- Email: [email protected]
Sbeutectic composite
Scripta Materialia, Vol. 41, No. 8, pp. 803– 807, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter
Pergamon
PII S1359-6462(99)00228-6
MAGNETIC PROPERTIES OF DIRECTIONALLY SOLIDIFIED MnSb/Sb EUTECTIC COMPOSITE Ye Pan and Guoxiong Sun Department of Mechanical Engineering, Southeast University, Nanjing 210018, China (Received September 1, 1998) (Accepted July 8, 1999)
Introduction MnSb is the only ferromagnetic compound in the first-row transition-metal antimonides; it is highly anisotropic and proposed to be an important magnetic material for a variety of potential applications [1–3]. The controlled eutectics consisting of MnSb and other components may constitute functional in-situ composites without any contamination and wetting problem between the components because of multiphase cooperative growth. The anisotropic MnSb/Sb composite is expected to possess interesting magnetic properties if MnSb is embedded in a diamagnetic antimony matrix as regular eutectic by directional solidification. However, very little has been published on MnSb/Sb eutectic composites. In the composite, the shape, size, orientation and distribution of magnetic components are important parameters which determine the quality of the materials. Consideration of these factors leads to the investigation on the relationships between microstructure, magnetic property and temperature dependence of the property in this system.
Experiments The eutectic alloy (90.5wt%Sb and 9.5wt%Mn) was prepared from commercially pure Mn (ⱖ 99.7wt%) and Sb (ⱖ99.5wt%) by vacuum induction melting. Excess Mn was added to compensate the loss during melting, and the alloys were held at 1073K for 40 minutes to ensure homogeneity. The directional solidifications were carried out in an evacuated (1.33mPa) quartz crucible using the Bridgman technique with temperature gradient GL ⫽ 108K/cm and growth velocities V ⫽ 1⬃25m/s. For metallographic observations, the specimens were sectioned along the parallel- and perpendicular-to-solidification directions, polished, and etched using a dilute HCl acid solution. The interrod spacing and rod size were measured over the range of accessible growth velocities by statistically determining the rod density Nr and rod diameter d. was defined as: ⫽ N⫺1/2 . r Magnetization of cylindrical samples was measured parallel to the solidification direction along applied fields up to 64⫻104A/m using a vibrating sample magnetometer (Princeton Applied Research Corporation, Model 155). Magnetizations as a function of temperature and applied field were carried out in the present work.
803
804
MnSb/Sb EUTECTIC COMPOSITE
Vol. 41, No. 8
Figure 1. Microstructures of directionally solidified MnSb/Sb eutectic composite: (a) longitudinal section; (b) transverse section.
Results and Discussion The microstructures of directionally solidified MnSb/Sb eutectic composite are shown in Fig. 1. It is evident that the MnSb rod axis is oriented along solidification direction; the rods are arranged in a regular lattice-like array and are well distributed throughout the transverse section of the sample, which shows an idealized in situ composite structure with high shape anisotropy. It has been found from metallographic examination that both interrod spacing and rod diameter d decrease with increasing growth velocity V. Measured values are listed in Table 1, indicating the aligned MnSb rods well embedded in a Sb matrix on a micron scale. The regularity of the eutectic is suggested to be determined by crystal growth in nonfacetednonfaceted (nf-nf) or faceted-nonfaceted (f-nf) types that depend on the entropy of solution, ⌬S␣. It has not been reported in the literature whether the MnSb phase morphology is nonfaceted or faceted; also it is difficult to calculate ⌬S␣ because of the complicated crystallography of the intermetallic compound. The ⌬S␣ of the matrix Sb is approximately 22J/K䡠mol, close to the critical value of 23J/K䡠mol TABLE 1 Measured Average Intrerrod Spacing and Rod Diameter d V, m/s , m d, m
1
2
3
5
8
10
12
15
17
20
25
15.1 5.7
9.7 4.9
7.1 3.6
6.9 3.1
5.7 2.8
5.5 2.6
5.1 2.4
4.9 2.1
4.6 1.9
4.3 1.8
3.4 1.5
Vol. 41, No. 8
MnSb/Sb EUTECTIC COMPOSITE
805
Figure 2. Non-faceted characteristic of MnSb.
