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α-Fe magnets
Journal of Alloys and Compounds 372 (2004) 267–271
Remanent enhancement of nanocomposite (Nd, Sm)2 Fe14 B/␣-Fe magnets M. Zhang a,∗ , Z.D. Zhang a,b , X.K. Sun a,b , W. Liu a,b , D.Y. Geng a,b , X.M. Jin a,b , C.Y. You a,b , X.G. Zhao a,b a
Shenyang National Laboratory for Materials Science and International Centre for Materials Physics, Institute of Metal Research, Chinese Academia of Sciences, 72 Wenhua Road, Shenyang 110016, China b International Centre for Materials Physics, Chinese Academia of Sciences, 72 Wenhua Road, Shenyang 110016, China Received 8 July 2003; received in revised form 29 September 2003; accepted 29 September 2003
Abstract We have systemically studied structure and magnetic properties of nanocomposite magnets Nd8−x Smx Fe88 B4 (x = 0–2.5) prepared by mechanical milling and subsequent heat-treatment. The remanence increases monotonically with Sm substitution for Nd. The value of the maximum energy product exhibits a peak at x = 0.4. The extension of the exchange coupling length is induced by a decrease of the anisotropy of the magnetic-hard phase. This results in that more magnetically soft grains participate well the exchange coupling, improving the magnetic properties of the nanocomposite magnets. Compared with Y substitution for Nd, the Sm substitution for Nd in the Nd2 Fe14 B/␣-Fe nanocomposite magnets has stronger effect on decreasing the magnetic anisotropy, thus it has stronger effect on increasing the exchange coupling length. © 2003 Elsevier B.V. All rights reserved. PACS: 75.50.Ww; 75.75.+a; 81.07.Bc; 81.20.Ev Keywords: Exchange coupling; Nanocomposite magnets; Mechanical milling
1. Introduction Nanocomposites of magnetically hard and soft phases have a substantial potential for application of permanent magnets, since their remanence could be higher than those in isotropic single-phase magnetically hard materials. Remanence enhancement originates from the exchange coupling of magnetic moments across interfaces between the magnetically hard and soft phases, because the exchange coupling could turn the moments of the magnetically soft phase to rotate toward an average direction of the nearest neighboring magnetically hard phases. It has been demonstrated on several occasions [1,2] that the crystallite size of the phases, in particular, that of the soft phase, is important for realizing the remanence enhancement and determining the maximum energy product of the nanocomposite magnets. Much effort has been made to optimize the magnetic properties, by using various preparation methods, such as, mechani∗
Corresponding author. E-mail address: [email protected] (M. Zhang).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.09.143
cal alloying, melt spinning or sputtering [3–6], or element substitution [7,8]. But the values of the maximum energy product (BH)max achieved experimentally have been much smaller than the ones predicted theoretically. The difference between the theoretical and experimental values of (BH)max is primarily due to the difficulty in obtaining the optimum size distribution of magnetically soft grains employed for the theoretical model. For an optimum exchange coupling, the model of Kneller and Hawig gave that the size of the magnetically soft grains should be about twice as large as the domain wall width of the hard phase [1]. It is usually difficult to prepare the nanocomposite magnets with the magnetically soft phase having such small grain size. Recently, the value of (BH)max = 203.52 kA/m3 was achieved, due to a better control of grain sizes and distribution of the magnetic phases in multilayers prepared by sputtering [9]. Zhao et al. systematically investigated (Nd, Sm)2 Fe14 B/ Fe3 B nanocomposite magnets by melt spinning [10]. An enhancement of the exchange coupling length was found to relate with the reduction of the magnetocrystalline anisotropy of the magnetically hard phase by Sm substitution for Nd.
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But Zhang et al. thought that no obvious relation exists between the anisotropy of the hard phase and the remanence enhancement of the composite magnets [11]. It is still very interesting to have a further understanding of the exchange correlation length bcm = π(Am /2K1 )1/2 , where Am is the exchange energy of the magnetically soft phase, K1 is the first-order magnetocrystalline anisotropy constant of the magnetically hard one, and to realize in different systems the idea of increasing the energy product by decreasing the magnetocrystalline anisotropy K1 . In order to verify the relation between the exchange coupling and the magnetocrystalline anisotropy, in this work, we choose Nd2 Fe14 B/␣-Fe nanocomposite magnets prepared by mechanical milling and subsequent annealing. The effect of the exchange coupling length on the magnetic properties is explored, by adjusting the magnetocrystalline anisotropy of the magnetically hard phase by Sm substitution for Nd. The effects of Sm substitution for Nd are compared with those of Y substitution for Nd in the Nd2 Fe14 B/␣-Fe nanocomposite magnets.
