- Email: [email protected]
Influence of Nb addition on vacancy defects and magnetic properties of the nanocrystalline Nd–Fe–B permanent magnets
Journal of Magnetism and Magnetic Materials 382 (2015) 307–311
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of Nb addition on vacancy defects and magnetic properties of the nanocrystalline Nd–Fe–B permanent magnets Małgorzata Szwaja a, Piotr Gębara a,n, Jacek Filipecki b, Katarzyna Pawlik a, Anna Przybył a, Piotr Pawlik a, Jerzy J. Wysłocki a, Katarzyna Filipecka a a b
Institute of Physics, Czestochowa University of Technology, Czestochowa, Poland Institute of Physics, Jan Dlugosz University, Czestochowa, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 3 July 2014 Received in revised form 20 January 2015 Accepted 29 January 2015 Available online 30 January 2015
In present work, influence of Nb addition on vacancy defects and magnetic properties of nanocrystalline Nd–Fe–B permanent magnets, was investigated. Samples with composition (Nd,Fe,B)100 xNbx (where x ¼ 6,7,8) were studied in as-cast state and after annealing. Samples were prepared by arc-melting with high purity of constituent elements under Ar atmosphere. Ribbons were obtained by melt-spinning technique under low pressure of Ar. Ribbon samples in as-cast state had amorphous structure and soft magnetic properties. Positron annihilation lifetime spectroscopy PALS has been applied to detection of positron – trapping voids (vacancy defects). With increase of Nb in alloy increasing of vacancy defects concentration was observed. Heat treatment of the samples was carried out at various temperatures (from 923 K to 1023 K) for 5 min, in order to obtain nanocrystalline structure. The aim of present work was to determine the influence of Nb addition and annealing conditions on the vacancy defects and magnetic properties of the Nd–Fe–B- type alloys in as-cast state and after heat treatment. & 2015 Published by Elsevier B.V.
1. Introduction As the third-generation rare earth permanent magnets, Nd–Fe– B magnets have been extensively studied. Permanent magnets are indispensable in modern technology and their influence is still growing [1]. To improve the magnetic properties of Nd–Fe–B magnets, extensive efforts have been made via the substitution of other elements [2–5]. Interesting studies of magnetic properties of magnets produced from the Nd–Fe–B base alloy doped with 4 at% Nb are presented in [6–8]. The Nb addition has a significant influence on the glasses forming ability, but also retards the growth of nanocrystalline grains formed during heat treatment [8]. The annealing process is also an important factor in the formation of the microstructure and magnetic properties. Therefore, it is crucial to determine the effect of Nb admixture and heat treatment conditions on the phase constitution and magnetic properties of (Nd10Fe67B23)100 xNbx (where x ¼6, 7, 8) alloys in a form of ribbons. Positron annihilation results in the change the whole mass of both particles and their kinetic energy into energy of photons of electromagnetic radiation. Therefore, the study of photons generated in the process of annihilation, provides information on the state of an electron–positron pair. Annihilation is possible only in n
Corresponding author. E-mail address: [email protected] (P. Gębara).
http://dx.doi.org/10.1016/j.jmmm.2015.01.076 0304-8853/& 2015 Published by Elsevier B.V.
the case when conservation laws are fulfilled, namely conservation of energy, momentum, angular momentum, charge, and parity. High-energy positron annihilation is proceeded by a termalization phenomenon, which consists in the rapid loss of energy due to scattering of positrons and the excitation center [9,10]. The aim of present paper was the studies of the vacancy defects and magnetic properties of the (Nd10Fe67B23)100 xNbx (where x ¼6, 7, 8) alloys.
2. Experimental material and methods Alloy ingots with nominal composition (Nd10Fe67B23)100 xNdx (where x ¼6, 7, 8) were prepared by arc-melting from high purity constituent elements of under a Ti-gettered argon atmosphere. In order to homogenize alloy, the samples were re-melted several times. The ribbon samples were prepared by melt-spinning technique under the Ar protective atmosphere. The linear speed of the copper roll surface of 35 m/s was used in the process. Subsequently the ribbon samples were sealed off in a quartz tube under a low pressure of argon to maintain the purity of atmosphere of heat treatment. In order to obtain a nanocrystalline microstructure, the samples were annealed at temperatures ranging from 923 K to 1023 K for 5 min, and subsequently rapidly cooled in water. The phase analysis of these samples was studied using Bruker D8 Advance diffractometer with CuKα radiation. The room temperature hysteresis loops were measured by LakeShore 7307
308
M. Szwaja et al. / Journal of Magnetism and Magnetic Materials 382 (2015) 307–311
vibrating sample magnetometer at external magnetic fields up to 2 T. Positron lifetime PALS measurements were performed at room temperature using a spectrometer ORTEC, based on a “start–stop” method [11]. Time resolution amounts to FWHM ¼250 ps (full width at half maxima). The sample, along with the source of positrons (Na22 isotope with an activity 4 l05 Bq prepared using 6 μm thick Kapton foil), formed the so-called “sandwich” system. Positron lifetime spectra were analyzed using the LT computer program [12].