which distinguishes between faceted and nonfaceted phase morphology [4]. Therefore, it is necessary to identify the growth behavior of MnSb through the morphology of the primary MnSb phase in the alloy slightly off the eutectic composition. Fig. 2 shows the typical dendrites of MnSb in Sb-9.8wt%Mn hypereutectic alloy by directional solidification. Apparently, MnSb is nonfaceted. The Sb matrix may exhibit a weakly faceted feature due to a value of ⌬S␣ slightly less than the critical value. However, in a rodlike eutectic, facets cannot form in the MnSb/Sb interface groove in the major phase (i.e. matrix) since this groove is a region of negative curvature. In this case, the MnSb/Sb eutectic is of nf-nf type and the resultant structure is regular. The magnetizations of the directionally solidified samples with the magnetic field applied at 290K and 520K are shown in Fig. 3. At 290K, as shown in Fig. 3a, with the increase of applied field H, the magnetization shows a linear increase and tends to become saturated at H⬎56⫻104A/m. The four curves show similar variations of magnetization with H. Clearly, the samples solidified at various velocities have a similar magnetic behavior. The measured saturation magnetization, Ms, is found to be around 25A䡠m2/kg except for V ⫽ 15m/s. It can be converted to a value of 83.3A䡠m2/kg for MnSb rods if the diamagnetic Sb matrix (70% by volume) is subtracted in calculating the magnetization. It was demonstrated from electron diffraction patterns and x-ray diffraction patterns that the MnSb rods preferentially grew in the [001] direction; and the orientation relation, [001]MnSb//rod axis//growth direction, was identified [5,6]. In this case, therefore, a high applied field required for saturation
Figure 3. Magnetization curves of MnSb/Sb eutectic composites at different temperatures: (a) 290K; (b) 520K.
806
MnSb/Sb EUTECTIC COMPOSITE
Vol. 41, No. 8
Figure 4. The temperature dependence of the magnetization at a field of 8⫻104A/m.
suggests that the magnetization is along the hard axis. Thus, it is possible to combine shape anisotropy and magneto-crystalline anisotropy to define the magnetic behavior of the composites. A notable change of magnetic behavior is observed at 520K, as shown in Fig. 3b. At H ⫽ 7⬃8⫻104A/m, the mass saturation magnetization yields Ms ⫽ 16.5A䡠m2/kg for the sample solidified at 2m/s. With growth velocity up to 10m/s, an increase in Ms, 19.5A䡠m2/kg, is noted as a result of the finer-scale microstructure, as seen in Table 1. However, the magnetization of the four samples appears to be saturated at almost the same value of the field. An applied field of 12⫻104A/m parallel to the solidification direction is sufficient to saturate MnSb/Sb composites solidified at all growth velocities studied. It may be suggested that an easy axis of magnetization is brought about by magnetic domain wall motion at a much smaller applied field. The results show the spin reorientation with increasing temperature, the easy direction of magnetization shifting from the c-plane to the c-axis. It should be noted that Ms decreases to a certain extent for the samples solidified at higher growth velocities, from 16A䡠m2/kg at 15m/s to 15.6A䡠m2/kg at 25m/s, corresponding to ⫽ 4.9m, d ⫽ 2.1m and ⫽ 3.4m, d ⫽ 1.5m respectively. This may be attributed to the appearance of a cellular eutectic in which MnSb rods are out of parallel alignment, resulting in a deviation of rod axis from the applied field direction. It was confirmed that MnSb/Sb eutectic growth with a planar interface could be ensured below a critical velocity of 12m/s in a temperature gradient of GL ⫽ 108K/cm [7]. The growth velocity dependence of the magnetization is in good agreement with the results. Consequently, both size and orientation of ferromagnetic MnSb play an important role in magnetic behavior of the composite. Fig. 4 shows the variation of magnetization with temperature at a constant applied field of 8⫻104A/m. As temperature decreases the magnetization shows a sharp increase at the Curie temperature followed by an approach to the saturation state of ferromagnetic ordering. Then, the magnetization decreases gradually with further decreasing the temperature. This phenomenon can be considered as a result of spin reorientation from the easy direction to the hard direction of the magnetization. The reorientation is an atomic spin process and occurs in a range of temperature. The peaks exist at 520⬃530K for all samples measured, and their magnitudes depend on growth velocity. Conclusions Magnetic anisotropic MnSb/Sb eutectic composites in which aligned MnSb rods are embedded in the Sb matrix on a micron scale have been produced by directional solidification. The eutectic growth is characterized by nonfaceted-nonfaceted type. Both interrod spacing and rod diameter decrease with
