- Email: [email protected]
3MnO3 polycrystalline films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 269 (2004) 38–41
The effects of lattice distortion on the electrical properties of magnetic La2/3Sr1/3MnO3 polycrystalline films X.L. Jiang*, Q.Y. Xu, G. Ni, H. Sang, F.M. Zhang, Y.W. Du National Laboratory of Solid State Microstructures, Center for Advanced Studies in Science and Technology of Microstructures, Nanjing University, Nanjing 210093, China Received 12 November 2002; received in revised form 15 May 2003
Abstract Using magnetron sputtering technique, a series of La2/3Sr1/3MnO3 polycrystalline films with various thickness, t; were prepared on Si (1 0 0) substrates with oxidized surface. The electrical transport properties of the film materials were investigated. It has been found that with the change of the film thickness the resistivity appears to be a significant change at thickness around 73 nm. XRD studies revealed that there is a corresponding change for the lattice parameters when the thickness of the films is less than 73 nm. It is believed that the observed big change of the resistivity at the thickness of around 73 nm is attributed to the enhancement of scattering of the conduction electrons, which is resulted from the distortion of the lattice structure. r 2003 Elsevier B.V. All rights reserved. PACS: 75.30.Kz; 75.70.–i; 71.45.Gm Keywords: Distortion; Lattice; LaSrMnO; Polycrystalline film
1. Introduction The discovery of colossal magnetoresistance (CMR) [1,2] in the perovskite manganites R1xAxMnO3, where R and A are rare-earth and alkaline-earth elements, respectively, has attracted renewed interests in these compounds. The resistivity of these materials has a peak at the metal– insulator transition temperature (Tp ), which is accompanied by a paramagnetic–ferromagnetic (PM–FM) transition at a very close temperature *Corresponding author. Tel.: +86-25-3592911; fax: +86-253595535. E-mail address: [email protected] (X.L. Jiang).
(Tc ) [3]. This behavior is usually modeled with double-exchange (DE) theory [4–6] and the John– Teller (JT) distortion [7]. Among these manganite materials, La1xSrxMnO3 group, especially La2/ 3Sr1/3MnO3 and La0.7Sr0.3MnO3, has been studied extensively due to the high Curie temperature and nearly 100% spin polarization of the itinerant electrons [8,9], which can be used widely in fabricating magnetic tunnelling junctions [10–13]. Many factors can influence the magnetic and electric transport properties in these manganite materials, such as oxygen concentration [14], pressure [15], etc. during the fabrication. While for the thin films, thickness can be critical to affect their structures and properties. Recently, many
0304-8853/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0304-8853(03)00558-4
ARTICLE IN PRESS X.L. Jiang et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 38–41 0.050 240
0.045 0.040
220
0.035 0.030 0.025
200
Tp (K)
resistivity (Ωm)
investigations have been done on the thicknessdependent transport properties of the epitaxial manganite perovskite films [16–18]. These results show that the thickness-dependent properties mainly come from the compress or tensile strain, which is induced by the change of concentration of Sr. However, the effect of the lattice distortion on the physical properties at fixed concentration of Sr in La2/3Sr1/3MnO3 polycrystalline films could be another interesting issue, which is what presented in this work.
39
0.020 0.015
180
0.010 50
100
150
200
250
300
350
t (nm) Fig. 1. Thickness dependence of the room temperature resistivity and M–I transition temperature Tp ; respectively.