3. Results and discussion The XRD scans measured for (Nd10Fe67B23)100 xNbx, where x ¼6, 7, 8 alloy ribbons in as-cast state and after annealing at 923 K and 943 K for 5 min, are shown in Fig. 1. XRD scans for the as-cast ribbon samples revealed their fully amorphous structure, except for the x¼ 6 alloy, where some low intensity reflexes coming from the crystalline phases are present. Annealing at 923 K and 943 K for 5 min resulted in significant changes in the crystal structure of the material. Broadened peaks originating from crystalline phase are observed on the diffraction patterns. It was shown that the hard magnetic Nd2Fe14B, the paramagnetic Nd1 þ εFe4B4 and soft magnetic metastable Nd2Fe23B3 phases are present in the samples. However, it was impossible to clearly identify phases from the XRD scans due to the overlapping of reflexes coming from different crystalline phases. The XRD pattern measured for the (Nd10Fe67B23)100 xNbx (where x¼ 7) alloy ribbon samples annealed at temperatures from 963 K to 1023 K for 5 min, are shown in Fig. 2. The short time annealing for 5 min at 963 K and at higher temperatures leads to nucleation and growth of the crystalline phases. The phases observed for the annealed ribbons were: the hard magnetic Nd2Fe14B, the paramagnetic Nd1 þ εFe4B4 and soft magnetic metastable Nd2Fe23B3 phases. Complementary Mössbauer spectra (MS) analysis was carried out for selected samples. The example of MS studies is presented in Fig. 3 for the x ¼8 alloy ribbon annealed at 1003 K for 5 min. The analysis was carried out taking into account a presence of hard magnetic Nd2Fe14B phase that was represented by six Zeeman lines corresponding to the magnetically nonequivalent positions of the Fe atoms in its unit cell (16k1, 16k2, 8j1, 8j2, 4e and 4c of relative intensities 4:4:2:2:1:1). Furthermore, the paramagnetic Nd1 þ εFe4B4 phase was denoted by a doublet component line and the metastable Nd2Fe23B3 phase was marked by three magnetically nonequivalent positions of the Fe atoms of the highest population (48e1, 48e2, 48e3). For complete fitting an additional line corresponding to the continuous hyperfine field distribution was added (Fig. 3). The continuous line may be attributed to the remaining amorphous or highly disordered phase. The quantitative analysis have shown that the dominant phase formed during annealing was the paramagnetic Nd1 þ εFe4B4 phase (Table 1), which reduces the saturation magnetization of the samples. Presence of large fractions of magnetically disordered and hard magnetic phases are responsible for the ferromagnetic behavior of the alloy. However, the fractions of constituent phases change significantly with the Nb addition, that impacts magnetic properties of annealed ribbons. The hysteresis loops of (Nd10Fe67B23)100 xNbx, where x ¼6, 7, 8 alloy ribbon samples annealed at various temperatures, are shown in Fig. 4. The samples of all alloys in as-cast state and annealed at temperatures lower than 943 K reveal soft magnetic properties. Annealing at higher temperatures resulted in an evolution of microstructure and phase constitution thus leading to significant changes of their magnetic properties (Fig. 5). The
Fig. 1. X-ray diffraction patterns measured for (Nd10Fe67B23)100 xNbx, where x¼ 6 (a), x ¼7 (b), x¼ 8 (c) alloy ribbon samples in as-cast state and annealed at 923 K and 943 K for 5 min.
hysteresis loops of (Nd10Fe67B23)100 xNbx (where x¼6, 7, 8) alloy ribbons annealed at 963 K, 983 K and 1003 K for 5 min, are shown in Fig. 4. For higher annealing temperatures (above 963 K) a hard magnetic properties are induced. The wasp-tail shaped hysteresis loops measured for the samples annealed at lower temperatures indicate presence of low fractions of hard magnetic phase. The dependences of polarization
M. Szwaja et al. / Journal of Magnetism and Magnetic Materials 382 (2015) 307–311
309
Fig. 2. X-ray diffraction patterns measured for (Nd10Fe67B23)100 xNbx where x ¼7 alloy ribbon samples annealed at 963–1023 K for 5 min.
