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Positive magnetoresistance feature in ultrathin La0.67Ca0.33MnO3 films
Solid State Communications 136 (2005) 528–532 www.elsevier.com/locate/ssc
Positive magnetoresistance feature in ultrathin La0.67Ca0.33MnO3 films J.P. Zhong*, S.B. Yang, J. Yuan, B. Xu, L.X. Cao, X.G. Qiu, B.R. Zhao National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China Received 2 September 2005; accepted 15 September 2005 by S. Das Sarma Available online 3 October 2005
Abstract The ultrathin epitaxial La0.67Ca0.33MnO3 (LCMO) films with thickness from 8 to 50 nm on (001) SrTiO3 (STO) substrates were prepared and investigated, and the unique MR effects were distinctly observed. The positive magnetoresistance (MR) effects were observed in the high temperature region for all the present ultrathin films. The films with thickness in the range from 17 to 30 nm not only show the positive MR effect at high temperature (OTMI), but also in the low temperature region (%96 K for 17 nm, %108 K for 30 nm). It is suggested that the positive MR effect occurred at low temperature could be attributed to the scattering of the domain wall on charge carriers, while the positive MR effect occurred at temperature higher than TMI is attributed to the transport dominated by the magnetic polarons which formed due to the trap of charge carriers with spin in local distorted lattice in such highly strained ultrathin LCMO films. q 2005 Elsevier Ltd. All rights reserved. PACS: 75.47.GK; 75.47.Km Keywords: A. Ultrathin film; B. Epitaxy; D. Electronic transport; D. Magnetoresistance
1. Introduction The discovery of unusual magnetic and electric properties in perovskite manganites has led to a constantly increasing interest for both basic science and applications. The notable property is the extremely large change in resistivity under the magnetic field, known as the colossal magnetoresistance (CMR) effect. This effect is explained mainly by double-exchange interaction [1], electron– phonon interaction [2,3], and electronic phase-separation [4–7]. But it is found that for these materials the MR effect in epitaxial thin films is quite different from that in bulk materials. Therefore, to deeply understand the mechanism of MR effect, the role of charge carriers, and their interaction with spins and lattice distortions should be * Corresponding author. Tel./fax: C86 10 82649193. E-mail address: [email protected] (J.P. Zhong).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.09.018
studied more carefully. Recently, there have been attempts to clarify the role of film thickness, or rather the influence of strain on the MR effect in manganites. In general, the filmsubstrate lattice mismatch will make the layers close to the interface to be strained. When the film grows to a critical thickness, the strain will be released through the formed dislocations. Such critical thickness should be in nanometer range. Therefore, to understand the strain effects, the preparation and investigation of nanometer scale films, ultrathin films, should be necessary. Some aspects related to strain effect have been addressed, for example, the magnetoresistance [8], microstructure [9], crystallographic domain structure [10], and magnetic anisotropy [11] of the manganites films with various thicknesses have been investigated. Usually, the value of the colossal magnetoresistance, defined as (r(H)Kr(0))/r(0), is negative. The positive MR, however, is observed in some more unusual cases, involving
J.P. Zhong et al. / Solid State Communications 136 (2005) 528–532
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doped or oxygen-deficient samples [12–18], which more or less related to the strains. So that, to deeply and systematically search the occurrence of the positive MR in manganites, and to develop the possible application, making the typical and strongly strained system may be an effective way. In this paper, we report the results from the studies on transport properties of strained ultrathin (8–50 nm) epitaxial LCMO films. The positive MR effect was clearly observed, which occurs not only at temperature higher than TMI, but also in some cases, occurs in the low temperature region (for the films with thickness of 17 and 30 nm). In addition, the strong thickness dependence of negative MR effects for these ultrathin LCMO films were also observed, the thinner the film is, and the stronger the MR effect is.
