Micromagnetic simulations of reversal magnetization in cerium-containing magnets

Single-grain models with different cerium contents or structural parameters have been introduced to investigate the reversal magnetization behaviors in cerium-containing magnets. All the micromagnetic simulations are carried out via the object oriented micromagnetic framework(OOMMF). As for single(Nd,Ce)_2 Fe_(14)B type grain, the coercivity decreases monotonously with the increase of the cerium content. Four types of grain structure have been compared: single(Nd,Ce)_2 Fe_(14)B type, core((Nd,Ce)_2 Fe_(14)B)-shell(Nd_2 Fe_(14)B) type with 2 nm thick shell, core(Ce_2 Fe_(14)B)-shell(Nd_2 Fe_(14)B) type, and core(Nd_2 Fe_(14)B)-shell(Ce_2 Fe_(14)B) type. It is found that core((Nd,Ce)_2 Fe_(14)B)-shell(Nd_2 Fe_(14)B)type grain with 2 nm thick shell always presents the largest coercivity under the same total cerium content. Furthermore,the relationship between the coercivity and the shell thickness t in core((Nd,Ce)_2 Fe_(14)B)-shell(Nd_2 Fe_(14)B) type grain has been studied. When the total cerium content is kept at 20.51 at.%, the analyzed results show that as t varies from 1 nm to 7 nm, the coercivity gradually ascends at the beginning, then quickly descends after reaching the maximum value when t = 5 nm. From the perspective of the positions of nucleation points, the reasons why t affects the coercivity are discussed in detail.

Preparation and Magnetic Properties of (Fe_7Co_3)_(0.15)(SiO_2)_(0.85)Granular Solids Using the Sol-Gel Method

(Fe7Co3)0.15(SiO2)0.85 granular alloy solid was prepared successfully using sol-gel method. The samples with different reducing temperatures were investigated by X-ray diffractometer(XRD),transmission electron micrography(TEM) and vibrating sample magnetometer(VSM). The av-erage particIe sizes of the samples were also calculated from Scherrer formula. The magnetic properties of (Fe7Co3 )o. 15 (SiO2)0.85 were studied in detail.

High cerium content(Nd-Pr-Ce)_(2)Fe_(14)B melt-spun ribbons with high coercivity

The effects of wheel speeds on the magnetic properties and microstructures of[(Nd,Pr)_(1-x)Ce_(x)]-Fe-B meltspun ribbons were investigated.Compared to melt-spun ribbons with low cerium(Ce)content(x=0.2),amorphous formation can be suppressed at high wheel speed in the ribbons with a relatively high Ce content(x=0.8),and with the increase of the wheel speed,the magnetic properties of the ribbons with high Ce content raise continuously.At high wheel speed,the coercivity mechanism of melt-spun ribbons is mainly pinning field,and the high wheel speed introduces a large number of defects into the matrix as pinning sites.Furthermore,ribbons with high Ce content form a fine and uniform grain structure,so the grain boundary area enlarges,which further hinders the movement of domain walls.The combined effects of the above factors enable ribbons with high Ce content to have excellent magnetic properties(Hcj=0.977 T,Mr=64 m^(2) A/kg)at the wheel speed of 35 m/s.

Superior corrosion resistance and corrosion mechanism of dual-main-phase (Ce_(15)Nd_(85))_(30)Fe_(bal)B_(1)M magnets in different solutions