2. Experimental
3. Results and discussions To investigate the metal–insulator transition of the film materials with different thickness, the temperature dependence of the resistivity was measured from 77 K to room temperature for the films. The transition temperatures, Tp ; between the metallic and insulating behaviors for the films were obtained, as shown in Fig. 1. It can be seen that the Tp for the films with thickness over 73 nm varies slightly around 210 K, which is lower than
0.00 [R(H)-R(0)]/R(0)
The samples of La2/3Sr1/3MnO3 thin films were fabricated on Si (1 0 0) substrates using magnetron sputtering, in which the silicon substrates were first oxidized in a thermal furnace in order to form a SiO2 layer. The target was a disk of La2/3Sr1/ 3MnO3 with a diameter of 6 cm. The base pressure was 1.0 104 Pa. Operation pressure of about 1.0 Pa for Ar (99.99%) was employed during the film deposition. The substrate temperature was kept at room temperature in the deposition of the films. The films with different thickness of 9, 18, 37, 73, 110, 147, 220 and 330 nm were annealed at 700 C for 10 h for achieving perovskite structure. The electrical resistivity and magneto-resistance (MR) of the crystallized film materials were measured at different conditions with conventional 4-probe method. The crystalline structures of the film materials were analyzed by X-ray diffraction (XRD, Rigaku diffractometer).
bulk
-0.02 -0.04 -0.06 -0.08
film
-0.10 -15
-10
-5
0 5 H (kOe)
10
15
Fig. 2. Magnetic field dependence of the resistivity for the bulk and film (thicker than 73 nm) of La2/3Sr1/3MnO3 polycrystalline materials.
that for the correspondent bulk materials [8]. Unfortunately, as the too high resistivity (out of the measurement limit of the system) of the films thinner than 73 nm, the corresponding values were unavailable. The resistivity of the films at room temperature for the films with different thickness are plotted in the figure too. It is noted that the resistivity for the films thicker than 73 nm is around 0.015 Om. However, a big increase to around 0.045 Om for the resistivity was observed when the thickness of the films drops to 73 nm. Measurement of the MR was performed. However, as mentioned above, only data for the films thicker than 73 nm are obtained. Shown in Fig. 2 is a typical curve for the field dependence of the MR. The applied field used was up to 1 T and parallel to the sample surface. For comparison, the MR curve of the bulk materials was plotted in the figure. It is
ARTICLE IN PRESS X.L. Jiang et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 38–41
40
clear that the field dependence of MR for t > 73 nm samples is linear. Fig. 3 shows the XRD spectra of the samples with different thickness. It is clearly seen that the films can be classified into two groups according to the spectra. One group is for films thicker than 73 nm and the other one is for those thinner than 73 nm. As we know, for rhombohedric structure, the distance between two neighboring planes with orientation of (HKL) can be expressed as 1 2 dHKL ¼
ðH 2 þ K 2 þ L2 Þsin2 a þ 2ðHK þ HL þ KLÞðcos2 a cos aÞ ; a2 ð1 3 cos2 a þ 2 cos3 aÞ
310
220
210 200
110
211
where a and a are the lattice spacing and angle, respectively. With the expression, the lattice parameters of the structure can be calculated from the diffraction peaks. It was found that the lattice parameters, a and a; of the rhombohedric structure ( respectively, for are around 59.175 and 5.542 A, the films thicker than 73 nm, which is consistent to the parameters of the bulk [19]. However, for the films thinner than 73 nm, it can be seen that the peak (2 2 0) shifts towards lower diffraction angle. The correspondent lattice parameters of the rhombohedric structures were calculated as ( Therefore, there exists a 64.805 and 5.515 A. distortion of the rhombohedric structure when the
Intensity (a. u.)
