Hard magnetic property and δM(H) plot for sintered NdFeB magnet

doi:10.1016/S0304-8853(99)00562-4

Journal of Magnetism and Magnetic Materials

Volume 208, Issue 3, 15 January 2000, Pages 239-243

https://doi.org/10.1016/S0304-8853(99)00562-4 Get rights and content

Abstract

The hard magnetic properties and the interactions between the grains for sintered Nd₁₆Fe₇₃Co₅B₆ magnets are investigated by using δM(H) plot technique. The results show that the δM(H) plot of NdFeB sintered magnet can explain the effects of the microstructure (size, shape and orientation of the grains) and the intergrain interactions on the hard magnetic properties of the magnet. However, the value of δM(H) is positive when the applied field is not strong enough, which means that the common δM(H) plot theory is not completely consistent with the sintered NdFeB magnet.

Introduction

The macroscopic magnetic properties of the magnetic material are tightly related to the interactions between the grains. It is very difficult to strictly calculate the intergrain interactions due to the complexity of the microstructure in the material. An experimental technique, δM(H) plot technique, is commonly adopted to investigate the intergrain interactions and their effects on the properties of magnetic material. Beardsley [1] and O'Grady et al. [2] studied the demagnetization processes in thin film media and the interactions in fine particle systems by using δM(H) plot. Folks [3] and Panagiotopoulos et al. [4] investigated the interaction mechanisms in melt-quenched NdFeB magnet and the exchange spring behavior in nanocomposite hard magnetic materials, respectively, using this technique. In this paper, we adopt the δM(H) plot technique to study the effects of microstructure (size, shape and orientation of the grains) and intergrain interactions on the hard magnetic properties for sintered NdFeB magnets.

Section snippets

Interactions between the grains in the magnetic materials

The interactions between the grains in magnetic materials can be classified as the long-range magnetostatic (dipolar) interaction and the exchange coupling interaction between the adjacent grains. Both interactions reduce the coercivity of the magnet. However, the degree of the effects of both the interactions is determined by the microstructure and the magnetization state of magnet [5].

The long-range magnetostatic interaction arises from the surface charges at the grain boundaries and the

Experiments and results

The magnets with the composition of Nd₁₆Fe₇₃Co₅B₆ are prepared by using conventional sintering process. The crushed ingots were milled in an alcohol medium for 80 (A) and 120 (B) min, respectively. The milled powders are pressed into cylinders with φ8×8 mm. Some samples are pressed without any alignment field (A₁, B₁), and other samples are pressed with an alignment magnetic field of 1 Tesla (T) (A₂, B₂). The shaped samples are sintered in Ar at 1100°C for 60 min, annealed at 600°C for 60 min and

Discussions and conclusion

The experimental results of Nd₁₆Fe₇₃Co₅B₆ sintered magnets show that the coercivity for the misaligned magnet is greater than that for the aligned magnets, and there is no obvious remanence enhancement (B_r(misaligned magnet)≈0.5B_r(aligned magnet)). The dependence of the coercivity on the degree of grain alignment can be explained by the starting field theory [6]. No obvious remanence enhancement implies that the exchange coupling interaction between the grains is weak for Nd₁₆Fe₇₃Co₅B₆ sintered

Acknowledgements

This work is supported by the National Natural Science Foundation of China (S-9971026) and the Natural Science Foundation of Shandong province (991175303).

References (6)

I. Panagiotopoulos et al.
J. Magn. Magn. Mater.
(1996)
I.A. Beardsley et al.
IEEE Trans. Magn.
(1991)
K. O'Grady et al.
IEEE Trans. Magn.
(1993)

There are more references available in the full text version of this article.

