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Intrinsic magnetic properties of Nd16Fe72−xCoxC11B carbides
Journal of Magnetism and Magnetic Materials 223 (2001) 273}278
Intrinsic magnetic properties of Nd Fe Co C B carbides \V V C. Paduani *, S. Rivoirard , J.-L. Soubeyroux , P. de Rango , R. Tournier Laboratoire de Cristallographie/CRETA-CNRS, BP 166, 38042 Grenoble Cedex 09, France Departamento de Fn& sica, Universidade Federal de Santa Catarina, CP476, UFSC, Floriano& polis CEP 88040-900 Floriano& polis, SC, Brazil Received 23 May 2000; received in revised form 29 September 2000
Abstract An investigation of the magnetic properties of the Nd Fe Co C B series is described. The as-cast alloys show \V V the coexistence in equilibrium of the Nd Fe C phase together with secondary phases, such as a-Fe and the very corrosive s (hexagonal) phase. After one week of heat treatment, the main 2 : 14 : 1 phase in the alloys is veri"ed. It is veri"ed that the addition of 1 at% B reduces the presence of a-Fe in this compound, whereas the substitution of Co for Fe causes an increase of the critical temperature. For x"10 a maximum is observed in both the remanence and coercivity, at 4 kG and 3 kOe, respectively. 2001 Elsevier Science B.V. All rights reserved. PACS: 23.23#x; 56.65.Dy Keywords: Permanent magnets; Carbides; Rare-earth; Transition metal
1. Introduction The "rst studies of rare-earth-transition-metal carbides were reported about two decades ago [1], and since then the development of these materials with high coercivities has been the subject of extensive studies [2,3]. Although very high coercivities are not obtained in sintered borides, an opposite behavior is observed in the carbides. In view of the fact that the carbides are less stable than the borides, usually they are not found in as-cast alloys. In spite of its high saturation magnetization of about 1281.2 emu/cm (1.61 T), the Nd Fe B compound has a rather low Curie temperature (3123C), which
* Corresponding author. Tel.: #55-48-2319434; fax #55-483319946/9688. E-mail address: [email protected] (C. Paduani).
imposes an upper limit for practical application at about 1003C. Several alternatives for iron in Nd Fe B have been tried in an attempt to im prove the thermal properties of this compound. Among them, Co in Nd (Fe Co ) B alloys is \V V known to increase the Curie temperature (¹ ). ! The lattice parameters decrease monotonically, while the tetragonal form is maintained in the entire range of composition (0,1) [4,5]. The saturation magnetization varies with composition and reaches a maximum at about x"0.1, with an average atomic magnetic moment of about 2.13 k . The "rst-order anisotropy coe$cient K also decreases with Co concentration. These studies have also shown that Fe atoms prefer the 8j site, whereas the Co atoms prefer the 16k site, avoiding the 8j one [6]. Neutron di!raction studies on Nd Fe C have established the occurrence of a tetragonal crystal
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 2 7 7 - 4
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structure for this compound (phase U), which is isotypic with Nd Fe B [7]. At room temperature, the magnetic moment arrangement is ferromagnetic with the moments parallel to the c-axis, while at the liquid He temperature it is found to be canted away from the c-axis. The average magnetic moment of the Nd atoms is 2.95 k at room temperature, and for the iron atoms, the neutron di!raction results have indicated 2.44 k , while MoK ssbauer experiments give 1.9 k [8]. The Nd Fe C phase is almost stable at 8003C and, by substituting a small amount of B for C, the tetragonal phase could be obtained with an appropriate heat treatment of the castings [3]. Due to nucleation di$culties and slow growth kinetic, this phase (U) has been found only in samples annealed at 8503C for 20 days [9]. This phase was also found to coexist in equilibrium with either ferromagnetic (+8 wt% of a-Fe and Nd Fe C ) and/or very V corrosive phases. However, this situation can be changed by adding small amounts of other elements, e.g., B, which accelerates the formation of the U phase, reducing the annealing time from weeks to hours. Above 8903C, the main equilibrium phase is Nd Fe C , and below 8303C, the solid state reac V tion forming the tetragonal U phase is very sluggish [10]. The Curie temperature of the former phase is 1433C, whereas for Nd Fe C , it was obtained at 2653C from DSC results. The lattice