Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3−xMnO3 compounds

Physica B 407 (2012) 145–152

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Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3  xMnO3 compounds T.D. Thanh a, L.H. Nguyen b, D.H. Manh a, N.V. Chien a, P.T. Phong b,n, N.V. Khiem c, L.V. Hong a, N.X. Phuc a a

Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay District, Ha Noi, Viet Nam Nha Trang Pedagogic College, Nguyen Chanh Street, Nha Trang City, Khanh Hoa Province, Viet Nam c Hong Duc University, 307 Le Lai Street, Thanh Hoa City, Viet Nam b

a r t i c l e i n f o

abstract

Article history: Received 8 July 2011 Received in revised form 1 October 2011 Accepted 3 October 2011 Available online 10 October 2011

A systematic investigation of the structural, magnetic and electrical properties of a series of nanocrystalline La0.7SrxCa0.3 xMnO3 materials, prepared by high energy ball milling method and then annealed at 900 1C has been undertaken. The analysis of the XRD data using the Win-metric software shows an increase in the unit cell volume with increasing Sr ion concentration. The La0.7SrxCa0.3 xMnO3 compounds undergo a structural orthorhombic-to-monoclinic transition at x¼ 0.15. Electric and magnetic measurements show that both the Curie temperature and the insulator-to-metal transition temperature increase from 259 K and 253 K correspondingly for La0.7Ca0.3MnO3 (x¼0) to 353 K and 282 K, respectively, for La0.7Sr0.3MnO3 (x¼ 0.3). It is argued that the larger radius of Sr2 þ ion than that of Ca2 þ is the reason to strengthen the double-exchange interaction and to give rise to the observed increase of transition temperatures. Using the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the resistivity versus temperature data measured in the range of 50–320 K and found that the activation barrier decreased with the raising Sr2 þ ion concentration. & 2011 Elsevier B.V. All rights reserved.

Keywords: Manganite Electron–electron scattering Metal–insulator transitions Phase segregation Nanoparticle Resistivity

1. Introduction Recently, the perovskite-like rare-earth manganese compounds have been an intensive topic in scientific studies and potential technological application because of their charge and orbital ordering, magnetic phase transitions, colossal magnetoresistance (CMR) effect as well as magnetocaloric effect (MCE) [1–3]. The strongly correlated structural, magnetic and transport properties have opened new areas of studying these materials from fundamental and application viewpoints [4–6]. The magneto-transport properties of manganites are known to depend on many factors, such as the external pressure [7,8], magnetic field [9,10], temperature [11], structure and chemical composition [1,12,13]. The last factor includes the doping and oxidation state of manganese, the nature and density of structural defects [12], the average A-site cation size /rAS [14] and related pffiffiffi parameters: the tolerance factor (t) defined as t ¼ ð/rA S þ rO Þ= 2ðrMn þ rO Þ and variance (mean square deviation) of the cation size, s2 ¼ Syi r 2i /r A S2 [15–21]. Some of authors prepared the La0.67SrxCa0.33 xMnO3 compounds to search for high ferromagnetic–paramagnetic transition temperatures (TC) and high magnetoresistance (MR). They found that the structure changes from orthorhombic to rhombohedral

n

Corresponding author. Tel.: þ84 58 3523812; fax: þ84 58 3523841. E-mail address: [email protected] (P.T. Phong).

0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.10.006

during the variation of x [22–25]. It has also been shown that the double exchange interaction (DE) between pairs of Mn3 þ and Mn4 þ ions is responsible for the ferromagnetic and metallic properties of these manganese oxides, which can be controlled by changing the doping level. Interestingly, because Sr2 þ ion has larger ionic radius than Ca2 þ ion, it is possible to enhance remarkably the magnetic and electrical properties of manganites. The mentioned studies, however, only focus on the relationship between the magnetic transport properties with the changing of the tolerance factor (t) and the variation of the cation size in La0.67Ca0.33 xSrxMnO3. Namely, Ulyanov et al. [24] observed that there was a jump of the temperature TC at the orthorhombic– rhombohedral phase transition in La0.7SrxCa0.3 xMnO3 compounds. Conversely, Lalitha and Reddy [25] showed that TC increased continuously within creasing strontium concentration. Similarly, Bose et al. [26] reported a decrease in ferromagnetic–paramagnetic for La0.875Sr0.125 xCaxMnO3 compounds as Ca2 þ ion concentration is increased. In order to provide a systematic point of view, we have prepared La0.7SrxCa0.3 xMnO3 (0.00rxr0.30) compounds with the use of high energy ball milling combined with thermal annealing and carefully studied their structural, electrical and magnetic properties. The effect of changing the structural factors e.g. tolerance factor (t), average A-site cation size /rAS and variance (mean square deviation /s2S) of the cation size on the magnetic and magneto-transport and the electrical property of La0.7SrxCa0.3 xMnO3 compounds have also been investigated under

