Substrate-dependent structural and magnetic properties of Sr2FeMoO6 nanostructured double perovskite thin films

ARTICLE IN PRESS Physica B 405 (2010) 3259–3266

Contents lists available at ScienceDirect

Physica B journal homepage: www.elsevier.com/locate/physb

Substrate-dependent structural and magnetic properties of Sr2FeMoO6 nanostructured double perovskite thin films Deepak Kumar, Davinder Kaur n Fuctional Nanomaterials Research Laboratory, Department of Physics and Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee 247 667, Uttrakhand, India

a r t i c l e in fo

abstract

Article history: Received 4 October 2009 Received in revised form 20 February 2010 Accepted 23 April 2010

We report the synthesis of nanostructured Sr2FeMoO6 (SFMO) thin films on SrTiO3 (0 0 1), LaAlO3 (0 0 1) and MgO (0 0 1) substrates by pulsed laser deposition technique. The influence of substrate nature on structural and magnetic properties of thin films has been systematically investigated. The films deposited on SrTiO3 and LaAlO3 were single crystalline with preferred (0 0 4) orientation without any impurity peak, whereas the films deposited on MgO substrate exhibit impurity peaks of SrFeO3 along with (0 0 4) reflection of SFMO. The out of plane lattice parameter, ‘c’ and grain size have been shown to be influenced by nature of substrate. Further, the magnetic properties of the films, including the coercive field, remnant magnetization, saturation magnetization and magnetoresistance were also observed to be strongly dependent on the choice of substrate. The investigations reveal an increase in magnetization with decrease in film–substrate lattice mismatch. The films on SrTiO3 substrate exhibit a saturation magnetic moment of 3.24mB/f.u. at 5 K, with a Curie temperature larger than 320 K, whereas the films deposited on MgO substrate are observed to have low magnetization, which is related to the high proportion of antisite defects in these films. The magnitude of MR was observed to be smaller in case of films deposited on SrTiO3 substrate and in addition the variation in MR with applied field was found to be linear in these films, whereas the films on MgO substrate were found to exhibit higher value of MR with two different regimes: the low-field region (Ho 20 kOe) with a steeper slope and the highfield behavior with a less pronounced slope. The present study elucidates the effects of substrate nature on magnetization and that the magnetoresistance behavior of SFMO thin films are useful in magnetic sensors and spintronics application. & 2010 Elsevier B.V. All rights reserved.

Keywords: Double perovskites Pulsed laser deposition Magnetoresistance

1. Introduction Recently, half metallic ferromagnet Sr2FeMoO6 (SFMO) has attracted much attention due to its large room temperature magnetoresistance (MR) and high curie temperature of Tc  410 K making it a potential candidate for spintronic device applications such as spin valves and magnetic tunnel junctions [1]. SFMO is an ordered double perovskite (A2BB0 O6) with regular arrangement of corner sharing FeO6 and MoO6 octahedra, alternating along all three directions of the crystal [2–4]. The Sr cations reside in the cuboctahedral vacancies created by the corner sharing octahedral. The Fe3 + and Mo5 + ions are spatially distributed alternately in the B site of the perovskite structure. The antiferromagnetic coupling of Fe3 + (S¼5/2) and Mo5 + (S¼ 1/2) ions leads to saturated moment Ms per formula unit of 4mB. However the reported value of Ms is smaller than this predicted value due to presence of naturally grown antisite defects, where the partial Fe(Mo) ions are misplaced at Mo(Fe) sites [5,6].

n

Corresponding author. Tel.: + 91 1332 285407; fax: +91 1332 273560. E-mail address: [email protected] (D. Kaur).

0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.04.056

Polycrystalline bulk samples have been extensively investigated for fundamental studies aimed to establish the structural and fundamental properties of SFMO [2–13]. However, epitaxial or highly oriented thin films of this compound are necessary for application in spintronics, which has not proven to be easy. There are several constraints on the growth parameters of the films dictated by the difficulty of avoiding the segregation of spurious phases and by the necessity of minimizing disorder in the double perovskite A2BB’O6 lattice. It was observed that often the magnetization is reduced due to antisite defects, which causes the sample to locally separate into SrFeO3 and SrMoO3 areas that in turn reduce the magnetization because SrFeO3 is antiferromagnetic with TN ¼ 134 K and SrMoO3 is paramagnetic [13]. Due to severe constraints and narrow space of deposition parameters for growth of high-quality films of these double perovskites, there are only few reports on SFMO thin films [14–19] as compared to very large number of studies on polycrystalline bulk material. Among various thin film deposition techniques, PLD has been recognized as a promising versatile technique for the deposition of stoichiometric films of SFMO and other metal oxides at high deposition rate [17–21]. Moreover, PLD has an added advantage of being a non-equilibrium process. Thin films with Tc higher than

