- Email: [email protected]
mechanical alloying
Journal of Magnetism and Magnetic Materials 205 (1999) 249}254
NiFe O ultra"ne particles prepared by co-precipitation/mechanical alloying Y. Shi*, J. Ding, X. Liu, J. Wang Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore Received 5 April 1999; received in revised form 22 June 1999
Abstract Nickel ferrite ultra"ne particles have been prepared by the combination of co-precipitation and mechanical alloying. Sodium chloride was added in order to avoid agglomeration. This work has shown that nickel ferrite phase can be formed directly during mechanical milling of hydroxide precursor. Ultra"ne NiFe O particles were obtained after a simple washing process. The XRD study showed that sodium chloride can be e$ciently removed during the washing with deionized water. These ultra"ne particles had a fairly uniform structure and a mean particle size of 10 nm from the TEM measurement. The ultra"ne powder possessed good soft magnetic properties and superparamagnetic behavior. 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 75.50.Gg; 75.50.Tt; 75.60.Ej Keywords: Mechanical alloying; Ferrite; Ultra"ne particles; MoK ssbauer spectroscopy; Magnetic materials
1. Introduction The synthesis of ultra"ne magnetic particles is intensively investigated in recent years because of their potential applications in high-density magnetic recording and magnetic #uids [1,2]. Various preparation techniques, such as sol}gel pyrolysis method [3], hydrothermal technique [4] and mechanical alloying [5}7], are used to produce ferrite nanoparticles. As it is well known, co-precipitation is an economical way in the production of ultra"ne powders [8,9]. However, coarse particles may form after the dehydration process due * Corresponding author. Tel.: #65-874-7899; fax: #65-7763604. E-mail address: [email protected] (Y. Shi)
to agglomeration. In this paper, we describe the synthesis of nickel ferrite by the combination of co-precipitation and mechanical alloying and subsequent heat treatment, and outline the magnetic properties of the specimens. Experiments were conducted to determine the optimum milling conditions in order to obtain uniform ferrite nanoparticles with good soft magnetic properties. 2. Experimental procedure Nickel ferrite powder was prepared through two steps: (1) co-precipitation processing route, and (2) mechanical alloying of the co-precipitation precursor. The starting materials used in this study were FeCl powder (purity 99%), NiCl powder (purity 96%), NaOH pellet.
0304-8853/99/$ - see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 5 0 4 - 1
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Y. Shi et al. / Journal of Magnetism and Magnetic Materials 205 (1999) 249}254
In the co-precipitation processing route, the solution of metallic salts containing Ni> and Fe> and ions in the ratio of (1.1 : 2), which is close to the ionic ratio of 1 : 2 for the ferrite phase NiFe O . The small excess of Ni was thought to balance the Ni loss because of the certain solubility of nickel ions in water during processing. The mixtures of nickel hydroxide and iron hydroxide precursor were formed when the sodium hydroxide solution was added. In the mechanical alloying process, the precursor was dried in a freeze-dryer. The mixture of the precursor powder and sodium chloride powder was milled for various times in a Fritsch Planetary miller, using a hardened steel vial and several grinding balls. The 15 mm diameter steel balls were used, and the ball-to-powder mass charge ratio was 13 : 1. The weight ratio of precursor and sodium chloride was 1 : 6. The mixture was milled at 200 rpm for 30 h and then further milled at 300 rpm for 30 and 33 h, respectively. The as-milled powders and powders after heat treatment were washed with deionized water for several times. After removal of sodium chloride by washing, nickel ferrite nanoparticles could be obtained. The structure was measured using X-ray di!raction (Philips PW 1820 di!ractometer with Cu K a radiation). The microstructure and particle sizes were studied with a transmission electron microscope (JEM 100 CX II TEM). MoK ssbauer studies of nano-size nickel ferrite particles with the purpose of investigating magnetic microstructure with Ranger Scienti"c Instruments (MS-1200). Magnetic measurements were carried out using a superconducting vibration sample magnetometer (Oxford Instruments) at room temperature. The saturation magnetization was measured at the maximum "eld of 90 kOe.
