γ-Rays shine on a molecular monolayer
The investigation of single molecule magnets brings Mössbauer spectroscopy firmly into the realm of surface science
The paper in Nature Communications is here: http://rdcu.be/F7hp
Written by Andrea Cornia and Roberta Sessoli
Andrea Cornia: Interfaces are not only important channels through which materials communicate with their surroundings and display their functionality. These thin portions of matter may also give rise to totally unforeseen behaviors, like ferromagnetism in otherwise diamagnetic metals.1 The exciting opportunity of engineering and controlling interfaces, however, comes at the cost of adapting investigation techniques to a new target: probing minuscule amounts of materials. Especially difficult to detect are the magnetic properties of soft molecules arranged on a metal surface as a single layer in a spintronic device. After our joint labs started to delve into the subject in 2000, it took almost a decade (and an untold number of sleepless nights at synchrotron light sources) to properly sharpen our working methods and fully exploit the exceptional surface sensitivity of X-ray magnetic circular dichroism (XMCD).2 In retrospect, the result was inspiring and definitely worth the effort: molecules with a directionally bistable magnetic moment maintain their memory effect when sitting on a metal surface. The XMCD technique suffers however from a major drawback: due to the finite rate at which magnetic fields can be swept and X-ray polarization switched, spin dynamics timescales shorter than about 100 s are impossible to follow. When dealing with the tetrairon molecules (Fe4) we synthesize in Modena, the only way out is freezing the sample down to prohibitively low temperatures. To tell the truth, accessing much shorter timescales (1-1000 ns) was not the only reason why we started thinking to Mössbauer spectroscopy as an alternative technique.
Roberta Sessoli: In our endeavour to understand how magnetic molecules are affected by being in contact with a metallic substrate we took great advantage of state-of-the-art theoretical tools. During his PhD thesis in Florence, Dr. Alessandro Lunghi, now at Trinity College in Dublin, developed an ab initio molecular dynamics approach to evidence structural modifications of the adsorbed Fe4 molecules and the resulting alteration of magnetic parameters. The video shows one of the twelve calculated trajectories reported in the original article.3 Many different potential minima are accessible to the molecules and a monolayer of Fe4 complexes should consequently present a distribution of configurations. But to our surprise, all XMCD-based experiments had failed to evidence any significant difference between a monolayer and the bulk material.
This puzzle was solved, as described in our Nature Communications article, after I attended a User Meeting at the European Synchrotron Radiation Facility (ESRF) in Grenoble. During an inspiring presentation on thin films of metallic iron I was stroken by the exceptional capabilities of the Mössbauer ID18 beamline at ESRF in terms of energy resolution, sensitivity, temperature range, etc.. I immediately visited the beamline scientists, and the project took off.
ID18 works with a focalized beam at grazing incidence and the sensitivity is increased by three orders of magnitude compared to standard radioactive sources; at the same time the energy resolution remains comparable thanks to a high-quality single crystal of 57FeBO3 which acts as a secondary source of gamma rays.
The paper in Nature Communications is here: http://go.nature.com/2nCRZDY
1. Al Ma’Mari, F. et al. Beating the Stoner criterion using molecular interfaces. Nature 524, 69–73 (2015).
2. Mannini, M. et al. Quantum tunnelling of the magnetization in a monolayer of oriented single-molecule magnets. Nature 468, 417–21 (2010).
3. Lunghi, A. et al. Single molecule magnets grafted on gold: magnetic properties from ab initio molecular dynamics. J. Mater. Chem. C 3, 7294-304 (2015).