, the detailed magnetic domain structure of a Nd-Fe-B hot-deformed magnet has been studied for the initial magnetization and demagnetization processes by using a soft X-ray magnetic circular dichroism (XMCD) microscopy. The observed magnetization reversal behavior for the initial magnetization process is quite different from that previously considered. The multi-domain states move from grains to grains through the gradual displacement of domain wall passing through many grains, and then the massive domain wall displacement takes place. The latter part of domain wall displacement is similar with that found in the demagnetization process near the coercivity. Moreover, we found that there are several strong pinning sites which are identical for both the initial magnetization and the demagnetization processes.

Micromagnetic Simulation of Nitrogenation Effect on the Magnetic Properties of Sm2Fe17N3 Alloy

Nitrogenation is an indispensable process for the formation of Sm₂Fe₁₇N₃ and has a major influence on its magnetic properties. In this work, the effect of nitrogenation on the magnetic properties of Sm₂Fe₁₇N₃ was investigated by micromagnetic simulation. The normal coercivity, remanence, and the maximum energy product of incompletely nitrided Sm₂Fe₁₇N₃ with a nonnitrided Sm₂Fe₁₇ core size less than 16 nm approach nearly 99% of their values in fully nitrided Sm₂Fe₁₇N₃. Thus, a small nonnitrided Sm₂Fe₁₇ core does not affect the effective utilization of Sm₂Fe₁₇N₃ permanent magnets. The magnetization reversal mode of incompletely nitrided Sm₂Fe₁₇N₃ with different sizes of nonnitrided Sm₂Fe₁₇ cores was evaluated, providing an in-depth fundamental understanding of the demagnetization processes in Sm₂Fe₁₇N₃ particles. This work could be useful for optimizing nitrogenation conditions to improve the magnetic properties of Sm₂Fe₁₇N₃ permanent magnets.

The Demagnetization Processes of Nd-Fe-B Sintered Magnets and Ferrite Magnets as Demonstrated by Soft X-Ray Magnetic Circular Dichroism Microscopy

The Demagnetization Processes of Nd-Fe-B Sintered Magnets and Ferrite Magnets as Demonstrated by Soft X-Ray Magnetic Circular Dichroism Microscopy

Experimental and Simulation Studies of Partial Demagnetization Process of Permanent Magnets in Electric Motors

For the analysis of the process of partial demagnetization of permanent magnets (PMs) in electric machines, a field model of coupled electromagnetic and thermal phenomena was proposed. In the proposed approach the non-linearity of the magnetic circuit, the effect of temperature on the magnetic, electrical and thermal properties of the materials as well as the developed method for modeling the process of partial demagnetization of the PMs were taken into account. In order to verify the usefulness and effectiveness of the developed algorithm as well as software for analyzing the impact of temperature and the process of partial demagnetization of the magnets on the operation of a PM motor, the results of calculations were compared with the results of experimental studies. The experimental tests were carried out on a specially designed and constructed measuring test stand. The results of the research on the process of partial demagnetization of magnets in the line start permanent magnet synchronous motor (LSPMSM) are presented and the conclusions resulting therefrom have been formulated.

Phases of Magmatic Activities in South Wadi Hodein-Shalatein, Egypt: Resolving controversy about their age using Paleomagnetism and AMS

ABSTRACT Several exposures of basaltic intrusions are distributed near Red Sea coast at Wadi Hodein Southern Egypt. There is some disagreement about the palaeomagnetic pole results of these basalts; some claim it is Tertiary in age, while others claim it is of Cretaceous age. To our knowledge, no Anisotropy of Magnetic Susceptibility (AMS) results have ever been published from these rocks. Therefore, palaeomagnetic and AMS studies were performed on these basalts in order to identify phases of magmatism, paleomagnetic pole position and types of magmatic intrusions. In the present study, eight sites (219 oriented core specimens) were sampled from eight location. Rock magnetic studies reflect the presence of magnetite as the chief mineral in these rocks. Alternating field technique in association with Thermal Demagnetisation process revealed presence of one magmatic phase took place in Cretaceous age with VGP (65°N, 250°E; A 95 = 5.3). AMS results showed that the basaltic intrusions in the study area are still holding their primary fabrics.

