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Magnetic domain structures of overquenched Nd–Fe–B permanent magnets studied by electron holography
Journal of Magnetism and Magnetic Materials 238 (2002) 68–74
Magnetic domain structures of overquenched Nd–Fe–B permanent magnets studied by electron holography Y.-G. Park*, D. Shindo Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1-1 Katahira, 2-Chome, Aobaku, Sendai 980-8577, Japan Received 14 February 2001; received in revised form 3 August 2001
Abstract Microstructures and magnetic domain structures of overquenched Nd–Fe–B permanent magnets have been investigated in detail by transmission electron microscopy. While magnetic domain boundaries are clarified by Lorentz microscopy, magnetization distribution in the domains is clearly observed by electron holography. In the as-quenched magnet, the size of the magnetic domains is in the range from 200 to 500 nm and the direction of the magnetic lines of force changes gradually in wide region, while in the annealed one having the crystalline phase of Nd2Fe14B, the direction of the magnetic lines of force changes drastically especially at the grain boundaries. Furthermore, the direction of the magnetic lines of force changes more drastically in the specimen annealed at 893 K than the specimen annealed at 843 K. This result clearly indicates that the magnetocrystalline anisotropy is enhanced with the increase of annealing temperature, resulting in strong domain wall pinning. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.14.Nm; 63.37.Lp; 75.50.Ww; 75.60.d Keywords: Electron holography; Reconstructed phase image; Magnetization distribution; Magnetic flux density; Domain wall pinning
1. Introduction It is well known that the outstanding hard magnetic properties [1–3] appear in ternary Nd– Fe–B alloys depending sensitively on microstructures and magnetic domain structures. In order to improve their magnetic properties, such as coercivity, remanence and energy product, a detailed understanding of correlation between microstructures and magnetic domain structures is necessary. *Corresponding author. Tel.: +81-22-217-5171; fax: +8122-217-5211. E-mail address: [email protected] (Y.-G. Park).
Theoretical consideration and experimental observation have been carried out to understand the magnetic hardening mechanism in Nd–Fe–B magnets. Previously, it was shown that the magnetic hardening is attributed to nucleation [4,5] or domain wall pinning [6–9]. Durst and . Kronmuller [4] proposed that the hardening proceeds from nucleation at the distributed surface layer of Nd2Fe14B grains, surrounded by a thin intergranular nonmagnetic Nd-rich phase, in sintered magnets. Hadjipanayis et al. [6,9] and Mishra [8] suggested that the hardening originates from domain wall pinning at grain boundaries in melt-spun magnets, based on the Lorentz microscope observation.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 6 9 9 - 0
Y.-G. Park, D. Shindo / Journal of Magnetism and Magnetic Materials 238 (2002) 68–74
So far the magnetic structure, mainly magnetic domain boundaries have been observed by Lorentz microscopy. In order to understand the magnetic properties in detail, further detailed experiments on magnetic structures, such as magnetization distribution and movement of domain walls are necessary. Recently, by using a field-emission gun and a biprism, electron holography has been carried out to image the magnetization distribution in various materials [10]. Actually, in sintered Nd–Fe–B permanent magnets, the magnetization distribution was observed by electron holography [11]. In this paper, magnetization distribution in overquenched Nd– Fe–B permanent magnets was studied by electron holography. Especially, from detailed observation of magnetization distribution by electron holography, we focused our attention on the difference in the magnetization distribution due to the heat treatment in relation to the magnetic property.
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electron microscopy studies were made from ribbon fragments by thinning with low-energy (3.5 keV) Ar ion beam at a 151 inclination. Transmission electron microscope (TEM) image at relatively low magnification and high-resolution electron microscope (HREM) image were obtained with a JEM-ARM 1250 TEM at the accelerating voltage 1250 kV. The Lorentz microscope images were also observed with the highvoltage electron microscope. Electron holography was carried out with a JEM-3000F TEM operating at 300 kV which was installed with a thermal fieldemission gun and a biprism. Detailed experimental conditions for electron holography were presented in the previous paper [12]. The second quadrant of the hysteresis loop was measured by vibrating sample magnetometer. Differential scanning calorimetry was used to measure crystallization temperature of overquenched magnets.
