A

Fnemisd©

K

1

cceg(eeisiMhtsn

0d

Materials Science and Engineering B 139 (2007) 128–133

Structural and magnetoresistive property of the LaCaMnO thin filmsprepared by a new method: Precursor film sintering

Hui Liu a,∗, Ying Luo b, Ming Li c

a Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904, United Statesb Charles L. Brown, Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, VA 22904, United States

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

Received 28 September 2006; received in revised form 14 November 2006; accepted 26 January 2007

bstract

We synthesized La0.67Ca0.33MnO3 thin films on LaAlO3 (1 0 0) single-crystal substrates by a newly invented precursor film sintering technique.TIR analysis shows that water evaporation and partial decomposition of PAM are the major reactions during the dry-off step, and oxidation ofitrate ions and the formation of Mn O Mn bonds only take place in the sintering step. LCMO thin films prepared by the PFS method are allpitaxially-grown thin films with good quality, and have the same crystal structure as the epitaxially-grown LCMO thin films prepared by the PLDethod. The critical sintering temperature was carefully studied based on XRD patterns and SEM images. It is found that the MR of thin films

s influenced by the critical sintering temperature, the length of sintering and oxygen temper. Besides, the advantages of the PFS method includeimple equipments, common chemical compounds and a variety of shapes of the prepared thin films. Therefore, it is promising to fabricate electricevices by LCMO thin films with CMR effects via the PFS method in the future. 2007 Elsevier B.V. All rights reserved.

baM

isLwa[Ls9tm

eywords: LaCaMnO; Thin film; Precursor film sintering; Magnetoresistance

. Introduction

The manganites A1−xBxMnO3 (“A” representing trivalentations, such as La, Nd, Pr and “B” representing doped divalentations, such as Ca, Sr, Ba) have drawn remarkable inter-sts for their very large magnetoresistance effects known asiant magnetoresistance (GMR) or colossal magnetoresistanceCMR) [1–7]. Among them La1−xCaxMnO3 (LCMO) has beenxtensively studied because their magnetic and electric prop-rties can be understood on the basis of the double-exchangenteraction theory [8,9]. Due to the changes of the crystaltructure and the valence of Mn ions when doped by Caons, semiconducting-type LaMnO3 compounds with a low

n4+/Mn3+ ratio transform into metal-like materials with aigh Mn4+/Mn3+ ratio. Balbashiov et al. [10] suggested that

he metal–semiconductor transformation in LCMO thin filmshould be accompanied by a magnetic transition from a paramag-etic phase into a ferromagnetic phase. A theoretical calculation

∗ Corresponding author.E-mail address: hl5h@virginia.edu (H. Liu).

asttmii

921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2007.01.049

y Jacobs et al. [11] implied that strong electron–phonon inter-ctions arising from the John–Teller splitting of the d levels ofn must be considered to play an equally important role.So far, the major methods of preparing LCMO thin films

nclude ion beam sputtering, pulsed laser deposition (PLD) andolid state reactions. For example, Chahara et al. [12] prepareda0.72Ca0.25MnOz films by an ion beam sputtering method, fromhich the magnetoresistance (defined as �R/R0) of 53% was

chieved in a magnetic field of 1 × 104 Oe at 220 K. Jin et al.13] reported that the epitaxial LaCaMnO films deposited onaAlO3 (1 0 0) substrates prepared by the pulsed laser depo-ition (PLD) had a large MR as high as 99.9% at 77 K and2.8% at room temperature at a magnetic field of 6 T, respec-ively. However, LCMO thin films are highly costly since PLD

ethod or ion beam sputtering requires expensive equipmentsnd complicated processing. These years, the polymer precur-or method and sol–gel method have been successfully adoptedo prepare some ceramic powders or films [14,15], but produc-

ion of LCMO thin films is still not cost-reduced because these

ethods require some special precursors. Therefore, develop-ng a simple and low cost method to prepare LCMO thin filmss essential for realizing their potential applications.

