La- and Mn-sites deficient LaMnO₃ as a model system for studying paramagnetic magnetic correlations and spin dynamics in doped manganites

To probe magnetic ordering in single crystals of La_1-xMnO ₃ (x = 0.01, 0.05, 0.13) and in ceramics of LaMn_1-yO ₃ (y = 0, 0.02, 0.06), the X-band electron magnetic resonance measurements were carried out in the temperature range 5 K ≤ T ≤ 600 K. The temperature dependences of doubly integrated intensity of electron paramagnetic resonance signal and its linewidth were fitted with known theoretical models modified for taking into account the different mechanisms of spin relaxation. Both experimental data and fitting results evidence that vacancies in Mn- and La-sites dope the carriers, which induces ferromagnetic double exchange interaction in parent LaMnO₃. However, strong structural and chemical disorder of La_1-xMnO₃ crystals makes the ferromagnetic ground state eventually impossible even at x = 0.13. In marked contrast, better structural/chemical homogeneity together with a stronger impact of Mn-site vacancies on mixed manganese valence and double exchange are characteristic for LaMn_1-yO₃ ceramics. As a result, the LaMn_0.94O₃ compound appears to be ferromagnetic-like ordered and demonstrates band-like character of the doped carriers. It is shown that 'self-doped' LaMnO₃ may be considered as a model system for studying both transition from antiferromagnetic to ferromagnetic-like magnetic ground state in La manganites upon doping and the influence of structural/chemical disorder on such a transition.