2. Experiment The alloys of Nd8−x Smx Fe88 B4 (x = 0–2.5) were prepared by arc-melting twice under purified argon, with starting materials of 99.5 wt.% purity. Boron was added in the form of a Fe–B master alloy containing 20.8 wt.% boron. The ingots were crashed to prepare the alloy powders with a particle size less than 150 m. Then the alloy powders were milled under a purified argon atmosphere for 5 h in a high-energy ball mill. The weight ratio of ball to powder was 40:1. The as-milled powders were annealed at various temperatures of 580–720 ◦ C in a vacuum better than 2×10−3 Pa for 20 min. X-ray diffraction (XRD) analysis was conducted using Cu K␣ radiation with a Rigaku D/Max-␥A diffractometer equipped with a graphite crystal monochromator. For magnetic measurements at room temperature, the powders were embedded in epoxy resin with a weight ratio of 1:1 between the magnetic powders and the resin to form magnetically isotropic magnets. The magnetic properties were measured at room temperature using a pulsed magnetometer in fields up to 6 T. The results of the magnetic measurements were corrected using an experimentally determined effective demagnetization factor of 0.28.
3. Result and discussion XRD results show that after mechanically milled for 5 h, all alloys consist of an amorphous matrix in which nano-grains of ␣-Fe are embedded [see Fig. 1(a)]. After the heat treatment, a nano-crystalline phase of Nd2 Fe14 B-type coexists with ␣-Fe in nanoscale [see Fig. 1(b) and (c)]. The optimum magnetic properties are achieved, after annealed at 630 ◦ C for 20 min. The mean grain sizes of ␣-Fe are estimated by X-ray diffraction using the Scherrer’s
Fig. 1. X-ray diffraction patterns of mechanically milled Nd8−x Smx Fe88 B4 alloys, (a) as milled, x = 0.0, (b) after annealed at 630 ◦ C, x = 0.0, (c) after annealed at 630 ◦ C, x = 0.4.
formula. When x equals to 0.0 and 0.4, the mean grain sizes of ␣-Fe are 17 and 18 nm, respectively. No significant dependence of the grain size on the concentration of the rare-earth elements is observed, according to the XRD analysis. The values (17 and 18 nm) of the mean grain sizes of ␣-Fe, obtained in this work by XRD, are close to those obtained by XRD (18–25 nm) and TEM (20–24 nm) in our previous work for Nd2 (Fe, Co, Mo)14 B/␣-Fe nanocomposite magnets [12]. The relative XRD intensity of the Nd2 Fe14 B-type phase is too weak to evaluate the grain size. The mean grain size of the Nd2 Fe14 B-type phase, as determined by TEM observations, was in range of 36–41 nm, for the alloys annealed at 650 ◦ C for 20 min [12]. The mean grain size of the Nd2 Fe14 B-type phase of the present alloys should be comparable with those reported previously. For detailed structural and microstructural aspects for the Nd2 Fe14 B-based nanocomposite magnets, the readers are referred to our recent work [12]. The relationship between nanostructure, exchange coupling and magnetic properties of the Nd2 (Fe, Co, Mo)14 B/␣-Fe nanocomposite magnets prepared by mechanical alloying (MA) was studied systematically. Three different categories of grain interfaces were directly observed by HRTEM, which correspond to different types of the exchange couplings in the Nd2 Fe14 B/␣-Fe-type nanocomposite magnets [12]. The demagnetization curves and the corresponding magnetic properties are showed in Figs. 2 and 3, respectively. The coercivity decreases monotonically with increasing Sm content as expected, because of the decrease of the magnetocrystalline anisotropy of the magnetically hard phase. The remanence monotonically increases with increasing x, due to the enhancement of the exchange coupling. The maximum energy product first increases and reaches its maximum with increasing x, due to the competition effect between an increase of the remanence and a decrease of the coercivity. For a higher Sm content, it decreases as a result of the further deterioration of the coercivity (Fig. 3). In our recent work, we studied systemically the structure and magnetic properties of Nd8−x Yx Fe88 B4 alloys [13,14]. As compared in Fig. 3, a different effect on remanence was
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Fig. 2. Demagnetization curves of Nd8−x Smx Fe88 B4 alloys annealed at 630 ◦ C for 20 min.