d Engineering B 139 (2007) 128–133 129

fiCmsiiwsetifi

2

iapLskipbdt

wdsrttPttoictp4

sd

Fp

m2irat

3

spp

TC

D

(

(

(

H. Liu et al. / Materials Science an

In this paper, we report the epitaxial growth of LCMO thinlms (with a nominal structure of La0.67Ca0.33MnO3) with theMR effects by a precursor film sintering method called as PFSethod. The PFS method includes two heat treatments, dry-off

tep and sintering step. The change of the precursor film dur-ng the dry-off step was characterized by Fourier transformationnfrared spectroscopy (FTIR). Then, the sintering temperature,hich is critical in this method, was carefully studied because it

trongly affects the quality of the obtained thin films. The CMRffects of the LCMO thin films sintered at different sinteringemperatures were reported. At last, effects of long-time sinter-ng and oxygen temper on the magnetoresistive property of thinlms were measured.

. Experiment

Polyacrylamide (PAM) was first dissolved in a beaker of de-onized water at the room temperature. To adjust the viscositynd the concentration of the polymer precursor film, the weightercentage of PAM in the solution was controlled at 22.5%. Thena(NO3)3, Ca(NO3)2 and Mn(NO3)2 were added into the PAMolution. The mixture was stirred by a glass rod for 10 min toeep the solution homogeneous. A qualified precursor solutions colorless, transparent and without any undissolvable com-onents. After the homogeneous precursor film was obtainedy spin coating on a LaAlO3 (1 0 0) single-crystal substrate,rying off at low temperature in an oven and sintering at highemperature in a tube-furnace were sequentially carried out.

Based on the thermal gravimetric analysis (not shown here),e have designed three low-temperature dry-off routes. Threery-off routes and resultant thin films after sintering 20 min areummarized in Table 1. It was figured out that the route c gaveise to thin films with relatively good quality. Therefore, all ofhin films discussed in this paper were prepared by followinghe route c. The sintering temperature is also a critical factor inFS method, which will be discussed in the next section. After

wo heat treatments, thin films were naturally cooled downo room temperature inside the tube-furnace. The thicknessf the obtained LCMO films on the LaAlO3 (1 0 0) substrates determined to be about 35 nm. For a thicker film, the spinoating, drying and sintering processes were repeated untilhe film with desired thickness was obtained. The thin films,repared by 10 times of the above process, with a thickness of

00 nm, were used for measurements.

The preparation of the samples for FTIR analysis includespin coating the polymer precursor film onto a KBr flake andrying it off at 120 ◦C for 1 day.

orfia

able 1omparison of three designed routes of the dry-off step and resultant thin films

rying routes Precursor film After dryin

a) Drying 20 min at 120 ◦C, then sintering at800 ◦C

Colorless Colorless

b) Drying 20 min at 200 ◦C, then sintering at800 ◦C

Colorless Colorless, b

c) Slowly drying from 120 to 200 ◦C in20 min, then sintering at 800 ◦C

Colorless Colorless

ig. 1. In situ Fourier transformation infrared (FTIR) spectroscopies of polymerrecursor films obtained at different temperatures.

The standard four-probe technique was used to measure theagnetoresistance (MR) of the film in a temperature range from

00 to 300 K with an applied field up to 5.5 T. The MR reportedn this paper is defined as (R0 − RH)/R0 × 100% (RH and R0 areesistance with and without the applied field, respectively). Thepplied field is parallel to the film plane and perpendicular tohe current direction.

