Elsevier

Solid State Communications

Volume 239, July 2016, Pages 37-43
Solid State Communications

Electric, magnetic, and thermo-electric properties of Cr doped La0.8Ca0.2Mn1−xCrxMnO3 manganites

https://doi.org/10.1016/j.ssc.2016.04.012Get rights and content

Highlights

  • Pristine and Cr-doped samples show metal–insulator transition.

  • TMI and TC both decrease with Cr-content.

  • Magnetic data shows that MZFC and MFC bifurcate below Curie temperature.

  • High temperature thermoelectric power data follows small polaron hopping model.

Abstract

A detailed study of the structural, magnetic, magneto-transport and thermoelectric properties of polycrystalline La0.8Ca0.2Mn1−xCrxMnO3 (0<x<0.07) compounds is presented in the present communication. XRD studies confirm that the samples are pure and single phased. We have observed from electrical and magnetic studies that all the samples are exhibiting metal–insulator transition temperature, TMI and ferromagnetic–paramagnetic (FM–PM) transition temperature, TC. With increasing Cr-content, both TMI and TC are observed to decrease. The electrical resistivity data is analyzed using different theoretical models at various regions viz., metallic, insulating and percolation region. Analysis in the metallic region (T<TMI) suggests that the electron–electron scattering in predominant in the region whereas the insulating region (T>TMI) is well described using SPH model. However, the resistivity data in the whole temperature range is analyzed using a phenomenological model based on phase segregation of ferromagnetic metallic and paramagnetic insulating regions. Thermoelectric power, S measurements were performed to understand the conduction mechanism and to ascertain the types of charge carrier responsible for conduction. It is observed that pristine as well as Cr-doped compounds show positive value of S which demonstrates that the charge carriers are holes.

Introduction

Doped perovskite manganites with general formula R1−xAxMnO3 (where R is trivalent rare earth ion and A is divalent alkaline earth ion) are of interest since the unusual discovery of an exotic property like colossal magneto-resistance (CMR) in them [1], [2], [3], [4], [5]. They also exhibit interesting electrical and magnetic properties [3], [4], [5], [6], [7] which have drawn widespread scientific and technological interest in these materials. Explicitly, La1−xCaxMnO3 (LCMO) has drawn immense importance owing to their technological applications as they exhibit huge magneto-resistance (MR) and magneto-caloric effect (MCE) [8], [9], [10], [11], [12], [13]. In addition to their technological applications, these materials are of immense interest as the mechanism(s) responsible for their exotic transport and magnetic properties are not yet well understood [14]. It is well-established that double exchange (DE) model [15] alone is not sufficient to explain the large change in resistivity observed in these manganites. In addition to the DE, Jahn–Teller distortion of Mn3+ ions plays important role in the transport mechanism [16], [17].

Many researchers have put efforts to understand the physics behind the conduction phenomenon by modifying the MnO6 octahedron environment, which plays key role in determining the properties of these materials [18]. This can be done in two ways viz., doping at the Mn-site or at the rare-earth site. The former is more effective as compared to the latter, since it directly affects the MnO6 octahedron, which acts like the heart of DE interaction. There are many reports [19], [20], [21], [22], [23], [24], [25] on Mn-site doping by various ions of different electronic configuration. Xun et al. [20] and Neeraj et al. [21], have reported that Cr doping at Mn site leads evolution of an extra peak, in addition to the intrinsic peak that arises due to DE interaction, in the temperature dependent resistivity plots. They have attributed this extra peak to the antiferromagnetic (AFM) interaction that arises due to super exchange (SE) mechanism between Cr3+/Cr3+ and/or Cr3+/Mn4+ ions. Mollah et al. [22] have reported that the Cr doped samples exhibit FM insulating state at low temperatures. Among the dopants reported, Cr substitution is interesting since the electronic configuration of Mn4+ and Cr3+ are similar and hence Cr3+ can actively participate in DE. It is also well established that Cr doping has a rather slow decreasing effect on TC and TMI of the parent compound [22], [23]. Although there are large number of reports on Cr-doped compounds, there seems to be only two reports [23], [26] on thermo-electric power (TEP) measurement on Cr-doped system. Therefore, we have undertaken detailed and exhaustive investigations of structural, magnetic, magneto-transport and thermo-power properties of Cr doped La0.8Ca0.2Mn1−xCrxO3 (0≤x≤0.07) system.

Section snippets

Experimental details

The polycrystalline samples of La0.8Ca0.2Mn1−xCrxO3 (0≤x≤0.07) were prepared by employing solid state reaction route. Stoichiometric proportion of high purity precursors (La2O3, Ca2CO3, Cr2O3 and MnO2) were mixed thoroughly in an agate mortar and calcined at 1000 and 1250 °C for 24 h with three intermediate grindings. These powders were pressed into pellets and sintered at 1400 °C for 30 h with two intermediate grindings and finally furnace cooled to room temperature. The samples were characterized

Magnetic studies

We have performed the temperature dependent magnetization (in the temperature range of 2–350 K) for the samples La0.8Ca0.2Mn1−xCrxO3 (0≤x≤0.07) under zero field cooled (ZFC) and field cooled (FC) (at 250 Oe) conditions and the plots are depicted in Fig. 3. The sharp rise in ZFC and FC curves as the temperature decreases from room temperature observed for all samples corresponds to phase transition from paramagnetic to ferromagnetic ordering. The transition temperature TC is evaluated from the

Conclusion

Systematic investigation on structural, magneto-transport, magnetic and thermal properties of La0.8Ca0.2Mn1−xCrxO3 (0<x<0.07) compounds have been carried out. Structural studies confirmed that all the samples are single phased and crystallize in rhombohedral symmetry with R-3C space group. Electrical and magnetic studies, respectively, showed that the TMI and TC shift towards lower temperatures side with increasing x. It is observed from magnetic studies that the values of MZFC and MFC

Acknowledgments

One of the authors (S.O.M) acknowledges Manipal University (Grant no. TMAPAI_CHAIR/15) for providing financial support to pursue Ph.D. The authors are thankful to Dr. Rajeev Rawat and Mr. Sachin Kumar, UGC-DAE, CSR Indore for resistivity and MR measurement.

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