acquired for Nd8−x Yx Fe88 B4 [13]. It can be seen that the remanence increases and then decreases with increasing Y content, different from the monotonic increase upon Sm content. It is a common point that a small amount of Sm/Y substitution for Nd increases the exchange correlation length bcm , due to the decrease of the magnetocrystalline anisotropy of the magnetically hard phase. The remanence is improved by enhancing the exchange coupling between the magnetically hard and soft phase of the nanocomposite magnets. When more Sm/Y substitutes for Nd, the effects on the remanence are different, which can be attributed to the different magnetic status of Sm and Y ions in the R2 Fe14 B-type lattice. It is known that the Sm ions in Sm2 Fe14 B contribute to a negative anisotropy constant K1 , while the Y ions in Y2 Fe14 B have no contribution (except for the dilution effect) to the magnetic anisotropy because it is non-magnetic [15]. Thus it is clear that in the R2 Fe14 B-type phase, the
Sm ions have stronger effects on the magnetic anisotropies than the Y ions do (since the Sm substitution goes forward to negative and the Y substitution goes to zero). Namely, the magnetic anisotropy constant K1 decreases more rapidly with the Sm substitution than the Y substitution for Nd. Because Sm2 Fe14 B shows the strong easy-plane anisotropy, the excessive Sm substitution for Nd would intensively decrease the anisotropy magnetocrystalline of the magnetically hard phase. It means that the exchange correlation length is increased more quickly than in the case of Y substitution in nanocomposite magnets. Therefore, the exchange coupling would be enhanced more obviously in the case of the Sm substitution. Although the Y substitution for Nd also enhances the exchange coupling, due to the reduced magnetocrystalline anisotropy, it has weaker effect than the Sm substitution. Furthermore, it would deteriorate the direct exchange coupling between the magnetic moments in nanocomposite magnets because Y is nonmagnetic. The effects of Y substitution for Nd on the remanence are determined by competition of these two factors [13,14]. When the Y content is lower, the effect of enhanced exchange coupling, due to the reduced magnetocrystalline anisotropy of the magnetically hard phase is important; when the Y content is higher, the effect of deteriorating the direct exchange coupling between magnetic moments, due to the substitution of nonmagnetic Y for Nd, becomes dominant. The typical smooth demagnetization curves of an exchange-coupled two-phase system can be noticed (Fig. 2). However, we cannot drive the conclusion that the magnetically hard and soft phases have enough strong exchange coupling. This is because this kind of curve can persist in a proper range where the magnetic properties have already decreased, due to the weakening of the exchange coupling [16]. Nevertheless, we could study the change of the exchange coupling by the reduced remanence. In Nd2 Fe14 B/␣-Fe nanocomposite magnets, the magnetically soft phase ␣-Fe is non-uniaxial, which contributes a greater reduced remanence than the uniaxial magnetically hard phase Nd2 Fe14 B. In order to properly evaluate the effect of remanence enhancement of nanocomposite magnets, the theoretical reduced remanence mth of magnetically isotropic magnets is calculated, according to the formula: mth =
Fig. 3. Composition dependence of the coercivity i Hc , the remanence Jr (solid squares, denoted by Sm), the maximum energy product (BH)max and reduced remanence Jr /Js of mechanical milled Nd8−x Smx Fe88 B4 alloys, after annealing at 630 ◦ C for 20 min. The remanence Jr (solid circles, denoted by Y) of mechanical milled Nd8−x Yx Fe88 B4 alloys, after annealing at 630 ◦ C for 20 min is also shown. The dashed line is for the theoretical calculation of reduced remanence.
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Jr (0.5 × (1 − vsoft )Jshard ) + (0.832 × vsoft Jssoft ) = Js (1 − vsoft )Jshard + vsoft Jssoft (1)
where vsoft is the volume fraction of the magnetically soft phase, for the present nanocomposite magnets, with a volume ratio of 70:30 for hard to soft phase given by the rare earth contents of the starting compositions. Jssoft stands for the spontaneous polarization of the magnetically soft phase. Jshard stands for the spontaneous polarization of the magnetically hard phase, which can be described as a function of Sm content x using the following equation: Jshard = 1 − 18 x JsNd + 18 xJSm (2) s
