We describe an accurate periodic boundary condition （PBC） called the symmetric PBC in the calculation of the magnetostatie interaction field in the finite-differentiation-method fast-Fourier-transform （FDM-FFT） mi...We describe an accurate periodic boundary condition （PBC） called the symmetric PBC in the calculation of the magnetostatie interaction field in the finite-differentiation-method fast-Fourier-transform （FDM-FFT） micromagneties. The micromagnetic cells in the regular mesh used by the FDM-FFT method are finite-sized elements, but not geometrical points. Therefore, the key PBC operations for FDM-FFT methods are splitting and relocating the micromagnetic cell surfaces to stay symmetrically inside the box of half-total sizes with respect to the origin. The properties of the demagnetizing matrix of the split micromagnetic cells are discussed, and the sum rules of demagnetizing matrix are fulfilled by the symmetric PBC.展开更多
Microwave permeability spectra of single Co nanotube under equilibrium state have been studied by micromagnetics simulation.More than four obvious resonance peaks have been found(11.72,24.20,33.18 and 39.55 GHz).Such ...Microwave permeability spectra of single Co nanotube under equilibrium state have been studied by micromagnetics simulation.More than four obvious resonance peaks have been found(11.72,24.20,33.18 and 39.55 GHz).Such large resonance frequency cannot be found in other traditional magnetic materials.The configurations of magnetic moments along the nanotube have been simulated.The results show that the top end of nanotube has a"flow-out"pattern of magnetic moments configuration.The bottom end has a"flow-in"pattern of magnetic moments configuration.The magnetic moments within the main body of nanotube are aligned perfectly along the length of nanotube.The magnitude of natural resonance peak is strongly related to the volume fraction of a zone,which has the same orientation of magnetic moments.Large microwave permeability values have been found for single nanotube.The generalized Snoek’s law has been used to validate the micromagnetics simulations in this paper.展开更多
In this paper, a new high accuracy numerical method for the thin-film problems of micron and submicron size ferromagnetic elements is proposed. For the computation of stray field, we use the finite element method(FEM)...In this paper, a new high accuracy numerical method for the thin-film problems of micron and submicron size ferromagnetic elements is proposed. For the computation of stray field, we use the finite element method(FEM) by introducing a semi-discrete artificial boundary condition [1, 2]. In our numerical experiments about the domain patterns and their movement, we can see that the results are accordant to that of experiments and other numerical methods. Our method are very convenient to deal with arbitrary shape of thin films such as a polygon with high accuracy.展开更多
Magnetic nanostructures with nonhomogeneous magnetic properties exhibit distinct magnon modes,and their interactions are crucial for understanding magnetization dynamics.In this work,we numerically investigate the mag...Magnetic nanostructures with nonhomogeneous magnetic properties exhibit distinct magnon modes,and their interactions are crucial for understanding magnetization dynamics.In this work,we numerically investigate the magnon-magnon coupling in a nanodisk with radially varying magnetic anisotropy by using micromagnetic simulations.By introducing perpendicular magnetic anisotropy into the inner region of the nanodisk,a radially chiral spin texture is observed.The presence of the chiral spin texture results in coupling between the ferromagnetic resonance mode of the whole disk and the higher-order confined modes in the outer region.Moreover,we find that the coupling strength is highly sensitive to the perpendicular magnetic anisotropy,the saturation magnetization,and the interfacial Dzyaloshinskii-Moriya interaction.Our findings could enrich the understanding of the dynamic characteristics of chiral nanomagnets and suggest a possible route to harnessing tunable magnon-magnon coupling for spin-based quantum information processing.展开更多
The low coercivity is the major factor inhibiting the large-scale commercial utilization of Nd-Ce-Fe-B sintered magnets.In this work,we achieved a record-high coercivity of 15.04 kOe in Ga-doped Nd-Ce-Fe-B sintered ma...The low coercivity is the major factor inhibiting the large-scale commercial utilization of Nd-Ce-Fe-B sintered magnets.In this work,we achieved a record-high coercivity of 15.04 kOe in Ga-doped Nd-Ce-Fe-B sintered magnets with 30 wt%Ce replacing Nd,demonstrating enormous potential.The Ga-doped Nd-Ce-Fe-B magnets with higher boron(HB)and lower boron(LB)content are designed.The coercivity of the HB magnet increases slightly from 10.80 to 12.26 kOe after annealing,attributed to the optimized distribution of grain boundary(GB)phases.In contrast,the coercivity of the LB magnet remarkably increases from 8.13 to 15.04 kOe after annealing.Microstructural observations indicate that the narrow GB phase in the as-sintered magnet is rich in Fe,and the strong exchange coupling of adjacent grains resulted in low coercivity.The evolution of Ga-rich phases reveals a potential formation mechanism of the RE_(6)Fe_(13)Ga phase,that is the RE-Fe amorphous phase and REGa phase in the as-sintered magnet combine to form the RE_(6)Fe_(13)Ga phase and RE-Ga amorphous phase during post-sinter annealing(RE:rare earth).Moreover,the GB phase of the annealed magnet transforms into a Fe-lean phase with a thickness of 16.4 nm.Magnetization and demagnetization behavior characterizations reveal that the exchange decoupling of adjacent grains induced by the optimized GB phases is the main reason for the remarkable coercivity enhancement,which is also validated by micromagnetic simulations.展开更多