parameters of the U phase are a"8.83 As and c"12.2 As . The saturation magnetization of powdered #akes prepared by melt spinning was estimated as 1114 emu/cm (1.40 T) at room temperature, and the anisotropy "eld was found to be close to the Nd Fe B value. The coercive "eld obtained is 10 kOe. The e!ect of the addition of Co on the magnetic properties of Nd}Fe}C ingot magnets has also been investigated. This study found that the substitution of Co can lead to a signi"cant decrease in the annealing time required to magnetically harden the ingots [11]. It is known that the upper limit of the temperature stability range of the tetragonal phase in these systems increases with boron content. The experimental results of Buschow et al. [11] showed that the coercivity is zero in all as-cast Nd Fe Co C B alloys prepared in an arc \V V furnace. However, it increases with increasing an-
nealing time and remains almost constant above 10 h of heat treatment at 9503C. A maximum in the coercivity was observed in the concentration dependence of these alloys, which shifts towards a higher Co concentration in samples annealed for longer times. For x"4, the coercivity is about 11.31 kOe, the remanence is 6.3 kG, and the maximum energy product is (BH) "
7;10 ergcm\. The structure and phase transformation of Nd}Fe}C alloys made by mechanical alloying were studied by Sui et al. [12]. The thermal stability of Nd Fe C increases by increasing the C content, reaches a maximum and then decreases. The experiments have also indicated that, in order to make Nd Fe C the main phase, the Nd : C ratio must be kept within a given range. With less or more carbon, one obtains the Nd Fe C or a-Fe V as the main phase, respectively. For instance, at an annealing temperature of 9003C, by adding carbon to Nd Fe C alloys, one observes an increase \V V of the amount of Nd Fe C, as well as a decrease of that of Nd Fe C . For x(8, it is predominant V in the alloy. If x"12, the upper temperature limit for the formation of Nd Fe C increases by in creasing the C content, which in turn decreases with further additions. For alloys annealed at 9003C, a maximum is observed for the maximum energy product (BH) , remanence and coercivity
for x"8.5. In this paper, we investigate the e!ect of Co substitution on the hard magnetic properties of Nd Fe Co C B alloys, and we have focused \V V on the e!ect of using higher carbon contents in addition to 1 at% of B on the hard magnetic properties and phase stability of these compounds.
2. Experiment The Nd Fe Co C B alloys with composi \V V tions x"0, 10 and 26 were prepared by induction melting of the nominal compositions in a watercooled copper crucible, under pure argon atmosphere. Subsequently, they were annealed for one week at 8703C, also under an argon atmosphere. X-ray di!raction patterns were taken with Cu}Karadiation. All of the as-cast alloys were found to coexist in equilibrium with the U phase, the soft
C. Paduani et al. / Journal of Magnetism and Magnetic Materials 223 (2001) 273}278
Fig. 1. X-ray di!raction patterns of Nd Fe Co C B \V V alloys after annealing at 8703C for one week. The bottom spectrum is a simulation for the 2 : 14 : 1 phase. The Co concentration is indicated in each case.
ferromagnetic a-Fe, the Nd Fe C phase, as well V as the very corrosive s (hexagonal) phase. This latter has been found in #akes that were relatively rich in C, with lattice spacings a"8.61 As and c"10.26 As , where a composition of Nd Fe C was assumed [10]. After the heat treatment, the main phase was found to have the Nd Fe C (U) structure, as can be seen in Fig. 1. The "rst peak for iron BCC should appear near 2h+453, as a shoulder at the higher neighboring peak, whereas the main peak for the Nd Fe C phase appears at V 2h+42.43. This last peak is clear in the spectra shown in Fig. 1. The disordering temperature of the specimens was determined using a magnetobalance. The temperature range examined was from room temperature to 7833C at a heating/cooling rate of 53C/min. These experimental curves are shown in Fig. 2. The magnetization measurements were carried out at room temperature by using a DC extraction-type magnetometer with an applied external "eld up to 70 kG. The density of these alloys was assumed to be 7.6 g/cm. The analysis of the demagnetizing curve, obtained after magnetizing the ingots in a #ux density of 70 kG, provided the coercivity and remanence values. The corresponding hysteresis loops are shown in Fig. 3.