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where JsNd and JsSm is the spontaneous polarization of Nd2 Fe14 B and Sm2 Fe14 B, respectively. The values of 0.5 and 0.832 correspond to the reduced remanence of the magnetically hard and soft phases, respectively. The dash curve in Fig. 3 shows the theoretical value of the reduced remanence for various Sm contents, while the solid curve gives the experimental ones for samples annealed at 630 ◦ C for 20 min. The spontaneous polarization of Nd2 Fe14 B and Sm2 Fe14 B is 1.6 and 1.52 T, respectively. The Sm substitution for Nd from 0 to 2.5, the calculated mth decreases slightly and the tendency cannot be distinguished more clearly (mth appears a straight line) in Fig. 3. It is clear from the experimental data that the effect of remanent enhancement exists in all the samples. The reduced remanence increases from 0.680 to 0.806 with increasing the Sm content. According to the theory of Kneller and Hawig [1], to obtain a sufficiently strong exchange coupling, the optimum grain size (D) of the magnetically soft phase must be smaller than 2bcm . Sm substitution for Nd reduces the magnetocrystalline anisotropy of the magnetically hard phase Nd2 Fe14 B, and increases the exchange correlation length bcm , so that a larger volume inside a magnetically soft grain can be coupled to the surrounded magnetically hard grains. Therefore, the larger fraction of the grains is sufficiently involved with the exchange coupling, enhancing the reduced remanence. Another plausible explanation for the increase of the remanence with increasing Sm content might be the formation of an additional magnetically soft phase (Sm2 Fe14 B) that could increase the total amount of the magnetically soft phases in the system. When the amount of Sm substitution for Nd is small in the system, only (Nd,Sm)2 Fe14 B forms, which is magnetically hard. When more Nd is substituted by Sm, a small amount of Sm2 Fe14 B could form. However, the spontaneous polarization of Sm2 Fe14 B (1.52 T) is lower than that of Nd2 Fe14 B and much lower than that of ␣-Fe (2.15 T). The formation of Sm2 Fe14 B decreases the saturation magnetization of the magnets. Furthermore, the lattice of Sm2 Fe14 B is tetragonal, which cannot contribute the high remanent ratio (0.832) as the same as what the cubic ␣-Fe is. Thus, if there were no the enhancement of the exchange coupling due to the decrease of the anisotropy, the formation of Sm2 Fe14 B would decrease the magnetic properties very rapidly. Although the formation of the Sm2 Fe14 B-based phase would have some contributions to the remanence, the most reasonable explain of the remanence enhancement in this system should be attributed to the enhancement of exchange coupling due to the extension of the exchange correlation length bcm . In order to understand the exchange couple enhancement, the susceptibility curves are calculated from the hysteresis curves [17]. As shown in the inset of Fig. 4, the susceptibility curves of all samples have a narrow peak near the coercive field. The existence of only one peak means that the magnetically hard and soft phases are small in terms of the grain size, having a uniform grain size distribution. Each magnetic-soft grain couples well with its neighboring mag-
Fig. 4. Composition dependence of the area ratio defining the squareness of hysteresis loop in Nd8−x Smx Fe88 B4 alloys. The dash curve is a second-order polynomial fit to the experimental data. The inset shows the susceptibility curves dJ/dH.
netically hard grains and its moments rotate together with the cluster. It is noted that the peak intensity increases with increasing the Sm content, indicating the enhancement of the exchange coupling between the magnetically hard and soft phases. The peak shifts to lower applied field, due to the decrease of the coercivity (caused by the decrease of the anisotropy field of the magnetically hard phase) because of Sm substitution for Nd. It is also interesting to calculate the area ratio for a more clear assessment of the exchange-coupling enhancement, which is defined as the ratio of the area below the demagnetization curve in the second quadrant to the product of coercivity and remanence [10]. As shown in Fig. 4, the area ratios increase monotonically as the Sm content increases, which might originate from the increasing of the exchange correlation length bcm upon the decrease of the magnetocrystalline anisotropy of the magnetically hard phase and a reinforcement of the exchange coupling, corresponding to a better squareness of their demagnetization curves. It is consistent with the previous analysis above.
4. Conclusion The Sm substitution for Nd in mechanically milled nanocomposite (Nd, Sm)2 Fe14 B/␣-Fe magnets could increase the maximum energy product in a certain range of composition. Compared with the theoretical results of the simply dilution, the reduced remanence is improved apparently by increasing the Sm content, because the Sm substitution for Nd improves the exchange coupling. These can be attributed to an extension of the exchange coupling length, due to a decrease of the anisotropy of the magnetically hard phase, so that more magnetically soft grains participate the exchange coupling well. Compared with the Y substitution for Nd, more Sm substitution for Nd, has a different effect
M. Zhang et al. / Journal of Alloys and Compounds 372 (2004) 267–271
on remanence, which is originated from the different magnetic status of Sm/Y ions in the R2 Fe14 B lattice. The Sm substitution for Nd has stronger effect on decreasing the magnetic anisotropy, thus it has stronger effect on increasing the exchange coupling length. This work verifies a new thought to improve the magnetic properties of nanocomposite magnets by adjusting the magnetocrystalline anisotropy.
Acknowledgements This work has been supported by the National Nature Science Foundation Committee of China (Grant Numbers 59725103 and 50071062) and the Science and Technology Commissions of Shenyang.
