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Tailoring the physical properties of manganite thin films by tuning the epitaxial strain
Physica B 327 (2003) 257–261
Tailoring the physical properties of manganite thin films by tuning the epitaxial strain P.X. Zhanga,b,*, H. Zhanga,b, L.M. Chaa,b, H.-U. Habermeierb,a a
Institute of Advanced Material for Photoelectronics, KUST, Kunming 650051, China b Max-Planck-Institute, FKF, Stuttgart, D-70569, Germany
Abstract Through a proper choice of the mismatch between substrate and films, the physical properties of manganite thin films can be tailored We show that two types of manganite thin films of the Ruddlesden–Popper family, n ¼ N and n ¼ 2; demonstrate a dramatic variation of their physical properties. It is proved that the property variation can be tuned precisely by controlling the lattice mismatch and/or the film thickness. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Manganite; Thin film; Strain; Bilayer
We shall show that two types of manganite thin films, which belong to the n ¼ N and n ¼ 2 member of Ruddlesden–Popper family, can be prepared with very different physical properties when they are grown on different substrates and/or with different thickness. The effect is mainly due to the manipulating of the epitaxial strain in the films. Several authors have studied the strain effect in manganite thin films [1–5]. The resistivity r in a system of adiabatic small polaron hopping [6] can be expressed as r ¼ AT expðEA =KB TÞ:
ð1Þ
Here, EA is the activation energy of the hopping polarons. In fact both A and EA can be tuned by manipulating the epitaxial strain in the films. TC and the magnetic moments are also linked to the epitaxial strain. According to Varma [7], and Chen *Corresponding author. Max-Planck-Institute, FKF, Stuttgart, Germany, D-70569. Fax: +49-711-689-1372. E-mail address: [email protected] (P.X. Zhang).
[8], the strain effect of TC is expressed as d ln TC d ln W da 2 : ¼ de de de Here W is the electronic bandwidth of the manganite in the polaron model, and a is the oxygen isotope exponent. Both W and a can be influenced and tuned by the strains. There are two limitations, which have to be considered in the manipulation. One is a lower thickness limitation, which originates from the island formation at very small thickness. Due to the island formation the stress is hardly applied to the film grown on it, hence the effect of the stress on the physical properties is difficult to control. Another limitation is from the relaxation of the stress for relatively thick films. It is well known that there is a critical thickness, above which the stress will be relieved by formation of large amount of dislocations. For the perovskite ABO3 structure there are a set of substrates suitable for
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 5 2 - 0
P.X. Zhang et al. / Physica B 327 (2003) 257–261
258
Table 1 The in-plane lattice parameters of some substrates and the manganite films Substrate
SrTiO3
LaAlO3
LaGaO3
NdGaO3
LaSrGaO4
LaSrAlO4
LaSrAlTiO
( a (A) ( b (A) ( c (A)
3.905 3.905 3.905
3.792 3.792 3.792
3.874 3.874 3.874
3.837 3.853 3.886
3.839 3.839 12.688
3.742 3.754
3.868
Intensity (a.b.)
15
10
20nm 30nm
40nm
5
75nm
200nm 0 10
20
30
40
50
60
70
80
90
2θ (degree)
(a) 3.871 3.870
c axis parameter (A)
3.869 3.868 3.867 3.866 3.865 3.864 3.863 0
(b)
20
40
60
80
100
120
140
160
180
200
220
Thickness (nm)
Fig. 1. (a) X-ray diffraction patterns of the La0.9Sr0.1MnO3 films grown on SrTiO3 (1 0 0). (b). Thickness dependence of the lattice parameter c of La0.9Sr0.1MnO3 films on SrTiO3 (1 0 0).
