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Influence of tetragonal distortion on magnetic and magneto-optical properties of copper ferrite films
Journal of Physics and Chemistry of Solids 61 (2000) 863–867 www.elsevier.nl/locate/jpcs
Influence of tetragonal distortion on magnetic and magneto-optical properties of copper ferrite films C. Baubet a, Ph. Tailhades a,*, C. Bonningue a, A. Rousset a, Z. Simsa b a
Laboratoire de Chimie des Mate´riaux Inorganiques et Energe´tiques, Universite´ Paul Sabatier, ESA-CNRS 5070, 118, route de Narbonne, 31062 Toulouse Cedex 4, France b Institute of Physics ASCR, Cukrovarnicka 10, 162 53 Prague 6, Czech Republic Received 28 July 1999; accepted 29 October 1999
Abstract A series of copper ferrite films was prepared using RF magnetron sputtering techniques. Annealing of the films at 450 and 650⬚C in air brought about changes of the tetragonal distortion of the spinel lattice enabling to investigate the influence of the tetragonal distortion on magnetic and magneto-optical properties. Faraday rotations up to 2⬚/mm at 442 nm, coercive fields up to 1800 Oe and high squareness of the hysteresis loops up to 0.9 could be attained by 2 h annealing at 650⬚C followed by slow cooling. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Thin films; D. Magnetic properties; D. Optical properties; Tetragonal distortion
1. Introduction Copper ferrite, CuFe2O4, has been intensively studied in the past either as a powder [1,2] or in a bulk ceramic form [3–7]. A renewal of interest in thin copper ferrite films has recently appeared [8–12]. Due to the presence of Jahn– Teller (JT) distortion cupric ions in octahedral sites, copper ferrite occurs at room temperature in a tetragonal structure with lattice parameter ratio c=a of about 1.06. A phase transition from the low temperature tetragonal to the high temperature cubic lattice occurs at about 390⬚C [2,3]. It is the result of the disorientation of JT distortions due to a thermal motion of lattice at high temperatures. This transition was found to be influenced by both the distribution of copper ions on the two sublattices and the oxygen non-stoichiometry [3–5,13]. Changing the distribution of cupric ions on the two cationic sublattices can modify the magnitude of the tetragonal distortion, characterised by the c=a ratio. By measuring magnetic moments on quenched samples, Ne´el * Corresponding author. Tel.: ⫹33-5-61-55-61-74; fax: ⫹33-5-6155-61-63. E-mail address: [email protected] (Ph. Tailhades).
[14] showed that low activation energy is needed for the migration of Cu 2⫹ ions in the spinel lattice. Such a migration from tetrahedral to octahedral sites can occur at temperatures of 250–300⬚C. Higher energy is needed for the inverse process so that the migration of cupric ions from octahedral to tetrahedral sites occurs only at temperatures above 400⬚C. Samples quenched from these temperatures display larger saturation magnetisation and lower c=a ratios [3,4,13–15]. When the samples are quenched from still higher temperatures (⬎600⬚C) a part of cupric ions is reduced into the cuprous ions decreasing, still further, the c=a ratio. A stable cubic structure is formed at room temperature when the total number of cupric ions in octahedral sites decreases below its critical value, i.e. 0.8 cupric ions per formula unit [1,3,4,16]. To obtain a single phase cubic copper ferrite a quench temperature higher than 760⬚C is needed [1,4,6]. Modifications of the tetragonal distortion of acicular copper ferrite powders [13,15] have lead to significant changes of magnetic properties like coercive force Hc and saturation magnetisation Ms. The purpose of this work is to find out the relationship between the distortion of the spinel lattice and the magnetic and magneto-optical properties of CuFe2O4 thin films.
