ARTICLE IN PRESS

0921-4526/$ - se

doi:10.1016/j.ph

�CorrespondiE-mail addr

(M. Li).

Physica B 395 (2007) 33–38

www.elsevier.com/locate/physb

The epitaxial growth of the PrCaSrMnO and LaCaMnO/PrCaSrMnO/LaCaMnO multilayer thin films with CMR effects prepared by a new

method: Precursor Film Sintering

Hui Liua, Ying Luob, Ming Lic,�

aDepartment of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904, USAbCharles L. Brown Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, VA 22904, USA

cDepartment of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China

Received 6 December 2006; accepted 9 February 2007

Abstract

We have successfully synthesized PrSrCaMnO thin films (with a nominal structure of Pr0.7Sr0.05Ca0.25MnO3) and some heterostructure

thin films on LaAlO3 (1 0 0) single-crystal substrates by a newly invented precursor film sintering technique. The XRD analysis showed

that the PCSMO thin films by the PFS method consisted of epitaxially grown single-phase perovskite structures, oriented to the c-axis

direction and without secondary phases. LPL, PLP and LCMO thin films prepared by the PFS method were all epitaxially grown thin

films with good quality, which were confirmed by XRD patterns. The effect of the critical sintering temperature was carefully studied

based on XRD patterns and SEM images. It was found that the sintering temperature higher than 1200 1C gave rise to high-quality

epitaxial thin films, but a secondary phase appeared at this temperature. Large values of the MR of PCSMO, LCMO and LPL thin films

have been observed. Besides, the advantages of the PFS method include simple equipments, common chemical compounds and a variety

of shapes of the prepared thin films.

r 2007 Elsevier B.V. All rights reserved.

PACS: 68.55.Jk

Keywords: PrCaSrMnO; LaCaMnO; Thin film; Precursor film sintering; XRD; EDS; SEM; CMR; Epitaxial growth

1. Introduction

The perovskite manganites A1�xBxMnO3 (‘‘A’’ repre-senting trivalent cation such as La, Nd, Pr and ‘‘B’’representing doped divalent cation such as Ca, Sr and Ba)have drawn remarkable interest for their very largemagnetoresistance effects known as giant magnetoresis-tance (GMR) or colossal magnetoresistance (CMR) [1–7].These years, besides LaCaMnO (LCMO) systems, CMReffects of PrCaMnO thin films have been observed [8,9].Especially, a partial substitution of Ca cation by Sr cationin the SmCaMnO system gives rise to a resistance variation

e front matter r 2007 Elsevier B.V. All rights reserved.

ysb.2007.02.018

ng author.

esses: hl5h@virginia.edu (H. Liu), lim@tsinghua.edu.cn

up to 50,000% at 92.5K with H ¼ 5T [10]. A high value ofthe CMR in these compounds was reported by Maignanet al. [11] that Pr0.7Sr0.05Ca0.25MnO3 exhibited an MR ratioof 10 [11] by applying a magnetic field of 5T at 30K, whichwas close to the upper limit of the CMR that can bereached in these compounds. Later, Hejtmanek et al. [12]explained the CMR effects in the PrCaSrMnO thin films bythe paramagnetic–antiferromagnetic and antiferromagne-tic–ferromagnetic transitions which exist in these thin films.The applications of PrCaMnO materials are also

promising. Asamitsu et al. [13] found that an electricalcurrent can trigger the collapse of the low temperature,electrically insulating charge-ordered state to a metallicferromagnetic state of Pr1�xCaxMnO3 and suggested thatsuch a phenomenon provides a route for fabricatingmicrometer- or nanometer-scale electromagnets. Also

ARTICLE IN PRESS

20 30 40 50 60 70 80

0.0

5.0x105

1.0x105

1.5x105

2.0x105

2.5x105

3.0x105

3.5x105

23.08

(001)

(002)

23.46

47.14

47.96

75.12

73.68

Inte

nsity

2θ (°)

PCSMD

PCSMD

(003)PCSMD

Fig. 1. X-ray diffraction pattern of the PCSMO thin film.

