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NiCuZn ferrite thin films grown by a sol–gel method and rapid thermal annealing
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 309 (2007) 75–79 www.elsevier.com/locate/jmmm
NiCuZn ferrite thin films grown by a sol–gel method and rapid thermal annealing Feng Liua,, Chen Yanga, Tianling Rena, A.Z. Wangb, Jun Yuc, Litian Liua a
Institute of Microelectronics, Tsinghua University, Beijing 100084, PR China Integrated Electronics Laboratory, Department of ECE, Illinois Institute of Technology, USA c Institute of Microelectronics, Huazhong University of Science and Technology, Wuhan 430074, PR China b
Received 24 May 2005; received in revised form 25 April 2006 Available online 10 July 2006
Abstract Ni0.8xCu0.2ZnxFe2O4 thin films were fabricated by sol–gel method and rapid thermal annealing (RTA). The X-diffraction and atomic force microscopy (AFM) measurements indicate that the films have a single-phase spinel structure with calcining temperature TX600 1C. The saturation magnetization Ms and coercivity Hc of the films as a function of the film composition were investigated by alternating gradient magnetometer (AGM). It has been found that the Ms increases firstly and then decreases with the increasing of Zn content. The Ms reaches the maximum value about 271.56 emu/cm3 for x ¼ 0.45, meanwhile the Hc reaches the minimum value about 15.62 Oe for x ¼ 0.4. The processing parameters were optimized, which includes the coating concentration, the annealing temperature and time for the films. The results indicate that the coating concentration of 0.4 mol/L, crystallization at 600 1C and annealing for 5 min are suitable for NiCuZn ferrite thin film. r 2006 Elsevier B.V. All rights reserved. Keywords: Ni0.8xCu0.2ZnxFe2O4 thin films; Sol–gel method; Rapid thermal annealing
1. Introduction NiCuZn ferrites are excellent soft magnetic materials in high-frequency devices due to their low cost, high resistivity and low eddy current losses, which have been studied extensively for multiplayer chip inductor applications [1–3]. But up to now, the applications of NiCuZn ferrite thin films in high-frequency integrated circuits have been reported scarcely. The successful growth of magnetic thin films is an important step towards their future incorporation as inductors and transformers into integrated circuits operating at high frequency. Attempts have been made by researchers to deposit NiZn ferrite films by a variety of techniques including pulsed laser deposition (PLD) [4], RF magnetron sputtering [5,6], spin-spray ferrite plating [7], alternative sputtering [8] and sol–gel method [9,10]. Compared to other methods of preparation, sol–gel method offers excellent composition control, low temperature processing and short Corresponding author. Tel.: +86 10 6278 9151; fax: +86 10 6277 1130.
E-mail address: [email protected] (F. Liu). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.06.014
fabrication times at comparatively low cost. However, the fabrication of NiCuZn ferrite thin films using sol–gel method has not been studied much. While, rapid thermal annealing (RTA) is popular in semiconductor processes, which not only takes short pre-heating and annealing time for crystallization but also reduces the interference between the film and substrates [11]. It has been found that using sol–gel method in conjunction with RTP is useful to achieve the low-temperature fabrication of thin films, which is an efficient way to fabricate integrated thin-film devices. In this paper, Ni0.8xCu0.2ZnxFe2O4 thin films were fabricated using sol–gel method and RTA. Their structural properties and the effects of the film composition, the crystallization temperature, the calcination time and the density on magnetic properties of the NiCuZn thin films were investigated. 2. Experimental The analytical grade iron nitrate, nickel nitrate, dimethylformamide, zinc acetate and copper nitrate were
