Professor Paolo G. Radaelli OSI

Abstract:

Magnetic domains at the surface of a ferroelectric monodomain BiFeO3 single crystal have been imaged by hard X-ray magnetic scattering. Magnetic domains up to several hundred microns in size have been observed, corresponding to cycloidal modulations of the magnetization along the wave-vector k=2\pi(\delta,\delta,0) and symmetry equivalent directions. The rotation direction of the magnetization in all magnetic domains, determined by diffraction of circularly polarized light, was found to be unique and in agreement with predictions of a combined approach based on a spin-model complemented by relativistic density-functional simulations. Imaging of the surface shows that the largest adjacent domains display a 120 degree vortex structure.

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.