3. Results and discussion 3.1. Structural studies by X-ray diwraction technique X-ray di!raction spectra for the powder at di!erent processing stages are shown in Fig. 1. Except for the broadened humps, no clear di!raction peaks of crystalline phases were observed in the XRD pattern of the hydroxide precursor. In order to study the nickel ferrite formation process during
the heat treatment, the co-precipitated precursor was annealed at di!erent temperatures for 2 h in air. The precursor exhibited paramagnetism and the magnetization measured at 90 kOe was 0.6 emu/g, showing no presence of the ferrimagnetic nickel ferrite phase. The magnetization of annealed samples increased with the annealing temperature from 0.6 emu/g for the precursor to 10 emu/g (nearly 20% theoretical magnetization for nickel ferrite) after annealing at 4003C, indicating the partial formation of nickel ferrite phase. Magnetization further increased to 45 emu/g after annealing at 11003C. After annealing at 13003C, the magnetization of the annealed sample is very close to that reported previously for nickel ferrite in the bulk form [10]. This result indicates that the initialization of the formation of the nickel ferrite phase starts at approximately 4003C and the completion of the formation requires a heat at 11003C or higher temperature. This also has been con"rmed by the X-ray di!raction study on annealing samples. After milling with sodium chloride at 300 rpm for 33 h and removing of sodium chloride by washing, the di!raction diagram indicates that the NiFe O powder was formed and no other phases are detectable. The calculated lattice parameters of the samples (the as-milled and the subsequently annealed powders) are consistent with that of the pure NiFe O phase. The powder annealed at 6503C has sharp di!raction peaks. The sharpness of the major peaks and di!raction line narrowing of the annealed sample is probably due to a better ordered structure or the increase of grain size. 3.2. Microstructural studies by TEM technique All the samples were washed with deionized water before being measured by the transmission electron microscopy. TEM micrographs are shown in Fig. 2. The precursor displays a relatively broad particle size distribution in the range 5}25 nm, as shown in Fig. 2a. It can also be seen that the particle size of the nickel ferrite that formed directly by annealing the precursor powder at 6503C for 2 h without mechanical alloying grows up to around 50 nm. The nickel ferrite particles formed directly during the mechanical milling have a uniform structure and the mean particle size is around
Y. Shi et al. / Journal of Magnetism and Magnetic Materials 205 (1999) 249}254
251
Fig. 1. XRD patterns of the precursor, the powder after mechanical milling and after subsequent heat treatment at 6503C.
10 nm, as shown in Fig. 2c. Fig. 2d is the micrograph of the ferrite particles, which are annealed at 6503C before the removal of sodium chloride. It can be seen there is no signi"cant di!erence in the particle size between ferrite particles in Figs. 2c and d. This result shows that the ferrite particles were formed in the sodium chloride matrix and the particle size growth can be avoided during the subsequent heat treatment because of the separation by sodium chloride. 3.3. Magnetic studies by VSM technique The formation of the nickel ferrite phase during mechanical milling is demonstrated in Fig. 3. The co-precipitated precursor exhibits paramagnetic,
indicating that the hydroxide precursor is paramagnetic at room temperature. After milling for 30 h at a relatively lower milling energy (200 rpm), the low magnetization implies that just a small amount of the ferrimagnetic nickel ferrite is formed. The magnetization signi"cantly increases after milling at 300 rpm for 30 h. The magnetization after milling at 300 rpm for 33 h is 6.9 emu/g. Considering the present sodium chloride, the saturation magnetization of the ferrimagnetic nickel ferrite is approximately 49 emu/g, which is close to the saturation magnetization for bulk nickel ferrite (50 emu/g [10]). This result indicates that nearly all the hydroxide precursor is converted into nickel ferrite. As shown in Fig. 4, the magnetization of the washed sample is 45 emu/g in an applied "eld of
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Fig. 2. Transmission electron micrographs of powders: (a) precursor, (b) after annealing at 6503C for 2 h, (c) after mechanical milling, and (d) after mechanical milling, and annealing at 6503C for 2 h.
Fig. 3. Hysteresis loops of samples at di!erent process stages: (a) precursor, (b) after milling for 300 h at 200 rpm, (c) after milling for 30 h at 300 rpm, and (d) after milling for 33 h at 300 rpm.
Fig. 4. Hysteresis loops of washed and unwashed Ni-ferrite samples annealed at 6503C for 2 h.
Y. Shi et al. / Journal of Magnetism and Magnetic Materials 205 (1999) 249}254
Fig. 5. Saturation magnetization, M , measured at a "eld of 90 kOe as a function of the annealing temperature for samples after milling at 300 rpm for 33 h.