0022-3697/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(99)00385-6
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Table 1 Saturation magnetisation Ms, annealing temperatures and cooling conditions (SC, slowly cooled; Q, quenched) and c=a ratio for copper ferrite films, powders or bulk crystals at 300 K Ms (emu/g)
Annealing and cooling conditions
c=a ratio
Reference
26 47 25 38 8 21
900⬚C, SC 900⬚C, Q 1050⬚C, SC 1100⬚C, SC As-sputtered 450⬚C, SC (100⬚C/h)
1.056 1.000 1.055 1.055 – 1.03
[5] [5] [6] [18] This work This work
2. Film preparation and characterisation A suitable target of copper ferrite was prepared by sintering of CuFe2O4 powder precursor that resulted from the heat treatment of an iron copper hydroxide. Radio frequency sputtering was used to deposit thin films of CuFe2O4 on glass substrates. Two kinds of substrates were used: microscope slides and Corning 1737F glass plates with dimensions 0:5 × 20 × 40 mm3 : Before the deposition both kinds of substrates were chemically etched and physically cleaned by argon milling. The deposition was carried out in pure argon gas under pressure of 0.5 Pa using a RF microwave power density of about 2 W/cm 2. The distance between the sample and target was 5 cm yielding a deposition rate of about 18:7 ^ 0:3 nm=min: Samples with typical thickness of about 2 mm were used for magnetisation measurements while all other physical and chemical characterisations were made on 150 nm thick films. In order
to increase the size of the crystallites the annealing treatments were performed in air at 450⬚C for 10 h and at 650⬚C for 5 h for films deposited on the microscope slides and on Corning glass substrates, respectively. All samples were then cooled in air to room temperature at a rate of 100⬚C/h. The film composition was determined by energy dispersion X-ray spectroscopy and was found to be close to the starting nominal composition. Grazing-angle X-ray diffraction (XRD) measurements were performed using a Siemens D5000 diffractometer. The grazing angle between the incident beam and the sample plane was fixed at 1⬚. Magnetisation measurements at a maximum field of 20 kOe, applied perpendicularly to the film plane, were made by means of a quantum design, model MPMS 5S SQUID magnetometer over a temperature range 5–300 K. Faraday rotation hysteresis loop measurements were carried out with a blue laser beam l 442 nm in a variable magnetic field ranging from ⫺20 to 20 kOe. To enable determination of the temperature dependence of coercivity, the same set-up was equipped with a heater allowing temperatures up to 400⬚C.
3. Results and discussion 3.1. As-sputtered films Grazing-angle XRD measurements of the as-sputtered thin films did not show any diffraction peak corresponding to a crystallised phase as e.g. copper ferrite, tenorite (CuO) or hematite (a-Fe2O3) or any other phase containing Cu and/ or Fe even though a weak fraction of oxygen (⬍0.3%) was present during the deposition. Atomic force microscopy did
Fig. 1. Grazing angle X-ray diffraction of copper thin film annealed at 650⬚C. In the inset are shown the splittings of the (404) and (440) diffraction peaks; ⴱ denotes the diffraction from the support.
C. Baubet et al. / Journal of Physics and Chemistry of Solids 61 (2000) 863–867
Fig. 2. Temperature dependence of the saturation magnetisation of copper ferrite film annealed at 450⬚C. Below 40 K a superparamagnetic-like behaviour of the residual amorphous ferrite phase is seen.
