Role of spin-lattice coupling in mediating exotic properties in Brownmillerites

Spin-lattice coupling and spin-reorientation are interesting phenomenon with exciting consequential properties. In materials with strong spin-lattice coupling and a broad spin-reorientation transition, the combined effect of both the remarkable phenomenon can give rise to many fascinating properties. We have studied the role of spin-lattice coupling in mediating exotic properties across the spin-reorientation transition in Brownmillerites (AA′BB′O5). Different structural and spectroscopic studies have been employed to study the Brownmillerite systems Ca2Fe2-xAlxO5 and Ca2FeCoO5. The complex spin structure across the spin reorientation is shown to modulate the crystal structure in Ca2Fe2-xAlxO5 resulting in anomalous thermal expansion properties and magneto-lattice coupling at room temperature. Also, the detailed magnetic structure of this compound is elucidated. Till date, spiral magnetic order was proposed to be responsible for induced polarization in many of the multiferroic systems. In renowned multiferroic compound TbMnO3, it was suggested that the Mn spin reorientation changes the exchange interaction energy and then brings about the lattice modulation owing to a finite spontaneous polarization, however, no direct evidence was provided. Our studies on Ca2FeCoO5 have shown the direct relationship between the spin disorder due to broad spin reorientation transition and the changes in the polarization susceptibility induced due to the structural modulation. In Ca2FeCoO5, the spin disorder across the spin reorientation is shown to modify the lattice through spin-lattice coupling and create a local non-centrosymmetry in an otherwise centrosymmetric compound, giving rise the giant magneto-dielectric effects. 

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