P.X. Zhang et al. / Physica B 327 (2003) 257–261
the tailoring. Table 1 lists some of the frequently used substrates. We define the in-plane mismatch by m ¼ ðab as Þ=ab 100% with abðsÞ are the lattice parameter of the bulk (substrate) materials. The La0.9Sr0.1MnO3 compound was chosen as an example, which is the n ¼ N member of the Ruddlesden–Popper family. According to the phase diagram of La1x.SrxMnO3, this doping level corresponds to the sharp transition region, where the physical properties are strongly affected
259
by a small variation of doping, hence possibly by the strains. This favors the observation of small changes in the measured physical properties. The pulsed laser deposition method was used to prepare the thin films. The same deposition condition was adopted in preparing a series of films, with thickness from 10 to 200 nm on the substrates. Cox and co-workers [9] have shown that La0.9Sr0.1MnO3 is a distorted perovskite with ( Two substrates of single a ¼ 3:93; and c ¼ 3:86 A.
La0.9Sr0.1MnO3/NdGaO3 (100)Pnmb 2.00E+008
Resistance (Ω)
1.50E+008 5nm 10nm 15nm 20nm 30nm 40nm 75nm 120nm 200nm
1.00E+008
5.00E+007
0.00E+000 100
150
200
250
300
Temperature (K)
(a)
20nm 30nm 40nm 75nm 200nm
1000
Resistivity (Ω cm)
100
10
1
0.1
0.01 0
(b)
50
100
150
200
250
300
350
Temperature (K)
Fig. 2. (a) Resistance vs. temperature of La0.9Sr0.1MnO3 films on NdGaO3 (1 1 0)cubic. (b) Resistivity vs. temperature of the films on SrTiO3 (1 0 0).
P.X. Zhang et al. / Physica B 327 (2003) 257–261
260 0
20
40
60
80
100
120
140
160
180
200
0.32
220 0.26
La0.9SrMnO3/SrTiO3 (100) La0.9Sr0.1MnO3/NdGaO3 (100)Pnma
0.24
0.22
0.20
0.18
EA (eV)
2EA (eV)
0.30
0.28 0.16
0.14
0.26 0
20
40
60
80
100
120
140
160
180
200
0.12 220
thickness (A) Fig. 3. Thickness dependence of activation energy EA for the films grown on SrTiO.
crystalline SrTiO3 (1 0 0) and NdGaO3 (1 0 0)Pnma were used. Our experiments show that the films, grown on NdGaO3, are really compressed, while those on SrTiO3 are under tensile stress. To prove the phase purity, X-ray diffraction was performed. Fig. 1(a) shows the X-ray diffraction from films grown on SrTiO3. The X-ray diffraction provides information on the involved phases and shows little of other phases than the La0.9Sr0.1MnO3 perovskite. Fig. 1(b) shows the thickness dependence of the calculated c-axis lattice constants of the films grown on STO. It clearly shows that the films are under tensile strains. The resistivity was measured by the four-probe method and results are shown in Fig. 2(a,b). For films grown on NdGaO3, the resistivity is decreased with increasing thickness, and vice versa for films on SrTiO3. The activation energies calculated from the two sets of curves are shown in Fig. 3. It is apparent that the thickness dependence of the polaron formation energy in the two types of films changes oppositely. In addition, the changing of EA with variation of the film thickness shows a continuous behavior. This offers the chance to finely tune the physical properties of the films. There was a lot of discussion on the effect of strains on the physical properties in various
manganite thin films [5]. However, some controversies exist. It is evident from our results that tensile strains in the limited thickness range for films of n ¼ N will lead to the reduction of the resistivity, and to an increase of the ferromagnetic interaction. While the compressive strains demonstrate opposite effect. The structure of the La22xSr12xMn2O7 system can be derived from the perovskite type manganite by inserting an additional metal-oxide block in each double (R1xAx)MnO3 blocks. Due to this insertion, an intrinsic anisotropy in both the structural and the physical properties is developed. For example, the resistivity in the c-direction may be 3 orders of magnitude higher than that in the ab-plane [10]. The structure also leads to a complicated spin ordering and to a complex temperature dependence [11–14]. We have prepared the bilayer manganite thin films with the composition of La1.36Sr1.64Mn2O7 by PLD technique. Two types of substrates, SrTiO3 (1 0 0), and NdGaO3 (1 1 0) were selected. The a-axis lattice parameter of bulk ( and it can be La1.36Sr1.64Mn2O7 is 3.855 A, seen from Table 1 that the films on SrTiO3 will be grown with compressing strains. We will show that the films on NdGaO3 are under tensile stress.