2. Experiment We used pulsed laser deposition (PLD) method to fabricate the ultrathin La0.67Ca0.33MnO3 films. For preparing these films the stoichiometric target of La0.67Ca0.33MnO3 was prepared by conventional solid state reaction. A 308 nm XeCl excimer laser (Lambda Physik Compex205) was used. The films were deposited on (001) STO single-crystal substrates under the following conditions: the background vacuum was better than 1!10K3 Pa, the substrate temperature was 750 8 the laser radiation energy density was 2 J/cm2. To get suitable deposition rate, the pulse repetition rate was controlled as 1 Hz, and the oxygen pressure of 50 Pa was used. After deposition an in situ annealing was performed in an oxygen pressure of 0.8 atm and at temperature of 800 8C for 30–60 min. The X-ray diffraction shows that all LCMO ultrathin films are highly c-axised oriented. In addition, by using the atomic force microscopy (AFM), all films show high quality surface. The ˚ . We also root-mean-square (rms) roughness is within 2 A used AFM to determine the film thickness and calibrate the deposition rate. Transport measurements with standard four terminal method were performed in a superconducting quantum interference device (MPMS-5 SQUID) magnetometer with the magnetic field parallel to the film plane and parallel to the current.
Fig. 1. Temperature dependence of resistivity r(T) of ultrathin La0.67Ca0.33MnO3 (LCMO) films on (001) SrTiO3 (STO) substrates with thickness from 10 to 30 nm.
decreases. It is suggested that this behavior can be clarified taking into account the stress due to film-substrate lattice mismatch. It may be understood that the distorted lattice due to strain suppresses the ferromagnetic phase. Fig. 2(a)–(d) shows the temperature dependence of the MR effects for the LCMO films with thickness of 10, 12, 17 and 30 nm in the magnetic fields of 200 Oe to1 T. The MR is defined as (r(H)Kr(0))/r(0), where r(H) and r(0) are the resistances in a magnetic field H and at zero field, respectively. In these ultrathin LCMO films, the unique MR effects were observed. First, all of these ultrathin LCMO films show negative MR peak near the metal– insulator transition temperature. Fig. 3 shows the film thickness dependence of the highest MR values at magnetic field 1 T. Obviously, the MR value strongly depends on the film thickness, the thinner the film is, and the larger the MR is. The largest MR (K93.1%) is found to occur for the film with thickness of 8 nm. The most important characteristic for these ultrathin films is the positive MR effect. It was
3. Results and discussion Fig. 1 shows the temperature dependence of the resistivity r for the ultrathin LCMO films deposited on STO substrate. All these films show a peak in r–T curve which is a typical feature of metal–insulator transition for LCMO. Corresponding to the film thickness 10, 12, 17 and 30 nm, the metal–insulator transition temperature TMI are 166, 174, 198 and 208 K, respectively. It is obviously that the resistivity and the metal–insulator transition temperature TMI are strongly dependent on the film thickness. When the thickness increases, TMI goes up and the resistivity
Fig. 2. Temperature dependence of magnetoresistance (r(H)Kr(0)) /r(0) of La0.67Ca0.33MnO3 films on (001) STO substrate with different fields (a) 10 nm; (b) 12 nm; (c): 17 nm; (d) 30 nm.
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Fig. 3. Film thickness dependence of the highest negative MR values at magnetic field 1 T.
