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Low temperature processing of epitaxial La1−xCaxMnO3 thin films by pulsed laser deposition
Applied Surface Science 127–129 Ž1998. 491–495
Low temperature processing of epitaxial La 1yxCa x MnO 3 thin films by pulsed laser deposition Y.S. Leung ) , K.H. Wong Department of Applied Physics, The Hong Kong Polytechnic UniÕersity, Hung Hom, Kowloon, Hong Kong, China
Abstract Magnetoresistive perovskite-like La 0.7 Ca 0.3 MnO 3 ŽLCMO. thin films have been successfully grown on Ž100.LaAlO 3 ŽLAO. at a relatively low substrate temperature of 6008C–7508C by pulsed laser deposition ŽPLD. method. Epitaxial LCMO films are obtained at deposition temperature of 6508C or above without post-annealing. The as-deposited films grown at higher substrate temperatures showed lower resistance and higher semiconductor-to-metal transition temperature. For films post-annealed for 1 h at their respective deposition temperatures, a large reduction in their electrical resistivity and an up-shifting of their transition temperature were observed. The maximum magnetoresistance ratio ŽMR., yD RŽ1 T.rRŽ0., for the films deposited and annealed at 6508C and 7508C were 37% and 50%, respectively. q 1998 Elsevier Science B.V. PACS: 68.55.y a; 75.70.Pa; 81.15.Fg Keywords: PLD; LCMO; GMR; Epitaxial growth; Thin films
1. Introduction Perovskite-like La 1y x A x MnO 3 ŽA s Ba, Ca, Sr, and Pb. has recently attracted a lot of attention due to its unusual giant magnetoresistance ŽGMR., in which the electrical resistance of the materials changes by more than an order of magnitude under the effect of a magnetic field w1–5x. Considerable effort has been focused on studies of epitaxial growth of La 0.7 Ca 0.3 MnO 3 ŽLCMO. thin films for potential device applications w3,6x. Pulsed laser deposition ŽPLD. is known to be an excellent method for growing perovskite films such as superconducting
)
Corresponding author. Tel.: q852-2766-5677; fax: q8522333-7629; e-mail: [email protected].
YBa 2 Cu 3 O 7 and piezoelectric PbZr xTi 1yxO 3 films w7–10x. It has been demonstrated that good quality LCMO films on single crystal substrate can be obtained by the PLD method w1x. Usually, the films were either grown at a relatively high substrate temperature of above 7508C or the as-deposited films annealed under ambient oxygen at high temperature Ž9008C. for a prolonged period Ž) 3 h. w1,11x. Magnetoresistance ratio ŽMR. of ) 80% under the influence of a high magnetic field ŽG 6 T. has been reported w2,3,12x. However, for compatibility with device fabrication processes, film growth and postdeposition annealing at high temperature should be avoided. So far, little effort has been made to study the characteristics of low temperature processing of LCMO thin films and to locate the threshold temperature for epitaxial growth of LCMO thin films.
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 7 . 0 0 6 8 0 - 6
492
Y.S. Leung, K.H. Wong r Applied Surface Science 127–129 (1998) 491–495
In this paper, we present studies of PLD of LCMO films at 6008C–7508C. High quality epitaxial growth films were obtained at substrate temperature of 6508C or above. Post-deposition annealing in ambient oxygen showed improved crystallinity and reduced resistivities. The resistance change after post-annealing is most prominent for films grown at 6508C. In addition, our samples showed lower resistance and higher transition temperature, Tc , at increased deposition temperatures from 6508C to 7008C. This is in marked contrast to the results reported for PLD of LCMO films at ) 7508C, where the Tc of the as-deposited LCMO films decreased and the resisitivity of the films became larger when higher deposition temperature was used w13x. Excellent MR values were obtained in all our epitaxially grown as-deposited and post-annealed films at processing temperatures as low as 6508C.
temperature was monitored by a thermocouple fixed beneath the heater face plate. No correction has been made for the LAO substrate surface temperature, which is estimated to be ; 308C lower than the measured ones. For some samples, in-situ post-annealing at the same deposition temperature were carried out in 1 atm oxygen for 1 h. The structural quality of the as-deposited and post-annealed films were characterized by the X-ray diffraction ŽXRD. technique using a Philips four-circle diffractometer and Ni-filtered CuK a radiation. The resistivity–temperature Ž R–T . curves were measured by the usual four-point probe method and by cooling the samples from room temperature to liquid nitrogen temperature. The MR of the films were obtained by placing the sample in a uniform magnetic field Ž H s 1 T. provided by a vibrating sample magnetometer. The field applied was parallel to the film surface and the current flow direction.