We proposed a new measure to optimize the comprehensive magnetic properties of SmCo_(5)alloy.By compounding Fe-15Ni-3Al-1Ti(FNAT)alloy with high saturation magnetization and Sm(Co,Cu)_(5) matrix alloy in the liquid st...We proposed a new measure to optimize the comprehensive magnetic properties of SmCo_(5)alloy.By compounding Fe-15Ni-3Al-1Ti(FNAT)alloy with high saturation magnetization and Sm(Co,Cu)_(5) matrix alloy in the liquid state,an innovative two-phase separation microstructure or cellular microstructure is formed after melt-spinning using the phase separation effect of the two alloys.At the same time,the element concentration,relative phase content,and microstructure are adjusted by adding different contents of FNAT alloy.The results show that FNAT addition promotes the as-spun ribbons phase separation(or spinodal decomposition)into Co-rich SmCo_(5)-and Sm-Ni-rich CeCo_(5)-or Sm_(2)Co_(7)-type phases.Adding 3 wt.%FNAT increases the coercivity,saturation magnetization,and remanence of the ribbons by 320.6%,39.8%,and 82.8%,respectively.Adding 5 wt.%FNAT promotes forming the Sm_(2)(Co,M)_(7) cell-wall phase and increases the coercivity and remanence by 272.7%and 48.1%,respectively.Finally,the corresponding microstructure evolution models,magnetization,and demagnetization mechanisms are proposed.展开更多
The complexity and intricacy of the brain,which is composed of billions of neurons,pose significant challenges to its study.Understanding neural connections and communication at the single-cell level is crucial for un...The complexity and intricacy of the brain,which is composed of billions of neurons,pose significant challenges to its study.Understanding neural connections and communication at the single-cell level is crucial for unraveling the brain’s functions.This study presents a novel strategy that utilizes magnetic nanoparticles(MNPs)and magnetic fields to manipulate neurons,thereby creating customized small-scale neural circuits for studying neural connections.To establish the feasibility of this approach,the effects of MNPs on neurons were initially investigated,demonstrating their low toxicity.Subsequently,a micromagnet array(MMA)chip was employed to manipulate the neurons,facilitating their precise arrangement on the electrodes.Over several days,the neurons extended their axons and established connections with neighboring cells,forming small-scale circular neural circuits.These artificially engineered circuits offer a simplified and controlled environment for studying neural networks in contrast to naturally occurring biological networks.Furthermore,electrophysiological recordings were conducted to investigate the connections between the manipulated neurons.This study introduces a customized small-scale neural circuit platform with electrode-specific recording and stimulating capabilities,enabling the study of neuron-to-neuron interactions at the single-cell level.By leveraging MNPs and an MMA chip,this research offers a powerful tool for studying neural connections and advancing our understanding of the brain’s intricate workings.展开更多
In this study,DyF_(3)powder was sprayed onto the polar and side surfaces of the magnets to determine the anisotropic diffusion mechanism of Dy in the sintered Nd-Fe-B magnet.The coercivity and squareness of the magnet...In this study,DyF_(3)powder was sprayed onto the polar and side surfaces of the magnets to determine the anisotropic diffusion mechanism of Dy in the sintered Nd-Fe-B magnet.The coercivity and squareness of the magnet in which the diffusion of Dy is perpendicular to the c-axis(a-magnet)are lower than those of the magnet with the diffusion of Dy parallel to the c-axis(c-magnet).Compared with the c-magnet,the a-magnet has a longer Dy-enrichment region from the diffusion surface,where Dy is enriched in the 2:14:1 grain.By contrast,the Dy concentration in the grain boundaries beyond the Dy enrichment region is lower in the a-magnet.Moreover,the Dy shells beyond the Dy enrichment region in the a-magnet are distributed on the side surfaces of the 2:14:1 grains but not on the polar surfaces.Based on the micromagnetic simulation,the Dy shells on the polar surfaces of the grains are more effective in enhancing coercivity.According to first-principle calculations,Dy migrating through 001 into the Nd vacancy in the Nd_(2)Fe_(14)B crystal has a higher diffusion barrier,thus indicating that the lattice diffusion of Dy parallel to the c-axis is more difficult.展开更多
A sintered Nd-Y-Fe-B magnet was designed and manufactured by the multi-main-phase process.Unevenly distributed Y in the magnet decreases the adverse magnetic weakening effect of Y on the coercivity.Grain boundary diff...A sintered Nd-Y-Fe-B magnet was designed and manufactured by the multi-main-phase process.Unevenly distributed Y in the magnet decreases the adverse magnetic weakening effect of Y on the coercivity.Grain boundary diffusion process(GBDP)was conducted to further enhance the coercivity of the Nd-Y-Fe-B magnet.The coercivity increases significantly from 884 to 1741 kA/m after GBDP with Pr_(60)Tb_(10)Cu_(30)alloy.The mechanism of the coercivity enhancement is discussed based on the microstructure analysis.Micromagnetic simulation reveals that when the diffused Tb-rich shell thickness is lower than 12 nm the c-plane shell(perpendicular to the c-axis)is much more effective in enhancing the coercivity than the side plane shell(parallel to the c-axis).But when the Tb-rich shell thickness is above12 nm the side plane shell contributes more to the coercivity enhancement.The results in this work can help to design and manufacture Nd-Fe-B magnets with low cost and high magnetic properties.展开更多
The competition between dimensionality and ordering in multiferroic materials is of great interest for both fundamental physics and potential applications. Combining first-principles calculations with micromagnetic si...The competition between dimensionality and ordering in multiferroic materials is of great interest for both fundamental physics and potential applications. Combining first-principles calculations with micromagnetic simulations, we investigate recently synthesized ultrathin perovskite bismuth ferrite(BFO) films. Our numerical results reveal that, at the monolayer limit, the ferroelectricity of BFO is missing because the octahedral distortions are constrained. However, the monolayer bismuth ferrite is a topological antiferromagnetic metal with tunable bimeron magnetic structure. The dual topologically non-trivial characteristics make monolayer bismuth ferrite a multifunctional building block in future spintronic devices.展开更多