Table 1

Lattice constants of the LaAlO3 (1 0 0) substrate and the PCSMO thin film

LaAlO3 (Substrate) PCSMO (Thin film)

Indices 2y (deg) A Indices 2y (deg) A

(0 0 1) 23.46 3.792 (0 0 1) 23.08 3.844

(0 0 2) 47.98 1.896 (0 0 2) 47.14 1.922

(0 0 3) 75.12 1.264 (0 0 3) 73.68 1.283

Lattice constant ¼ 3.791 A Lattice constant ¼ 3.846 A

H. Liu et al. / Physica B 395 (2007) 33–3834

Fiebig et al. [14] suggested that manipulation of the phasetransition of Pr0.7Ca0.3MnO3 may be useful in theconstruction of optical switches. Epitaxial perovskiteheterostructures are also very important for applicationsof thin films. For example, Hwang et al. [15] reportedsignificant improvement in the MR response of perovskitemanganites for small applied fields by the use of hetero-structures with soft ferromagnets. At room temperature, a5900-fold enhancement of the MR response at 10Oe inLa2/3Ca1/3MnO3 has been achieved using (Mn,Zn)Fe2O4 asthe soft ferromagnet.

However, the PrCaSrMnO (PCSMO) thin films are notonly highly costly, but the shapes of the obtained films arelimited by the size of the processing chamber, since thesefilms are usually prepared by the pulsed laser deposition(PLD) method [16], for which expensive equipments andcomplicated processing are required. Therefore, developinga simple and low-cost method to prepare the PCSMO thinfilm and other thin films with the CMR effects is essentialfor realizing their potential applications. The well-knownmethod widely used for preparing ceramic films, sol–gelprocess, however, is not suitable for preparing PCSMOfilms because of its special requirements. These years, thepolymer precursor method has been adopted to preparesome ceramic powders or films [17,18]. For example,Pechini process [19] that is used to prepare ceramic filmscan remarkably reduce the cost, but special precursors formaking the PCSMO thin films are not fit for massproduction.

In this paper, we report the epitaxial growth of thePCSMO thin films (with a nominal structure of Pr0.7Ca0.25Sr0.05MnO3), the LCMO thin films (with a nominalstructure of La0.67Ca0.33MnO3), the LCMO/PCSMO/LCMO (LPL) multilayer thin films and the PCSMO/LCMO/PCSMO (PLP) multilayer thin films with the CMReffects by the precursor film sintering method called as PFSmethod. The epitaxial growth of PCSMO thin films wasconfirmed by the XRD pattern and the lattice constant wascalculated. Then, the critical sintering temperature in thePFS method was carefully studied based on XRD patternsand SEM images because the sintering temperaturestrongly affected the quality of the thin films prepared bythe PFS method. After that, we prepared the PCSMO,LCMO, PLP and LPL thin films, and their structures werecharacterized by XRD analyses. Finally, CMR effects ofthe obtained PCSMO, LCMO and LPL thin films weremeasured.

2. Experiment

Polyacrylamide (PAM) was first dissolved in a beaker ofde-ionized water at room temperature. To adjust theviscosity and the concentration of the polymer precursorfilm, the weight percentage of PAM in the solution wascontrolled at 0.5%. Then, Pr(NO3)3, Ca(NO3)2, Sr(NO3)2and Mn(NO3)2 were added into the PAM solution in aratio of 0.7:0.25:0.05:1 to prepare the PCSMO thin film

with a nominal structure of Pr0.75Ca0.25Sr0.05MnO3. At thesame time, the polymer precursor solution for makingLCMO thin film with a nominal structure of La0.67Ca0.33MnO3 was prepared with the same method. Finally,two polymer precursor solutions were separately stirredby a glass rod for 10min to keep them homogeneous.A qualified precursor solution is colorless, transparent andwithout any undissolvable components.The PCSMO thin film and the LPL multilayer thin film