3. Results and discussion 2. Experimental procedure Overquenched specimens were produced by the melt-spinning method. Some of the specimens were annealed for 1 min between 843 and 893 K. The nominal composition of the specimen is given as Fe–10 at% Nd–6 at% B containing a small amount of Pr and Co. Thin foils for transmission
Figs. 1(a) and (b) show the variation in coercivity as a function of annealing temperature and the second quadrant demagnetization curves for three specimens indicated by the circles of A; B and C in (a), respectively. It is seen that the coercivity increases with the increase of annealing temperature in Fig. 1(a). As shown in Fig. 1(b), the
Fig. 1. (a) Variation in coercivity as a function of annealing temperature. (b) Second quadrant demagnetization curves for the three specimens indicated by the circles of A; B and C in (a), respectively.
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demagnetization curve for an as-quenched magnet (A) having a low coercivity of 4.75 kOe decreases rapidly with the applied field. The curve for a magnet (B) annealed at 843 K having relatively a low coercivity of 7.8 kOe shows a shoulder as indicated by an arrow. This is considered to consist of two components; one has low coercivity and the other has relatively high coercivity. The curve for a magnet (C) annealed at 893 K having a high coercivity of 9.2 kOe does not show a dip in contrast with the demagnetization curve for the magnet annealed at 843 K. Figs. 2(a) and (b) show a selected area diffraction pattern and a bright-field image in the magnet annealed at 843 K, respectively. From a selected area diffraction pattern and a bright-field image, it is seen that most of the film consists of the
Fig. 2. Selected area diffraction pattern (a) and a bright-field image (b) obtained from crystalline phase in a magnet annealed at 843 K. (c) Selected area diffraction pattern obtained from amorphous phase. (d) Enlarged HREM image obtained from the area indicated by an arrow in (b).
crystalline phase, while some part consists of the amorphous phase as shown in the diffraction pattern of Fig. 2(c). This inhomogeneity is due to the low annealing temperature in comparison with a crystallization temperature of 850 K. All diffraction spots in the diffraction pattern such as those shown in Fig. 2(a) are consistent with the tetragonal Nd2Fe14B structure (space group P42/mnm) with lattice parameters a ¼ 0:881 and c ¼ 1:22 nm. The grain sizes of Nd2Fe14B crystalline phases are not uniform, being from 40 to 150 nm. Fig. 2(d) shows an enlarged HREM image obtained from the area indicated by an arrow in (b). The lattice image clearly indicates the absence of the second phase, such as the Nd-rich amorphous phase [8]. From similar electron diffraction patterns and electron microscope images, it was found that the as-quenched magnet was composed of the amorphous phase, while the magnet annealed at 893 K consisted of only the Nd2Fe14B crystalline phase with a grain size of 100–600 nm. In order to investigate microstructural changes at the grain boundaries with the increase of annealing temperature, high-resolution electron microscopy observation was also carried out in the magnet annealed at 893 K. However, no microstructural differences were observed at the boundaries in the magnets annealed at 843 and 893 K. Fig. 3(a) shows a Lorentz microscope image of the as-quenched magnet in amorphous state. The Lorentz microscope image was observed by the Foucault mode, and the white and black bands correspond to the magnetic domains whose sizes are in the range from 200 to 500 nm. By the Fourier transform in the previous paper [12], the reconstructed phase image of Fig. 3(b) could be obtained from an electron hologram. In the reconstructed phase image, the direction and the density of the white lines just correspond to the direction and the density of magnetic lines of force projected along the electron beam. Since the phase shift is represented by cosine function, there is a constant flux of h=e (=4.1 1015 Wb) between two adjacent white lines in the reconstructed phase image, and no phase amplification in Fig. 3(b) is done. From the reconstructed phase image of Fig. 3(b), it is seen that the direction of magnetic lines of force indicated by arrows slowly changes
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Fig. 3. The Lorentz microscope image (a) observed by the Foucault mode and a reconstructed phase image (b) obtained by the Fourier transform in the as-quenched magnet.