The electrochemical corrosion behavior of both (Ce_(15)Nd_(85))_(30)Fe_(bal)B_(1)M sintered magnets prepared with dual-main-phase method and N45-type magnets was studied in 3.5 wt% NaCl,1.1 wt% NaH_(2)PO_(4),and 2.5 wt% NaOH solutions,respectively.The (Ce_(15)Nd_(85))_(30)Fe_(bal)B_(1)M sintered magnets perform superior corrosion resistance than N45-type magnets in the tested solutions.In general,two kinds of magnets exhibit the best corrosion resistance property in 2.5 wt% NaOH solution,while the worst in 3.5 wt% NaCl solution.Microstructures of samples before and after corrosion were investigated.With the addition of Ce by means of dual-main-phase method,which is conducive to facilitating low-temperature sintering and grain refinement,uniform grain size and(Nd,Ce)-rich phase distribution form,give rise to narrow and thin corrosion channels.Moreover,the corrosion rate of the(Nd,Ce)-rich phase is lower than that of Nd-rich one.Acco rdingly,corro sion re sistance of (Ce_(15)Nd_(85))_(30)Fe_(bal)B_(1)M sintered magnet is superior when compared with the commercial N45-type magnet which has comparable magnetic properties.

Magnetic properties,crystalline and magnetic microstructures of dual-main-phase(Nd,Ce)-Fe-B sintered magnets

The(Nd,Ce)-Fe-B sintered magnets were prepared by a dual-main-phase(DMP)process under the same conditions using different combinations of Nd-Fe-B with Ce-Fe-B and(Nd,Ce)-Fe-B strips.The crystalline and magnetic microstructures of DMP(Nd,Ce)-Fe-B magnets with the remanence of 11.92-12.68 kGs,the intrinsic coercivity of 3.97-5.31 kOe,and the maximum energy product of 23.08-32.99 MGOe have been investigated.Magnetic force microscope(MFM)investigations reveal that the DMP(Nd,Ce)-Fe-B magnets show maze-like patterns,which are like that of standard anisotropy Nd-Fe-B sintered magnets by and large.However,much finer domain structures mixing with coarse ones can be observed obviously in DMP(Nd,Ce)-Fe-B sintered magnets.The size distribution of the domain width of the DMP(Nd,Ce)-Fe-B magnet is not uniform obviously.The average domain width is W=0.912μm,and the fine domain width has only 0.216μm.The smaller domain width and more branch domain patterns exist in poorer coercivity DMP magnets.This is caused by the non-uniform Ce composition of poorer property DMP magnets and the grain boundary microstructure that is not conducive to improving the coercivity.Furthermore,it is found that some domains of rare-earth-rich grain boundary phases exhibit the characteristics of plate-like patterns rather than no-contrast by using MFM,indicating their ferromagnetism.Obvious correlations between the crystalline microstructure,chemical composition of phases,and magnetic structure were demonstrated for the DMP magnets.

Micron-scale 1:5H-based precipitated phase with the lamellar structure of sintered Sm_(2)Co_(17)-based magnets and its potential application

The excellent thermal stability of magnetic properties of Sm_(2)Co_(17)-based magnets is their most impor-tant feature.However,this stability is reduced when the maximum energy product of Sm_(2)Co_(17)-based magnets is improved,which is mainly determined by the Fe/Cu distribution of the 2:17R cell and 1:5H cell boundary phases.During the demagnetization process,the Cu-rich 1:5H cell boundary phase with a width of 2-15 nm obstructs the motion of the domain walls,yielding coercivity.Herein,we report a micron-scale Cu/Zr-rich and Fe-lean 1:5H-based precipitated phase with a lamellar structure,probably induced by Sm_(2)O_(3) doping.This structure enables the separate regulation of Fe and Cu distribution for Sm_(2)Co_(17)-based magnets with Fe-rich 2:17R cell phases and Cu-rich 1:5H cell boundary phases,consid-erably optimizing the thermal stability of magnetic properties.This discovery can be further developed to produce Sm_(2)Co_(17)-based magnets with high performance and excellent thermal stability of magnetic properties.