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T.D. Thanh et al. / Physica B 407 (2012) 145–152

the light of the phenomenological percolation model based on phase segregation. The obtained results are in good agreement with the experimental data measured in La0.7SrxCa0.3  xMnO3 nanocrystalline samples.

2. Experimental La0.7SrxCa0.3 xMnO3 (LSCMO) (x¼ 0.0; 0.05; 0.10; 0.15; 0.20; 0.25 and 0.30) compounds were prepared using oxide La2O3, carbonates CaCO3, SrCO3 and MnO by high energy ball milling and thermal processing methods. Single phase La0.7SrxCa0.3  xMnO3 powders were obtained after 4 h of milling time in the ambient atmosphere. The powder was then pressed into circular pellets and annealed at 900 1C for 5 h. The structural characterization was done by employing the X-ray diffraction (XRD) technique at room temperature in the 2y range of (201–751) with a step size of 0.031 ˚ radiation and the surface morphology using CuKa (l ¼1.5406 A) was observed by scanning electron microscopy (SEM). To determine the structural parameters of the samples the commercial Win-metric software was used. The magnetic measurements were performed by utilizing a vibrating sample magnetometer (VSM) in the temperature range from 80 to 400 K. The electrical transport behaviors were measured by a standard four-probe method in the temperature range from 50 to 320 K for all samples using Keithley instruments. In the present studies, the parameters, t and s2 were calculated using nine co-ordinate radii for A-site ions [27].

3. Results and discussion For the analysis of the transform of phase formation and unit cell parameters, the observed XRD data were further processed using the Win-metric software. The XRD patterns of the LSCMO samples are shown in Fig. 1. It is clear from the spectra that they show single

5000

Intensity (a.u.)

x = 0.3 x = 0.25 x = 0.2 x = 0.15 x = 0.1 x = 0.05 x = 0.0

0

40

30

20

50 2 (degree)

60

70

80

Fig. 1. XRD patterns of La0.7SrxCa0.3  xMnO3 compounds.

phase without detectable secondary phase. The values of the cell parameters of all the samples are listed in Table 1. It should be noticed that the lattice parameters increase with the Strontium content. A similar result is also found in Ref. [25]. The increase in the lattice parameters and unit cell volume can be related to the big ˚ In order to size of the Sr2 þ ion (r Sr2 þ ¼1.31 A˚ and r Ca2 þ ¼1.18 A). corroborate the experimental observations, the results have been compared with the pffiffiffi Goldsmidt tolerance factor (t) given by Ref. [14] t ¼ ð/rA S þr O Þ= 2ðr Mn þ rO Þ, where rA, rB and rO are the ionic radii of A, B, and O site atoms in ABO3, respectively. The structure of CMR manganites is of perovskite if their tolerance factor lies in the limits of 0.75o t o1 and in an ideal case the value is unity. From the calculated values of all the samples (see Table 1), one may conclude that they all might have a stable perovskite structure. In the region with t o0.969, the compounds exhibit an orthorhombic structure, and for 0.968o t o r0.974 the structure observed is of monoclinic type. It means the La0.7SrxCa0.3 xMnO3 compounds undergo a structural orthorhombic-to-monoclinic transition at x¼0.15. Contrary to our result, Radaelli et al. [15] and Ulyanov et al. [24] reported that near /rAS¼1.227 A˚ (amount of Ca approximately equals amount of Sr), La0.7SrxCa0.3 xMnO3 compounds show the structural phase transition from orthorhombic (Pbnm) phase to the rhombohedral ðR3cÞ one. The difference in the high Sr phase structure may be originated from the different processes used for preparations. The monoclinic or non-symmetric structure observed in our samples is supposed to be due to the defects and surface roughness of the particles created during the high energy ball milling process. On the other hand, all these samples have the tolerance factor,t between 0.976 and 0.979. This value range of tolerance factor proves that the Mn–O–Mn angle is much closer to 1801 in the cubic structure (t ¼1). This only causes a little distortion in MnO6 and results in reducing the localization of the charge and increasing the conduction, which is further supported by the resistivity data of the samples in all temperature range. The average crystallite sizes of the samples were obtained using a commercial WIN-CRYSIZE program packet based on the Warren–Averbach formalism to be of about 56 nm. Fig. 2 shows the representative FESEM images of the three samples: La0.7Ca0.3MnO3 (x¼0), La0.7Sr0.15Ca0.15MnO3 (x ¼0.15) and La0.7Sr0.3MnO3 (x ¼0.3), with the average particle size 60 nm. The relatively uniform spherical particles tend to keep with one another to form larger clusters. In comparison with the average crystallite size deduced from the XRD data the particle sizes provided by FESEM micrographs are in good agreement. The temperature dependence of magnetization M (T) under an applied field of 100 Oe for the series La0.7SrxCa0.3  xMnO3 (x ¼0.00–0.30) is shown in Fig. 3. All the samples show a steep paramagnetic–ferromagnetic (PM–FM) transition. The Curie temperature TC, which is defined by taking the derivative dM/dT of M–T curve, is listed in Table 2 for all the samples. TC increases from 259 to 353 K as Sr doping increases from 0.00 to 0.30. Generally, this increase in TC can be interpreted in terms of increasing the average A-site cation size /rAS. This can be better