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[4] B.C. Cui, X.K. Sun, W. Liu, Z.D. Zhang, D.Y. Geng, X.G. Zhao, J. Appl. Phys. 87 (2000) 5335. [5] X.Z. Zhang, Y. Guan, L. Yang, J.W. Zhang, Appl. Phys. Lett. 79 (2001) 2426. [6] J.P. Liu, D.J. Sellmyer, J. Appl. Phys. 83 (1998) 6608. [7] B.C. Cui, X.K. Sun, W. Liu, Z.D. Zhang, D.Y. Geng, X.G. Zhao, J. Appl. Phys. 87 (2000) 5335. [8] C.Y. You, X.K. Sun, W. Liu, B.Z. Cui, X.G.ZhaoD.Y. Geng, Z.D. Zhang, J. Phys. D: Appl. Phys. 35 (2002) 943. [9] W. Liu, Z.D. Zhang, J.P. Liu, L.J. Chen, L.L. He, Y. Liu, X.K. Sun, D.J. Sellmyer, Adv. Mater. 14 (2002) 1832. [10] T. Zhao, Q.F. Xiao, Z.D. Zhang, M. Dahlgren, R. Grössinger, K.H.J. Buschow, F.R. de Boer, Appl. Phys. Lett. 75 (1999) 2298. [11] H.W. Zhang, B.H. Li, J. Wang, B.G. Shen, J. Phys. D: Appl. Phys. 33 (2000) 3022. [12] B.Z. Cui, X.K. Sun, L.Y. Xiong, S.T. Cao, X.X. Zhang, W. Liu, D.Y. Geng, Z.D. Zhang, J. Alloys Compd. 340 (2002) 242. [13] M. Zhang, Z.D. Zhang, X.K. Sun, W. Liu, D.Y. Geng, X.M. Jin, C.Y. You, X.G. Zhao, J. Appl. Phys. 92 (2002) 5569. [14] M. Zhang, W.J. Ren, Z.D. Zhang, X.K. Sun, W. Liu, D.Y. Geng, X.G. Zhao, R. Grössinger, D. Triyono, J. Appl. Phys. 94 (2003) 2602. [15] Z.D. Zhang, Y.K. Huang, X.K. Sun, Y.C. Chuang, F.R. de Boer, R.J. Radwañski, J. Less-Common Met. 152 (1989) 67. [16] T. Schrefl, R. Fischer, J. Fidler, H. Kromüller, J. Appl. Phys. 76 (1994) 7053. [17] Y.H. Gao, J.H. Zhu, Y.Q. Weng, E.B. Park, C.J. Yang, J. Magn. Magn. Mater. 191 (1999) 146.
Remanent enhancement of nanocomposite (Nd, Sm)2 Fe14 B/␣-Fe magnets M. Zhang a,∗ , Z.D. Zhang a,b , X.K. Sun a,b , W. Liu a,b , D.Y. Geng a,b , X.M. Jin a,b , C.Y. You a,b , X.G. Zhao a,b a
Shenyang National Laboratory for Materials Science and International Centre for Materials Physics, Institute of Metal Research, Chinese Academia of Sciences, 72 Wenhua Road, Shenyang 110016, China b International Centre for Materials Physics, Chinese Academia of Sciences, 72 Wenhua Road, Shenyang 110016, China Received 8 July 2003; received in revised form 29 September 2003; accepted 29 September 2003
Abstract We have systemically studied structure and magnetic properties of nanocomposite magnets Nd8−x Smx Fe88 B4 (x = 0–2.5) prepared by mechanical milling and subsequent heat-treatment. The remanence increases monotonically with Sm substitution for Nd. The value of the maximum energy product exhibits a peak at x = 0.4. The extension of the exchange coupling length is induced by a decrease of the anisotropy of the magnetic-hard phase. This results in that more magnetically soft grains participate well the exchange coupling, improving the magnetic properties of the nanocomposite magnets. Compared with Y substitution for Nd, the Sm substitution for Nd in the Nd2 Fe14 B/␣-Fe nanocomposite magnets has stronger effect on decreasing the magnetic anisotropy, thus it has stronger effect on increasing the exchange coupling length. © 2003 Elsevier B.V. All rights reserved. PACS: 75.50.Ww; 75.75.+a; 81.07.Bc; 81.20.Ev Keywords: Exchange coupling; Nanocomposite magnets; Mechanical milling
1. Introduction Nanocomposites of magnetically hard and soft phases have a substantial potential for application of permanent magnets, since their remanence could be higher than those in isotropic single-phase magnetically hard materials. Remanence enhancement originates from the exchange coupling of magnetic moments across interfaces between the magnetically hard and soft phases, because the exchange coupling could turn the moments of the magnetically soft phase to rotate toward an average direction of the nearest neighboring magnetically hard phases. It has been demonstrated on several occasions [1,2] that the crystallite size of the phases, in particular, that of the soft phase, is important for realizing the remanence enhancement and determining the maximum energy product of the nanocomposite magnets. Much effort has been made to optimize the magnetic properties, by using various preparation methods, such as, mechani∗
Corresponding author. E-mail address: [email protected] (M. Zhang).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.09.143
cal alloying, melt spinning or sputtering [3–6], or element substitution [7,8]. But the values of the maximum energy product (BH)max achieved experimentally have been much smaller than the ones predicted theoretically. The difference between the theoretical and experimental values of (BH)max is primarily due to the difficulty in obtaining the optimum size distribution of magnetically soft grains employed for the theoretical model. For an optimum exchange coupling, the model of Kneller and Hawig gave that the size of the magnetically soft grains should be about twice as large as the domain wall width of the hard phase [1]. It is usually difficult to prepare the nanocomposite magnets with the magnetically soft phase having such small grain size. Recently, the value of (BH)max = 203.52 kA/m3 was achieved, due to a better control of grain sizes and distribution of the magnetic phases in multilayers prepared by sputtering [9]. Zhao et al. systematically investigated (Nd, Sm)2 Fe14 B/ Fe3 B nanocomposite magnets by melt spinning [10]. An enhancement of the exchange coupling length was found to relate with the reduction of the magnetocrystalline anisotropy of the magnetically hard phase by Sm substitution for Nd.