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F. Liu et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 75–79
used as raw materials. According to the stoichiometric composition of Ni0.8xCu0.2ZnxFe2O4, the specified weights of metal nitrate were first fully dissolved in dimethylformamide to form a mixed solution. After the stirring of the solution for 1 h, acetic acid was added to adjust the concentration of the solution to 0.3, 0.4 and 0.5 mol/L. Meanwhile polyvinglpyrrolidone (PVP, C.R., molecular weight 30,000, Germany) was added as a kind of surfactant, which can keep the colloidal particles of chelate from combining with each other. The as-prepared solution was then continuously stirred for 2 h and placed for 36 h at room temperature to form the stable sol–gel precursors that used for following process. In order to integrate these magnetic thin films into the Si integrated circuit, the thin films were spin coated on (1 0 0)oriented n-type Si substrates (900–1000 O cm) with 2000 A˚ SiO2 layer formed by wet thermal oxidization. The speed and time of spin were controlled to get a uniform thin film. The obtained films were dried at 120 1C for 10 min to remove the mixed solvents, and then heated at 400 1C for 30 min to pyrolyze and exclude the organic substances. The cycles of the operation of spin coating, drying and heating were repeated to reach the required thickness of the films. Finally, RTA process was used to crystallize the thin film in oxygen. Annealing temperature was from 400 to 700 1C with an increasing speed of 150 1C/s. The annealing time was about 1–10 min and then the wafers were cooled quickly. The phase identification of the thin films was performed using X-ray diffraction (XRD) with CuKa radiation. The structural properties of the films were characterized by atomic force microscopy (AFM) and scanning electron microscope (SEM). The quasi-static magnetic properties of the thin films were measured by alternating gradient magnetometer (AGM) at room temperature.
Fig. 1. AFM photo for the film calcined at 650 1C.
3. Results and discussion The structural properties of the films were characterized by AFM and SEM firstly. Fig. 1 shows the surface morphology of the Ni0.4Cu0.2Zn0.4Fe2O4 film calcined at 650 1C. It has been found that the film is composed of ballshaped grains with the average size of 30 nm and its root mean square roughness is about 10 nm. No cracks and other defects can be found in the AFM image. Meantime, the thickness of the films was estimated by the cross-sectional SEM image of the samples. The ferrite film thickness was estimated to 4000 A˚ for the sample that is composed of Si substrate, SiO2 layer and Ni0.4Cu0.2Zn0.4Fe2O4 layer calcined at 600 1C. The SEM results indicate that the effect of crystallization temperature and composition of the film on the thickness of the sample is negligible. Fig. 2 shows the XRD patterns of the Ni0.8xCu0.2Znx Fe2O4 (0.25pxp0.5) thin films that come from the 0.4 mol/L coating solution and annealed in oxygen at 600 1C for 5 min. For all the films, the single-phase spinel structure is obtained and the main peak is (3 1 1) peak,
Fig. 2. XRD patterns of Ni0.8xCu0.2ZnxFe2O4 films with different composition annealed in oxygen for 5 min at 600 1C.
