Hard Magnetic Grains Research Articles

2D Simulation of Nd2Fe14B/α‐Fe Nanocomposite Magnets with Random Grain Distributions Generated by a Monte Carlo Procedure

The magnetic properties of Nd2Fe14B/α‐Fe nanocomposite magnets consisting of two nanostructured hard and soft magnetic grains assemblies were simulated for 2D case with random grain distributions generated by a Monte Carlo procedure. The effect of the soft phase volume fraction on the remanence Br, coercivity Hc, squareness γ, and maximum energy product (BH) max has been simulated for the case of Nd2Fe14B/α‐Fe nanocomposite magnets. The simulation results showed that, for the best case, the (BH) max can be gained up only a several tens of percentage of the origin hard magnetic phase, but not about hundred as theoretically predicted value. The main reason of this discrepancy is due to the fact that the microstructure of real nanocomposite magnets with their random feature is deviated from the modeled microstructure required for implementing the exchange coupling interaction between hard and soft magnetic grains. The hard magnetic shell/soft magnetic core nanostructure and the magnetic field assisted melt‐spinning technique seem to be prospective for future high‐performance nanocomposite magnets.

Fe64B22.8Nd6.6Y3.9Nb2.7 bulk nanocomposite magnets with improved size and magnetic properties

Abstract

Fe65B22Nd9Mo4 bulk nanocomposite permanent magnets produced by crystallizing amorphous precursors

The Fe65B22Nd9Mo4 nanocomposite permanent magnets in the form of a rectangular cross sectioned rod have been prepared by annealing the amorphous precursors. The thermal behavior, structure and magnetic properties of the magnets have been investigated by differential scanning calorimetry, X-ray diffractometry, electron microscopy and magnetometry techniques. The as-cast Fe65B22Nd9Mo4 alloy showed soft magnetic properties, which changed into magnetically hard after annealing. Results provoke that the magnetic properties of the alloy are sensitive to thermal processing conditions. The optimum hard magnetic properties with a remanence (Br) of 0.56T, coercivity (iHc) of 920.7kA/m and maximum energy product (BH)max of 50.15kJ/m3 were achieved after annealing the alloy at 983K for 10min. The good magnetic properties of Fe65B22Nd9Mo4 magnets are ascribed to the exchange coupling between the nano-scaled soft α-Fe, Fe3B and hard Nd2Fe14B magnetic grains.

Study of the magnetic interaction in nanocrystalline Pr–Fe–Co–Nb–B permanent magnets

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr–Fe–Co–Nb–B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity.The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd2Fe14B grains.

Magnetic Performance of a Nanocomposite Permanent Material

We build a sandwiched structure model in which the intergranular phase (IP) is homogeneously distributed between soft and hard magnetic grains, and gives a continuously anisotropic expression of the coupling part under the assumption that the IP weakens the intergrain exchange-coupling interaction. Based on the idea that the hardening mechanism is of the pinning type, we calculate the effect of the IP's thickness d and its anisotropy constant K1(0) on the intrinsic coercivity of a nanocomposite permanent material. The calculated results indicate that the domain wall goes twice through irreversible domain wall displacement during the process of moving from soft to hard magnetic grains, and the intrinsic coercivity increases with increasing d, but decreases with increasing K1(0). When d and K1(0) take 2 nm and 0.7Kh, respectively, with Kh being the anisotropy constant in the inner part of the hard magnetic grain, the calculated intrinsic coercivity is in good agreement with the experimental data.

Structural and magnetic properties of NdFeB and NdFeB/Fe films with Mo addition

The influence of the Mo addition on the microstructure and magnetic properties of Nd-Fe-B and Nd-Fe-B/Fe films was studied. The coercivity is a key parameter in the control of technical performances of Nd-Fe-B films. A small amount of about 1 at.% Mo can enhance the coercivity of Nd-Fe-B film by controlling the growth of soft and hard magnetic grains. A coercivity of 22.1 kOe, a remanence ratio, Mr/Ms, of 0.83 and a maximum energy product of 8 MGOe were obtained for Ta/[NdFeBMo(1at.%)(540nm)/Ta films annealed at 650°C for 20 minutes due to Mo precipitates formed at the Nd2Fe14B phase boundaries which prevent the nucleation and expansion of the magnetic domains. Simultaneous use of Mo as addition and the stratification of Nd-Fe-B-Mo films using Fe as spacer layer are important tools for the improvement of the hard magnetic properties of Nd-Fe-B films. The Ta/[NdFeBMo(1at.%)(180nm)/Fe(1nm)]×3/Ta multilayer film annealed at 620°C exhibits an increase in the coercivity from 12.1 kOe to 22.8 kOe, in the remanence ratio from 0.77 to 0.80, and in the maximum energy product from 4.5 to 7.1 MGOe in comparison with Ta/Nd-Fe-B/Ta film. As compared to Ta/Nd-Fe-B/Ta film, the Ta/[NdFeBMo(1at.%)(180nm)/Fe(1nm)]×3/Ta film presents a decrease in the crystallization temperature of about 30°C.

