phys. stat. sol. (c) 3, No. 9, 3175–3179 (2006) / DOI 10.1002/pssc.200567109

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Magneto-transport properties of lanthanum

and calcium deficient manganites

I. Walha1, W. Boujelben1,

M. Koubaa1, 2, A. Cheikh-Rouhou*, 1, and A. M. Haghiri-Gosnet2

1 Laboratoire de Physique des Matériaux, Faculté des Sciences de Sfax, B. P. 802, 3018 Sfax, Tunisia 2 Institut d’Electronique Fondamentale, IEF/ UMR 8622, Université Paris Sud, Bâtiment 220,

91405 Orsay, Cedex, France

Received 5 September 2005, revised 8 January 2006, accepted 26 April 2006

Published online 28 August 2006

PACS 75.47.Lx, 75.60.Ej

We present a comparative study of the physical properties of the La0.5Ca0.5MnO3, La0.450.05Ca0.5MnO3

and La0.5Ca0.450.05MnO3 powder samples. Our synthesized samples have been elaborated using the con-

ventional solid-state reaction technique they crystallize in the orthorhombic perovskite structure with

Pnma space group. Resistivity measurements show that the defect effects in the A site on the electrical

properties are substantially different for both lacunar samples. Electrical investigations show a metallic-

semiconducting transition when temperature deceases for the calcium-deficient sample, however we ob-

serve a semiconducting behavior in the whole temperature range 20-300 K in the lanthanum-deficient one.

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

The perovskite manganese oxides with general formula Ln1-xAxMnO3 where Ln is a rare-earth and A is a divalent alkaline-earth or a monovalent alkali-metal are of considerable interest owing to a colossal resis-tivity decrease in an applied magnetic field near the metal-insulator transition temperature [1, 2]. A lot of studies have been carried out to elucidate the many interesting properties of these perovskites [3–8]. Especially the understanding of charge ordering phenomena in La0.5Ca0.5MnO3 has been the subject of considerable attention over the past few years [9–14]. These investigations have revealed an important interplay among the charge, spin, lattice and orbital degrees of freedom in addition to the well-known spin-charge coupling essential to the double exchange mechanism [15, 16]. The aim of this paper is to study the effects of a small amount of vacancy in the A site of La0.5Ca0.5MnO3 manganite, we investigate the structural and magneto-transport properties in both defect La0.450.05Ca0.5MnO3 and La0.5Ca0.450.05MnO3 samples.

2 Experimental

Powder samples La0.5Ca0.5MnO3, La0.450.05Ca0.5MnO3 and La0.5Ca0.450.05MnO3 have been elaborated using the conventional solid-state reaction by mixing La2O3, MnO2 and CaCO3 up to 99.9% purity in the desired proportions. The precursors are mixed in an agate mortar and then heated in air at 1000 °C for 60 hours. A systematic annealing at high temperature is necessary to ensure a complete reaction. In fact, the powders are pressed into pellets (of about 1 mm thickness) and sintered at 1400 °C in air for 60 hours with intermediate regrinding and repelling.

* Corresponding author: e-mail: abdcheikhrouhou@yahoo.fr

3176 I. Walha et al.: Magneto-transport properties of lanthanum and calcium deficient manganites

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

Phase purity, homogeneity and cell dimensions were determined by X-ray diffraction at room tempera-ture. Unit cell dimensions were obtained by least-square calculations. Resistivity and magnetoresistance measurements as a function of the temperature and the magnetic ap-plied field were carried out on dense ceramic pellets using the conventional four-probe method.

3 Results and discussion

A vacancy in the A site implies a partial conversion of Mn3+ to Mn4+ leading to an increase in the Mn4+ content above 50% (65% Mn4+ in La0.450.05Ca0.5MnO3 and 60% Mn4+ in La0.5Ca0.450.05MnO3). The Mn3+ and Mn4+ contents have been checked by chemical analysis. We list in table 1, the chemical analy-sis results. Chemical analysis confirmed the theoretical Mn3+ and Mn4+ contents in our lacunar samples. This vacancy leads also to a change in the average ionic radius <rA> of the A-site.

Table 1 Chemical analysis results for lacunar La0.5Ca0.5-xxMnO3 and La0.5-xxCa0.5MnO3 samples.

X-ray diffraction patterns of our samples showed that our samples are in single phase with no detectable secondary phases (Fig. 1).

Fig. 1 X-ray powder diffraction patterns of La0.5Ca0.5MnO3, La0.450.05Ca0.5MnO3 and La0.5Ca0.450.05MnO3 samples.

Structural study shows that our synthesized samples cristallise in an orthorhombic perovskite structure with Pnma space group. Lanthanum and calcium deficiencies do not modify the La0.5Ca0.5MnO3 struc-ture.

