Oxide electronics

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.