were prepared on LaAlO3 (1 0 0) single crystalline sub-strates by following a two-step method for the heattreatment. At the same time, a PLP thin film and anLCMO thin film were prepared for comparison. Afterhomogeneous precursor films were spin coated on sub-strates, the low-temperature dry-off step was carried out inan oven. The temperature of the oven was set to increasefrom 90 to 180 1C with a rate of 5 1C/min. Then, the high-temperature sintering step was performed in a tubefurnace. With a rate of 20 1C/min, the temperature in thetube furnace was raised to a critical sintering temperatureand held for 20min to realize the crystallization of thefilms. After the two-step heat treatment, thin films werenaturally cooled to room temperature inside the tubefurnace. The thickness of the obtained thin films on theLaAlO3 (1 0 0) substrate is determined to be about 35 nm.For a thicker film or an LCMO/PSCMO/LCMO multi-layer thin film, the spin coating, drying and sinteringprocesses were repeated using corresponding precursorsolutions until thin films with desired thickness were

ARTICLE IN PRESSH. Liu et al. / Physica B 395 (2007) 33–38 35

obtained. The PCSMO and LCMO thin films wereprepared by repeating the above spin-coating, drying andsintering processes for 15 times. Each layer of the PLP andLPL films was prepared by five times of the above process.

The standard four-probe technique was used to measurethe magnetoresistance (MR) of thin films in a temperaturerange from 80 to 300K with an applied field of 5.5 T. TheMR of the LCOM and LPL thin film is defined as(R0�RH)/R0� 100% (RH and R0 are resistances with andwithout the applied field, respectively). But the MR of thePCSMO thin film is defined as (R0�RH)/RH� 100% toreflect the colossal change. The applied field is parallel tothe film plane and perpendicular to the current direction.

3. Results and discussions

The PCSMO, PLP, LPL and LCMO thin films for XRDwere grown on a single-crystal substrate of LaAlO3 (1 0 0),and the sintering temperature was 1200 1C. The thickness

20 30 40 50 60 70 80

LCMO

LPL

PLP

PCSMO

Intens

ity

22.50 22.75 23.00 23.25 23.50 23.75 24.00

23.04

23.1823.04

23.06 23.14

23.06

LCMO

LPL

PLP

PCSMO

Intens

ity

a

b

2θ (°)

2θ (°)

Fig. 2. (a) X-ray diffraction patterns of the LCMO, LPL, PLP and

PCSMO thin films and (b) (0 0 1) diffraction peaks are enlarged.

of the thin film was about 500 nm which was calculatedby a SEM image of the cross-section of the sample (notshown here).A standard y–2y XRD pattern of the PCSMO thin film

is shown in Fig. 1. It is noted that strong characteristicpeaks of the PCSMO thin film are present on the low-angleside of the intrinsic peaks of the single-crystal substrate ofLaAlO3 (1 0 0) and no peaks from the polycrystallinePCSMO are observed. Moreover, the intensities ofPCSMO peaks are pretty high, which can be attributedto the epitaxial growth of the thin film. Based on the XRDpattern, we calculated the lattice constant of the PCSMOthin film and compared it with the lattice constant of thesubstrate, referring to Table 1. The ‘Indices’ column is theindices of crystallographic plan. It is found that the latticeconstant of PCSMO thin film is 3.846 A and the latticeconstant of LaAlO3 (1 0 0) is 3.791 A, which is anotherevidence for the epitaxial growth of the PCSMO thin filmprepared by the PFS method. The XRD pattern reveals

22.0 22.5 23.0 23.5 24.0 24.5 25.0

Inte

nsity

20 30 40 50 60 70 80

Inte

nsity

2θ (°)

2θ (°)

1400°C

1200°C

1000°C

800°C

1400°C

1200°C

1000°C

800°C

a

b

Fig. 3. (a) X-ray diffraction patterns of the PCSMO thin films sintered at

different temperatures and (b) (0 0 1) diffraction peaks are enlarged.