in wide region. It is reasonably considered that the gradual change in the direction of the magnetic lines of force in the as-quenched magnet without strong pinning sites corresponds well to lower coercivity than the annealed one below. Since the magnetic domain structures in the Lorentz microscope images observed at the relatively thick crystal region by high-voltage electron microscope are basically consistent with those in the Lorentz microscope images and the reconstructed phase images observed with the 300 kV electron microscope, we consider that the magnetic domain structures presented reflect the domain structure in bulk specimens. Fig. 4(a) shows a Lorentz microscope image of the magnet annealed at 843 K, while Fig. 4(b) shows an enlarged Lorentz microscope image observed near the specimen edge. In the Lorentz microscope image (a) observed by the Fresnel
Fig. 4. The Lorentz microscope images observed by the Fresnel (a) and the Foucault mode (b) in the magnet annealed at 843 K. (c) Reconstructed phase image.
mode, the bright lines such as those indicated by arrowheads correspond to the magnetic domain wall contrast, which seems to be pinned at the grain boundaries. In Fig. 4(b), obtained by the Foucault mode, there are many dark and bright grains. It is seen that some of the dark and bright grains aligned along the horizontal line as indicated by arrows of D and B; respectively, forming large magnetic domains. Fig. 4(c) shows a
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reconstructed phase image with no phase amplification. While the positions of the domain walls and thus the shape of magnetic domains are specified from the Lorentz microscope images of (a) and (b), the reconstructed phase image (c) obtained by electron holography clearly shows the direction (arrows) and magnitudes of the magnetic lines of force in the specimen. In the reconstructed phase image (c), the direction of the magnetic lines of force gently changes from place to place especially at the grain boundaries indicated by the broken lines. It is considered that due to the poor crystallinity at grain boundaries, the magnetocrystalline anisotropy at the boundaries in the magnet annealed 843 K is weakened. The gradual magnetization distribution is rather consistent with the magnetic domain structure in the Lorentz microscope image of (b). Since the magnet annealed at 843 K contains small amount of the amorphous phase, it contributes to the rapid decrease of the demagnetization curve shown in Fig. 1(b). Fig. 5(a) shows a Lorentz microscope image obtained by the Fresnel mode in the magnet annealed at 893 K, while Fig. 5(b) shows an enlarged Lorentz microscope image near the specimen edge. It is seen that the magnetic domain wall contrast (arrowheads) in the magnet annealed at 893 K is sharper than that in the magnet annealed at 843 K in Fig. 4(a). In the enlarged Lorentz microscope image of Fig. 5(b), magnetic domain walls are found inside some large grains indicated as ‘‘DW’’. Fig. 5(c) shows a reconstructed phase image of the same area as that of (b). The reconstructed phase image reveals that the direction of the magnetic lines of force abruptly changes at the grain boundaries, being different from the reconstructed phase image of Fig. 4(c). Here, we investigate the change of the direction of the magnetic lines of force around the grain boundaries in the reconstructed phase images of Figs. 4(c) and 5(c). Around the grain boundaries as indicated by the square regions in Figs. 4(c) and 5(c), the angle (y) between two straight lines which are extrapolated along the magnetic lines of force in the adjacent grains were measured as shown in the inset of Figs. 6(a) and (b). In the diagrams of Figs. 6(a) and (b), the direction and magnitude of
Fig. 5. (a, b) The Lorentz microscope images observed by the Fresnel mode in the magnet annealed at 893 K. (c) Reconstructed phase image with no phase amplification. Grain boundaries and magnetic domain boundaries seen in (b) are indicated by dotted lines.
