ESR evidence for partial melting of the orbital order inLaMnO3below the Jahn-Teller transition

Abstract

We report on high-temperature electron spin resonance studies of a detwinned LaMnO3 single crystal across the Jahn-Teller transition at TJT = 750 K. The anisotropy of the linewidth and gfactor reflects the local Jahn-Teller distortions in the orbitally ordered phase. A clear jump in the linewidth accompanies the Jahn-Teller transition at TJT = 750 K confirming that the transition is of first order. Already at T * = 550 K a significant decrease of the reduced linewidth is observed. This temperature scale is discussed with respect to the interaction of the eg-electrons of the Mn 3+ -ions and the elastic field of the cooperative distortions. Our results support a partial melting of the orbital order along the antiferromagnetically coupled b-axis at T * . The remaining two-dimensional orbital ordering within the ferromagnetically coupled ac-plane finally disappears together with the cooperative distortion at TJT. Moreover in our discussion we show that elastic strain field interactions can explain the melting of the orbital order and, thus, has to be taken into account to explain the orbital ordering in LaMnO3.

Electronic, structural, and magnetic properties ofLaMnO3phase transition at high temperature

“…Upon heating the system above the Néel temperature (140K) [17], LaMnO 3 transforms from the A-AFM state into a paramagnetic state characterized by timefluctuating short-range spin ordering between Mn atoms [18] consistent with the Curie-Weiss law. At T JT = 750 K a first order structural phase transition takes place with both an IMT [9,10] and an orbital order-disorder transition [19]. These transitions are driven by the reduction of the cooperative JT distortions [12,13] which result in a crystal structure similar to the Pbnm crystallographic symmetry but with nearly equal a and b lattice constants (referred as pseudo-cubic).…”

Understanding Complex Interplay among Different Instabilities in Multiferroic BiMn7O12 Using 57Fe Probe Mössbauer Spectroscopy

“…As was noted in several refs. [51][52][53], the local disordering of (Mn 3+ O 6 ) polyhedra can start at a temperature (T*) that is significantly lower than the temperature of the structural phase transition T JT (>>T*). However, there is still no reliable experimental data on the changes in the structure and electronic state of manganite, which take place in this "intermediate" temperature range.…”

“…The temperature dependences of hyperfine magnetic fields (B hf ) and principal components {V ii } X,Y,Z of the tensor of the electrical field gradient (EFG) gained using these methods reproduce the corresponding dependences M(T) and P s (T). Meanwhile, the relationship B hf = αM is usually linear for 57 Fe, 55 Mn, and 53 Cr nuclei, and the dependencies of the EFG tensor parameters V ii = f(P s ) and η = f ′ (P s ) (where η = (V XX − V YY )/V ZZ is the asymmetry parameter) demonstrate nontrivial behavior. In some works, quadratic dependences V ii = a + bP s 2 and η ∝ P s 2 are used for approximation [31][32][33].…”