ARTICLE IN PRESS 3260

D. Kumar, D. Kaur / Physica B 405 (2010) 3259–3266

300 K have been obtained by a careful selection of the deposition parameters. In particular, the critical role played by the oxygen background pressure and by the substrate temperature has been clearly demonstrated. Oxygen pressure higher than 10  4 mbar has been reported to be deleterious for the stabilization of single phase SFMO, because the more stable SrMoO4 phase can nucleate [22]. Substrate temperature has been reported to affect the vertical unit cell size of SFMO films deposited by PLD, as at low substrate temperature the atomic mobility seems to be insufficient to achieve a perfect arrangement of the double perovskite configuration. Asano et al. [18] observed that the films deposited at low temperatures has semiconductor like behavior of resistivity, whereas those deposited at high temperatures were metallic. Westerburg et al. [16] observed small negative MR of  3% at very large magnetic field of 80 kOe at 300 K in case of their PLD films. Manako et al. [17] have also reported that SFMO films could only be obtained in a narrow range of deposition temperature and oxygen partial pressure. Shinde et al. [19] have also reported that best quality SFMO films could only be grown at particular substrate temperature and oxygen partial pressure. Trolio et al. [22] have demonstrated the influence of laser fluence on structural and magnetic properties of SFMO films. In the case of SFMO thin films, the role of substrates is also very important because it can limit the growth direction and growth rate. There are reports on synthesis of SFMO thin films on different substrates such as SrTiO3 [22], MgO [23] and LaAlO3 [24]. However, there are limited studies on systematic investigation of influence of substrate on microstructure and magnetic properties of these films. In the present study we have tried to investigate the influence of substrate type such as SrTiO3, LaAlO3 and MgO on growth and properties of SFMO thin films. The grain size, c-axis lattice constant, saturation magnetization and magnetoresistance has been shown to be influenced by the nature of substrate. These studies are useful in magnetic sensors and spintronics application.

2. Experimental SFMO thin films were grown by pulsed laser deposition technique using a 15 mm diameter target. The details of process setup are given elsewhere [25]. The targets were prepared as polycrystalline powders by solid state reaction method. Stoichiometric amounts of high-purity powders of SrCO3, Fe2O3 and MoO3 were ground and calcined at 900 1C for 12 h. The resulted powders were reground and pressed into pellets under a pressure of about 80 MPa and then sintered at 1000 1C for 24 h in argon atmosphere. The excimer laser KrF with wavelength of 248 nm was used for deposition. The pulse repetition rate was 10 Hz with laser fluence of about 2–3 J cm  2. The target substrate distance was kept at 40 mm and working pressure was 5  10  5 Torr. The deposition temperature and time was kept at 825 1C and 25 min, respectively. Single crystal SrTiO3 (0 0 1), LaAlO3 (0 0 1) and MgO (0 0 1) wafers were used as substrates and were cleaned sequentially in acetone, methanol and deionized water prior to deposition. The orientation and crystallinity of the films were investigated ˚ using Bruker AXS D-8 advanced diffractometer of Cu Ka (1.54 A) in y–2y geometry. To obtain a profile fitting with good signal, polycrystalline silicon powder was used for instrumental correction. The surface morphology of the films was studied by means of atomic force microscopy in semi-contact mode (AFM, NTMDT) with silicon nitride (Si3N4) tip of 10 nm radius. The root-meansquare (RMS) roughness of the surface was calculated from AFM scans over substrate areas of 3 mm  3 mm, and the RMS roughness was calculated three times at a different spot for each sample. The thickness of the films was measured using cross-

sectional Field Emission Scanning Electron Microscopy (FESEM) and was kept constant  200 nm for all the films. The magnetic properties of the films were measured using superconducting quantum interference device (Quantum Design) in an applied field of 77 T. Magnetoresistance was measured using a computer controlled system made up of a Keithley-220 sourcemeter, a Keithley-2000 nanovoltmeter and a LakeShore-340 temperature controller by the conventional four-terminal technique with magnetic field up to 110 kOe. The contacts over the sample were made by silver paint.

