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Fe2O3 magnetic composite synthesized by mechanical alloying
Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
Ni/Fe2O3 magnetic composite synthesized by mechanical alloying Y. Shia,*, J. Dinga, Serene L.H. Tana, Z. Hub a
Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore b Department of Chemistry, Nanjing University, People’s Republic of China Received 17 April 2001; received in revised form 20 July 2001
Abstract Mechanical alloying of Ni and Fe2O3 resulted in the formation of a disordered structure, when only broadened X-ray diffraction peaks corresponding to wustite . appeared. After annealing at 500–9001C, a nanocomposite mixture of ferrite and (Ni, Fe) was formed. The investigation showed that Fe atoms concentrated in the ferrite phase, while the intermetallic structure was Ni-rich. r 2001 Elsevier Science B.V. All rights reserved. . Keywords: Mechanical alloying; Ferrites; Disordered; Mossbauer; Composite
1. Introduction Although nickel ferrites have been commercialized in high frequency application [1] for a long time, they still have been attracting intense research interests due to its some new promising applications, such as ferrofluids, gas sensor and pigment [2–4]. However, the relative lower saturation magnetization of nickel ferrite, compared to those of metallic compounds, limits its application performance. Recently, magnetic Fe3O4/Fe nanocomposites have shown enhanced magnetization values compared to that of Fe3O4 [5]. This work indicates that nanocomposites of ferrites/Fe may be promising for microwave applications, *Corresponding author. Tel.: +65-874-7899; fax: +65-7763604. E-mail address: [email protected] (Y. Shi).
if the electric resistance remains unchanged when high magnetization may increase magnetic permeability. The purpose of our investigation is to study the structure and magnetic properties of nanocomposites of Ni/Fe2O3 synthesized by mechanical alloying and subsequent heat treatment. Structure and magnetic properties are reported in this paper.
2. Experiment Mixture of Ni and Fe2O3 powders (the mole ratio of Ni to Fe2O3 was 1 : 1) were loaded together with 15 mm diameter stainless steel grinding balls into a hardened stainless steel vial. The ball to powder mass ratio was 8 : 1. The mechanical
0304-8853/03/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 4 8 4 - X
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Y. Shi et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
alloying was performed in a Spex 8000 Mixer/Mill. As-milled samples were annealed under vacuum at temperatures in a range of 200–9001C. The structure was studied using X-ray diffraction (Philips PW 1820 diffractometer with Cu Ka radiation). The microstructure and powder morphology were examined using transmission electron microscope (TEM, JEOL (100CX)) and a field-emission scanning electron microscope (SEM, Philips FEG-SEM XL30). The thermal properties of the as-milled powder were studied with a differential scanning calorimeter (DSC, DuPont Instrument Thermal Analyst 2100 series). Fe-containing phases analysis was carried . out with the 57Fe-Mossbauer spectrometer (Ranger Scientific Instruments MS-1200). Magnetic measurements were carried out using a superconducting vibration sample magnetometer (Oxford Instruments) with a maximum field of 90 kOe in the temperature range from 5 K to room temperature. All powders were pressed into 5 mm diameter pellets before magnetic measurement.
3. Result and discussion
After milling for 2 h, the XRD patterns still showed a mixture of the starting phases, Ni and Fe2O3. Further milling to 5 h, the diffraction peak of wustite . phase appeared. At same time, the diffraction peaks of Ni and Fe2O3 become broadened and the intensity of them also reduced. Increasing the milling time to 18 h, the main peaks of Ni and Fe2O3 disappeared and peaks of wustite . phase become obvious. Finally, only the characteristic diffraction peaks for wustite . phase can be detected in the powders subjected to 32 h of mechanical alloying. The 32 h milled powder was subjected to subsequent heat treatment under vacuum at different temperatures in order to study the evolution of the as-milled powder as a function of annealing temperature. Fig. 2 shows X-ray diffraction patterns of the as-milled and annealed samples. There are several broad and weak diffraction peaks, which appear in the X-ray diffraction pattern of the as-milled powder. All the diffraction peaks could be well identified with the wustite . structure. This result indicates that a wustite . similar phase was formed after the mechanical alloying of the mixture of Ni+Fe2O3. The wide line broadening can be explained by small grain size and high disorder. No obvious
Fig. 1 shows XRD patterns of Ni/Fe2O3 powder blend mechanically alloyed for different times.
Fig. 1. X-ray diffraction patterns of Ni/Fe2O3 powders blend mechanically alloyed for various times.