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But Zhang et al. thought that no obvious relation exists between the anisotropy of the hard phase and the remanence enhancement of the composite magnets [11]. It is still very interesting to have a further understanding of the exchange correlation length bcm = π(Am /2K1 )1/2 , where Am is the exchange energy of the magnetically soft phase, K1 is the first-order magnetocrystalline anisotropy constant of the magnetically hard one, and to realize in different systems the idea of increasing the energy product by decreasing the magnetocrystalline anisotropy K1 . In order to verify the relation between the exchange coupling and the magnetocrystalline anisotropy, in this work, we choose Nd2 Fe14 B/␣-Fe nanocomposite magnets prepared by mechanical milling and subsequent annealing. The effect of the exchange coupling length on the magnetic properties is explored, by adjusting the magnetocrystalline anisotropy of the magnetically hard phase by Sm substitution for Nd. The effects of Sm substitution for Nd are compared with those of Y substitution for Nd in the Nd2 Fe14 B/␣-Fe nanocomposite magnets.
2. Experiment The alloys of Nd8−x Smx Fe88 B4 (x = 0–2.5) were prepared by arc-melting twice under purified argon, with starting materials of 99.5 wt.% purity. Boron was added in the form of a Fe–B master alloy containing 20.8 wt.% boron. The ingots were crashed to prepare the alloy powders with a particle size less than 150 m. Then the alloy powders were milled under a purified argon atmosphere for 5 h in a high-energy ball mill. The weight ratio of ball to powder was 40:1. The as-milled powders were annealed at various temperatures of 580–720 ◦ C in a vacuum better than 2×10−3 Pa for 20 min. X-ray diffraction (XRD) analysis was conducted using Cu K␣ radiation with a Rigaku D/Max-␥A diffractometer equipped with a graphite crystal monochromator. For magnetic measurements at room temperature, the powders were embedded in epoxy resin with a weight ratio of 1:1 between the magnetic powders and the resin to form magnetically isotropic magnets. The magnetic properties were measured at room temperature using a pulsed magnetometer in fields up to 6 T. The results of the magnetic measurements were corrected using an experimentally determined effective demagnetization factor of 0.28.
3. Result and discussion XRD results show that after mechanically milled for 5 h, all alloys consist of an amorphous matrix in which nano-grains of ␣-Fe are embedded [see Fig. 1(a)]. After the heat treatment, a nano-crystalline phase of Nd2 Fe14 B-type coexists with ␣-Fe in nanoscale [see Fig. 1(b) and (c)]. The optimum magnetic properties are achieved, after annealed at 630 ◦ C for 20 min. The mean grain sizes of ␣-Fe are estimated by X-ray diffraction using the Scherrer’s
Fig. 1. X-ray diffraction patterns of mechanically milled Nd8−x Smx Fe88 B4 alloys, (a) as milled, x = 0.0, (b) after annealed at 630 ◦ C, x = 0.0, (c) after annealed at 630 ◦ C, x = 0.4.
formula. When x equals to 0.0 and 0.4, the mean grain sizes of ␣-Fe are 17 and 18 nm, respectively. No significant dependence of the grain size on the concentration of the rare-earth elements is observed, according to the XRD analysis. The values (17 and 18 nm) of the mean grain sizes of ␣-Fe, obtained in this work by XRD, are close to those obtained by XRD (18–25 nm) and TEM (20–24 nm) in our previous work for Nd2 (Fe, Co, Mo)14 B/␣-Fe nanocomposite magnets [12]. The relative XRD intensity of the Nd2 Fe14 B-type phase is too weak to evaluate the grain size. The mean grain size of the Nd2 Fe14 B-type phase, as determined by TEM observations, was in range of 36–41 nm, for the alloys annealed at 650 ◦ C for 20 min [12]. The mean grain size of the Nd2 Fe14 B-type phase of the present alloys should be comparable with those reported previously. For detailed structural and microstructural aspects for the Nd2 Fe14 B-based nanocomposite magnets, the readers are referred to our recent work [12]. The relationship between nanostructure, exchange coupling and magnetic properties of the Nd2 (Fe, Co, Mo)14 B/␣-Fe nanocomposite magnets prepared by mechanical alloying (MA) was studied systematically. Three different categories of grain interfaces were directly observed by HRTEM, which correspond to different types of the exchange couplings in the Nd2 Fe14 B/␣-Fe-type nanocomposite magnets [12]. The demagnetization curves and the corresponding magnetic properties are showed in Figs. 2 and 3, respectively. The coercivity decreases monotonically with increasing Sm content as expected, because of the decrease of the magnetocrystalline anisotropy of the magnetically hard phase. The remanence monotonically increases with increasing x, due to the enhancement of the exchange coupling. The maximum energy product first increases and reaches its maximum with increasing x, due to the competition effect between an increase of the remanence and a decrease of the coercivity. For a higher Sm content, it decreases as a result of the further deterioration of the coercivity (Fig. 3). In our recent work, we studied systemically the structure and magnetic properties of Nd8−x Yx Fe88 B4 alloys [13,14]. As compared in Fig. 3, a different effect on remanence was
M. Zhang et al. / Journal of Alloys and Compounds 372 (2004) 267–271
Fig. 2. Demagnetization curves of Nd8−x Smx Fe88 B4 alloys annealed at 630 ◦ C for 20 min.