2. Experiments 3. Results and discussion LCMO thin films with thickness of 200 nm were prepared by the PLD method. Single crystal Ž100.LaAlO 3 ŽLAO. substrates were used throughout for all film growth processes in the present studies. Both the LCMO and the LAO have cubic structures ˚ and a s 3.792 A, ˚ with lattice parameters a s 3.89 A respectively. Lattice mismatch is, therefore, less than 3%. The LCMO ceramic oxide target was prepared by using the standard solid state reaction technique, in which a mixture of proper amount of high purity La 2 O 3 , CaCO 3 and MnO 2 powders were repeatedly ground and calcined. It was then pressed into circular pellets of 2 cm diameter and 0.5 cm thick, and sintered at temperature of 11008C–13008C for 3 h in air. The nominal composition of the LCMO target was determined by energy dispersive X-ray microanalysis ŽEDX.. Composition of all deposited films showed a little deviation from the target stoichiometry. In the PLD experiments, an XeCl excimer laser Ž l s 308 nm. running at a repetition rate of 10 Hz was focused by a silica lens onto the flat surface of the rotating target. The laser fluence was estimated to be 1.5 Jrcm2 . The substrate to target distance was kept at 4 cm. Films were deposited under 200 mTorr ambient oxygen at four different substrate temperatures of 6008C, 6508C, 7008C and 7508C. The heater
3.1. The epitaxial growth of La 0 .7 Ca 0 .3 MnO3 thin films Fig. 1 show the u –2 u scans of the films deposited at 6008C, 6508C and 7508C. Apart from films grown at 6008C, all as-deposited films were well crystallized. From the figure, Ž h00. LCMO peaks can be seen clearly for films deposited at 6508C and 7508C. No trace of other LCMO reflections was observed. Films grown at 6008C, however, are largely amorphous and show very weak LCMO peaks. The v-scan rocking curve for the Ž200. LCMO reflections for films deposited at 6508C shows a narrow profile with a full width half maximum ŽFWHM. of 1.28. This suggests a good film crystalline orientation. Full epitaxial growth of the LCMO films at 6508C is confirmed by the XRD 3608 f-scans for the Ž220. reflections of the LCMO films and the LAO substrate shown in Fig. 2. Qualitatively similar but better epitaxial relations were found on films deposited at higher substrate temperatures. All post-annealed films showed improved crystallinity. For example, the FWHM of the rocking curve of the film grown at 6508C narrows down from 1.28 to 0.98 after annealing.
Y.S. Leung, K.H. Wong r Applied Surface Science 127–129 (1998) 491–495
Fig. 1. The XRD pattern of normal u –2 u scan of the films deposited at 6008C, 6508C and 7508C, respectively.
3.2. The electrical and magnetic properties of La 0 .7 Ca 0 .3 MnO3 thin films The deposition temperature and the post-annealing process have distinct effect on the electrical and magnetic properties of the LCMO thin films. The R–T curves of the thin films fabricated at 6508C and 7008C with and without post-annealing are shown in Fig. 3. The values of resistivity of the films are normalized for the ease of comparison. It is seen from this figure that the Tc of the as deposited films shifted from 125 K to 190 K as the deposition temperatures changes from 6508C to 7008C. Post-deposition annealing, on the other hand, raised the Tc further by about 20 K in both cases Ž145 K and 210 K for 6508C and 7008C, respectively.. The absolute electrical resistance of the LCMO films showed strong dependence on the deposition temperatures and the post-annealing process. As depicted in Fig. 4, the maximum resistivities of the as deposited films decreased fairly smoothly with increasing deposition temperatures. These results are clearly different from those reported by Zhang et al. w13x, in which their LCMO films grown at a relatively higher substrate
493
Fig. 2. The X-ray f-scan of the Ž220. reflections of LCMO films and LAO substrate.
temperature of ; 9008C showed the opposite trend. They attributed the effect being caused by oxygen deficiency in the LCMO films. Apparently, oxygen depletion due to high deposition temperatures has not occurred in our samples prepared at 6008C– 7508C. Post-annealing has, in general, the effect of reducing the resistance of the films. The most noticeable
Fig. 3. The R – T curves of LCMO as-deposited films AŽ6508C., X X BŽ7008C. and post-annealed films A Ž6508C., B Ž7008C..