Fig. 2. X-ray diffraction patterns of the as-milled and annealed samples.
Y. Shi et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
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Fig. 3. SEM micrographs for the as-milled sample, and the samples annealed under vacuum at different temperature.
difference could be seen after the as-milled sample was heated at 2001C under vacuum. After a heat treatment at 5001C, the peaks of wustite . phase disappeared. Meanwhile, some new peaks were generated. These peaks were attributed to magnetite phase (nickel ferrite) and BCC phase (it could be Ni or NiFe alloy), respectively. For the sample annealed at 7001C, the peaks of magnetite phase became much sharper due to the increased particle size, and the BCC phase was identified as NiFe alloy due to its characteristic peak at 651. After annealed at 9001C, a new peak was generated at 51.51 and the peak at 651 disappeared. Because the peaks at 441 and 51.51 correspond to FCC Ni, the sample annealed at 9001C was identified as a composite of magnetite phase and FCC Ni phase. The Scherrer method was used for the estimation of the crystallite size of the annealed samples. The crystalline size for 5001C sample
measured from the broadening of X-ray diffraction peaks is in the range of about 10–20 nm. The crystalline size for the samples annealed at 7001C and 9001C is around 50 and 90 nm, respectively. Fig. 3 shows SEM micrographs. The as-milled powder consists of submicron particles. The detailed study with a higher magnification reveals a disordered structure on the surface of the asmilled powder. The sample probably consisted of wustite . grain embedded in an amorphous matrix, in the combination with the XRD examination as discussed before. After annealing at 2001C, the surface was fully covered with grains with a mean grain size of approximately 10 nm. The typical grain size of the samples annealed at 5001C and 9001C is around 10 and 100 nm, respectively. This result indicates a nanocomposite formed after mechanical alloying followed by annealing. Similar results were obtained in the TEM study. The
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Y. Shi et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
Table 1 The particle sizes of as-milled and subsequently annealed samples, obtained from three different measurements Sample
As-milled 2001C 5001C 7001C 9001C
Particle size (nm) From XRD
From SEM
From TEM
F F 15 50 90
10–20 10–20 10–20 50–70 90–130
5–10 10–20 10–20 50–70 90–150
particle sizes of the as-milled and subsequently annealed samples, obtained from above three methods (XRD, SEM and TEM), are listed in Table 1. In the differential scanning calorimetric study, an exothermic peak appeared at approximately 4001C. The thermal reaction was associated with the formation of the mixture of BCC phase and nickel ferrite from the wustite-similar . structure after mechanical milling. In order to understand better the whole process . of phase formation, Mossbauer measurements . were done. The Mossbauer spectra of the asmilled and the annealed samples are shown in Fig. 4. The as-milled sample shows superparamagnetic and can be analyzed with three doublets. Two of them assigned to the wustite . phase [6]. One is typical for Fe2+ in an octahedral oxygen environment. The other one is that of tetrahedrally coordinated Fe3+ with an oxygen environment similar to that in Fe3O4. The third one may be attributed to the superparamagnetic magnetite phase (nickel ferrite). This nickel ferrite phase has not been identified from the XRD pattern (Fig. 2) due to its very small grain size. For the sample annealed at 2001C, no obvious change has been found. Magnetic splitting was shown in the spectra of the samples annealed at 5001C and higher temperatures. For the sample annealed at 5001C, a large doublet superimposed on the sextets indicates that a substantial part of . the grains are superparamagnetic. The Mossbauer spectra of the samples annealed at 7001C and 9001C revealed the presence of two magnetic
. Fig. 4. Mossbauer spectra for the as-milled and annealed samples.
components. The first component (including two sextets) has parameters very close to bulk nickel ferrite (hyperfine field 500 kOe) and the other one has a hyperfine field significantly reduced (454 kOe). The first component, which is
Y. Shi et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
characterized by sharp lines, can be assigned to nickel ferrite grain and the second one, with much broader lines, may be to grain boundary nickel ferrite [7,8]. In the spectrum of the sample annealed at 7001C, the failure to add the site corresponding to Fe ion in NiFe alloy showed
. Fig. 5. Mossbauer spectra of the sample annealed at 9001C taken at 78 K.