3. Results and discussion 3.1. Structural properties Fig. 1 shows the X-ray diffraction (XRD) pattern of SFMO thin films deposited on various substrates at fixed deposition temperature of 825 1C and pressure of 5  10  5 Torr. The samples S1, S2 and S3 are the films deposited on SrTiO3, LaAlO3 and MgO substrate, respectively. The XRD pattern of films S1 and S2 showed only diffraction peaks corresponding to SFMO (0 0 l) reflections and the substrate, indicating that the films were oriented along the c-axis, perpendicular to the growth plane. However, the XRD pattern of sample S3 showed additional minute spurious peak at 2y ¼42.91 due to small quantity of secondary phase, i.e. strontium ferrate (SrFeO3  a 0o a o0.5), which has been reported to affect the magnetic behavior of SFMO films. The strontium ferrate phase develops when antisite disorder occurs in the double perovskite lattice [26,27]. We are able to observe (1 0 1) reflection [inset of Fig. 1] depicting the ordering of Fe/Mo ions at the B/B0 sites in these films. Further the intensity ratio of (1 0 1) and (0 0 4) reflections was calculated and antisite defects were determined for samples S1, S2 and S3 in similar manner as reported by Venimadhav et al. [46] The percentage of antisite defects calculated by this method is presented in Table 2. It is clear that the films grown on MgO substrate have less degree of B-site ordering in comparison to films grown on LaAlO3 and SrTiO3 substrate. The inset of Fig. 1 shows the c-axis lattice parameter of these films, calculated from XRD results. The value of the c-axis lattice constant was 8.023 A˚ (S1), 7.95 A˚ (S2) and 7.94 A˚ (S3). For all the films, the c-axis was observed to be elongated in ˚ [28,29], which could be comparison to the bulk c-axis (7.879 A) attributed to strain [30–32] originating from the film–substrate lattice mismatch or cationic offstoichiometry [33] or to the presence of disorder in the Fe and Mo sublattices [18,19]. The film–substrate lattice mismatch was calculated for all the samples and is presented in Table 1. We note that SrTiO3 has the best lattice match with SFMO among these substrates with lattice mismatch of  1.14%. The negative value indicates that the substrate lattice parameter is smaller than the film lattice parameter and the film will grow in a compressive strain environment. SrTiO3 (0 0 1) is a cubic perovskite with a lattice parameter c, such that when doubled 2c¼7.81 A˚ it is smaller ˚ than the corresponding SFMO lattice constant c ¼7.879 A. Therefore, SFMO films sustain an in-plane compressive strain when deposited on SrTiO3 substrate. On the other hand MgO was observed to have maximum lattice mismatch of 6.41%. The positive lattice mismatch means that the substrate has a large lattice parameter than the film lattice parameter and the film will grow in a tensile strain condition. The large value of lattice mismatch in case of films deposited on MgO substrate could probably be the reason for high density of antisite defects in these films and is in agreement with the reported results [18]. Further using XRD data the crystallite size of the films was also calculated and was found to be 27.6, 16.0 and 12.7 nm for sample S1, S2 and

ARTICLE IN PRESS D. Kumar, D. Kaur / Physica B 405 (2010) 3259–3266

3261

8.04 (a)

(004)

19 20 43 (101)

18

19

44 45 46 (004)

7.96

c (Å)

18

8.00

MgO

(101)

20

45 46 47

STO

19

Bulk SFMO

7.88 S1

S3

S2

LAO

18

44 45 46 47 (004) (004)

Intensity (a.u.)

7.92 20 (101)

2θ (Degree)

S1

S2 * SrFeO3 + SrMoO4

30

*

S3

+ 40

50

60

70

2θ (degree) Fig. 1. X-ray diffraction patterns of SFMO thin films deposited on STO, LAO and MgO substrates. Inset (a) shows the (1 0 1) and (0 0 4) reflections and in the inset (b) we have shown the c-axis lattice constants of S1, S2 and S3 samples.

Table 1 Calculated crystallite size, lattice parameter and microstrain for SFMO thin films deposited on MgO, LAO and STO substrates. Substrate

SrTiO3 LaAlO3 MgO SFMO

Lattice parameter ‘c’

Lattice mismatch (%)

˚ Substrate (A)

˚ Film (A)

7.81 7.642 8.406 7.897

8.023 7.95 7.94 

 1.14  3.27 6.41 

S3, respectively. The small value of crystallite size observed in sample S3 supports the presence of disorder in the Fe and Mo sublattices. The surface morphology of the films was studied using atomic force microscopy (AFM) in tapping mode and is shown in Fig. 2(a–c). It clearly depicts that the sample S1 is homogeneous, smooth, and uniform with average roughness of 3.2 nm while sample S2 and S3 show roughness of 7.6 and 11.3 nm, respectively. The grain size of the films was found to be 83.7 nm (S1), 48.5 nm (S2) and 36.4 nm (S3). The smallest grain size observed in case of films deposited on MgO substrate was in confirmation with the XRD results. However, overall particle size shown by AFM was much larger as compared with that calculated from the XRD results (Table 1). This was because of the fact that the XRD gave the average mean crystallite size while AFM showed agglomeration of the particles. The XRD and AFM data can be reconciled by the fact that smaller primary particles have a large surface free energy and would, therefore, tend to agglomerate faster and grow into larger grains.

3.2. Magnetic properties The influence of substrate on magnetic properties of SFMO thin films was carried out using a SQUID magnetometer. The area of the substrates used for the film deposition was 0.5  0.5 cm2.