which is in agreement with Ref. [8]. The peak about 331 is associated with the Si substrates. As can be seen from the XRD pattern the phase structure of the films do not change obviously with the increase of Zn concentration. Compared with the standard NiCuZn ferrite, the main peak (3 1 1) is more prominent, which is relative to the Si substrates. In additional, it can be seen that the diffraction peaks shift to large angles. It can be interpreted that the radius of Cu2+ is smaller than that of Zn2+, which causes the effective lattice constant decrease. MicroMag 2900 AGM was used to test the hysteresis loop and coercivity of the thin films at room temperature. Fig. 3 is the composition dependence of the saturation magnetization Ms and coercivity Hc for Ni0.8xCu0.2Znx
ARTICLE IN PRESS F. Liu et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 75–79
Fe2O4 (0.25pxp0.5) films calcined at 600 1C. It can be found that the saturation magnetization Ms of the films increases firstly and the decreases with the increasing Zn content. For x ¼ 0:45 sample, the Ms reaches the maximum value at about 271 emu/cm3, has the same trend with that of the bulk materials. While, the Hc reaches the minimum value about 15.62 Oe for x ¼ 0:4. The Hc of the thin films is greater than the bulk material, which is caused by the smaller granular size than the bulk material. XRD patterns of the Ni0.4Cu0.2Zn0.4Fe2O4 thin films calcined at different temperature are shown in Fig. 4. It indicates that the thin film begins to crystallize at about 500 1C and is well-crystallized at a temperature above 650 1C, which is much lower than the required temperature in traditional ceramic method. From 400 to 600 1C, diffraction peaks from intermediate phase are found in the X-ray pattern at 2y ¼ 29.21, which might be the formation of (Ni, Fe)Fe2O4. The intermediate phase disappears when annealing temperature reaches 600 1C. The maximum value of crystall sizes is 18.7 nm calculated by Scherrer equation for the sample calcined at 600 1C. Therefore, the XRD result confirms that sol–gel method can reduce the crystallization temperature effectively. Fig. 5 is the temperature dependence of the saturation magnetization Ms and coercivity Hc for the Ni0.4Cu0.2Zn0.4 Fe2O4 thin films that come from the 0.4 mol/L sol–gel solution. As it can be seen in the figure, the Ms of the sample fired at 400 1C has a certain value about 80 emu/ cm3, which was analyzed to be related to the intermediate phase of Fe3O4. During 400–450 1C, Ms and Hc decrease with the increase in annealing temperature, which is relative to the degradation of the intermediate phase and the transform of part of Fe3+ into Fe2+ ions. When the annealing temperature reaches 500 1C, though the intermediate phase continues to degrade, the spinel structure of NiCuZn ferrite begins to form, which induces the increase of the Ms and Hc. Subsequently, the Ms increases drastically with the increasing annealing temperature and
Fig. 3. Composition dependence of Ms and Hc for Ni0.8xCu0.2ZnxFe2O4 films calcined at 600 1C.
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Fig. 4. XRD patterns of the Ni0.4Cu0.2Zn0.4Fe2O4 films calcined at different temperature.
Fig. 5. Temperature dependence of Ms and Hc for the Ni0.4Cu0.2Zn0.4 Fe2O4 thin films
the tendency of increase becomes slightly at a temperature above 600 1C, which is relative to the degree of the crystallization. The maximum of Ms is 235.654 emu/cm3, less than the bulk material, which indicates that the density of the thin-film fabricated by sol–gel method is hard to be identical with the bulk material. This figure also shows that the coercivity Hc increases with the increase of temperature and grain size, which is not consistent with the tendency that Hc decreases with the increase in grain size. It can be supposed to be caused by the factors as follows. For the thin film, the thermal stress is the main factor influences the Hc. When temperature increases, the thermal stress increase correspondingly, leading to the Hc increasing. On the other hand, the grain size of the samples is 10–40 nm and single-domain nanocrystalline may cause the increase of the Hc. Fig. 6 is the annealing time dependence of the saturation magnetization Ms and coercivity Hc for the Ni0.4Cu0.2Zn0.4
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Fig. 6. Annealing time dependence of Ms and Hc of the samples.
