Session: Physical Chemistry Poster:

Table-top extreme ultraviolet (XUV) light sources provide the ability to probe core-to-valence transitions on a femtosecond timescale while retaining the elemental and oxidation state specificity and sensitivity to changes in local environment and spin-state of other x-ray spectroscopic techniques. Using this technique, transition metal oxide semiconductors have been studied at the M_2,3-edge (3p → 3d) to understand their charge carrier dynamics, while other transition metal complexes have garnered more interest for their spin-crossover behavior. These systems could enable ultrafast optical control of information processing via photoinduced spin crossover, however their application requires pairing with additional functionality such as long-range magnetic order. One notable example are bimetallic Prussian Blue analogues, but these metal complexes are limited to operate below 150 K due to loss of magnetic ordering. In contrast, cobalt ferrite is a ferrimagnetic semiconductor with a magnetic ordering temperature of above 300 K and extremely favorable magnetic properties. The mixed valence electronic structure of cobalt ferrite is reminiscent of the well-known photoswitch cobalt-iron Prussian Blue analogue. To investigate the potential for photoswitching in this material, we employ transient XUV spectroscopy to probe charge and spin dynamics with element-specific resolution on the femtosecond time scale. Results show that photoexciting cobalt ferrite thin films with 400 nm light excites a metal-to-metal charge transfer transition, as electrons from Co²⁺ localize to Fe³⁺ metal centers. This drives the crossover of native high-spin Co²⁺ to low-spin Co³⁺ corroborated by comparing experimental results to charge-transfer multiplet simulations. The time scale for photoinduced spin-crossover is 405 ± 29 fs as determined by global kinetic analysis. This result establishes the existence of efficient photoswitching in a new class of room-temperature ferrimagnetic spinel ferrites.

(Left) Experimental cobalt ferrite transient spectrum compared to simulations for high- and low-spin Co<sup>3+</sup> (Right) State population for high- and low-spin states determined by kinetic analysis with amplitude coefficients

(Left) Experimental cobalt ferrite transient spectrum compared to simulations for high- and low-spin Co³⁺ (Right) State population for high- and low-spin states determined by kinetic analysis with amplitude coefficients

Stephen Londo

Graduate Research Assistant, The Ohio State University

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