. Results and discussions

Fig. 1 shows in situ Fourier transformation infrared (FTIR)pectroscopies of the polymer precursor films from room tem-erature to 340 ◦C. Because the FTIR spectroscopy of therecursor film at room temperature is not a simple summation

f the FTIR spectroscopies of PAM and the nitrate acid, someeactions between components took place during the precursorlm was dried off at 120 ◦C for 1 day. The peaks between 3000nd 3600 cm−1 are obvious in the temperature range from room

g After sintering

Black and erasable grainy film

ut many white powder appears Light brown and translucent thin film,with some black grainy areaFlat, translucent and light brown thin film

1 and

tTifsrmtphibWPttdtsa

diT

Fts

c(

tossostfitpstd

tt

30 H. Liu et al. / Materials Science

emperature to 200 ◦C, but they disappear higher than 260 ◦C.hese peaks can be explained by the presence of water and PAM

n the precursor film at low temperature since these peaks comerom the bonds of O H and N H. It is noted that the inten-ity of one of the characteristic peaks of PAM at 1626 cm−1

educes and vanishes at 280 ◦C as temperature elevates, whicheans that PAM continuously decomposes as the increase of

emperature. One prominent peak change is two sets of sharpeaks near 1400 cm−1 gradually changing to a broad peak withigh intensity, which is a reflection of the oxidation of nitrateons. Moreover, the characteristic peak coming from Mn O Mnonds at 550 cm−1 appears at temperature higher than 280 ◦C.e suggest that water evaporation and partial decomposition of

AM are the major reactions during the dry-off step, and oxida-ion of nitrate ions and the formation of Mn O Mn bonds onlyake place in the sintering step. In fact, reactions in the systemuring the dry-off step are quite complicated. More informa-ion is required to recognize exact reactions during the dry-offtep. The reactions in the sintering step have been discussed innother paper [16].

Fig. 2(a) is the XRD patterns of LCMO thin films prepared atifferent sintering temperatures from 800 to 1400 ◦C. Fig. 2(b)s a �-scan pattern of an LCMO thin film sintered at 1200 ◦C.he thickness of the thin film was about 500 nm which was

ig. 2. (a) X-ray diffraction patterns of LCMO thin films sintered at differentemperatures and (b) �-scan pattern of the (1 1 1) direction, the thin film wasintered at 1200 ◦C.

fcmitepfiattssfi

dStst8fiiatfbtt2sciwls1

Engineering B 139 (2007) 128–133

alculated by a SEM image of the cross-section of the samplenot shown here).

Based on XRD patterns, we calculated the lattice constants ofhe LCMO thin films and the LaAlO3 (1 0 0) substrate. It turnsut that the lattice constants of the LCMO thin films and theubstrate are 3.861 and 3.791 A, respectively. It is noted thattrong characteristic peaks of the LCMO thin films are presentn the low angle sides of intrinsic peaks of the single-crystalubstrate of LaAlO3 (1 0 0) and no peaks from the polycrys-alline of LCMO are observable. Furthermore, there are onlyour peaks present every 90◦ in the �-scan pattern, whichs another evidence for the epitaxial growth of the LCMOhin films. Here, we conclude that the LCMO thin films pre-ared by PFS method epitaxially grow up on LaAlO3 (1 0 0)ingle-crystal substrate and they have the same crystal struc-ure as the epitaxially grown films prepared by the PLD methodoes.

From the XRD pattern at 800 ◦C, no diffraction peaks fromhe film can be observed. There are two possible reasons: one ishat the sintering temperature of 800 ◦C is not high enough toorm LCMO thin films by PFS method; the other is that formedlusters are too small to give diffraction patterns. Based on theeasured MR of the thin film, the presence of small cluster

s more reasonable. The (0 0 1) and (0 0 2) diffraction peaks ofhe LCMO thin film appear when the sintering temperature islevated to 1000 ◦C or higher temperature. As the sintering tem-erature elevates, the intensities of all diffraction peaks of thinlms increase, which indicates that the extent of crystallinitynd the quality of thin films have been improved. Moreover,here is no appreciable changes in the diffraction angle fromhe enlarged XRD patterns of the (0 0 1) diffraction peaks (nothown here). Therefore, it can be concluded that the higher theintering temperature, the better the quality of the LCMO thinlms.