found that the positive MR effect is strongly dependent on the film thickness. With different thickness, the positive MR occurs in different temperature region. For the films with thickness %12 nm, the positive MR effect was observed only in the higher temperature region (OTMI), and the positive MR values increase with decreasing magnetic field. In the lowest measured field (200 Oe), the highest positive MR of 10 nm thick LCMO film is 9.2% at 230 K. The transition from negative MR to positive MR is found to occur at temperature about 174 K. With increasing magnetic field, the transition temperature increases. When the applied field is 1 T, the highest positive MR of 10 nm LCMO film is 0.069% at 295 K, and the transition temperature is found to occur at about 285 K. For other ultrathin films with different thickness, the similar magnetic field dependences of negative MR-positive MR transition temperature and positive MR value were also observed. For the films with thickness of 12, 17 and 30 nm, the highest positive MR values which occur at 230, 244 and 250 K are 4.56, 4.08 and 7.185%, respectively. It should be particularly noted that for the cases of thickness of 17 and 30 nm (Fig. 2(c) and (d)), the obviously positive MR effect was also clearly observed in the low temperature region. For these cases, the positive MR values also increase with the decrease of the magnetic field. In the lowest magnetic field (200 Oe), the highest positive MR of 17 nm film (Fig. 2(c)) is 4.4% at 6 K. The transition from negative MR to positive MR is found to occur at temperature about 98 K. For the case of 30 nm thick film (Fig. 2(d)), the highest positive MR value is 14.7% at 6 K. The negative MR-positive MR transition temperature is about 110 K. For both films, the transition temperature decreases with increasing magnetic field. When the applied field is 1 T, the positive MR effect disappears and only negative MR effect can be observed. To understand the origin of the positive MR effect, the magnetism of these ultrathin films was investigated. Fig. 4 shows the magnetic hysteresis loops of the typical sample (the 17 nm thick LCMO film) at temperatures of 240, 150
Fig. 4. Magnetic hysteresis loops of 17 nm thick La0.67Ca0.33MnO3 film on (001) STO substrate with different temperature 50, 150 and 240 K.
and 50 K. At the temperature of 50 K, the saturation field is about 700 Oe, and the saturation magnetization and the remanence value are about 2.3!10K4 and 1.9!10K4 emu, respectively. At the temperature of 150 K, the saturation field is about 600 Oe, and the saturation magnetization and the remanence value are about 9.8!10K5 and 7.7!10K5 emu, respectively. In the case of 240 K, the magnetic hysteresis almost disappears and the film matrix is paramagnetic. It is clearly observed that the ferromagnetic domains inside the film matrix increase during the cooling process. For comparison, the magnetic hysteresis loops of 17 nm thick film at 50 and 10 K are plotted in Fig. 5. It is clearly seen that when the temperature was decreased from 50 to 10 K, the hysteresis has a large change, means that the ferromagnetic domains obviously change with magnetic field in the low temperature region. This may be the main reason of the positive MR effect. Because, in the low temperature region the applied magnetic field still can easily induce the domain wall motion. This would increase
Fig. 5. Magnetic hysteresis loops of 17 nm thick La0.67Ca0.33MnO3 film on (001) STO substrate at the temperatures of 10 and 50 K.
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the domain wall scattering on the charge carriers, and then induce the increase of the resistivity, i.e. the magnetic field enhances resistance and showing an obvious positive MR effect. At the higher magnetic field (1 T), however, the magnetization becomes saturated, then the domain wall motion and rotation caused by external field should be neglected and the positive MR effect disappears. When the film thickness increased to above 40 nm, the positive MR disappears in either high temperature or low temperature regions. This indicates that such MR effect is the feature of ultrathin films of LCMO. The positive MR effect in all these ultrathin LCMO films which occurred in the higher temperature region (above TMI), could not be induced by scattering of the charge carriers at the domain wall. Because the film matrix is paramagnetic in the high temperature region and no ferromagnetic domain exists. In the previous reports, the origin of the positive MR effect (which occurs at high temperature region) was suggested as the grain boundary scattering [19,20]. For our ultrathin ˚ , and no obvious LCMO films, the flatness is better than 2 A grain boundaries were observed, so the grain boundary scattering may not be the reason. We tend to suggest that the present positive MR effect may be attributed to the transport dominated by the magnetic polarons. For the present ultrathin films, the growth strain is mainly caused by the substrate-film lattice mismatch which should exist throughout the entire film matrix, and may not be released by defects since the film is too thin to form the defects (for such nanometer thick films the dislocation, cracks and other kind of defects may not be formed). Shown in Fig. 6 is a typical clear look of the HRTEM image of the substrate-film interface for the LCMO film with thickness of 30 nm, a good epitaxial growth throughout the entire film is observed. Therefore, the unreleased strain may
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directly induce the change in Mn–O–Mn bond angle and bond length, and result in a lattice distortion. Such local lattice distortion will produce a potential minimum to trap the electron [3]. When the temperature reaches around TMI, the trap well will trap spin polarized electrons, and form magnetic polarons. Therefore, we suggest that the polaron hopping may be the dominant factor for the conductance of such ultrathin LCMO films at temperatures higher than and around TMI. When the magnetic field is applied, the size of these polarons becomes larger (the number of polarons decreases) [21], and the hopping conductance will be suppressed. This will naturally lead to the positive MR effect. With further increase of magnetic field, the MR effect turns from positive to negative. This may be attributed to the formation of ferromagnetic domains near and below TC under the relatively higher magnetic field. In this case, the ferromagnetic domains can contribute to the percolation conductance in ultrathin films.