494
Y.S. Leung, K.H. Wong r Applied Surface Science 127–129 (1998) 491–495
Fig. 4. The peak resistivities of the LCMO films as a function of processing temperatures; B are the as-deposited films and ' are the post-annealed films. The solid curves are drawn for viewing purpose.
change, however, occurs in films deposited at 6508C, in which the film resistance dropped by more than an order of magnitude from 4230 m V cm to 310 m V cm after annealing at 6508C for 1 h. It is important to note that for the annealed films, the resistance remained more or less the same at about 300 m V cm for deposition temperatures of 6508C, 7008C and 7508C. The marked change of resistance for films annealed at 6508C may imply that the single phase perovskite LCMO formation starts at such temperature. Annealing can, thus, promote the growth of the
highly conducting perovskite LCMO films and a sudden drop in the film’s resistance results. Higher annealing temperature, however, only improves the crystallinity of the already conducting LCMO films and has a minor effect on lowering the film resistance. Under the influence of a magnetic field aligned parallel to the plane of the LCMO films, the resistance is suppressed resulting in a noted GMR effect. Fig. 5 showed the R–T curves and the MR profiles of the LCMO films deposited and post-annealed at 6508C. A maximum MR of 37% at 110 K was observed. For the films fabricated under higher deposition temperatures, the MR became larger. MR values of 37%, 40% and 50% were obtained at deposition temperature of 6508C, 7008C and 7508C, respectively. The reason is that the crystallinity of the films improved at higher deposition temperatures.
4. Summary Epitaxial growth of La 0.7 Ca 0.3 MnO 3 films on LaAlO 3 Ž100. substrates were obtained under a relatively low processing temperature of 6508C. It has been shown that both the resistivity and the transition temperatures, Tc , improved at higher deposition temperatures from 6508C to 7008C. The effect is most prominent for films prepared at 6508C, in which the single phase conductive LCMO is believed to have crystallized and oriented films began to grow. A MR of 37% is obtained for films grown at 6508C under a relatively low field of 1 T. Our studies suggest that epitaxial LCMO thin films prepared by PLD method can be obtained at the processing temperature as low as 6508C.
Acknowledgements Fig. 5. The R – T curves at B s 0 T Žsolid line. and B s1 T Ždash line. presented and the values of the resistivities referred to the right-hand axis. The MR profiles of the 6508C post-annealed LCMO films presented by the solid squares. The values of the MR are determined by Ž R Bs 0 T y R Bs1 T .r R Bs0 T and referred to the left-hand axis.
This work is supported by a Research Grant of the Hong Kong Polytechnic University under Code No. 0350.536.A3.110. One of the author ŽY.S. Leung. is grateful for the award of a research studentship the Hong Kong Polytechnic University.
Y.S. Leung, K.H. Wong r Applied Surface Science 127–129 (1998) 491–495
References w1x S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 Ž1994. 413. w2x M. McCormack, S. Jin, T.H. Tiefel, R.M. Fleming, J.M. Phillips, R. Ramesh, Appl. Phys. Lett. 64 Ž1994. 3045. w3x J.Y. Gu, K.H. Kim, T.W. Noh, K.-S. Suh, J. Appl. Phys. 78 Ž1995. 6151. w4x G.Q. Gong, C.L. Canedy, G. Xiao, J. Appl. Phys. 79 Ž1996. 4538. w5x R.E. Treece, P. Dorsey, M. Rubinstein, J.M. Byers, J.S. Horwitz, E. Donovan, D.B. Chrisey, Mater. Res. Soc. Symp. Proc. 384 Ž1995. 427.
495
w6x R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 Ž1993. 2331. w7x C.-S. Huang, I.-N. Lin, J.Y. Lee, T.-Y. Tseng, Jpn. J. Appl. Phys. 33 Ž1994. 4058. w8x C. Prouteau, J.F. Hamet, B. Mercey, M. Hervieu, B. Raveau, D. Robbes, L. Coudrier, G. Ben, Physica C 248 Ž1995. 108. w9x M.H. Yeh, K.S. Liu, J. Appl. Phys. 77 Ž1995. 5335. w10x S.K. Hau, K.H. Wong, P.W. Chan, C.L. Choy, Appl. Phys. Lett. 66 Ž1995. 245. w11x S. Jin, M. McCormack, T.H. Tiefel, R. Ramesh, J. Appl. Phys. 76 Ž1994. 6929. w12x S.-Y. Bae, S.X. Wang, Appl. Phys. Lett. 69 Ž1996. 121. w13x W. Zhang, W. Boyd, M. Elliot, W. Herrenden-Harkerand, Appl. Phys. Lett. 69 Ž1996. 3929.