90 kOe, and obviously is still not saturated, possibly due to the result of superparamagnetism. As shown in Fig. 4, magnetization cannot be saturated with the maximum "eld of 9 T. The magnetization curves can be well described using the Langevin function [11], indicating a superparamagnetic behavior at room temperature. To con"rm superparamangetism, low-temperature measurement was done using an Fe-MoK ssbauer spectrometer. MoK ssbauer spectrum taken at room temperature showed a superparamagnetic doublet, while lowtemperature MoK ssbauer showed ferromagnetic sextets. After the extrapolation of the magnetization versus reversal "eld [11], the saturation magnetization was estimated to be 49 emu/g, which also supports the conclusion that nearly all hydroxide precursor is converted into nickel ferrite. The ferrite powder milled at 300 rpm for 33 h is annealed at di!erent temperature for 2 h. Fig. 5 is the saturation magnetization of the samples annealed at di!erent temperatures up to 7003C. The magnetization increases slightly from 6.9 to 7.1 emu/g after annealing at 1503C. At higher temperatures, the magnetization decreases continuously to 6.3 emu/g after annealing at 500}6003C. This decrease may be associated with the structural imperfections in the as-milled state. As shown in the XRD patterns (Fig. 1), the as-milled powder may have a disordered ferrite structure, since the asmilled powder has broadened di!raction lines and
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there is no signi"cant change in the particle size between the as-milled and the annealed particles (see Fig. 2). This decrease in magnetization is possibly attributed to a shift in the nickel concentration during the ferrite phase. The pure iron oxide Fe O possesses a saturation magnetization of 90 emu/g, while the nickel ferrite NiFe O has a magneti zation of 50 emu/g. It is possible that the ferrite phase formed directly during the mechanical milling has a lower nickel concentration. It is shown in Fig. 4, that a six-fold increase in the saturation magnetization for the sample after washing can be observed and compared with that of the unwashed sample. This result was expected because the starting material was mixed with six (weight) times sodium chloride. As shown in the XRD pattern of the sample, no trace of sodium chloride is evident, indicating that the simple washing process can e$ciently remove sodium chloride. It is further con"rmed by the VSM result, that the saturation magnetization is near the magnetization for the bulk nickel ferrite. It can be seen that the washed nickel ferrite particles have superparamagnetic behavior and low coercivity.
4. Conclusions Nickel ferrite ultra"ne particles have been produced by the co-precipitation from Fe> and Ni> solutions followed by mechanical alloying. In this paper, we have presented a new technique for the synthesis of nickel ferrite nanoparticles through which nickel ferrite phase can be formed directly during the ball milling. The formation of NiFe O was con"rmed by the X-ray di!raction. Sodium chloride was added in order to avoid agglomeration and therefore achieve ultra"ne particles. The TEM study con"rmed that the nickel ferrite particles prepared by this way have a fairly uniform structure and a mean particle size of around 10 nm. The magnetic properties were measured using a VSM at room temperature. Superparamagnetism and low coercivity were observed for the ultra"ne nickel ferrite particles. This work shows that high-quality ultra"ne nickel ferrite particles can be produced by the
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combination of co-precipitation and mechanical milling followed by a simple washing process. This soft-magnetic nanopowder is interesting in applications, such as magnetic coatings and ferro-#uids. References [1] M. Ozaki, MRS Bull. 12 (1989) 35. [2] V.E. Fertman, Magnetic Fluids Guidebook: Properties and Application, Hemisphere, New York, 1990. [3] Jae-Gwang Lee, Hi Minlee, Chul Sung Kim, J. Magn. Magn. Mater. 177}181 (1998) 900.
[4] K.J. Davis, K.O. Grady, S. Morup, J. Magn. Magn. Mater. 149 (1995) 14. [5] J. Ding, W.M. Miao, P.G. McCormick, R. Street, Appl. Phys. Lett. 67 (1995) 3804. [6] J. Ding, P.G. McCormick, R. Street, J. Magn. Magn. Mater. 171 (1997) 309. [7] V. Sepelak, A. Buchal, K.D. Becker, Mater. Sci. Forum. 278}281 (1998) 862. [8] W. Roos, J. Am. Ceram. Soc. 63 (1980) 601. [9] S.E. Jacobo, M.A. Blesa, J. Mater. Sci. 32 (1997) 1025. [10] R.S. Tebble, D.J. Craik, Magnetic materials, Elsevier, Amsterdam, 1969, p. 271. [11] S. Chikazumi, S.H. Charap, Physics of Magnetism, Krieger, New York, Florida, 1964.