Vol. 41, No. 8
MnSb/Sb EUTECTIC COMPOSITE
807
increasing growth velocity. The magnetization as a function of temperature and applied field has been determined along the solidification direction. The composites exhibit a hard axis of magnetization with mass saturation magnetization Ms ⫽ 25A䡠m2/kg at 290K. The temperature dependence of magnetization at a fixed field shows that the easy axis shifts to the c-axis within a range of temperature. At 520K, the saturation values are attained at a low applied field of 12⫻104A/m, where Ms, from 15.6A䡠m2/kg to 19.5A䡠m2/kg, depends on growth velocity. The higher Ms is corresponding to highly aligned MnSb rods as a result of planar interface growth of MnSb/Sb eutectic. Acknowledgments The present investigation was supported by the Applied Science Research Foundation of Jiangsu Province, China. The authors are grateful to State Key Laboratory for Magnetism of Institute of Physics, Chinese Academy of Sciences, for the magnetization measurements. References 1. 2. 3. 4. 5. 6. 7.
C. V. Narasimha, S. L. Pinjare, and K. V. S. Rama Rao, J Magn Magn Mater. 50, 107 (1985). G. Markandeyulu and K. V. S. Rama Rao, J Magn Magn Mater. 67, 215 (1987). V. S. Bai and K. V. S. Rama Rao, J Appl Phys. 55, 2167 (1984). M. N. Croker, R. S. Fidler, and R. W. Smith, Proc R Soc. A335, 15 (1973). Y. Pan and G. X. Sun, J Mater Sci. 33, 763 (1998). Y. Pan and G. X. Sun, Proc 4th Special Symposium on Advanced Materials, Nagoya, Japan, 138 (1998). Y. Pan and G. X. Sun, Chinese J Mater Res, in press.
Pergamon
PII S1359-6462(99)00228-6
MAGNETIC PROPERTIES OF DIRECTIONALLY SOLIDIFIED MnSb/Sb EUTECTIC COMPOSITE Ye Pan and Guoxiong Sun Department of Mechanical Engineering, Southeast University, Nanjing 210018, China (Received September 1, 1998) (Accepted July 8, 1999)
Introduction MnSb is the only ferromagnetic compound in the first-row transition-metal antimonides; it is highly anisotropic and proposed to be an important magnetic material for a variety of potential applications [1–3]. The controlled eutectics consisting of MnSb and other components may constitute functional in-situ composites without any contamination and wetting problem between the components because of multiphase cooperative growth. The anisotropic MnSb/Sb composite is expected to possess interesting magnetic properties if MnSb is embedded in a diamagnetic antimony matrix as regular eutectic by directional solidification. However, very little has been published on MnSb/Sb eutectic composites. In the composite, the shape, size, orientation and distribution of magnetic components are important parameters which determine the quality of the materials. Consideration of these factors leads to the investigation on the relationships between microstructure, magnetic property and temperature dependence of the property in this system.
Experiments The eutectic alloy (90.5wt%Sb and 9.5wt%Mn) was prepared from commercially pure Mn (ⱖ 99.7wt%) and Sb (ⱖ99.5wt%) by vacuum induction melting. Excess Mn was added to compensate the loss during melting, and the alloys were held at 1073K for 40 minutes to ensure homogeneity. The directional solidifications were carried out in an evacuated (1.33mPa) quartz crucible using the Bridgman technique with temperature gradient GL ⫽ 108K/cm and growth velocities V ⫽ 1⬃25m/s. For metallographic observations, the specimens were sectioned along the parallel- and perpendicular-to-solidification directions, polished, and etched using a dilute HCl acid solution. The interrod spacing and rod size were measured over the range of accessible growth velocities by statistically determining the rod density Nr and rod diameter d. was defined as: ⫽ N⫺1/2 . r Magnetization of cylindrical samples was measured parallel to the solidification direction along applied fields up to 64⫻104A/m using a vibrating sample magnetometer (Princeton Applied Research Corporation, Model 155). Magnetizations as a function of temperature and applied field were carried out in the present work.