Low temperature processing of epitaxial La 1yxCa x MnO 3 thin films by pulsed laser deposition Y.S. Leung ) , K.H. Wong Department of Applied Physics, The Hong Kong Polytechnic UniÕersity, Hung Hom, Kowloon, Hong Kong, China
Abstract Magnetoresistive perovskite-like La 0.7 Ca 0.3 MnO 3 ŽLCMO. thin films have been successfully grown on Ž100.LaAlO 3 ŽLAO. at a relatively low substrate temperature of 6008C–7508C by pulsed laser deposition ŽPLD. method. Epitaxial LCMO films are obtained at deposition temperature of 6508C or above without post-annealing. The as-deposited films grown at higher substrate temperatures showed lower resistance and higher semiconductor-to-metal transition temperature. For films post-annealed for 1 h at their respective deposition temperatures, a large reduction in their electrical resistivity and an up-shifting of their transition temperature were observed. The maximum magnetoresistance ratio ŽMR., yD RŽ1 T.rRŽ0., for the films deposited and annealed at 6508C and 7508C were 37% and 50%, respectively. q 1998 Elsevier Science B.V. PACS: 68.55.y a; 75.70.Pa; 81.15.Fg Keywords: PLD; LCMO; GMR; Epitaxial growth; Thin films
1. Introduction Perovskite-like La 1y x A x MnO 3 ŽA s Ba, Ca, Sr, and Pb. has recently attracted a lot of attention due to its unusual giant magnetoresistance ŽGMR., in which the electrical resistance of the materials changes by more than an order of magnitude under the effect of a magnetic field w1–5x. Considerable effort has been focused on studies of epitaxial growth of La 0.7 Ca 0.3 MnO 3 ŽLCMO. thin films for potential device applications w3,6x. Pulsed laser deposition ŽPLD. is known to be an excellent method for growing perovskite films such as superconducting
)
Corresponding author. Tel.: q852-2766-5677; fax: q8522333-7629; e-mail: [email protected].
YBa 2 Cu 3 O 7 and piezoelectric PbZr xTi 1yxO 3 films w7–10x. It has been demonstrated that good quality LCMO films on single crystal substrate can be obtained by the PLD method w1x. Usually, the films were either grown at a relatively high substrate temperature of above 7508C or the as-deposited films annealed under ambient oxygen at high temperature Ž9008C. for a prolonged period Ž) 3 h. w1,11x. Magnetoresistance ratio ŽMR. of ) 80% under the influence of a high magnetic field ŽG 6 T. has been reported w2,3,12x. However, for compatibility with device fabrication processes, film growth and postdeposition annealing at high temperature should be avoided. So far, little effort has been made to study the characteristics of low temperature processing of LCMO thin films and to locate the threshold temperature for epitaxial growth of LCMO thin films.
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 7 . 0 0 6 8 0 - 6
492
Y.S. Leung, K.H. Wong r Applied Surface Science 127–129 (1998) 491–495
In this paper, we present studies of PLD of LCMO films at 6008C–7508C. High quality epitaxial growth films were obtained at substrate temperature of 6508C or above. Post-deposition annealing in ambient oxygen showed improved crystallinity and reduced resistivities. The resistance change after post-annealing is most prominent for films grown at 6508C. In addition, our samples showed lower resistance and higher transition temperature, Tc , at increased deposition temperatures from 6508C to 7008C. This is in marked contrast to the results reported for PLD of LCMO films at ) 7508C, where the Tc of the as-deposited LCMO films decreased and the resisitivity of the films became larger when higher deposition temperature was used w13x. Excellent MR values were obtained in all our epitaxially grown as-deposited and post-annealed films at processing temperatures as low as 6508C.
temperature was monitored by a thermocouple fixed beneath the heater face plate. No correction has been made for the LAO substrate surface temperature, which is estimated to be ; 308C lower than the measured ones. For some samples, in-situ post-annealing at the same deposition temperature were carried out in 1 atm oxygen for 1 h. The structural quality of the as-deposited and post-annealed films were characterized by the X-ray diffraction ŽXRD. technique using a Philips four-circle diffractometer and Ni-filtered CuK a radiation. The resistivity–temperature Ž R–T . curves were measured by the usual four-point probe method and by cooling the samples from room temperature to liquid nitrogen temperature. The MR of the films were obtained by placing the sample in a uniform magnetic field Ž H s 1 T. provided by a vibrating sample magnetometer. The field applied was parallel to the film surface and the current flow direction.
