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Magnetic properties of the single magnetic domain particles of Sm2Fe17Nx compounds
Journal of Alloys and Compounds, 193 (1993) 235-238 JALCOM 2176
235
Magnetic properties of the single magnetic domain particles of Sm2Fe17Nx compounds K. K o b a y a s h i * , T. I r i y a m a * a n d T. Y a m a g u c h i * * Central Laboratory*, Analytical Research Center**, Asahi Chemical Industry Co., Ltd., 2-1 Samefima, Fuji 416 (Japan)
H . K a t o a n d Y. N a k a g a w a Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980 (Japan)
Abstract The magnetic material Sm2Fe17Nx (0
1. Introduction
2. Experimental details
The magnetic material Sm2Fe~7Nx (SFN) has recently gained widespread interest as a potential material for permanent magnets [1]. When pulverized to about 5 /xm or less, it exhibits characteristics of single magnetic domain particles (SMDPs) [2], of which the upper limit of SMDP size can be determined by the colloid scanning electron microscopy (SEM) method [3]. We have prepared Sm2FelTNx (0
Sm2Fe17 alloy was prepared by high frequency melting of Fe and Sm metals (purity, 99.9%) in an Ar gas atmosphere at 1823 K, followed by pouring of the melt into an Fe mold and cooling to room temperature. The ingots thus obtained were annealed at 1523 K for 3 h in the Ar gas atmosphere. The annealed ingots were coarsely crushed to 20-100 tzm and the resulting powder was heated at 738 K for 2 h in the NH3 (0.35 atm)-H2(0.65 atm) mixed gas atmosphere and annealed in the Ar gas atmosphere at 738 K for 0.5 h. Fine pulverization of the annealed powder was conducted by ball-milling in cyclohexane and by jet-milling with dry N2 gas. About 5 vol.% of the resulting fine powder was identified by electron probe microanalysis (EPMA) as being the SmlFe3 phase with an Ni content of about 30 at.%. The finding for this Ni content was also supported by a separate introduction of Ni into an SmlFe3 alloy powder in the NH3-H2 mixed gas atmosphere at 738 K, which resulted in a similar Ni content. Therefore it was calculated that the Ni content in the Sm2Fe17Nx phase with the Th2Znl7 structure was about 94% of the value obtained for the SFN powder examined in this study. The upper limit of the SMDP size was determined by colloid SEM observation [3] for a domain wall in SFN particles 2-10 /xm in diameter. Submicrometersized Fe304 powder and SFN particles were dispersed
0925-8388/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
236
K. Kobayashi et al. / Magnetic properties of SMDPs of Sm2FelzNx
in oleic acid and the precipitated particles were lightly washed in acetone and dried. The distribution of Fe304 powder on the SFN particles was then observed by SEM to determine the existence of a domain wall. The microstructure of the SFN particles was observed by transmission electron microscopy (TEM). The SFN powder was dispersed in commercial epoxy resin with a hardening agent. After hardening at room temperature, the specimen was sliced into thin films, thinned by Ar ion exposure, and observed using TEM with an acceleration voltage of 400 kV. The magnetic properties were measured using a vibrating sample magnetometer. The O content was determined by gas analysis. The specific surface area of the powder was determined by the Blain method. The grain size was measured by laser diffraction. The distributions of Sm, Fe, N and O in the surface region of the particle were determined by Auger electron spectroscopy (AES).
lo4
\
a: Specific Surface
% b ~ ~ b :Laser Diffraction
'~ \
\
".,
MnBi \ •
.i
.
10o
.
.
.
.
.
.
.
i
.
(b)
Fig. 1. SEM images of the distribution of submierometer-sized magnetite particles on the SFN particles of about (a) 3 /.Lmsize and (b) 7 /~m size.
.
.
.
101
.
.
.
.