Bit patterned recording(BPR)has attracted much attention due to its promising potential in achieving high densities in magnetic storage devices.The materials with strong perpendicular magnetic anisotropy(PMA)are alway...Bit patterned recording(BPR)has attracted much attention due to its promising potential in achieving high densities in magnetic storage devices.The materials with strong perpendicular magnetic anisotropy(PMA)are always preferred in designing the BPR.Here,the patterned Co/Ni multilayers showing d-d hybridization induced PMA was studied.In particular,we record the ultrafast spin dynamics by means of time-resolved scanning magneto-optical Kerr effect(TRMOKE)microscopy.We are able to acquire the“snapshot”magnetic maps of the sample surface because of both the femtosecond temporal and submicrometer spatial resolution in our TRMOKE microscopy.Furthermore,the spatially inhomogeneous ultrafast demagnetization was observed in experiment,and this has been evidenced by simulations.展开更多
We propose a hybrid coercivity mechanism for exchange-coupled hard/soft multilayers, which incorporates elements of both the traditional nucleation and pinning mechanisms based on both threedimensional(3 D) and one-di...We propose a hybrid coercivity mechanism for exchange-coupled hard/soft multilayers, which incorporates elements of both the traditional nucleation and pinning mechanisms based on both threedimensional(3 D) and one-dimensional(1 D) micromagnetic calculations. The magnetic reversal starts with the nucleation of the domain wall near the defects or soft phases, which ends by the pinning usually in the same place. Therefore, pinning near the nucleation centers are the dominant coercivity mechanism for both exchange-coupled nanocomposites and so-called single-phased permanent magnets. Our proposed coercivity mechanism and calculated results agree very well with available experimental data,especially the recently reported high energy products achieved in NdFeB and SmCo based hard/soft multilayers. The hybrid coercivity mechanism can be readily extended to single-phased permanent magnets with defects and other magnetic systems.展开更多
Magnetization configurations were calculated under various magnetic fields for nanocrystalline Pr-Fe-B permanent magnets by micromagnetic finite element method.According to the configurations during demagnetization pr...Magnetization configurations were calculated under various magnetic fields for nanocrystalline Pr-Fe-B permanent magnets by micromagnetic finite element method.According to the configurations during demagnetization process, the mechanism of magnetization reversal was analyzed.For the Pr2Fe14B with 10 nm grains or its composite with 10vol.% α-Fe, the coercivity was determined by nucleation of reversed domain that took place at grain boundaries.However, for Pr2Fe14B with 30 nm grains, coercivity was controlled by pinning of the nucle-ated domain.For Pr2Fe14B/α-Fe with 30vol.% α-Fe, the demagnetization behavior was characterized by continuous reversal of α-Fe moment.展开更多
The relation between microscopic properties (e.g., layer thickness, easy axis orientation) and the macroscopic magnetic properties such as remanent magnetization of the ferromagnetic multilayer system is investigated ...The relation between microscopic properties (e.g., layer thickness, easy axis orientation) and the macroscopic magnetic properties such as remanent magnetization of the ferromagnetic multilayer system is investigated based on a simple micromagnet approach. We concentrate on a multilayer design with periodic boundary condition, where alternating soft/hard layers build a nanostructured multilayer. For any easy axis direction in the soft and hard layers a simple explicit expression of remanence of the system has been derived analytically. We find that the remanence clearly depends on the thickness of the soft magnetic layer and is nearly independent of the thickness of hard magnetic layer. On the other hand, the remanence increases upon reducing the angle enclosed by the saturation magnetization and the easy axis directions of soft magnetic layer. However, it is unsensitive to the easy axis direction of hard magnetic layer, but there exists a maximum remanence for a certain easy axis direction of hard magnetic layer.展开更多
Multiscale simulation is a key research tool in the quest for new permanent magnets.Starting with first principles methods,a sequence of simulation methods can be applied to calculate the maximum possible coercive fie...Multiscale simulation is a key research tool in the quest for new permanent magnets.Starting with first principles methods,a sequence of simulation methods can be applied to calculate the maximum possible coercive field and expected energy density product of a magnet made from a novel magnetic material composition.Iron(Fe)-rich magnetic phases suitable for permanent magnets can be found by means of adaptive genetic algorithms.The intrinsic properties computed by ab initio simulations are used as input for micromagnetic simulations of the hysteresis properties of permanent magnets with a realistic structure.Using machine learning techniques,the magnet’s structure can be optimized so that the upper limits for coercivity and energy density product for a given phase can be estimated.Structure property relations of synthetic permanent magnets were computed for several candidate hard magnetic phases.The following pairs(coercive field(T),energy density product(kJ·m^-3))were obtained for iron-tin-antimony(Fe3Sn0.75Sb0.25):(0.49,290),L10-ordered iron-nickel(L10 FeNi):(1,400),cobalt-iron-tantalum(CoFe6Ta):(0.87,425),and manganese-aluminum(MnAl):(0.53,80).展开更多