3. Results and discussion As shown in Fig. 1, the results of the X-ray powder di!raction analysis at room temperature
275
show that these compounds have a Nd Fe C type structure over the entire range of concentration studied. The di!raction patterns were analyzed by means of Rietveld's pro"le re"nement technique. The results for the lattice parameters are shown in Table 1, where one can see that, when substituting Co for Fe, these compounds still have the same tetragonal structure and both the lattice constants a and c change very little over the concentration range studied. These results agree with the known experimental values of a"8.83 As and c"12.20 As for Nd Fe C [10]. No other minor phases could be identi"ed in the Co-free sample except for a small amount of a-Fe. Fig. 2 shows the results of the measurements with the magnetobalance. The Curie temperatures obtained for these alloys are also shown in Table 1. In Fig. 2, the lower (upper) curve indicates the heating (cooling) process. For the cobalt-free sample, the heating curve in the upper part of Fig. 2 shows the disordering temperature occurring at about 2803C. The solubility of carbon in Nd Fe C at 8503C V has been estimated at x+0.3, and the maximum ¹ value at +1973C [10]. Besides, the Curie tem! peratures of Nd Fe C and a-Fe are 2703C and 7703C, respectively, whereas for the Nd Fe C V phase, it is in the range 1463C(¹ (1973C [12]. ! Thus, the addition of 1 at% B seems to slightly increase the Curie temperature of the Nd Fe C phase. Below the decomposition temperature of 8903C, the U phase is known to be more stable than Nd Fe C [11]. Nevertheless, that sample V seems to have no traces of this latter phase. At this higher C composition, the solid-state transformation arising from the heat treatment enhances the transformation of the Nd Fe C V phase into the tetragonal 2 : 14 : 1 phase. The small peak appearing at about 7743C is thought to be due to the decomposition reaction giving rise to the formation of a-Fe at the expense of the U phase, with the subsequent collapse of the magnetization taking place at the ¹ ! value for BCC iron. In the cooling curve, the steep increase observed means that the reversible magnetization process is occurring with the sample richer in a-Fe. At the transition temperature for the Nd Fe C phase, the smaller step con"rms this assumption.
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Fig. 3. Magnetic hysteresis loops for Nd Fe Co C B \V V alloys obtained at room temperature.
Fig. 2. Magnetization versus temperature scanning curves, recorded at a rate of 53C/min. The insets indicate the composition for each alloy.
In the second diagram at the middle of Fig. 2, one sees the transition temperature occurring at
¹ "4293C, besides a weak transition at about ! 2423C, which is due to the presence of a small amount of a carbon-rich Nd Fe C phase. The V peak near 8003C no longer appears, and no formation of a-Fe is evident in this case. The disordering process is reversible for the main phase, but the transition registered on heating at lower temperature is no longer observed in the cooling process. Furthermore, at the highest Co concentration shown in the bottom diagram of Fig. 2, a ¹ value ! of 5793C was found, which is the highest value in this series. No other transitions are observed in this sample. As mentioned above, the U phase was also
C. Paduani et al. / Journal of Magnetism and Magnetic Materials 223 (2001) 273}278
277
Table 1 Lattice parameters, Curie temperature, remanence and coercive "eld for Nd Fe Co C B alloys \V V x
0 10 26
Lattice Constants a (As )
c (As )
8.83 ($0.01) 8.81 ($0.01) 8.80 ($0.02)
12.10 ($0.02) 12.06 ($0.01) 12.00 ($0.04)
found to coexist in equilibrium with a-Fe (+8 wt%), as well as with the Nd Fe C phase, V even after annealing for more than 20 days. Our results con"rm that 1 at% B reduces the presence of BCC iron in the sample treated with a shorter annealing time. The hysteresis loops obtained at room temperature are shown in Fig. 3. One observes in these diagrams a small coercivity (3 kOe) for the sample with x"10, and a small remanence of about 4 kG (see Table 1). The sample has a high saturation magnetization. Even at 70 kG, the magnetization is still not completely saturated. In annealed meltspun #akes with the composition Nd Fe C the same behavior is observed. From hysteresis loops recorded up to 150 kG, a saturation magnetization of about 14 kG was estimated [10]. In an ingot magnet with the composition Nd Fe Co C B annealed for 9 h at 9503C, a remanence of 6.3 kG has been veri"ed [11]. The irregularities appearing in the hysteresis loop at the middle diagram of Fig. 3 can be attributed to the presence of a small amount of a minor phase. In fact, by observing the middle diagram of Fig. 2, corresponding to the same sample, the scanning temperature measurements have also indicated the presence of a secondary phase, which is believed to be the Nd Fe C phase. Our results indicate that V the substitution of low concentrations of Co for Fe in these carbides with high C content leads to a substantial increase in the coercivity, as well as in the remanence, although giving almost the same value for the saturation magnetization. It is worth mentioning that the results of earlier studies on Nd (Fe Co ) B alloys also found that the \V V magnetization passes through a slight maximum at lower Co concentration around x"0.1 [4]. As one can observe in these diagrams, the addition of
¹ (3C) !