2. Experiments 3. Results and discussion LCMO thin films with thickness of 200 nm were prepared by the PLD method. Single crystal Ž100.LaAlO 3 ŽLAO. substrates were used throughout for all film growth processes in the present studies. Both the LCMO and the LAO have cubic structures ˚ and a s 3.792 A, ˚ with lattice parameters a s 3.89 A respectively. Lattice mismatch is, therefore, less than 3%. The LCMO ceramic oxide target was prepared by using the standard solid state reaction technique, in which a mixture of proper amount of high purity La 2 O 3 , CaCO 3 and MnO 2 powders were repeatedly ground and calcined. It was then pressed into circular pellets of 2 cm diameter and 0.5 cm thick, and sintered at temperature of 11008C–13008C for 3 h in air. The nominal composition of the LCMO target was determined by energy dispersive X-ray microanalysis ŽEDX.. Composition of all deposited films showed a little deviation from the target stoichiometry. In the PLD experiments, an XeCl excimer laser Ž l s 308 nm. running at a repetition rate of 10 Hz was focused by a silica lens onto the flat surface of the rotating target. The laser fluence was estimated to be 1.5 Jrcm2 . The substrate to target distance was kept at 4 cm. Films were deposited under 200 mTorr ambient oxygen at four different substrate temperatures of 6008C, 6508C, 7008C and 7508C. The heater
3.1. The epitaxial growth of La 0 .7 Ca 0 .3 MnO3 thin films Fig. 1 show the u –2 u scans of the films deposited at 6008C, 6508C and 7508C. Apart from films grown at 6008C, all as-deposited films were well crystallized. From the figure, Ž h00. LCMO peaks can be seen clearly for films deposited at 6508C and 7508C. No trace of other LCMO reflections was observed. Films grown at 6008C, however, are largely amorphous and show very weak LCMO peaks. The v-scan rocking curve for the Ž200. LCMO reflections for films deposited at 6508C shows a narrow profile with a full width half maximum ŽFWHM. of 1.28. This suggests a good film crystalline orientation. Full epitaxial growth of the LCMO films at 6508C is confirmed by the XRD 3608 f-scans for the Ž220. reflections of the LCMO films and the LAO substrate shown in Fig. 2. Qualitatively similar but better epitaxial relations were found on films deposited at higher substrate temperatures. All post-annealed films showed improved crystallinity. For example, the FWHM of the rocking curve of the film grown at 6508C narrows down from 1.28 to 0.98 after annealing.
Y.S. Leung, K.H. Wong r Applied Surface Science 127–129 (1998) 491–495
Fig. 1. The XRD pattern of normal u –2 u scan of the films deposited at 6008C, 6508C and 7508C, respectively.
3.2. The electrical and magnetic properties of La 0 .7 Ca 0 .3 MnO3 thin films The deposition temperature and the post-annealing process have distinct effect on the electrical and magnetic properties of the LCMO thin films. The R–T curves of the thin films fabricated at 6508C and 7008C with and without post-annealing are shown in Fig. 3. The values of resistivity of the films are normalized for the ease of comparison. It is seen from this figure that the Tc of the as deposited films shifted from 125 K to 190 K as the deposition temperatures changes from 6508C to 7008C. Post-deposition annealing, on the other hand, raised the Tc further by about 20 K in both cases Ž145 K and 210 K for 6508C and 7008C, respectively.. The absolute electrical resistance of the LCMO films showed strong dependence on the deposition temperatures and the post-annealing process. As depicted in Fig. 4, the maximum resistivities of the as deposited films decreased fairly smoothly with increasing deposition temperatures. These results are clearly different from those reported by Zhang et al. w13x, in which their LCMO films grown at a relatively higher substrate
493
Fig. 2. The X-ray f-scan of the Ž220. reflections of LCMO films and LAO substrate.
temperature of ; 9008C showed the opposite trend. They attributed the effect being caused by oxygen deficiency in the LCMO films. Apparently, oxygen depletion due to high deposition temperatures has not occurred in our samples prepared at 6008C– 7508C. Post-annealing has, in general, the effect of reducing the resistance of the films. The most noticeable
Fig. 3. The R – T curves of LCMO as-deposited films AŽ6508C., X X BŽ7008C. and post-annealed films A Ž6508C., B Ž7008C..