Hysteresis loops,energy products and magnetic moment distributions of perpendicularly oriented Nd2Fe(14)B/α-Fe exchange-spring multilayers are studied systematically based on both three-dimensional(3D)and one-dimensi...Hysteresis loops,energy products and magnetic moment distributions of perpendicularly oriented Nd2Fe(14)B/α-Fe exchange-spring multilayers are studied systematically based on both three-dimensional(3D)and one-dimensional(1D)micromagnetic methods,focused on the influence of the interface anisotropy.The calculated results are carefully compared with each other.The interface anisotropy effect is very palpable on the nucleation,pinning and coercive fields when the soft layer is very thin.However,as the soft layer thickness increases,the pinning and coercive fields are almost unchanged with the increment of interface anisotropy though the nucleation field still monotonically rises.Negative interface anisotropy decreases the maximum energy products and increases slightly the angles between the magnetization and applied field.The magnetic moment distributions in the thickness direction at various applied fields demonstrate a progress of three-step magnetic reversal,i.e.,nucleation,evolution and irreversible motion of the domain wall.The above results calculated by two models are in good agreement with each other.Moreover,the in-plane magnetic moment orientations based on two models are different.The 3D calculation shows a progress of generation and disappearance of vortex state,however,the magnetization orientations within the film plane calculated by the 1D model are coherent.Simulation results suggest that negative interface anisotropy is necessarily avoided experimentally.展开更多
A three-dimensional finite element micromagnetic algorithm was developed to study the magnetization reversal of the SmCo 5/Sm 2Co 17 based magnets. The influences of the microstructure and magnetic parameters on ...A three-dimensional finite element micromagnetic algorithm was developed to study the magnetization reversal of the SmCo 5/Sm 2Co 17 based magnets. The influences of the microstructure and magnetic parameters on the coercivity were studied based on the model consisting of 64 irregular cells according to the experimental microstructure. Numerical results show that the coercivity increases with increasing the 2∶17-type cell size. Large cell boundary thickness leads to small coercivity. The drop of anisotropy constant of 1∶5 phase leads to the coercivity reducing, while the effect of exchange constant of 1∶5 phase on coercivity is contrary to that of exchange constant. The calculated field dependence of coercivity can be predicted by an inhomogeneous domain-wall pinning model. The microstructure parameter was analyzed by comparing the calculated coercivity.展开更多
The hysteresis loops as well as the spin distributions of Sm-Co/a-Fe bilayers have been investigated by both three- dimensional （3D） and one-dimensional （1D） micromagnetic calculations, focusing on the effect of t...The hysteresis loops as well as the spin distributions of Sm-Co/a-Fe bilayers have been investigated by both three- dimensional （3D） and one-dimensional （1D） micromagnetic calculations, focusing on the effect of the interface exchange coupling under various soft layer thicknesses ts. The exchange coupling coefficient Alas between the hard and soft ,layers varies from 1.8 x10-6 erg/cm to 0.45 x 10-6 erg/cm, while the soft layer thickness increases from 2 nm to 10 nm. As the exchange coupling decreases, the squareness of the loop gradually deteriorates, both pinning and coercive fields rise up monotonically, and the nucleation field goes down. On the other hand, an increment of the soft layer thickness leads to a significant drop of the nucleation field, the deterioration of the hysteresis loop squareness, and an increase of the remanence. The simulated loops based on the 3D and 1D methods are consistent with each other and in good agreement with the measured loops for Sm-Co/a-Fe multilayers.展开更多
The magnetization reversal process of nano-size rectangle-shaped NiFe film elements with different aspect ratios have been investigated under the orthogonally applied magnetic fields by micromagnetic simulation. Diffe...The magnetization reversal process of nano-size rectangle-shaped NiFe film elements with different aspect ratios have been investigated under the orthogonally applied magnetic fields by micromagnetic simulation. Different magnetization reversal modes can appear depending on whether the bias field is applied or not. When there is no bias field, double “C” state is the initial reversal state. However, when there is a bias field, “S” state is the starting mode. The larger the aspect ratio is, the larger the switching field is. But, when the aspect ratio is larger than 3, the increase of the switching field ceases. These results can provide useful information to the application of the patterned NiFe film with rectangular elements.展开更多
The magnetization reversal mechanisms for Ni nanowires with different diameters were investigated by micromagnetic simulations. The results show that the reversal mechanisms are significantly dependeht on the diameter...The magnetization reversal mechanisms for Ni nanowires with different diameters were investigated by micromagnetic simulations. The results show that the reversal mechanisms are significantly dependeht on the diameter of wire. For very thin wires, the reversal occurs by pseudo-coherent rotation. With increasing diameter, magnetization reversal takes place via different nucleation （the transverse domain wall and the vortex domain wall） and subsequent propagation. The reason of transition from the transverse domain wall to the vortex domain wall is given by analytical studies. With further increase of the diameter, the reversal nuclear domain wall becomes tundishoshaped form. As the diameter increases, the width of wall becomes larger.展开更多
基金Supported by the National Natural Science Foundation of China under Grant Nos 51171086 and 51371101
文摘We describe an accurate periodic boundary condition （PBC） called the symmetric PBC in the calculation of the magnetostatie interaction field in the finite-differentiation-method fast-Fourier-transform （FDM-FFT） micromagneties. The micromagnetic cells in the regular mesh used by the FDM-FFT method are finite-sized elements, but not geometrical points. Therefore, the key PBC operations for FDM-FFT methods are splitting and relocating the micromagnetic cell surfaces to stay symmetrically inside the box of half-total sizes with respect to the origin. The properties of the demagnetizing matrix of the split micromagnetic cells are discussed, and the sum rules of demagnetizing matrix are fulfilled by the symmetric PBC.