Fig. 3. Mössbauer spectra for the (Nd10Fe67B23)100 xNbx, where x¼ 8 alloy ribbon samples annealed at 1023 K for 5 min with corresponding probability of hyperfine field Bhf (inset). Table 1 Weight content of recognized phases for alloys with different Nb addition. Fig. 4. The hysteresis loops measured for the (Nd10Fe67B23)100 xNbx alloys, where x¼ 6 (a), x ¼ 7 (b), x ¼ 8 (c) in the form of ribbons annealed at 923–1023 K for 5 min.
(Nd10Fe67B23)100 xNbx V [wt%] Nd2Fe14B Nd2Fe23B3 Nd1 þ εFe4B4 Amorphous phase x¼6 x¼7 x¼8
17.2 31.6 18.6
28.8 15.3 1.9
41.2 42.6 48.6
12.8 10.5 30.9
remanence Jr, the maximum magnetic energy density (BH)max and coercivity JHc on the annealing temperature for the investigated alloys are shown in Fig. 5 It was shown, that increase of Nb contents resulted in significant rise of JHC and (BH)max for annealed samples. Furthermore, a change from two-stage to the squareshape demagnetization curve with the Nb content was observed for ribbons annealed at temperatures of optimal magnetic properties for particular composition (Fig. 4). This suggests that larger
content of Nb in the alloy composition results in formation of more uniform microstructure of the annealed samples, thus leading to exchange interactions between grains of soft and hard magnetic phases. The microstructure of the x ¼8 alloy ribbon annealed at 1003 K was shown in Fig. 6. Nanocrystalline grains of diameters from 10 to 45 nm were measured for this alloy. The nanocrystalline microstructure was also confirmed by electron diffraction studies. The hard magnetic properties of nanocrystalline magnets are attributed to the nucleation of reversed domains [13,14] or to pinning of domain walls [13]. In both models of magnetization reversal processes, a presence of nucleation or pinning centers are taken into account. Identification of such centers in case of studied
310
M. Szwaja et al. / Journal of Magnetism and Magnetic Materials 382 (2015) 307–311
Fig. 6. The TEM image and corresponding to this area electron diffraction for the ribbon sample of the Nd9.2Fe61.64B21.16Nb8 alloy annealed at 1003 K for 5 min.
Table 2 Mean positron lifetime values τ1, τ2 and their intensities. Content x
τ1 [ns]
I1 [%]
(Nd10Fe67B23)100 xNbx in as-cast state 6 0.1787 0.003 94.09 7 0.67 7 0.1747 0.003 92.977 0.67 8 0.1727 0.003 91.88 7 0.67
Fig. 5. Temperature dependences of the coercivity JHc (a), remanence Jr (b) and maximum energy product (BH)max (c) obtained for different content of Nb addition.
samples can be performed using positron lifetime spectra. These spectra obtained for amorphous as well as nanocrystallized samples can be best fitted by two lifetime components: τ1 and τ2 with respective intensities I1 and I2. Mean positron lifetime values τ1, τ2 and their intensities I1 and I2 are collected in Table 2. Their errors are the result of mathematical analysis. The component τ1 is responsible for free annihilation of positrons and the annihilation with electrons of point defects of vacancy type. This value can be attributed to the annihilation within the amorphous phase. The second component τ2 is close to the calculated positron lifetime of microvoids and is thus attributable to the annihilation of positrons in microvoids in the amorphous matrix as well as at the grain boundaries of the crystalline phases formed during annealing.
τ2 [ns]
I2 [%]
0.554 7 0.041 0.5277 0.041 0.4927 0.041
5.91 70.37 7.03 70.37 8.12 70.37
(Nd10Fe67B23)100 xNbx after annealing at 1003 K 6 0.1727 0.003 86.81 7 0.57 0.456 7 0.050 7 0.168 7 0.003 83.85 7 0.57 0.4377 0.050 8 0.1557 0.003 81.137 0.57 0.4787 0.051
13.19 70.37 16.15 70.37 18.90 70.37
Increase of Nb content in the amorphous and nanocrystalline samples reduces positron lifetime values τ1 and τ2. The process of annealing at 1003 K causes a decrease in the lifetimes τ1 and τ2 for all investigated specimens that can be related to relaxation of the microstructure. The process of heat treatment resulted in a significant increase of the intensity I2 at the expense of the intensity of the I1 component. Similar results were obtained for Fe–Cu–Nb– Si–B and Fe–Si–B amorphous and nanocrystallized samples [15,16]. In case of as-cast samples, the short lifetime τ1 (with the peak intensity larger than 90% ) corresponds to the amorphous matrix. Other one of the intensity lower than 10% is associated to a trapping of positrons by microvoids present in the amorphous matrix and at the grain boundaries of nanocrystalline phases. The annealing of ribbons causes growth of concentration of the microvoids at the grain boundaries of the crystalline phases. The increase of coercivity of the annealed ribbons with the Nb contents might be related to the increase of I2 intensity, which can be attributed to areas of lower magnetic anisotropy, that can be considered as centers of nucleation and growth of reversed domains.
