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

Abstract

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 $^{139}\text{L}\text{a}$ and $^{55}\text{M}\text{n}$ 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 ${\text{La}}_{0.67}{\text{Ca}}_{0.33}{\text{MnO}}_{3}$, is not a random uncorrelated process but the signature of the formation of a genuine spin-glass state, which for $T<{T}_{c}$ consolidates with the ferromagnetic (FM) state into a single thermodynamic phase. Comparison with NMR measurements performed on ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$ systems for $0.0\ensuremath{\le}x\ensuremath{\le}0.41$ and ODM ${\text{La}}_{0.70}{\text{Sr}}_{0.30}{\text{MnO}}_{3}$, 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 ${T}_{c}$ a discontinuous first-orderlike change in both the FM and the ``glassy'' Edwards-Anderson order parameters.