Disentangling electronic, lattice, and spin dynamics in the chiral helimagnet Cr1/3NbS2

We investigate the static and ultrafast magneto-optical response of the hexagonal chiral helimagnet $Cr_{1/3}NbS_{2}$ above and below the helimagnetic ordering temperature. The presence of a magnetic easy plane contained within the crystallographic ab-plane is confirmed, while degenerate optical pump-probe experiments reveal significant differences in the dynamic between the parent, $NbS_{2}$, and Cr-intercalated compounds. Time resolved magneto-optical Kerr effect measurements show a two-step demagnetization process, where an initial, sub-ps relaxation and subsequent buildup ($\tau > 50$ ps) in the demagnetization dynamic scale similarly with increasing pump fluence. Despite theoretical evidence for partial gapping of the minority spin channel, suggestive of possible half metallicity in $Cr_{1/3}NbS_{2}$, such a long demagnetization dynamic likely results from spin lattice-relaxation as opposed to minority state blocking. However, comparison of the two-step demagnetization process in $Cr_{1/3}NbS_{2}$ with other 3d intercalated transition metal dichalcogenides reveals a behavior that is unexpected from conventional spin-lattice relaxation, and may be attributed to the complicated interaction of local moments with itinerant electrons in this material system.

Laser-induced two-step demagnetization process study in Ni–Mn–Sn Heusler alloy film

The ultrafast magnetization dynamic processes are investigated in a wide time scale range for different laser pump fluences in Ni- and Mn-rich Heusler alloy film of Ni54.3Mn31.9Sn13.8 composition using time-resolved magneto-optical Kerr effect (TR-MOKE) in perpendicular magnetic field geometry. For all fluences used, two distinct types of magnetization dynamics of different time scales: ultrafast up to 2 picoseconds and slower from 2 ps to hundreds of ps, were observed. The description of the two-step de- and remagnetization processes was performed in the frame of modified model developed, based on the microscopic three-temperature model (M3TM). The model is based on the assumption that two magnetic sublattices of the Heusler alloy which are weakly exchange coupled due to Curie temperature proximity are responsible for the fast and slow magnetization dynamics. It is demonstrated excellent agreement of the model prediction with experimental data for all the fluences used. It is shown that the values of model parameters determined — demagnetization rates R and spin-flip probabilities asf for one sublattice are of the order reported for the Ni film. It has been found that R parameter value for the second sublattice related to Mn site is over two orders, and asf is up to one order of magnitude lower than in Ni site. It is shown that strong reduction of the spin-flip probability and electron–phonon coupling, and large magnetic moment in the Mn sublattice as compared to the Ni one is the cause of the observed demagnetization slowing down effect in the Heusler alloy studied.

Ultra-long spin relaxation in two-dimensional ferromagnet Cr2Ge2Te6 flake

By utilizing time-resolved magneto-optical Kerr effect technique, we investigated both demagnetization and remagnetization processes of a typical two-dimensional (2D) van der Waals (vdW) ferromagnetic (FM) semiconductor Cr2Ge2Te6 (CGT) and two comparative Te-based crystals, i.e. three-dimensional (3D) FM metal Cr3Te4 (CT) and 2D vdW FM metal Fe3GeTe2 (FGT). Different from 3D CT showing one-step laser-induced demagnetization, two-steps demagnetization behavior is dominated in 2D vdW FGT and CGT, which indicates that the 2D vdW magnets have relatively weaker electron-spin coupling. Of particular interest, we discovered an ultra-long remagnetization behavior in the 2D vdW CGT flake. The laser pumping induced spin demagnetization would not recover even in the time scale of 3500 ps (experimental setup limitation). To the best of our knowledge, this is the longest remagnetization process observed so far and it might be one of the particular characterizations of 2D vdW ferromagnet semiconductors. Based on the modified three temperature model, the key roles played by the crystal dimensionality and thermal diffusion anisotropy were revealed in the spin dynamics of 2D vdW magnets. Our findings would not only open the door to the thoroughly understanding of the origin of the magnetism in 2D FM materials but also undoubtedly find suitable applications in spintronics and magnonics devices.

Sub-100 femtosecond time scale spin dynamics in epitaxial Fe3O4 thin film

We have investigated the ultrafast laser-induced magnetization and reflectivity dynamics in the Fe3O4 thin film using all-optical pump–probe measurements. The high-quality of our sample is verified by the temperature-dependent static magnetic properties, showing a clear Verwey transition. Although the Fe3O4 is predicted to be a half-metal, the demagnetization process upon laser incident shows a transition-metal-like behavior. The demagnetization characteristic time has been found to be in sub-100 fs (fs) time scale, suggesting the dominant interaction of hole spin-flip processes. The spin relaxation process has been found to be around 600 fs, which is attributed to the electron–phonon coupling in accordance to the observed two electron–phonon coupling modes in reflectivity dynamics. Our results provide new insight into the ultrafast laser-induced spin dynamics in half-metallic ferrimagnet.

