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Low power RF sputter deposition of oriented copper ferrite films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e793–e794
Low power RF sputter deposition of oriented copper ferrite films Prasanna D. Kulkarnia, M. Desaib, N. Venkataramania,*, Shiva Prasadb, R. Krishanc a
ACRE, IIT Bombay, Powai Mumbai 400076, India Department of Physics, IIT Bombay, Powai Mumbai 400076, India c Laboratoire de Magnetisme et d’optique de Versailles, CNRS, 78935 Versailles, France b
Abstract Thin copper ferrite films have been deposited on amorphous quartz substrates. The films are annealed in air and either quenched in liquid nitrogen or slow cooled from the annealing temperature. The quenched as well as slow cooled films, show (1 1 1) peaks of copper ferrite along with the (3 1 1) peak. This indicates strong (1 1 1) orientation of the films. From the MH loops, squareness value (MR/MS) has been calculated for both type of films. The squareness value of the parallel MH loop is double the value of the perpendicular MH loop. r 2004 Elsevier B.V. All rights reserved. PACS: 75.60. d; 75.70. i Keywords: Magnetic thin films; Copper ferrite; Orientation; Nanocrystalline materials
1. Introduction In an oriented film, MH loop may be different when taken along easy and hard directions. The squareness ratio (MR/MS) and the coercivity is generally high, when the loop is taken along the easy direction in comparison to the hard one. In hexagonal strontium ferrite films such magnetic property attributes could be obtained by just changing the deposition conditions [1]. Similar studies on cubic lithium zinc ferrite thin films remained inconclusive [2]. Recent deposition studies on copper ferrite thin films have yielded interesting results. Copper ferrite films deposited at a RF power of 200 W, though nonoriented show a wide range of coercivity and magnetization values by varying the exsitu thermal treatment. It has been seen from the transmission electron microscopy-selected area diffraction patterns *Corresponding author. Tel.: +91-22-25767657; fax: +9122-25723480. E-mail address: [email protected] (N. Venkataramani).
(TEM-SAD) that, as deposited copper ferrite films are cubic, the films quenched in liquid nitrogen retain the cubic phase whilst the slow cooled films are tetragonal [3]. However, deposition at lower RF power of 50 W resulted in well oriented films. In the present study, the results of the copper ferrite films, prepared by RF sputtering, at 50 W RF power are reported.
2. Experimental The copper ferrite films were deposited using a Leybold Z400 RF sputtering system. The deposition conditions are as in our earlier work on copper ferrite films [3], except the RF power during deposition is now ( Annealing has 50 W. The film thickness is B2400 A.
been carried out for 2 h, at 800 C, followed by either quenching in liquid nitrogen or slow cooling. X-ray diffraction (XRD) and the magnetization studies on a vibrating sample magnetometer (VSM), up to a field of 0.7 T have been carried out.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.01.020
ARTICLE IN PRESS P.D. Kulkarni et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e793–e794
Intensity (a.u.)
(111)
3000
3000
1500
1500
0 -1500
(311)
-3000
(222) (333)
20
M (Gauss)
M (Gauss)
e794
40 2θ (degree)
60
Fig. 1. X-ray diffraction pattern for 50 W RF power deposited copper ferrite, annealed and quenched from 800 C.
3. Results and discussion Fig. 1 shows the XRD pattern for quenched cubicphase copper ferrite film. Miller indices for tetragonal phase have been appropriately transformed to facilitate the comparison of the XRD of the tetragonal copper ferrite phase with cubic phase. The lattice parameter ( Four copper calculated from the XRD is 8.2170.09 A. ferrite peaks, (1 1 1), (2 2 2), (3 3 3) are observed along with the 100% intensity (3 1 1) peak. The intensities for (1 1 1) and (2 2 2) peaks are about 80% of the (3 1 1) peak and the (3 3 3) peak is about 30% intense. The intensities as reported in JCPDS for bulk copper ferrite (1 1 1), (2 2 2) and (3 3 3) peaks are 13%, 8% and 25%, respectively. This shows a strong (1 1 1) orientation of the quenched cubic phase copper ferrite films. The orientation feature has also been seen from texture studies carried out on these copper ferrite films on a texture goniometer. The slow cooled 50 W tetragonal film also shows a similar XRD pattern with significant (1 1 1) orientation. However, a copper ferrite film deposited at 200 W, shows (3 1 1), (2 2 0), (4 0 0), (5 1 1) and (4 4 0) peaks with no specific orientation feature [3]. Fig. 2 shows the perpendicular MH loop on the left and the parallel loop on the right for a 50 W quenched copper ferrite film. The high field susceptibility can be clearly seen in MH loops, as in all other ferrite thin film systems [2]. From the parallel MH loop, the spontaneous magnetization obtained by slope correction of MH curve is B2300 G. The saturation magnetization for copper ferrite film deposited at 200 W RF power and quenched from 800 C was B3000 G [4]. Copper ferrite is an inverse spinel. The magnetization value in cubic copper ferrite is higher than the tetragonal counterpart on account of some Cu2+ ions occupying the tetrahedral site. Large changes in the magnetization value result
0
-1500
-8000 -4000 0 4000 8000 H (Oe)
-3000 -8000 -4000
0 4000 8000 H (Oe)
Fig. 2. Perpendicular and parallel MH loops for the copper ferrite.