Crystallite size (nm)

Surface Roughness (nm)

XRD

AFM

27.62 15.95 12.71 

83.7 48.5 36.4 

3.2 7.6 11.3

The data have been corrected for the background signal from the substrate using equation Mfilm ðHÞ ¼ Mtotal ðHÞwsubstrate H where wsubstrate is the susceptibility of the substrate, Mtotal the magnetization of (film+substrate) and H the magnetic field applied parallel to the film surface. Fig. 3(a) and 3(b) show the M–H curves of S1, S2 and S3 at 5 and 300 K. Inset of Fig. 3(a) shows the low-field region of the loop, showing coercivity (Hc) of  1163 Oe, and remanent magnetization of about (Mr)  0.88mB/ f.u. for sample S3. The value of coercivity (Hc)  800 Oe and remanent magnetization 0.91mB/f.u. was observed in case of sample S2, whereas sample S1 showed Hc  716 Oe and Mr  0.96mB/f.u. at 5 K. It is worth noting that the value of Hc for sample S3 was 1.6 times higher than sample S1, further indicating more disorder in case of sample S3. The ratio of Hc values measured at 5 and 300 K was 7.31 for sample S1, 6.25 for sample S2 and 5.41 for sample S3 (Table 2). Assuming that the film density is close to bulk theoretical value and film volume is only filled with the SFMO unit cells, the value of saturation magnetization Ms at 5 K was 3.24mB/f.u. (S1), 2.83mB/f.u. (S2) and 2.21mB/f.u. (S3). Such a value is lower than theoretical 4mB/f.u. expected on the basis of the ferromagnetic coupling between Fe3 + and Mo5 + configurations suggesting the presence of antisite defects in these films. The reported experimental results on value of saturation magnetization (Ms) are very

ARTICLE IN PRESS 3262

D. Kumar, D. Kaur / Physica B 405 (2010) 3259–3266

Fig. 2. AFM images of SFMO thin films deposited on (a) STO, (b) LAO and (c) MgO substrates.

different in thin films as well as bulk. Recently, Li et al. [34] have reported the value of Ms is less than 3.2mB/f.u. at 10 K in case of their bulk samples prepared by sol–gel method. For single crystal of SFMO, the reported value of Ms is 3.2mB/f.u. at 10 K [29]. Westerburg et al. [16] have reported Ms value of 4mB/f.u. for SFMO thin films grown under Ar atmosphere and have observed the (1 0 1) peak indicative of a significant degree of B-site ordering. Trolio et al. [22] have reported saturation magnetization of 3.4mB/ f.u. in case of films prepared by pulsed laser deposition on SrTiO3 substrate at low laser fluence, whereas Shinde et al. [19] have reported saturation magnetization of 3.28mB/f.u. in case of films prepared by a similar technique.

In most cases of thin films and bulk SFMO, the value of Ms is shown to be less than the theoretical value. Since the ionic sizes of Fe + 3 and Mo + 5 are similar, there is a finite concentration of missite disorder in Sr2FeMoO6, which interchanges the position of Fe and Mo sites in a random fashion. The most significant effect of disordering is to reduce the net magnetization of the sample. However, the nature and origin of this decrease in the magnetization in the presence of disorder is still a matter of debate in the literature [38,39]. There are two distinct ways that the net magnetization may be reduced in Sr2FeMoO6 in the presence of mis-site disorders. One possibility is that the disorder destroys the specific spin arrangement of Fe and Mo sublattices without any significant effect on the individual magnetic moments at these sites. This can be achieved by transforming the ferromagnetic coupling between some of the Fe sites to an antiferromagnetic coupling. This view has been preferred by most in recent times, under the assumption that Fe–O–Fe interactions, induced by the mis-site disorder in place of Fe–O–Mo, will be antiferromagnetically driven by the superexchange. Alternately, the magnetic moments at each individual site may decrease due to the different chemical environments induced by the disorder, without affecting the nature of the spin order within the Fe and Mo sublattices. The real situation may even be a combination of both these effects, with a simultaneous reduction in the magnetic moments at different sites as well as a change in the nature of the magnetic coupling between different sites. In order to understand the nature of magnetic exchange in double perovskites, X-ray magnetic circular dichroism (XMCD) has been used to find the value of magnetic moment on the B and B0 site of double perovskites. For Sr2FeMoO6 single crystal, using XMCD measurements, Besse et al. [35] have shown for the first time the spin moments of +3.05 and  0.32mB for Fe3 + and Mo5 + ions, respectively, which gives a direct confirmation of the ferrimagnetic ordering and also predicts that a localized spin carried on Fe sites and a delocalized electron of Mo are responsible for the half metallicity of Sr2FeMoO6 compound. This is in agreement with a kinetic energy driven exchange model proposed by Sarma et al. [41], which explain the strong ferromagnetic exchange in double perovskites despite the large distance between the magnetic ions. Subsequently, Majewski et al. [36,37] have also investigated the magnetic moment of the 5d shell of the nonmagnetic W ion in the double perovskites Sr2CrWO6 and Ca2CrWO6 using XMCD. In both compounds a finite negative spin and positive orbital magnetic moment was detected. These studies suggest that there is a correlation between the magnitude of the magnetic moment at the nonmagnetic ion and the magnetic ordering temperature Tc in the double perovskites. The relation between the antisite defect (of concentration x) and saturation magnetization is given by Ms ¼4–8xmB/formula unit (f.u.). This can be easily understood if one considers that the saturation magnetization of perfect SFMO (4mB/f.u.) is calculated assuming an antiferromagnetic coupling between Fe + 3 (3d5; S¼5/2) and Mo + 5 (4d1; S¼1/2) spins as previously reported for double perovskite systems [40], and that the interaction between two Fe magnetic moments through the 2p oxygen orbitals is also antiferromagnetic. The value of antisite defect in our case was found to be highest (22.37%) for S3 sample, 14.6% for S2 and lowest (9.5%) for S1 sample. The results indicate that decrease in concentration of antisite defects and corresponding increase in magnetization are according to the substrate order MgO4LaAlO3 4SrTiO3. The differences in magnetic properties are likely arise from variations in the lattice–substrate mismatch, which can induce different epitaxial strain states, sample inhomogeneities and phase separation. Since the lattice mismatch of SFMO/MgO (6.41%) is larger than those of SFMO/LAO ( 3.47%) and SFMO/STO ( 1.14%), the degree of lattice distortion and the amount of inhomogeneities in SFMO/MgO are expected to be larger than those in the SFMO/LAO and SFMO/STO. The lattice distortion and