NiFe O ultra"ne particles prepared by co-precipitation/mechanical alloying Y. Shi*, J. Ding, X. Liu, J. Wang Department of Materials Science, National University of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore Received 5 April 1999; received in revised form 22 June 1999
Abstract Nickel ferrite ultra"ne particles have been prepared by the combination of co-precipitation and mechanical alloying. Sodium chloride was added in order to avoid agglomeration. This work has shown that nickel ferrite phase can be formed directly during mechanical milling of hydroxide precursor. Ultra"ne NiFe O particles were obtained after a simple washing process. The XRD study showed that sodium chloride can be e$ciently removed during the washing with deionized water. These ultra"ne particles had a fairly uniform structure and a mean particle size of 10 nm from the TEM measurement. The ultra"ne powder possessed good soft magnetic properties and superparamagnetic behavior. 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 75.50.Gg; 75.50.Tt; 75.60.Ej Keywords: Mechanical alloying; Ferrite; Ultra"ne particles; MoK ssbauer spectroscopy; Magnetic materials
1. Introduction The synthesis of ultra"ne magnetic particles is intensively investigated in recent years because of their potential applications in high-density magnetic recording and magnetic #uids [1,2]. Various preparation techniques, such as sol}gel pyrolysis method [3], hydrothermal technique [4] and mechanical alloying [5}7], are used to produce ferrite nanoparticles. As it is well known, co-precipitation is an economical way in the production of ultra"ne powders [8,9]. However, coarse particles may form after the dehydration process due * Corresponding author. Tel.: #65-874-7899; fax: #65-7763604. E-mail address: [email protected] (Y. Shi)
to agglomeration. In this paper, we describe the synthesis of nickel ferrite by the combination of co-precipitation and mechanical alloying and subsequent heat treatment, and outline the magnetic properties of the specimens. Experiments were conducted to determine the optimum milling conditions in order to obtain uniform ferrite nanoparticles with good soft magnetic properties. 2. Experimental procedure Nickel ferrite powder was prepared through two steps: (1) co-precipitation processing route, and (2) mechanical alloying of the co-precipitation precursor. The starting materials used in this study were FeCl powder (purity 99%), NiCl powder (purity 96%), NaOH pellet.
0304-8853/99/$ - see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 5 0 4 - 1
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Y. Shi et al. / Journal of Magnetism and Magnetic Materials 205 (1999) 249}254
In the co-precipitation processing route, the solution of metallic salts containing Ni> and Fe> and ions in the ratio of (1.1 : 2), which is close to the ionic ratio of 1 : 2 for the ferrite phase NiFe O . The small excess of Ni was thought to balance the Ni loss because of the certain solubility of nickel ions in water during processing. The mixtures of nickel hydroxide and iron hydroxide precursor were formed when the sodium hydroxide solution was added. In the mechanical alloying process, the precursor was dried in a freeze-dryer. The mixture of the precursor powder and sodium chloride powder was milled for various times in a Fritsch Planetary miller, using a hardened steel vial and several grinding balls. The 15 mm diameter steel balls were used, and the ball-to-powder mass charge ratio was 13 : 1. The weight ratio of precursor and sodium chloride was 1 : 6. The mixture was milled at 200 rpm for 30 h and then further milled at 300 rpm for 30 and 33 h, respectively. The as-milled powders and powders after heat treatment were washed with deionized water for several times. After removal of sodium chloride by washing, nickel ferrite nanoparticles could be obtained. The structure was measured using X-ray di!raction (Philips PW 1820 di!ractometer with Cu K a radiation). The microstructure and particle sizes were studied with a transmission electron microscope (JEM 100 CX II TEM). MoK ssbauer studies of nano-size nickel ferrite particles with the purpose of investigating magnetic microstructure with Ranger Scienti"c Instruments (MS-1200). Magnetic measurements were carried out using a superconducting vibration sample magnetometer (Oxford Instruments) at room temperature. The saturation magnetization was measured at the maximum "eld of 90 kOe.