Fe2O4 films that come from the 0.3 mol/L sol–gel solution. As seen in the figure, it can be found that the Ms and Hc of the films increase with the increasing annealing time. This tendency can be due to the increasing size of crystal grains and the crystallization of the samples is more perfect with the annealing time increasing, which leads to higher saturation magnetization Ms and coercivity Hc. The concentration of sol–gel solution is another important parameter relevant to the magnetic properties of the Ni0.4Cu0.2Zn0.4Fe2O4 thin films. Fig. 7 shows the effect of the mol concentration of sol–gel solution on the Ms and Hc of the thin films. In fact, the concentration of solution affects directly the calcination density of the samples. As the concentration of the solution increases, the density of the samples increases, which causes the increase of Ms. Meanwhile, the coercivity Hc of the films decreases firstly and then increases, which is relative to calcination density of the films too. Under a certain value, the lower the concentration of the solution is, the higher the rate of air space for the films is. Therefore, the increase of Hc due to the air space is the main factor. Moreover, the density and thickness of the films increase with the increasing concentration of the solution, which brings up the increasing thermal stress. When the concentration of the solution is above a certain value, the thermal stress becomes prominent effect on the Hc. It makes the Hc increases with the increasing concentration of the solution. Therefore there is an optimal value of the concentration for the solution to obtain the smallest Hc. From our results, the suitable value is 0.4 mol/L that corresponds to Hc ¼ 15.62 Oe. The magnetic hysteresis loop of the Ni0.4Cu0.2Zn0.4 Fe2O4 thin film calcined at 600 1C for 5 min is shown in Fig. 8. The sample comes from the 0.4 mol/L sol–gel solution. It indicated that the sample fabricated by sol–gel method and RTA process exhibits the typical behavior of soft ferrite, which has Ms ¼ 228.877 emu/cm3 and Hc ¼ 15.62 Oe.
Fig. 7. Ms and Hc of the Ni0.4Cu0.2Zn0.4Fe2O4 thin films dependence on mol concentration of sol–gel solution.
Fig. 8. Magnetic hysteresis loop of the Ni0.4Cu0.2Zn0.4Fe2O4 thin film calcined at 600 1C.
4. Conclusions Ni0.8xCu0.2ZnxFe2O4 (0.25pxp0.5) thin films were fabricated by sol–gel method and RTA process. The analysis of XRD for the films indicates sol–gel method can lower the crystallization temperature of the magnetic thin film effectively. The measured results of quasi-static magnetic properties for the Ni0.8xCu0.2ZnxFe2O4 (0.25p xp0.5) thin films show that the saturation magnetization Ms of the films increases firstly and the decreases. The Ms reaches the maximum value at about 271 emu/cm3 for x ¼ 0:45. Meantime the Hc reaches the minimum value at about 15.62 Oe for x ¼ 0:4. In addition, the experimental results indicate that the coating concentration of 0.4 mol/L, crystallization at 600 1C and annealing for 5 min are suitable for NiCuZn ferrite thin film. The Ni0.4Cu0.2Zn0.4 Fe2O4 sample obtained from above parameters exhibits excellent soft magnetic performance, which has Ms ¼ 228.877 emu/cm3 and Hc ¼ 15.62 Oe.
ARTICLE IN PRESS F. Liu et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 75–79
References [1] T. Nakamura, J. Magn. Magn. Mater. 168 (1997) 285. [2] B. Li, Z.-X. Yue, X.-W. Qi, J. Zhou, Z.-L. Gui, L.-T. Li, Mater. Sci. Eng. B. 99 (2003) 252. [3] C.Y. Tsay, K.S. Liu, T.F. Lin, I.N. Ling, J. Magn. Magn. Mater. 209 (2000) 189. [4] N. Wakiya, K. Shinozaki, N. Mizutani, Appl. Phys. Lett. 85 (2004) 1199.
79
[5] M. Desai, S. Prasad, N. Venketaramani, J. Appl. Phys. 91 (2002) 7592. [6] J.-S. Lee, B.-I. Lee, S.-K. Joo, IEEE Trans. Magn. 35 (1999) 3415. [7] C.M. Fu, H.S. Hsu, Y.C. Chao, N. Matsushita, M. Abe, J. Appl. Phys. 93 (2003) 7127. [8] J. Gao, Y. Cui, Z. Yang, Mater. Sci. Eng. B. 110 (2004) 111. [9] R.E. Vande Leest, F. Roozeboom, Appl. Surf. Sci. 187 (2002) 68. [10] S.-Y. Bae, C.-S. Kim, Y.-J. Oh, J. Appl. Phys. 85 (1999) 5226. [11] S.A. Campbell, The Science and Engineering of Microelectronic Fabrication, Publishing house of electronics industry, Beijing, 2003.