Distribution and characterization of screening current and induced field in HTS quasi-isotropic and simply stacked strands

With the development of the second-generation high temperature superconducting tapes (2G HTS tapes), there have been several high current-carrying conductors proposed by different groups. However, since the conductors are generally stacked by 2G HTS tapes with a high aspect ratio, the perpendicular AC magnetic field can induce the screening current on the wide surface of tapes, then generates a magnetic field during the process of excitation and demagnetization, which affect the spatial uniformity and temporal drift as well as field quality in HTS magnets. In this paper, as potential conductors can be used in fusion magnets, the screening effect characteristics of quasi-isotropic strand (Q-IS) and simply stacked strand (SSS) are performed numerically and experimentally. We plot the penetration diagrams of the screening current for both strands under applied changing vertical magnetic field. The dependence of the screening current induced field (SCIF) on the amplitude of the external magnetic field is analyzed, the SCIF hysteresis loop is obtained. We also discuss the spatial distribution of SCIF for both strands. It is shown that SCIF of Q-IS has quasi-isotropic distribution and considerable amplitude reduction comparing with the SSS. The study provides a valuable reference for the application of Q-IS in superconducting magnets, especially nuclear fusion magnets.

Sub-15-fs X-ray pump and X-ray probe experiment for the study of ultrafast magnetization dynamics in ferromagnetic alloys.

In this paper, we present a new setup for the measurement of element-specific ultrafast magnetization dynamics in ferromagnetic thin films with a sub-15-fs time resolution. Our experiment relies on a split and delay approach which allows us to fully exploit the shortest X-rays pulses delivered by X-ray Free Electrons Lasers (close to the attosecond range), in an X-ray pump - X-ray probe geometry. The setup performance is demonstrated by measuring the ultrafast elemental response of Ni and Fe during demagnetization of ferromagnetic Ni and Ni80Fe20 (Permalloy) samples upon resonant excitation at the corresponding absorption edges. The transient demagnetization process is measured in both reflection and transmission geometry using, respectively, the transverse magneto-optical Kerr effect (T-MOKE) and the Faraday effect as probing mechanisms.

Analysis of Remagnetization Processes of High-Anisotropic Alloy after Heat Treatment

The simulation of the Sm(Co, Fe, Cu, Zr)7.5 type alloy domain structure formation after various thermal treatments was carried out by FMRM program based on a phenomenological approach to the analysis of the uniaxial highly anisotropic ferromagnets demagnetization processes. It is shown that the domain structure of the alloy in the thermally demagnetized state expands as the coercive force of the alloy decreases. It is noted that the domains size increasing process is associated not only with a decrease in the coercive force but also with a change in the influence of the magnetostatic interaction.

An approach to the transformer core residual magnetism evaluation and demagnetization

Aiming at the problem of transformer inrush current suppression, a method of transformer core residual magnetism evaluation and demagnetization is proposed. Firstly, by setting the amplitude and time of DC current, different transformer core residual magnetism states are obtained. Secondly, under different residual magnetism conditions, the high-voltage winding of the transformer is charged and discharged in the forward and reverse direction alternately. The current change data in the process is collected, and the correlation coefficient of the current data in the forward and reverse charging processes is calculated, which represents the transformer core residual magnetism states. Finally, the DC current with alternating change direction and gradually decreasing amplitude is applied at the high voltage winding to demagnetize, and the variation of residual magnetism is analyzed during the demagnetization process.

Effect of hydrogenation-disproportionation-desorption-recombination powder processing on the demagnetization process of Nd-Fe-B sintered magnets analyzed by soft X-ray magnetic circular dichroism microscopy

The demagnetization process of a variety of Nd-Fe-B-based magnets produced using different manufacturing techniques are studied by soft X-ray magnetic circular dichroism microscopy. The four magnets studied include a hot deformed magnet, sintered magnets produced from either a N2 jet-milled or a He jet-milled fine powder, and a magnet produced by hydrogenation-disproportionation-desorption-recombination (HDDR) followed by He jet-milling. The coercivity of the HDDR processed magnet is lower than the magnets produced with N2 jet-milled powder even though the size of the grains is smaller. By following the magnetic domain reversal behavior during the demagnetization process, it was found that, while the magnets produced with N2 jet-milled powder and the hot deformed magnet presents a predominance of the reversed magnetic domain propagation, in the HDDR processed magnet the nucleation of individual grains occurs before the reversed magnetic domains propagate. This behavior is associated with a decreased nucleation field of each grain.