4. Conclusions In summary, we have fabricated the ultrathin La0.67Ca0.33MnO3 (LCMO) films on (001) SrTiO3 substrates by pulsed laser deposition. The unique MR effects were observed in these ultrathin LCMO films. It is clearly observed that all of these ultrathin LCMO films show positive MR effect at temperature higher than TMI. For the 10 nm thick LCMO film, the highest positive-MR of 9.2% was obtained at 230 K under the applied field of 200 Oe. It is particularly noted that the 17 and 30 nm thick LCMO films not only show positive MR effect in high temperature region, but also show the obviously positive MR effect in the low temperature region. For the 17 nm thick LCMO film, the highest positiveMR of 4.4% was obtained at 6 K under the applied field of 200 Oe, while the 30 nm thick LCMO film shows highest positive MR value of 14.7% at 6 K. We suggest that the positive MR effect occurred at low temperature could be attributed to the scattering of domain wall to the charge carriers. While the positive MR effect occurred at the temperature region higher than TMI is attributed to the transport dominated by the magnetic polarons, which formed due to the trap of electrons with spin in the local distorted lattice region in such highly strained ultrathin LCMO films. It also should be noted that the positive MR disappears in either high temperature or low temperature regions when the film thickness is larger than 40 nm, indicating that such positive MR effect is the feature of ultrathin films of LCMO.
Acknowledgements
Fig. 6. HRTEM view of LCMO/STO interface of the 30 nm thick La0.67Ca0.33MnO3 film.
The authors would like to thank W.W. Huang and X.Y. Qi for their measurement support. This work is supported by the State Key programme for Basic Research of China and the National Natural Science Foundation of China.
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References [1] C. Zener, Phys. Rev. 82 (1951) 403. [2] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [3] A.J. Millis, Nature 392 (1998) 147. [4] M. Uehara, S. Mori, C.H. Chen, S.W. Cheong, Nature 399 (1999) 560. [5] A. Moreo, M. Mayr, A. Feiguin, S. Yunoki, E. Dagotto, Phys. Rev. Lett. 84 (2000) 5568. [6] M. Mayr, A. Moreo, J.A. Verge´s, J. Arispe, A. Feiguin, E. Dagotto, Phys. Rev. Lett. 86 (2001) 135. [7] L.W. Zhang, C. Israel, A. Biswas, R.L. Greene, A.D. Lozanne, Science 298 (2002) 805. [8] G. Harzheim, J. Schubert, L. Beckers, W. Zander, D. Meertens, C. Ostho¨ver, Ch. Buchal, Mater. Sci. Eng. B 56 (1998) 147. [9] B. Wiedenhorst, C. Ho¨fener, Y. Lu, J. Klein, L. Alff, R. Gross, Appl. Phys. Lett. 74 (1999) 3636. [10] R.A. Rao, D. Lavric, T.K. Nath, C.B. Eom, L. Wu, F. Tsui, Appl. Phys. Lett. 73 (1998) 3294.