4pM (kG)
H (kOe)
280 ($1) 429 ($1) 579 ($1)
0.8 ($0.1) 4.0 ($0.1) 0.5 ($0.1)
0.2 ($0.1) 3.0 ($0.1) 0.2 ($0.1)
1 at% B tends to inhibit the appearance of coercivity in the carbides. As mentioned above, it is known that, in order to make Nd Fe C , the Nd : C ratio in these com pounds must be kept within a certain range. A carbon content in excess of this ratio increases the a-Fe content and inhibits the formation of the 2 : 14 : 1 phase in these carbides. The present investigation has indicated that the addition of B indeed inhibits the Fe segregation. Room-temperature experimental results of MoK ssbauer spectroscopy of Nd Fe C have indicated that 52% of the iron atoms in this compound are in the Nd Fe C phase, 41% in the a-Fe phase and that the remainder constitutes a paramagnetic phase [10]. On the other hand, the present investigation has also shown that increasing the Co concentration in this series leads to an increase in the disordering temperature, while the remanence, as well as the coercivity, passes through a maximum at lower Co content and decreases thereafter. It is clear that increasing the C and B concentrations and heat treating for one week do not contribute to the development of a microstructure favorable to increasing the hard magnetic properties of this compound. Finally, our results con"rm that the U phase can be formed by long-time annealing in the solid state below 9003C. Also it is veri"ed that 1 at% B accelerates this phase transformation and allows formation of this phase after 1 week of heat treatment, against 21 days required for alloys with lower C content. However, in this case, we have observed that no a-Fe is present in these carbides with the introduction of Co, while giving rise to an enhanced coercivity at lower concentrations in this series. It is known that the intrinsic coercivity of annealed Nd Fe C alloys increases with W \ W W both the Nd and C contents to y"16, and decrease
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C. Paduani et al. / Journal of Magnetism and Magnetic Materials 223 (2001) 273}278
thereafter. The present results also indicate that the addition of 1 at% B tends to inhibit the appearance of the coercivity in this compound, as well as reducing the presence of BCC iron.
Acknowledgements This work was partially supported by CAPES, Brazilian agency and Laboratoire de Cristallographie/CRETA-Centre National de la Recherche Scienti"que, Grenoble, France.
References [1] H.H. Stadelmaier, H.K. Park, Z. Metallkd. 72 (1981) 417. [2] N.C. Liu, H.H. Stadelmaier, Mater. Lett. 4 (1986) 377.
