Unusual peculiarities of paramagnet to ferromagnet phase transition in La0.88MnO2.91

Study of the linear and nonlinear magnetic responses as well as electron spin resonance (ESR) is presented for insulating La0.88MnO2.91 with TC≈216K. According to the data on the second harmonic of magnetization (M2), this compound exhibits an unconventional paramagnet to ferromagnet (PF) phase transition. A critical behavior of the system is found to agree with a prediction for a 3D isotropic ferromagnet in the P-region above T*≈247 K. Below T*, an anomalous phase develops whose properties differ radically from those of a magnet exhibiting an usual PF transition. This phase originates in a ferromagnetically ordered state and reveals a strong nonlinear behavior in the weak fields. The ESR data, which are obtained in the essentially higher field and frequency than those in the M2 measurements, agree with the conventional critical behavior above 225 K, indicating a strong suppression of the anomalous response by the field. These peculiarities are close to those found in the usually doped Nd1−xBaxMnO3 (x=0.23, 0.25) manganites. It supports an assumption on universal character of this phenomenon, namely, formation of a heterogeneous magnetic state with the coexistence of the anomalous and traditional phases. Since this compound is near a border of a metal to insulator transition, the anomalous phase is assumed to exhibit a ferromagnetic metallic state whereas the traditional one is a ferromagnetic insulator. Formation of this nontrivial state is attributed to a strong coupling of the orbital, Jahn–Teller phonons and charge degrees of freedom with the magnetic ones.