[11] T.K. Nath, R.A. Rao, D. Lavric, C.B. Eom, Appl. Phys. Lett. 74 (1999) 1615. [12] S.R. Shinde, S.B. Ogale, R.L. Greene, T. Venkatesan, K. Tsoi, S.-W. Cheong, A.J. Millis, J. Appl. Phys. 93 (2003) 1605. [13] F.-C. Zhang, W.-R. Chen, W.-Z. Gong, B. Xu, X.-G. Qiu, B.R. Zhao, Chin. Phys. 13 (2004) 783. [14] P. Chen, D.Y. Xing, Y.W. Du, Phys. Rev. B 64 (2001) 104402. [15] X.X. Zhang, J.M. Hernandez, Europhys. Lett. 47 (1999) 487. [16] K. Ghosh, et al., Appl. Phys. Lett. 73 (1998) 689. [17] M. Paraskevopoulos, et al., J. Phys.: Condens. Matter 12 (2000) 3993. [18] N.H. Hong, J. Saki, J.G. Noudem, F. Gervais, M. Gervais, J. Magn. Magn. Mater. 272–276 (2004) 1826. [19] N.D. Mathur, G. Burnell, S.P. Isaac, T.J. Jackson, B.S. Teo, J.L. MacManus-Driscoll, L.F. Cohen, J.E. Evetts, M.G. Blamire, Nature 387 (1997) 266. [20] C. Srinitiwarawong, M. Ziese, Appl. Phys. Lett. 73 (1998) 1140. [21] J.M. Teresa, M.R. Ibarra, P.A. Algarabel, C. Ritter, C. Marquina, J. Biasco, J. Garcia, A. del Moral, Z. Arnold, Nature 386 (1997) 256.
Positive magnetoresistance feature in ultrathin La0.67Ca0.33MnO3 films J.P. Zhong*, S.B. Yang, J. Yuan, B. Xu, L.X. Cao, X.G. Qiu, B.R. Zhao National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China Received 2 September 2005; accepted 15 September 2005 by S. Das Sarma Available online 3 October 2005
Abstract The ultrathin epitaxial La0.67Ca0.33MnO3 (LCMO) films with thickness from 8 to 50 nm on (001) SrTiO3 (STO) substrates were prepared and investigated, and the unique MR effects were distinctly observed. The positive magnetoresistance (MR) effects were observed in the high temperature region for all the present ultrathin films. The films with thickness in the range from 17 to 30 nm not only show the positive MR effect at high temperature (OTMI), but also in the low temperature region (%96 K for 17 nm, %108 K for 30 nm). It is suggested that the positive MR effect occurred at low temperature could be attributed to the scattering of the domain wall on charge carriers, while the positive MR effect occurred at temperature higher than TMI is attributed to the transport dominated by the magnetic polarons which formed due to the trap of charge carriers with spin in local distorted lattice in such highly strained ultrathin LCMO films. q 2005 Elsevier Ltd. All rights reserved. PACS: 75.47.GK; 75.47.Km Keywords: A. Ultrathin film; B. Epitaxy; D. Electronic transport; D. Magnetoresistance
1. Introduction The discovery of unusual magnetic and electric properties in perovskite manganites has led to a constantly increasing interest for both basic science and applications. The notable property is the extremely large change in resistivity under the magnetic field, known as the colossal magnetoresistance (CMR) effect. This effect is explained mainly by double-exchange interaction [1], electron– phonon interaction [2,3], and electronic phase-separation [4–7]. But it is found that for these materials the MR effect in epitaxial thin films is quite different from that in bulk materials. Therefore, to deeply understand the mechanism of MR effect, the role of charge carriers, and their interaction with spins and lattice distortions should be * Corresponding author. Tel./fax: C86 10 82649193. E-mail address: [email protected] (J.P. Zhong).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.09.018
studied more carefully. Recently, there have been attempts to clarify the role of film thickness, or rather the influence of strain on the MR effect in manganites. In general, the filmsubstrate lattice mismatch will make the layers close to the interface to be strained. When the film grows to a critical thickness, the strain will be released through the formed dislocations. Such critical thickness should be in nanometer range. Therefore, to understand the strain effects, the preparation and investigation of nanometer scale films, ultrathin films, should be necessary. Some aspects related to strain effect have been addressed, for example, the magnetoresistance [8], microstructure [9], crystallographic domain structure [10], and magnetic anisotropy [11] of the manganites films with various thicknesses have been investigated. Usually, the value of the colossal magnetoresistance, defined as (r(H)Kr(0))/r(0), is negative. The positive MR, however, is observed in some more unusual cases, involving