Samples %Mn4+

theoretical %Mn4+

experimental % Relative

error La0.5Ca0.5MnO3 50 50.6 1.2 La0.450.05Ca0.5MnO3 65 63.78 1.8 La0.5Ca0.450.05MnO3 60 59.75 0.5

10 20 30 40 50

7.99 103

I(a.u)

2θ(degree)

200

121

220

040

042

321

400

161

402

440

600

280244

361

602

444

165

640

La0.45 0.05

Ca0.5

MnO3

284

031

020

La0.5

Ca0.45 0.05

MnO3

La0.5

Ca0.5

MnO3

phys. stat. sol. (c) 3, No. 9 (2006) 3177

www.pss-c.com © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In Table 2, we list the lattice parameters and the unit cell volume of our synthesized samples. Calcium-deficient leads to an increase of the unit cell volume however we observe a decrease of the unit cell vol-ume in the lanthanum-deficient sample. Such result is in concordance with those observed in lacunar Pr0.5Sr0.5-xxMnO3 and Pr0.5-xxSr0.5MnO3 samples [17].

Table 2 Crystallographic data for lacunar La0.5Ca0.5-xxMnO3 and La0.5-xxCa0.5MnO3 samples

Figure 2 reproduces the temperature dependence of the zero-field resistivity ρ(T) for La0.5Ca0.5MnO3, La0.450.05Ca0.5MnO3 and La0.5Ca0.450.05MnO3 samples.

Fig. 2 Resistivity evolution as a function of temperature for La0.5Ca0.5MnO3, La0.450.05Ca0.5MnO3 and

La0.5Ca0.450.05MnO3 samples at H = 0 T.

Deficiency effects on the electrical properties are spectacular. The parent compound La0.5Ca0.5MnO3 is semiconducting in the whole temperature range 20-300 K with a change in the slope of the curve at about 100 K. The lanthanum-deficient sample shows a semiconducting behaviour in the whole temperature range 20-300 K with no change in the slope of the curve. Moreover, we observe an important increase of the resistivity value at low temperatures. However, for the calcium-deficient sample we observe a semi conduction-metal transition with deceasing temperature at about 160 K. At high temperatures (T > 160 K) we observe the same evolution of the resistivity versus temperature for all samples. The effect of the vacancy is observed only at low temperatures. The resistivity value for calcium deficient sample at a fixed temperature (T = 40 K) is 10–7 smaller than that in lanthanum deficient one. Previous studies on Pr0.5Sr0.5-xxMnO3 and Pr0.5-xxSr0.5MnO3 [17] lacunar compounds show a similar effect of the vacancy content on the physical properties. We have plotted in Fig. 3 the resistivity evolution as a function of temperature at several magnetic ap-plied field for our synthesized samples. For the parent compound La0.5Ca0.5MnO3, we observe a semi-

Samples a(Å) b(Å) c(Å)% v(Å3) La0.5Ca0.5MnO3 5.4291 7.6934 5.4218 226.45 La0.450.05Ca0.5MnO3 5.4283 7.6900 5.4121 225.92 La0.5Ca0.450.05MnO3 5.4302 7.6961 5.4221 226.62

0 50 100 150 200 250 300

100

101

102

103

104

105

106

107

108

109

H=0T

ρ(Ω

cm)

T(K)

La0.5

Ca0.5

MnO3

La0.5 0.05

Ca0.5

MnO3

La0.5

Ca0.45 0.05

MnO3

3178 I. Walha et al.: Magneto-transport properties of lanthanum and calcium deficient manganites

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

conducting-metallic transition with deceasing temperature for a magnetic applied field of 4 T and 8 T. With increasing the magnetic applied field the peak of the resistivity becomes broader and shifts to high temperature. Moreover, the resistivity values at low temperatures decrease with increasing the magnetic applied field [18]. For La0.5Ca0.5O3, the magnetic applied field of 8 T induces a semiconducting-metallic transition at about 125 K; it also induces a steep decrease of the resistivity at low temperatures. For La0.5Ca0.450.05MnO3 the effect of the magnetic applied field of 8 T is to lower the resistivity at low temperature, the peak of the resistivity becomes broader and shifts to higher temperatures.

Fig. 3 The resistivity evolution as a function of temperature at several magnetic applied field for La0.5Ca0.5MnO3 (a),

La0.5Ca0.450.05MnO3 (b) and La0.450.05Ca0.5MnO3 (c) samples.