102
Grain Size/)Jm Fig. 2. Relationship between grain size and coercivity of SFN powder. The data on BaO-6Fe203 and MnSi are from ref. 7.
o/
0.8
The colloid SEM photographs of the SFN particles are shown in Fig. 1. The particle about 3/~m in diameter (Fig. l(a)) clearly shows the SMDP characteristic of a magnetic powder collection on opposite sides. The 7 /~m particle (Fig.1 (b)), in contrast, exhibits the collection of the magnetite powder on its surface, indicating the presence of a domain wall. The upper limit of the SMDP size, as determined from these and other SEM photographs, is 3-4 /zm. Figure 2 shows the relationship between the coercivity of the SFN particles and their grain size, the grain size being either calculated from the specific surface area measured by the Blain method or directly determined by laser diffraction. This relationship is similar to that shown by the typical SMDP materials of BaO. 6Fe203 and MnBi [7].
\
BaO.6Fe203
3. Results
(a)
a~ ~
v c
0.7
eO
0.6
0.5 0'.3
0'.4
Specific Surface (m2/g) Fig. 3. Relationship between specificsurface area and O content of SFN powders. The O content of the jet-milled SFN powders was found to be directly proportional to their specific surface area, as shown in Fig. 3. Figure 4 shows the TEM photographs of an SFN particle about 2/~m in diameter The point of focus is near the center plane of the particle in Fig. 4(a) and at the upper portion of the particle in Fig. 4(b). Both photographs show an amorphous-like layer at the particle surface, apparently to a depth of about 10 nm in Fig. 4(a) and about 20 nm in Fig. 4(b). The actual depth is thought to be 10 nm in both cases, as can be seen in Fig. 5. Figure 6 shows the distributions of the Sm, Fe, N and O elements near the surface of the particle, as measured by AES. Oxygen segregation is clearly observable and the O is largely confined to a surface
IC Kobayashi et al. / Magnetic properties of SMDPs of
S m 2 F e l T Nx
237
4. Discussion 0111
(a)
(bl
Fig. 4. T E M images of SFN particle, showing nearly amorphous surface layer. The focus point is (a) near the center plane or (b) at the upper plane of the particle.
lOnm~
'o
j2 /
L 20rim
Fig. 5. Schematic representation of the microstructure of SMDP of SFN.
//~_s
L.
The finely powdered SmzFe17Nx clearly exhibits the characteristics of an SMDP material. The results of the present study indicate the upper limit of the particle size for these characteristics is about 3-4 /~m. This value is consistent with our previously reported findings that compacted powder magnets composed of this material exhibit the highest maximum energy product when the size of their component particles is in the range 1-5 tzm [2], [5]. The results of the TEM observations and the AES measurements show the SFN particle to contain a surface region with a much higher O content than that in the inner portion. The thickness of this surface region is about 10 nm. If it may be presumed that the surface region thickness of the 50 tzm particle (prior to fine pulverization) is the same as that of the 2.5/zm particle (following fine pulverization), it then follows that the relative volume of the O-rich surface region is about 20 times larger in the smaller particle. From the analysis of the O distribution, moreover, it can be calculated that about 95% of the total O in the particle exists in the surface region. A schematic view of the SMDP is shown in Fig. 5. Most metals possess a thin oxidized surface layer, and Mott and Cabrerra [8] have proposed a theory to explain its formation. The oxidized layer observed in the present study would seem to be amenable to a similar theoretical treatment. It must be noted, however, that in SFN this thin layer is clearly related to the magnetic properties of SMDP, and particularly to its coercivity. We have found that increasing the total O content to about 30% beyond the content of the SMDP observed in the present study results in a 3%-4% reduction in the saturation magnetization and a 50% reduction in the coercivity. The details will be reported elsewhere.