Study on preparation and properties of CeO_2/epoxy resin composite coating on sintered NdFeB magnet

The CeO2/epoxy resin composite coating was deposited on NdFeB substrate by cathode electrophoresis method for enhancing the anticorrosion and anti-wear performances. The morphologies and structures were characterized by a scanning electron microscope and an X-ray diffractometer. The micro hardness of the composite coating was evaluated by a microhardness tester. The corrosive behaviors of the coatings were studied by potentiodynamic polarization curve, electrochemical impedance spectroscopy and neutral salt spray tests. The concentration of CeO2 nanoparticles（NPs） in the electrophoresis bath was optimized according to the coating structures and anticorrosion performances. The results show that CeO2 NPs can enhance the microhardness of the composite coatings. Moreover, the nanoparticles disperse uniformly in the matrix when the concentration is lower than 30 g/L. The microhardness of CeO2/epoxy resin（30 g/L） composite coating is about 63% higher than that of the blank epoxy resin coating. And the NSS time of the CeO2/epoxy resin（30 g/L） composite coated sample can reach 1248 h.Meanwhile, the composite coatings possess no deteriorate influence on the magnetic properties of NdFeB substrates. The anticorrosion mechanisms of the composite coatings on the NdFeB substrate are deeply discussed.

Coercivity enhancement of nanocrystalline hot-deformed Nd-Fe-B magnets by low-melting eutectic MM-Cu(MM=La,Ce,Pr,Nd) alloys addition

MM85Cu15(MM = La,Ce,Pr,Nd) eutectic alloys were added into the hot-deformed Nd-Fe-B magnets to enhance the coercivity.It is found that three endothermic peaks occur on the differential scanning calorimetry curve of the MM-Cu melt-spun ribbons at 432.2,451.1 and 516.5℃.The peaks substantially correspond to three types of MM-Cu low-melting eutectic phase.The coercivity of magnets increases when the MM-Cu content is lower than 4 wt%,and then keeps almost no change with the content further increasing to 5 wt%.The coercivity of the hot-deformed magnets with 4 wt% and without MM-Cu addition is 948 and 683 kA/m,respectively.Nearly all the platelet-shaped grains are isolated by the thickened intergranular phase after MM-Cu addition.Moreover,the average grain size of the magnets with MM-Cu addition decreases compared with that of the magnet without MM-Cu addition.Scanning electron microscopy images show that the areal fraction of the RE-rich grain boundary phase increases from 8.6% to 15.1% after MM-Cu addition.The La,Ce together with Cu and Ga aggregate at the grain boundary regions separating neighboring grains and smoothing the grain boundaries.Therefore,both the thickened grain boundary and decreased mean grain size result in the enhancement of coercivity after MM-Cu eutectic alloy addition.

Optimization of microstructures and magnetic properties of Sm(Co_(bal)Fe_(0.227)Cu_(0.07)Zr_(0.023))_(7.6) magnets by sintering treatment

Magnetic properties and microstructures of Sm(Co_(bal)Fe_(0.227)Cu_(0.07)Zr_(0.023_)_(7.6) sintered magnets were optimized by sintering treatment. Results show that the knee-point magnetic field, Hknee, is twofold up and the intrinsic coercivity Hcjincreases by 40%, ranging from 21.64 to 30.39 kOe at the cost of a little decrease of Brfrom 10.84 to 10.31 kGs with sintering temperature decreasing from 1488 to 1473 K. And the average domain width is narrower and more uniform for the specimen sintered at 1473 K than that of the specimen sintered at 1488 K. It is impressive that the density of lamellar phase increases from ~0.050 to ~0.058 nm^(-1) with the sintering temperature decreasing from 1488 to 1473 K. Moreover, the average cellular size is about ~84 nm for the magnets sintered at 1473 K, which is 80% of that of the magnets sintered at 1488 K(~97 nm). And the cell boundary width of the magnets sintered at 1473 K(~7 nm) is only half average width of the magnets sintered at 1488 K(~14 nm). It is found that the Cu content in the cell boundaries is much higher(~17 at%) in the magnets sintered at 1473 K compared to that of the magnets sintered at 1488 K(~10 at%). It can be concluded that smaller cells and narrower cell boundaries together with higher gradient of Cu content are key points for obtaining the optimum Hkneeand Hcj.