the arrow indicate the deflection angle (y) and the number of the deflection angles observed in the magnetic lines of force at the grain boundaries in Figs. 4(c) and 5(c). While big arrows in the diagram of Fig. 6(a) are confined at the small angle less than 20 1, many big arrows exist in the range from 301 to 701 in the diagram of Fig. 6(b). From this statistical investigation, it is considered that the magnetocrystalline anisotropy is enhanced
Y.-G. Park, D. Shindo / Journal of Magnetism and Magnetic Materials 238 (2002) 68–74
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Fig. 6. The deflection angles (y) and their numbers measured at the grain boundaries in the magnet annealed at 843 K (a) and 893 K (b), respectively. The insets in (a) and (b) correspond to the square regions in Figs. 4(c) and 5(c), respectively. The dotted lines in the insets indicate grain boundaries.
due to the improvement of crystallinity at the boundaries with the increase of annealing temperature. Although the microstructural change with the increase of annealing temperature in the two magnets is not identified by using highresolution electron microscopy, the enhancement of the magnetocrystalline anisotropy in each grain is clearly identified by electron holography study. The enhancement of the magnetocrystalline anisotropy with the increase of annealing temperature in each grain is consistent with high coercivity of the magnet annealed at 893 K in Fig. 1(b). Since the density of white lines in the reconstructed phase image directly corresponds to the magnetic flux density, the magnetic flux density can be evaluated if thickness of the crystal is known. By using electron energy-loss spectroscopy [13] assuming the mean free path for inelastic electron scattering is the same as that for a-Fe [11], the magnetic flux density of the Nd2Fe14B crystal was evaluated to be about 1.5 T, being consistent with the saturation magnetization of the Nd2Fe14B phase, i.e., 1.6 T.
4. Conclusions The results obtained by electron holography and Lorentz microscopy on overquenched Nd–Fe–B magnets are summarized as follows. In the asquenched magnet, the size of the magnetic domains is in the range from 200 to 500 nm and the direction of the magnetic lines of force is found to change gradually in wide region. In the magnet annealed at 843 K containing a small amount of
the amorphous phase, the direction of the magnetic lines of force changes gently at the grain boundaries. In the magnet annealed at 893 K, the direction of the magnetic lines of force whose density is evaluated to be 1.5 T, changes drastically at the grain boundaries, indicating the good crystallinity at the boundaries resulting in the enhancement of the magnetocrystalline anisotropy. It is considered that the increase of the magnetocrystalline anisotropy with the increase of annealing temperature results in the increase of high coercivity of overquenched Nd2Fe14B magnets.
Acknowledgements The authors gratefully acknowledge Dr. H. Hiroyoshi, for providing them with the overquenched Nd2Fe14B samples. This work was partly supported by Special Coordination Funds for Promoting Science and Technology on ‘‘Nanohetero Metallic Materials’’ from the Science and Technology Agency.
References [1] J.J. Croat, J.F. Herbst, R.W. Lee, F.E. Pinkerton, Appl. Phys. Lett. 44 (1984) 148. [2] M. Sagawa, S. Hirosawa, H. Yamamoto, Jpn. J. Appl. Phys. 26 (1987) 785. [3] A.S. Kim, F.E. Camp, J. Appl. Phys. 79 (1996) 5035. . [4] K.D. Durst, H. Kronmuller, J. Magn. Magn. Mater. 68 (1987) 63. [5] M. Sagawa, S. Hirosawa, MRS Meeting, San Diego, CA, 1987, p. 650.
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[6] G.C. Hadjipanayis, K.R. Lawless, J. Appl. Phys. 57 (1985) 4097. [7] F.E. Pinkerton, D.J. Van Wingerden, J. Appl. Phys. 60 (1986) 3685. [8] R.J. Mishra, J. Magn. Magn. Mater. 54–57 (1986) 450. [9] G.C. Hadjipanayis, W. Gong, J. Magn. Magn. Mater. 66 (1987) 390.