3. Results and discussion 3.1. Structural studies by X-ray diwraction technique X-ray di!raction spectra for the powder at di!erent processing stages are shown in Fig. 1. Except for the broadened humps, no clear di!raction peaks of crystalline phases were observed in the XRD pattern of the hydroxide precursor. In order to study the nickel ferrite formation process during
the heat treatment, the co-precipitated precursor was annealed at di!erent temperatures for 2 h in air. The precursor exhibited paramagnetism and the magnetization measured at 90 kOe was 0.6 emu/g, showing no presence of the ferrimagnetic nickel ferrite phase. The magnetization of annealed samples increased with the annealing temperature from 0.6 emu/g for the precursor to 10 emu/g (nearly 20% theoretical magnetization for nickel ferrite) after annealing at 4003C, indicating the partial formation of nickel ferrite phase. Magnetization further increased to 45 emu/g after annealing at 11003C. After annealing at 13003C, the magnetization of the annealed sample is very close to that reported previously for nickel ferrite in the bulk form [10]. This result indicates that the initialization of the formation of the nickel ferrite phase starts at approximately 4003C and the completion of the formation requires a heat at 11003C or higher temperature. This also has been con"rmed by the X-ray di!raction study on annealing samples. After milling with sodium chloride at 300 rpm for 33 h and removing of sodium chloride by washing, the di!raction diagram indicates that the NiFe O powder was formed and no other phases are detectable. The calculated lattice parameters of the samples (the as-milled and the subsequently annealed powders) are consistent with that of the pure NiFe O phase. The powder annealed at 6503C has sharp di!raction peaks. The sharpness of the major peaks and di!raction line narrowing of the annealed sample is probably due to a better ordered structure or the increase of grain size. 3.2. Microstructural studies by TEM technique All the samples were washed with deionized water before being measured by the transmission electron microscopy. TEM micrographs are shown in Fig. 2. The precursor displays a relatively broad particle size distribution in the range 5}25 nm, as shown in Fig. 2a. It can also be seen that the particle size of the nickel ferrite that formed directly by annealing the precursor powder at 6503C for 2 h without mechanical alloying grows up to around 50 nm. The nickel ferrite particles formed directly during the mechanical milling have a uniform structure and the mean particle size is around
Y. Shi et al. / Journal of Magnetism and Magnetic Materials 205 (1999) 249}254
251
Fig. 1. XRD patterns of the precursor, the powder after mechanical milling and after subsequent heat treatment at 6503C.
10 nm, as shown in Fig. 2c. Fig. 2d is the micrograph of the ferrite particles, which are annealed at 6503C before the removal of sodium chloride. It can be seen there is no signi"cant di!erence in the particle size between ferrite particles in Figs. 2c and d. This result shows that the ferrite particles were formed in the sodium chloride matrix and the particle size growth can be avoided during the subsequent heat treatment because of the separation by sodium chloride. 3.3. Magnetic studies by VSM technique The formation of the nickel ferrite phase during mechanical milling is demonstrated in Fig. 3. The co-precipitated precursor exhibits paramagnetic,
indicating that the hydroxide precursor is paramagnetic at room temperature. After milling for 30 h at a relatively lower milling energy (200 rpm), the low magnetization implies that just a small amount of the ferrimagnetic nickel ferrite is formed. The magnetization signi"cantly increases after milling at 300 rpm for 30 h. The magnetization after milling at 300 rpm for 33 h is 6.9 emu/g. Considering the present sodium chloride, the saturation magnetization of the ferrimagnetic nickel ferrite is approximately 49 emu/g, which is close to the saturation magnetization for bulk nickel ferrite (50 emu/g [10]). This result indicates that nearly all the hydroxide precursor is converted into nickel ferrite. As shown in Fig. 4, the magnetization of the washed sample is 45 emu/g in an applied "eld of
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Fig. 2. Transmission electron micrographs of powders: (a) precursor, (b) after annealing at 6503C for 2 h, (c) after mechanical milling, and (d) after mechanical milling, and annealing at 6503C for 2 h.
Fig. 3. Hysteresis loops of samples at di!erent process stages: (a) precursor, (b) after milling for 300 h at 200 rpm, (c) after milling for 30 h at 300 rpm, and (d) after milling for 33 h at 300 rpm.
Fig. 4. Hysteresis loops of washed and unwashed Ni-ferrite samples annealed at 6503C for 2 h.
Y. Shi et al. / Journal of Magnetism and Magnetic Materials 205 (1999) 249}254
Fig. 5. Saturation magnetization, M , measured at a "eld of 90 kOe as a function of the annealing temperature for samples after milling at 300 rpm for 33 h.