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that the fraction of Fe ions in NiFe alloy is very small. The sample annealed at 9001C was studied . with the Mossbauer spectrometer at 78 K. It confirmed that no sextet could be found for the . BCC Fe–Ni phase (Fig. 5). The Mossbauer spectrum could be well fitted with three sextets that can be assigned to the nickel ferrite phase. This result indicates that Fe atoms mainly concentrate in the ferrite structure, while nickel atoms mainly appear in the intermetallic phase. The FCC structure of the intermetallic phase from the XRD study indicated that the intermetallic phase was based on Ni. Hysteresis loops of the as-milled sample and the samples annealed at 5001C and 9001C at room temperature are shown in Fig. 6. The as-milled and 5001C samples show a superparamagnetic behavior, the magnetization cannot be saturated even at 90 kOe magnetic field. The magnetization of asmilled sample is much higher than that of typical antiferromagnetic materials. This relative higher magnetization may be attributed to the contribution of nickel ferrite instead of Fe contamination
Fig. 6. Hysteresis loops of the as-milled sample and the samples annealed at 5001C and 9001C, taken at room temperature under 90 kOe magnetic field.
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Y. Shi et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
from mill device or balls [9], because pure iron has . not been detected in the Mossbauer spectra of all the as-milled and annealed samples. The analysis . results of above Mossbauer spectra have suggested
Fig. 7. Effect of annealing temperature on saturation magnetization, Ms.
the existence of nickel ferrite phase in as-milled sample. The results of magnetic measurements are shown in Fig. 7. The saturation magnetization increased with the annealing temperature and nearly reached its maximum value at 9001C. Moreover, it was found that the magnetization shows a very slight increase at 2001C and a very rapid increase above 2001C. This can be explained by the formation of the ferrimagnetic and ferromagnetic phase in the higher temperature annealed samples. Hysteresis loops of as-milled and 5001C annealed samples cooled from 298 to 4.2 K under 90 kOe magnetic field are shown in Fig. 8. The shifted hysteresis loop and unidirectional anisotropy in as-milled sample that cooled in the magnetic field are attributed to the exchange coupling of ferrimagnetic (or ferromagnetic) with antiferromagnetic constituents. The biased hysteresis loop has not been found in the sample annealed at 5001C. It is expected because antiferromagnetic wustite . phase has decomposed into
Fig. 8. Hysteresis loops of as-milled and 5001C annealed samples, taken at 4.2 K after field cooling (FC) under 90 kOe.
Y. Shi et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
ferrimagnetic magnetite phase and ferromagnetic phase at 5001C and the exchange coupling also disappeared at the same time.
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The nanocrystalline composites, obtained through decomposing the as-milled powder, exhibit well soft magnetic properties. The saturation magnetization is over 67 emu/g that is much higher than that of nickel ferrite (50 emu/g).
4. Conclusion A metastable wustite . similar structure phase is formed after milling the mixture of nickel and Fe2O3 powders. After subsequently annealed at 500–9001C under vacuum, the decomposition of wustite . phase resulted in the formation of magnetite phase and ferromagnetic phase. The whole structural evolution process has been well studied through analysis of X-ray diffraction patterns and . the Mossbauer spectra. It has been found that Fe atoms concentrated in the ferrite phase, while the intermetallic phase was Ni-rich. The shifted hysteresis loop and unidirectional anisotropy were found in as-milled sample cooled in magnetic field. This revealed that a ferrimagnetic (or ferromagnetic) phase and an antiferromagnetic phase coexist in the as-milled powders.
References [1] R.F. Soohoo, in:W.L.Everitt (Ed.), Theory and Application of Ferrites, Prentice-Hall Inc., Engelwood Cliffs, NJ, 1960. [2] V.E. Fertman, Magnetic Fluids Guidebook: Properties and Application, 1990. [3] C.V.G. Reddy, S.V. Manorama, V.J. Rao, Sensors Actuators B-Chemical 55 (1999) 90. [4] S.M. ElSawy, S.H. Salah, Corros. Prev. Control 44 (1997) 135. [5] J. Ding, W.F. Miao, R. Street, P.G. McCormick, Scripta Mater. 35 (1996) 1307. . [6] N.N. Greenwood, Mossbauer Spectroscopy, Chapman & Hall, London, 1971. [7] D.V. Dimitrov, G.C. Hadjipanayis, V. Papaefthymiou, A. Simopoulos, J. Vac. Sci. Technol. 15 (1997) 1473. [8] B. Fulz, H. Kuwano, H. Ouyang, J. Appl. Phys. 77 (1995) 3458. [9] S.R. Mekala, J. Ding, J. Alloys Comp. 296 (2000) 152.
