Hexagonal Sr0.6Ba0.4MnO3 Room Temperature Magneto-electric: Field Tunable Tunneling State and Entangled Pseudospin Clusters.

Hexagonal Sr0.6Ba0.4MnO3 undergoes ferroelectric (FE) and antiferromagnetic (AFM) orderings having competing energetics within a narrow range close to the room temperature. Magnetic properties appear to be governed by intricate exchange interactions among Mn4+ ions within and in adjacent Mn2O9 bioctahedra, contingent upon local structural changes. Calculations based on our model spin-Hamiltonian reveal that the linear AFM fluctuations between the Mn4+ ions of two oxygen-linked bi-octahedra result in short range correlations, manifest as a smooth drop in magnetization below 325 K. On the other hand, the non-linear intra-bioctahedral Mn-O-Mn AFM interactions, which get enhanced and acquire long-range character below 225 K, cause a slope-break in magnetization vs. temperature. Consequently, dual magneto-electric effects are obtained, as the high-temperature soft mode FE phase first transforms below 325K into a tunneling state, and then crosses over into a reentrant cluster glass phase around ~225K. Vitreous character of the electrical cluster glass phase is examined by the indispensable aging and rejuvenation effects, similar to the spin glasses. Field tunability of non-FE regimes, defining pseudospin composites of entangled spin and dipole, offers a prototype for room temperature atypical magneto-electricity. Frustration-competition involving spin-exchange and local-strain is reckoned as responsible for the dual/+-ve nature of magneto-electricity obtained in adjacent temperature regimes.