J.P. Zhong et al. / Solid State Communications 136 (2005) 528–532
529
doped or oxygen-deficient samples [12–18], which more or less related to the strains. So that, to deeply and systematically search the occurrence of the positive MR in manganites, and to develop the possible application, making the typical and strongly strained system may be an effective way. In this paper, we report the results from the studies on transport properties of strained ultrathin (8–50 nm) epitaxial LCMO films. The positive MR effect was clearly observed, which occurs not only at temperature higher than TMI, but also in some cases, occurs in the low temperature region (for the films with thickness of 17 and 30 nm). In addition, the strong thickness dependence of negative MR effects for these ultrathin LCMO films were also observed, the thinner the film is, and the stronger the MR effect is.
2. Experiment We used pulsed laser deposition (PLD) method to fabricate the ultrathin La0.67Ca0.33MnO3 films. For preparing these films the stoichiometric target of La0.67Ca0.33MnO3 was prepared by conventional solid state reaction. A 308 nm XeCl excimer laser (Lambda Physik Compex205) was used. The films were deposited on (001) STO single-crystal substrates under the following conditions: the background vacuum was better than 1!10K3 Pa, the substrate temperature was 750 8 the laser radiation energy density was 2 J/cm2. To get suitable deposition rate, the pulse repetition rate was controlled as 1 Hz, and the oxygen pressure of 50 Pa was used. After deposition an in situ annealing was performed in an oxygen pressure of 0.8 atm and at temperature of 800 8C for 30–60 min. The X-ray diffraction shows that all LCMO ultrathin films are highly c-axised oriented. In addition, by using the atomic force microscopy (AFM), all films show high quality surface. The ˚ . We also root-mean-square (rms) roughness is within 2 A used AFM to determine the film thickness and calibrate the deposition rate. Transport measurements with standard four terminal method were performed in a superconducting quantum interference device (MPMS-5 SQUID) magnetometer with the magnetic field parallel to the film plane and parallel to the current.
Fig. 1. Temperature dependence of resistivity r(T) of ultrathin La0.67Ca0.33MnO3 (LCMO) films on (001) SrTiO3 (STO) substrates with thickness from 10 to 30 nm.
decreases. It is suggested that this behavior can be clarified taking into account the stress due to film-substrate lattice mismatch. It may be understood that the distorted lattice due to strain suppresses the ferromagnetic phase. Fig. 2(a)–(d) shows the temperature dependence of the MR effects for the LCMO films with thickness of 10, 12, 17 and 30 nm in the magnetic fields of 200 Oe to1 T. The MR is defined as (r(H)Kr(0))/r(0), where r(H) and r(0) are the resistances in a magnetic field H and at zero field, respectively. In these ultrathin LCMO films, the unique MR effects were observed. First, all of these ultrathin LCMO films show negative MR peak near the metal– insulator transition temperature. Fig. 3 shows the film thickness dependence of the highest MR values at magnetic field 1 T. Obviously, the MR value strongly depends on the film thickness, the thinner the film is, and the larger the MR is. The largest MR (K93.1%) is found to occur for the film with thickness of 8 nm. The most important characteristic for these ultrathin films is the positive MR effect. It was
3. Results and discussion Fig. 1 shows the temperature dependence of the resistivity r for the ultrathin LCMO films deposited on STO substrate. All these films show a peak in r–T curve which is a typical feature of metal–insulator transition for LCMO. Corresponding to the film thickness 10, 12, 17 and 30 nm, the metal–insulator transition temperature TMI are 166, 174, 198 and 208 K, respectively. It is obviously that the resistivity and the metal–insulator transition temperature TMI are strongly dependent on the film thickness. When the thickness increases, TMI goes up and the resistivity
Fig. 2. Temperature dependence of magnetoresistance (r(H)Kr(0)) /r(0) of La0.67Ca0.33MnO3 films on (001) STO substrate with different fields (a) 10 nm; (b) 12 nm; (c): 17 nm; (d) 30 nm.