ARTICLE IN PRESS D. Kumar, D. Kaur / Physica B 405 (2010) 3259–3266

4

3

Sample S1

3263

Sample S2

3 2

0 -1 -2 -3

1

0

0 -1 -2

-1

1

0

-1

-800 -400 0 400 800 Magnetic Field (Oe)

-4 -10000

5K 300K

1

Magnetization (μB/f.u.)

Magnetization (μΒ/f.u.)

5K 300K

1

Magnetization (μB/f.u.)

Magnetization (μΒ/f.u.)

2

-5000 0 Magnetic Field (Oe)

5000

-800 -400 0 400 800 Magnetic Field (Oe)

-3

10000

-10000

-5000 0 Magnetic Field (Oe)

5000

10000

Sample S3

5K 300K

1

0

Magnetization (μB/f.u.)

Magnetization (μΒ/f.u.)

2

-1

1.0 0.5 0.0

-0.5

-2

-1.0 -1000 -500 0 500 1000 Magnetic Field (Oe)

-15000

-10000

-5000

0

5000

10000

15000

Magnetic Field (Oe) Fig. 3. Magnetization hysteresis loops of (a) S1, (b) S2 and (c) S3 samples measured at 5 and 300 K. The magnetic field is applied parallel to the film plane. Inset shows the data for the low-field regions.

Table 2 Various magnetic parameters of SFMO thin films deposited on MgO, LAO and STO substrates. Sample

S1 S2 S3

Coercivity Hc (Oe)

5K

300 K

716 800 1163

98 128 215

Ratio of Hc 5 K/300 K

7.31 6.25 5.41

Remnant Magnetization Mr (mB/f.u.)

Saturation Magnetization Ms (mB/f.u.)

5K

300 K

5K

300 K

0.96 0.91 0.88

0.19 0.18 0.17

3.24 2.83 2.21

1.62 1.32 1.1

sample inhomogeneities decrease the degree of ferromagnetic long-range order, inducing the decrease in magnetization. Fig. 4 shows the magnetization versus temperature of sample S1, S2 and S3. The measurements were performed after field cooling the sample from 300 K down to 5 K with a magnetic field H¼200 Oe parallel to the film surface. The magnetization curve for sample S1 reflects continuous decrease in magnetization with temperature, which suggests Tc above 320 K. In case of sample S3, no evidence of long-range ferromagnetic order was observed and

Intensity Ratio I (1 0 1)/I (0 0 4)

0.034 0.025 0.017

Antisite Defect (%)

Magnetoresistance MR (%)

From XRD

FIM Model

70 K

200 K

300 K

12 17 24

9.5 14.6 22.37

 0.8  4.7  16.5

 0.3  3.1  12.9

 0.18  1.5  3.6

the magnetic moment was found to increase rapidly with the decrease in temperature. 3.3. Magnetoresistance The magnetoresistance (MR) is defined as MRð%Þ ¼

RðHÞRð0Þ  100 Rð0Þ

ARTICLE IN PRESS 3264

D. Kumar, D. Kaur / Physica B 405 (2010) 3259–3266

1.5

1.2

1.2

Magnetization (µB/f.u.)

Magnetization (µB/f.u.)

Sample S1

H = 200 Oe

0.9

0.6

Sample S2

0.9 H = 200 Oe 0.6

0.3

0.3 0

50

100

150

200

250

300

0

350

50

100

250

300

Temperature (K)

150 200 Temperature (K)

250

300

350

1.0 Sample S3

Magnetization (µB/f.u.)

0.8

0.6 H = 200 Oe 0.4

0.2

0

50

100

150 200 Temperature (K)

350

Fig. 4. Temperature-dependent magnetization of (a) S1, (b) S2 and (c) S3 samples measured from 5 to 320 K at a constant magnetic field of 200 Oe.

