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

TL;DR

This study uses high-temperature NMR to explore how lattice distortions induce spin-glass states and influence phase transitions in optimally doped manganites, revealing the interplay between lattice effects and magnetic ordering.

Contribution

It demonstrates that lattice distortions control the stability of spin-glass phases and the order of phase transitions in manganites, supported by experimental NMR data and theoretical modeling.

Findings

Lattice distortions induce a genuine spin-glass state in ODM manganites.

The nature of the PM-FM transition depends on lattice distortions, being first or second order.

Discontinuous changes in order parameters at $T_c$ are explained by the compressible random bond-random field Ising model.

Abstract

The physics underlying the complex glassy phenomena, which accompany the formation of polarons in optimally doped manganites (ODM) is a cumbersome issue with many unexplained aspects. In this article we present $^{139}$La and $^{55}$Mn NMR in the temperature range 80K - 900K of ODM La$_{0.67}$Ca$_{0.33}$MnO$_3$. We show that local lattice distortions, established in the Paramagnetic (PM) phase for $T<700$K, induce a genuine spin-glass state, which for $T<T_c$ consolidates with the Ferromagnetic (FM) state into a single thermodynamic phase. Comparative NMR experiments on La$_{0.77}$Ca$_{0.23}$MnO$_3$, La$_{0.59}$Ca$_{0.41}$MnO$_3$, and La$_{0.70}$Sr$_{0.30}$MnO$_3$ demonstrate the dominant role of lattice distortions, which appear to control (i) the stability of the spin glass phase component and (ii) the kind (1st or 2nd 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 (1st order like) change of both the FM and the "glassy" Edwards-Anderson (EA) order parameters.