owing to small changes in the Cu2+ ion redistribution [5]. The magnetization studies on slow cooled films reflects similar features though the saturation magnetization for tetragonal phase is B1500 G. Even though, XRD patterns show similar peaks for quenched and slow cooled films, higher magnetization value of the quenched films shows that these films are in the cubic phase. Hence, in the cubic phase of the copper ferrite thin films strong orientation features have been recorded during low-RF power deposition. The squareness (MR/MS) value for the parallel MH loop is 0.25, along the orientation direction and about 0.49 in the perpendicular direction. The shape of MH loop is similar for the cubic and tetragonally oriented copper ferrite films and for the nonoriented films deposited at 200 W RF power. This is different from c-axis oriented hexagonal strontium ferrite films, which showed a large MR/MS value for the perpendicular loop and a low MR/MS for the parallel loop [1]. This shows that in the studied copper ferrite thin films, the shape of MH loop is not determined by magnetocrystalline anisotropy alone [6]. In conclusion, copper ferrite films can be sputter deposited with a strong (l l l) orientation at a lower RF power of 50 W. The MH loop of such a film shows that the loop is not controlled only by magnetocrystalline anisotropy alone and the magnetization realized in the quenched cubic phase also depends on the RF power employed.
Acknowledgements We thank ERPIP wing of the DRDO, India for financial support.
References [1] [2] [3] [4] [5] [6]
R. Acharya, et al., Appl. Phys. Lett. 64 (1994) 1579. J. Dash, et al., J. Appl. Phys. 86 (1999) 3303. M. Desai, et al., IEEE Trans. Magn. MAG-38 (2002) 3012. M. Desai, et al., J. Magn. Magn. Mater. 246 (2002) 266. Ph. Tailhades, et al., J. Solid State Chem. 141 (1998) 56. H. Nagata, et al., IEEE Trans. Magn. MAG-8 (1972) 451.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e793–e794
Low power RF sputter deposition of oriented copper ferrite films Prasanna D. Kulkarnia, M. Desaib, N. Venkataramania,*, Shiva Prasadb, R. Krishanc a
ACRE, IIT Bombay, Powai Mumbai 400076, India Department of Physics, IIT Bombay, Powai Mumbai 400076, India c Laboratoire de Magnetisme et d’optique de Versailles, CNRS, 78935 Versailles, France b
Abstract Thin copper ferrite films have been deposited on amorphous quartz substrates. The films are annealed in air and either quenched in liquid nitrogen or slow cooled from the annealing temperature. The quenched as well as slow cooled films, show (1 1 1) peaks of copper ferrite along with the (3 1 1) peak. This indicates strong (1 1 1) orientation of the films. From the MH loops, squareness value (MR/MS) has been calculated for both type of films. The squareness value of the parallel MH loop is double the value of the perpendicular MH loop. r 2004 Elsevier B.V. All rights reserved. PACS: 75.60. d; 75.70. i Keywords: Magnetic thin films; Copper ferrite; Orientation; Nanocrystalline materials
1. Introduction In an oriented film, MH loop may be different when taken along easy and hard directions. The squareness ratio (MR/MS) and the coercivity is generally high, when the loop is taken along the easy direction in comparison to the hard one. In hexagonal strontium ferrite films such magnetic property attributes could be obtained by just changing the deposition conditions [1]. Similar studies on cubic lithium zinc ferrite thin films remained inconclusive [2]. Recent deposition studies on copper ferrite thin films have yielded interesting results. Copper ferrite films deposited at a RF power of 200 W, though nonoriented show a wide range of coercivity and magnetization values by varying the exsitu thermal treatment. It has been seen from the transmission electron microscopy-selected area diffraction patterns *Corresponding author. Tel.: +91-22-25767657; fax: +9122-25723480. E-mail address: [email protected] (N. Venkataramani).
(TEM-SAD) that, as deposited copper ferrite films are cubic, the films quenched in liquid nitrogen retain the cubic phase whilst the slow cooled films are tetragonal [3]. However, deposition at lower RF power of 50 W resulted in well oriented films. In the present study, the results of the copper ferrite films, prepared by RF sputtering, at 50 W RF power are reported.