not show any grain structure. However, SQUID magnetometer measurements performed perpendicular to the film plane, revealed a ferrimagnetic compound with a specific saturation magnetisation of 8.2 emu/g at 300 K. This value is very low compared with the saturation magnetisation of copper ferrite found in bibliographical data (Table 1). Low magnetisation values in our films point to the presence of a nanocrystalline form of copper ferrite as it was already mentioned elsewhere [8]. 3.2. Annealed films In order to improve the quality of the as-prepared films these were additionally annealed for 2 h at 450 or 650⬚C as mentioned above. As seen from Fig. 1 only a tetragonally distorted phase was present in the films annealed at 650⬚C. The distortion is revealed by the splitting of the (440) and (404) diffraction peaks (see the inset of Fig. 1), because of the predominant 具310典 orientation of the crystallites [10]. A similar X-ray behaviour was observed with 450⬚C annealed films. In the 2 mm thick films, where the XRD patterns are well developed, a small peak corresponding to CuO phase was detected. An unknown magnetic impurity was also revealed by the saturation magnetisation measurements
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shown in Fig. 2. Saturation magnetisation versus temperature curve in the 50–300 K range is typical for the spinel ferrites with prevailing magnetic intersublattice interactions. The spontaneous magnetisation obtained by the extrapolation of this curve to 0 K amounts to 25 emu/g which is slightly lower than the experimental values referred for pure copper ferrites (see Table 1). Such a low value could be the result of the presence of a small amount of weak magnetic (antiferromagnetic) phase of CuO. Moreover, an anomalous increase of s s at low temperatures like the one below 40 K in Fig. 2 is often found in the amorphous or fine nanocrystalline ferrites [8]. As revealed by Kaneyoshi [17], the departures from the regular crystal lattice introduce a spread in the sublattices exchange interactions. As a consequence, in certain crystal regions the weakly coupled spin orderings might exist displaying a superparamagnetic behaviour like that observed in Fig. 2. In the present 450 and 650⬚C annealed samples it is obviously due to the incomplete transformation of original amorphous “as-sputtered” layers into the nanocrystalline films. A small amount of the CuO phase in the annealed films could be the result of the lack of oxygen in the sputtering atmosphere during the process of films growth. The subsequent annealings were obviously carried out at too low temperatures to enable a complete diffusion of the oxygen into the spinel phase. Indeed, according to Kolta et al. [19], a minimal temperature of 780⬚C is necessary to allow for the formation of CuFe2O4 from solid/solid reactions. A mean crystallite size of 78 ^ 15 nm was determined for 150 nm thick films annealed at 650⬚C, from the XRD linebroadening by applying the Scherrer method. This value is in between the smallest (30 nm) and the largest (90 nm) crystallite diameters as observed by atomic force microscopy. A better crystallisation of the annealed films as compared with the as-sputtered ones results in a higher coercivity. Its variation with temperature displays a transition at about 380⬚C as seen in Fig. 3. This transition corresponds to a change in crystal symmetry from the tetragonal to cubic as was observed by Tang et al. [3] for Cu0.97Fe2.03O4.
3.3. Additional heat treatments
Fig. 3. Temperature dependence of the coercivity Hc of copper ferrite film annealed at 450⬚C.
In order to investigate the influence of the tetragonal distortion on the magnetic and magneto-optical properties of the copper ferrite films, two samples were additionally heat treated in the same way as Villette et al. did with copper ferrite powders [13,15]. At the beginning both films samples were annealed at 650⬚C. The first one was quenched in air (Q sample) and the second was slowly cooled at 10⬚C/h to room temperature (SC sample). After this initial treatment both samples were additionally annealed for 10 h at increasing temperatures followed by quenching. After each treatment, structural characterisation and magnetic and
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Fig. 4. Variation in tetragonal deformation c=a with the annealing temperature (Ta) for slow cooled (SC) and quenched (Q) CuFe2O4 thin films.
Fig. 6. Variation in remanant Faraday rotation u FR at 442 nm with the annealing temperature Ta for slowly cooled (SC) and quenched (Q) copper ferrite films.
magneto-optical measurements were performed at room temperature. In Fig. 4 the variation of c=a as a function of the annealing temperature Ta for quenched and slowly cooled samples is represented. We can expect that for the SC film, the cupric ions are localised only in the octahedral sites before annealing treatments. Such a cationic distribution corresponds to a thermodynamic equilibrium at room temperature. The distortion of the spinel lattice is then maximal and the c=a ratio attains values close to 1.05; see Fig. 4. It is interesting to note that for such a cationic distribution the out-of-plane coercivity also reaches its maximum values—see Fig. 5. For CuFe2O4 powders it was already observed [13] that the coercivity increases with the increase of c=a ratio because of the structural anisotropy induced by the tetragonal distortion. A similar interpretation can be proposed for the films studied in the present paper. The remanant Faraday rotation and the magneto-optical squareness (remanant Faraday rotation/saturation Faraday rotation) become also maximal when the cupric ions are located in octahedral sites as demonstrated in Figs. 6 and 7. This perpendicular anisotropy could arise from the preferential 具310典 orientation of our films. Indeed, this direction is close to the [100] easy magnetisation axis of the copper ferrite [20]. The annealing treatments carried out at temperatures Ta
higher than 400⬚C involve a migration of the cupric ions from octahedral to tetrahedral sites as it is demonstrated by the decrease in c=a ratio in Fig. 4 and by decrease of coercivity as shown in Fig. 5. Moreover, it is generally accepted that CuFe2O4 samples quenched from a temperature higher than 600⬚C are oxygen deficient [1,13]. Cuprous and cupric ions could then be located in both octahedral and tetrahedral sites. The resulting distortion of the spinel lattice becomes very low and the ferrite is not in thermodynamic equilibrium at room temperature. Fig. 5 shows that the rate of the cationic migration is very small below 400⬚C. In fact, annealing times of only 2 h are not sufficient to allow for a complete migration of the cupric ions into octahedral sites as found in Fig. 5. It turned out that at least 10 h were necessary to reach equilibrium. This is the reason we used dwell times of 10 h for all the annealing treatments. As seen from Figs. 4–6 the two branches of each curve, SC and Q, joined each other above 400⬚C resulting probably in an identical cationic distribution. Consequently, it can be assumed that the SC and Q samples have approximately the same cationic distribution when annealed at temperatures higher than 400⬚C.
