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Abstract:

Spin-lattice coupling and ferroelectric polarization associated with the magnetic ordering in a geometrically frustrated delafossite AgFeO2 have been studied by high resolution neutron powder diffraction and dielectric measurements. The cycloidal magnetic ground state found in this material and the character of the lattice distortions are unprecedented in the family of the delafossite compounds, implying a peculiar mechanism lifting the geometrical frustration. © 2013 Owned by the authors, published by EDP Sciences.

Abstract:

We have performed dielectric measurements and neutron diffraction experiments on the delafossite AgFeO 2. A ferroelectric polarization P 300μC/m2 was observed in a powder sample, below 9AK. The neutron diffraction experiment demonstrated successive magnetostructural phase transitions at T N1=15K and T N2=9K. The magnetic structure for 9K≤T≤15K is a spin-density wave with a temperature dependent incommensurate modulation k=(-1,q,12), q 0.384. Below 9AK, the magnetic structure turns into elliptical cycloid with the incommensurate propagation vector k=(-12,q,12), q 0.2026 Based on the deduced magnetic point-group symmetry m1 ′ of the low-temperature polar phase, we conclude that the ferroelectric polarization in AgFeO 2 is perpendicular to the monoclinic b axis and is driven by the inverse Dzyaloshinskii-Moriya effect with two orthogonal components p 1 r ij×(S i×S j) and p 2 S i×S j. © 2012 American Physical Society.

Abstract:

Orbitally ordered states evolving coincidentally with structural distortions and magnetic ordering provide a novel route to controlling electronic and magnetic properties of materials through external fields. We report an unprecedented magneto-orbital helix in CaMn7O12, found to give rise to the largest magnetically induced ferroelectric polarisation measured to date. Characterisation of the structural modulation using x-ray diffraction, and analysis of magnetic exchange shows that orbital order is crucial in stabilising a chiral magnetic structure, thus allowing for electric polarisation. Additionally, the presence of a global structural rotation enables the coupling between this polarisation and magnetic helicity required for multiferrocitiy. These novel principles open up the possibility of discovering new high-temperature multiferroics.