2. Experimental The copper ferrite films were deposited using a Leybold Z400 RF sputtering system. The deposition conditions are as in our earlier work on copper ferrite films [3], except the RF power during deposition is now ( Annealing has 50 W. The film thickness is B2400 A.
been carried out for 2 h, at 800 C, followed by either quenching in liquid nitrogen or slow cooling. X-ray diffraction (XRD) and the magnetization studies on a vibrating sample magnetometer (VSM), up to a field of 0.7 T have been carried out.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.01.020
ARTICLE IN PRESS P.D. Kulkarni et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e793–e794
Intensity (a.u.)
(111)
3000
3000
1500
1500
0 -1500
(311)
-3000
(222) (333)
20
M (Gauss)
M (Gauss)
e794
40 2θ (degree)
60
Fig. 1. X-ray diffraction pattern for 50 W RF power deposited copper ferrite, annealed and quenched from 800 C.
3. Results and discussion Fig. 1 shows the XRD pattern for quenched cubicphase copper ferrite film. Miller indices for tetragonal phase have been appropriately transformed to facilitate the comparison of the XRD of the tetragonal copper ferrite phase with cubic phase. The lattice parameter ( Four copper calculated from the XRD is 8.2170.09 A. ferrite peaks, (1 1 1), (2 2 2), (3 3 3) are observed along with the 100% intensity (3 1 1) peak. The intensities for (1 1 1) and (2 2 2) peaks are about 80% of the (3 1 1) peak and the (3 3 3) peak is about 30% intense. The intensities as reported in JCPDS for bulk copper ferrite (1 1 1), (2 2 2) and (3 3 3) peaks are 13%, 8% and 25%, respectively. This shows a strong (1 1 1) orientation of the quenched cubic phase copper ferrite films. The orientation feature has also been seen from texture studies carried out on these copper ferrite films on a texture goniometer. The slow cooled 50 W tetragonal film also shows a similar XRD pattern with significant (1 1 1) orientation. However, a copper ferrite film deposited at 200 W, shows (3 1 1), (2 2 0), (4 0 0), (5 1 1) and (4 4 0) peaks with no specific orientation feature [3]. Fig. 2 shows the perpendicular MH loop on the left and the parallel loop on the right for a 50 W quenched copper ferrite film. The high field susceptibility can be clearly seen in MH loops, as in all other ferrite thin film systems [2]. From the parallel MH loop, the spontaneous magnetization obtained by slope correction of MH curve is B2300 G. The saturation magnetization for copper ferrite film deposited at 200 W RF power and quenched from 800 C was B3000 G [4]. Copper ferrite is an inverse spinel. The magnetization value in cubic copper ferrite is higher than the tetragonal counterpart on account of some Cu2+ ions occupying the tetrahedral site. Large changes in the magnetization value result
0
-1500
-8000 -4000 0 4000 8000 H (Oe)
-3000 -8000 -4000
0 4000 8000 H (Oe)
Fig. 2. Perpendicular and parallel MH loops for the copper ferrite.
owing to small changes in the Cu2+ ion redistribution [5]. The magnetization studies on slow cooled films reflects similar features though the saturation magnetization for tetragonal phase is B1500 G. Even though, XRD patterns show similar peaks for quenched and slow cooled films, higher magnetization value of the quenched films shows that these films are in the cubic phase. Hence, in the cubic phase of the copper ferrite thin films strong orientation features have been recorded during low-RF power deposition. The squareness (MR/MS) value for the parallel MH loop is 0.25, along the orientation direction and about 0.49 in the perpendicular direction. The shape of MH loop is similar for the cubic and tetragonally oriented copper ferrite films and for the nonoriented films deposited at 200 W RF power. This is different from c-axis oriented hexagonal strontium ferrite films, which showed a large MR/MS value for the perpendicular loop and a low MR/MS for the parallel loop [1]. This shows that in the studied copper ferrite thin films, the shape of MH loop is not determined by magnetocrystalline anisotropy alone [6]. In conclusion, copper ferrite films can be sputter deposited with a strong (l l l) orientation at a lower RF power of 50 W. The MH loop of such a film shows that the loop is not controlled only by magnetocrystalline anisotropy alone and the magnetization realized in the quenched cubic phase also depends on the RF power employed.
Acknowledgements We thank ERPIP wing of the DRDO, India for financial support.
References [1] [2] [3] [4] [5] [6]
R. Acharya, et al., Appl. Phys. Lett. 64 (1994) 1579. J. Dash, et al., J. Appl. Phys. 86 (1999) 3303. M. Desai, et al., IEEE Trans. Magn. MAG-38 (2002) 3012. M. Desai, et al., J. Magn. Magn. Mater. 246 (2002) 266. Ph. Tailhades, et al., J. Solid State Chem. 141 (1998) 56. H. Nagata, et al., IEEE Trans. Magn. MAG-8 (1972) 451.