90 kOe, and obviously is still not saturated, possibly due to the result of superparamagnetism. As shown in Fig. 4, magnetization cannot be saturated with the maximum "eld of 9 T. The magnetization curves can be well described using the Langevin function [11], indicating a superparamagnetic behavior at room temperature. To con"rm superparamangetism, low-temperature measurement was done using an Fe-MoK ssbauer spectrometer. MoK ssbauer spectrum taken at room temperature showed a superparamagnetic doublet, while lowtemperature MoK ssbauer showed ferromagnetic sextets. After the extrapolation of the magnetization versus reversal "eld [11], the saturation magnetization was estimated to be 49 emu/g, which also supports the conclusion that nearly all hydroxide precursor is converted into nickel ferrite. The ferrite powder milled at 300 rpm for 33 h is annealed at di!erent temperature for 2 h. Fig. 5 is the saturation magnetization of the samples annealed at di!erent temperatures up to 7003C. The magnetization increases slightly from 6.9 to 7.1 emu/g after annealing at 1503C. At higher temperatures, the magnetization decreases continuously to 6.3 emu/g after annealing at 500}6003C. This decrease may be associated with the structural imperfections in the as-milled state. As shown in the XRD patterns (Fig. 1), the as-milled powder may have a disordered ferrite structure, since the asmilled powder has broadened di!raction lines and
253
there is no signi"cant change in the particle size between the as-milled and the annealed particles (see Fig. 2). This decrease in magnetization is possibly attributed to a shift in the nickel concentration during the ferrite phase. The pure iron oxide Fe O possesses a saturation magnetization of 90 emu/g, while the nickel ferrite NiFe O has a magneti zation of 50 emu/g. It is possible that the ferrite phase formed directly during the mechanical milling has a lower nickel concentration. It is shown in Fig. 4, that a six-fold increase in the saturation magnetization for the sample after washing can be observed and compared with that of the unwashed sample. This result was expected because the starting material was mixed with six (weight) times sodium chloride. As shown in the XRD pattern of the sample, no trace of sodium chloride is evident, indicating that the simple washing process can e$ciently remove sodium chloride. It is further con"rmed by the VSM result, that the saturation magnetization is near the magnetization for the bulk nickel ferrite. It can be seen that the washed nickel ferrite particles have superparamagnetic behavior and low coercivity.
4. Conclusions Nickel ferrite ultra"ne particles have been produced by the co-precipitation from Fe> and Ni> solutions followed by mechanical alloying. In this paper, we have presented a new technique for the synthesis of nickel ferrite nanoparticles through which nickel ferrite phase can be formed directly during the ball milling. The formation of NiFe O was con"rmed by the X-ray di!raction. Sodium chloride was added in order to avoid agglomeration and therefore achieve ultra"ne particles. The TEM study con"rmed that the nickel ferrite particles prepared by this way have a fairly uniform structure and a mean particle size of around 10 nm. The magnetic properties were measured using a VSM at room temperature. Superparamagnetism and low coercivity were observed for the ultra"ne nickel ferrite particles. This work shows that high-quality ultra"ne nickel ferrite particles can be produced by the
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combination of co-precipitation and mechanical milling followed by a simple washing process. This soft-magnetic nanopowder is interesting in applications, such as magnetic coatings and ferro-#uids. References [1] M. Ozaki, MRS Bull. 12 (1989) 35. [2] V.E. Fertman, Magnetic Fluids Guidebook: Properties and Application, Hemisphere, New York, 1990. [3] Jae-Gwang Lee, Hi Minlee, Chul Sung Kim, J. Magn. Magn. Mater. 177}181 (1998) 900.
[4] K.J. Davis, K.O. Grady, S. Morup, J. Magn. Magn. Mater. 149 (1995) 14. [5] J. Ding, W.M. Miao, P.G. McCormick, R. Street, Appl. Phys. Lett. 67 (1995) 3804. [6] J. Ding, P.G. McCormick, R. Street, J. Magn. Magn. Mater. 171 (1997) 309. [7] V. Sepelak, A. Buchal, K.D. Becker, Mater. Sci. Forum. 278}281 (1998) 862. [8] W. Roos, J. Am. Ceram. Soc. 63 (1980) 601. [9] S.E. Jacobo, M.A. Blesa, J. Mater. Sci. 32 (1997) 1025. [10] R.S. Tebble, D.J. Craik, Magnetic materials, Elsevier, Amsterdam, 1969, p. 271. [11] S. Chikazumi, S.H. Charap, Physics of Magnetism, Krieger, New York, Florida, 1964.