Effects of grain boundary ternary alloy doping on corrosion resistance of(Ce,Pr,Nd)-Fe-B permanent magnets

To satisfy the application of different environments,grain boundary doping is commonly used in the preparation of sintered magnets to improve the coercivity and the corrosion resistance.In this paper,the alloys were prepared by mixing different ratios of the master alloy(Ce,Pr,Nd)-Fe-B and the sintering aid(Pr,Nd)-Al.The coercivity of sintered(Ce,Pr,Nd)-Fe-B magnet is substantially enhanced by doping 2 wt%of(Pr,Nd)-Al,while the maximum energy product decreases slightly.We systematically investigated the corrosion behavior and micro structure of the sintered magnets in order to determine the mechanism of the degradation.The sintered(Ce,Pr,Nd)-Fe-B magnets with 2 wt% of(Pr,Nd)-Al addition exhibit the decreasing corrosion rate compared with others,due to the distribution of intergranular phases.The electrode potential difference between the main phase and the RE-rich phase is reduced by the addition of Al,improving the potential and stability of RE-rich phase due to the higher electrode potential of Al than that of Nd,Pr or Ce.In addition,the element distribution of the magnets doped by(Pr,Nd)-Al indicates that the Al-rich shell formed at the marginal area of the Ce-rich phase improves its stability.Therefo re,intergranular adding te rnary(Pr,Nd)-Al alloy powders results in both high coe rcivity and good corrosion resistance synchronously.

Anisotropic corrosion behavior of sintered(Ce_(0.15)Nd_(0.85))_(30)Fe_(bal)B permanent magnets

In this work, the corrosion behavior of the surface parallel to the oriented c-axis(c‖) and perpendicular to c-axis(c⊥)in the(Ce_(0.15)Nd_(0.85))_(30)Fe_(bal)B dual main phase magnet was studied. With the addition of Ce,the volume fraction of RE-rich phase shown in the backscattered electron(BSE) images is basically approximate on the two surfaces. The free corrosion potential(E_(corr)) of the c‖ surface is more negative,which shows the worse corrosion resistance from the perspective of thermodynamics. While the reaction kinetics parameters with the smaller free corrosion current(i_(corr)) and the larger transfer resistance(R_(ct)) react the opposite conclusion in 3.5 wt% NaCl solution. Moreover, the c‖ surface performs smaller ions concentration of corrosion product and less damaged corrosion morphology compared to c⊥ surface after the free corrosion. The inconformity is not affected by the RE-rich phase, but by the anisotropy of the grains that the c‖ surface has larger density of the atoms and the lower ratio of(Nd, Ce)/Fe.

Dependence of magnetic properties on microstructure and composition of Ce-Fe-B sintered magnets

In this paper,dependence of magnetic properties on microstructure and composition of Ce-Fe-B sintered magnets with Cu-doped Ce-rich alloy addition was investigated.It shows that the maximum energy product(BH)(max)and coercivity H(cj)of Ce-Fe-B sintered magnet are improved from 6.76 to 9.13 MGOe by 35.1%,and from 1.44 to 1.67 kOe by 16.0%,respectively,via adding 5 wt%liquid phase alloy of Ce(35.58)Fe(57.47)Cu6 B(0.95)(at%).Compared with the magnet without Cerich alloy addition,the volume fraction of the grain-boundary phase with low melting point increases in the magnet with Ce-rich alloy additio n,which is be ne ficial to imp roving the microstructure and promoting the coercivity enhancement of the magnet.In the Ce-Fe-B magnet with Ce-rich alloy addition,Cu and Ce enrich in the grain boundaries of the magnet after annealing,therefore the as-annealed magnet has a higher coercivity than the as-sintered magnet.A distinct Fe-rich layer with the average thickness of 60 nm is found in the grain boundaries in the magnet without Ce-rich alloy addition,but it seems that Fe-rich phase disappears in the magnet with Ce-rich alloy addition.The present work suggests that the further improvement of coercivity in the Ce-Fe-B sintered magnets is expectable by designing the composition and structure of added liquid phase alloys.