4. Conclusion It was shown that the rapidly solidified (Nd10Fe67B23)100 xNbx (where x ¼6, 7, 8) alloy ribbons have not fully amorphous structure and soft magnetic properties. The heat treatment of these ribbons at temperatures higher than 963 K leads to the nucleation and growth of the Nd2Fe14B hard magnetic phase, the Nd2Fe23B3 metastable phase and the Nd1 þ εFe4B4 paramagnetic phase. Furthermore, larger alloy doping with Nb result in gradual increase of coercivity. In the as-cast samples, more than 90%, of the positrons were trapped in vacancy-sized point defects and other positrons were trapped by microvoids. Along with the appearance of
M. Szwaja et al. / Journal of Magnetism and Magnetic Materials 382 (2015) 307–311
nanocrystallites and their growth due to annealing the concentration of microvoids increased in the inter-granular amorphous phase. This might be related to the increase of coercivity with the Nb contents for annealed ribbons, for which the increase of I2 might be caused by rise population of lower magnetic anisotropy areas, that can be considered as centers of nucleation and growth of reversed domains.
References [1] [2] [3] [4] [5]
J.I. Betancourt, Rev. Mex. Fis. 48 (4) (2002) 283–289. F.M. Ahmed, I.R. Harris, J. Magn. Magn. Mater. 320 (2008) 2808–2813. Z.H. Hu, M.G. Zhu, W. Li, F.Z. Lian, J. Magn. Magn. Mater. 320 (2008) 96–99. L.Q. Yu, Y.H. Wen, M. Yan, J. Magn. Magn. Mater. 283 (2004) 353–356. A. Przybył, K. Pawlik, P. Pawlik, P. Gębara, J.J. Wysłocki, Phase composition and magnetic properties of (Pr,Dy)–Fe–Co–(Ni–Mn)–B–Zr–Ti alloys, J. Alloy. Compd. 536S (2012) S333. [6] R. Tamura, S. Kobayashi, T. Fukuzaki, M. Isobe, Y. Ueda, J. Phys.: Conf. Ser. 144 (2009) 012068. [7] M. Szwaja, P. Pawlik, J.J. Wysłocki, P. Gębara, Arch. Metall. Mater. 57 (1) (2012)
311
233–236. [8] J. Zhang, K.Y. Lim, Y.P. Feng, Y. Li, Scr. Mater. 56 (2007) 943–946. [9] W. Shao-jie, T. Zhong-Xun, T. De-Chong, Positron Annihilation, in: P.C. Jain, R. M. Singru, K.P. Gopinathan (Eds.), World Scientific Publishing Co. Pte. Ltd., Singapore, 1985, p. 924. [10] U.A. Arifow, P.U. Arifow, Fizika Miedliennych Pozitronow, FAN, Taszkient, 1971. [11] A. Kocela, J. Filipecki, P. Korzekwa, E. Golis, Investigation of the free volume changes in one day hydrogel and one day silicone-hydrogel contact lenses by means of positron annihilation lifetime spectroscopy, Polym. Med. 42 (1) (2012) 61–68. [12] J. Kansy, Microcomputer program for analysis of positron annihilation lifetime spectra, Nucl. Instr. and Meth. Phys. Res. A. 374 (1996) 235–244. [13] H. Kronmüller, K.D. Durst, M. Sagawa, Analysis of the magnetic hardening mechanism in Re-FeB permanent magnets, J. Magn. Magn. Mater. 74 (1988) 291–302. [14] D. Givord, A. Lienard, P. Tenaud, T. Viadieu, J. Magn. Magn. Mater. 67 (1987) L281. [15] R. Wirschum. H.-E. Schaefer, in: A.S. Edelstein, R.C. Cammarata (Eds.) Nanomaterials: Synthesis, Properties and Application, Institute of Physics. Bristol. (1996) p. 277. [16] K. Sato, H. Murakami, Y. Kobayashi, W. Sprengel, H.-E. Schaefer, Nanocrystallization-induced structural evolution of intergranular amorphous phase in Fe78B13Si9 alloy, J. Non-Cryst. Solids 353 (2007) 1882–1885.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of Nb addition on vacancy defects and magnetic properties of the nanocrystalline Nd–Fe–B permanent magnets Małgorzata Szwaja a, Piotr Gębara a,n, Jacek Filipecki b, Katarzyna Pawlik a, Anna Przybył a, Piotr Pawlik a, Jerzy J. Wysłocki a, Katarzyna Filipecka a a b
Institute of Physics, Czestochowa University of Technology, Czestochowa, Poland Institute of Physics, Jan Dlugosz University, Czestochowa, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 3 July 2014 Received in revised form 20 January 2015 Accepted 29 January 2015 Available online 30 January 2015