The surface morphologies of the LCMO thin films sintered atifferent temperatures are shown in Fig. 3(a–d) investigated byEM. There is no observable crystalline structure of the LCMO

hin film at the sintering temperature of 800 ◦C, which is con-istent with the XRD pattern shown in Fig. 2. The sinteringemperature of 1000 ◦C leads to a different morphology from00 ◦C. The formation of the LCMO thin film has been con-rmed since a lot of clusters can be clearly observed in the SEM

mage. It can be seen that the thin film is composed of clusters,nd some of clusters are connected by bridges, which seemshat the LCMO thin film grows in a three dimension island-likeorm. The average cluster size is approximately 1um, estimatedy the SEM image. When the sintering temperature is elevatedo 1200 ◦C, formation of clusters and holes in between clus-ers become more obvious. The average cluster size is about00 nm, much smaller than that of clusters in the thin filmintered at 1000 ◦C. Instead of clusters, a slightly corrugated,omplete thin film is generated when the sintering temperaturencreases to 1400 ◦C. The thin film has a terrace-like surface

ith facets, and seems to grow in a two dimension layer-by-

ayer mode. Few holes can be observed from SEM images. Wepeculate that LCMO thin film melts at a temperature higher than200 ◦C. Solid state reactions take place at the sintering temper-

H. Liu et al. / Materials Science and Engineering B 139 (2007) 128–133 131

at: (

afaiitttF

taialoritbptvvpd

snttgrown thin films prepared by PLD method, the LCMO thinfilms we report here are epitaxial thin films with a large valueof MR.

Fig. 3. SEM images of the LCMO thin films sintered

ture lower than the melting temperature, which gives rise to theormation of clusters and holes. Because of high mobilities ofll elements at the sintering temperature higher than the melt-ng temperature, they diffuse, aggregate and then form flat andntegrated thin films as the temperature naturally cools downo room temperature. Furthermore, it is found that the LCMOhin film sintered at 1200 ◦C for 8 h has same morphology as thehin film sintered at 1400 ◦C for 20 min does, which is shown inig. 4.

Fig. 5 shows the temperature dependence of the resistance ofhe LCMO thin films with and without a magnetic field, as wells the MR from 200 to 300 K. For individual thin films, the MRncreases as the temperature increases and reaches a maximumround 270 K. The MR of these thin films remains in a highevel even at room temperature. For example, the maximum MRf the thin film sintered at 1200 ◦C is 80% at 264 K. Even atoom temperature, the MR still reaches 45%. The MR behav-or of LCMO thin films prepared by PFS method is similar tohe MR of a single crystal of La0.55(CaPb)0.35MnO3 reportedy Liu et al. [17]. Moreover, it is found that the sintering tem-erature has influence on the maximal value of MR of thesehin films. As the sintering temperature elevated, the highest

alue of MR increases, but the temperature where the highestalue of MR is obtained becomes lower. It is noted that the tem-erature dependence of the resistance of thin films sintered atifferent temperatures with a magnetic field of 5.5 T is almost

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

ame, but the resistance peaks of these thin films without a mag-etic field are gradually shifted from 284 to 264 K. We suggesthat MR behavior may be explained by the metal–semiconductorransformation in the thin film. Compared with the epitaxially

Fig. 4. A SEM image of a LCMO thin film sintered at 1200 ◦C for 8 h.

132 H. Liu et al. / Materials Science and Engineering B 139 (2007) 128–133

F ic fielt 1200

8boo

(

ig. 5. Resistances of the LCMO thin films without (H = 0 T) and with a magnethin films. Thin films sintered at: (a and b) 800 ◦C; (c and d) 1000 ◦C; (e and f)

Fig. 6 shows the MR of the thin film sintered at 1200 ◦C for

h and the thin film sintered at 1200 ◦C for 20 min followedy a oxygen temper at 900 ◦C for 1 h. Long-time sintering andxygen temper after the sintering have no observable effectsn the crystal structure of the thin film based on XRD patterns

taf[

d (H = 5.5 T) at different temperatures and the magnetoresistance of the LCMO◦C; (g and h) 1400 ◦C.

not shown here), but they are helpful to improve the MR of

hin films, since both of these processes provide more oxygentoms to thin films, reduce the oxygen defects in thin films andacilitate the double-exchange mechanism proposed by Zener18].

