Effect of K doping on the physical properties of La0.65Ca0.35−xKxMnO3 (0⩽x⩽0.2) perovskite manganites

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Abstract

The effects of K doping in the A-site on the structural, magnetic and magnetocaloric properties in La0.65Ca0.35−xKxMnO3 (0⩽x⩽0.2) powder samples have been investigated. Our samples have been synthesized using the solid-state reaction method at high temperature. The parent compound La0.65Ca0.35MnO3 is an orthorhombic (Pbnm space group) ferromagnet with a Curie temperature TC of 248 K. X-ray diffraction analysis using the Rietveld refinement show that all our synthesized samples are single phase and crystallize in the orthorhombic structure with Pbnm space group for x⩽0.1 and in the rhombohedral system with R3¯c space group for x=0.2 while La0.65Ca0.2K0.15MnO3 sample exhibits both phases with different proportions. Magnetization measurements versus temperature in a magnetic applied field of 50 mT indicate that all our investigated samples display a paramagnetic–ferromagnetic transition with decreasing temperature. Potassium doping leads to an enhancement in the strength of the ferromagnetic double-exchange interaction between Mn ions, and makes the system ferromagnetic at room temperature. Arrott plots show that all our samples exhibit a second-order magnetic-phase transition. The value of the critical exponent, associated with the spontaneous magnetization, decreases from 0.37 for x=0.05 to 0.3 for x=0.2. A large magnetocaloric effect (MCE) has been observed in all samples, the value of the maximum entropy change, |ΔSm|max, increases from 1.8 J/kg K for x=0.05 to 3.18 J/kg K for x=0.2 under a magnetic field change of 2 T. For x=0.15, the temperature dependence of |ΔSm| presents two maxima which may arise from structural inhomogeneity.

Introduction

After the discovery of colossal magnetoresistance (CMR), spectacular decrease of resistivity under a magnetic applied field, by Jin et al. [1] in La–Ca–Mn–O epitaxial thin film, the physical properties of the hole-doped manganites RE1−xMxMnO3 (RE=La, Pr and M=Ca, Sr, Ba) have been investigated intensively [2], [3], [4]. Manganites are expected to offer various technical applications such as spintronics-based devices [5], [6]. These oxides provide a rich variety of structural, magnetic and transport properties depending on the doping concentration (Mn3+/Mn4+ ratio), the average size of the A-site cations 〈rA〉 which controls the effective electron bandwidth W and the size mismatch at this A-site [7], [8], [9], [10]. The widely studied La1−xCaxMnO3 system, which has a relatively intermediate one-electron bandwidth (W), shows at low-temperature ferromagnetic insulator phase for 0.02⩽x⩽0.18 and ferromagnetic metal phase in the range 0.2⩽x⩽0.5 [11]. The double-exchange (DE) mechanism between adjacent Mn3+ and Mn4+ ions has been used first to explain the correlation between magnetic and transport properties [12]. However, both experimental and theoretical studies indicate that DE model alone cannot explain the magneto-transport properties and other factors such as the Jahn–Teller distortion of Mn3+ ion (electron–phonon coupling) and phase separation play a key role to understand the CMR physics [13], [14], [15].

More recently, an interesting property has been found in the ferromagnetic manganites near the Curie temperature TC, the magnetocaloric effect (MCE) [16], [17]. The origin of this effect is based on the adiabatic demagnetization: the application of a magnetic field in a ferromagnetic material induces a spin reorientation thus decreasing the spin entropy. This process is accompanied by a rise of the lattice entropy when the field is applied adiabatically. On the contrary, if we remove off the magnetic applied field, the spin system tends to randomize which increases the spin entropy, reduces the lattice one and consequently lowers the temperature of the system. The main requirements for a magnetic material to possess a large magnetic entropy change are the large spontaneous magnetization as well as the sharp drop in the magnetization associated with the ferromagnetic to paramagnetic transition at TC [18], [19]. It has been well-established that the magnitude of the MCE in manganites is comparable to that of pure Gd [20], [21]. For ΔH=1.5 T, both Gd and La2/3Ca1/3MnO3 samples exhibit a maximum magnetic entropy change, |ΔSm|max, of 4.2 J kg/K at 293 K and 4.3 J kg/K at 253 K, respectively [22]. In order to attain large magnetic entropy changes induced by low magnetic field changes at room temperature, many researchers have reported the effects of partial substitution in the A site of La or Ca by other elements. Hanh et al. [23] found |ΔSm|max of 3.72 J/kg K upon a magnetic applied field change of 1.35 T in La0.7Ca0.25Pb0.05MnO3 sample. Zhang et al. [24] investigated the MCE properties in La0.65−xEuxCa0.35MnO3 and found that for x=0.05, |ΔSm|max reaches 5.78 J/kg K upon a magnetic applied field change of 1.5 T. However, |ΔSm| does not extend over a large temperature range as it should be expected in magnetic refrigeration. In the present work, we elaborated by the solid-state method at high temperature the La0.65Ca0.35−xKxMnO3 powder samples and investigated the effect of the potassium substitution on the structural, magnetic and magnetocaloric properties.