Optimization of both coercivity and knee-point magnetic field of Sm2Co17-type magnets via solid solution process

It is confirmed that phase homogenization is very important for improving the magnetic properties of 2:17-type Sm-Co sintered magnets,In this work,the influence of solid solution process on microstructure and magnetic properties of the Sm(CobalFe0.233Cu0.073Zr0.024)7.6 sintered magnets was systematically studied.With the solid-solution treating duration(tS)increasing from 0 to 4 h,intrinsic coercivity(Hcj)increases from 12.83 to 36.54 kOe,magnetic field at knee-point(Hknee)increases from2.76 to 19.14 kOe,and the maximum energy product increases from 19.79 to 29.48 MGOe.The electron probe microanalyzer results reveal that there mainly exist gray and dark regions besides"white"rare earth-rich phase,and the conte nt of Sm,Fe and Cu elements for the two kinds of regions changes a lot for the specimens,Furthermore,with tS increasing up to 4 h,the elements content deviation between the gray and dark regions becomes small gradually from 3.94 at%to 0.27 at%,7.66 at%to 0.21 at%and 7.27 at%to 0.16 at%for Sm,Fe and Cu elements,respectively.Moreover,transmission electron microscopy results show that the distribution of cell size is much more concentrated for aged specimens when tS is 4 h.It is also found that the Cu concentration at cell boundaries for the 4 h solid-solution treatment case shows relatively higher values and greater concentration gradient(1.94 at%/nm).It is verified that sufficient solution treatment duration is prerequisite to form these homogeneous microstructural features,which are the key points for obtaining both high Hcj and Hknee.

Micromagnetic simulations on demagnetization processes in anisotropic Nd_2Fe_(14)B magnets

Individual grains with diverse dimensional parameters were introduced to investigate the magnetization reversals in anisotropic Nd2 Fe_(14)B magnets. The micromagnetic simulations were carried out via Object Oriented MicroMagnetic Framework(OOMMF). With the same bottom area and height, analysis results show that the coercive fields for different bottom shapes are of similar values. Designed as a cubic grain,the coercive field presents descending tendency as grain volume ascends. Under constant grain volume,with aspect ratio increasing, the coercive field decreases in the beginning and increases soon. Based on the demagnetization field vector, the effects of bottom shape, grain volume and aspect ratio on the coercive field can be explained. The nucleation point is chosen to discuss. Its synthetic field and reversal field are calculated by parallelogram law and inverse external field equation, respectively. The synthetic field equal to the reversal field is defined as critical field, which always shows the same tendency as the coercive field for all cases of this study. It can be concluded that critical field is qualified to be a reference index to measure the magnitude of coercive field.

Abnormal corrosion behavior of dual-main phase sintered(Ce,Nd)-Fe-B magnets in different sodium solutions

Sintered(Ce,Nd)-Fe-B magnets with high cost performance were prepared by substituting of Ce for Nd with the dual-main phase method.The corrosion behaviors of dual-main phase sintered(CexNd1-x)30Fe69B1(x=0.15 wt%,0.3 wt%) magnets,which were named as Ce15 and Ce30,were studied in 3.5 wt% NaCl,6.6 wt% Na2 SO4 and 3.0 wt% NaOH aqueous solutions,respectively.The galvanic corrosion of Ce30 is stronger than that of Ce15.However,the mass loss value of Ce30 is smaller than Ce15 after free corrosion for 33 d in NaCl solution.In the other two solutions,all the results show that the corrosion resistance of Ce30 is better than that of Ce15.The scanning electron microscope results show that the crack and shedding of the magnetic phase of Ce30 is less than that of Ce15.Combined above,the abnormal corrosion behavior shows that Ce30 magnets perform better corrosion resistance than Ce15,and the degradation degree of magnetic phase mainly affects the corrosion resistance of magnets.