Fig. 5. Variation in coercivity (Hc) with the annealing temperature (Ta) for slow cooled (SC) and quenched (Q) CuFe2O4 thin films.
Fig. 7. Variation in magneto-optical squareness with the annealing temperature Ta for slowly cooled (SC) and quenched (Q) copper ferrite films.
C. Baubet et al. / Journal of Physics and Chemistry of Solids 61 (2000) 863–867
High coercivities (up to 1800 Oe) can be reached for the annealed and slowly cooled copper ferrite films. Moreover, a maximum value of 0.9 (Fig. 7) for the squareness of the magneto-optical hysteresis loop (a particularly high value for non-epitaxial ferrite thin films) and a remanant Faraday rotation of 2⬚/mm have been measured. Higher values of coercive field (up to 3000 Oe) could be reached by replacing a small part of cupric ions by bivalent cobalt ions. Such results have already been reported for powders [15]. These characteristics make copper ferrite films attractive for magneto-optical recording technologies in the blue spectral region. 4. Conclusions A series of CuFe2O4 thin films was prepared by RF sputtering. When heat treated at 650⬚C the films showed a tetragonal distorted structure. The c=a ratio of their lattice constants ranged from 1.05 to 1.03 for slowly cooled (SC) and quenched (Q) samples, respectively. It is found that annealing at temperatures above 400⬚C enables the cupric ions to migrate between tetrahedral and octahedral positions. Octahedral Cu 2⫹ are partially displaced into tetrahedral sites when the samples are annealed at temperatures higher than 400⬚C. Copper ferrite films with highly distorted spinel structure display magnetic anisotropy perpendicular to the film plane. Such films have a fairly high coercivity (1800 Oe) and a high remanant Faraday rotation of 2⬚/mm at 442 nm. Acknowledgements This work was supported by the Franco-Czech project BARRANDE no. 88057.