A feasible approach for preparing remanence enhanced NdFeB based permanent magnetic composites

NdFeB based permanent magnetic nanocomposites were prepared by depositing soft magnetic Fe, Co, or Fe65Co35 nanoparticles on the melt spun NdFeB powders with near single phase composition by a chemical reduction method. The effects of the reduction process, the composition, and the concentrations of metal ions on the magnetic properties of nanocomposites were investigated. Introducing and increasing soft nanoparticle content improved the remanence and maximum energy product of the nanocomposites at the expense of coercivity. Fe65Co35 coated NdFeB powders had higher remanence and energy product than Co or Fe coated powders. The inter-grain exchange coupling between hard and soft magnetic grains was demonstrated by the smooth demagnetization curve with high remanent polarization. Remanence enhanced bulk magnets were also fabricated by consolidating the nanocomposite powders using spark plasma sintering (SPS) technique.

Experimental determination of the magnetization dependent part of the demagnetizing field in hard magnetic materials

A method for extracting the magnetization dependent part of the demagnetizing field from minor hysteresis loops is described. It applies to hard magnetic materials with irreversible magnetization switching. The method’s validity is tested on the simulated magnetization curves of an assembly of hard magnetic grains, as well as on a thin NdFeB film with out of plane magnetization. Effective demagnetization factors are extracted from the analysis. These factors are smaller than the usually applied sample shape dependent demagnetizing factors.

The influence of microstructure on magnetic properties of nanocrystalline Fe–Pt–Nb–B permanent magnet ribbons

A FePt-based hard-magnetic nanocomposite of exchange spring type was prepared by isothermal annealing of melt-spun Fe52Pt28Nb2B18 (atomic percent) ribbons. The relationship between microstructure and magnetic properties was investigated by qualitative and quantitative structural analysis based on the x-ray diffraction, transmission electron microscopy, and F57e Mössbauer spectrometry on one hand and the superconducting quantum interference device magnetometry on the other hand. The microstructure consists of L10-FePt hard-magnetic grains (15–45 nm in diameter) dispersed in a soft magnetic medium composed by A1 FePt, Fe2B, and boron-rich (FeB)PtNb remainder phase. The ribbons annealed at 700 °C for 1 h exhibit promising hard-magnetic properties at room temperature: Mr/Ms=0.69; Hc=820 kA/m and (BH)max=70 kJ/m3. Strong exchange coupling between hard and soft magnetic phases was demonstrated by a smooth demagnetizing curve and positive δM-peak in the Henkel plot. The magnetic properties measured from 5 to 750 K reveals that the hard characteristics remains rather stable up to 550 K, indicating a good prospect for the use of these permanent magnets in a wide temperature range.

Effect of post-sintering annealing on microstructure and coercivity of Al 85Cu 15-added Nd–Fe–B sintered magnets

Effects of post-sintering annealing on the microstructure and coercivity have been investigated for the Al 85Cu 15-added (Pr, Nd) 14.8Fe 78.7B 6.5 sintered magnets. It is found that the optimum annealing temperature at which the coercivity i H c reaches a maximum decreases from 550 °C for the magnets added with 0.3% and 0.6% Al 85Cu 15 to 480 °C for the magnets added with 0.9% and 1.2% Al 85Cu 15. The decrease in optimum annealing temperature is related to the precipitation of Al–Cu or (Pr, Nd)−Cu liquid phase among (Pr, Nd)-rich phases during annealing. Existence of Al–Cu or (Pr, Nd)−Cu liquid phase is beneficial to dissolve the irregularities of (Pr, Nd) 2Fe 14B grain interface and increase the quantities of (Pr, Nd)-rich phases at the grain boundary, thus optimizing the grain boundary microstructure. The modifications of the microstructure are helpful to decouple the exchange interaction between (Pr, Nd) 2Fe 14B hard magnetic grains, thereby increasing the coercivity.

Nano-sized disorders in hard magnetic grains and their influence on magnetization reversal at artificial Nd/Nd 2Fe 14B interfaces

We have investigated the effect of lattice disorders near the surface of hard magnetic Nd 2Fe 14B grains on coercivity using artificial interfaces created by sputter depositing Nd on polished surface of Nd–Fe–B sintered magnets. The interfacial structure was manipulated by annealing the coated samples at 550 °C in vacuum with and without Ta cap. Nano-beam electron diffraction revealed a few nm thick disordered layer within the Nd 2Fe 14B phase at the Nd/Nd 2Fe 14B interface of a low coercivity sample, while a high coercivity sample showed a well-defined crystal structure of Nd 2Fe 14B near the NdO x /Nd 2Fe 14B interface.