Cellular microstructure modification and high temperature performance enhancement for Sm2Co17-based magnets with different Zr contents

Influence of Zr contents on high-temperature magnetic performance of Sm(CoFeCuZr)(x=0.025,0.03,0.035,0.04) magnets were investigated.As x increases from 0.025 to 0.04,the temperature coefficient of intrinsic coercivity(H) is optimized from-0.1673% K^(-1)to-0.1382% K^(-1)and the Hat 773 K gradually increases from 556.32 kA m^(-1)to 667 kA m^(-1).The microstructure and microchemistry of different Zr-content magnets were revealed by a transmission electron microscope equipped with EDS.The increasing Zr content induces that the average size of cells decreases from ~76 nm to ~56 nm and the weight fraction of 1:5 H cell boundary phase increases from ~25% to ~37% as well,resulting decreasing of the average Cu content at cell boundaries from 13.59 at% to ~8.52 at%.It is found that the Cu-lean characteristic at cell boundary phase is the reason that gives rise to higher magnetic properties at elevated temperatures for x=0.04 magnet.

Effect of magnetic layer thickness on magnetic properties of Ce-Fe-B thin films

The Ce;Fe;B thin films with a notable out-of-plane c-axis texture were prepared by DC magnetron sputtering on a Ta buffer layer. The morphological and magnetic properties were investigated. The thickness of the magnetic layer had a dramatic effect on the formation of Ce;Fe;B phase,and excellent magnetic properties(H;≈4.25 kOe, M;/M;≈0.81) were observed for the Ce-Fe-B film with the thickness d;= 200 nm. The results of the hysteresis loops for Ce-Fe-B film（d;= 200 nm） at various measured temperatures show that a decoupling between the hard and the soft phases is observed at low temperatures, which is due to the regions with quite low anisotropy provided by the a-Fe. Moreover. it is clear that significantly various magnetization behaviors between the films with d;= 200 and 300 nm were observed with a similar trend due to the existence of the a-Fe soft phase.

The microstructure and magnetization reversal behavior of melt-spun (Nd_(1-x)Ce_x)-Fe-B ribbons

In this study, the alloy ingots with nominal compositions of （Nd1_xCex）31FebalCoo.2Gao.1B （x = 0, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%） were prepared and then melt-spun to form nanocrystalline ribbons at the wheel speed of 20 m/s. XRD results show that all melt-spun ribbons exhibit the tetragonal structure （Nd,Ce）2Fe14B phase with the space group P42]rnmm. The Curie temperature and lattice constant decrease with the increase of Ce content. The Curie temperature decreases gradually from 306 to 247 ~C with the increase of Ce content. Those results indicate that Ce element has been incorporated into Nd2Fe14B main crystalline phase and formed （Ce,Nd）-Fe-B hard magnetic phase. It is also found that the remanence ratio （Mr/Ms） decreases from 0.693 to 0.663 and the coercivity （Hc） decreases from 18.7 to 14.2 kOe with the increase of Ce content. However, a relatively high coercivity of 18.3 kOe for （Ndl xCex）31FebalCoo.2Gao.lB （x = 0.2） melt-spun ribbon is achieved. The coercivity is sensitive to microstructure. The AFM patterns show the sample （x - 0.2） has the most uniform and finest micro- structure. The magnetization reversal behavior （0M plots） is discussed in detail. The positive 0M value is observed in every sample, which confirms the existence of exchange coupling interaction. Evidently, the OM maximum value reaches 0.9 in the sample （x - 0.2）. It is indicated that the intergranular exchange coupling effect is the strongest, which is consistent with coercivity enhancing.