Electrons in an iron-based molecule have been spotted crossing between four different spin states in less than 50 fs (5 × 10–14 s), which is the fastest ever observation of such a transition. The measurements were made by researchers in Switzerland, who say that the speed of the transition means that the electrons jump directly from the initial low-spin state to the final high-spin state without going through any intermediate states. A new theoretical model is now needed to describe such rapid transitions, and their discovery has implications for future devices based on the spin of the electron.
Electrons in atoms, molecules and solids exist in different spin states, and electrons can make transitions between these states. Spintronics makes use of such states to process and store information, and has the potential to deliver devices that are smaller and more energy efficient than conventional electronics. These states are quantum-mechanical in nature and therefore could also be used as building blocks for quantum computers, which could out-perform conventional computers on certain tasks. But before the full potential of spintronics can be realized, researchers need to gain a better understanding of the nature of spin transitions – or crossovers, as they are called in iron-based molecules.
Iron(II) complexes are molecules with an iron atom that shares two of its electrons with other atoms in the molecule. These complexes could play an important role in spintronics because their electron spins are able to cross over through four spin states. This is unlike most materials, in which the electron spin will only shift through one or two states. The ability to switch through four spin states makes these materials potentially useful for many spintronic applications, including magneto-optical data storage.
100,000 times faster
In 2009 researchers at the Laboratory of Ultrafast Spectroscopy (LSU) at Ecole Polytechnique Fédérale de Lausanne in Switzerland measured electrons in an iron(II) complex shifting through four spin states – in less than 150 fs. This was 100,000 times faster than any recorded spin crossover and the researchers suspected the process was even faster.
Majed Chergui, head of the LSU, hypothesized that the speed of the switch meant it was a direct event and the electrons do not pass through the two intermediate spin states – but not everyone in the research community agreed with this. To test this theory, Chergui and colleagues had to measure the speed of the electron shift. To do this, researchers at LSU developed an ultrafast ultraviolet spectroscopy experiment that was capable of measuring faster spin crossovers than had been possible. In their latest research, Chergui and his colleague Gerald Auböck have shown that the electrons actually jump through the four spin states in less than 50 fs, which is the time-resolution limit of their equipment.
This shows that it is a direct spin transition, Chergui claims, because there simply is not enough time for the intermediate spin states to develop. “I would even put it the other way round, because there are no intermediate steps, the switch is extremely fast,” he told physicsworld.com.
Despite the measurement, the researchers do not know exactly what is happening during the transition. This is because the observation does not fit with current theories of spin crossover, which assume that electrons pass through the intermediate states. Chergui says that these theories apply to other molecular systems with crossovers that are much slower than those observed at LSU – crossovers that take hundreds of picoseconds, or even nanoseconds.
‘Quasistatic regimes’
“All modelling of spin transitions is based on the old models, which are correct, but they are not applicable in this case,” Chergui says. He says that the current models are correct for “quasistatic regimes” where the spin transitions occur on much longer timescales than other molecular processes. He adds that the fast transitions occur in the “dynamic regime” on the same timescale that atoms are vibrating and moving around in the molecule.
Chergui is calling on theoreticians to come up with new models to explain the dynamic spin transitions they have seen, so that the technological and scientific implications of the ultrafast switches can be better understood. In the meantime, researchers at LSU are developing new techniques to enable them to measure the actual time it takes for the spin crossover to occur, which they believe is around 20 fs.
Coen de Graaf, a quantum chemist at Universitat Rovira i Virgili in Spain, who had previously proposed a mechanism involving intermediate states for spin crossover in iron(II) complexes, says: “In the paper it is argued that the timescale of the deactivation process from excited singlet to final quintet is even shorter than assumed so far, and that the [final spin state] populates on the very same timescale at which the [initial spin state] disappears. This indeed puts serious doubts on the possibility of other intermediate states.”
The research is described in Nature Chemistry.