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Fig. 3. Film thickness dependence of the highest negative MR values at magnetic field 1 T.
found that the positive MR effect is strongly dependent on the film thickness. With different thickness, the positive MR occurs in different temperature region. For the films with thickness %12 nm, the positive MR effect was observed only in the higher temperature region (OTMI), and the positive MR values increase with decreasing magnetic field. In the lowest measured field (200 Oe), the highest positive MR of 10 nm thick LCMO film is 9.2% at 230 K. The transition from negative MR to positive MR is found to occur at temperature about 174 K. With increasing magnetic field, the transition temperature increases. When the applied field is 1 T, the highest positive MR of 10 nm LCMO film is 0.069% at 295 K, and the transition temperature is found to occur at about 285 K. For other ultrathin films with different thickness, the similar magnetic field dependences of negative MR-positive MR transition temperature and positive MR value were also observed. For the films with thickness of 12, 17 and 30 nm, the highest positive MR values which occur at 230, 244 and 250 K are 4.56, 4.08 and 7.185%, respectively. It should be particularly noted that for the cases of thickness of 17 and 30 nm (Fig. 2(c) and (d)), the obviously positive MR effect was also clearly observed in the low temperature region. For these cases, the positive MR values also increase with the decrease of the magnetic field. In the lowest magnetic field (200 Oe), the highest positive MR of 17 nm film (Fig. 2(c)) is 4.4% at 6 K. The transition from negative MR to positive MR is found to occur at temperature about 98 K. For the case of 30 nm thick film (Fig. 2(d)), the highest positive MR value is 14.7% at 6 K. The negative MR-positive MR transition temperature is about 110 K. For both films, the transition temperature decreases with increasing magnetic field. When the applied field is 1 T, the positive MR effect disappears and only negative MR effect can be observed. To understand the origin of the positive MR effect, the magnetism of these ultrathin films was investigated. Fig. 4 shows the magnetic hysteresis loops of the typical sample (the 17 nm thick LCMO film) at temperatures of 240, 150
Fig. 4. Magnetic hysteresis loops of 17 nm thick La0.67Ca0.33MnO3 film on (001) STO substrate with different temperature 50, 150 and 240 K.
and 50 K. At the temperature of 50 K, the saturation field is about 700 Oe, and the saturation magnetization and the remanence value are about 2.3!10K4 and 1.9!10K4 emu, respectively. At the temperature of 150 K, the saturation field is about 600 Oe, and the saturation magnetization and the remanence value are about 9.8!10K5 and 7.7!10K5 emu, respectively. In the case of 240 K, the magnetic hysteresis almost disappears and the film matrix is paramagnetic. It is clearly observed that the ferromagnetic domains inside the film matrix increase during the cooling process. For comparison, the magnetic hysteresis loops of 17 nm thick film at 50 and 10 K are plotted in Fig. 5. It is clearly seen that when the temperature was decreased from 50 to 10 K, the hysteresis has a large change, means that the ferromagnetic domains obviously change with magnetic field in the low temperature region. This may be the main reason of the positive MR effect. Because, in the low temperature region the applied magnetic field still can easily induce the domain wall motion. This would increase
Fig. 5. Magnetic hysteresis loops of 17 nm thick La0.67Ca0.33MnO3 film on (001) STO substrate at the temperatures of 10 and 50 K.