基金supported by the National Natural Science Foundation of China(Grant No.61271039)the Scientific Foundation of Young Scientists of Sichuan Province(Grant No.2012JQ0053)the Program for New Century Excellent Talents in Universities(NCET-11-0060)
文摘Microwave permeability spectra of single Co nanotube under equilibrium state have been studied by micromagnetics simulation.More than four obvious resonance peaks have been found(11.72,24.20,33.18 and 39.55 GHz).Such large resonance frequency cannot be found in other traditional magnetic materials.The configurations of magnetic moments along the nanotube have been simulated.The results show that the top end of nanotube has a"flow-out"pattern of magnetic moments configuration.The bottom end has a"flow-in"pattern of magnetic moments configuration.The magnetic moments within the main body of nanotube are aligned perfectly along the length of nanotube.The magnitude of natural resonance peak is strongly related to the volume fraction of a zone,which has the same orientation of magnetic moments.Large microwave permeability values have been found for single nanotube.The generalized Snoek’s law has been used to validate the micromagnetics simulations in this paper.
基金This work was supported by the Climbing Program of National Key Project of Foundation.
文摘In this paper, a new high accuracy numerical method for the thin-film problems of micron and submicron size ferromagnetic elements is proposed. For the computation of stray field, we use the finite element method(FEM) by introducing a semi-discrete artificial boundary condition [1, 2]. In our numerical experiments about the domain patterns and their movement, we can see that the results are accordant to that of experiments and other numerical methods. Our method are very convenient to deal with arbitrary shape of thin films such as a polygon with high accuracy.
基金supported by the National Key Research and Development Program of China(Grant No.2023YFF0718400)the National Natural Science Foundation of China(Grant Nos.12474119 and 12074189)。
文摘Magnetic nanostructures with nonhomogeneous magnetic properties exhibit distinct magnon modes,and their interactions are crucial for understanding magnetization dynamics.In this work,we numerically investigate the magnon-magnon coupling in a nanodisk with radially varying magnetic anisotropy by using micromagnetic simulations.By introducing perpendicular magnetic anisotropy into the inner region of the nanodisk,a radially chiral spin texture is observed.The presence of the chiral spin texture results in coupling between the ferromagnetic resonance mode of the whole disk and the higher-order confined modes in the outer region.Moreover,we find that the coupling strength is highly sensitive to the perpendicular magnetic anisotropy,the saturation magnetization,and the interfacial Dzyaloshinskii-Moriya interaction.Our findings could enrich the understanding of the dynamic characteristics of chiral nanomagnets and suggest a possible route to harnessing tunable magnon-magnon coupling for spin-based quantum information processing.
基金supported by the National Natural Science Foundation of China(Nos.52261037,52088101)the Key research project of Jiangxi Province(No.20203ABC28W006)the Double-Thousand Plan of Jiangxi Province(No.jxsq2023101057).
文摘The low coercivity is the major factor inhibiting the large-scale commercial utilization of Nd-Ce-Fe-B sintered magnets.In this work,we achieved a record-high coercivity of 15.04 kOe in Ga-doped Nd-Ce-Fe-B sintered magnets with 30 wt%Ce replacing Nd,demonstrating enormous potential.The Ga-doped Nd-Ce-Fe-B magnets with higher boron(HB)and lower boron(LB)content are designed.The coercivity of the HB magnet increases slightly from 10.80 to 12.26 kOe after annealing,attributed to the optimized distribution of grain boundary(GB)phases.In contrast,the coercivity of the LB magnet remarkably increases from 8.13 to 15.04 kOe after annealing.Microstructural observations indicate that the narrow GB phase in the as-sintered magnet is rich in Fe,and the strong exchange coupling of adjacent grains resulted in low coercivity.The evolution of Ga-rich phases reveals a potential formation mechanism of the RE_(6)Fe_(13)Ga phase,that is the RE-Fe amorphous phase and REGa phase in the as-sintered magnet combine to form the RE_(6)Fe_(13)Ga phase and RE-Ga amorphous phase during post-sinter annealing(RE:rare earth).Moreover,the GB phase of the annealed magnet transforms into a Fe-lean phase with a thickness of 16.4 nm.Magnetization and demagnetization behavior characterizations reveal that the exchange decoupling of adjacent grains induced by the optimized GB phases is the main reason for the remarkable coercivity enhancement,which is also validated by micromagnetic simulations.
基金supported by the General Program from the National Natural Science Foundation of China(NNSFC)(No.52371185)Central Government Guides Local Funds for Science and Technology Development(No.216Z1008G)the Natural Science Foundation of Hebei province,China(No.E2022202017).