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References [1] J. Mexmain, Ann. Chim. 4 (1969) 429. [2] T. Inoue, S. Iida, J. Phys. Soc. Jpn 13 (1958) 656. [3] X.X. Tang, A. Manthiram, J.B. Goodenough, J. Solid State Chem. 79 (1989) 250. [4] H. Ohnishi, T. Teranishi, J. Phys. Soc. Jpn 16 (1) (1961) 35. [5] K. Obayashi, S. Iiida, J. Phys. Soc. Jpn 23 (4) (1967) 776. [6] K.S.R.C. Murthy, S. Mahanty, J. Ghose, Mater. Res. Bull. 22 (1987) 1665. [7] P.N. Kamble, A.S. Vaingankar, Ind. J. Pure Appl. Phys. 28 (1990) 542. [8] G. Srinivasan, B. Uma Maheshwar Rao, J. Zhao, S. Seehra, Appl. Phys. Lett. 59 (1991) 372. [9] M.M. Ibrahim, M.S. Seehra, G. Srinivasan, J. Appl. Phys. 75 (1994) 6822. [10] C. Despax, Ph. Tailhades, C. Baubet, C. Villette, A. Rousset, Thin Solid Films 293 (1997) 22. [11] T. Tanaka, M. Chiba, H. Okimura, Y. Koizumi, J. Phys. IV C1 (1997) 501. [12] L. Stichauer, Z. Simsa, Ph. Tailhades, C. Despax, J. Phys. IV C1 (1997) 729. [13] C. Villette, Ph. Tailhades, A. Rousset, J. Solid State Chem. 117 (1995) 64. [14] L. Ne´el, C.R. Acad. Sci. Paris (1950) 190. [15] Ph. Tailhades, C. Villette, A. Rousset, K.R. Kannan, C.N.R. Rao, M. Lenglet, Solid State Chem. 141 (1998) 56. [16] J. Janicki, J. Pietrzak, A. Porebska, J. Suwalski, Phys. Status Solidi (a) 72 (1982) 95. [17] T. Kayenoshi, Solid State Commun. 69 (1989) 91. [18] P.B. Pandya, H.H. Joshi, R.G. Kulkarni, J. Mater. Sci. Lett. 10 (1991) 474. [19] G.A. Kolta, S.Z. El-Tawil, A.A. Ibragim, N.S. Felix, Therm. Acta 43 (1981) 279. [20] N.T. Malafaev, A.A. Muraklovskii, J.A. Popkov, I. Onysziewicz, J. Magn. Magn. Mat. 89 (1990) 8.
Influence of tetragonal distortion on magnetic and magneto-optical properties of copper ferrite films C. Baubet a, Ph. Tailhades a,*, C. Bonningue a, A. Rousset a, Z. Simsa b a
Laboratoire de Chimie des Mate´riaux Inorganiques et Energe´tiques, Universite´ Paul Sabatier, ESA-CNRS 5070, 118, route de Narbonne, 31062 Toulouse Cedex 4, France b Institute of Physics ASCR, Cukrovarnicka 10, 162 53 Prague 6, Czech Republic Received 28 July 1999; accepted 29 October 1999
Abstract A series of copper ferrite films was prepared using RF magnetron sputtering techniques. Annealing of the films at 450 and 650⬚C in air brought about changes of the tetragonal distortion of the spinel lattice enabling to investigate the influence of the tetragonal distortion on magnetic and magneto-optical properties. Faraday rotations up to 2⬚/mm at 442 nm, coercive fields up to 1800 Oe and high squareness of the hysteresis loops up to 0.9 could be attained by 2 h annealing at 650⬚C followed by slow cooling. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Thin films; D. Magnetic properties; D. Optical properties; Tetragonal distortion
1. Introduction Copper ferrite, CuFe2O4, has been intensively studied in the past either as a powder [1,2] or in a bulk ceramic form [3–7]. A renewal of interest in thin copper ferrite films has recently appeared [8–12]. Due to the presence of Jahn– Teller (JT) distortion cupric ions in octahedral sites, copper ferrite occurs at room temperature in a tetragonal structure with lattice parameter ratio c=a of about 1.06. A phase transition from the low temperature tetragonal to the high temperature cubic lattice occurs at about 390⬚C [2,3]. It is the result of the disorientation of JT distortions due to a thermal motion of lattice at high temperatures. This transition was found to be influenced by both the distribution of copper ions on the two sublattices and the oxygen non-stoichiometry [3–5,13]. Changing the distribution of cupric ions on the two cationic sublattices can modify the magnitude of the tetragonal distortion, characterised by the c=a ratio. By measuring magnetic moments on quenched samples, Ne´el * Corresponding author. Tel.: ⫹33-5-61-55-61-74; fax: ⫹33-5-6155-61-63. E-mail address: [email protected] (Ph. Tailhades).