330 nm 220 nm 147 nm 110 nm 220
73 nm
films get thinner. Moreover, it can be noticed that there is a peak around 2y ¼ 31 for the films thinner than 73 nm, which is thought due to the mixed phase of La2O3 although very limited in quantity. This interesting observation on the lattice distortion when the film thickness becomes smaller could be attributed to two causes: (1) the possible deviation of the composition of Sr in the materials from the value for thicker films and (2) the stress change between the film and the substrate. In the first possibility, the deviation of the concentration of Sr in the rhombohedric structure could be due to the enrichment of La2O3. The change of Sr concentration will cause the change of the lattice parameters of the structure [8]. While, for the latter possibility, due to the lattice constant mismatch between the substrate and film materials, the stress caused could distort the structure of the films, especially more significant when the film is thinner. It is believed that the second possibility is more likely instead of the first one for the case under investigation. If it were the first case, the distortion would occur gradually with the change of the film thickness. However, this is not the case as the abrupt change of the distortion was observed for film thickness at around 73 nm. For the second case, the stress could accumulate with the increase of the film thickness. When the film gets thicker and thicker, the accumulated stress becomes bigger and bigger, and the distortion disappears at some critical point, or 73 nm as observed in this work. Therefore, it is reasonable to believe that the sharp change of the resistivity at film thickness of 73 nm is resulted from the corresponding abrupt distortion of the structure at the same thickness as the distortion of the structure enhances the scattering of the conduction electrons in the materials.
37 nm
La2O3
4. Conclusions
18 nm 9 nm
20
30
40
50
2θ
60
70
80
Fig. 3. XRD spectra of the polycrystalline La2/3Sr1/3MnO3 films with different thickness at room temperature.
With a series of La2/3Sr1/3MnO3 polycrystalline films with different thickness on SiO2 coated on Si (1 0 0) substrate, the electrical and magnetic properties of the materials were studied. A sharp change of the resistivity at critical thickness of
ARTICLE IN PRESS X.L. Jiang et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 38–41
about 73 nm for the films was observed. The corresponding abrupt change of the lattice parameters was also revealed by XRD. It is believed the resistivity change is due to the distortion of the structure, which is resulted from the stress between the film material and the substrates.
Acknowledgements The authors would like to thank Dr. Zhou Yugang for thickness measurement. This work was supported in part by G1999064508, NSFC grant No. 19890310(4).
References [1] R. Von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [3] P. Schiffer, A.P. Ramirez, W. Bao, S.-W. Cheong, Phys. Rev. Lett. 75 (1995) 3336. [4] C. Zener, Phys. Rev. 82 (1951) 403.
41
[5] P.W. Anderson, H. Hasegawa, Phys. Rev. 100 (1955) 675. [6] P.G. deGennes, Phys. Rev. 118 (1960) 141. [7] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [8] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [9] J.-H. Park, E. Vescovo, H.-J. Kim, C. Kwon, R. Ramesh, T. Venkatesam, Nature 392 (1998) 794. [10] J. O’Donnell, A.E. Andrus, S. Oh, E.V. Colla, J.N. Eckstein, Appl. Phys. Lett. 76 (2000) 1914. [11] H.Q. Yin, J.-S. Zhou, J.B. Goodenough, Appl. Phys. Lett. 77 (2000) 714. [12] Y. Lu, X.W. Xi, G.Q. Gong, G. Xiao, A. Gupta, P. Lecoeur, J.Z. Sun, Y.Y. Wang, V.P. Dravid, Phys. Rev. B 54 (1996) R8357. [13] T. Obata, T. Manako, Y. Shimakwa, Y. Kubo, Appl. Phys. Lett. 74 (1999) 290. [14] G. Zhao, K. Conder, H. Keller, K.A. Muller, . Nature 381 (1996) 676. [15] J.-S. Zhou, J.B. Goodenough, A. Asamitsu, Y. Tokura, Phys. Rev. Lett. 79 (1997) 3234. [16] J.Z. Sun, D.W. Abraham, R.A. Rao, C.B. Eom, Appl. Phys. Lett. 74 (1999) 3017. [17] A. Barman, G. Koren, Appl. Phys. Lett. 77 (2000) 1674. [18] H.S. Wang, E. Wertz, Y.F. Hu, Q. Li, D.G. Schlom, J. Appl. Phys. 87 (2000) 7409. [19] K. Cherif, J. Dhahri, H. Vincent, S. Zemni, E. Dhahri, M. Oumezzine, Physica B 321 (2002) 48.