[3] N.C. Liu, H.H. Stadelmaier, G. Schneider, J. Appl. Phys. 61 (1987) 3574. [4] Y. Matsuura, S. Hirosawa, H. Yamamoto, S. Fujimura, M. Sagawa, Appl. Phys. Lett. 46 (3) (1985) 308. [5] Ying-Chang Yang, Wen-Wang Ho, Hai-Ying Chen, Jin Wang, Jian Lan, J. Appl. Phys. 57 (1) (1985) 4118. [6] K. Ichinose, K. Fujiwara, M. Oyasato, H. Nagai, A. Tsujimura, J. Phys. C 8 (12) (1988) 595. [7] R.B. Helmholdt, K.H.J. Buschow, J. Less-Common Met. 144 (1988) L33. [8] C.J.M. Denissem, D.B. de Mooij, K.H.J. Buschow, J. LessCommon Met. 139 (1988) 291. [9] B. Grieb, K. Fritz, E.-Th. Henig, J. Appl. Phys. 70 (10) (1991) 6447. [10] R. Coehoorn, J.P.W.B. Duchateau, C.J.M. Denissen, J. Appl. Phys. 65 (2) (1989) 704. [11] K.H.J. Buschow, D.B. de Mooij, G. Martinek, J. Alloys Compounds 190 (1992) 95. [12] Y.C. Sui, Z.D. Zhang, Q.F. Xiao, W. Liu, T. Zhao, X.G. Zhao, Y.C. Chuang, J. Alloys Compounds 267 (1998) 215.
Intrinsic magnetic properties of Nd Fe Co C B carbides \V V C. Paduani *, S. Rivoirard , J.-L. Soubeyroux , P. de Rango , R. Tournier Laboratoire de Cristallographie/CRETA-CNRS, BP 166, 38042 Grenoble Cedex 09, France Departamento de Fn& sica, Universidade Federal de Santa Catarina, CP476, UFSC, Floriano& polis CEP 88040-900 Floriano& polis, SC, Brazil Received 23 May 2000; received in revised form 29 September 2000
Abstract An investigation of the magnetic properties of the Nd Fe Co C B series is described. The as-cast alloys show \V V the coexistence in equilibrium of the Nd Fe C phase together with secondary phases, such as a-Fe and the very corrosive s (hexagonal) phase. After one week of heat treatment, the main 2 : 14 : 1 phase in the alloys is veri"ed. It is veri"ed that the addition of 1 at% B reduces the presence of a-Fe in this compound, whereas the substitution of Co for Fe causes an increase of the critical temperature. For x"10 a maximum is observed in both the remanence and coercivity, at 4 kG and 3 kOe, respectively. 2001 Elsevier Science B.V. All rights reserved. PACS: 23.23#x; 56.65.Dy Keywords: Permanent magnets; Carbides; Rare-earth; Transition metal
1. Introduction The "rst studies of rare-earth-transition-metal carbides were reported about two decades ago [1], and since then the development of these materials with high coercivities has been the subject of extensive studies [2,3]. Although very high coercivities are not obtained in sintered borides, an opposite behavior is observed in the carbides. In view of the fact that the carbides are less stable than the borides, usually they are not found in as-cast alloys. In spite of its high saturation magnetization of about 1281.2 emu/cm (1.61 T), the Nd Fe B compound has a rather low Curie temperature (3123C), which
* Corresponding author. Tel.: #55-48-2319434; fax #55-483319946/9688. E-mail address: [email protected] (C. Paduani).