Table 1 Refined cell parameters obtained for the La0.7Ca0.3  xSrxMnO3 compounds. Sample

x¼ 0.0

x¼ 0.05

x¼ 0.10

x ¼0.15

x ¼0.20

x ¼0.25

x ¼0.30

Space group Structure type ˚ a (A)

Pnma Orthorhombic 5.4705

Pnma Orthorhombic 5.4762

Pnma Orthorhombic 5.4759

Pc/2 Monoclinic 5.4802

Pc/2 Monoclinic 5.4763

Pc/2 Monoclinic 5.4767

Pc/2 Monoclinic 5.4791

˚ b (A)

5.4561

5.4567

5.7326

5.4610

5.4619

5.4663

5.4738

˚ c (A) b (deg.) V (A˚ 3)

7.7315

7.7348

7.7326

7.7440

7.7496

7.7489

7.7537

90.0 230.764

90.0 231.132

90.0 231.453

90.463 231.751

90.531 231.791

90.548 231.969

90.542 232.533

T.D. Thanh et al. / Physica B 407 (2012) 145–152

147

Fig. 2. FESEM images of LSCMO samples with x¼ 0 (a), x¼ 0.15 and x¼ 0.3 (c).

6 H = 100 Oe

5

x = 0.3 M (emu/g)

4 3 2 1 x = 0.0 0 100

150

200

250 T (K)

300

350

400

Fig. 3. Temperature dependence of the magnetization at 100 Oe for the LSCMO.

Table 2 Experimental data of LSCMO samples. x

˚ rA (A)

s2 (  104) (A˚ 2)

t

T C (K)a

DT C (K)b

TMI (K)

0.0 0.05 0.10 0.15 0.20 0.25 0.30

1.2048 1.2095 1.2142 1.2189 1.2236 1.2283 1.2330

0.53737 5.7886 10.595 14.963 18.886 22.370 25.409

0.968 0.970 0.972 0.974 0.976 0.977 0.979

258.88 280.92 300.83 324.19 330.57 341.14 352.61

2.57 2.49 2.42 2.31 2.32 2.21 2.21

253 262 267 269 276 278 282

a Curie transition temperature (TC) from the magnetization determined from a Lorentzian function fit of the derivative curves. b Width of the magnetization transition (DTC) is FWHM of Lorentzian function

seen, and quantified, in Fig. 4 where the derivatives of the magnetization (dM/dT) data for the all samples are shown. The peaks in the derivative curves have been fitted with a Gaussian and a Lorentzian function to obtain the TC values and the transition widths (full width at half maximum, FWHM), DTC. The fits are invariably better with the Lorentzian function and the obtained results, TC and DTC, using these fits are given in Table 2. The variation of TC with /rAS is displayed in Fig. 5. The increasing of x value leads to the monotonic increase of temperature TC in the whole x range, and smooth changes in each orthorhombic and monoclinic phase. Our accurate estimation of TC (Fig. 4), moreover, allows observing a clear change of the increase rate at the

