Our aim is to understand how light can be used to probe, and eventually control, electrons in magnetic molecular materials. We use ultrafast magneto-optical (MO) pump-probe techniques, which are one of very few methods that can directly probe the spin state of electrons on timescales relevant for photoexcitation (fs to ps). The MO signal is obtained by carefully measuring the change in polarisation state of the probe pulse. To understand the photophysics of the materials, we also use transient absorption spectroscopy.
Ultrafast spectroscopy of molecule-based magnets
We are interested in a class of magnetic materials, which have previously not been studied from the perspective of ultrafast magnetism and are based on molecular building blocks. A promising class of materials are the Prussian blue analogues (PBAs), since they are magnetic and their optical properties have been well studied. These are cyanobridged bimetallic compounds, composed of transition-metal ions, with a face-centred cubic lattice. Due to the chemical flexibility of these materials, interesting magnetic and optical properties are obtained and it is possible to systematically study how the material composition and properties affect the magnetisation dynamics.
Key papers:
Observation of excited state absorption in the V-Cr Prussian blue analogue
L. Hedley, M. D. Horbury, F. Liedy, J. O. Johansson, Chem. Phys. Lett. 687, 125-130 (2017).
Directly probing spin dynamics in a molecular magnet with femtosecond time-resolution
J. O. Johansson, J.-W. Kim, E. Allwright, D. Rogers, N. Robertson, J.-Y. Bigot, Chem. Sci., 7, 7061-7067 (2016).
Ultrafast photoinduced dynamics in Prussian blue analogues
K. Barlow, J. O. Johansson: Physical Chemistry Chemical Physics, 23, 8118–8131 (2021)
Single-molecule magnets and exchange-coupled polynuclear metal clusters
We are also interested in single-molecule magnets (SMMs), which are molecules that show magnetic hysteresis below a certain temperature TB (the blocking temperature). This can be achieved with only a few metal centres in the molecule and arises due to an energy barrier to magnetisation reversal. We carry out transient absorption spectroscopy and collaborate with theory groups to understand dynamics in these large exchange-coupled systems.
Key papers:
Photoinduced dynamics in an exchange-coupled trinuclear iron cluster
F. Liedy, R. Shi, M. Coletta, J. Vallejo, E. K. Brechin, G. Lefkidis, W. Huebner, J. O. Johansson, J. Magn. Magn. Mater. 501, 166476 (2020).
Vibrational coherences in manganese single-molecule magnets after ultrafast photoexcitation
F. Liedy, J. Eng, R. McNab, R. Inglis, T. J. Penfold, E. K. Brechin, J. O. Johansson, Nature Chemistry, 12, 452–458 (2020).
Thin film fabrication
We are interested in developing new thin films based on magnetic molecular materials but also exploit other properties, such as electrochromics, to create switchable multifunctional materials. We typically study thin films of molecular materials, which we make ourselves using a variety of techniques such as layer-by-layer (LBL) and electrochemical deposition and have developed an expertise in optimising film properties to best suit optical measurements. We characterise the films using equipment at the UoE such as AFM, SEM, XPS, ellipsometry and UV/VIS, IR and Raman spectroscopies.
Key papers:
Electrochromic bilayers of Prussian blue and its Cr analogue
L. Hedley, L. Porteous, D. Hutson, N. Robertson, J.O. Johansson, J. Mater. Chem. C, 6, 512 (2018).
Electrochromic Thin Films of the V-Cr Prussian Blue Analogue Molecular Magnet
L. Hedley, N. Robertson and J.O. Johansson, Electrochimica Acta, 236, 97 (2017).
Preparation of thin films of molecule-based magnets for optical measurements
H. A. Lewis, J. Kirkpatrick, J. O. Johansson: Thin Solid Films, 732, 138767 (2021)
Experiments
Ultrafast Transient Absorption Spectroscopy
Our transient absorption(TA) setup is located at the ultrafast laser lab (G36) in the School of Chemistry. We use the output from a Light Conversion Pharos-PH2 regenerative amplifier to pump a Light Conversion Orpheus-F NOPA that produces pump pulses having width cs. 40 fs. A part of the beam from Pharos is used to generate a broadband white-light continuum probe by focusing into a CaF2 plate. The white light is split into two parts: one to probe the sample and the other to use as a reference that increases the S/N ratio. Two fast CCD cameras with 512× 58 active pixels allows synchronous read out of both the probe and reference at 1 kHz. We have a cryostat that enables low temperature measurements down to 4.5 K.
Time Resolved Magnetic Circular Dichroism Spectroscopy
We can perform the time resolved magnetic circular dichroism (TRMCD) with the same setup with the inclusion of a quarter wave-plate that produces circularly polarized light from linearly polarized light and an electromagnet. The electromagnet gives a maximum 0.75 T magnetic field in the sample position.
Magnetic Circular Dichroism Spectroscopy
We have a JASCO J-710 spectropolarimeter where we can measure magnetic circular dichroism incorporating magnetic field. The spectrometer runs with a Xe arc lamp that gives a scan range of 200-800 nm. With the help of the cryostat, we can perform low temperature measurements down to 4.5 K.