Journal of Magnetism and Magnetic Materials 269 (2004) 38–41
The effects of lattice distortion on the electrical properties of magnetic La2/3Sr1/3MnO3 polycrystalline films X.L. Jiang*, Q.Y. Xu, G. Ni, H. Sang, F.M. Zhang, Y.W. Du National Laboratory of Solid State Microstructures, Center for Advanced Studies in Science and Technology of Microstructures, Nanjing University, Nanjing 210093, China Received 12 November 2002; received in revised form 15 May 2003
Abstract Using magnetron sputtering technique, a series of La2/3Sr1/3MnO3 polycrystalline films with various thickness, t; were prepared on Si (1 0 0) substrates with oxidized surface. The electrical transport properties of the film materials were investigated. It has been found that with the change of the film thickness the resistivity appears to be a significant change at thickness around 73 nm. XRD studies revealed that there is a corresponding change for the lattice parameters when the thickness of the films is less than 73 nm. It is believed that the observed big change of the resistivity at the thickness of around 73 nm is attributed to the enhancement of scattering of the conduction electrons, which is resulted from the distortion of the lattice structure. r 2003 Elsevier B.V. All rights reserved. PACS: 75.30.Kz; 75.70.–i; 71.45.Gm Keywords: Distortion; Lattice; LaSrMnO; Polycrystalline film
1. Introduction The discovery of colossal magnetoresistance (CMR) [1,2] in the perovskite manganites R1xAxMnO3, where R and A are rare-earth and alkaline-earth elements, respectively, has attracted renewed interests in these compounds. The resistivity of these materials has a peak at the metal– insulator transition temperature (Tp ), which is accompanied by a paramagnetic–ferromagnetic (PM–FM) transition at a very close temperature *Corresponding author. Tel.: +86-25-3592911; fax: +86-253595535. E-mail address: [email protected] (X.L. Jiang).
(Tc ) [3]. This behavior is usually modeled with double-exchange (DE) theory [4–6] and the John– Teller (JT) distortion [7]. Among these manganite materials, La1xSrxMnO3 group, especially La2/ 3Sr1/3MnO3 and La0.7Sr0.3MnO3, has been studied extensively due to the high Curie temperature and nearly 100% spin polarization of the itinerant electrons [8,9], which can be used widely in fabricating magnetic tunnelling junctions [10–13]. Many factors can influence the magnetic and electric transport properties in these manganite materials, such as oxygen concentration [14], pressure [15], etc. during the fabrication. While for the thin films, thickness can be critical to affect their structures and properties. Recently, many
0304-8853/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0304-8853(03)00558-4
ARTICLE IN PRESS X.L. Jiang et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 38–41 0.050 240
0.045 0.040
220
0.035 0.030 0.025
200
Tp (K)
resistivity (Ωm)
investigations have been done on the thicknessdependent transport properties of the epitaxial manganite perovskite films [16–18]. These results show that the thickness-dependent properties mainly come from the compress or tensile strain, which is induced by the change of concentration of Sr. However, the effect of the lattice distortion on the physical properties at fixed concentration of Sr in La2/3Sr1/3MnO3 polycrystalline films could be another interesting issue, which is what presented in this work.
39
0.020 0.015
180
0.010 50
100
150
200
250
300
350
t (nm) Fig. 1. Thickness dependence of the room temperature resistivity and M–I transition temperature Tp ; respectively.