acquired for Nd8−x Yx Fe88 B4 [13]. It can be seen that the remanence increases and then decreases with increasing Y content, different from the monotonic increase upon Sm content. It is a common point that a small amount of Sm/Y substitution for Nd increases the exchange correlation length bcm , due to the decrease of the magnetocrystalline anisotropy of the magnetically hard phase. The remanence is improved by enhancing the exchange coupling between the magnetically hard and soft phase of the nanocomposite magnets. When more Sm/Y substitutes for Nd, the effects on the remanence are different, which can be attributed to the different magnetic status of Sm and Y ions in the R2 Fe14 B-type lattice. It is known that the Sm ions in Sm2 Fe14 B contribute to a negative anisotropy constant K1 , while the Y ions in Y2 Fe14 B have no contribution (except for the dilution effect) to the magnetic anisotropy because it is non-magnetic [15]. Thus it is clear that in the R2 Fe14 B-type phase, the
Sm ions have stronger effects on the magnetic anisotropies than the Y ions do (since the Sm substitution goes forward to negative and the Y substitution goes to zero). Namely, the magnetic anisotropy constant K1 decreases more rapidly with the Sm substitution than the Y substitution for Nd. Because Sm2 Fe14 B shows the strong easy-plane anisotropy, the excessive Sm substitution for Nd would intensively decrease the anisotropy magnetocrystalline of the magnetically hard phase. It means that the exchange correlation length is increased more quickly than in the case of Y substitution in nanocomposite magnets. Therefore, the exchange coupling would be enhanced more obviously in the case of the Sm substitution. Although the Y substitution for Nd also enhances the exchange coupling, due to the reduced magnetocrystalline anisotropy, it has weaker effect than the Sm substitution. Furthermore, it would deteriorate the direct exchange coupling between the magnetic moments in nanocomposite magnets because Y is nonmagnetic. The effects of Y substitution for Nd on the remanence are determined by competition of these two factors [13,14]. When the Y content is lower, the effect of enhanced exchange coupling, due to the reduced magnetocrystalline anisotropy of the magnetically hard phase is important; when the Y content is higher, the effect of deteriorating the direct exchange coupling between magnetic moments, due to the substitution of nonmagnetic Y for Nd, becomes dominant. The typical smooth demagnetization curves of an exchange-coupled two-phase system can be noticed (Fig. 2). However, we cannot drive the conclusion that the magnetically hard and soft phases have enough strong exchange coupling. This is because this kind of curve can persist in a proper range where the magnetic properties have already decreased, due to the weakening of the exchange coupling [16]. Nevertheless, we could study the change of the exchange coupling by the reduced remanence. In Nd2 Fe14 B/␣-Fe nanocomposite magnets, the magnetically soft phase ␣-Fe is non-uniaxial, which contributes a greater reduced remanence than the uniaxial magnetically hard phase Nd2 Fe14 B. In order to properly evaluate the effect of remanence enhancement of nanocomposite magnets, the theoretical reduced remanence mth of magnetically isotropic magnets is calculated, according to the formula: mth =
Fig. 3. Composition dependence of the coercivity i Hc , the remanence Jr (solid squares, denoted by Sm), the maximum energy product (BH)max and reduced remanence Jr /Js of mechanical milled Nd8−x Smx Fe88 B4 alloys, after annealing at 630 ◦ C for 20 min. The remanence Jr (solid circles, denoted by Y) of mechanical milled Nd8−x Yx Fe88 B4 alloys, after annealing at 630 ◦ C for 20 min is also shown. The dashed line is for the theoretical calculation of reduced remanence.