•
5. Conclusions
The Sm2Fe17Nx magnetic material prepared in the N H 3 - H 2 mixed gas atmosphere exhibits best the mag0 Depth
"40 from
surface
'
'
'BO
of particle/nm
Fig. 6. Concentration distribution of Sm, Fe, N and O in the surface portion of the SFN particle measured by AES.
region up to a depth of about 10 nm, which corresponds to the amorphous-like layer observed by TEM.
netic properties when x is about 3.0. Powders obtained by the fine pulverization of this material clearly exhibit the characteristics of single magnetic domain particles, in particle sizes of up to 3-4/zm. Oxygen in the particle is segregated to the amorphous-like surface layer 10 nm thick, as shown by TEM and AES. The calculation based on the observed O distribution curve indicates that the surface layer contains about 95% of the total amount of O in the particle.
238
K. Kobayashi et al. / Magnetic properties of SMDPs of Sm2Pet7Nx
Acknowledgments We are very grateful to Mr. N. Imaoka, Mr. A. Sudo, Ms. N. Kashiwaya, Mr. T. Fukuda and Mr. M. Stever for valuable discussion and experiments.
References 1 J. M. D. Coey and Hong Sun, J. Magn. Magn. Mater., 87 (1990) L251. 2 K. Kobayashi, T. Iriyama, N. Imaoka, A. Sudo and N. Kashiwaya, Euro. Patent, 0-417-733-A2, 1990.
3 K. Goto, M. Ito and T. Sakurai, Jpn. J. AppL Phys., 19 (1980) 1339. 4 K. H. J. Buschow, R. Coehoorn, D. B. de Mooij, K. de Waard and T. H. Jacobs, Z Magn. Magn. Mater., 92 (1990) L35. 5 T. Iriyama, K. Kobayashi, N. Imaoka, T. Fukuda, H. Kato and Y. Nakagawa, IEEE Trans. Magn., 28 (5) (1992) 2326, paper presented at Intermag Conf., 1992. 6 K. Kobayashi, T. Iriyama, N. Imaoka and T. Fukuda, Rare Earths, 19 (1991) 31--43 (in Japanese). 7 Y. Iwama, Kohshitugiseizabyoh (Hard Magnetic Materials) Maruzen, Tokyo, 1976, p. 15 (in Japanese). 8 N. Cabrerra and N. F. Mott, Rep. Prog. Phys., 12 (1948--49) 163.
235
Magnetic properties of the single magnetic domain particles of Sm2Fe17Nx compounds K. K o b a y a s h i * , T. I r i y a m a * a n d T. Y a m a g u c h i * * Central Laboratory*, Analytical Research Center**, Asahi Chemical Industry Co., Ltd., 2-1 Samefima, Fuji 416 (Japan)
H . K a t o a n d Y. N a k a g a w a Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980 (Japan)
Abstract The magnetic material Sm2Fe17Nx (0
1. Introduction
2. Experimental details
The magnetic material Sm2Fe~7Nx (SFN) has recently gained widespread interest as a potential material for permanent magnets [1]. When pulverized to about 5 /xm or less, it exhibits characteristics of single magnetic domain particles (SMDPs) [2], of which the upper limit of SMDP size can be determined by the colloid scanning electron microscopy (SEM) method [3]. We have prepared Sm2FelTNx (0
Sm2Fe17 alloy was prepared by high frequency melting of Fe and Sm metals (purity, 99.9%) in an Ar gas atmosphere at 1823 K, followed by pouring of the melt into an Fe mold and cooling to room temperature. The ingots thus obtained were annealed at 1523 K for 3 h in the Ar gas atmosphere. The annealed ingots were coarsely crushed to 20-100 tzm and the resulting powder was heated at 738 K for 2 h in the NH3 (0.35 atm)-H2(0.65 atm) mixed gas atmosphere and annealed in the Ar gas atmosphere at 738 K for 0.5 h. Fine pulverization of the annealed powder was conducted by ball-milling in cyclohexane and by jet-milling with dry N2 gas. About 5 vol.% of the resulting fine powder was identified by electron probe microanalysis (EPMA) as being the SmlFe3 phase with an Ni content of about 30 at.%. The finding for this Ni content was also supported by a separate introduction of Ni into an SmlFe3 alloy powder in the NH3-H2 mixed gas atmosphere at 738 K, which resulted in a similar Ni content. Therefore it was calculated that the Ni content in the Sm2Fe17Nx phase with the Th2Znl7 structure was about 94% of the value obtained for the SFN powder examined in this study. The upper limit of the SMDP size was determined by colloid SEM observation [3] for a domain wall in SFN particles 2-10 /xm in diameter. Submicrometersized Fe304 powder and SFN particles were dispersed