[10] A. Tonomura, Electron Holography, 2nd Edition, Springer, New York, 1999. [11] Y. Zhu, M.R. McCartney, J. Appl. Phys. 84 (1998) 3267. [12] Y.-G. Park, D. Shindo, M. Okada, Mater. Trans. JIM 41 (2000) 1132. [13] R.F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope, 2nd Edition, Plenum Press, New York, 1996.
Magnetic domain structures of overquenched Nd–Fe–B permanent magnets studied by electron holography Y.-G. Park*, D. Shindo Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1-1 Katahira, 2-Chome, Aobaku, Sendai 980-8577, Japan Received 14 February 2001; received in revised form 3 August 2001
Abstract Microstructures and magnetic domain structures of overquenched Nd–Fe–B permanent magnets have been investigated in detail by transmission electron microscopy. While magnetic domain boundaries are clarified by Lorentz microscopy, magnetization distribution in the domains is clearly observed by electron holography. In the as-quenched magnet, the size of the magnetic domains is in the range from 200 to 500 nm and the direction of the magnetic lines of force changes gradually in wide region, while in the annealed one having the crystalline phase of Nd2Fe14B, the direction of the magnetic lines of force changes drastically especially at the grain boundaries. Furthermore, the direction of the magnetic lines of force changes more drastically in the specimen annealed at 893 K than the specimen annealed at 843 K. This result clearly indicates that the magnetocrystalline anisotropy is enhanced with the increase of annealing temperature, resulting in strong domain wall pinning. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.14.Nm; 63.37.Lp; 75.50.Ww; 75.60.d Keywords: Electron holography; Reconstructed phase image; Magnetization distribution; Magnetic flux density; Domain wall pinning
1. Introduction It is well known that the outstanding hard magnetic properties [1–3] appear in ternary Nd– Fe–B alloys depending sensitively on microstructures and magnetic domain structures. In order to improve their magnetic properties, such as coercivity, remanence and energy product, a detailed understanding of correlation between microstructures and magnetic domain structures is necessary. *Corresponding author. Tel.: +81-22-217-5171; fax: +8122-217-5211. E-mail address: [email protected] (Y.-G. Park).
Theoretical consideration and experimental observation have been carried out to understand the magnetic hardening mechanism in Nd–Fe–B magnets. Previously, it was shown that the magnetic hardening is attributed to nucleation [4,5] or domain wall pinning [6–9]. Durst and . Kronmuller [4] proposed that the hardening proceeds from nucleation at the distributed surface layer of Nd2Fe14B grains, surrounded by a thin intergranular nonmagnetic Nd-rich phase, in sintered magnets. Hadjipanayis et al. [6,9] and Mishra [8] suggested that the hardening originates from domain wall pinning at grain boundaries in melt-spun magnets, based on the Lorentz microscope observation.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 6 9 9 - 0
Y.-G. Park, D. Shindo / Journal of Magnetism and Magnetic Materials 238 (2002) 68–74
So far the magnetic structure, mainly magnetic domain boundaries have been observed by Lorentz microscopy. In order to understand the magnetic properties in detail, further detailed experiments on magnetic structures, such as magnetization distribution and movement of domain walls are necessary. Recently, by using a field-emission gun and a biprism, electron holography has been carried out to image the magnetization distribution in various materials [10]. Actually, in sintered Nd–Fe–B permanent magnets, the magnetization distribution was observed by electron holography [11]. In this paper, magnetization distribution in overquenched Nd– Fe–B permanent magnets was studied by electron holography. Especially, from detailed observation of magnetization distribution by electron holography, we focused our attention on the difference in the magnetization distribution due to the heat treatment in relation to the magnetic property.