Modeling of Intergrain Exchange Coupling for Quantitative Predictions of $\delta m$ Plots

The study of irreversible switching processes via δm-plots is a common experimental technique to assess intergrain interactions in nanoscaled magnetic materials. Theoretical predictions, on the other hand, of the influence of grain size, texture and field direction on the shape and absolute values of δm-plots are missing. In this paper direct intergrain exchange coupling is evaluated in a combined numerical/analytical approach for a model system of two coupled, hard magnetic grains. The influence of grain size and easy axis orientation on the outcome of a δm-analysis is determined quantitatively. As expected from the short range characteristic of direct exchange coupling, the field-integrated δm-signal decreases with increasing grain size. However, whereas directly coupled grains with parallel easy axis orientation possess large positive δm -values, for increasing misorientation angle the integrated δm-signal reduces and reaches even negative values. Such an effect of negative δm is so far only associated with indirect magnetostatic interactions. In a statistical extension of the model, grain ensembles with Gaussian distribution of their texture axis along one well defined sample direction are examined. The shape, width and height of the calculated δm-plots bear characteristic features known from experimental studies on nanocrystalline, well textured SmCo5 films.

Effect of annealing time on the exchange coupling interactions and microstructure of nanocomposite Pr 7.5Dy 1Fe 71Co 15Nb 1B 4.5 ribbons

The influence of annealing time on the magnetic properties and microstructure of nanocomposite Pr 7.5Dy 1Fe 71Co 15Nb 1B 4.5 ribbons was systematically investigated by the methods of vibrating sample magnetometer (VSM), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Interaction domains derived from strong exchange coupling interactions between hard and soft magnetic grains were imaged using magnetic force microscopy (MFM). Maximum remanence, intrinsic coercivity, and maximum energy product values were obtained in the ribbons annealed at 700°C for 15 min, which were composed of Pr 2(Fe, Co) 14B, α-(Fe, Co), and slight Pr 2(Fe, Co) 17 phases. Although J r H d, and ( BH) max decreased gradually with further increase of annealing time, it is emphasized that comparatively high J r and H ci and ( BH) max were obtained in a wide annealing time period of 15 to 360 min. The shape of initial magnetization curves and hysteresis loops change as a function of annealing time, indicating different magnetization reversal routes, which can be fully explained by the corresponding microstructure.

AN ANALYTICAL MODEL ON REMANENCE ENHANCEMENT IN NANOSIZED PERMANENT/COMPOSITE MAGNETS

Remanence enhancement has been calculated for a range of configurations, including 1D (one-dimensional), 2D and 3D arrays of hard magnetic grains, composites of hard and soft magnetic grains and triple-films. A linear relationship between the remanence Mr and the reduced domain wall width W/L has been derived analytically, which is consistent with available experimental and numerical data. For composite materials with a soft phase inserted among hard phases, the soft grain behaves like a massive inter-grain domain-wall and results in more effective remanence enhancement. In view of this, a unified formula is reached which can be applied to both composite and single phased magnets with in-plane random easy axis distribution.

Highly coercive rapidly solidified Sm–Co alloys

Highly coercive (Hc up to 37kOe at 300K), high remanent permanent magnets have been achieved by rapid solidification of binary Sm–Co alloys and Sm–Co alloys modified with Nb and C. Rapidly solidified SmCox alloys with x ranging from 5 to 11.5 formed predominantly a solid solution TbCu7-type SmCo7 phase, although hcp Co was observed for x>7.3. A coercivity value of 10kOe was observed for x<6.1, even though the microstructural scale was on the order of 1μm. The coercivity decreased significantly with the presence of the hcp Co phase, which formed initially as ∼80nm grains and, at higher x, as primary dendrites. Additions of 3at.% Nb or 3 and 5at.% C profoundly affected the coercivity values. Transmission electron microscopy (TEM) investigations revealed the origin of the improved coercivity. The addition of Nb resulted in a significant reduction in microstructural scale. The SmCo7 grain size decreased systematically with Nb content, reaching 150–200nm at 3at.% Nb. The addition of C also significantly enhanced the coercivity, which systematically increased with C content and reached 37kOe at 5at.% C. The effect of C, however, resulted in morphological changes as TEM revealed the formation of an intergranular phase that effectively isolated the hard magnetic SmCo7 grains from one another, reducing magnetic interactions. Excellent isotropic energy products of 6–8MGOe were also achieved.