0925-8388/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
236
K. Kobayashi et al. / Magnetic properties of SMDPs of Sm2FelzNx
in oleic acid and the precipitated particles were lightly washed in acetone and dried. The distribution of Fe304 powder on the SFN particles was then observed by SEM to determine the existence of a domain wall. The microstructure of the SFN particles was observed by transmission electron microscopy (TEM). The SFN powder was dispersed in commercial epoxy resin with a hardening agent. After hardening at room temperature, the specimen was sliced into thin films, thinned by Ar ion exposure, and observed using TEM with an acceleration voltage of 400 kV. The magnetic properties were measured using a vibrating sample magnetometer. The O content was determined by gas analysis. The specific surface area of the powder was determined by the Blain method. The grain size was measured by laser diffraction. The distributions of Sm, Fe, N and O in the surface region of the particle were determined by Auger electron spectroscopy (AES).
lo4
\
a: Specific Surface
% b ~ ~ b :Laser Diffraction
'~ \
\
".,
MnBi \ •
.i
.
10o
.
.
.
.
.
.
.
i
.
(b)
Fig. 1. SEM images of the distribution of submierometer-sized magnetite particles on the SFN particles of about (a) 3 /.Lmsize and (b) 7 /~m size.
.
.
.
101
.
.
.
.
102
Grain Size/)Jm Fig. 2. Relationship between grain size and coercivity of SFN powder. The data on BaO-6Fe203 and MnSi are from ref. 7.
o/
0.8
The colloid SEM photographs of the SFN particles are shown in Fig. 1. The particle about 3/~m in diameter (Fig. l(a)) clearly shows the SMDP characteristic of a magnetic powder collection on opposite sides. The 7 /~m particle (Fig.1 (b)), in contrast, exhibits the collection of the magnetite powder on its surface, indicating the presence of a domain wall. The upper limit of the SMDP size, as determined from these and other SEM photographs, is 3-4 /zm. Figure 2 shows the relationship between the coercivity of the SFN particles and their grain size, the grain size being either calculated from the specific surface area measured by the Blain method or directly determined by laser diffraction. This relationship is similar to that shown by the typical SMDP materials of BaO. 6Fe203 and MnBi [7].
\
BaO.6Fe203
3. Results
(a)
a~ ~
v c
0.7
eO
0.6
0.5 0'.3
0'.4
Specific Surface (m2/g) Fig. 3. Relationship between specificsurface area and O content of SFN powders. The O content of the jet-milled SFN powders was found to be directly proportional to their specific surface area, as shown in Fig. 3. Figure 4 shows the TEM photographs of an SFN particle about 2/~m in diameter The point of focus is near the center plane of the particle in Fig. 4(a) and at the upper portion of the particle in Fig. 4(b). Both photographs show an amorphous-like layer at the particle surface, apparently to a depth of about 10 nm in Fig. 4(a) and about 20 nm in Fig. 4(b). The actual depth is thought to be 10 nm in both cases, as can be seen in Fig. 5. Figure 6 shows the distributions of the Sm, Fe, N and O elements near the surface of the particle, as measured by AES. Oxygen segregation is clearly observable and the O is largely confined to a surface
IC Kobayashi et al. / Magnetic properties of SMDPs of
S m 2 F e l T Nx
237
4. Discussion 0111
(a)
(bl
Fig. 4. T E M images of SFN particle, showing nearly amorphous surface layer. The focus point is (a) near the center plane or (b) at the upper plane of the particle.
lOnm~
'o
j2 /
L 20rim
Fig. 5. Schematic representation of the microstructure of SMDP of SFN.