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electron microscopy studies were made from ribbon fragments by thinning with low-energy (3.5 keV) Ar ion beam at a 151 inclination. Transmission electron microscope (TEM) image at relatively low magnification and high-resolution electron microscope (HREM) image were obtained with a JEM-ARM 1250 TEM at the accelerating voltage 1250 kV. The Lorentz microscope images were also observed with the highvoltage electron microscope. Electron holography was carried out with a JEM-3000F TEM operating at 300 kV which was installed with a thermal fieldemission gun and a biprism. Detailed experimental conditions for electron holography were presented in the previous paper [12]. The second quadrant of the hysteresis loop was measured by vibrating sample magnetometer. Differential scanning calorimetry was used to measure crystallization temperature of overquenched magnets.
3. Results and discussion 2. Experimental procedure Overquenched specimens were produced by the melt-spinning method. Some of the specimens were annealed for 1 min between 843 and 893 K. The nominal composition of the specimen is given as Fe–10 at% Nd–6 at% B containing a small amount of Pr and Co. Thin foils for transmission
Figs. 1(a) and (b) show the variation in coercivity as a function of annealing temperature and the second quadrant demagnetization curves for three specimens indicated by the circles of A; B and C in (a), respectively. It is seen that the coercivity increases with the increase of annealing temperature in Fig. 1(a). As shown in Fig. 1(b), the
Fig. 1. (a) Variation in coercivity as a function of annealing temperature. (b) Second quadrant demagnetization curves for the three specimens indicated by the circles of A; B and C in (a), respectively.
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demagnetization curve for an as-quenched magnet (A) having a low coercivity of 4.75 kOe decreases rapidly with the applied field. The curve for a magnet (B) annealed at 843 K having relatively a low coercivity of 7.8 kOe shows a shoulder as indicated by an arrow. This is considered to consist of two components; one has low coercivity and the other has relatively high coercivity. The curve for a magnet (C) annealed at 893 K having a high coercivity of 9.2 kOe does not show a dip in contrast with the demagnetization curve for the magnet annealed at 843 K. Figs. 2(a) and (b) show a selected area diffraction pattern and a bright-field image in the magnet annealed at 843 K, respectively. From a selected area diffraction pattern and a bright-field image, it is seen that most of the film consists of the
Fig. 2. Selected area diffraction pattern (a) and a bright-field image (b) obtained from crystalline phase in a magnet annealed at 843 K. (c) Selected area diffraction pattern obtained from amorphous phase. (d) Enlarged HREM image obtained from the area indicated by an arrow in (b).
crystalline phase, while some part consists of the amorphous phase as shown in the diffraction pattern of Fig. 2(c). This inhomogeneity is due to the low annealing temperature in comparison with a crystallization temperature of 850 K. All diffraction spots in the diffraction pattern such as those shown in Fig. 2(a) are consistent with the tetragonal Nd2Fe14B structure (space group P42/mnm) with lattice parameters a ¼ 0:881 and c ¼ 1:22 nm. The grain sizes of Nd2Fe14B crystalline phases are not uniform, being from 40 to 150 nm. Fig. 2(d) shows an enlarged HREM image obtained from the area indicated by an arrow in (b). The lattice image clearly indicates the absence of the second phase, such as the Nd-rich amorphous phase [8]. From similar electron diffraction patterns and electron microscope images, it was found that the as-quenched magnet was composed of the amorphous phase, while the magnet annealed at 893 K consisted of only the Nd2Fe14B crystalline phase with a grain size of 100–600 nm. In order to investigate microstructural changes at the grain boundaries with the increase of annealing temperature, high-resolution electron microscopy observation was also carried out in the magnet annealed at 893 K. However, no microstructural differences were observed at the boundaries in the magnets annealed at 843 and 893 K. Fig. 3(a) shows a Lorentz microscope image of the as-quenched magnet in amorphous state. The Lorentz microscope image was observed by the Foucault mode, and the white and black bands correspond to the magnetic domains whose sizes are in the range from 200 to 500 nm. By the Fourier transform in the previous paper [12], the reconstructed phase image of Fig. 3(b) could be obtained from an electron hologram. In the reconstructed phase image, the direction and the density of the white lines just correspond to the direction and the density of magnetic lines of force projected along the electron beam. Since the phase shift is represented by cosine function, there is a constant flux of h=e (=4.1 1015 Wb) between two adjacent white lines in the reconstructed phase image, and no phase amplification in Fig. 3(b) is done. From the reconstructed phase image of Fig. 3(b), it is seen that the direction of magnetic lines of force indicated by arrows slowly changes
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Fig. 3. The Lorentz microscope image (a) observed by the Foucault mode and a reconstructed phase image (b) obtained by the Fourier transform in the as-quenched magnet.