In present work, influence of Nb addition on vacancy defects and magnetic properties of nanocrystalline Nd–Fe–B permanent magnets, was investigated. Samples with composition (Nd,Fe,B)100 xNbx (where x ¼ 6,7,8) were studied in as-cast state and after annealing. Samples were prepared by arc-melting with high purity of constituent elements under Ar atmosphere. Ribbons were obtained by melt-spinning technique under low pressure of Ar. Ribbon samples in as-cast state had amorphous structure and soft magnetic properties. Positron annihilation lifetime spectroscopy PALS has been applied to detection of positron – trapping voids (vacancy defects). With increase of Nb in alloy increasing of vacancy defects concentration was observed. Heat treatment of the samples was carried out at various temperatures (from 923 K to 1023 K) for 5 min, in order to obtain nanocrystalline structure. The aim of present work was to determine the influence of Nb addition and annealing conditions on the vacancy defects and magnetic properties of the Nd–Fe–B- type alloys in as-cast state and after heat treatment. & 2015 Published by Elsevier B.V.
1. Introduction As the third-generation rare earth permanent magnets, Nd–Fe– B magnets have been extensively studied. Permanent magnets are indispensable in modern technology and their influence is still growing [1]. To improve the magnetic properties of Nd–Fe–B magnets, extensive efforts have been made via the substitution of other elements [2–5]. Interesting studies of magnetic properties of magnets produced from the Nd–Fe–B base alloy doped with 4 at% Nb are presented in [6–8]. The Nb addition has a significant influence on the glasses forming ability, but also retards the growth of nanocrystalline grains formed during heat treatment [8]. The annealing process is also an important factor in the formation of the microstructure and magnetic properties. Therefore, it is crucial to determine the effect of Nb admixture and heat treatment conditions on the phase constitution and magnetic properties of (Nd10Fe67B23)100 xNbx (where x ¼6, 7, 8) alloys in a form of ribbons. Positron annihilation results in the change the whole mass of both particles and their kinetic energy into energy of photons of electromagnetic radiation. Therefore, the study of photons generated in the process of annihilation, provides information on the state of an electron–positron pair. Annihilation is possible only in n
Corresponding author. E-mail address: [email protected] (P. Gębara).
http://dx.doi.org/10.1016/j.jmmm.2015.01.076 0304-8853/& 2015 Published by Elsevier B.V.
the case when conservation laws are fulfilled, namely conservation of energy, momentum, angular momentum, charge, and parity. High-energy positron annihilation is proceeded by a termalization phenomenon, which consists in the rapid loss of energy due to scattering of positrons and the excitation center [9,10]. The aim of present paper was the studies of the vacancy defects and magnetic properties of the (Nd10Fe67B23)100 xNbx (where x ¼6, 7, 8) alloys.
2. Experimental material and methods Alloy ingots with nominal composition (Nd10Fe67B23)100 xNdx (where x ¼6, 7, 8) were prepared by arc-melting from high purity constituent elements of under a Ti-gettered argon atmosphere. In order to homogenize alloy, the samples were re-melted several times. The ribbon samples were prepared by melt-spinning technique under the Ar protective atmosphere. The linear speed of the copper roll surface of 35 m/s was used in the process. Subsequently the ribbon samples were sealed off in a quartz tube under a low pressure of argon to maintain the purity of atmosphere of heat treatment. In order to obtain a nanocrystalline microstructure, the samples were annealed at temperatures ranging from 923 K to 1023 K for 5 min, and subsequently rapidly cooled in water. The phase analysis of these samples was studied using Bruker D8 Advance diffractometer with CuKα radiation. The room temperature hysteresis loops were measured by LakeShore 7307
308
M. Szwaja et al. / Journal of Magnetism and Magnetic Materials 382 (2015) 307–311
vibrating sample magnetometer at external magnetic fields up to 2 T. Positron lifetime PALS measurements were performed at room temperature using a spectrometer ORTEC, based on a “start–stop” method [11]. Time resolution amounts to FWHM ¼250 ps (full width at half maxima). The sample, along with the source of positrons (Na22 isotope with an activity 4 l05 Bq prepared using 6 μm thick Kapton foil), formed the so-called “sandwich” system. Positron lifetime spectra were analyzed using the LT computer program [12].