//~_s
L.
The finely powdered SmzFe17Nx clearly exhibits the characteristics of an SMDP material. The results of the present study indicate the upper limit of the particle size for these characteristics is about 3-4 /~m. This value is consistent with our previously reported findings that compacted powder magnets composed of this material exhibit the highest maximum energy product when the size of their component particles is in the range 1-5 tzm [2], [5]. The results of the TEM observations and the AES measurements show the SFN particle to contain a surface region with a much higher O content than that in the inner portion. The thickness of this surface region is about 10 nm. If it may be presumed that the surface region thickness of the 50 tzm particle (prior to fine pulverization) is the same as that of the 2.5/zm particle (following fine pulverization), it then follows that the relative volume of the O-rich surface region is about 20 times larger in the smaller particle. From the analysis of the O distribution, moreover, it can be calculated that about 95% of the total O in the particle exists in the surface region. A schematic view of the SMDP is shown in Fig. 5. Most metals possess a thin oxidized surface layer, and Mott and Cabrerra [8] have proposed a theory to explain its formation. The oxidized layer observed in the present study would seem to be amenable to a similar theoretical treatment. It must be noted, however, that in SFN this thin layer is clearly related to the magnetic properties of SMDP, and particularly to its coercivity. We have found that increasing the total O content to about 30% beyond the content of the SMDP observed in the present study results in a 3%-4% reduction in the saturation magnetization and a 50% reduction in the coercivity. The details will be reported elsewhere.
•
5. Conclusions
The Sm2Fe17Nx magnetic material prepared in the N H 3 - H 2 mixed gas atmosphere exhibits best the mag0 Depth
"40 from
surface
'
'
'BO
of particle/nm
Fig. 6. Concentration distribution of Sm, Fe, N and O in the surface portion of the SFN particle measured by AES.
region up to a depth of about 10 nm, which corresponds to the amorphous-like layer observed by TEM.
netic properties when x is about 3.0. Powders obtained by the fine pulverization of this material clearly exhibit the characteristics of single magnetic domain particles, in particle sizes of up to 3-4/zm. Oxygen in the particle is segregated to the amorphous-like surface layer 10 nm thick, as shown by TEM and AES. The calculation based on the observed O distribution curve indicates that the surface layer contains about 95% of the total amount of O in the particle.
238
K. Kobayashi et al. / Magnetic properties of SMDPs of Sm2Pet7Nx
Acknowledgments We are very grateful to Mr. N. Imaoka, Mr. A. Sudo, Ms. N. Kashiwaya, Mr. T. Fukuda and Mr. M. Stever for valuable discussion and experiments.
References 1 J. M. D. Coey and Hong Sun, J. Magn. Magn. Mater., 87 (1990) L251. 2 K. Kobayashi, T. Iriyama, N. Imaoka, A. Sudo and N. Kashiwaya, Euro. Patent, 0-417-733-A2, 1990.
3 K. Goto, M. Ito and T. Sakurai, Jpn. J. AppL Phys., 19 (1980) 1339. 4 K. H. J. Buschow, R. Coehoorn, D. B. de Mooij, K. de Waard and T. H. Jacobs, Z Magn. Magn. Mater., 92 (1990) L35. 5 T. Iriyama, K. Kobayashi, N. Imaoka, T. Fukuda, H. Kato and Y. Nakagawa, IEEE Trans. Magn., 28 (5) (1992) 2326, paper presented at Intermag Conf., 1992. 6 K. Kobayashi, T. Iriyama, N. Imaoka and T. Fukuda, Rare Earths, 19 (1991) 31--43 (in Japanese). 7 Y. Iwama, Kohshitugiseizabyoh (Hard Magnetic Materials) Maruzen, Tokyo, 1976, p. 15 (in Japanese). 8 N. Cabrerra and N. F. Mott, Rep. Prog. Phys., 12 (1948--49) 163.