Where R(H) is the resistance of the sample at temperature T in the presence of an applied magnetic field of strength H. MR of SFMO thin films with temperature on different substrates was measured using a four probe resistivity measurement at magnetic field up to 110 kOe. The area of the substrates used for the film deposition was 0.5  0.5 cm2. The magneoresistance (MR) measured for samples S1, S2 and S3 at three different temperatures of 70, 200 and 300 K is shown in Fig. 5. The MR magnitude for all the samples at room temperature, compared to that seen at 70 K, is somewhat reduced, being 3–5 times smaller, which could be due to spin thermal fluctuations being dominant at high temperature. The values of magnetoresistance at 70 K for samples S1, S2 and S3 were  0.8%,  4.7% and  16.5%, respectively (Table 2). MR was observed to be minimum for films deposited on SrTiO3 substrate (sample S1) and maximum for films deposited on MgO substrate (sample S3) with higher concentration of disorder defects. There are three basic mechanisms that give rise to a high magnetoresistive response in SFMO material: the extrinsic tunnel magnetoresistance (TMR), accounting for a large fraction of the observed magnetoresistance in the polycrystalline SFMO bulk, mainly at low temperatures and low fields [41]; the intrinsic magnetoresistive effect associated with the ferromagnetic transition, appearing also in single crystals and enhanced at temperatures near Tc, and also a third component related to the presence of antisite defects. This latter contribution can be

understood in terms of the existence of antiferromagnetic (AF) correlations at the antisite defects (i.e. between two consecutive Fe cations). This antiferromagnetism introduces a significant spin disorder in the system, which in turn, leads to an increase in the material resistivity, due to the enhanced spin-polarized electron scattering. The application of an external magnetic field suppresses this spin disorder to a great extent, lowering the resistivity and thus inducing a remarkable magnetoresistive effect and is consistent with our MR results on sample S3. It was interesting to note that magnitude of MR in case of films deposited on SrTiO3 substrate (S1) at all measured temperatures, i.e. 70, 200 and 300 K, was smaller (Table 2) and in addition to the variation in MR with applied field was almost linear, which is in agreement with the reported results on epitaxial films [19]. On the other hand, in case of films deposited on MgO substrate (S3) the magnitude of MR was largest at all measured temperatures and the variation in magnetoresistance with applied magnetic field showed two different regions: the low-field region (Ho20 kOe) with a steeper slope and the high-field behavior with a less pronounced slope without showing any sign of saturation up to the highest magnetic field (110 kOe). We note the present type of MR in case of sample S3, i.e. sharp increase in the magnitude of MR at low applied field and then a slow increase at large magnetic field has also been observed in polycrystalline SFMO [1] and some other half-metallic oxides [42,43], as well as in the granular alloys

ARTICLE IN PRESS D. Kumar, D. Kaur / Physica B 405 (2010) 3259–3266

0

3265

0 STO

STO

-2

-2 LAO

-6

Magnetoresistance (%)

Magnetoresistance (%)

-4

-8 -10 -12

LAO

-4 -6 -8 -10

-14 MgO

T = 70 K

-16 -18 0

2

4

6

8

10

T = 200 K

-12

12

MgO

-14 0

2

4

Magnetic Field (T)

6 8 Magnetic Field (T)

10

12

0.0 STO

Magnetoresistance (%)

-0.5 -1.0 LAO

-1.5 -2.0 -2.5 -3.0 T = 300 K

MgO

-3.5 -4.0 0

2

4

6 8 Magnetic Field (T)

10

12

Fig. 5. Magnetoresistance of SFMO thin film on STO, LAO and MgO substrates at (a) 70 K, (b) 200 K and (c) 300 K.

containing some transition metal [44,45]. The origin of this lowfield MR in SFMO has been interpreted in terms of tunnelling; MR at the grain boundaries. In this process, the hopping of the spinpolarized electron between microcrystalline grains (each with their own magnetic domains) is critically affected by the relative angle of the magnetization directions, and hence may be controlled by an external magnetic field via the domain-rotation process. We speculate on the basis of the observed features, that a similar process is dominant in case of SFMO films on MgO substrate. Fig. 6 shows the variation in MR with temperature for samples S1, S2 and S3. Maximum change in values of MR was observed in case of films deposited on MgO substrate. We obtain values of MR (70 K), MR (200 K) and MR (300 K) as large as 16.5%,  12.9% and  3.6%, respectively. We note that a steep low-field MR curve against the field was observed not only at 70 K but also at room temperature 300 K. Such a low-field-sensitive MR, as well as its gradually increasing magnitude with decrease in temperature, signals that the spin-dependent scattering is not due to the bulk phenomenon arising from the thermal fluctuation of spins, but occurs at the grain boundaries or at the magnetic domain boundaries.