[14] showed that low activation energy is needed for the migration of Cu 2⫹ ions in the spinel lattice. Such a migration from tetrahedral to octahedral sites can occur at temperatures of 250–300⬚C. Higher energy is needed for the inverse process so that the migration of cupric ions from octahedral to tetrahedral sites occurs only at temperatures above 400⬚C. Samples quenched from these temperatures display larger saturation magnetisation and lower c=a ratios [3,4,13–15]. When the samples are quenched from still higher temperatures (⬎600⬚C) a part of cupric ions is reduced into the cuprous ions decreasing, still further, the c=a ratio. A stable cubic structure is formed at room temperature when the total number of cupric ions in octahedral sites decreases below its critical value, i.e. 0.8 cupric ions per formula unit [1,3,4,16]. To obtain a single phase cubic copper ferrite a quench temperature higher than 760⬚C is needed [1,4,6]. Modifications of the tetragonal distortion of acicular copper ferrite powders [13,15] have lead to significant changes of magnetic properties like coercive force Hc and saturation magnetisation Ms. The purpose of this work is to find out the relationship between the distortion of the spinel lattice and the magnetic and magneto-optical properties of CuFe2O4 thin films.
0022-3697/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(99)00385-6
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C. Baubet et al. / Journal of Physics and Chemistry of Solids 61 (2000) 863–867
Table 1 Saturation magnetisation Ms, annealing temperatures and cooling conditions (SC, slowly cooled; Q, quenched) and c=a ratio for copper ferrite films, powders or bulk crystals at 300 K Ms (emu/g)
Annealing and cooling conditions
c=a ratio
Reference
26 47 25 38 8 21
900⬚C, SC 900⬚C, Q 1050⬚C, SC 1100⬚C, SC As-sputtered 450⬚C, SC (100⬚C/h)
1.056 1.000 1.055 1.055 – 1.03
[5] [5] [6] [18] This work This work
2. Film preparation and characterisation A suitable target of copper ferrite was prepared by sintering of CuFe2O4 powder precursor that resulted from the heat treatment of an iron copper hydroxide. Radio frequency sputtering was used to deposit thin films of CuFe2O4 on glass substrates. Two kinds of substrates were used: microscope slides and Corning 1737F glass plates with dimensions 0:5 × 20 × 40 mm3 : Before the deposition both kinds of substrates were chemically etched and physically cleaned by argon milling. The deposition was carried out in pure argon gas under pressure of 0.5 Pa using a RF microwave power density of about 2 W/cm 2. The distance between the sample and target was 5 cm yielding a deposition rate of about 18:7 ^ 0:3 nm=min: Samples with typical thickness of about 2 mm were used for magnetisation measurements while all other physical and chemical characterisations were made on 150 nm thick films. In order
to increase the size of the crystallites the annealing treatments were performed in air at 450⬚C for 10 h and at 650⬚C for 5 h for films deposited on the microscope slides and on Corning glass substrates, respectively. All samples were then cooled in air to room temperature at a rate of 100⬚C/h. The film composition was determined by energy dispersion X-ray spectroscopy and was found to be close to the starting nominal composition. Grazing-angle X-ray diffraction (XRD) measurements were performed using a Siemens D5000 diffractometer. The grazing angle between the incident beam and the sample plane was fixed at 1⬚. Magnetisation measurements at a maximum field of 20 kOe, applied perpendicularly to the film plane, were made by means of a quantum design, model MPMS 5S SQUID magnetometer over a temperature range 5–300 K. Faraday rotation hysteresis loop measurements were carried out with a blue laser beam l 442 nm in a variable magnetic field ranging from ⫺20 to 20 kOe. To enable determination of the temperature dependence of coercivity, the same set-up was equipped with a heater allowing temperatures up to 400⬚C.
3. Results and discussion 3.1. As-sputtered films Grazing-angle XRD measurements of the as-sputtered thin films did not show any diffraction peak corresponding to a crystallised phase as e.g. copper ferrite, tenorite (CuO) or hematite (a-Fe2O3) or any other phase containing Cu and/ or Fe even though a weak fraction of oxygen (⬍0.3%) was present during the deposition. Atomic force microscopy did
Fig. 1. Grazing angle X-ray diffraction of copper thin film annealed at 650⬚C. In the inset are shown the splittings of the (404) and (440) diffraction peaks; ⴱ denotes the diffraction from the support.