803
804
MnSb/Sb EUTECTIC COMPOSITE
Vol. 41, No. 8
Figure 1. Microstructures of directionally solidified MnSb/Sb eutectic composite: (a) longitudinal section; (b) transverse section.
Results and Discussion The microstructures of directionally solidified MnSb/Sb eutectic composite are shown in Fig. 1. It is evident that the MnSb rod axis is oriented along solidification direction; the rods are arranged in a regular lattice-like array and are well distributed throughout the transverse section of the sample, which shows an idealized in situ composite structure with high shape anisotropy. It has been found from metallographic examination that both interrod spacing and rod diameter d decrease with increasing growth velocity V. Measured values are listed in Table 1, indicating the aligned MnSb rods well embedded in a Sb matrix on a micron scale. The regularity of the eutectic is suggested to be determined by crystal growth in nonfacetednonfaceted (nf-nf) or faceted-nonfaceted (f-nf) types that depend on the entropy of solution, ⌬S␣. It has not been reported in the literature whether the MnSb phase morphology is nonfaceted or faceted; also it is difficult to calculate ⌬S␣ because of the complicated crystallography of the intermetallic compound. The ⌬S␣ of the matrix Sb is approximately 22J/K䡠mol, close to the critical value of 23J/K䡠mol TABLE 1 Measured Average Intrerrod Spacing and Rod Diameter d V, m/s , m d, m
1
2
3
5
8
10
12
15
17
20
25
15.1 5.7
9.7 4.9
7.1 3.6
6.9 3.1
5.7 2.8
5.5 2.6
5.1 2.4
4.9 2.1
4.6 1.9
4.3 1.8
3.4 1.5
Vol. 41, No. 8
MnSb/Sb EUTECTIC COMPOSITE
805
Figure 2. Non-faceted characteristic of MnSb.
which distinguishes between faceted and nonfaceted phase morphology [4]. Therefore, it is necessary to identify the growth behavior of MnSb through the morphology of the primary MnSb phase in the alloy slightly off the eutectic composition. Fig. 2 shows the typical dendrites of MnSb in Sb-9.8wt%Mn hypereutectic alloy by directional solidification. Apparently, MnSb is nonfaceted. The Sb matrix may exhibit a weakly faceted feature due to a value of ⌬S␣ slightly less than the critical value. However, in a rodlike eutectic, facets cannot form in the MnSb/Sb interface groove in the major phase (i.e. matrix) since this groove is a region of negative curvature. In this case, the MnSb/Sb eutectic is of nf-nf type and the resultant structure is regular. The magnetizations of the directionally solidified samples with the magnetic field applied at 290K and 520K are shown in Fig. 3. At 290K, as shown in Fig. 3a, with the increase of applied field H, the magnetization shows a linear increase and tends to become saturated at H⬎56⫻104A/m. The four curves show similar variations of magnetization with H. Clearly, the samples solidified at various velocities have a similar magnetic behavior. The measured saturation magnetization, Ms, is found to be around 25A䡠m2/kg except for V ⫽ 15m/s. It can be converted to a value of 83.3A䡠m2/kg for MnSb rods if the diamagnetic Sb matrix (70% by volume) is subtracted in calculating the magnetization. It was demonstrated from electron diffraction patterns and x-ray diffraction patterns that the MnSb rods preferentially grew in the [001] direction; and the orientation relation, [001]MnSb//rod axis//growth direction, was identified [5,6]. In this case, therefore, a high applied field required for saturation
Figure 3. Magnetization curves of MnSb/Sb eutectic composites at different temperatures: (a) 290K; (b) 520K.