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Jr (0.5 × (1 − vsoft )Jshard ) + (0.832 × vsoft Jssoft ) = Js (1 − vsoft )Jshard + vsoft Jssoft (1)
where vsoft is the volume fraction of the magnetically soft phase, for the present nanocomposite magnets, with a volume ratio of 70:30 for hard to soft phase given by the rare earth contents of the starting compositions. Jssoft stands for the spontaneous polarization of the magnetically soft phase. Jshard stands for the spontaneous polarization of the magnetically hard phase, which can be described as a function of Sm content x using the following equation: Jshard = 1 − 18 x JsNd + 18 xJSm (2) s
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where JsNd and JsSm is the spontaneous polarization of Nd2 Fe14 B and Sm2 Fe14 B, respectively. The values of 0.5 and 0.832 correspond to the reduced remanence of the magnetically hard and soft phases, respectively. The dash curve in Fig. 3 shows the theoretical value of the reduced remanence for various Sm contents, while the solid curve gives the experimental ones for samples annealed at 630 ◦ C for 20 min. The spontaneous polarization of Nd2 Fe14 B and Sm2 Fe14 B is 1.6 and 1.52 T, respectively. The Sm substitution for Nd from 0 to 2.5, the calculated mth decreases slightly and the tendency cannot be distinguished more clearly (mth appears a straight line) in Fig. 3. It is clear from the experimental data that the effect of remanent enhancement exists in all the samples. The reduced remanence increases from 0.680 to 0.806 with increasing the Sm content. According to the theory of Kneller and Hawig [1], to obtain a sufficiently strong exchange coupling, the optimum grain size (D) of the magnetically soft phase must be smaller than 2bcm . Sm substitution for Nd reduces the magnetocrystalline anisotropy of the magnetically hard phase Nd2 Fe14 B, and increases the exchange correlation length bcm , so that a larger volume inside a magnetically soft grain can be coupled to the surrounded magnetically hard grains. Therefore, the larger fraction of the grains is sufficiently involved with the exchange coupling, enhancing the reduced remanence. Another plausible explanation for the increase of the remanence with increasing Sm content might be the formation of an additional magnetically soft phase (Sm2 Fe14 B) that could increase the total amount of the magnetically soft phases in the system. When the amount of Sm substitution for Nd is small in the system, only (Nd,Sm)2 Fe14 B forms, which is magnetically hard. When more Nd is substituted by Sm, a small amount of Sm2 Fe14 B could form. However, the spontaneous polarization of Sm2 Fe14 B (1.52 T) is lower than that of Nd2 Fe14 B and much lower than that of ␣-Fe (2.15 T). The formation of Sm2 Fe14 B decreases the saturation magnetization of the magnets. Furthermore, the lattice of Sm2 Fe14 B is tetragonal, which cannot contribute the high remanent ratio (0.832) as the same as what the cubic ␣-Fe is. Thus, if there were no the enhancement of the exchange coupling due to the decrease of the anisotropy, the formation of Sm2 Fe14 B would decrease the magnetic properties very rapidly. Although the formation of the Sm2 Fe14 B-based phase would have some contributions to the remanence, the most reasonable explain of the remanence enhancement in this system should be attributed to the enhancement of exchange coupling due to the extension of the exchange correlation length bcm . In order to understand the exchange couple enhancement, the susceptibility curves are calculated from the hysteresis curves [17]. As shown in the inset of Fig. 4, the susceptibility curves of all samples have a narrow peak near the coercive field. The existence of only one peak means that the magnetically hard and soft phases are small in terms of the grain size, having a uniform grain size distribution. Each magnetic-soft grain couples well with its neighboring mag-
Fig. 4. Composition dependence of the area ratio defining the squareness of hysteresis loop in Nd8−x Smx Fe88 B4 alloys. The dash curve is a second-order polynomial fit to the experimental data. The inset shows the susceptibility curves dJ/dH.
netically hard grains and its moments rotate together with the cluster. It is noted that the peak intensity increases with increasing the Sm content, indicating the enhancement of the exchange coupling between the magnetically hard and soft phases. The peak shifts to lower applied field, due to the decrease of the coercivity (caused by the decrease of the anisotropy field of the magnetically hard phase) because of Sm substitution for Nd. It is also interesting to calculate the area ratio for a more clear assessment of the exchange-coupling enhancement, which is defined as the ratio of the area below the demagnetization curve in the second quadrant to the product of coercivity and remanence [10]. As shown in Fig. 4, the area ratios increase monotonically as the Sm content increases, which might originate from the increasing of the exchange correlation length bcm upon the decrease of the magnetocrystalline anisotropy of the magnetically hard phase and a reinforcement of the exchange coupling, corresponding to a better squareness of their demagnetization curves. It is consistent with the previous analysis above.
4. Conclusion The Sm substitution for Nd in mechanically milled nanocomposite (Nd, Sm)2 Fe14 B/␣-Fe magnets could increase the maximum energy product in a certain range of composition. Compared with the theoretical results of the simply dilution, the reduced remanence is improved apparently by increasing the Sm content, because the Sm substitution for Nd improves the exchange coupling. These can be attributed to an extension of the exchange coupling length, due to a decrease of the anisotropy of the magnetically hard phase, so that more magnetically soft grains participate the exchange coupling well. Compared with the Y substitution for Nd, more Sm substitution for Nd, has a different effect
M. Zhang et al. / Journal of Alloys and Compounds 372 (2004) 267–271
on remanence, which is originated from the different magnetic status of Sm/Y ions in the R2 Fe14 B lattice. The Sm substitution for Nd has stronger effect on decreasing the magnetic anisotropy, thus it has stronger effect on increasing the exchange coupling length. This work verifies a new thought to improve the magnetic properties of nanocomposite magnets by adjusting the magnetocrystalline anisotropy.
Acknowledgements This work has been supported by the National Nature Science Foundation Committee of China (Grant Numbers 59725103 and 50071062) and the Science and Technology Commissions of Shenyang.
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