structural transition point. Namely, instead of the whole region ˚ the ones observed for orthorhombic increase rate of 3277 K/A, ˚ respectively. and monoclinic phase are 3510 K/A˚ and 2016 K/A, This jump of TC increase rate was also observed in (Nd1  xLax)0.5Sr0.5MnO3 [28] and La0.7Ca0.3  xSrxMnO3 [24] perovskite manganites at the rhombohedral–orthorhombic phase transition. However, most interestingly, the TC is reduced linearly with the dispersion of A-cation radius /s2S (see Fig. 6). These data confirm the A-site mismatch, for which /s2S is one of the key factors to control the magnetic transition temperatures (TC) of perovskite manganites. On the other hand, the TC is reduced as because of the influence of the average size of the A-site cation in the ABO3 structure, with different size ions occupying the A-site, there will be a size disorder among the ions. The structural disorder produced a strong local stress in MnO6 octahedral and modifying the Mn–O–Mn angles resulting in the change of lattice parameters and magnetic properties [14]. A systematic investigation of M–H measurements of all the samples was carried out by applying a magnetic field from 0 Oe to 10 kOe at 300 K (see Fig. 7). From the plots, one may observe that the magnetization of all the samples increases with magnetic field. Therefore, the samples may need higher magnetic fields ( 410 kOe) for saturation. Further, the M–H behavior exhibited by the samples with xr0.10 is found to be very different from those by the samples with x Z0.15. For example, the slope of these curves of the samples increases with noticeable irreversibility when the magnetic field decreases. The resistivity versus temperature and MR curves for LSCMO samples are shown in Fig. 8. Here, MR is defined as the ratio of (r0-rH)/r0, where r0 and rH stand for the resistivity at 0 and 3 kOe, respectively. The r(T) curves reveal that the samples exhibit high-temperature insulating behaviors at dr/dTo0 to low-temperature metallic behaviors at dr/dT 40. The values of metallic-insulating transition temperature (TMI) of all samples are showed in Table 2. On the other hand, the r(T) curves also expose that the increase of Sr-doping concentration decreases the resistivity, but enhances the TMI temperature. The shift of TMI to higher ˚ temperatures is due to the ionic radius of Ca2 þ ion (1.18 A) ˚ [27]. However, as can be smaller than that of Sr2 þ ion (1.31 A) seen in Table 2, the TMI and TC values of the series of LSCMO samples are not the same temperature. For single crystal, thin film and polycrystals with a large grain size (  mm) of La0.7 Ca0.3MnO3 and La0.7Sr0.3MnO3 compositions, the electronic transition from insulator to metal is accompanied by a simultaneous paramagnetic to ferromagnetic transition, in other words the two transitions appear at almost the same temperature, similarly to the established phase diagram. The origin of ferromagnetism and the close correlation between the magnetic and transport properties in La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 are basically interpreted within the framework of the double-exchange model as proposed by Zener.

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0

-0.05

-0.1

x=0

100

150

200 T (K)

250

300

dM/dT (emu/gK)

0 dM/dT (emu/gK)

dM/dT (emu/gK)

0

-0.05 -0.1 -0.15 -0.2 100

x = 0.15

150

200

250 T (K)

-0.05 -0.1 -0.15 x = 0.3

-0.2

300

350

400

-0.25 100

150

200

250 T (K)

300

350

400

Fig. 4. Derivatives of the magnetization (dM/dT) and the fitted Lorentzian function curve (line) of the samples with x ¼0.0 (a), x ¼0.15 (b) and x ¼0.30 (c).

60

y = -3682.7 + 3277.9x R= 0.98137

360

50

TC(x)

340

M (emu/g)

TC (x) (K)

40

320 300

x=0 x = 0.05 x = 0.1 x = 0.15 x = 0.2 x = 0.25 x = 0.3

30 20

280 10

T = 300 K

260 1.208

1.214

1.221

1.228

1.234

(Å)

TC(x) (K)

360

y = 260.01 + 3.7856x R= 0.99266

340

TC(x)

320 300 280 260 5

10

15

0

2

4

6

8

10

H (kOe)

Fig. 5. Dependence average cation radius /rAS of temperature TC. The line is fitted curve by linear equation with R¼ 0.98137.

0

0

20

25

σ4 (Å4) Fig. 6. Dependence of temperature TC against A-site mismatch /s2S. The line is fitted curve by linear equation with R ¼0.99266.