in wide region. It is reasonably considered that the gradual change in the direction of the magnetic lines of force in the as-quenched magnet without strong pinning sites corresponds well to lower coercivity than the annealed one below. Since the magnetic domain structures in the Lorentz microscope images observed at the relatively thick crystal region by high-voltage electron microscope are basically consistent with those in the Lorentz microscope images and the reconstructed phase images observed with the 300 kV electron microscope, we consider that the magnetic domain structures presented reflect the domain structure in bulk specimens. Fig. 4(a) shows a Lorentz microscope image of the magnet annealed at 843 K, while Fig. 4(b) shows an enlarged Lorentz microscope image observed near the specimen edge. In the Lorentz microscope image (a) observed by the Fresnel
Fig. 4. The Lorentz microscope images observed by the Fresnel (a) and the Foucault mode (b) in the magnet annealed at 843 K. (c) Reconstructed phase image.
mode, the bright lines such as those indicated by arrowheads correspond to the magnetic domain wall contrast, which seems to be pinned at the grain boundaries. In Fig. 4(b), obtained by the Foucault mode, there are many dark and bright grains. It is seen that some of the dark and bright grains aligned along the horizontal line as indicated by arrows of D and B; respectively, forming large magnetic domains. Fig. 4(c) shows a
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reconstructed phase image with no phase amplification. While the positions of the domain walls and thus the shape of magnetic domains are specified from the Lorentz microscope images of (a) and (b), the reconstructed phase image (c) obtained by electron holography clearly shows the direction (arrows) and magnitudes of the magnetic lines of force in the specimen. In the reconstructed phase image (c), the direction of the magnetic lines of force gently changes from place to place especially at the grain boundaries indicated by the broken lines. It is considered that due to the poor crystallinity at grain boundaries, the magnetocrystalline anisotropy at the boundaries in the magnet annealed 843 K is weakened. The gradual magnetization distribution is rather consistent with the magnetic domain structure in the Lorentz microscope image of (b). Since the magnet annealed at 843 K contains small amount of the amorphous phase, it contributes to the rapid decrease of the demagnetization curve shown in Fig. 1(b). Fig. 5(a) shows a Lorentz microscope image obtained by the Fresnel mode in the magnet annealed at 893 K, while Fig. 5(b) shows an enlarged Lorentz microscope image near the specimen edge. It is seen that the magnetic domain wall contrast (arrowheads) in the magnet annealed at 893 K is sharper than that in the magnet annealed at 843 K in Fig. 4(a). In the enlarged Lorentz microscope image of Fig. 5(b), magnetic domain walls are found inside some large grains indicated as ‘‘DW’’. Fig. 5(c) shows a reconstructed phase image of the same area as that of (b). The reconstructed phase image reveals that the direction of the magnetic lines of force abruptly changes at the grain boundaries, being different from the reconstructed phase image of Fig. 4(c). Here, we investigate the change of the direction of the magnetic lines of force around the grain boundaries in the reconstructed phase images of Figs. 4(c) and 5(c). Around the grain boundaries as indicated by the square regions in Figs. 4(c) and 5(c), the angle (y) between two straight lines which are extrapolated along the magnetic lines of force in the adjacent grains were measured as shown in the inset of Figs. 6(a) and (b). In the diagrams of Figs. 6(a) and (b), the direction and magnitude of
Fig. 5. (a, b) The Lorentz microscope images observed by the Fresnel mode in the magnet annealed at 893 K. (c) Reconstructed phase image with no phase amplification. Grain boundaries and magnetic domain boundaries seen in (b) are indicated by dotted lines.
