Transport properties and magnetic behavior in the polycrystalline lanthanum-deficient manganate perovskites (≈La1−xMnO3)

The rhombohedral lanthanum manganates ≈La1 − xMnO3 (x = 0.0, 0.05, 0.1) were prepared by a soft chemistry route to test first as catalysts. These three weak ferromagnets exhibit a structural transition at Tt slightly below Tc. ≈La0.9MnO3 undergoes a group-subgroup R3̄c ↔ R3c transition most likely of second order. ≈LaMnO3 does not show any insulator-to-metal transition, but does show a semiconductor-semiconductor at Tt ~ 130 K. Analysis of the transport data (conductivity and thermopower) indicates hopping of polarons at both low and high temperature. An effective magnetic moment of 3.50 μB/Mn is deduced from the spontaneous magnetization value at 5 K. The slight difference from the spin-only value (3.67 μB) is assigned to a spincanting effect which is reflected in the divergence of the FC and ZFC susceptibility curves at low fields (H ≤ 10−2 T) and low temperature. Compared with recent results on thin films, the compositions x = 0.05, 0.1 are expected to show magnetoresistive effects as well as a semiconductor-to-metal transition. If it is verified on bulk samples which all contain a similar Mn4+ content of 35%, the structural transition at Tt could be considered as magnetically driven.