P.X. Zhang et al. / Physica B 327 (2003) 257–261
261
Table 2 Bilayer manganite films grown on different substrates and the influence on the physical properties Substrate
Orientation
Mismatch 300 K (%)
TMI (K)
rðTMI Þ (Om)
SrTiO3 LaAlSrTiO3 NdGaO3 LaSrGaO4
(0 0 1) STO (1 1 0) (1 0 0)
+1.02 +0.32 0.44(4.5) 0.42
80 88 150 220
13 124 0.27 0.07
Cha et al. reported that the peak positions of TMI for the two films are very different. For films grown on NdGaO3 TMI is 150 K, while that from films on SrTiO3 is just around 80 K. The resistivity of the La1.36Sr1.64Mn2O7 film grown on STO is about 2 orders of magnitudes higher than that grown on a NGO substrate in nearly the whole measured temperature range. A similar effect was observed in several other cases. Table 2 lists the results from films grown on several substrates. It is evident that with positive mismatch or tensile stress in the films, the resistivities are high and TMI are low, while for negative mismatches or a compressive stress, the resistivities are low and TMI are high for the La1.36Sr1.64Mn2O7 thin films grown on substrates. Thus, we have demonstrated that the physical properties, such as the resistivity, TC of the manganite films grown on substrates for both the n ¼ 2 and n ¼ N structures can be precisely tuned by manipulating the epitaxial strain. It is proved that by proper choice of substrate, and/or controlling the film thickness within certain ranges, the tuning can be easily realized. There are thickness limitations for this manipulation. The small thickness limitation comes from the island formation, and the large thickness limitation is due to the stress relaxation in the epitaxial growth of films. For most of the CMR films grown on properly choused substrates, 10–200 nm is the best range for tailoring the physical properties
of the films. One can obtain sometimes novel properties by this technique. Much progress has been made in this direction. However it is important to investigate the aging effect or the instability of these films, before they can be used in industry.
References [1] C. Zener, Phys. Rev. 82 (1951) 403; J.B. Goodenough, Phys. Rev. 100 (1955) 564. [2] A.J. Millis, R. Mueller, B.I. Shraiman, Phys. Rev. B 54 (1996) 5405. [3] Y. Yamada, O. Hino, S. Nohdo, R. Kanao, Phys. Rev. Lett. 77 (1996) 904. [4] Y. Tokura, N. Nagaosa, Science 288 (2000) 462. [5] W. Prellier, P. Lecoeur, B. Mercey, J. Phys. Condens. Matter 13 (2001) R915. [6] D. Emin, T. Holstein, Ann. Phys. (NY) 53 (1969) 439. [7] C.M. Varma, Phys. Rev. B 54 (1996) 7328. [8] X.J. Chen, S. Soltan, H. Zhang, H.-U. Habermeier, Phys. Rev. B 65 (2002) 174402. [9] D.E. Cox, T. Iglesias, E. Moshopoulou, K. Hirota, K. Takahashi, Y. Endoh, Phy. Rev. B64 (2001) 024431. [10] T. Kimura, Y. Tomioka, H. Kuwahara, A. Asamitsu, M. Tamura, Y. Tokura, Science 274 (1996) 1698. [11] Y. Konishi, T. Kimura, et al., Appl. Phys. Lett. 73 (1998) 3004. [12] S. Parashar, A. Moreira dos Santos, et al., Int. J. Inorg. Mater. 2 (2000) 651. [13] Despina Louca, G.H. Kwei, J.F. Mitchell, Phys. Rev. Lett. 80 (1998) 3811. [14] C.D. Potter, M. Swwiatek, S.D. Bader, et al., Phys. Rev. B57 (1998) 72.