Ni/Fe2O3 magnetic composite synthesized by mechanical alloying Y. Shia,*, J. Dinga, Serene L.H. Tana, Z. Hub a
Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore b Department of Chemistry, Nanjing University, People’s Republic of China Received 17 April 2001; received in revised form 20 July 2001
Abstract Mechanical alloying of Ni and Fe2O3 resulted in the formation of a disordered structure, when only broadened X-ray diffraction peaks corresponding to wustite . appeared. After annealing at 500–9001C, a nanocomposite mixture of ferrite and (Ni, Fe) was formed. The investigation showed that Fe atoms concentrated in the ferrite phase, while the intermetallic structure was Ni-rich. r 2001 Elsevier Science B.V. All rights reserved. . Keywords: Mechanical alloying; Ferrites; Disordered; Mossbauer; Composite
1. Introduction Although nickel ferrites have been commercialized in high frequency application [1] for a long time, they still have been attracting intense research interests due to its some new promising applications, such as ferrofluids, gas sensor and pigment [2–4]. However, the relative lower saturation magnetization of nickel ferrite, compared to those of metallic compounds, limits its application performance. Recently, magnetic Fe3O4/Fe nanocomposites have shown enhanced magnetization values compared to that of Fe3O4 [5]. This work indicates that nanocomposites of ferrites/Fe may be promising for microwave applications, *Corresponding author. Tel.: +65-874-7899; fax: +65-7763604. E-mail address: [email protected] (Y. Shi).
if the electric resistance remains unchanged when high magnetization may increase magnetic permeability. The purpose of our investigation is to study the structure and magnetic properties of nanocomposites of Ni/Fe2O3 synthesized by mechanical alloying and subsequent heat treatment. Structure and magnetic properties are reported in this paper.
2. Experiment Mixture of Ni and Fe2O3 powders (the mole ratio of Ni to Fe2O3 was 1 : 1) were loaded together with 15 mm diameter stainless steel grinding balls into a hardened stainless steel vial. The ball to powder mass ratio was 8 : 1. The mechanical
0304-8853/03/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 4 8 4 - X
14
Y. Shi et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
alloying was performed in a Spex 8000 Mixer/Mill. As-milled samples were annealed under vacuum at temperatures in a range of 200–9001C. The structure was studied using X-ray diffraction (Philips PW 1820 diffractometer with Cu Ka radiation). The microstructure and powder morphology were examined using transmission electron microscope (TEM, JEOL (100CX)) and a field-emission scanning electron microscope (SEM, Philips FEG-SEM XL30). The thermal properties of the as-milled powder were studied with a differential scanning calorimeter (DSC, DuPont Instrument Thermal Analyst 2100 series). Fe-containing phases analysis was carried . out with the 57Fe-Mossbauer spectrometer (Ranger Scientific Instruments MS-1200). Magnetic measurements were carried out using a superconducting vibration sample magnetometer (Oxford Instruments) with a maximum field of 90 kOe in the temperature range from 5 K to room temperature. All powders were pressed into 5 mm diameter pellets before magnetic measurement.
3. Result and discussion
After milling for 2 h, the XRD patterns still showed a mixture of the starting phases, Ni and Fe2O3. Further milling to 5 h, the diffraction peak of wustite . phase appeared. At same time, the diffraction peaks of Ni and Fe2O3 become broadened and the intensity of them also reduced. Increasing the milling time to 18 h, the main peaks of Ni and Fe2O3 disappeared and peaks of wustite . phase become obvious. Finally, only the characteristic diffraction peaks for wustite . phase can be detected in the powders subjected to 32 h of mechanical alloying. The 32 h milled powder was subjected to subsequent heat treatment under vacuum at different temperatures in order to study the evolution of the as-milled powder as a function of annealing temperature. Fig. 2 shows X-ray diffraction patterns of the as-milled and annealed samples. There are several broad and weak diffraction peaks, which appear in the X-ray diffraction pattern of the as-milled powder. All the diffraction peaks could be well identified with the wustite . structure. This result indicates that a wustite . similar phase was formed after the mechanical alloying of the mixture of Ni+Fe2O3. The wide line broadening can be explained by small grain size and high disorder. No obvious
Fig. 1 shows XRD patterns of Ni/Fe2O3 powder blend mechanically alloyed for different times.
Fig. 1. X-ray diffraction patterns of Ni/Fe2O3 powders blend mechanically alloyed for various times.