This observation is similar to the results obtained by Hueso et al. [29], in which they report constancy in TC as a function of grain size, otherwise TMI shift to lower temperature. In order to explain the underlying physics behind the difference between TMI and TC and the broadening of TMI maximum in nanocrystalline manganites, Dey

Fig. 7. Magnetization (M) vs. applied field (H) for the LSCMO compounds.

and Nath [30] have used spin polarized tunneling model and the concept of a core–shell-type structure of Zhang et al. [31]. In our case, the TC, being an intrinsic characteristic, does not show any significant change as dependent on the sintering temperature (here, value of TC of the nanocrystalline samples is very close to that of the bulk samples), whereas TMI, an extrinsic property strongly depends on the synthesis conditions and microstructure, namely it shifts to lower temperature. As can be seen in Fig. 9b, we also obtain that a linear relationship between the TMI and the A-site mismatch /s2S of the samples is better than that of TMI and average cation radius /rAS (see Fig. 9a). As it can be observed in Fig. 8, in the explored temperature range (50–320 K), the resistivity of the samples decreases as Sr2 þ content increases. This improvement of conductivity may be due to a partial substitution of the Ca2 þ ions onto the Sr2 þ ions, which reduces the value of /rAS. Consequently, /rAS becomes too small to fill the space in the cube centers and the oxygen tends to move towards the center, which reduces the dA–O and dMn–O bond distances. The result is that a lattice distortion becomes more pronounced and provides a local trap for eg electrons, possibly causing the phase or the domain separation. Moreover, the hopping amplitude of the charge carriers between Mn3 þ and Mn4 þ naturally decreases as y becomes smaller than 1801, leading to local lattice distortions of the MnO6 octahedral. By consequence, the tendencies of charge localization increase due to the reduction in the mobility of the carriers. These effects result in the decrease of TMI with increasing Ca2 þ (Fig. 8) [32].

T.D. Thanh et al. / Physica B 407 (2012) 145–152

2

28 H=0 H = 3kOe

MR

25

24

MR 20

12

x=0

0.6

15 x = 0.05

0.4

10

MR (%)

16

1

MR (%)

20

8

0.5

5

4 0 0.4

0.2 0

0 H=0 H = 3kOe

MR

0.35

20

H=0 H = 3kOe

MR

20

15 x = 0.2

0.15

10

MR

H=0 H = 3kOe

0.2

MR (%)

0.2

5 0

20

MR

ρ (Ω.cm)

H=0 H = 3kOe

10

x = 0.15

0.1

0 0.25

0.2 0.15

5

0.16

15

0.25

20 15

0.16 x = 0.25

10

MR (%)

10

MR (%)

x = 0.1

0.24

ρ (Ω.cm)

15

MR (%)

0.3

0.32 ρ (Ω.cm)

H=0 H = 3kOe

0.8

ρ (Ω.cm)

ρ (Ω.cm)

1.5

ρ (Ω.cm)

149

0.12 5

5

0.1

0.08 100

150

200 T (K)

250

300

350

0 50

100

150

0.25

250

300

350

20 H=0 H = 3kOe

0.2 ρ (Ω.cm)

200 T (K)

MR 15

0.15

10

x = 0.3 0.1

MR (%)

50

5 0 350

0.05 50

100

150

200 T (K)

250

300

Fig. 8. Temperature dependence of resistivity at 0 and 3 kOe and MR for LCSMO compounds with various x value.

285

285 280

y = -915.98 + 972.64x R= 0.98287

275

270 265

TMI

TMI(x) (K)

275 TMI(x) (K)

y = 253.98 + 1.1071x R= 0.99093

280

270 265

260

260

255

255

TMI

250

250 1.208

1.214

1.221

1.228

1.234

(Å)

0

5

10

15

20

25

σ4 (Å4)

Fig. 9. Dependence of temperature TMI on average cation radius /rAS (a), and on A-site mismatch /s2S (b). The lines are fitted by linear equation.