Journal of Magnetism and Magnetic Materials 309 (2007) 75–79 www.elsevier.com/locate/jmmm
NiCuZn ferrite thin films grown by a sol–gel method and rapid thermal annealing Feng Liua,, Chen Yanga, Tianling Rena, A.Z. Wangb, Jun Yuc, Litian Liua a
Institute of Microelectronics, Tsinghua University, Beijing 100084, PR China Integrated Electronics Laboratory, Department of ECE, Illinois Institute of Technology, USA c Institute of Microelectronics, Huazhong University of Science and Technology, Wuhan 430074, PR China b
Received 24 May 2005; received in revised form 25 April 2006 Available online 10 July 2006
Abstract Ni0.8xCu0.2ZnxFe2O4 thin films were fabricated by sol–gel method and rapid thermal annealing (RTA). The X-diffraction and atomic force microscopy (AFM) measurements indicate that the films have a single-phase spinel structure with calcining temperature TX600 1C. The saturation magnetization Ms and coercivity Hc of the films as a function of the film composition were investigated by alternating gradient magnetometer (AGM). It has been found that the Ms increases firstly and then decreases with the increasing of Zn content. The Ms reaches the maximum value about 271.56 emu/cm3 for x ¼ 0.45, meanwhile the Hc reaches the minimum value about 15.62 Oe for x ¼ 0.4. The processing parameters were optimized, which includes the coating concentration, the annealing temperature and time for the films. The results indicate that the coating concentration of 0.4 mol/L, crystallization at 600 1C and annealing for 5 min are suitable for NiCuZn ferrite thin film. r 2006 Elsevier B.V. All rights reserved. Keywords: Ni0.8xCu0.2ZnxFe2O4 thin films; Sol–gel method; Rapid thermal annealing
1. Introduction NiCuZn ferrites are excellent soft magnetic materials in high-frequency devices due to their low cost, high resistivity and low eddy current losses, which have been studied extensively for multiplayer chip inductor applications [1–3]. But up to now, the applications of NiCuZn ferrite thin films in high-frequency integrated circuits have been reported scarcely. The successful growth of magnetic thin films is an important step towards their future incorporation as inductors and transformers into integrated circuits operating at high frequency. Attempts have been made by researchers to deposit NiZn ferrite films by a variety of techniques including pulsed laser deposition (PLD) [4], RF magnetron sputtering [5,6], spin-spray ferrite plating [7], alternative sputtering [8] and sol–gel method [9,10]. Compared to other methods of preparation, sol–gel method offers excellent composition control, low temperature processing and short Corresponding author. Tel.: +86 10 6278 9151; fax: +86 10 6277 1130.
E-mail address: [email protected] (F. Liu). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.06.014
fabrication times at comparatively low cost. However, the fabrication of NiCuZn ferrite thin films using sol–gel method has not been studied much. While, rapid thermal annealing (RTA) is popular in semiconductor processes, which not only takes short pre-heating and annealing time for crystallization but also reduces the interference between the film and substrates [11]. It has been found that using sol–gel method in conjunction with RTP is useful to achieve the low-temperature fabrication of thin films, which is an efficient way to fabricate integrated thin-film devices. In this paper, Ni0.8xCu0.2ZnxFe2O4 thin films were fabricated using sol–gel method and RTA. Their structural properties and the effects of the film composition, the crystallization temperature, the calcination time and the density on magnetic properties of the NiCuZn thin films were investigated. 2. Experimental The analytical grade iron nitrate, nickel nitrate, dimethylformamide, zinc acetate and copper nitrate were