Fig. 2. X-ray diffraction patterns of the as-milled and annealed samples.
Y. Shi et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
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Fig. 3. SEM micrographs for the as-milled sample, and the samples annealed under vacuum at different temperature.
difference could be seen after the as-milled sample was heated at 2001C under vacuum. After a heat treatment at 5001C, the peaks of wustite . phase disappeared. Meanwhile, some new peaks were generated. These peaks were attributed to magnetite phase (nickel ferrite) and BCC phase (it could be Ni or NiFe alloy), respectively. For the sample annealed at 7001C, the peaks of magnetite phase became much sharper due to the increased particle size, and the BCC phase was identified as NiFe alloy due to its characteristic peak at 651. After annealed at 9001C, a new peak was generated at 51.51 and the peak at 651 disappeared. Because the peaks at 441 and 51.51 correspond to FCC Ni, the sample annealed at 9001C was identified as a composite of magnetite phase and FCC Ni phase. The Scherrer method was used for the estimation of the crystallite size of the annealed samples. The crystalline size for 5001C sample
measured from the broadening of X-ray diffraction peaks is in the range of about 10–20 nm. The crystalline size for the samples annealed at 7001C and 9001C is around 50 and 90 nm, respectively. Fig. 3 shows SEM micrographs. The as-milled powder consists of submicron particles. The detailed study with a higher magnification reveals a disordered structure on the surface of the asmilled powder. The sample probably consisted of wustite . grain embedded in an amorphous matrix, in the combination with the XRD examination as discussed before. After annealing at 2001C, the surface was fully covered with grains with a mean grain size of approximately 10 nm. The typical grain size of the samples annealed at 5001C and 9001C is around 10 and 100 nm, respectively. This result indicates a nanocomposite formed after mechanical alloying followed by annealing. Similar results were obtained in the TEM study. The
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Y. Shi et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
Table 1 The particle sizes of as-milled and subsequently annealed samples, obtained from three different measurements Sample
As-milled 2001C 5001C 7001C 9001C
Particle size (nm) From XRD
From SEM
From TEM
F F 15 50 90
10–20 10–20 10–20 50–70 90–130
5–10 10–20 10–20 50–70 90–150
particle sizes of the as-milled and subsequently annealed samples, obtained from above three methods (XRD, SEM and TEM), are listed in Table 1. In the differential scanning calorimetric study, an exothermic peak appeared at approximately 4001C. The thermal reaction was associated with the formation of the mixture of BCC phase and nickel ferrite from the wustite-similar . structure after mechanical milling. In order to understand better the whole process . of phase formation, Mossbauer measurements . were done. The Mossbauer spectra of the asmilled and the annealed samples are shown in Fig. 4. The as-milled sample shows superparamagnetic and can be analyzed with three doublets. Two of them assigned to the wustite . phase [6]. One is typical for Fe2+ in an octahedral oxygen environment. The other one is that of tetrahedrally coordinated Fe3+ with an oxygen environment similar to that in Fe3O4. The third one may be attributed to the superparamagnetic magnetite phase (nickel ferrite). This nickel ferrite phase has not been identified from the XRD pattern (Fig. 2) due to its very small grain size. For the sample annealed at 2001C, no obvious change has been found. Magnetic splitting was shown in the spectra of the samples annealed at 5001C and higher temperatures. For the sample annealed at 5001C, a large doublet superimposed on the sextets indicates that a substantial part of . the grains are superparamagnetic. The Mossbauer spectra of the samples annealed at 7001C and 9001C revealed the presence of two magnetic
. Fig. 4. Mossbauer spectra for the as-milled and annealed samples.
components. The first component (including two sextets) has parameters very close to bulk nickel ferrite (hyperfine field 500 kOe) and the other one has a hyperfine field significantly reduced (454 kOe). The first component, which is
Y. Shi et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
characterized by sharp lines, can be assigned to nickel ferrite grain and the second one, with much broader lines, may be to grain boundary nickel ferrite [7,8]. In the spectrum of the sample annealed at 7001C, the failure to add the site corresponding to Fe ion in NiFe alloy showed
. Fig. 5. Mossbauer spectra of the sample annealed at 9001C taken at 78 K.