ARTICLE IN PRESSH. Liu et al. / Physica B 395 (2007) 33–3836

that the PCSMO thin film prepared by the PFS methodconsists of epitaxially grown single-phase perovskitestructures, oriented to the c-axis direction and withoutsecondary phases.

Fig. 2 shows the XRD patterns of the PCSMO, PLP,LPL and LCMO thin films sintered at 1200 1C. Allcharacteristic diffraction peaks from thin films are presenton the low-angle sides of intrinsic peaks of the single-crystal substrate of LaAlO3 (1 0 0) and no peaks frompolycrystals are observed. Epitaxial growth of the thinfilms on substrates was confirmed by the high intensities oftheir diffraction peaks. The lattice constants of LCMO andPCSMO thin films are 3.861 and 3.846 A, respectively,calculated from their XRD patterns. However, there aretwo new diffraction peaks at 23.181 and 23.141 in the XRDpatterns of the LPL and PLP thin films. We speculate thatthe new peaks can be related to the lattice mismatch at theLCMO/PCSMO interface even though the mismatch isvery small (0.3%). Another possible reason for the newpeaks is that La ions diffuse into the lattice of PCSMO andPr ions diffuse into the lattice of LCMO and they form newphases.

Fig. 3(a) and (b) shows the XRD patterns of PCSMOthin films prepared at different sintering temperatures from800 to 1400 1C. From the XRD pattern at 800 1C, nodiffraction peaks from the film can be observed, whichmeans that the sintering temperature of 800 1C is not highenough to form PCSMO thin films. The (0 0 1) and (0 0 2)diffraction peaks of the PCSMO thin film appear when thesintering temperature is elevated to 1000 1C or higher thanthat. As the sintering temperature increases, the intensities

Fig. 4. SEM images of the PCSMO thin films sintered at

of all diffraction peaks of PCSMO films increase, whichindicates that the extent of crystallinity and the quality ofthin films are improved. From the enlarged XRD patternsof the (0 0 1) diffraction peaks as shown in Fig. 3(b), thereis no appreciable change in the diffraction angle. Further-more, the full-width-half-maximum of the (0 0 1) diffrac-tion peaks stay unchanged with increasing sinteringtemperatures, which reflects the stable grain size withincreased sintering temperatures. Therefore, it can beconcluded from XRD patterns that the critical sinteringtemperature is around 1000 1C, and the quality of thePCSMO thin films becomes better as the sinteringtemperature is raised higher than the critical temperature.The surface morphologies of the PCSMO thin films

sintered at different temperatures are shown in Fig. 4(a)–(d)investigated by SEM. It is clear that there is no observablecrystalline structure of the PCSMO thin film when thesintering temperature is 800 1C, which is consistent with theXRD pattern shown in Fig. 3. Several crater-like pitsappear on the surface since the polymer precursor filmgenerates bubbles at the beginning of the sintering step.The sintering temperature of 1000 1C leads to a differentmorphology from 800 1C. The formation of the PCSMOthin film has been confirmed since a lot of clusters can beclearly observed in the SEM image. It can be seen that thethin film is composed of clusters, and some of clusters areconnected by bridges, which seems that the PCSMO thinfilm grows in a three-dimensional island-like form. Thegrain size is in a wide range from 50 to 400 nm, estimatedby the SEM image. When the sintering temperature iselevated to 1200 1C, instead of clusters, a slightly corrugated,

(a) 800 1C, (b) 1000 1C, (c) 1200 1C and (d) 1400 1C.

ARTICLE IN PRESS

Pr

Mn

Ca

Sr

300

200

100

0

0 10μm6μm

a

b

Fig. 5. (a) SEM image of the PCSMO thin film sintered at 1200 1C used for energy dispersive spectroscopy (EDS) analysis. (b) The atomic percentage of

Pr, Ca, Sr and Mn along the scanning line (the long straight line in yellow in (a)). The short yellow line in the SEM image is drawn to guide eyes to find the

relative position.