imposes an upper limit for practical application at about 1003C. Several alternatives for iron in Nd Fe B have been tried in an attempt to im prove the thermal properties of this compound. Among them, Co in Nd (Fe Co ) B alloys is \V V known to increase the Curie temperature (¹ ). ! The lattice parameters decrease monotonically, while the tetragonal form is maintained in the entire range of composition (0,1) [4,5]. The saturation magnetization varies with composition and reaches a maximum at about x"0.1, with an average atomic magnetic moment of about 2.13 k . The "rst-order anisotropy coe$cient K also decreases with Co concentration. These studies have also shown that Fe atoms prefer the 8j site, whereas the Co atoms prefer the 16k site, avoiding the 8j one [6]. Neutron di!raction studies on Nd Fe C have established the occurrence of a tetragonal crystal
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 2 7 7 - 4
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C. Paduani et al. / Journal of Magnetism and Magnetic Materials 223 (2001) 273}278
structure for this compound (phase U), which is isotypic with Nd Fe B [7]. At room temperature, the magnetic moment arrangement is ferromagnetic with the moments parallel to the c-axis, while at the liquid He temperature it is found to be canted away from the c-axis. The average magnetic moment of the Nd atoms is 2.95 k at room temperature, and for the iron atoms, the neutron di!raction results have indicated 2.44 k , while MoK ssbauer experiments give 1.9 k [8]. The Nd Fe C phase is almost stable at 8003C and, by substituting a small amount of B for C, the tetragonal phase could be obtained with an appropriate heat treatment of the castings [3]. Due to nucleation di$culties and slow growth kinetic, this phase (U) has been found only in samples annealed at 8503C for 20 days [9]. This phase was also found to coexist in equilibrium with either ferromagnetic (+8 wt% of a-Fe and Nd Fe C ) and/or very V corrosive phases. However, this situation can be changed by adding small amounts of other elements, e.g., B, which accelerates the formation of the U phase, reducing the annealing time from weeks to hours. Above 8903C, the main equilibrium phase is Nd Fe C , and below 8303C, the solid state reac V tion forming the tetragonal U phase is very sluggish [10]. The Curie temperature of the former phase is 1433C, whereas for Nd Fe C , it was obtained at 2653C from DSC results. The lattice parameters of the U phase are a"8.83 As and c"12.2 As . The saturation magnetization of powdered #akes prepared by melt spinning was estimated as 1114 emu/cm (1.40 T) at room temperature, and the anisotropy "eld was found to be close to the Nd Fe B value. The coercive "eld obtained is 10 kOe. The e!ect of the addition of Co on the magnetic properties of Nd}Fe}C ingot magnets has also been investigated. This study found that the substitution of Co can lead to a signi"cant decrease in the annealing time required to magnetically harden the ingots [11]. It is known that the upper limit of the temperature stability range of the tetragonal phase in these systems increases with boron content. The experimental results of Buschow et al. [11] showed that the coercivity is zero in all as-cast Nd Fe Co C B alloys prepared in an arc \V V furnace. However, it increases with increasing an-
nealing time and remains almost constant above 10 h of heat treatment at 9503C. A maximum in the coercivity was observed in the concentration dependence of these alloys, which shifts towards a higher Co concentration in samples annealed for longer times. For x"4, the coercivity is about 11.31 kOe, the remanence is 6.3 kG, and the maximum energy product is (BH) "
7;10 ergcm\. The structure and phase transformation of Nd}Fe}C alloys made by mechanical alloying were studied by Sui et al. [12]. The thermal stability of Nd Fe C increases by increasing the C content, reaches a maximum and then decreases. The experiments have also indicated that, in order to make Nd Fe C the main phase, the Nd : C ratio must be kept within a given range. With less or more carbon, one obtains the Nd Fe C or a-Fe V as the main phase, respectively. For instance, at an annealing temperature of 9003C, by adding carbon to Nd Fe C alloys, one observes an increase \V V of the amount of Nd Fe C, as well as a decrease of that of Nd Fe C . For x(8, it is predominant V in the alloy. If x"12, the upper temperature limit for the formation of Nd Fe C increases by in creasing the C content, which in turn decreases with further additions. For alloys annealed at 9003C, a maximum is observed for the maximum energy product (BH) , remanence and coercivity
for x"8.5. In this paper, we investigate the e!ect of Co substitution on the hard magnetic properties of Nd Fe Co C B alloys, and we have focused \V V on the e!ect of using higher carbon contents in addition to 1 at% of B on the hard magnetic properties and phase stability of these compounds.
2. Experiment The Nd Fe Co C B alloys with composi \V V tions x"0, 10 and 26 were prepared by induction melting of the nominal compositions in a watercooled copper crucible, under pure argon atmosphere. Subsequently, they were annealed for one week at 8703C, also under an argon atmosphere. X-ray di!raction patterns were taken with Cu}Karadiation. All of the as-cast alloys were found to coexist in equilibrium with the U phase, the soft
C. Paduani et al. / Journal of Magnetism and Magnetic Materials 223 (2001) 273}278
Fig. 1. X-ray di!raction patterns of Nd Fe Co C B \V V alloys after annealing at 8703C for one week. The bottom spectrum is a simulation for the 2 : 14 : 1 phase. The Co concentration is indicated in each case.
