Andrew Boothroyd

Abstract:

Photo-excitation in complex oxides1 transfers charge across semicovalent bonds, drastically perturbing spin and orbital orders2. Light may then be used in compounds like magnetoresistive manganites to control magnetism on nanometre lengthscales and ultrafast timescales. Here, we show how ultrafast resonant soft x-ray diffraction can separately probe the photo-induced dynamics of spin and orbital orders in La0.5Sr1.5MnO4. Ultrafast melting of CE antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand the (1/4,1/4,0) peak, reflecting orbital order, is only partially reduced. Cluster calculations aid our interpretation by considering different magnetically ordered states accessible after photo-excitation. Nonthermal coupling between light and magnetism emerges as a primary aspect of photo-induced phase transitions in manganites.

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:

We report measurements of the magnetic excitation spectrum of the layered antiferromagnet La2 CoO4 by time-of-flight neutron inelastic scattering. In the energy range probed in our experiments (0-250 meV) the magnetic spectrum consists of spin-wave modes with strong in-plane dispersion extending up to 60 meV, and a nearly dispersionless peak at 190 meV. The spin-wave modes exhibit a small (∼1meV) dispersion along the magnetic zone boundary. We show that the magnetic spectrum can be described very well by a model of a Heisenberg antiferromagnet that includes the full spin and orbital degrees of freedom of Co2⊃+ in an axially distorted crystal field. The collective magnetic dynamics are found to be controlled by dominant nearest-neighbor exchange interactions, strong XY-like single-ion anisotropy and a substantial unquenched orbital angular momentum. © 2010 The American Physical Society.