Spin dynamics and internal motion in magnetically dilute manganites probed by EPR

EPR spectra of a series of single crystal samples of LaGa_1?xMn_xO₃ manganites were studied in a broad range of temperatures (20–300 K) and compositions (0.02≤x≤1). Evolution of the spectra under the action of exchange interactions of Mn³⁺ ions leads to the formation of a single Lorentzian line for x≥0.2. The results confirm the transition from antiferromagnetic to ferromagnetic spin ordering during diamagnetic dilution of the manganite with gallium, beginning with x=0.8. For x=0.1, the EPR spectra exhibit an anomalous broadening and splitting on cooling of the samples, which is interpreted as manifestations of a thermoactivated internal motion with a characteristic energy of about 50 meV. The results are compared with published data on the nuclear spin-lattice relaxation of gallium. The model of thermoactivated reorientations of the electron orbitals of Mn³⁺ ions subject to the Jahn-Teller effect is discussed.