494
Y.S. Leung, K.H. Wong r Applied Surface Science 127–129 (1998) 491–495
Fig. 4. The peak resistivities of the LCMO films as a function of processing temperatures; B are the as-deposited films and ' are the post-annealed films. The solid curves are drawn for viewing purpose.
change, however, occurs in films deposited at 6508C, in which the film resistance dropped by more than an order of magnitude from 4230 m V cm to 310 m V cm after annealing at 6508C for 1 h. It is important to note that for the annealed films, the resistance remained more or less the same at about 300 m V cm for deposition temperatures of 6508C, 7008C and 7508C. The marked change of resistance for films annealed at 6508C may imply that the single phase perovskite LCMO formation starts at such temperature. Annealing can, thus, promote the growth of the
highly conducting perovskite LCMO films and a sudden drop in the film’s resistance results. Higher annealing temperature, however, only improves the crystallinity of the already conducting LCMO films and has a minor effect on lowering the film resistance. Under the influence of a magnetic field aligned parallel to the plane of the LCMO films, the resistance is suppressed resulting in a noted GMR effect. Fig. 5 showed the R–T curves and the MR profiles of the LCMO films deposited and post-annealed at 6508C. A maximum MR of 37% at 110 K was observed. For the films fabricated under higher deposition temperatures, the MR became larger. MR values of 37%, 40% and 50% were obtained at deposition temperature of 6508C, 7008C and 7508C, respectively. The reason is that the crystallinity of the films improved at higher deposition temperatures.
4. Summary Epitaxial growth of La 0.7 Ca 0.3 MnO 3 films on LaAlO 3 Ž100. substrates were obtained under a relatively low processing temperature of 6508C. It has been shown that both the resistivity and the transition temperatures, Tc , improved at higher deposition temperatures from 6508C to 7008C. The effect is most prominent for films prepared at 6508C, in which the single phase conductive LCMO is believed to have crystallized and oriented films began to grow. A MR of 37% is obtained for films grown at 6508C under a relatively low field of 1 T. Our studies suggest that epitaxial LCMO thin films prepared by PLD method can be obtained at the processing temperature as low as 6508C.
Acknowledgements Fig. 5. The R – T curves at B s 0 T Žsolid line. and B s1 T Ždash line. presented and the values of the resistivities referred to the right-hand axis. The MR profiles of the 6508C post-annealed LCMO films presented by the solid squares. The values of the MR are determined by Ž R Bs 0 T y R Bs1 T .r R Bs0 T and referred to the left-hand axis.
This work is supported by a Research Grant of the Hong Kong Polytechnic University under Code No. 0350.536.A3.110. One of the author ŽY.S. Leung. is grateful for the award of a research studentship the Hong Kong Polytechnic University.
Y.S. Leung, K.H. Wong r Applied Surface Science 127–129 (1998) 491–495
References w1x S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 Ž1994. 413. w2x M. McCormack, S. Jin, T.H. Tiefel, R.M. Fleming, J.M. Phillips, R. Ramesh, Appl. Phys. Lett. 64 Ž1994. 3045. w3x J.Y. Gu, K.H. Kim, T.W. Noh, K.-S. Suh, J. Appl. Phys. 78 Ž1995. 6151. w4x G.Q. Gong, C.L. Canedy, G. Xiao, J. Appl. Phys. 79 Ž1996. 4538. w5x R.E. Treece, P. Dorsey, M. Rubinstein, J.M. Byers, J.S. Horwitz, E. Donovan, D.B. Chrisey, Mater. Res. Soc. Symp. Proc. 384 Ž1995. 427.
495
w6x R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 Ž1993. 2331. w7x C.-S. Huang, I.-N. Lin, J.Y. Lee, T.-Y. Tseng, Jpn. J. Appl. Phys. 33 Ž1994. 4058. w8x C. Prouteau, J.F. Hamet, B. Mercey, M. Hervieu, B. Raveau, D. Robbes, L. Coudrier, G. Ben, Physica C 248 Ž1995. 108. w9x M.H. Yeh, K.S. Liu, J. Appl. Phys. 77 Ž1995. 5335. w10x S.K. Hau, K.H. Wong, P.W. Chan, C.L. Choy, Appl. Phys. Lett. 66 Ž1995. 245. w11x S. Jin, M. McCormack, T.H. Tiefel, R. Ramesh, J. Appl. Phys. 76 Ž1994. 6929. w12x S.-Y. Bae, S.X. Wang, Appl. Phys. Lett. 69 Ž1996. 121. w13x W. Zhang, W. Boyd, M. Elliot, W. Herrenden-Harkerand, Appl. Phys. Lett. 69 Ž1996. 3929.