Magnetic Structure and Magnetization Reversal in Nanocomposite Pr<sub>2</sub>Fe<sub>14</sub>B/α-Fe Ribbons

Nanocomposite Pr7.5Dy1Fe71Co15Nb1B4.5 ribbons were prepared by melt-spinning and subsequent annealing. Interaction domains were imaged using magnetic force microscopy (MFM) because of strong exchange coupling between hard and soft magnetic grains. Coercivity was determined by exchange coupling pinning field. But the magnetization reversal of nanocomposite magnets were different from that of traditional single phase permanent magnets.

電子線ホログラフィーによるNd-Fe-B系ナノコンポジット磁石の磁区構造解析

Magnetic flux distribution in Nd-Fe-B based nanocomposite magnets was investigated at nanometer scale by electron holography. Both Nd4.5Fe77B18.5 and Nd-Fe-B-Ti-C, the latter of which was newly developed and commercialized under tradename of SPRAX, are found to consist of hard and soft magnetic grains of 10~30 nm in diameter. Through the comparison of reconstructed phase images of Nd4.5Fe77B18.5 and Nd-Fe-B-Ti-C, it is found that the lines of magnetic flux of Nd-Fe-B-Ti-C fluctuate more largely than Nd4.5Fe77B18.5 in both demagnetized and remanent states. The fluctuation in the distribution of lines of magnetic flux is considered to result from the randomness in the crystal orientation of the hard magnetic grains with high anisotropy. It is also pointed out that the fluctuated distribution of lines of magnetic flux is consistent with the high coercivity and high maximum energy product of Nd-Fe-B-Ti-C.

Amorphization and ultrafine-scale recrystallization in shear bands formed in shock-consolidated Pr2Fe14B∕α-Fe nanocomposite magnets

Amorphization and ultrafine-scale recrystallization within shear bands formed in shock-consolidated Pr2Fe14B∕α-Fe nanocomposite magnetic powder compacts have been observed using transmission electron microscopy. The shear bands span through multiple grain lengths and truncate preexisting ∼25nm hard and soft magnetic phase grains, resulting in further grain size refinement. The shear bands contain nanocrystallites (<10nm size) interdispersed in an amorphous matrix, which suggests the occurrence of shock-induced phase transition in localized regions of the shear bands, and provides insight into the process of deformation of nanocrystalline materials under coupled high-strain-rate and high-pressure conditions.

Effects of composition and temperature of crystallization on the microstructure and magnetic properties of NdFeB thin films for perpendicular recording media

NdFeB thin films were synthesized by a dc magnetron sputtering system. The influence of composition and temperature of crystallization on the microstructure and magnetic properties of NdFeB thin films was investigated. It was found that excessive Nd and B addition was helpful in promoting granular structure and reducing grain growth as well as in modifying magnetic properties. The excess elements were precipitated and well distributed around the shells of hard magnetic grains. This actively decreased the intergranular interactions between hard grains. Films made on heated substrate were also compared to those fabricated by post annealing. The former method was proved to be very effective to produce hard NdFeB thin films with good perpendicular anisotropy on a W underlayer, whereas films made by the latter process were isotropic. The maximum coercivity was obtained near 10 kOe in films annealed at 600 °C. The intergranular interactions were characterized by δM plots. It was found that annealed samples have a much stronger exchange coupling interaction than do the in situ deposited samples. Therefore, the magnetization reversal process in annealed films is mainly controlled by domain wall motion while it is more likely to be dominated by incoherent magnetization reverse in in situ deposited thin films, defined by the dependence of coercivity Hc(θ) on the measuring angle θ. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanoscale structure and magnetic properties of nd-fe-nb-b-c permanent magnets

We investigated the nanoscale structure and magnetic properties of Nd/sub 2/(FeNb)/sub 14/(BC)//spl alpha/-Fe. We studied several alloy compositions and the effect of C and Nb on the properties, and thus produced a new alloy of Nd/sub 9/Fe/sub 85.5/Nb/sub 1.0/B/sub 4/C/sub 0.5/. Its mean grain size is about 40 nm. Grain refinement enhanced the remanence, because of greater ferromagnetic exchange coupling between the hard and soft magnetic phase grains. Annealing improved the properties further. By optimizing the processing conditions, we obtained relatively high performance [J/sub r/=1.099 T, H/sub ci/=518.3 kA/m, J/sub r//J/sub s/=0.82, (BH)/sub max/=137.8 kJ/m/sup 3/] when the alloy was annealed at 700/spl deg/C for 15 min. We examined the magnetic domain structure in the nanophase alloy by magnetic force microscopy. The length of the magnetic contrast in the alloy is in the range of 450-550 nm, much bigger than the mean grain size. Here, we interpret the length in terms of interaction domains, which originate from the exchange coupling effect.