Dynamics of electronic order in Magnetoresistive Manganites studied with time-resolved x-ray scattering

Authors: Henri P. Ehrke (University of Oxford)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: September 2010

Abstract: This thesis presents studies on how photoexcitation disturbs the ordering of the electronic degree of freedom located at the manganese 3d orbitals. Throughout the thesis the model compound La0.5Sr1.5MnO4 has been studied. This material exhibits two dimensional ordering of the charges and the orbitals (COO) and at a lower temperature also a three dimensional antiferromagnetic spin order. In the first approach the sample was photoexcited at 560 nm and the ensuing dynamics was probed through the optical anisotropy at 630 nm. To this end two independent tunable noncollinear optical parameteric amplifiers (NOPA) were used. The cross correlation between them was measured to be 16 fs, much shorter than what previously has been used. The majority of the COO signal at 25 K was found to disappear with a time constant limited by the time-integral of the cross correlation between pump and probe pulse. This hints towards an electrically-driven melting of the COO. To be more sensitive to the photo-induced changes in the COO, the technique of resonant soft x-ray diffraction was transferred from the static to the time-resolved domain. A diffractometer has been designed, built and comissioned which allows time-resolved experiments on the single-layered manganite La0.5Sr1.5MnO4. In this way it was possible to separate the dynamics of the orbital from the spin ordering by measuring the intensities of the associated diffraction peaks. The time resolution was as good as 10 ps. On this timescale the orbital peak responded profoundly differently compared to the magnetic ordering peak. It was found that it is possible to melt the magnetic peak completely within the time resolution, while the orbital ordered peak was much less affected. Through calculation and careful comparison with static data a temperature-driven effect could be excluded. Therefore, the experiments suggest that the quenching of the magnetic order is an electrically-driven process.

Subject Areas: Physics

Instruments: I06-Nanoscience

Added On: 10/10/2017 10:29

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