Hard magnetic property and δm(h) plot for sintered NdFeB magnet

Transcription

1 Journal of Magnetism and Magnetic Materials 208 (2000) 239}243 Hard magnetic property and δm(h) plot for sintered NdFeB magnet R.W. Gao *, D.H. Zhang, W.Li, X.M. Li, J.C. Zhang Physics Department, Shandong University, Shangong, Jinan , People's Republic of China Central Iron and Steel Research Institute, Beijing , People's Republic of China Received 15 February 1999; received in revised form 4 August 1999 Abstract The hard magnetic properties and the interactions between the grains for sintered magnets are investigated by using δm(h) plot technique. The results show that the δm(h) plot of NdFeB sintered magnet can explain the e!ects 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 "eld is not strong enough, which means that the common δm(h) plot theory is not completely consistent with the sintered NdFeB magnet Elsevier Science B.V. All rights reserved. PACS: Bb; Gm; Ej Keywords: Sintered magnet; Hard magnetic property; Magnetostatic interaction between the grains; Exchange coupling interaction; δm(h) plot technique 1. Introduction The macroscopic magnetic properties of the magnetic material are tightly related to the interactions between the grains. It is very di$cult to strictly calculate the intergrain interactions due to the complexity of the microstructure in the material. An experimental technique, δm(h) plot technique, is commonly adopted to investigate * Corresponding author. Tel.: # ; fax: # address: xmliu@sdu.edu.cn (R.W. Gao) the intergrain interactions and their e!ects on the properties of magnetic material. Beardsley [1] and O'Grady et al. [2] studied the demagnetization processes in thin "lm media and the interactions in "ne particle systems by using δm(h) plot. Folks [3] and Panagiotopoulos et al. [4] investigated the interaction mechanisms in meltquenched NdFeB magnet and the exchange spring behavior in nanocomposite hard magnetic materials, respectively, using this technique. In this paper, we adopt the δm(h) plot technique to study the e!ects of microstructure (size, shape and orientation of the grains) and intergrain interactions on the hard magnetic properties for sintered NdFeB magnets /00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S ( 9 9 )

2 240 R.W. Gao et al. / Journal of Magnetism and Magnetic Materials 208 (2000) 239} Interactions between the grains an δm(h) plot theory 2.1. Interactions between the grains in the magnetic materials The interactions between the grains in magnetic materials can be classi"ed 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 e!ects 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 volume charges due to the inhomogeneous distribution of the moments. The strengths of magnetostatic interactions are dependent on the size, the shape and the orientation of the grains. The magnetostatic interaction makes the coercivity decrease for the magnet composed of large and nonregular grains, especially for the magnets composed of ideally aligned grains. The exchange coupling interaction between grains makes the grains align in parallel and decreases the e!ect of the anisotropy. The strength of the exchange interaction depends on the size and coupling degree of grains. For the magnet composed of small (nanocrystalline) and tightly coupling grains, the exchange interactions obviously increase the remanence and decrease the coercivity, especially for the magnets composed of randomly aligned grains. For the sintered NdFeB magnets, the grains are large and the intergranular boundaries are mostly separated by the non-magnetic or weak-magnetic phases [5]. So, the e!ects of the exchange interaction between grains should be weak and the e!ects of the magnetostatic interaction should be strong for NdFeB sintered magnets δm(h) plot describing the interactions between the grains A useful technique to monitor the intergrain interactions is by using the δm(h) plot. Based on the di!erent magnetization and demagnetization processes, there are two principal remanence curves: one is the isothermal remanent magnetization (IRM) curve M (H) which is obtained by progressive magnetizing of a thermally demagnetized sample, and the other is the DC demagnetization remanent magnetization (DCD) curve M (H) which is gotten by demagnetizing a previously saturated sample [2]. For a magnet composed of non-interacting single domain particles, there is the Wolhfarth relation [2] M (H)"M (R)!2M (H), (1) where H is the applied reverse "eld, M (R) is the isothermal remanence after saturation. When M (H) and M (H) are normalized by M (R), i.e., m (H)"M (H)/M (R), m (H)"M (H)/M (R), Eq. (1) can be rewritten as m (H)"1!2m (H). (1a) Provided there exist interactions between the grains, the relation of M (H) and M (H) deviates from Eq. (1) or Eq. (1a). The e!ect of interactions can be examined by the Henkel plot [2] δm(h)"m (H)![1!2m (H)]. (2) δm(h) plot curves indicate the type and strength of the dominant interaction mechanism between grains. The positive δm(h) plot suggests that the intergrain interactions support the magnetized state and the exchange coupling interaction is dominant. The negative δm(h) plot suggests that the intergrain interactions promote the demagnetized state and the magnetostatic interaction is dominant [2,4]. In the following we give the δm(h) plots and study the properties of intergrain interactions for Nd Fe Co B sintered magnets. 3. Experiments and results The magnets with the composition of 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 8 8 mm. Some samples are pressed without any alignment "eld (A, ), and other samples are pressed with an