4. Conclusion In conclusion, highly crystalline SFMO thin films with (0 0 4) orientation were grown on different substrates using pulsed laser deposition technique. Both structural and magnetic properties of SFMO films were shown to be influenced by choice of substrate. The investigations reveal an increase in magnetization with decrease in film–substrate lattice mismatch. The films deposited on SrTiO3 substrate with best lattice match with SFMO were found to exhibit a maximum saturation magnetic moment, whereas the films deposited on MgO substrate with large misfit were observed to have low magnetization, which is related to the high proportion of antisite defects in these films. The magnitude of MR was observed to be smaller in case of films deposited on SrTiO3 substrate and in addition the variation in MR with applied field was found to be linear in these films, whereas the films on MgO substrate were found to exhibit higher value of MR with two different regimes: the low-field region (Ho20 kOe) with a steeper slope and the high-field behavior with a less pronounced slope. Such a low-field-sensitive MR, as well as its gradually increasing magnitude with decrease in temperature, suggests that

ARTICLE IN PRESS 3266

D. Kumar, D. Kaur / Physica B 405 (2010) 3259–3266

0 -2

Magnetoresistance (%)

-4 -6 -8 STO LAO MgO

-10 -12 -14 -16 -18 50

100

150 200 Temperature (K)

250

300

Fig. 6. Variation in magnetoresistance with temperature for films deposited on STO, LAO and MgO substrates.

the spin-dependent scattering is not due to the bulk phenomenon arising from the thermal fluctuation of spins, but occurs at the grain boundaries or at the magnetic domain boundaries.

Acknowledgement The financial support provided by Ministry of Communications and Information Technology (MIT), India, under Nanotechnology Initiative Program with Reference no. 20(11)/2007-VCND and DRDO ARMREB, New Delhi, is highly acknowledged. The authors are also thankful to Prof. V. Vankataraman, IISc Bangalore, for providing research facilities for 11 T magnetic field measurements. The author Deepak Kumar is thankful to CSIR for award of senior research fellowship. References [1] K.-I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677 (London). [2] K.I. Kobayashi, T. Kimura, Y. Tomioka, H. Sawada, K. Terakura, Y. Tokura, Phys. Rev. B 59 (1999) 11 159. [3] F.K. Patterson, C.W. Moeller, R. Ward, Inorg. Chem. 2 (1963) 196. [4] F.S. Galasso, F.C. Douglas, R.J. Kasper, J. Chem. Phys. 44 (1966) 1672. [5] J.M. Greneche, M. Venkatesan, R. Suryanarayanan, J.M.D. Coey, Phys. Rev. B 63 (2001) 174403. [6] J. Linden, T. Yamamoto, M. Karppinen, Y. Yamauchi, T. Pietari, Appl. Phys. Lett. 76 (2000) 2925. [7] A.P. Douvalis, M. Venkatesan, P. Velasco, C.B. Fitzgerald, J.M.D. Coey, J. Appl. Phys. 93 (2003) 8071. [8] O. Chmaissem, R. Kruk, B. Dabrowski, D.E. Brown, X. Xiong, S. Kolesnik, J.D. Jorgensen, C.W. Kimball, Phys. Rev. B 62 (2000) 14197. [9] D.D. Sarma, Sugata Ray, K. Tanaka, M. Kobayashi, A. Fujimori, P. Sanyal, H.R. Krishnamurthy, Dasgupta, Phys. Rev. Lett. 98 (2007) 157205.