Cited by (39)

Influence of exchange coupling interaction on coercivity in 2:17-type SmCo-based high temperature permanent magnets
2023, Scripta Materialia
The intergranular exchange coupling interaction in Sm(Co_balFe_0.10Cu_0.10Zr_0.03)_7.2 high temperature permanent magnets with different isothermal aging time was studied. The magnet isothermally aging for 24 h with higher coercivity at room temperature exhibits a weaker exchange coupling, while that aging for 4 h with a relatively small temperature coefficient of coercivity shows a stronger exchange coupling. Microstructure and microchemistry analysis indicated that Cu gradually enriches in the cell boundaries with the prolongation of isothermal aging time, decreasing the exchange coupling strength. Theoretical analysis results revealed that with the increase of temperature, the cells gradually get exchange decoupled and demagnetized independently, and the exchange coupling strength between cells decreases, which slows down the decline of coercivity. It is proposed that an optimized temperature coefficient of coercivity requires a stronger exchange coupling at room temperature, conducing to a more significant compensation effect with the increase of temperature.
Magnetization reversal processes in sintered permanent magnets Sm(Co, Fe, Zr, Cu)<inf>Z</inf>
2022, Journal of Magnetism and Magnetic Materials
Citation Excerpt :
The positive value of δM(H) means that in the field of H a larger fraction of particles switched in the sample during magnetization than during the magnetization reversal. The nature of this can be related both to the exchange interaction [38,39] and to the magnetization reversal [33] and cannot be explicitly determined from the Kelly plot. In the proposed model of magnetization reversal in a thermally demagnetized sample, in the magnets form extended domains along the c-axis with zigzag domain walls [17-21,25,26].
The magnetic hysteresis properties of commercially available samples of sintered permanent magnets with the composition of Sm(Co_0.796–xFe_0.177Cu_xZr_0.027)_6.63 (x = 0.117 – 0.131) were investigated. To analyze the processes of magnetization and magnetization reversal, the magnetization curves from the thermally demagnetized state, major hysteresis loops, recoil curves, the dependence of the reversible magnetic susceptibility vs. magnetic fields strength, Kelly plots and angular dependences of the coercivity were used. These results indicate that magnetization reversal processes are more complicated than pinning. A model of magnetization reversal processes is proposed to explain why in some experiments the magnets exhibit a pinning mechanism of magnetization reversal and in some experiments a nucleation mechanism.
Linear structures of Nd-Fe-B magnets: Simulation, design and implementation in mineral processing – A review
2019, Minerals Engineering
Citation Excerpt :
It is necessary to take them into account especially for magnets working at higher temperatures, e.g. 150 °C. The magnetic properties of a sintered magnetic material are closely related to the interactions between the grains (Gao et al., 2000), while the magnetic field of a given magnet is a function of the physical structure of the magnet, its chemical composition, material homogeneity, and dimensions. In the simplest case, the structure consists of two different phases, namely a magnetically hard Nd2Fe14B phase with domains of a volume of 0.001–1 mm3, and a non-magnetic granular phase from practically pure neodymium (Fig. 2).
This paper presents the different types of linear structures from mainly large blocks made from permanent Nd-Fe-B magnets with a high value of the maximum energy product and their implementation in mineral processing facilities. The methods of the assemblage of these large blocks which allow control the speed of the attraction of magnets during this assembly are described and discussed. The most serious problem in this case is the safe handling of large magnetic forces, both during the assembly of the magnets into the large blocks and subsequently during the insertion of these blocks into the iron circuit for final implementation. Linear assemblies can be of various designs, both in the individual and opposing arrangements, depending on the purpose for which they were designed and constructed. In technological practice, linear magnetic structures find significant application in the purification of raw materials using the methods of magnetic separation. Specifically, magnetic filtration using Nd-Fe-B magnets is a very effective method of eliminating undesirable metal impurities from the feedstock. Another method is the use of suspended magnetic separators, designed for the removal of undesirable ferromagnetic particles from the layer of the material transported by a conveyor belt. All the methods are described and discussed here in this critical review.
Structure and magnetic properties of (Sm <inf>0.9</inf> Zr <inf>0.1</inf> )Fe <inf>11</inf> Ti alloys with ThMn <inf>12</inf> -type structure
2019, Journal of Magnetism and Magnetic Materials
Citation Excerpt :
Thus, it can be concluded that among alloys annealed at 973, 1023 and 1073 K the last one had better exchange coupling of grains. But further increase of δmmax at higher Ta was mainly due to change of the hysteresis type into the nucleation [35]. Nonzero values at small magnetic fields were other features of δm-plots caused by the effect of soft magnetic phases.