文摘We proposed a new measure to optimize the comprehensive magnetic properties of SmCo_(5)alloy.By compounding Fe-15Ni-3Al-1Ti(FNAT)alloy with high saturation magnetization and Sm(Co,Cu)_(5) matrix alloy in the liquid state,an innovative two-phase separation microstructure or cellular microstructure is formed after melt-spinning using the phase separation effect of the two alloys.At the same time,the element concentration,relative phase content,and microstructure are adjusted by adding different contents of FNAT alloy.The results show that FNAT addition promotes the as-spun ribbons phase separation(or spinodal decomposition)into Co-rich SmCo_(5)-and Sm-Ni-rich CeCo_(5)-or Sm_(2)Co_(7)-type phases.Adding 3 wt.%FNAT increases the coercivity,saturation magnetization,and remanence of the ribbons by 320.6%,39.8%,and 82.8%,respectively.Adding 5 wt.%FNAT promotes forming the Sm_(2)(Co,M)_(7) cell-wall phase and increases the coercivity and remanence by 272.7%and 48.1%,respectively.Finally,the corresponding microstructure evolution models,magnetization,and demagnetization mechanisms are proposed.
基金supported by Westlake Universitythe Research Center for Industries of the Future of Westlake University (No. WU2022C040).
文摘The complexity and intricacy of the brain,which is composed of billions of neurons,pose significant challenges to its study.Understanding neural connections and communication at the single-cell level is crucial for unraveling the brain’s functions.This study presents a novel strategy that utilizes magnetic nanoparticles(MNPs)and magnetic fields to manipulate neurons,thereby creating customized small-scale neural circuits for studying neural connections.To establish the feasibility of this approach,the effects of MNPs on neurons were initially investigated,demonstrating their low toxicity.Subsequently,a micromagnet array(MMA)chip was employed to manipulate the neurons,facilitating their precise arrangement on the electrodes.Over several days,the neurons extended their axons and established connections with neighboring cells,forming small-scale circular neural circuits.These artificially engineered circuits offer a simplified and controlled environment for studying neural networks in contrast to naturally occurring biological networks.Furthermore,electrophysiological recordings were conducted to investigate the connections between the manipulated neurons.This study introduces a customized small-scale neural circuit platform with electrode-specific recording and stimulating capabilities,enabling the study of neuron-to-neuron interactions at the single-cell level.By leveraging MNPs and an MMA chip,this research offers a powerful tool for studying neural connections and advancing our understanding of the brain’s intricate workings.
基金supported by the National Natural Science Foundation of China(52361033)National Key Research and Development Program(2022YFB3505400)+1 种基金the Main Discipline and Technology Leaders Training Plan of Jiangxi Province(2022BCJ23007)the Jiangxi Province Postgraduate Innovation Project(YC2022-S693)。
文摘In this study,DyF_(3)powder was sprayed onto the polar and side surfaces of the magnets to determine the anisotropic diffusion mechanism of Dy in the sintered Nd-Fe-B magnet.The coercivity and squareness of the magnet in which the diffusion of Dy is perpendicular to the c-axis(a-magnet)are lower than those of the magnet with the diffusion of Dy parallel to the c-axis(c-magnet).Compared with the c-magnet,the a-magnet has a longer Dy-enrichment region from the diffusion surface,where Dy is enriched in the 2:14:1 grain.By contrast,the Dy concentration in the grain boundaries beyond the Dy enrichment region is lower in the a-magnet.Moreover,the Dy shells beyond the Dy enrichment region in the a-magnet are distributed on the side surfaces of the 2:14:1 grains but not on the polar surfaces.Based on the micromagnetic simulation,the Dy shells on the polar surfaces of the grains are more effective in enhancing coercivity.According to first-principle calculations,Dy migrating through 001 into the Nd vacancy in the Nd_(2)Fe_(14)B crystal has a higher diffusion barrier,thus indicating that the lattice diffusion of Dy parallel to the c-axis is more difficult.
基金Project supported by the National Natural Science Foundation of China(51901087)China Postdoctoral Science Foundation(2021M701504)。
文摘A sintered Nd-Y-Fe-B magnet was designed and manufactured by the multi-main-phase process.Unevenly distributed Y in the magnet decreases the adverse magnetic weakening effect of Y on the coercivity.Grain boundary diffusion process(GBDP)was conducted to further enhance the coercivity of the Nd-Y-Fe-B magnet.The coercivity increases significantly from 884 to 1741 kA/m after GBDP with Pr_(60)Tb_(10)Cu_(30)alloy.The mechanism of the coercivity enhancement is discussed based on the microstructure analysis.Micromagnetic simulation reveals that when the diffused Tb-rich shell thickness is lower than 12 nm the c-plane shell(perpendicular to the c-axis)is much more effective in enhancing the coercivity than the side plane shell(parallel to the c-axis).But when the Tb-rich shell thickness is above12 nm the side plane shell contributes more to the coercivity enhancement.The results in this work can help to design and manufacture Nd-Fe-B magnets with low cost and high magnetic properties.