Tailoring the physical properties of manganite thin films by tuning the epitaxial strain P.X. Zhanga,b,*, H. Zhanga,b, L.M. Chaa,b, H.-U. Habermeierb,a a
Institute of Advanced Material for Photoelectronics, KUST, Kunming 650051, China b Max-Planck-Institute, FKF, Stuttgart, D-70569, Germany
Abstract Through a proper choice of the mismatch between substrate and films, the physical properties of manganite thin films can be tailored We show that two types of manganite thin films of the Ruddlesden–Popper family, n ¼ N and n ¼ 2; demonstrate a dramatic variation of their physical properties. It is proved that the property variation can be tuned precisely by controlling the lattice mismatch and/or the film thickness. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Manganite; Thin film; Strain; Bilayer
We shall show that two types of manganite thin films, which belong to the n ¼ N and n ¼ 2 member of Ruddlesden–Popper family, can be prepared with very different physical properties when they are grown on different substrates and/or with different thickness. The effect is mainly due to the manipulating of the epitaxial strain in the films. Several authors have studied the strain effect in manganite thin films [1–5]. The resistivity r in a system of adiabatic small polaron hopping [6] can be expressed as r ¼ AT expðEA =KB TÞ:
ð1Þ
Here, EA is the activation energy of the hopping polarons. In fact both A and EA can be tuned by manipulating the epitaxial strain in the films. TC and the magnetic moments are also linked to the epitaxial strain. According to Varma [7], and Chen *Corresponding author. Max-Planck-Institute, FKF, Stuttgart, Germany, D-70569. Fax: +49-711-689-1372. E-mail address: [email protected] (P.X. Zhang).
[8], the strain effect of TC is expressed as d ln TC d ln W da 2 : ¼ de de de Here W is the electronic bandwidth of the manganite in the polaron model, and a is the oxygen isotope exponent. Both W and a can be influenced and tuned by the strains. There are two limitations, which have to be considered in the manipulation. One is a lower thickness limitation, which originates from the island formation at very small thickness. Due to the island formation the stress is hardly applied to the film grown on it, hence the effect of the stress on the physical properties is difficult to control. Another limitation is from the relaxation of the stress for relatively thick films. It is well known that there is a critical thickness, above which the stress will be relieved by formation of large amount of dislocations. For the perovskite ABO3 structure there are a set of substrates suitable for
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 5 2 - 0
P.X. Zhang et al. / Physica B 327 (2003) 257–261
258
Table 1 The in-plane lattice parameters of some substrates and the manganite films Substrate
SrTiO3
LaAlO3
LaGaO3
NdGaO3
LaSrGaO4
LaSrAlO4
LaSrAlTiO
( a (A) ( b (A) ( c (A)
3.905 3.905 3.905
3.792 3.792 3.792
3.874 3.874 3.874
3.837 3.853 3.886
3.839 3.839 12.688
3.742 3.754
3.868
Intensity (a.b.)
15
10
20nm 30nm
40nm
5
75nm
200nm 0 10
20
30
40
50
60
70
80
90
2θ (degree)
(a) 3.871 3.870
c axis parameter (A)
3.869 3.868 3.867 3.866 3.865 3.864 3.863 0
(b)
20
40
60
80
100
120
140
160
180
200
220
Thickness (nm)
Fig. 1. (a) X-ray diffraction patterns of the La0.9Sr0.1MnO3 films grown on SrTiO3 (1 0 0). (b). Thickness dependence of the lattice parameter c of La0.9Sr0.1MnO3 films on SrTiO3 (1 0 0).
