Andrew Boothroyd

Abstract:

Orbital order has been proposed theoretically for more than half a century, and has subsequently been observed in a huge range of materials. Charge ordering is generally accompanied by a transition from a metal to an insulator. In manganites, colossal magneto-resistance (CMR) is associated with competition between charge and orbital antiferromagnetic insulating phases and ferromagnetic metallic phases. Initial models of charge and orbital order in the manganites concentrated on the chequerboard charge order structure, observed in many half doped systems. Incommensurate charge ordering and structural distortions have been identified in La0.6Sr2.4Mn2O7 through the observation of superlattice reflections using resonant x-ray scattering. Such structural distortions are indicative of concomitant orbital order, leading to our proposal of an incommensurate charge-orbital density wave. Superlattice structural distortion reflections were observed with a modulation vector (0.178, 0.178, 0), and charge order reflections with a modulation vector (0.356, 0.356, 0). Low temperature charge order melting, observed in the half doped bilayer manganite, is not present due to the absence of long range magnetic order. © 2010 IOP Publishing Ltd.

Abstract:

The magnetic structures which endow TbMnO(3) with its multiferroic properties have been reassessed on the basis of a comprehensive soft x-ray resonant scattering (XRS) study. The selectivity of XRS facilitated separation of the various contributions (Mn L(2) edge, Mn 3d moments; Tb M(4) edge, Tb 4f moments), while its variation with azimuth provided information on the moment direction of distinct Fourier components. When the data are combined with a detailed group theory analysis, a new picture emerges of the ferroelectric transition at 28 K. Instead of being driven by the transition from a collinear to a noncollinear magnetic structure, as has previously been supposed, it is shown to occur between two noncollinear structures.