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that the fraction of Fe ions in NiFe alloy is very small. The sample annealed at 9001C was studied . with the Mossbauer spectrometer at 78 K. It confirmed that no sextet could be found for the . BCC Fe–Ni phase (Fig. 5). The Mossbauer spectrum could be well fitted with three sextets that can be assigned to the nickel ferrite phase. This result indicates that Fe atoms mainly concentrate in the ferrite structure, while nickel atoms mainly appear in the intermetallic phase. The FCC structure of the intermetallic phase from the XRD study indicated that the intermetallic phase was based on Ni. Hysteresis loops of the as-milled sample and the samples annealed at 5001C and 9001C at room temperature are shown in Fig. 6. The as-milled and 5001C samples show a superparamagnetic behavior, the magnetization cannot be saturated even at 90 kOe magnetic field. The magnetization of asmilled sample is much higher than that of typical antiferromagnetic materials. This relative higher magnetization may be attributed to the contribution of nickel ferrite instead of Fe contamination
Fig. 6. Hysteresis loops of the as-milled sample and the samples annealed at 5001C and 9001C, taken at room temperature under 90 kOe magnetic field.
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from mill device or balls [9], because pure iron has . not been detected in the Mossbauer spectra of all the as-milled and annealed samples. The analysis . results of above Mossbauer spectra have suggested
Fig. 7. Effect of annealing temperature on saturation magnetization, Ms.
the existence of nickel ferrite phase in as-milled sample. The results of magnetic measurements are shown in Fig. 7. The saturation magnetization increased with the annealing temperature and nearly reached its maximum value at 9001C. Moreover, it was found that the magnetization shows a very slight increase at 2001C and a very rapid increase above 2001C. This can be explained by the formation of the ferrimagnetic and ferromagnetic phase in the higher temperature annealed samples. Hysteresis loops of as-milled and 5001C annealed samples cooled from 298 to 4.2 K under 90 kOe magnetic field are shown in Fig. 8. The shifted hysteresis loop and unidirectional anisotropy in as-milled sample that cooled in the magnetic field are attributed to the exchange coupling of ferrimagnetic (or ferromagnetic) with antiferromagnetic constituents. The biased hysteresis loop has not been found in the sample annealed at 5001C. It is expected because antiferromagnetic wustite . phase has decomposed into
Fig. 8. Hysteresis loops of as-milled and 5001C annealed samples, taken at 4.2 K after field cooling (FC) under 90 kOe.
Y. Shi et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 13–19
ferrimagnetic magnetite phase and ferromagnetic phase at 5001C and the exchange coupling also disappeared at the same time.
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The nanocrystalline composites, obtained through decomposing the as-milled powder, exhibit well soft magnetic properties. The saturation magnetization is over 67 emu/g that is much higher than that of nickel ferrite (50 emu/g).
4. Conclusion A metastable wustite . similar structure phase is formed after milling the mixture of nickel and Fe2O3 powders. After subsequently annealed at 500–9001C under vacuum, the decomposition of wustite . phase resulted in the formation of magnetite phase and ferromagnetic phase. The whole structural evolution process has been well studied through analysis of X-ray diffraction patterns and . the Mossbauer spectra. It has been found that Fe atoms concentrated in the ferrite phase, while the intermetallic phase was Ni-rich. The shifted hysteresis loop and unidirectional anisotropy were found in as-milled sample cooled in magnetic field. This revealed that a ferrimagnetic (or ferromagnetic) phase and an antiferromagnetic phase coexist in the as-milled powders.
References [1] R.F. Soohoo, in:W.L.Everitt (Ed.), Theory and Application of Ferrites, Prentice-Hall Inc., Engelwood Cliffs, NJ, 1960. [2] V.E. Fertman, Magnetic Fluids Guidebook: Properties and Application, 1990. [3] C.V.G. Reddy, S.V. Manorama, V.J. Rao, Sensors Actuators B-Chemical 55 (1999) 90. [4] S.M. ElSawy, S.H. Salah, Corros. Prev. Control 44 (1997) 135. [5] J. Ding, W.F. Miao, R. Street, P.G. McCormick, Scripta Mater. 35 (1996) 1307. . [6] N.N. Greenwood, Mossbauer Spectroscopy, Chapman & Hall, London, 1971. [7] D.V. Dimitrov, G.C. Hadjipanayis, V. Papaefthymiou, A. Simopoulos, J. Vac. Sci. Technol. 15 (1997) 1473. [8] B. Fulz, H. Kuwano, H. Ouyang, J. Appl. Phys. 77 (1995) 3458. [9] S.R. Mekala, J. Ding, J. Alloys Comp. 296 (2000) 152.