ferromagnetic a-Fe, the Nd Fe C phase, as well V as the very corrosive s (hexagonal) phase. This latter has been found in #akes that were relatively rich in C, with lattice spacings a"8.61 As and c"10.26 As , where a composition of Nd Fe C was assumed [10]. After the heat treatment, the main phase was found to have the Nd Fe C (U) structure, as can be seen in Fig. 1. The "rst peak for iron BCC should appear near 2h+453, as a shoulder at the higher neighboring peak, whereas the main peak for the Nd Fe C phase appears at V 2h+42.43. This last peak is clear in the spectra shown in Fig. 1. The disordering temperature of the specimens was determined using a magnetobalance. The temperature range examined was from room temperature to 7833C at a heating/cooling rate of 53C/min. These experimental curves are shown in Fig. 2. The magnetization measurements were carried out at room temperature by using a DC extraction-type magnetometer with an applied external "eld up to 70 kG. The density of these alloys was assumed to be 7.6 g/cm. The analysis of the demagnetizing curve, obtained after magnetizing the ingots in a #ux density of 70 kG, provided the coercivity and remanence values. The corresponding hysteresis loops are shown in Fig. 3.
3. Results and discussion As shown in Fig. 1, the results of the X-ray powder di!raction analysis at room temperature
275
show that these compounds have a Nd Fe C type structure over the entire range of concentration studied. The di!raction patterns were analyzed by means of Rietveld's pro"le re"nement technique. The results for the lattice parameters are shown in Table 1, where one can see that, when substituting Co for Fe, these compounds still have the same tetragonal structure and both the lattice constants a and c change very little over the concentration range studied. These results agree with the known experimental values of a"8.83 As and c"12.20 As for Nd Fe C [10]. No other minor phases could be identi"ed in the Co-free sample except for a small amount of a-Fe. Fig. 2 shows the results of the measurements with the magnetobalance. The Curie temperatures obtained for these alloys are also shown in Table 1. In Fig. 2, the lower (upper) curve indicates the heating (cooling) process. For the cobalt-free sample, the heating curve in the upper part of Fig. 2 shows the disordering temperature occurring at about 2803C. The solubility of carbon in Nd Fe C at 8503C V has been estimated at x+0.3, and the maximum ¹ value at +1973C [10]. Besides, the Curie tem! peratures of Nd Fe C and a-Fe are 2703C and 7703C, respectively, whereas for the Nd Fe C V phase, it is in the range 1463C(¹ (1973C [12]. ! Thus, the addition of 1 at% B seems to slightly increase the Curie temperature of the Nd Fe C phase. Below the decomposition temperature of 8903C, the U phase is known to be more stable than Nd Fe C [11]. Nevertheless, that sample V seems to have no traces of this latter phase. At this higher C composition, the solid-state transformation arising from the heat treatment enhances the transformation of the Nd Fe C V phase into the tetragonal 2 : 14 : 1 phase. The small peak appearing at about 7743C is thought to be due to the decomposition reaction giving rise to the formation of a-Fe at the expense of the U phase, with the subsequent collapse of the magnetization taking place at the ¹ ! value for BCC iron. In the cooling curve, the steep increase observed means that the reversible magnetization process is occurring with the sample richer in a-Fe. At the transition temperature for the Nd Fe C phase, the smaller step con"rms this assumption.
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Fig. 3. Magnetic hysteresis loops for Nd Fe Co C B \V V alloys obtained at room temperature.
Fig. 2. Magnetization versus temperature scanning curves, recorded at a rate of 53C/min. The insets indicate the composition for each alloy.