3 R.W. Gao et al. / Journal of Magnetism and Magnetic Materials 208 (2000) 239} alignment magnetic "eld of 1 Tesla (T) (A, ). The shaped samples are sintered in Ar at 11003C for 60 min, annealed at 6003C for 60 min and cooled down rapidly. The remanent magnetization and demagnetization curves are measured by using HG-105-type hysteresis loop measurer with an applied "eld of 0.2}1.8 T. The saturated magnetization of samples is realized by using a pulse "eld of 4 T. The experimental results are as follows: The hard magnetic properties of samples prepared by di!erent processes are shown in Table 1. The data in Table 1 show: (1) The annealed samples have the larger coercivity and a slightly smaller remanence than that for unannealed samples. (2) With lengthening of mill time (APB), the sizes of magnetic powders diminish and the coercivity of magnet increases. (3) For the aligned magnets (number 2), the remanence obviously enhances and the coercivity decreases compared with the misaligned magnets (number 1). δm(h) plots of the samples with mill time 80 and 120 min are shown in Figs. 1 and 2. As seen from the Figures, the characteristics of δm(h) plots for sintered magnets are as follows: (1) The values of δm(h) are positive while the applied reverse "eld is small. With an increase of applied "eld δm(h) increases and reaches a maximum (δm(h) ), and then drops to small negative values. Fig. 1. δm(h) plots of sintered magnets with mill time 80 min. ( ) Misaligned sample, ( ) aligned sample, (- - -) unannealed sample, (*) annealed sample. Fig. 2. δm(h) plots of the sintered magnets with mill time 120 min. ( ) Misaligned sample, ( ) aligned sample, (- - -) unannealed sample, (*) annealed sample. Table 1 The coercivity μ, the remanence and the energy product (BH) for sintered magnets prepared by using di!erent processes Sample number Unannealed sample Annealed sample μ (¹) (¹) μ (¹) (¹) (BH) (KJ/m ) A B A B Note: A milling time 80 min. B milling time 120 min. 1 misaligned magnet. 2 aligned magnet. (2) The value of applied "eld corresponding to the maximum δm(h) is almost equal to the coercivity of the magnets. (For example, for the annealed and misaligned magnet with mill time 80 min, the value being about 1.3 T. See Fig. 1.) The peak value of δm(h) for the aligned magnet (pressed in alignment "eld) is much larger than that for the misaligned magnet. (3) For the unannealed samples, the magnetic "eld corresponding to a change in sign of δm(h) (from the positive to the negative value) enhances with the prolongation of milling time (compare Fig. 1 with Fig. 2). For the annealed

