J Supercond Nov Magn (2011) 24:1425–1431 DOI 10.1007/s10948-010-0846-1 ORIGINAL PAPER Low-field Magnetoresistance, Specific Heat and Magnetocaloric Effect in Sr Substituted Pr 0.7 Ca 0.3 MnO 3 Anjana Dogra · Sudhindra Rayaprol · Shovit Bhattacharya · Matthias Eul · Wilfried Hermes · Rainer Pöttgen Received: 28 July 2010 / Accepted: 29 August 2010 / Published online: 22 September 2010 © Springer Science+Business Media, LLC 2010 Abstract We investigate the effect of ionic size variation on the electrical and thermodynamic properties in a series of Pr 0.7 Ca 0.3−x Sr x MnO 3 (PCSMO) samples. The increase in Sr content results in an increase of the unit cell volume, as a bigger Sr 2+ ion replaces the smaller Ca 2+ ions. Resistivity measurements show that the increase in the Sr content also results in the induction of a metal–insulator transition (T MI ), which increases with increasing Sr content. The activation energy (E a ), calculated from the resistivity data, decreases with increasing Sr content confirming the metallic character. The effect of the magnetic field on resistivity and specific heat has also been studied. Keywords Manganites · Magnetoresistance · Specific heat · Magnetization A. Dogra · S. Bhattacharya Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India A. Dogra National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India S. Rayaprol ( ) UGC DAE Consortium for Scientific Research, R-5 Shed, BARC Campus, Trombay, Mumbai 400085, India e-mail: rayaprol@gmail.com S. Rayaprol e-mail: sudhindra@csr.res.in M. Eul · W. Hermes · R. Pöttgen Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, 48149 Münster, Germany 1 Introduction In the perovskite (ABO 3 type) manganites, when A is a triva- lent rare-earth ion, the effective valence of Mn is 3+. On doping the A-site with a divalent ion such as Ca 2+ or Sr 2+ , some Mn 3+ is converted to Mn 4+ by a mechanism known as double exchange [1, 2]. Doping of the A-site with a di- valent ion M (where M = Ca, Ba or Sr) leads to a situa- tion where, for R 0.5 M 0.5 MnO 3 , Mn is equally distributed as Mn 3+ and Mn 4+ . The equal proportion of Mn 3+/4+ leads to charge ordering, which competes with the double ex- change phenomenon giving rise to interesting situations in physics. The charge ordering occurs due to the interatomic Coulomb interactions. The mobile d -orbital electron can be localized on a Mn ion if the interatomic Coulomb interaction is of the same order as that of the conduction-electron band- width (W )[3, 4]. Therefore, by suitably tuning the A-site radius through substitution, W can be manipulated. Owing to the average A-site radius, the compound Pr 0.7 Ca 0.3 MnO 3 falls in the category of narrow-bandwidth materials [2]. Rare-earth manganites with a general formula R 1−x A x MnO 3 are found to be attractive materials for researchers due to their potential applications. Colossal magnetoresis- tance (CMR) is one of the important phenomena exhibited by the rare-earth manganites, which arises from the depen- dence of resistivity on the applied magnetic field. And since the resistivities of manganites are very much affected by the applied magnetic field, they are potential candidates for var- ious magnetoresistive applications. Along with CMR, the high-temperature coefficient of resistivity (TCR) and the co- efficient of magnetization (TCM) are also critical parame- ters to be investigated for applications such as the infra-red (IR) bolometer. TCR depicts the sharpness of the resistiv- ity drop whereas TCM gives an idea of the sharpness of the ferromagnetic transition.