Rapidly increasing demand for high-energy permanent magnets and volatility of the rear-earth market encourage a search of hard magnetic materials that can compete with those based on the Nd₂Fe₁₄B phase. Sm-Fe compounds with the ThMn₁₂-type crystal structure are considered as promising candidates for it. However, their synthesis and achievement of theoretically predicted magnetic properties are still challenging tasks that have not been solved yet. This paper addresses this problem. Its aim was to synthesize and to study magnetic properties of the (Sm_0.9Zr_0.1)Fe₁₁Ti compound with ThMn₁₂-type structure. Initially amorphous alloy was obtained by melt spinning and its subsequent heat treatments were performed. Structural and phase transformations of the (Sm_0.9Zr_0.1)Fe₁₁Ti alloy with annealing temperatures were studied along with magnetic properties. For the optimally annealed alloy features of magnetization reversal were discussed and temperature dependences of coercivity and maximum energy product were obtained.
Origins of inhomogeneous microstructure of hot-deformed nanocrystalline cylindrical Nd-Fe-B magnets and its effects on magnetization behaviors
2019, Journal of Alloys and Compounds
Citation Excerpt :
The maximum value of δM curve, δMmax, decreases gradually when the sampling position changes from the center to the edge. The P3 sample possesses the highest δMmax among all the samples, which could be attributed to the well alignment of the grains [42]. Usually, a thin grain boundary contributes to the improvement of the exchange coupling strength of grains.
To apply bulk hot-deformed magnets into commercial fields, it is quite necessary to deeply understand the origins and evolution of their inhomogeneous microstructure. In this work, the microstructure of hot-deformed nanocrystalline cylindrical Nd-Fe-B magnets with a dimension of ∼24.8 mm in diameter and ∼7.0 mm in height along the radial direction was investigated in order to correlate them with the inhomogeneous magnetic properties. The remanence was 1.42 T, 1.37 T, and 1.30 T for the samples taken from the center (sample P3), subcenter (sample P2), and edge (sample P1) of the hot-deformed magnet, respectively. Scanning electron microscopy images showed from the P3 to P1 the interval of strip-shaped Nd-rich layers increased. Meanwhile, the perpendicularity between the pressure direction and the Nd-rich layers deteriorated for the samples from the P3 to P1. Well oriented platelet-shaped grains with c-axis parallel to the pressure direction were observed in the P3 sample, and the orientation gradually deteriorated with the position changing from the center to the edge. It was also found that large Nd-rich phase aggregation areas occurred in the edge sample. The maximum value of Henkel plot decreased gradually for the samples from the P3 to P1, which was caused by the thickened grain boundary observed by transmission electron microscopy. The openness of recoil loops increased gradually from the center to the edge due to the aggregation of the Nd-rich phase. The inhomogeneous microstructure shows distinct effects on magnetic properties, therefore changing the magnetization behavior along the radial direction of hot-deformed magnets.
Preparation and magnetic properties of the Sr-hexaferrite with foam structure
2016, Journal of Magnetism and Magnetic Materials
Citation Excerpt :
The different magnetization values between these samples is attributed to a reconfiguration of the magnetic interactions, in particular the dipolar ones, which are long-range magnetostatic interaction that arises from the surface charges at the grain boundaries. The magnetostatic interactions depend strongly on the microstructure [20], and have demagnetizing nature. This reconfiguration among the magnetic interactions in the foams is promoted by a change in both, the microstructure of the grains and the structure of the pores.
This work reports an optimal way to fabricate strontium hexaferrite with porous-reticulated structure using a variation of the replication technique and taking two different precursors, one obtained from the coprecipitation and the other from the ceramic method. Changes made to the original replication technique include the addition of Arabic gum as binder, and the addition of ethylene glycol to form the ceramic sludge. In addition, some parameters such as the relation between solid material and liquid phase, the quantity of binder and the heat treatment were varied to obtain high quality magnetic foams. Two polymeric sponges were used as patterns, one with average pore size of 300 μm diameter and the other with 1100 μm. The characterization of the samples included the analysis of the structure and phase purity, the magnetic properties, the remanence properties, magnetic interactions and the microstructural characteristics. Results indicate that both, the powder precursors and the polymeric pattern play an important role in the configuration of the foam structure and this configuration has an important influence on the dipolar interactions which tend to demagnetize the samples. In addition, it was analyzed the behavior between the minimum value of the δM curves and the hysteresis properties.

View all citing articles on Scopus

View full text