the arrow indicate the deflection angle (y) and the number of the deflection angles observed in the magnetic lines of force at the grain boundaries in Figs. 4(c) and 5(c). While big arrows in the diagram of Fig. 6(a) are confined at the small angle less than 20 1, many big arrows exist in the range from 301 to 701 in the diagram of Fig. 6(b). From this statistical investigation, it is considered that the magnetocrystalline anisotropy is enhanced
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Fig. 6. The deflection angles (y) and their numbers measured at the grain boundaries in the magnet annealed at 843 K (a) and 893 K (b), respectively. The insets in (a) and (b) correspond to the square regions in Figs. 4(c) and 5(c), respectively. The dotted lines in the insets indicate grain boundaries.
due to the improvement of crystallinity at the boundaries with the increase of annealing temperature. Although the microstructural change with the increase of annealing temperature in the two magnets is not identified by using highresolution electron microscopy, the enhancement of the magnetocrystalline anisotropy in each grain is clearly identified by electron holography study. The enhancement of the magnetocrystalline anisotropy with the increase of annealing temperature in each grain is consistent with high coercivity of the magnet annealed at 893 K in Fig. 1(b). Since the density of white lines in the reconstructed phase image directly corresponds to the magnetic flux density, the magnetic flux density can be evaluated if thickness of the crystal is known. By using electron energy-loss spectroscopy [13] assuming the mean free path for inelastic electron scattering is the same as that for a-Fe [11], the magnetic flux density of the Nd2Fe14B crystal was evaluated to be about 1.5 T, being consistent with the saturation magnetization of the Nd2Fe14B phase, i.e., 1.6 T.
4. Conclusions The results obtained by electron holography and Lorentz microscopy on overquenched Nd–Fe–B magnets are summarized as follows. In the asquenched magnet, the size of the magnetic domains is in the range from 200 to 500 nm and the direction of the magnetic lines of force is found to change gradually in wide region. In the magnet annealed at 843 K containing a small amount of
the amorphous phase, the direction of the magnetic lines of force changes gently at the grain boundaries. In the magnet annealed at 893 K, the direction of the magnetic lines of force whose density is evaluated to be 1.5 T, changes drastically at the grain boundaries, indicating the good crystallinity at the boundaries resulting in the enhancement of the magnetocrystalline anisotropy. It is considered that the increase of the magnetocrystalline anisotropy with the increase of annealing temperature results in the increase of high coercivity of overquenched Nd2Fe14B magnets.
Acknowledgements The authors gratefully acknowledge Dr. H. Hiroyoshi, for providing them with the overquenched Nd2Fe14B samples. This work was partly supported by Special Coordination Funds for Promoting Science and Technology on ‘‘Nanohetero Metallic Materials’’ from the Science and Technology Agency.
References [1] J.J. Croat, J.F. Herbst, R.W. Lee, F.E. Pinkerton, Appl. Phys. Lett. 44 (1984) 148. [2] M. Sagawa, S. Hirosawa, H. Yamamoto, Jpn. J. Appl. Phys. 26 (1987) 785. [3] A.S. Kim, F.E. Camp, J. Appl. Phys. 79 (1996) 5035. . [4] K.D. Durst, H. Kronmuller, J. Magn. Magn. Mater. 68 (1987) 63. [5] M. Sagawa, S. Hirosawa, MRS Meeting, San Diego, CA, 1987, p. 650.
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