3. Results and discussion The XRD scans measured for (Nd10Fe67B23)100 xNbx, where x ¼6, 7, 8 alloy ribbons in as-cast state and after annealing at 923 K and 943 K for 5 min, are shown in Fig. 1. XRD scans for the as-cast ribbon samples revealed their fully amorphous structure, except for the x¼ 6 alloy, where some low intensity reflexes coming from the crystalline phases are present. Annealing at 923 K and 943 K for 5 min resulted in significant changes in the crystal structure of the material. Broadened peaks originating from crystalline phase are observed on the diffraction patterns. It was shown that the hard magnetic Nd2Fe14B, the paramagnetic Nd1 þ εFe4B4 and soft magnetic metastable Nd2Fe23B3 phases are present in the samples. However, it was impossible to clearly identify phases from the XRD scans due to the overlapping of reflexes coming from different crystalline phases. The XRD pattern measured for the (Nd10Fe67B23)100 xNbx (where x¼ 7) alloy ribbon samples annealed at temperatures from 963 K to 1023 K for 5 min, are shown in Fig. 2. The short time annealing for 5 min at 963 K and at higher temperatures leads to nucleation and growth of the crystalline phases. The phases observed for the annealed ribbons were: the hard magnetic Nd2Fe14B, the paramagnetic Nd1 þ εFe4B4 and soft magnetic metastable Nd2Fe23B3 phases. Complementary Mössbauer spectra (MS) analysis was carried out for selected samples. The example of MS studies is presented in Fig. 3 for the x ¼8 alloy ribbon annealed at 1003 K for 5 min. The analysis was carried out taking into account a presence of hard magnetic Nd2Fe14B phase that was represented by six Zeeman lines corresponding to the magnetically nonequivalent positions of the Fe atoms in its unit cell (16k1, 16k2, 8j1, 8j2, 4e and 4c of relative intensities 4:4:2:2:1:1). Furthermore, the paramagnetic Nd1 þ εFe4B4 phase was denoted by a doublet component line and the metastable Nd2Fe23B3 phase was marked by three magnetically nonequivalent positions of the Fe atoms of the highest population (48e1, 48e2, 48e3). For complete fitting an additional line corresponding to the continuous hyperfine field distribution was added (Fig. 3). The continuous line may be attributed to the remaining amorphous or highly disordered phase. The quantitative analysis have shown that the dominant phase formed during annealing was the paramagnetic Nd1 þ εFe4B4 phase (Table 1), which reduces the saturation magnetization of the samples. Presence of large fractions of magnetically disordered and hard magnetic phases are responsible for the ferromagnetic behavior of the alloy. However, the fractions of constituent phases change significantly with the Nb addition, that impacts magnetic properties of annealed ribbons. The hysteresis loops of (Nd10Fe67B23)100 xNbx, where x ¼6, 7, 8 alloy ribbon samples annealed at various temperatures, are shown in Fig. 4. The samples of all alloys in as-cast state and annealed at temperatures lower than 943 K reveal soft magnetic properties. Annealing at higher temperatures resulted in an evolution of microstructure and phase constitution thus leading to significant changes of their magnetic properties (Fig. 5). The
Fig. 1. X-ray diffraction patterns measured for (Nd10Fe67B23)100 xNbx, where x¼ 6 (a), x ¼7 (b), x¼ 8 (c) alloy ribbon samples in as-cast state and annealed at 923 K and 943 K for 5 min.
hysteresis loops of (Nd10Fe67B23)100 xNbx (where x¼6, 7, 8) alloy ribbons annealed at 963 K, 983 K and 1003 K for 5 min, are shown in Fig. 4. For higher annealing temperatures (above 963 K) a hard magnetic properties are induced. The wasp-tail shaped hysteresis loops measured for the samples annealed at lower temperatures indicate presence of low fractions of hard magnetic phase. The dependences of polarization
M. Szwaja et al. / Journal of Magnetism and Magnetic Materials 382 (2015) 307–311
309
Fig. 2. X-ray diffraction patterns measured for (Nd10Fe67B23)100 xNbx where x ¼7 alloy ribbon samples annealed at 963–1023 K for 5 min.