As showed in Fig. 8, for the two samples with x ¼0 and x ¼0.05 a sharp peak in resistivity versus temperature, r–T appears at the M–I temperature, which accompanies a jump in MR, whereas for other samples the changes happen more smoothly. These findings

indicate that because of the impact of replacement of Ca2 þ ion by Sr2 þ ion, the intrinsic MR decreases with Sr2 þ ion addition. With decreasing /rAS, magnetic ordering and increased MR occur at lower temperatures. It is important to note that for this series we

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T.D. Thanh et al. / Physica B 407 (2012) 145–152

results show that the resistivity data in the whole temperature (ToTMI) for all the samples is governed by the electron scattering process. These experimental data were fitted to theoretical ones using the equation

have obtained almost linear temperature dependence of MR at low temperatures; so that the highest MR, at 50 K, achieved for this series is 27% and 17% for the sample with x¼0.0 and x¼0.30, respectively. The strong increase of the MR values at the temperatures much lower than TC, To oTC, may originate from an enhanced contribution of grain boundary effects on the conductivity of different polycrystalline manganites. In order to understand the conduction mechanism in the LSCMO samples, the electrical resistivity data were analyzed by a new phenomenological model based on the phase segregation mechanism proposed by Li et al. [33]. This model not only clearly elucidates the transport mechanism in the whole measured temperature region of manganites but also could interpret well the transport mechanism in composites manganites [34–36]. We attempt to fit the r–T curves at both the zero field and the applied field (H¼ 3 kOe) according to any suitable models. The

H=0 H = 3 kOe

0.8

ρ (Ω.cm)

1.2

x=0

0.8

ð1Þ

where the temperature independent part r0 is the resistivity due to the domains, grain boundary and other temperature independent scattering mechanism. r2.T2 term represents the electrical resistivity due to the electron–electron scattering process and is generally dominant up to 100 K. On the other hand, the term r4.5.T4.5 is a combination of electron–electron, electron–magnon and electron–phonon scattering processes. Meanwhile, the conductivity in the high temperatures (T4 TMI), PM insulating phase is dominated by the hopping motion of the self-trapped small polarons [28]. The classical expression for the resistivity (r) in

H=0 H = 3 kOe

1.6 ρ (Ω.cm)

rFM ¼ r0 þ r2 T 2 þ r4:5 T 4:5

0.4

0.6

x = 0.05

0.4

0.2

0.4 H=0 H = 3 kOe

0.3

0.3

ρ (Ω.cm)

ρ (Ω.cm)

H=0 H = 3 kOe x = 0.1

x = 0.15 0.2

0.2 0.1 0.25

0.25

H=0 H = 3 kOe

0.2 ρ (Ω.cm)

ρ (Ω.cm)

0.2

H=0 H = 3 kOe

x = 0.2 0.15

0.1

x = 0.25 0.15

0.1

0.05 50

100

150

200 T (K)

250

300

50

100

150

200 T (K)

250

300

350

H=0 H = 3 kOe

0.2

ρ (Ω.cm)

350

0.16

x = 0.3

0.12

0.08 50

100

150

200 T (K)

250

300

350

Fig. 10. Model fitting of r–T curves: Solid lines are the resistivity of LSCMO compounds calculated using expression (3) corresponding to the parameters indicated in Table 3a and 3b. Points are the experimental data.

T.D. Thanh et al. / Physica B 407 (2012) 145–152

151

Table 3a Parameters obtained corresponding to the best fit to the experimental data of LSCMO compounds between 50 and 320 K in H ¼0. LCSMO

r0

x ¼0.0 x ¼0.05 x ¼0.10 x ¼0.15 x ¼0.20 x ¼0.25 x ¼0.30

0.196 0.155 0.139 0.098 0.105 0.098 0.091

(O cm)

r2  10  6 (O cm K  2)

r4.5  10  12(O cm K  4.5)

ra  10  8(O cm)

T Cmod ðK Þ

U0/kB (K)

Eg/kB (K)

17.864 8.445 5.271 4.142 1.790 1.683 1.531

2.349 1.219 1.143 1.056 0.817 0.783 0.737

9.732 7.364 5.266 4.211 1.981 1.461 1.445

251 259 263 265 270 275 280

4566 4426 3467 3065 2833 2691 2646

2865 2389 2407 2256 2248 2001 2006

Table 3b Parameters obtained corresponding to the best fit to the experimental data of LSCMO compounds between 50 and 320 K in H ¼3 kOe. LCSMO

r0

x ¼0.0 x ¼0.05 x ¼0.10 x ¼0.15 x ¼0.20 x ¼0.25 x ¼0.30

0.193 0.132 0.107 0.073 0.080 0.075 0.073

(O cm)

r2  10  6 (O cm K  2)

r4.5  10  12(O cm K  4.5)

ra  10  8(O cm)