Defining the MR at a given temperature as MR = ∆ρ⁄ρ = (ρ(0)-ρ(H))/ρ(0) where ρ(0) and ρ(H) are the resistivity in zero and at a magnetic applied field H respectively, we plot in Fig. 4 the MR evolution as a function of temperature at 8 T for La0.5Ca0.5MnO3, La0.450.05Ca0.5MnO3 and La0.5Ca0.450.05MnO3 sam-ples. The magnetoresistance effect depends strongly on the deficient element. In fact, the parent compound La0.5Ca0.5MnO3 and the lanthanum-deficient La0.450.05Ca0.5MnO3 sample show a MR effect of about 100% at low temperature. With increasing temperature the MR decreases and this decrease is more rap-idly for the lanthanum deficient sample than that for the parent compound. However, the MR evolution for the calcium deficient sample is different. The MR at 8T exhibits a maximum of about 95% at 185K.

50 100 150 200 250 300

100

101

102

103

104

105

106

107

108

109

1010

8T

0T

(c)

ρ(Ωcm)

T(K)

0 50 100 150 200 250 300

10

100

1000

ρ(Ωcm)

T(K)

0T

8T

(b)

0 50 100 150 200 250 300

100

101

102

103

104

105

106

107

0T

4T

8T

(a)

ρ(Ωcm)

T(K)

phys. stat. sol. (c) 3, No. 9 (2006) 3179

www.pss-c.com © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 4 Temperature dependence of the magnetoresistance at H = 8 T for La0.5Ca0.5MnO3, La0.450.05Ca0.5MnO3 and

La0.5Ca0.450.05MnO3 samples.

4 Conclusion

In this work we have investigated the effect of 5% lanthanum defect and calcium defect on the structural

and magneto-transport properties in the La0.5Ca0.5MnO3 sample. Resistivity properties exhibit different

behaviour depending on the vacancy type. The calcium deficient sample show a seconducting-metallic

transition when temperature decreases, while the lanthanum deficient sample is semiconducting in the

whole temperature range 20-300 K. The magnetoresistance effect exhibits a different behaviour depend-

ing on the vacancy nature. The MR at 8 T exhibits a maximum of 95% at 185 K for calcium deficient

sample La0.5Ca0.45MnO3 and remains constant at 100% below 100 K for both stoichiometric and lantha-

num samples.

Acknowledgements This work has been supported by the Tunisian Secretary of State for High Education, Scien-

tific Research and Technology.

References

[1] H. Y. Hwang, S. W. Cheong, P. G. Radelli, M. Marezio, and B. Battlog, Phys. Rev. Lett. 75, 914 (1995).

[2] A. Urushibara, Y. Morimoto, T. Arima, A. Amamitsu, G. Kido, and Y. Tokura, Phys. Rev. B 51, 14103 (1995).

[3] J. B. Goodenough, Phys. Rev. 100, 564 (1955).

[4] E. O. Wollan and W. C. Koeler, Phys. Rev. 100, 545 (1955).

[5] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Moritomo, and Y. Tokura, Science 270, 961 (1995).

[6] R. M. Kusters, J. Singleton, and D. A. Keen, Physica B 155, 362 (1989).

[7] S. Jin, T. H. Tiefel, M. Mc Cormack, R. A. Fastnacht, R. Ramech, and L. H. Chen., Science 264, 413 (1994).

[8] W. Boujelben, A. Cheikh-rouhou, M. Ellouze, and J. C. Joubert, J. Alloys Compd. 314, 15 (2001).

[9] H. Y. Hwang, S. W. Cheong, P. G. Radelli, M. Marezio, and B. Battlog, Phys. Rev. Lett. 75, 914 (1995).

[10] T.F. Millange, S. de Brion, and G. Chouteau, Phys. Rev. B 62, 5619 (2000).

[11] T. Hotta and E. Dagotto, Phys. Rev. B 61, 11879 (2000).

[12] P. Radaelli, D. E. Cox, M. Marezio, and S.-W. Cheong, Phys. Rev. B 55, 3015 (1997).

[13] R. Mahendiran, S. K. Tiwary, A. K. Raychaudhuri, and T. V. Ramakrishnan, Phys. Rev. B 53, 6 (1996).

[14] P. E. Schiffer, A. P. Ramirez, W. Bao, and S. W. Cheong, Phys. Rev. Lett. 75, 3336 (1995).

[15] C. Zener, Phys. Rev. 82, 403 (1951).

[16] P. W. Anderson and H. Hasegawa, Phys. Rev. 100, 675 (1955).

[17] S. Chaffai, W. Boujelben, M. Ellouze, A. Cheikh-rhouhou, and J. C. Joubert, Physica B 321, 74 (2002).

[18] I. Walha, W. Boujelben, M. Koubaa, A. Cheikh-rhouhou, and A. M. Haghiri-Goosnet, phys. stat. sol. (a) 201,

1416 (2004).

0 50 100 150 200 250 300

-40

-20

0

20

40

60

80

100

MR%

T(K)

La0.5

Ca0.5

MnO3

La0.5 0.05

Ca0.5

MnO3

La0.5

Ca0.45 0.05

MnO3