Fig. 3. Mössbauer spectra for the (Nd10Fe67B23)100 xNbx, where x¼ 8 alloy ribbon samples annealed at 1023 K for 5 min with corresponding probability of hyperfine field Bhf (inset). Table 1 Weight content of recognized phases for alloys with different Nb addition. Fig. 4. The hysteresis loops measured for the (Nd10Fe67B23)100 xNbx alloys, where x¼ 6 (a), x ¼ 7 (b), x ¼ 8 (c) in the form of ribbons annealed at 923–1023 K for 5 min.
(Nd10Fe67B23)100 xNbx V [wt%] Nd2Fe14B Nd2Fe23B3 Nd1 þ εFe4B4 Amorphous phase x¼6 x¼7 x¼8
17.2 31.6 18.6
28.8 15.3 1.9
41.2 42.6 48.6
12.8 10.5 30.9
remanence Jr, the maximum magnetic energy density (BH)max and coercivity JHc on the annealing temperature for the investigated alloys are shown in Fig. 5 It was shown, that increase of Nb contents resulted in significant rise of JHC and (BH)max for annealed samples. Furthermore, a change from two-stage to the squareshape demagnetization curve with the Nb content was observed for ribbons annealed at temperatures of optimal magnetic properties for particular composition (Fig. 4). This suggests that larger
content of Nb in the alloy composition results in formation of more uniform microstructure of the annealed samples, thus leading to exchange interactions between grains of soft and hard magnetic phases. The microstructure of the x ¼8 alloy ribbon annealed at 1003 K was shown in Fig. 6. Nanocrystalline grains of diameters from 10 to 45 nm were measured for this alloy. The nanocrystalline microstructure was also confirmed by electron diffraction studies. The hard magnetic properties of nanocrystalline magnets are attributed to the nucleation of reversed domains [13,14] or to pinning of domain walls [13]. In both models of magnetization reversal processes, a presence of nucleation or pinning centers are taken into account. Identification of such centers in case of studied
310
M. Szwaja et al. / Journal of Magnetism and Magnetic Materials 382 (2015) 307–311
Fig. 6. The TEM image and corresponding to this area electron diffraction for the ribbon sample of the Nd9.2Fe61.64B21.16Nb8 alloy annealed at 1003 K for 5 min.
Table 2 Mean positron lifetime values τ1, τ2 and their intensities. Content x
τ1 [ns]
I1 [%]
(Nd10Fe67B23)100 xNbx in as-cast state 6 0.1787 0.003 94.09 7 0.67 7 0.1747 0.003 92.977 0.67 8 0.1727 0.003 91.88 7 0.67
Fig. 5. Temperature dependences of the coercivity JHc (a), remanence Jr (b) and maximum energy product (BH)max (c) obtained for different content of Nb addition.
samples can be performed using positron lifetime spectra. These spectra obtained for amorphous as well as nanocrystallized samples can be best fitted by two lifetime components: τ1 and τ2 with respective intensities I1 and I2. Mean positron lifetime values τ1, τ2 and their intensities I1 and I2 are collected in Table 2. Their errors are the result of mathematical analysis. The component τ1 is responsible for free annihilation of positrons and the annihilation with electrons of point defects of vacancy type. This value can be attributed to the annihilation within the amorphous phase. The second component τ2 is close to the calculated positron lifetime of microvoids and is thus attributable to the annihilation of positrons in microvoids in the amorphous matrix as well as at the grain boundaries of the crystalline phases formed during annealing.
τ2 [ns]
I2 [%]
0.554 7 0.041 0.5277 0.041 0.4927 0.041
5.91 70.37 7.03 70.37 8.12 70.37
(Nd10Fe67B23)100 xNbx after annealing at 1003 K 6 0.1727 0.003 86.81 7 0.57 0.456 7 0.050 7 0.168 7 0.003 83.85 7 0.57 0.4377 0.050 8 0.1557 0.003 81.137 0.57 0.4787 0.051
13.19 70.37 16.15 70.37 18.90 70.37
Increase of Nb content in the amorphous and nanocrystalline samples reduces positron lifetime values τ1 and τ2. The process of annealing at 1003 K causes a decrease in the lifetimes τ1 and τ2 for all investigated specimens that can be related to relaxation of the microstructure. The process of heat treatment resulted in a significant increase of the intensity I2 at the expense of the intensity of the I1 component. Similar results were obtained for Fe–Cu–Nb– Si–B and Fe–Si–B amorphous and nanocrystallized samples [15,16]. In case of as-cast samples, the short lifetime τ1 (with the peak intensity larger than 90% ) corresponds to the amorphous matrix. Other one of the intensity lower than 10% is associated to a trapping of positrons by microvoids present in the amorphous matrix and at the grain boundaries of nanocrystalline phases. The annealing of ribbons causes growth of concentration of the microvoids at the grain boundaries of the crystalline phases. The increase of coercivity of the annealed ribbons with the Nb contents might be related to the increase of I2 intensity, which can be attributed to areas of lower magnetic anisotropy, that can be considered as centers of nucleation and growth of reversed domains.