In the second diagram at the middle of Fig. 2, one sees the transition temperature occurring at
¹ "4293C, besides a weak transition at about ! 2423C, which is due to the presence of a small amount of a carbon-rich Nd Fe C phase. The V peak near 8003C no longer appears, and no formation of a-Fe is evident in this case. The disordering process is reversible for the main phase, but the transition registered on heating at lower temperature is no longer observed in the cooling process. Furthermore, at the highest Co concentration shown in the bottom diagram of Fig. 2, a ¹ value ! of 5793C was found, which is the highest value in this series. No other transitions are observed in this sample. As mentioned above, the U phase was also
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277
Table 1 Lattice parameters, Curie temperature, remanence and coercive "eld for Nd Fe Co C B alloys \V V x
0 10 26
Lattice Constants a (As )
c (As )
8.83 ($0.01) 8.81 ($0.01) 8.80 ($0.02)
12.10 ($0.02) 12.06 ($0.01) 12.00 ($0.04)
found to coexist in equilibrium with a-Fe (+8 wt%), as well as with the Nd Fe C phase, V even after annealing for more than 20 days. Our results con"rm that 1 at% B reduces the presence of BCC iron in the sample treated with a shorter annealing time. The hysteresis loops obtained at room temperature are shown in Fig. 3. One observes in these diagrams a small coercivity (3 kOe) for the sample with x"10, and a small remanence of about 4 kG (see Table 1). The sample has a high saturation magnetization. Even at 70 kG, the magnetization is still not completely saturated. In annealed meltspun #akes with the composition Nd Fe C the same behavior is observed. From hysteresis loops recorded up to 150 kG, a saturation magnetization of about 14 kG was estimated [10]. In an ingot magnet with the composition Nd Fe Co C B annealed for 9 h at 9503C, a remanence of 6.3 kG has been veri"ed [11]. The irregularities appearing in the hysteresis loop at the middle diagram of Fig. 3 can be attributed to the presence of a small amount of a minor phase. In fact, by observing the middle diagram of Fig. 2, corresponding to the same sample, the scanning temperature measurements have also indicated the presence of a secondary phase, which is believed to be the Nd Fe C phase. Our results indicate that V the substitution of low concentrations of Co for Fe in these carbides with high C content leads to a substantial increase in the coercivity, as well as in the remanence, although giving almost the same value for the saturation magnetization. It is worth mentioning that the results of earlier studies on Nd (Fe Co ) B alloys also found that the \V V magnetization passes through a slight maximum at lower Co concentration around x"0.1 [4]. As one can observe in these diagrams, the addition of
¹ (3C) !
4pM (kG)
H (kOe)
280 ($1) 429 ($1) 579 ($1)
0.8 ($0.1) 4.0 ($0.1) 0.5 ($0.1)
0.2 ($0.1) 3.0 ($0.1) 0.2 ($0.1)
1 at% B tends to inhibit the appearance of coercivity in the carbides. As mentioned above, it is known that, in order to make Nd Fe C , the Nd : C ratio in these com pounds must be kept within a certain range. A carbon content in excess of this ratio increases the a-Fe content and inhibits the formation of the 2 : 14 : 1 phase in these carbides. The present investigation has indicated that the addition of B indeed inhibits the Fe segregation. Room-temperature experimental results of MoK ssbauer spectroscopy of Nd Fe C have indicated that 52% of the iron atoms in this compound are in the Nd Fe C phase, 41% in the a-Fe phase and that the remainder constitutes a paramagnetic phase [10]. On the other hand, the present investigation has also shown that increasing the Co concentration in this series leads to an increase in the disordering temperature, while the remanence, as well as the coercivity, passes through a maximum at lower Co content and decreases thereafter. It is clear that increasing the C and B concentrations and heat treating for one week do not contribute to the development of a microstructure favorable to increasing the hard magnetic properties of this compound. Finally, our results con"rm that the U phase can be formed by long-time annealing in the solid state below 9003C. Also it is veri"ed that 1 at% B accelerates this phase transformation and allows formation of this phase after 1 week of heat treatment, against 21 days required for alloys with lower C content. However, in this case, we have observed that no a-Fe is present in these carbides with the introduction of Co, while giving rise to an enhanced coercivity at lower concentrations in this series. It is known that the intrinsic coercivity of annealed Nd Fe C alloys increases with W \ W W both the Nd and C contents to y"16, and decrease
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thereafter. The present results also indicate that the addition of 1 at% B tends to inhibit the appearance of the coercivity in this compound, as well as reducing the presence of BCC iron.
Acknowledgements This work was partially supported by CAPES, Brazilian agency and Laboratoire de Cristallographie/CRETA-Centre National de la Recherche Scienti"que, Grenoble, France.
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