Ultrafast Spin Crossover in a Room-Temperature Ferrimagnet: Element-Specific Spin Dynamics in Photoexcited Cobalt Ferrite

Abstract

Transition metal complexes capable of photo-induced spin crossover have been widely investigated because of their potential to enable ultrafast optical control of information processing. However, any real application of photo-switchable molecules requires that spin crossover be paired with additional functionality such as long-range magnetic order. Important advances combining these functions are notably reported for a number of bimetallic Prussian Blue analogues; however, to date PBA-based magnetic photo-switches can only operate below 150 K due to loss of magnetic order. In contrast, cobalt ferrite is a ferrimagnetic semiconductor with a Curie temperature of 790 K and extremely favorable magnetic properties by comparison to state-of-the-art PBAs. The mixed valence electronic structure of cobalt ferrite is reminiscent of cobalt-iron PBA, which is a well-known photo-switch. To investigate the potential for photo-switching in this material, we employ transient XUV spectroscopy to probe charge and spin dynamics with element-speci fic resolution on the femtosecond time scale. Results show that 400 nm light excites a metal-to-metal charge transfer transition, which drives the crossover of high-spin Co2+ to low-spin Co3+ with a time constant of 405 ± 29 fs and an internal quantum efficiency of unity. This result establishes the existence of efficient photo-switching in a new class of robust ferrimagnetic spinel ferrites. File list (2) download file view on ChemRxiv CFO_Main Mauscript.pdf (0.98 MiB) download file view on ChemRxiv CFO_Supplementary_Information.pdf (840.14 KiB)

Ultrafast Optical Spin Switching in Ferrimagnetic Nickel Ferrite (NiFe2O4) Studied by XUV Reflection–Absorption Spectroscopy

“…Starting with the LMCT transitions (Figures a and b), it can clearly be seen that these do not agree well with experimental findings because the experimental spectrum displays features at both the Fe- and Ni-edges. This implies that the spectral signature at time delays greater than 0.3 ps resembles a MMCT transition state, and this is consistent with previous experiments on CFO . Additionally, reduction of Fe 3+ is predicted to result in an excited-state absorption peak at 53 eV and a ground-state bleach at 55 eV which can be seen in Figure a.…”

Extreme Ultraviolet Reflection–Absorption Spectroscopy: Probing Dynamics at Surfaces from a Molecular Perspective

“…Using ligand field multiplet simulations, the initial state (blue) is assigned as a HS Co 3+ and the final state (orange) is assigned as a LS Co 3+ state. 4 Based on this kinetic analysis, we find that the time constant for photoinduced spin crossover (SCO) is 405 ± 29 fs. This sub-picosecond time constant for SCO is similar to the case for several PBAs reported before.…”

“…22 XUV-RA measurements of ferrimagnetic spinel metal oxides also demonstrate the sensitivity of this method to spin state, oxidation state, and coordination geometry. 4,28 It should be noted that the advantages of XUV spectroscopy described in this section are general phenomena of core-level spectroscopy ranging from XUV spectral domain to soft and hard X-ray regime. However, the ability to measure core-hole spectra using a tabletop instrument with ultrafast time resolution combined with the surface specificity of XUV light make XUV-RA spectroscopy uniquely suited to provide a detailed understanding of dynamics at surfaces.…”

Small Polarons and Surface Defects in Metal Oxide Photocatalysts Studied Using XUV Reflection–Absorption Spectroscopy

“…Metal centers in oxide materials also undergo photoexcited spin transitions, and metal oxides are known to display an array of interesting magnetic properties. For example, α-Fe 2 O 3 and NiO are both antiferromagnets. , Other metal oxides, including the spinel ferrites, have particularly strong exchange interactions and display ferrimagnetic ordering up to Curie temperatures greater than 500 °C. , As an example of the role XUV spectroscopy can play in probing ultrafast spin dynamics at interfaces, we have recently investigated spin crossover in CoFe 2 O 4 using XUV reflection–absorption, and based on these results, we anticipate this tool will be useful to understand light-induced spin dynamics in a wide range of semiconductor magnets. In light of these emerging applications, we expect that XUV spectroscopy will continue to play an important role in elucidating charge and spin dynamics in a growing number of molecular and material systems.…”