4. Conclusion It was shown that the rapidly solidified (Nd10Fe67B23)100 xNbx (where x ¼6, 7, 8) alloy ribbons have not fully amorphous structure and soft magnetic properties. The heat treatment of these ribbons at temperatures higher than 963 K leads to the nucleation and growth of the Nd2Fe14B hard magnetic phase, the Nd2Fe23B3 metastable phase and the Nd1 þ εFe4B4 paramagnetic phase. Furthermore, larger alloy doping with Nb result in gradual increase of coercivity. In the as-cast samples, more than 90%, of the positrons were trapped in vacancy-sized point defects and other positrons were trapped by microvoids. Along with the appearance of
M. Szwaja et al. / Journal of Magnetism and Magnetic Materials 382 (2015) 307–311
nanocrystallites and their growth due to annealing the concentration of microvoids increased in the inter-granular amorphous phase. This might be related to the increase of coercivity with the Nb contents for annealed ribbons, for which the increase of I2 might be caused by rise population of lower magnetic anisotropy areas, that can be considered as centers of nucleation and growth of reversed domains.
References [1] [2] [3] [4] [5]
J.I. Betancourt, Rev. Mex. Fis. 48 (4) (2002) 283–289. F.M. Ahmed, I.R. Harris, J. Magn. Magn. Mater. 320 (2008) 2808–2813. Z.H. Hu, M.G. Zhu, W. Li, F.Z. Lian, J. Magn. Magn. Mater. 320 (2008) 96–99. L.Q. Yu, Y.H. Wen, M. Yan, J. Magn. Magn. Mater. 283 (2004) 353–356. A. Przybył, K. Pawlik, P. Pawlik, P. Gębara, J.J. Wysłocki, Phase composition and magnetic properties of (Pr,Dy)–Fe–Co–(Ni–Mn)–B–Zr–Ti alloys, J. Alloy. Compd. 536S (2012) S333. [6] R. Tamura, S. Kobayashi, T. Fukuzaki, M. Isobe, Y. Ueda, J. Phys.: Conf. Ser. 144 (2009) 012068. [7] M. Szwaja, P. Pawlik, J.J. Wysłocki, P. Gębara, Arch. Metall. Mater. 57 (1) (2012)
311
233–236. [8] J. Zhang, K.Y. Lim, Y.P. Feng, Y. Li, Scr. Mater. 56 (2007) 943–946. [9] W. Shao-jie, T. Zhong-Xun, T. De-Chong, Positron Annihilation, in: P.C. Jain, R. M. Singru, K.P. Gopinathan (Eds.), World Scientific Publishing Co. Pte. Ltd., Singapore, 1985, p. 924. [10] U.A. Arifow, P.U. Arifow, Fizika Miedliennych Pozitronow, FAN, Taszkient, 1971. [11] A. Kocela, J. Filipecki, P. Korzekwa, E. Golis, Investigation of the free volume changes in one day hydrogel and one day silicone-hydrogel contact lenses by means of positron annihilation lifetime spectroscopy, Polym. Med. 42 (1) (2012) 61–68. [12] J. Kansy, Microcomputer program for analysis of positron annihilation lifetime spectra, Nucl. Instr. and Meth. Phys. Res. A. 374 (1996) 235–244. [13] H. Kronmüller, K.D. Durst, M. Sagawa, Analysis of the magnetic hardening mechanism in Re-FeB permanent magnets, J. Magn. Magn. Mater. 74 (1988) 291–302. [14] D. Givord, A. Lienard, P. Tenaud, T. Viadieu, J. Magn. Magn. Mater. 67 (1987) L281. [15] R. Wirschum. H.-E. Schaefer, in: A.S. Edelstein, R.C. Cammarata (Eds.) Nanomaterials: Synthesis, Properties and Application, Institute of Physics. Bristol. (1996) p. 277. [16] K. Sato, H. Murakami, Y. Kobayashi, W. Sprengel, H.-E. Schaefer, Nanocrystallization-induced structural evolution of intergranular amorphous phase in Fe78B13Si9 alloy, J. Non-Cryst. Solids 353 (2007) 1882–1885.