Andrew Boothroyd

Abstract:

Controlling orbital occupancy is a fundamental prerequisite for orbitronics. It has been shown in the orthorhombic bilayer manganite Pr(Sr 0.1Ca0.9)2Mn2O7 that the direction of orbital order stripes can be influenced by controlling temperature or through inducing strain in the material. In this paper we have used resonant soft x-ray diffraction at the Mn L-edge to confirm the rotation of the orbital direction TOO2 and furthermore prove that there is no change in the occupied orbital type, however the orbital rotation causes a switch from 3x 2-r2 to 3y2-r2 on a single site. We find that unlike the tetragonal bilayer manganites, where an onset of A-type AFM quenches the orbital order, no such effect is found on the orbital order below TN. © 2010 IOP Publishing Ltd.

Abstract:

The temperature dependence of the magnetic order of stripe-ordered La 1.725Sr0.275NiO4 is investigated by neutron diffraction. Upon cooling, the widths of the magnetic Bragg peaks are observed to broaden. The degree of broadening is found to be very different for l = odd-integer and l = even-integer magnetic peaks. We argue that the observed behaviour is a result of competition between magnetic and charge order. © 2010 IOP Publishing Ltd.

Abstract:

We present ultrafast resonant soft-x-ray diffraction measurements of time-dependent orbital order in the single-layer-manganite La0.5Sr1.5MnO4. These experiments reveal the appearance of a metastable phase with reduced ordering, different from any thermal state of the system. © OSA / UP 2010.

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.