2. Experimental
3. Results and discussions To investigate the metal–insulator transition of the film materials with different thickness, the temperature dependence of the resistivity was measured from 77 K to room temperature for the films. The transition temperatures, Tp ; between the metallic and insulating behaviors for the films were obtained, as shown in Fig. 1. It can be seen that the Tp for the films with thickness over 73 nm varies slightly around 210 K, which is lower than
0.00 [R(H)-R(0)]/R(0)
The samples of La2/3Sr1/3MnO3 thin films were fabricated on Si (1 0 0) substrates using magnetron sputtering, in which the silicon substrates were first oxidized in a thermal furnace in order to form a SiO2 layer. The target was a disk of La2/3Sr1/ 3MnO3 with a diameter of 6 cm. The base pressure was 1.0 104 Pa. Operation pressure of about 1.0 Pa for Ar (99.99%) was employed during the film deposition. The substrate temperature was kept at room temperature in the deposition of the films. The films with different thickness of 9, 18, 37, 73, 110, 147, 220 and 330 nm were annealed at 700 C for 10 h for achieving perovskite structure. The electrical resistivity and magneto-resistance (MR) of the crystallized film materials were measured at different conditions with conventional 4-probe method. The crystalline structures of the film materials were analyzed by X-ray diffraction (XRD, Rigaku diffractometer).
bulk
-0.02 -0.04 -0.06 -0.08
film
-0.10 -15
-10
-5
0 5 H (kOe)
10
15
Fig. 2. Magnetic field dependence of the resistivity for the bulk and film (thicker than 73 nm) of La2/3Sr1/3MnO3 polycrystalline materials.
that for the correspondent bulk materials [8]. Unfortunately, as the too high resistivity (out of the measurement limit of the system) of the films thinner than 73 nm, the corresponding values were unavailable. The resistivity of the films at room temperature for the films with different thickness are plotted in the figure too. It is noted that the resistivity for the films thicker than 73 nm is around 0.015 Om. However, a big increase to around 0.045 Om for the resistivity was observed when the thickness of the films drops to 73 nm. Measurement of the MR was performed. However, as mentioned above, only data for the films thicker than 73 nm are obtained. Shown in Fig. 2 is a typical curve for the field dependence of the MR. The applied field used was up to 1 T and parallel to the sample surface. For comparison, the MR curve of the bulk materials was plotted in the figure. It is
ARTICLE IN PRESS X.L. Jiang et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 38–41
40
clear that the field dependence of MR for t > 73 nm samples is linear. Fig. 3 shows the XRD spectra of the samples with different thickness. It is clearly seen that the films can be classified into two groups according to the spectra. One group is for films thicker than 73 nm and the other one is for those thinner than 73 nm. As we know, for rhombohedric structure, the distance between two neighboring planes with orientation of (HKL) can be expressed as 1 2 dHKL ¼
ðH 2 þ K 2 þ L2 Þsin2 a þ 2ðHK þ HL þ KLÞðcos2 a cos aÞ ; a2 ð1 3 cos2 a þ 2 cos3 aÞ
310
220
210 200
110
211
where a and a are the lattice spacing and angle, respectively. With the expression, the lattice parameters of the structure can be calculated from the diffraction peaks. It was found that the lattice parameters, a and a; of the rhombohedric structure ( respectively, for are around 59.175 and 5.542 A, the films thicker than 73 nm, which is consistent to the parameters of the bulk [19]. However, for the films thinner than 73 nm, it can be seen that the peak (2 2 0) shifts towards lower diffraction angle. The correspondent lattice parameters of the rhombohedric structures were calculated as ( Therefore, there exists a 64.805 and 5.515 A. distortion of the rhombohedric structure when the
Intensity (a. u.)