C. Baubet et al. / Journal of Physics and Chemistry of Solids 61 (2000) 863–867
Fig. 2. Temperature dependence of the saturation magnetisation of copper ferrite film annealed at 450⬚C. Below 40 K a superparamagnetic-like behaviour of the residual amorphous ferrite phase is seen.
not show any grain structure. However, SQUID magnetometer measurements performed perpendicular to the film plane, revealed a ferrimagnetic compound with a specific saturation magnetisation of 8.2 emu/g at 300 K. This value is very low compared with the saturation magnetisation of copper ferrite found in bibliographical data (Table 1). Low magnetisation values in our films point to the presence of a nanocrystalline form of copper ferrite as it was already mentioned elsewhere [8]. 3.2. Annealed films In order to improve the quality of the as-prepared films these were additionally annealed for 2 h at 450 or 650⬚C as mentioned above. As seen from Fig. 1 only a tetragonally distorted phase was present in the films annealed at 650⬚C. The distortion is revealed by the splitting of the (440) and (404) diffraction peaks (see the inset of Fig. 1), because of the predominant 具310典 orientation of the crystallites [10]. A similar X-ray behaviour was observed with 450⬚C annealed films. In the 2 mm thick films, where the XRD patterns are well developed, a small peak corresponding to CuO phase was detected. An unknown magnetic impurity was also revealed by the saturation magnetisation measurements
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shown in Fig. 2. Saturation magnetisation versus temperature curve in the 50–300 K range is typical for the spinel ferrites with prevailing magnetic intersublattice interactions. The spontaneous magnetisation obtained by the extrapolation of this curve to 0 K amounts to 25 emu/g which is slightly lower than the experimental values referred for pure copper ferrites (see Table 1). Such a low value could be the result of the presence of a small amount of weak magnetic (antiferromagnetic) phase of CuO. Moreover, an anomalous increase of s s at low temperatures like the one below 40 K in Fig. 2 is often found in the amorphous or fine nanocrystalline ferrites [8]. As revealed by Kaneyoshi [17], the departures from the regular crystal lattice introduce a spread in the sublattices exchange interactions. As a consequence, in certain crystal regions the weakly coupled spin orderings might exist displaying a superparamagnetic behaviour like that observed in Fig. 2. In the present 450 and 650⬚C annealed samples it is obviously due to the incomplete transformation of original amorphous “as-sputtered” layers into the nanocrystalline films. A small amount of the CuO phase in the annealed films could be the result of the lack of oxygen in the sputtering atmosphere during the process of films growth. The subsequent annealings were obviously carried out at too low temperatures to enable a complete diffusion of the oxygen into the spinel phase. Indeed, according to Kolta et al. [19], a minimal temperature of 780⬚C is necessary to allow for the formation of CuFe2O4 from solid/solid reactions. A mean crystallite size of 78 ^ 15 nm was determined for 150 nm thick films annealed at 650⬚C, from the XRD linebroadening by applying the Scherrer method. This value is in between the smallest (30 nm) and the largest (90 nm) crystallite diameters as observed by atomic force microscopy. A better crystallisation of the annealed films as compared with the as-sputtered ones results in a higher coercivity. Its variation with temperature displays a transition at about 380⬚C as seen in Fig. 3. This transition corresponds to a change in crystal symmetry from the tetragonal to cubic as was observed by Tang et al. [3] for Cu0.97Fe2.03O4.
3.3. Additional heat treatments
Fig. 3. Temperature dependence of the coercivity Hc of copper ferrite film annealed at 450⬚C.
In order to investigate the influence of the tetragonal distortion on the magnetic and magneto-optical properties of the copper ferrite films, two samples were additionally heat treated in the same way as Villette et al. did with copper ferrite powders [13,15]. At the beginning both films samples were annealed at 650⬚C. The first one was quenched in air (Q sample) and the second was slowly cooled at 10⬚C/h to room temperature (SC sample). After this initial treatment both samples were additionally annealed for 10 h at increasing temperatures followed by quenching. After each treatment, structural characterisation and magnetic and
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Fig. 4. Variation in tetragonal deformation c=a with the annealing temperature (Ta) for slow cooled (SC) and quenched (Q) CuFe2O4 thin films.