J.P. Zhong et al. / Solid State Communications 136 (2005) 528–532
the domain wall scattering on the charge carriers, and then induce the increase of the resistivity, i.e. the magnetic field enhances resistance and showing an obvious positive MR effect. At the higher magnetic field (1 T), however, the magnetization becomes saturated, then the domain wall motion and rotation caused by external field should be neglected and the positive MR effect disappears. When the film thickness increased to above 40 nm, the positive MR disappears in either high temperature or low temperature regions. This indicates that such MR effect is the feature of ultrathin films of LCMO. The positive MR effect in all these ultrathin LCMO films which occurred in the higher temperature region (above TMI), could not be induced by scattering of the charge carriers at the domain wall. Because the film matrix is paramagnetic in the high temperature region and no ferromagnetic domain exists. In the previous reports, the origin of the positive MR effect (which occurs at high temperature region) was suggested as the grain boundary scattering [19,20]. For our ultrathin ˚ , and no obvious LCMO films, the flatness is better than 2 A grain boundaries were observed, so the grain boundary scattering may not be the reason. We tend to suggest that the present positive MR effect may be attributed to the transport dominated by the magnetic polarons. For the present ultrathin films, the growth strain is mainly caused by the substrate-film lattice mismatch which should exist throughout the entire film matrix, and may not be released by defects since the film is too thin to form the defects (for such nanometer thick films the dislocation, cracks and other kind of defects may not be formed). Shown in Fig. 6 is a typical clear look of the HRTEM image of the substrate-film interface for the LCMO film with thickness of 30 nm, a good epitaxial growth throughout the entire film is observed. Therefore, the unreleased strain may
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directly induce the change in Mn–O–Mn bond angle and bond length, and result in a lattice distortion. Such local lattice distortion will produce a potential minimum to trap the electron [3]. When the temperature reaches around TMI, the trap well will trap spin polarized electrons, and form magnetic polarons. Therefore, we suggest that the polaron hopping may be the dominant factor for the conductance of such ultrathin LCMO films at temperatures higher than and around TMI. When the magnetic field is applied, the size of these polarons becomes larger (the number of polarons decreases) [21], and the hopping conductance will be suppressed. This will naturally lead to the positive MR effect. With further increase of magnetic field, the MR effect turns from positive to negative. This may be attributed to the formation of ferromagnetic domains near and below TC under the relatively higher magnetic field. In this case, the ferromagnetic domains can contribute to the percolation conductance in ultrathin films.
4. Conclusions In summary, we have fabricated the ultrathin La0.67Ca0.33MnO3 (LCMO) films on (001) SrTiO3 substrates by pulsed laser deposition. The unique MR effects were observed in these ultrathin LCMO films. It is clearly observed that all of these ultrathin LCMO films show positive MR effect at temperature higher than TMI. For the 10 nm thick LCMO film, the highest positive-MR of 9.2% was obtained at 230 K under the applied field of 200 Oe. It is particularly noted that the 17 and 30 nm thick LCMO films not only show positive MR effect in high temperature region, but also show the obviously positive MR effect in the low temperature region. For the 17 nm thick LCMO film, the highest positiveMR of 4.4% was obtained at 6 K under the applied field of 200 Oe, while the 30 nm thick LCMO film shows highest positive MR value of 14.7% at 6 K. We suggest that the positive MR effect occurred at low temperature could be attributed to the scattering of domain wall to the charge carriers. While the positive MR effect occurred at the temperature region higher than TMI is attributed to the transport dominated by the magnetic polarons, which formed due to the trap of electrons with spin in the local distorted lattice region in such highly strained ultrathin LCMO films. It also should be noted that the positive MR disappears in either high temperature or low temperature regions when the film thickness is larger than 40 nm, indicating that such positive MR effect is the feature of ultrathin films of LCMO.
Acknowledgements
Fig. 6. HRTEM view of LCMO/STO interface of the 30 nm thick La0.67Ca0.33MnO3 film.
The authors would like to thank W.W. Huang and X.Y. Qi for their measurement support. This work is supported by the State Key programme for Basic Research of China and the National Natural Science Foundation of China.
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