330 nm 220 nm 147 nm 110 nm 220
73 nm
films get thinner. Moreover, it can be noticed that there is a peak around 2y ¼ 31 for the films thinner than 73 nm, which is thought due to the mixed phase of La2O3 although very limited in quantity. This interesting observation on the lattice distortion when the film thickness becomes smaller could be attributed to two causes: (1) the possible deviation of the composition of Sr in the materials from the value for thicker films and (2) the stress change between the film and the substrate. In the first possibility, the deviation of the concentration of Sr in the rhombohedric structure could be due to the enrichment of La2O3. The change of Sr concentration will cause the change of the lattice parameters of the structure [8]. While, for the latter possibility, due to the lattice constant mismatch between the substrate and film materials, the stress caused could distort the structure of the films, especially more significant when the film is thinner. It is believed that the second possibility is more likely instead of the first one for the case under investigation. If it were the first case, the distortion would occur gradually with the change of the film thickness. However, this is not the case as the abrupt change of the distortion was observed for film thickness at around 73 nm. For the second case, the stress could accumulate with the increase of the film thickness. When the film gets thicker and thicker, the accumulated stress becomes bigger and bigger, and the distortion disappears at some critical point, or 73 nm as observed in this work. Therefore, it is reasonable to believe that the sharp change of the resistivity at film thickness of 73 nm is resulted from the corresponding abrupt distortion of the structure at the same thickness as the distortion of the structure enhances the scattering of the conduction electrons in the materials.
37 nm
La2O3
4. Conclusions
18 nm 9 nm
20
30
40
50
2θ
60
70
80
Fig. 3. XRD spectra of the polycrystalline La2/3Sr1/3MnO3 films with different thickness at room temperature.
With a series of La2/3Sr1/3MnO3 polycrystalline films with different thickness on SiO2 coated on Si (1 0 0) substrate, the electrical and magnetic properties of the materials were studied. A sharp change of the resistivity at critical thickness of
ARTICLE IN PRESS X.L. Jiang et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 38–41
about 73 nm for the films was observed. The corresponding abrupt change of the lattice parameters was also revealed by XRD. It is believed the resistivity change is due to the distortion of the structure, which is resulted from the stress between the film material and the substrates.
Acknowledgements The authors would like to thank Dr. Zhou Yugang for thickness measurement. This work was supported in part by G1999064508, NSFC grant No. 19890310(4).
References [1] R. Von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [3] P. Schiffer, A.P. Ramirez, W. Bao, S.-W. Cheong, Phys. Rev. Lett. 75 (1995) 3336. [4] C. Zener, Phys. Rev. 82 (1951) 403.
41
[5] P.W. Anderson, H. Hasegawa, Phys. Rev. 100 (1955) 675. [6] P.G. deGennes, Phys. Rev. 118 (1960) 141. [7] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [8] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [9] J.-H. Park, E. Vescovo, H.-J. Kim, C. Kwon, R. Ramesh, T. Venkatesam, Nature 392 (1998) 794. [10] J. O’Donnell, A.E. Andrus, S. Oh, E.V. Colla, J.N. Eckstein, Appl. Phys. Lett. 76 (2000) 1914. [11] H.Q. Yin, J.-S. Zhou, J.B. Goodenough, Appl. Phys. Lett. 77 (2000) 714. [12] Y. Lu, X.W. Xi, G.Q. Gong, G. Xiao, A. Gupta, P. Lecoeur, J.Z. Sun, Y.Y. Wang, V.P. Dravid, Phys. Rev. B 54 (1996) R8357. [13] T. Obata, T. Manako, Y. Shimakwa, Y. Kubo, Appl. Phys. Lett. 74 (1999) 290. [14] G. Zhao, K. Conder, H. Keller, K.A. Muller, . Nature 381 (1996) 676. [15] J.-S. Zhou, J.B. Goodenough, A. Asamitsu, Y. Tokura, Phys. Rev. Lett. 79 (1997) 3234. [16] J.Z. Sun, D.W. Abraham, R.A. Rao, C.B. Eom, Appl. Phys. Lett. 74 (1999) 3017. [17] A. Barman, G. Koren, Appl. Phys. Lett. 77 (2000) 1674. [18] H.S. Wang, E. Wertz, Y.F. Hu, Q. Li, D.G. Schlom, J. Appl. Phys. 87 (2000) 7409. [19] K. Cherif, J. Dhahri, H. Vincent, S. Zemni, E. Dhahri, M. Oumezzine, Physica B 321 (2002) 48.