4 242 R.W. Gao et al. / Journal of Magnetism and Magnetic Materials 208 (2000) 239}243 samples, δm(h) is always positive for the extent of applied "eld. 4. Discussions and conclusion Fig. 3. δm(h) plot of the nanocrystalline Al Si [4]. The experimental results of 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 ( (misaligned magnet)+0.5 (aligned magnet)). The dependence of the coercivity on the degree of grain alignment can be explained by the starting "eld theory [6]. No obvious remanence enhancement implies that the exchange coupling interaction between the grains is weak for sintered magnets. This is consistent with the microstructure of the sintered magnets. The existence of non-magnetic or softmagnetic boundary phases between the hard grains and large grain sizes (μm in dimension) makes the exchange coupling interaction between the grains become weak. The variation of δm(h) with applied reverse "eld implies that the strengths of the two types of interactions are related to the magnetization state. The character of δm(h) being positive in small applied "elds is similar to that of nanocrystalline melt}spun single-phase Al Si magnets (see Fig. 3, Ref. [4]), which suggests that the exchange coupling interaction between the grains is dominant. This is true for the nanocrystalline magnets, but is not true for the sintered magnets. As discussed above, the exchange coupling interaction between hard grains is weak in the sintered magnets. The reason for positive δm(h) of the sintered magnets is the existence of an exchange coupling between the hard grains and the magnetically softer boundary phases [3]. In addition, in individual grain, there exists the exchange interaction between the domains with di!erent orientations, which is also responsible for the positive δm value. In a word, this discordance expresses that common δm(h) plot theory is only suitable for the magnets composed of single domain grains and is not completely in accordance with the sintered NdFeB magnet. However, some properties of NdFeB sintered magnets can be interpreted by using the δm(h) plot. (1) The relation between the "eld corresponding to the maximum of δm(h) and the coercivity. The value of applied reverse "eld corresponding to the maximum of δm(h) plot (δm(h) ) is about equal to the coercivity of the magnets. While the reverse "eld is equal to the coercivity of magnets, the projections of total magnetic moments for all grains in the direction of applied "eld counterbalance each other basically. The demagnetization effect of the magnetostatic interaction between the grains is very small (approaching 0), so the e!ect of the exchange coupling interaction is most remarkable. Hence δm(h) plot appears the maximum δm(h). (2) The e!ect of grain alignment on the maximum value of δm(h) plot. The value of δm for the aligned magnet is much larger than that for the misaligned magnet. According to the ordinary δm(h) plot theory, it seems that the exchange coupling interaction in aligned magnets is stronger than that in misaligned magnets. In fact, the grains in aligned magnet are already aligned by the applied alignment "eld while the powders are compressed. The exchange coupling interaction between the grains also tends to make the moments of adjacent grains align in parallel. Application of the alignment "eld has the same e!ect on the magnet as the exchange coupling interaction between the grains. Hence, application of alignment "eld for the aligned magnet corresponds to the enhancement of the exchange coupling interaction, which increases the value of δm.

5 R.W. Gao et al. / Journal of Magnetism and Magnetic Materials 208 (2000) 239} (3) The e!ect of the milling time and annealing on δm(h) plot. In case, other technological processes are the same except for the milling time, the average sizes of the grains in magnets are related to the milling time. Long time milling makes the powders of sample become tiny, the average sizes of grains in magnet small; consequently, the exchange coupling interaction is enhanced and the magnetostatic interaction reduced. Hence, the magnetic "eld corresponding to sign change of δm(h) is enhanced. The annealing process can improve the microstructure of grains, weaken the unregular edges and corners and reduce the magnetostatic interaction between the grains. So, the e!ect of exchange interaction is prominant and the value of δm(h) is always positive while applied "eld is not strong enough. Summarizing the above discussion, we get following conclusion: the δm(h) plot technique can be used to describe the e!ects of the microstructure (size, shape and orientation of the grains) and the intergrain interaction on the hard magnetic properties for NdFeB sintered magnets. The value of applied "eld at which δm(h) plot takes its maximum δm(h) is almost equal to the coercivity of the magnets. Application of an alignment "eld has the same e!ect on the magnet as the exchange coupling interaction. Both of these make the grains align, the remanence increase, the coercivity decrease, and the value of δm increase. With the decreasing grain sizes, the exchange coupling interaction enhances and the magnetostatic interaction reduces, the coercivity of the magnets increases, and the applied "eld corresponding to sign change of δm(h) increases. Annealing process makes the effect of magnetostatic interaction decrease and the value of δm(h) be positive while applied "eld is not enough strong. However, positive δm(h) (when applied "eld is not strong) expresses that the ordinary δm(h) plot theory is not completely consistent with the sintered NdFeB magnet. Acknowledgements This work is supported by the National Natural Science Foundation of China (S ) and the Natural Science Foundation of Shandong province ( ). References [1] I.A. Beardsley, J.G. Zhu, IEEE Trans. Magn. 27 (1991) [2] K. O'Grady, M. El-Hilo, R.W. Chantrell, IEEE Trans. Magn. 29 (1993) [3] L. Folks, R. Street, R. Woodward, J. Appl. Phys. 75 (10) (1994) [4] I. Panagiotopoulos, L. Withanawasam, G.C. Hadjipanayis, J. Magn. Magn. Mater. 152 (1996) 353. [5] J. Fidler, T. Scre#, J. Appl. Phys. 79 (8) (1996) [6] Gao Ruwei, Zhou Shouzeng, Zhang Deheng et al., J. Appl. Phys. 78 (1995) 1156.