Fig. 6. Variation in remanant Faraday rotation u FR at 442 nm with the annealing temperature Ta for slowly cooled (SC) and quenched (Q) copper ferrite films.
magneto-optical measurements were performed at room temperature. In Fig. 4 the variation of c=a as a function of the annealing temperature Ta for quenched and slowly cooled samples is represented. We can expect that for the SC film, the cupric ions are localised only in the octahedral sites before annealing treatments. Such a cationic distribution corresponds to a thermodynamic equilibrium at room temperature. The distortion of the spinel lattice is then maximal and the c=a ratio attains values close to 1.05; see Fig. 4. It is interesting to note that for such a cationic distribution the out-of-plane coercivity also reaches its maximum values—see Fig. 5. For CuFe2O4 powders it was already observed [13] that the coercivity increases with the increase of c=a ratio because of the structural anisotropy induced by the tetragonal distortion. A similar interpretation can be proposed for the films studied in the present paper. The remanant Faraday rotation and the magneto-optical squareness (remanant Faraday rotation/saturation Faraday rotation) become also maximal when the cupric ions are located in octahedral sites as demonstrated in Figs. 6 and 7. This perpendicular anisotropy could arise from the preferential 具310典 orientation of our films. Indeed, this direction is close to the [100] easy magnetisation axis of the copper ferrite [20]. The annealing treatments carried out at temperatures Ta
higher than 400⬚C involve a migration of the cupric ions from octahedral to tetrahedral sites as it is demonstrated by the decrease in c=a ratio in Fig. 4 and by decrease of coercivity as shown in Fig. 5. Moreover, it is generally accepted that CuFe2O4 samples quenched from a temperature higher than 600⬚C are oxygen deficient [1,13]. Cuprous and cupric ions could then be located in both octahedral and tetrahedral sites. The resulting distortion of the spinel lattice becomes very low and the ferrite is not in thermodynamic equilibrium at room temperature. Fig. 5 shows that the rate of the cationic migration is very small below 400⬚C. In fact, annealing times of only 2 h are not sufficient to allow for a complete migration of the cupric ions into octahedral sites as found in Fig. 5. It turned out that at least 10 h were necessary to reach equilibrium. This is the reason we used dwell times of 10 h for all the annealing treatments. As seen from Figs. 4–6 the two branches of each curve, SC and Q, joined each other above 400⬚C resulting probably in an identical cationic distribution. Consequently, it can be assumed that the SC and Q samples have approximately the same cationic distribution when annealed at temperatures higher than 400⬚C.
Fig. 5. Variation in coercivity (Hc) with the annealing temperature (Ta) for slow cooled (SC) and quenched (Q) CuFe2O4 thin films.
Fig. 7. Variation in magneto-optical squareness with the annealing temperature Ta for slowly cooled (SC) and quenched (Q) copper ferrite films.
C. Baubet et al. / Journal of Physics and Chemistry of Solids 61 (2000) 863–867
High coercivities (up to 1800 Oe) can be reached for the annealed and slowly cooled copper ferrite films. Moreover, a maximum value of 0.9 (Fig. 7) for the squareness of the magneto-optical hysteresis loop (a particularly high value for non-epitaxial ferrite thin films) and a remanant Faraday rotation of 2⬚/mm have been measured. Higher values of coercive field (up to 3000 Oe) could be reached by replacing a small part of cupric ions by bivalent cobalt ions. Such results have already been reported for powders [15]. These characteristics make copper ferrite films attractive for magneto-optical recording technologies in the blue spectral region. 4. Conclusions A series of CuFe2O4 thin films was prepared by RF sputtering. When heat treated at 650⬚C the films showed a tetragonal distorted structure. The c=a ratio of their lattice constants ranged from 1.05 to 1.03 for slowly cooled (SC) and quenched (Q) samples, respectively. It is found that annealing at temperatures above 400⬚C enables the cupric ions to migrate between tetrahedral and octahedral positions. Octahedral Cu 2⫹ are partially displaced into tetrahedral sites when the samples are annealed at temperatures higher than 400⬚C. Copper ferrite films with highly distorted spinel structure display magnetic anisotropy perpendicular to the film plane. Such films have a fairly high coercivity (1800 Oe) and a high remanant Faraday rotation of 2⬚/mm at 442 nm. Acknowledgements This work was supported by the Franco-Czech project BARRANDE no. 88057.
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