P.X. Zhang et al. / Physica B 327 (2003) 257–261
the tailoring. Table 1 lists some of the frequently used substrates. We define the in-plane mismatch by m ¼ ðab as Þ=ab 100% with abðsÞ are the lattice parameter of the bulk (substrate) materials. The La0.9Sr0.1MnO3 compound was chosen as an example, which is the n ¼ N member of the Ruddlesden–Popper family. According to the phase diagram of La1x.SrxMnO3, this doping level corresponds to the sharp transition region, where the physical properties are strongly affected
259
by a small variation of doping, hence possibly by the strains. This favors the observation of small changes in the measured physical properties. The pulsed laser deposition method was used to prepare the thin films. The same deposition condition was adopted in preparing a series of films, with thickness from 10 to 200 nm on the substrates. Cox and co-workers [9] have shown that La0.9Sr0.1MnO3 is a distorted perovskite with ( Two substrates of single a ¼ 3:93; and c ¼ 3:86 A.
La0.9Sr0.1MnO3/NdGaO3 (100)Pnmb 2.00E+008
Resistance (Ω)
1.50E+008 5nm 10nm 15nm 20nm 30nm 40nm 75nm 120nm 200nm
1.00E+008
5.00E+007
0.00E+000 100
150
200
250
300
Temperature (K)
(a)
20nm 30nm 40nm 75nm 200nm
1000
Resistivity (Ω cm)
100
10
1
0.1
0.01 0
(b)
50
100
150
200
250
300
350
Temperature (K)
Fig. 2. (a) Resistance vs. temperature of La0.9Sr0.1MnO3 films on NdGaO3 (1 1 0)cubic. (b) Resistivity vs. temperature of the films on SrTiO3 (1 0 0).
P.X. Zhang et al. / Physica B 327 (2003) 257–261
260 0
20
40
60
80
100
120
140
160
180
200
0.32
220 0.26
La0.9SrMnO3/SrTiO3 (100) La0.9Sr0.1MnO3/NdGaO3 (100)Pnma
0.24
0.22
0.20
0.18
EA (eV)
2EA (eV)
0.30
0.28 0.16
0.14
0.26 0
20
40
60
80
100
120
140
160
180
200
0.12 220
thickness (A) Fig. 3. Thickness dependence of activation energy EA for the films grown on SrTiO.
crystalline SrTiO3 (1 0 0) and NdGaO3 (1 0 0)Pnma were used. Our experiments show that the films, grown on NdGaO3, are really compressed, while those on SrTiO3 are under tensile stress. To prove the phase purity, X-ray diffraction was performed. Fig. 1(a) shows the X-ray diffraction from films grown on SrTiO3. The X-ray diffraction provides information on the involved phases and shows little of other phases than the La0.9Sr0.1MnO3 perovskite. Fig. 1(b) shows the thickness dependence of the calculated c-axis lattice constants of the films grown on STO. It clearly shows that the films are under tensile strains. The resistivity was measured by the four-probe method and results are shown in Fig. 2(a,b). For films grown on NdGaO3, the resistivity is decreased with increasing thickness, and vice versa for films on SrTiO3. The activation energies calculated from the two sets of curves are shown in Fig. 3. It is apparent that the thickness dependence of the polaron formation energy in the two types of films changes oppositely. In addition, the changing of EA with variation of the film thickness shows a continuous behavior. This offers the chance to finely tune the physical properties of the films. There was a lot of discussion on the effect of strains on the physical properties in various
manganite thin films [5]. However, some controversies exist. It is evident from our results that tensile strains in the limited thickness range for films of n ¼ N will lead to the reduction of the resistivity, and to an increase of the ferromagnetic interaction. While the compressive strains demonstrate opposite effect. The structure of the La22xSr12xMn2O7 system can be derived from the perovskite type manganite by inserting an additional metal-oxide block in each double (R1xAx)MnO3 blocks. Due to this insertion, an intrinsic anisotropy in both the structural and the physical properties is developed. For example, the resistivity in the c-direction may be 3 orders of magnitude higher than that in the ab-plane [10]. The structure also leads to a complicated spin ordering and to a complex temperature dependence [11–14]. We have prepared the bilayer manganite thin films with the composition of La1.36Sr1.64Mn2O7 by PLD technique. Two types of substrates, SrTiO3 (1 0 0), and NdGaO3 (1 1 0) were selected. The a-axis lattice parameter of bulk ( and it can be La1.36Sr1.64Mn2O7 is 3.855 A, seen from Table 1 that the films on SrTiO3 will be grown with compressing strains. We will show that the films on NdGaO3 are under tensile stress.