ARTICLE IN PRESS 76
F. Liu et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 75–79
used as raw materials. According to the stoichiometric composition of Ni0.8xCu0.2ZnxFe2O4, the specified weights of metal nitrate were first fully dissolved in dimethylformamide to form a mixed solution. After the stirring of the solution for 1 h, acetic acid was added to adjust the concentration of the solution to 0.3, 0.4 and 0.5 mol/L. Meanwhile polyvinglpyrrolidone (PVP, C.R., molecular weight 30,000, Germany) was added as a kind of surfactant, which can keep the colloidal particles of chelate from combining with each other. The as-prepared solution was then continuously stirred for 2 h and placed for 36 h at room temperature to form the stable sol–gel precursors that used for following process. In order to integrate these magnetic thin films into the Si integrated circuit, the thin films were spin coated on (1 0 0)oriented n-type Si substrates (900–1000 O cm) with 2000 A˚ SiO2 layer formed by wet thermal oxidization. The speed and time of spin were controlled to get a uniform thin film. The obtained films were dried at 120 1C for 10 min to remove the mixed solvents, and then heated at 400 1C for 30 min to pyrolyze and exclude the organic substances. The cycles of the operation of spin coating, drying and heating were repeated to reach the required thickness of the films. Finally, RTA process was used to crystallize the thin film in oxygen. Annealing temperature was from 400 to 700 1C with an increasing speed of 150 1C/s. The annealing time was about 1–10 min and then the wafers were cooled quickly. The phase identification of the thin films was performed using X-ray diffraction (XRD) with CuKa radiation. The structural properties of the films were characterized by atomic force microscopy (AFM) and scanning electron microscope (SEM). The quasi-static magnetic properties of the thin films were measured by alternating gradient magnetometer (AGM) at room temperature.
Fig. 1. AFM photo for the film calcined at 650 1C.
3. Results and discussion The structural properties of the films were characterized by AFM and SEM firstly. Fig. 1 shows the surface morphology of the Ni0.4Cu0.2Zn0.4Fe2O4 film calcined at 650 1C. It has been found that the film is composed of ballshaped grains with the average size of 30 nm and its root mean square roughness is about 10 nm. No cracks and other defects can be found in the AFM image. Meantime, the thickness of the films was estimated by the cross-sectional SEM image of the samples. The ferrite film thickness was estimated to 4000 A˚ for the sample that is composed of Si substrate, SiO2 layer and Ni0.4Cu0.2Zn0.4Fe2O4 layer calcined at 600 1C. The SEM results indicate that the effect of crystallization temperature and composition of the film on the thickness of the sample is negligible. Fig. 2 shows the XRD patterns of the Ni0.8xCu0.2Znx Fe2O4 (0.25pxp0.5) thin films that come from the 0.4 mol/L coating solution and annealed in oxygen at 600 1C for 5 min. For all the films, the single-phase spinel structure is obtained and the main peak is (3 1 1) peak,
Fig. 2. XRD patterns of Ni0.8xCu0.2ZnxFe2O4 films with different composition annealed in oxygen for 5 min at 600 1C.