Section snippets

Experimental techniques

Powder samples of La0.65Ca0.35−xKxMnO3 (0⩽x⩽0.2) were synthesized using the standard solid-state reaction method at high temperature, by mixing La2O3, CaCO3, K2CO3 and MnO2 up to 99.9% purity in the desired proportions. The starting materials were intimately mixed in an agate mortar and then heated in air up to 1000 °C for 60 h. The obtained powders were then pressed into pellets (of about 1 mm thickness) and sintered at 1100 °C in air for 60 h with intermediate regrinding and repelling. Finally,

Results and discussion

The X-ray diffraction patterns of all our synthesized La0.65Ca0.35−xKxMnO3 (0⩽x⩽0.2) powder samples were recorded at room temperature and based on these patterns, their crystal structures were refined by the Rietveld's profile-fitting method. The experimental diffraction profile was fitted with the pseudo-voigt profile function. The profile refinement is started with scale and background parameters followed by the unit cell parameters. Then, the peak asymmetry and preferred orientation

Conclusion

We have investigated structural, magnetic and magnetocaloric properties of La0.65Ca0.35−xKxMnO3 powder samples. Potassium substitution in La0.65Ca0.35MnO3 induces a structural transition from orthorhombic (Pbnm) to rhombohedral (R3¯c) phase. Rietveld refinement analyses show that the unit cell volume increases with increasing K+ amount. The Curie temperature TC shifts to higher values with increasing K+ content. All our samples exhibit a maximum of magnetic entropy change at the magnetic

Acknowledgments

This study has been supported by the Tunisian Ministry of Higher Education, Scientific Research and Technology.

References (40)

  • P.G. Radaelli et al.

    J. Solid State Chem.

    (1996)
  • J.L. Martínez et al.

    J. Magn. Magn. Mater.

    (1999)
  • E. Dagotto et al.

    Phys. Rep.

    (2001)
  • X. Bohigas et al.

    J. Magn. Magn. Mater.

    (2000)
  • Y. Xu et al.

    Cryst. Eng.

    (2002)
  • D.T. Hanh et al.

    J. Magn. Magn. Mater.

    (2007)
  • A.N. Ulyanov et al.

    Physica B

    (2005)
  • D.K. Mishra et al.

    Physica B

    (2007)
  • L. Pi et al.

    Solid State Commun.

    (2003)
  • M. Koubaa et al.

    Physica B

    (2008)
  • N.K. Singh et al.

    Solid State Commun.

    (2003)
  • Y. Xu et al.

    J. Magn. Magn. Mater.

    (2002)
  • D.L. Hou et al.

    J. Alloys Compd.

    (2004)
  • J.S. Kim et al.

    J. Magn. Magn. Mater.

    (2007)
  • S. Jin et al.

    Science

    (1994)
  • A.M. Haghiri-Gosnet et al.

    J. Phys. D: Appl. Phys.

    (2003)
  • E. Dagotto, in: The Physics of Manganites and Related Compounds, Springer Series in Solid-State Sciences, No. 136,...
  • J. Cilbert et al.

    C. R. Phys.

    (2005)
  • T. Venkatesan et al.

    Philos. Trans. R. Soc. London Ser. A

    (1998)
  • A.K. Pradhan et al.

    J. Appl. Phys.

    (2004)
  • Cited by (0)

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