基金supported by the National Natural Science Foundation of China (Grant No. 12174382)the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB0460000 and XDB28000000)the Innovation Program for Quantum Science and Technology (Grant Nos. 2024ZD0300104 and 2021ZD0302600)。
文摘The competition between dimensionality and ordering in multiferroic materials is of great interest for both fundamental physics and potential applications. Combining first-principles calculations with micromagnetic simulations, we investigate recently synthesized ultrathin perovskite bismuth ferrite(BFO) films. Our numerical results reveal that, at the monolayer limit, the ferroelectricity of BFO is missing because the octahedral distortions are constrained. However, the monolayer bismuth ferrite is a topological antiferromagnetic metal with tunable bimeron magnetic structure. The dual topologically non-trivial characteristics make monolayer bismuth ferrite a multifunctional building block in future spintronic devices.
基金Project supported by the National Key Research and Development Program of China(Grant No.2022YFA1403302)the National Natural Science Foundation of China(Grant Nos.52031015,U22A20115,and 12104030)+1 种基金the Natural Science Foundation of Zhejiang Province,China(Grant No.LZ25A040007)the Natural Science Foundation of Beijing(Grant No.1252026).
文摘Bit patterned recording(BPR)has attracted much attention due to its promising potential in achieving high densities in magnetic storage devices.The materials with strong perpendicular magnetic anisotropy(PMA)are always preferred in designing the BPR.Here,the patterned Co/Ni multilayers showing d-d hybridization induced PMA was studied.In particular,we record the ultrafast spin dynamics by means of time-resolved scanning magneto-optical Kerr effect(TRMOKE)microscopy.We are able to acquire the“snapshot”magnetic maps of the sample surface because of both the femtosecond temporal and submicrometer spatial resolution in our TRMOKE microscopy.Furthermore,the spatially inhomogeneous ultrafast demagnetization was observed in experiment,and this has been evidenced by simulations.
基金Project supported by National Natural Science Foundation of China(51771127,51571126,51772004)
文摘We propose a hybrid coercivity mechanism for exchange-coupled hard/soft multilayers, which incorporates elements of both the traditional nucleation and pinning mechanisms based on both threedimensional(3 D) and one-dimensional(1 D) micromagnetic calculations. The magnetic reversal starts with the nucleation of the domain wall near the defects or soft phases, which ends by the pinning usually in the same place. Therefore, pinning near the nucleation centers are the dominant coercivity mechanism for both exchange-coupled nanocomposites and so-called single-phased permanent magnets. Our proposed coercivity mechanism and calculated results agree very well with available experimental data,especially the recently reported high energy products achieved in NdFeB and SmCo based hard/soft multilayers. The hybrid coercivity mechanism can be readily extended to single-phased permanent magnets with defects and other magnetic systems.
基金supported by the National Natural Science Foundation of China (10574156)
文摘Magnetization configurations were calculated under various magnetic fields for nanocrystalline Pr-Fe-B permanent magnets by micromagnetic finite element method.According to the configurations during demagnetization process, the mechanism of magnetization reversal was analyzed.For the Pr2Fe14B with 10 nm grains or its composite with 10vol.% α-Fe, the coercivity was determined by nucleation of reversed domain that took place at grain boundaries.However, for Pr2Fe14B with 30 nm grains, coercivity was controlled by pinning of the nucle-ated domain.For Pr2Fe14B/α-Fe with 30vol.% α-Fe, the demagnetization behavior was characterized by continuous reversal of α-Fe moment.
文摘The relation between microscopic properties (e.g., layer thickness, easy axis orientation) and the macroscopic magnetic properties such as remanent magnetization of the ferromagnetic multilayer system is investigated based on a simple micromagnet approach. We concentrate on a multilayer design with periodic boundary condition, where alternating soft/hard layers build a nanostructured multilayer. For any easy axis direction in the soft and hard layers a simple explicit expression of remanence of the system has been derived analytically. We find that the remanence clearly depends on the thickness of the soft magnetic layer and is nearly independent of the thickness of hard magnetic layer. On the other hand, the remanence increases upon reducing the angle enclosed by the saturation magnetization and the easy axis directions of soft magnetic layer. However, it is unsensitive to the easy axis direction of hard magnetic layer, but there exists a maximum remanence for a certain easy axis direction of hard magnetic layer.
基金This work was supported by the EU H2020 project NOVAMAG(686056)and the Austrian Science Fund FWF(I3288-N36).Sergiu Arapan and Pablo Nieves acknowledge the European Regional Development Fund in the IT4Innovations National Supercomputing Center—path to exascale project(CZ 02.1.01/0.0/0.0/16-013/0001791)within the Operational Programme Research,Development and Education,and IT4Innovations computational resources allocated within projects OPEN-11-33,OPEN-14-23,and OPEN-17-14.
文摘Multiscale simulation is a key research tool in the quest for new permanent magnets.Starting with first principles methods,a sequence of simulation methods can be applied to calculate the maximum possible coercive field and expected energy density product of a magnet made from a novel magnetic material composition.Iron(Fe)-rich magnetic phases suitable for permanent magnets can be found by means of adaptive genetic algorithms.The intrinsic properties computed by ab initio simulations are used as input for micromagnetic simulations of the hysteresis properties of permanent magnets with a realistic structure.Using machine learning techniques,the magnet’s structure can be optimized so that the upper limits for coercivity and energy density product for a given phase can be estimated.Structure property relations of synthetic permanent magnets were computed for several candidate hard magnetic phases.The following pairs(coercive field(T),energy density product(kJ·m^-3))were obtained for iron-tin-antimony(Fe3Sn0.75Sb0.25):(0.49,290),L10-ordered iron-nickel(L10 FeNi):(1,400),cobalt-iron-tantalum(CoFe6Ta):(0.87,425),and manganese-aluminum(MnAl):(0.53,80).