which is in agreement with Ref. [8]. The peak about 331 is associated with the Si substrates. As can be seen from the XRD pattern the phase structure of the films do not change obviously with the increase of Zn concentration. Compared with the standard NiCuZn ferrite, the main peak (3 1 1) is more prominent, which is relative to the Si substrates. In additional, it can be seen that the diffraction peaks shift to large angles. It can be interpreted that the radius of Cu2+ is smaller than that of Zn2+, which causes the effective lattice constant decrease. MicroMag 2900 AGM was used to test the hysteresis loop and coercivity of the thin films at room temperature. Fig. 3 is the composition dependence of the saturation magnetization Ms and coercivity Hc for Ni0.8xCu0.2Znx
ARTICLE IN PRESS F. Liu et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 75–79
Fe2O4 (0.25pxp0.5) films calcined at 600 1C. It can be found that the saturation magnetization Ms of the films increases firstly and the decreases with the increasing Zn content. For x ¼ 0:45 sample, the Ms reaches the maximum value at about 271 emu/cm3, has the same trend with that of the bulk materials. While, the Hc reaches the minimum value about 15.62 Oe for x ¼ 0:4. The Hc of the thin films is greater than the bulk material, which is caused by the smaller granular size than the bulk material. XRD patterns of the Ni0.4Cu0.2Zn0.4Fe2O4 thin films calcined at different temperature are shown in Fig. 4. It indicates that the thin film begins to crystallize at about 500 1C and is well-crystallized at a temperature above 650 1C, which is much lower than the required temperature in traditional ceramic method. From 400 to 600 1C, diffraction peaks from intermediate phase are found in the X-ray pattern at 2y ¼ 29.21, which might be the formation of (Ni, Fe)Fe2O4. The intermediate phase disappears when annealing temperature reaches 600 1C. The maximum value of crystall sizes is 18.7 nm calculated by Scherrer equation for the sample calcined at 600 1C. Therefore, the XRD result confirms that sol–gel method can reduce the crystallization temperature effectively. Fig. 5 is the temperature dependence of the saturation magnetization Ms and coercivity Hc for the Ni0.4Cu0.2Zn0.4 Fe2O4 thin films that come from the 0.4 mol/L sol–gel solution. As it can be seen in the figure, the Ms of the sample fired at 400 1C has a certain value about 80 emu/ cm3, which was analyzed to be related to the intermediate phase of Fe3O4. During 400–450 1C, Ms and Hc decrease with the increase in annealing temperature, which is relative to the degradation of the intermediate phase and the transform of part of Fe3+ into Fe2+ ions. When the annealing temperature reaches 500 1C, though the intermediate phase continues to degrade, the spinel structure of NiCuZn ferrite begins to form, which induces the increase of the Ms and Hc. Subsequently, the Ms increases drastically with the increasing annealing temperature and
Fig. 3. Composition dependence of Ms and Hc for Ni0.8xCu0.2ZnxFe2O4 films calcined at 600 1C.
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Fig. 4. XRD patterns of the Ni0.4Cu0.2Zn0.4Fe2O4 films calcined at different temperature.
Fig. 5. Temperature dependence of Ms and Hc for the Ni0.4Cu0.2Zn0.4 Fe2O4 thin films
the tendency of increase becomes slightly at a temperature above 600 1C, which is relative to the degree of the crystallization. The maximum of Ms is 235.654 emu/cm3, less than the bulk material, which indicates that the density of the thin-film fabricated by sol–gel method is hard to be identical with the bulk material. This figure also shows that the coercivity Hc increases with the increase of temperature and grain size, which is not consistent with the tendency that Hc decreases with the increase in grain size. It can be supposed to be caused by the factors as follows. For the thin film, the thermal stress is the main factor influences the Hc. When temperature increases, the thermal stress increase correspondingly, leading to the Hc increasing. On the other hand, the grain size of the samples is 10–40 nm and single-domain nanocrystalline may cause the increase of the Hc. Fig. 6 is the annealing time dependence of the saturation magnetization Ms and coercivity Hc for the Ni0.4Cu0.2Zn0.4
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Fig. 6. Annealing time dependence of Ms and Hc of the samples.