100 150 200 250 300

0

50

100

150

200

250

300

Resis

tance (

Ohm

)

Temperature (K)

100 150 200 250 300

Temperature (K)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Magneto

resis

tance

180 200 220 240 260 280 300 320 340 360

0

20

40

60

80

100

120

Resis

tance (

Ohm

)

Temperature(K)

180 200 220 240 260 280 300 320 340 360

Temperature(K)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Magneto

resis

tance

50 100 150 200 250 300 350 50 100 150 200 250 300 350

0

2000000

4000000

6000000

8000000

10000000

H=5.5T

H = 0T

H = 0T

H = 0T

H = 5.5T

H = 5.5T

270 K

269 K

264K MR = 80.0%

264K MR = 77.5%

Resis

tance(O

hm

)

Temperature(K)

0

5

10

15

20

25 MR = 2400%

MR

(R0-R

h)/

Rh

Temperature(K)

a

c

e

b

d

f

Fig. 6. Temperature dependence of the resistance and the MR of: (a, b) the PCSMO thin film without (H ¼ 0T) and with a magnetic field (H ¼ 5.5T);

(c, d) the LCMO thin film; and (e, f) the LPL thin film.

H. Liu et al. / Physica B 395 (2007) 33–38 37

ARTICLE IN PRESSH. Liu et al. / Physica B 395 (2007) 33–3838

complete thin film with some pits is generated. However,there is a small cluster in most of the pits. The small clustermay be a secondary phase formed during the sintering. ThePCSMO thin film sintered at 1400 1C grows in a two-dimensional layer-by-layer mode and it has a terrace-likesurface with facets. No pits can be observed in the SEMimage, but large clusters in the triangular shape becomemore obvious. Considering that the PCSMO thin filmmelts at a temperature around or higher than 1200 1C, wespeculate that some elements diffuse and aggregate at hightemperature, and then form secondary-phase clusters asthin films naturally cool down to room temperature.Therefore, the thin film sintered at 1400 1C has a muchlower density of the secondary-phase clusters than the thinfilm sintered at 1200 1C because of higher mobilities of allelements at higher temperature.

The SEM image of the PCSMO thin film sintered at1200 1C used for energy dispersive spectroscopy (EDS)analysis is shown in Fig. 5(a) and the atomic percentages ofPr, Ca, Sr and Mn along the scanning line (the longstraight line in yellow in (a)) are shown in Fig. 5(b). It isfound that small clusters in the pits have a higher atomicpercentage of Mn, lower atomic percentages of Pr and Cathan the normal area of the thin film has, while the atomicpercentages of Sr in the clusters and in the thin film arethe same. Therefore, the cluster in the pits is a thermo-dynamically favored chemical compound with a higheratomic percentage of Mn than the PCSMO thin film thatwe need. Because of the limited number of the clusters inthe thin film, the new phase cannot be observed in XRDpatterns.

Fig. 6 shows the temperature dependence of theresistance of the PCSMO, LCMO and LPL thin filmswithout and with a magnetic field of 5.5 T, as well as theirMRs. The MR of the PCSMO thin film is 10% at roomtemperature, and it increases as the temperature reduces.The MR of the PCSMO thin film reaches the experimentalmaximum of 2400% at 80K. It can be seen that there is ametal-to-insulator transition at the temperature lower than80K, and the MR has a rapid ascending trend withdecreasing of temperature, which is in agreement withMollah’s report [20]. The maximumMR of the LCMO thinfilm is 80% at 264K, which is similar to the MR of a singlecrystal of La0.55(CaPb)0.35MnO3 reported by Liu et al. [21].The MR behavior of the LCMO film should mainly comefrom the suppression of the spin-dependent scattering ofpolarized eg electrons at the grain boundaries [22,23]. Themaximum MR of the LPL thin film is 77.5%, slightly lowerthan that of the LCMO thin film, but the position of thepeak of the MR curve is still at 264K.

