Spin order and lattice frustration in optimally doped manganites: A high-temperature NMR study

Understanding the complex glassy phenomena, which accompany polaron formation in optimally doped manganites (ODMs) is a cumbersome issue with many unexplained perspectives. Here, on the basis of ¹³⁹La and ⁵⁵Mn nuclear magnetic resonance (NMR) measurements, performed in the temperature range 80--900 K we show that glass freezing, observed in the paramagnetic (PM) phase of ODM La0.67Ca0.33MnO₃, is not a random uncorrelated process but the signature of the formation of a genuine spin-glass state, which for T<Tc consolidates with the ferromagnetic (FM) state into a single thermodynamic phase. Comparison with NMR measurements performed on La_1-xCaₓMnO₃ systems for 0.0≤x≤0.41 and ODM La0.70Sr0.30MnO₃, demonstrates the key role played by the local lattice distortions, which control (i) the stability of the spin-glass phase component and (ii) the kind (first or second order) of the PM-FM phase transition. The experimental results are in agreement with the predictions of the compressible random bond-random field Ising model, where consideration of a strain field induced by lattice distortions is shown to invoke at Tc a discontinuous first-orderlike change in both the FM and the ``glassy'' Edwards-Anderson order parameters.