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

We report on a comparative study of three manganites: Pr0.5Sr0.5MnO3, Pr0.5 Ca0.5MnO3, 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 develop s 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 simultaneousl y at T-N = T-CO = 160 K (185 K) upon cooling (heating), respectively. The s tudy is complemented by the data on two systems of 30% Mn4+, the ferromagne tic metal Pr0.7Sr0.1Ca0.2MnO3 and insulator Pr0.85K0.15MnO3, both with simi lar Curie temperature T(C)similar to 140 K. The electrical resistivity, the rmoelectric power, thermal conductivity, and volume thermal expansion data are analyzed with respect to observed magnetic transitions and reveal the s ignificantly distinct ground state of the A-type and CE-type antiferromagne ts. We suppose that phonons and charge carriers are coupled via the Jahn-Te ller 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.