H. Liu et al. / Materials Science and Engineering B 139 (2007) 128–133 133

F lm si

4

Lcerasatfipictpsfv

R

[

[

[

[

[[15] Y. Shimizu, T. Murata, J. Am. Ceram. Soc. 80 (1997) 2702.[16] H. Liu, Y. Luo, M. Li, J. Cryst. Growth 296 (2006) 207.[17] J.Z. Liu, I.C. Chang, S. Irons, P. Klavins, R.N. Shelton, K. Song, S.R.

Wasserman, Appl. Phys. Lett. 66 (1995) 3218.[18] C. Zener, Phys. Rev. 82 (1951) 403.

ig. 6. The MR of (a) the thin film sintered at 1200 ◦C for 8 h and (b) the thin fi

. Conclusion

In summary, we synthesized La0.67Ca0.33MnO3 thin films onaAlO3 (1 0 0) single-crystal substrates by a newly invented pre-ursor film sintering technique. FTIR analysis shows that watervaporation and partial decomposition of PAM are the majoreactions during the dry-off step, and oxidation of nitrate ionsnd the formation of Mn O Mn bonds only take place in theintering step. LCMO thin films prepared by the PFS methodre all epitaxially grown thin films with good quality, and havehe same crystal structure as the epitaxially grown LCMO thinlms prepared by the PLD method. The critical sintering tem-erature was carefully studied based on XRD patterns and SEMmages. It is found that the MR of thin films is influenced by theritical sintering temperature, the length of sintering and oxygenemper. Besides, the advantages of the PFS method include sim-le equipments, common chemical compounds and a variety ofhapes of the prepared thin films. Therefore, it is promising toabricate electric devices by LCMO thin films with CMR effectsia the PFS method in the future.

eferences

[1] C.W. Searle, S.T. Wang, Can. J. Phys. 48 (1969) 2023.[2] M. McCormack, S. Jin, T.H. Tiefel, R.M. Fleming, J.M. Phillips, Appl.

Phys. Lett. 64 (1994) 3045.

ntered at 1200 ◦C for 20 min followed by a oxygen temper at 900 ◦C for 1 h.

[3] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri, C.N.R. Rao, J. Solid StateChem. 114 (1995) 297.

[4] B. Raveau, A. Maignan, V. Caignaert, J. Solid State Chem. 117 (1995)424.

[5] C. Martin, A. Maignan, M. Hervieu, B. Raveau, Phys. Rev. B 60 (1999)12191.

[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. Growth275 (2005) e2415.

[8] P.W. Anderson, H. Hasegnwa, Phys. Rev. 100 (1955) 675.[9] P.G. de Gennes, Phys. Rev. 118 (1960) 141.10] A.M. Balbashiov, S.G. Karabashev, Ya.M. Mukovskiy, S.A. Zverkov, J.

Cryst. Growth 167 (1996) 365.11] T.H. Jacobs, K.H.J. bushow, G.F. Zhou, X. Li, F.R. De Boer, J. Magn.

Magn. Mater. 116 (1992) 220.12] K. Chahara, T. Ohna, M. Kasai, Y. Kozono, Appl. Phys. Lett. 63 (1993)

1990.13] S. Jin, M. McCormack, T.H. Tiefel, R. Ramesh, J. Appl. Phys. 76 (1994)

6929.14] V. Agarwal, M. Liu, J. Mater. Sci. 32 (1997) 619.