Fe2O4 films that come from the 0.3 mol/L sol–gel solution. As seen in the figure, it can be found that the Ms and Hc of the films increase with the increasing annealing time. This tendency can be due to the increasing size of crystal grains and the crystallization of the samples is more perfect with the annealing time increasing, which leads to higher saturation magnetization Ms and coercivity Hc. The concentration of sol–gel solution is another important parameter relevant to the magnetic properties of the Ni0.4Cu0.2Zn0.4Fe2O4 thin films. Fig. 7 shows the effect of the mol concentration of sol–gel solution on the Ms and Hc of the thin films. In fact, the concentration of solution affects directly the calcination density of the samples. As the concentration of the solution increases, the density of the samples increases, which causes the increase of Ms. Meanwhile, the coercivity Hc of the films decreases firstly and then increases, which is relative to calcination density of the films too. Under a certain value, the lower the concentration of the solution is, the higher the rate of air space for the films is. Therefore, the increase of Hc due to the air space is the main factor. Moreover, the density and thickness of the films increase with the increasing concentration of the solution, which brings up the increasing thermal stress. When the concentration of the solution is above a certain value, the thermal stress becomes prominent effect on the Hc. It makes the Hc increases with the increasing concentration of the solution. Therefore there is an optimal value of the concentration for the solution to obtain the smallest Hc. From our results, the suitable value is 0.4 mol/L that corresponds to Hc ¼ 15.62 Oe. The magnetic hysteresis loop of the Ni0.4Cu0.2Zn0.4 Fe2O4 thin film calcined at 600 1C for 5 min is shown in Fig. 8. The sample comes from the 0.4 mol/L sol–gel solution. It indicated that the sample fabricated by sol–gel method and RTA process exhibits the typical behavior of soft ferrite, which has Ms ¼ 228.877 emu/cm3 and Hc ¼ 15.62 Oe.
Fig. 7. Ms and Hc of the Ni0.4Cu0.2Zn0.4Fe2O4 thin films dependence on mol concentration of sol–gel solution.
Fig. 8. Magnetic hysteresis loop of the Ni0.4Cu0.2Zn0.4Fe2O4 thin film calcined at 600 1C.
4. Conclusions Ni0.8xCu0.2ZnxFe2O4 (0.25pxp0.5) thin films were fabricated by sol–gel method and RTA process. The analysis of XRD for the films indicates sol–gel method can lower the crystallization temperature of the magnetic thin film effectively. The measured results of quasi-static magnetic properties for the Ni0.8xCu0.2ZnxFe2O4 (0.25p xp0.5) thin films show that the saturation magnetization Ms of the films increases firstly and the decreases. The Ms reaches the maximum value at about 271 emu/cm3 for x ¼ 0:45. Meantime the Hc reaches the minimum value at about 15.62 Oe for x ¼ 0:4. In addition, the experimental results indicate that the coating concentration of 0.4 mol/L, crystallization at 600 1C and annealing for 5 min are suitable for NiCuZn ferrite thin film. The Ni0.4Cu0.2Zn0.4 Fe2O4 sample obtained from above parameters exhibits excellent soft magnetic performance, which has Ms ¼ 228.877 emu/cm3 and Hc ¼ 15.62 Oe.
ARTICLE IN PRESS F. Liu et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 75–79
References [1] T. Nakamura, J. Magn. Magn. Mater. 168 (1997) 285. [2] B. Li, Z.-X. Yue, X.-W. Qi, J. Zhou, Z.-L. Gui, L.-T. Li, Mater. Sci. Eng. B. 99 (2003) 252. [3] C.Y. Tsay, K.S. Liu, T.F. Lin, I.N. Ling, J. Magn. Magn. Mater. 209 (2000) 189. [4] N. Wakiya, K. Shinozaki, N. Mizutani, Appl. Phys. Lett. 85 (2004) 1199.
79
[5] M. Desai, S. Prasad, N. Venketaramani, J. Appl. Phys. 91 (2002) 7592. [6] J.-S. Lee, B.-I. Lee, S.-K. Joo, IEEE Trans. Magn. 35 (1999) 3415. [7] C.M. Fu, H.S. Hsu, Y.C. Chao, N. Matsushita, M. Abe, J. Appl. Phys. 93 (2003) 7127. [8] J. Gao, Y. Cui, Z. Yang, Mater. Sci. Eng. B. 110 (2004) 111. [9] R.E. Vande Leest, F. Roozeboom, Appl. Surf. Sci. 187 (2002) 68. [10] S.-Y. Bae, C.-S. Kim, Y.-J. Oh, J. Appl. Phys. 85 (1999) 5226. [11] S.A. Campbell, The Science and Engineering of Microelectronic Fabrication, Publishing house of electronics industry, Beijing, 2003.