T Cmod ðK Þ

U0/kB (K)

Eg/kB (K)

17.223 8.418 5.165 4.123 1.690 1.674 1.576

1.976 1.217 1.124 1.076 0.907 0.788 0.724

9.729 6.400 4.785 3.551 1.531 1.381 1.266

252 262 267 269 276 278 282

4620 4487 3486 3157 2741 2657 2630

2824 2375 2236 2177 2157 2006 1971

this temperature region is given by the expression,

rPM ¼ ra T expðEg =kB TÞ

ð2Þ

where Eg is the activation energy and kB is the Boltzmann constant. Finally, using the equation given by Li et al. [33], we fitted the experimental r–T data from 50 to 320 K for both the zero field and the applied field (H¼3 kOe) by, 2

4:5

r ¼ f ðr0 þ r2 T þ r4:5 T Þ þ ð1f Þra T expðEg =kB TÞ

ð3Þ

where f represents the volume fractions of the FM domain, and f can be expressed as f¼

1 f1 þexp½U 0 ð1T=T mod Þ=kB Tg C

ð4Þ

is a PI–FM transition temperature used in the model (here T mod C and near/equal to TC). The fitting of the experimental data, shown in Fig. 10, indicates that Eq. (3) is the best fit for the LSCMO samples. As can see from Table 3a and 3b, the values of the parameters r0, r2, r4.5 and ra vary systematically with varying the radius of A-site; these parameters decrease as the replacement of Sr2 þ ion increases. The change of the r2 terms in the samples shows that the behavior of half metallic of the system decreases as the amount of Ca increases, hence, its lowly converts to the insulating behavior. However, for all the LSCMO samples the domain/grain boundary (GB) contribution is high. Thus, the electrical resistivity in the metallic region can be explained on the basis of electron–electron and electron–magnon scattering mechanisms for the LSCMO samples. The values of Eg/kB are found to be in the range 2006–2865 and 1971–2824 with and without magnetic field, respectively, for our samples. It is interesting to see that these values are in agreement with the findings of ours in La0.7Ca0.3MnO3 with various grain sizes [37]. In addition, as a result, T mod is very close to the TMI. According to Rozenberg [38], in C polycrystalline samples, the effects of the GB-induced contribution to the conductivity are much more pronounced. In particular, in samples with sufficiently small grain size no peak is seen on r(T) near TC, but only a broadened maximum at TMI oTC appears. In relatively good polycrystalline samples with large grain size, the TMI is a sharp maximum and near TC. In most cases the contribution of GB effects was strongly emphasized. Let us be noted that, the

theoretical model based on a percolation approach predicts the appearance of a broad maximum on the r(T) curve at ToTC, and as a result, T mod is very close to TMI, which agrees with the extrinsic C property of the r(T) experimental data obtained on nanocrystalline manganite.

4. Conclusion In this work, we have investigated structural, electrical and magnetic properties of La0.7SrxCa0.3  xMnO3 (x¼0.00–0.30) compounds. The XRD refinement confirms an increase in the lattice volume with Sr2 þ ion doping. The La0.7SrxCa0.3  xMnO3 compounds undergo a structural orthorhombic-to-monoclinic transition at x ¼0.15. An electrical and magnetic measurement supports a suggestion that bigger Sr2 þ ion takes part in doubleexchange mechanism, hence increases the electrical and magnetic transition temperatures. With decrease in the Sr2 þ doping level the resistivity increases and the TIM of the samples shifts towards the lower temperatures. Results were explained in terms of cation mismatch leading to distortion in the Mn–O–Mn bond angle that consequently weakens the double exchange interaction. In addition, the temperature dependence of the resistivity of these samples can be explained using phenomenological model based on the phase segregation mechanism.

Acknowledgments This work was supported by the National Foundation for Science and Technology under Grant no. 103.02-2010.29. The authors are also thankful to the National Key Laboratory for Electronic and Devices of Institute of Materials Science. The fifth author (P.T. Phong) would like to thank Nha Trang Pedagogic College for the support in his research. References [1] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri, C.N.R. Rao, Appl. Phys. Lett. 68 (1996) 2291. [2] A.G. Gamzatov, A.B Batdalov, I.K Kamilov, Physica B 406 (2011) 2231.

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