P.X. Zhang et al. / Physica B 327 (2003) 257–261
261
Table 2 Bilayer manganite films grown on different substrates and the influence on the physical properties Substrate
Orientation
Mismatch 300 K (%)
TMI (K)
rðTMI Þ (Om)
SrTiO3 LaAlSrTiO3 NdGaO3 LaSrGaO4
(0 0 1) STO (1 1 0) (1 0 0)
+1.02 +0.32 0.44(4.5) 0.42
80 88 150 220
13 124 0.27 0.07
Cha et al. reported that the peak positions of TMI for the two films are very different. For films grown on NdGaO3 TMI is 150 K, while that from films on SrTiO3 is just around 80 K. The resistivity of the La1.36Sr1.64Mn2O7 film grown on STO is about 2 orders of magnitudes higher than that grown on a NGO substrate in nearly the whole measured temperature range. A similar effect was observed in several other cases. Table 2 lists the results from films grown on several substrates. It is evident that with positive mismatch or tensile stress in the films, the resistivities are high and TMI are low, while for negative mismatches or a compressive stress, the resistivities are low and TMI are high for the La1.36Sr1.64Mn2O7 thin films grown on substrates. Thus, we have demonstrated that the physical properties, such as the resistivity, TC of the manganite films grown on substrates for both the n ¼ 2 and n ¼ N structures can be precisely tuned by manipulating the epitaxial strain. It is proved that by proper choice of substrate, and/or controlling the film thickness within certain ranges, the tuning can be easily realized. There are thickness limitations for this manipulation. The small thickness limitation comes from the island formation, and the large thickness limitation is due to the stress relaxation in the epitaxial growth of films. For most of the CMR films grown on properly choused substrates, 10–200 nm is the best range for tailoring the physical properties
of the films. One can obtain sometimes novel properties by this technique. Much progress has been made in this direction. However it is important to investigate the aging effect or the instability of these films, before they can be used in industry.
References [1] C. Zener, Phys. Rev. 82 (1951) 403; J.B. Goodenough, Phys. Rev. 100 (1955) 564. [2] A.J. Millis, R. Mueller, B.I. Shraiman, Phys. Rev. B 54 (1996) 5405. [3] Y. Yamada, O. Hino, S. Nohdo, R. Kanao, Phys. Rev. Lett. 77 (1996) 904. [4] Y. Tokura, N. Nagaosa, Science 288 (2000) 462. [5] W. Prellier, P. Lecoeur, B. Mercey, J. Phys. Condens. Matter 13 (2001) R915. [6] D. Emin, T. Holstein, Ann. Phys. (NY) 53 (1969) 439. [7] C.M. Varma, Phys. Rev. B 54 (1996) 7328. [8] X.J. Chen, S. Soltan, H. Zhang, H.-U. Habermeier, Phys. Rev. B 65 (2002) 174402. [9] D.E. Cox, T. Iglesias, E. Moshopoulou, K. Hirota, K. Takahashi, Y. Endoh, Phy. Rev. B64 (2001) 024431. [10] T. Kimura, Y. Tomioka, H. Kuwahara, A. Asamitsu, M. Tamura, Y. Tokura, Science 274 (1996) 1698. [11] Y. Konishi, T. Kimura, et al., Appl. Phys. Lett. 73 (1998) 3004. [12] S. Parashar, A. Moreira dos Santos, et al., Int. J. Inorg. Mater. 2 (2000) 651. [13] Despina Louca, G.H. Kwei, J.F. Mitchell, Phys. Rev. Lett. 80 (1998) 3811. [14] C.D. Potter, M. Swwiatek, S.D. Bader, et al., Phys. Rev. B57 (1998) 72.