[10] H. Sakuma, T. Taniyama, Y. Kitamoto, Y. Yamazaki, J. Appl. Phys. 93 (2003) 2816. [11] K.-I. Kobayashi, T. Okuda, Y. Tomioka, T. Kimura, Y. Tokura, J. Magn. Magn. Mater. 218 (2000) 17. [12] D.D. Sarma, E.V. Sampathkumaran, Sugata Ray, R. Nagarajan, Subham Majumdar, Ashwani Kumar, G. Nalini, T.N. Guru Row, Solid State Commun. 114 (2000) 465. [13] T. Saitoh, M. Nakatake, A. Kakizaki, H. Nakajima, O. Morimoto, S. Xu, Y. Moritomo, N. Hamada, Y. Aiura, Phys. Rev. B 66 (2002) 035112. [14] D. Sanchez, M. Garcia-Hernandez, N. Auth, G. Jacob, J. Appl. Phys. 96 (2004) 2736. [15] A. Venimadhav, F. Sher, J.P. Attfield, M.G. Blamire, J. Magn. Magn. Mater. 269 (2004) 101. [16] W. Westerburg, D. Reisinger, G. Jakob, Phys. Rev. B 62 (2000) R767. [17] T. Manako, M. Izumi, Y. Konishi, K.-I. Kobayashi, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 74 (1999) 2215. [18] H. Asano, S.B. Ogale, J. Garrison, A. Orozco, Y.H. Li, E. Li, V. Smolyaninova, C. Galley, M. Downes, M. Rajeswari, R. Ramesh, T. Venkatesan, Appl. Phys. Lett. 74 (1999) 3696. [19] 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. [20] D. Kaur, J. Jesudasan, P. Raychaudhuri, Solid State Commun. 136 (2005) 369. [21] D. Kaur, K.V. Rao, Solid State Commun. 128 (2003) 391. [22] A. Di Trolio, R. Larciprete, A.M. Testa, D. Fiorani, P. Imperatori, S. Turchini, N. zema, J. Appl. Phys. 100 (2006) 013907. [23] R.P. Borges, S. Lhostis, M.A. Bari, J.J. Versluijs, J.G. Lunney, J.M.D. Coey, M. Besse, J.P. Contour, Thin Solid Films 5 (2003) 429. [24] X.B. Zhu, J.M. Dai, X.H. Li, B.C. Zhao, S.M. Liu, W.H. Song, Y.P. Sun, Mater. Lett. 59 (2005) 2366. [25] A. Kaushal, D. Kaur, Sol. Energy Mater. Sol. Cell 93 (2009) 193. [26] D.Y. Kim, Jin Soo Kim, Bae Ho Park, Jeon-Kook Lee, Jang Hee Kim, Je Hyun lee, joonyeon Chang, Hi-Jung Kim, Appl. Phys. Lett. 84 (2004) 5037. [27] M. Garcia-Hernandez, J.L. Martinez, M.J. Martinez-Lope, M.T. Casais, J.A. Alonso, Phys. Rev. Lett. 86 (2001) 2443. [28] B. Garcia-Landa, C. Ritter, M.R. Ibarra, J. Blasco, P.A. Algarabel, R. Mahendiran, J. Garcia, Solid State Commun. 110 (1999) 435. [29] Y. Tomioka, T. Okuda, Y. Okimoto, R. Kumai, K.-L. Kobayashi, Phys. Rev. B 61 (2000) 422. [30] P. Majewski, S. Geprags, A. Boger, M. Opel, L. Alff, R. Gross, J. Magn. Magn. Mater. 1154 (2005) 290. [31] J.B. Philipp, P. Majewski, D. Reisinger, S. Geprags, M. Opel, A. Erb, L. Alff, R. Gross, Acta Phys. Pol. A 105 (2004) 7. [32] J.B. Philipp, P. Majewski, L. Alff, A. Erb, R. Gross, T. Graf, M.S. Brandt, J. Simon, T. Walther, W. Mader, D. Topwal, D.D. Sarma, Phys. Rev. B 68 (2003) 144431. [33] M. Besse, F. Pailloux, A. Barthe´le´my, K. Bouzehouane, A. Fert, J. Olivier, O. Durand, F. Wyczisk, R. Bisaro, J.-P. Contour, J. Cryst. Growth 241 (2002) 448. [34] X.H. Li, Y.P. Sun, W.J. Lu, R. Ang, S.B. Zhang, X.B. Zhu, W.H. Song, J.M. Dai, Solid State Commun. 145 (2008) 98. [35] M. Besse, V. Cros, A. Barthelemy, H. Jaffres, J. Vogel, F. Petroff, A. Mirone, A. Tagliaferri, P. Bencok, P. Decorse, P. Berthet, Z. Szotek, W.M. Temmerman, S.S. Dhesi, N.B. Brookes, A. Rogalev, A. Fert, Europhys. Lett. 60 (2002) 608. ¨ [36] P. Majewski, S. Geprags, A. Boger, M. Opel, A. Erb, R. Gross, G. Vaitheeswaran, V. Kanchana, A. Delin, F. Wilhelm, A. Rogalev, L. Alff, Phys. Rev. B 72 (2005) 132402. [37] P. Majewski, S. Geprags, O. Sanganas, M. Opel, R. Gross, F. Wilhelm, A. Rogalev, L. Alff, Appl. Phys. Lett. 87 (2005) 202503. [38] H.Q. Yin, J.-S. Zhou, R. Dass, J.P. Zhou, J.T. McDevitt, J.B. Goodenough, J. Appl. Phys. 87 (2000) 6761. [39] Ll. Balcells, J. Navarro, M. Bibes, A. Roig, B. Martinez, J. Fontcuberta, Appl. Phys. Lett. 78 (2001) 781. [40] A.W. Sleight, J.F. Weiher, J. Phys. Chem. Solids 33 (1972) 679. [41] D.D. sarma, Curr. Opin. Solid State Mater. Sci. 5 (2001) 261. [42] H.Y. Hwang, S.W. Cheong, Nature 389 (1997) 942. [43] H.Y. Hwang, S.W. Cheong, Science 278 (1997) 1607. [44] D. Vanderbilt, Phys. Rev. B 41 (1990) 7892. [45] K. Lassonen, A. Pasquarello, R. Car, C. Lee, D. Vanderbilt, Phys. Rev. B 47 (1993) 10142. [46] A. Venimadhav, M.E. Vickers, M.G. Blamire, Solid State Commun. 130 (2004) 631.