Thermal conductivity and magnetic transitions in Mn3+/Mn4+ manganites

We report on a comparative study of three manganites: Pr0.5Sr0.5MnO3, Pr0.5Ca0.5MnO3, and Pr0.5Sr0.3Ca0.2MnO3, which all exhibit the same Mn3+/Mn4+ = 1:1 concentration but show a very distinct magnetic behavior. The Pr0.5Sr0.5MnO3 sample is a ferromagnet (FM) below T-C = 265 K and becomes the A-type antiferromagnet at T-N = 140 K. In Pr0.5Ca0.5MnO3 the charge order develops at T-CO = 245 K and the CE-type antiferromagnetism occurs below T-N = 175 K. The Pr0.5Sr0.3Ca0.2MnO3 sample exhibits the FM transition at T-C = 225K and the CE type antiferromagnetism and charge ordering occur simultaneously at T-N = T-CO = 160 K (185 K) upon cooling (heating), respectively. The study is complemented by the data on two systems of 30% Mn4+, the ferromagnetic metal Pr0.7Sr0.1Ca0.2MnO3 and insulator Pr0.85K0.15MnO3, both with similar Curie temperature T(C)similar to 140 K. The electrical resistivity, thermoelectric power, thermal conductivity, and volume thermal expansion data are analyzed with respect to observed magnetic transitions and reveal the significantly distinct ground state of the A-type and CE-type antiferromagnets. We suppose that phonons and charge carriers are coupled via the Jahn-Teller effect and the CMR effect observed in manganites originates thus from a mutual interplay of a strong electron-phonon interaction and spin order. (C) 1998 American Institute of Physics.