基金Project supported by the National Key Research and Development Program of China(Grant No.2016YFB0700900)the National Natural Science Foundation of China(Grant Nos.51571126 and 51861030)+1 种基金the Inner Mongolia Autonomous Region Natural Science Foundation of China(Grant No.2019MS01002)the Inner Mongolia Innovative Research Team of China(Grant No.3400102)。
文摘Hysteresis loops,energy products and magnetic moment distributions of perpendicularly oriented Nd2Fe(14)B/α-Fe exchange-spring multilayers are studied systematically based on both three-dimensional(3D)and one-dimensional(1D)micromagnetic methods,focused on the influence of the interface anisotropy.The calculated results are carefully compared with each other.The interface anisotropy effect is very palpable on the nucleation,pinning and coercive fields when the soft layer is very thin.However,as the soft layer thickness increases,the pinning and coercive fields are almost unchanged with the increment of interface anisotropy though the nucleation field still monotonically rises.Negative interface anisotropy decreases the maximum energy products and increases slightly the angles between the magnetization and applied field.The magnetic moment distributions in the thickness direction at various applied fields demonstrate a progress of three-step magnetic reversal,i.e.,nucleation,evolution and irreversible motion of the domain wall.The above results calculated by two models are in good agreement with each other.Moreover,the in-plane magnetic moment orientations based on two models are different.The 3D calculation shows a progress of generation and disappearance of vortex state,however,the magnetization orientations within the film plane calculated by the 1D model are coherent.Simulation results suggest that negative interface anisotropy is necessarily avoided experimentally.
文摘A three-dimensional finite element micromagnetic algorithm was developed to study the magnetization reversal of the SmCo 5/Sm 2Co 17 based magnets. The influences of the microstructure and magnetic parameters on the coercivity were studied based on the model consisting of 64 irregular cells according to the experimental microstructure. Numerical results show that the coercivity increases with increasing the 2∶17-type cell size. Large cell boundary thickness leads to small coercivity. The drop of anisotropy constant of 1∶5 phase leads to the coercivity reducing, while the effect of exchange constant of 1∶5 phase on coercivity is contrary to that of exchange constant. The calculated field dependence of coercivity can be predicted by an inhomogeneous domain-wall pinning model. The microstructure parameter was analyzed by comparing the calculated coercivity.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11074179 and 10747007)the National Basic Research Program of Chi(Grant No.2014CB643702)+3 种基金the Zhejiang Provincial Natural Science Foundation of China(Grant No.LY14E010006)the Construction Plan for Scientif Research Innovation Teams of Universities in Sichuan Province,China(Grant No.12TD008)the Scientific Research Foundation for the Returned Overse Chinese Scholars of the Education Ministry,Chinathe Program for Key Science and Technology Innovation Team of Zhejiang Province,China(Gra No.2013TD08)
文摘The hysteresis loops as well as the spin distributions of Sm-Co/a-Fe bilayers have been investigated by both three- dimensional （3D） and one-dimensional （1D） micromagnetic calculations, focusing on the effect of the interface exchange coupling under various soft layer thicknesses ts. The exchange coupling coefficient Alas between the hard and soft ,layers varies from 1.8 x10-6 erg/cm to 0.45 x 10-6 erg/cm, while the soft layer thickness increases from 2 nm to 10 nm. As the exchange coupling decreases, the squareness of the loop gradually deteriorates, both pinning and coercive fields rise up monotonically, and the nucleation field goes down. On the other hand, an increment of the soft layer thickness leads to a significant drop of the nucleation field, the deterioration of the hysteresis loop squareness, and an increase of the remanence. The simulated loops based on the 3D and 1D methods are consistent with each other and in good agreement with the measured loops for Sm-Co/a-Fe multilayers.
文摘The magnetization reversal process of nano-size rectangle-shaped NiFe film elements with different aspect ratios have been investigated under the orthogonally applied magnetic fields by micromagnetic simulation. Different magnetization reversal modes can appear depending on whether the bias field is applied or not. When there is no bias field, double “C” state is the initial reversal state. However, when there is a bias field, “S” state is the starting mode. The larger the aspect ratio is, the larger the switching field is. But, when the aspect ratio is larger than 3, the increase of the switching field ceases. These results can provide useful information to the application of the patterned NiFe film with rectangular elements.
基金supported by the National Natural Sci-ence Foundation of China under Grant No. 60571043 the Natural Science Foundation of Hunan Provinceof China under Grant No. 04JJ3078.
文摘The magnetization reversal mechanisms for Ni nanowires with different diameters were investigated by micromagnetic simulations. The results show that the reversal mechanisms are significantly dependeht on the diameter of wire. For very thin wires, the reversal occurs by pseudo-coherent rotation. With increasing diameter, magnetization reversal takes place via different nucleation （the transverse domain wall and the vortex domain wall） and subsequent propagation. The reason of transition from the transverse domain wall to the vortex domain wall is given by analytical studies. With further increase of the diameter, the reversal nuclear domain wall becomes tundishoshaped form. As the diameter increases, the width of wall becomes larger.