806
MnSb/Sb EUTECTIC COMPOSITE
Vol. 41, No. 8
Figure 4. The temperature dependence of the magnetization at a field of 8⫻104A/m.
suggests that the magnetization is along the hard axis. Thus, it is possible to combine shape anisotropy and magneto-crystalline anisotropy to define the magnetic behavior of the composites. A notable change of magnetic behavior is observed at 520K, as shown in Fig. 3b. At H ⫽ 7⬃8⫻104A/m, the mass saturation magnetization yields Ms ⫽ 16.5A䡠m2/kg for the sample solidified at 2m/s. With growth velocity up to 10m/s, an increase in Ms, 19.5A䡠m2/kg, is noted as a result of the finer-scale microstructure, as seen in Table 1. However, the magnetization of the four samples appears to be saturated at almost the same value of the field. An applied field of 12⫻104A/m parallel to the solidification direction is sufficient to saturate MnSb/Sb composites solidified at all growth velocities studied. It may be suggested that an easy axis of magnetization is brought about by magnetic domain wall motion at a much smaller applied field. The results show the spin reorientation with increasing temperature, the easy direction of magnetization shifting from the c-plane to the c-axis. It should be noted that Ms decreases to a certain extent for the samples solidified at higher growth velocities, from 16A䡠m2/kg at 15m/s to 15.6A䡠m2/kg at 25m/s, corresponding to ⫽ 4.9m, d ⫽ 2.1m and ⫽ 3.4m, d ⫽ 1.5m respectively. This may be attributed to the appearance of a cellular eutectic in which MnSb rods are out of parallel alignment, resulting in a deviation of rod axis from the applied field direction. It was confirmed that MnSb/Sb eutectic growth with a planar interface could be ensured below a critical velocity of 12m/s in a temperature gradient of GL ⫽ 108K/cm [7]. The growth velocity dependence of the magnetization is in good agreement with the results. Consequently, both size and orientation of ferromagnetic MnSb play an important role in magnetic behavior of the composite. Fig. 4 shows the variation of magnetization with temperature at a constant applied field of 8⫻104A/m. As temperature decreases the magnetization shows a sharp increase at the Curie temperature followed by an approach to the saturation state of ferromagnetic ordering. Then, the magnetization decreases gradually with further decreasing the temperature. This phenomenon can be considered as a result of spin reorientation from the easy direction to the hard direction of the magnetization. The reorientation is an atomic spin process and occurs in a range of temperature. The peaks exist at 520⬃530K for all samples measured, and their magnitudes depend on growth velocity. Conclusions Magnetic anisotropic MnSb/Sb eutectic composites in which aligned MnSb rods are embedded in the Sb matrix on a micron scale have been produced by directional solidification. The eutectic growth is characterized by nonfaceted-nonfaceted type. Both interrod spacing and rod diameter decrease with
Vol. 41, No. 8
MnSb/Sb EUTECTIC COMPOSITE
807
increasing growth velocity. The magnetization as a function of temperature and applied field has been determined along the solidification direction. The composites exhibit a hard axis of magnetization with mass saturation magnetization Ms ⫽ 25A䡠m2/kg at 290K. The temperature dependence of magnetization at a fixed field shows that the easy axis shifts to the c-axis within a range of temperature. At 520K, the saturation values are attained at a low applied field of 12⫻104A/m, where Ms, from 15.6A䡠m2/kg to 19.5A䡠m2/kg, depends on growth velocity. The higher Ms is corresponding to highly aligned MnSb rods as a result of planar interface growth of MnSb/Sb eutectic. Acknowledgments The present investigation was supported by the Applied Science Research Foundation of Jiangsu Province, China. The authors are grateful to State Key Laboratory for Magnetism of Institute of Physics, Chinese Academy of Sciences, for the magnetization measurements. References 1. 2. 3. 4. 5. 6. 7.
C. V. Narasimha, S. L. Pinjare, and K. V. S. Rama Rao, J Magn Magn Mater. 50, 107 (1985). G. Markandeyulu and K. V. S. Rama Rao, J Magn Magn Mater. 67, 215 (1987). V. S. Bai and K. V. S. Rama Rao, J Appl Phys. 55, 2167 (1984). M. N. Croker, R. S. Fidler, and R. W. Smith, Proc R Soc. A335, 15 (1973). Y. Pan and G. X. Sun, J Mater Sci. 33, 763 (1998). Y. Pan and G. X. Sun, Proc 4th Special Symposium on Advanced Materials, Nagoya, Japan, 138 (1998). Y. Pan and G. X. Sun, Chinese J Mater Res, in press.