4. Conclusion

In summary, we have successfully synthesized PrSrCaMnOthin films (with a nominal structure of Pr0.7Sr0.05Ca0.25MnO3) and some heterostructures on LaAlO3 (1 0 0) single-crystal substrates by a newly invented precursor film

sintering technique. The XRD analysis shows that thePCSMO thin films by the PFS method consists ofepitaxially grown single-phase perovskite structures, or-iented to the c-axis direction and without secondaryphases. LPL, PLP and LCMO thin films prepared by thePFS method are all epitaxially grown thin films with goodquality, which was confirmed by XRD patterns. Thecritical sintering temperature was carefully studied basedon XRD patterns and SEM images. It is found that thequality of the epitaxial growth of thin films has beenimproved as the sintering temperature is raised to higherthan 1200 1C, but a secondary phase appears at highertemperature. The CMR effects of PCSMO, LCMO andLPL thin films have been observed. Besides, the advantagesof the PFS method include simple equipments, commonchemical compounds and a variety of shapes of theprepared thin films. Therefore, it is promising to fabricateelectric devices by PCSMO or other thin film in hetero-structures with CMR effects via the PFS method in thefuture.

References

[1] C.W. Searle, S.T. Wang, Can. J. Phys. 48 (1969) 2023.

[2] H. Liu, et al., Mater. Sci. Eng. B (2007), doi:10.1016/j.mseb.

2007.01.049.

[3] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri, C.N.R. Rao,

J. Solid State Chem. 114 (1995) 297.

[4] B. Raveau, A. Maignan, V. Caignaert, J. Solid State Chem. 117

(1995) 424.

[5] H. Liu, Y. Luo, M. Li, J. Cryst. Growth 296 (2006) 207.

[6] X.B. Zhu, S.M. Liu, Z.G. Sheng, B.C. Zhao, W.J. Lu, W.H. Song,

J.M. Dai, Y.P. Sun, Phys. B: Condens. Matter 369 (2005) 299.

[7] Z.X. Cheng, H.F. Zhen, A.H. Li, X.L. Wang, H. Kimura, J. Cryst.

Growth 275 (2005) e2415.

[8] V. Caignaert, E. Suard, A. Maignan, C. Simon, B. Raveau, J. Magn.

Magn. Mater. 153 (1996) L260.

[9] J.G. Lin, J. Phys. Chem. Solids 62 (2001).

[10] V. Caignaert, A. Maignan, B. Raveau, Solid State Commun. 95

(1995) 357.

[11] A. Maignan, C. Simon, V. Caignaert, B. Raveau, Solid State

Commun. 96 (1995) 623.

[12] J. Hejtmanek, Z. Jirak, D. Sedmidubsky, A. Maignan, Ch. Simon,

V. Caignaert, C. Martin, B. Raveau, Phys. Rev. B 54 (1996) 11947.

[13] A. Asamitsu, Y. Tomioka, H. Kuwhara, Y. Tokura, Science 388

(1997) 50.

[14] M. Fiebig, K. Miyano, Y. Tomioka, Y. Tokura, Science 280 (1998)

1925.

[15] H.Y. Hwang, S.W. Cheong, B. Batlogg, Appl. Phys. Lett. 68 (1996)

3494.

[16] C. Mitra, P. Raychaudhuri, J. John, S. Dhar, A. Nigam, R. Pinto,

J. Appl. Phys. 89 (2001) 524.

[17] V. Agarwal, M. Liu, J. Mater. Sci. 32 (1997) 619.

[18] Y. Shimizu, T. Murata, J. Am. Ceram. Soc. 80 (1997) 2702.

[19] M.P. Pechini, U.S., 1967.

[20] S. Mollah, H.L. Huang, H.D. Yang, S. Pal, S. Taran, B.K.

Chaudhuri, J. Magn. Magn. Mater. 284 (2004) 383.

[21] J.Z. Liu, I.C. Chang, S. Irons, P. Klavins, R.N. Shelton, K. Song,

S.R. Wasserman, Appl. Phys. Lett. 66 (1995) 3218.

[22] X.L. Li, A. Gupta, G. Xiao, G.Q. Gong, Appl. Phys. Lett. 71 (1997)

1124.

[23] H.Y. Huang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. B 77

(1996) 2041.