Largest molecular spin found close to a quantum phase transition

Every single electron possesses a quantum mechanical intrinsic angular momentum, also called spin. The new magnetic molecule modelled at Bielefeld University and synthesized at the KIT reveals a spin in its ground state that is as large as that of 120 electrons combined. This makes it the largest spin ever to be observed in a single molecule. Magnetic molecules are molecules containing magnetic ions such as iron or gadolinium. The name of the magnetic molecule synthesized and studied by the research team is abbreviated to "Fe10Gd10." It has the geometric structure of a torus similar to a life-saving ring.

"In the case of the new molecule, this is joined by an unexpected property that also permits completely different applications," says Jürgen Schnack. Scientists in the interdisciplinary research project namely also found a so called quantum phase transition that strongly influences the property of the molecule. In quantum phase transitions, substances change their behaviour fundamentally at so called quantum critical points. An example of a "classical" phase transition is that of water when it begins to boil as it passes a certain temperature. Quantum phase transitions occur at a temperature of absolute zero. In the newly synthesized Fe10Gd10 molecule, ten thousand states are degenerate at the critical point. That means they have the same energy. On this absolutely flat energy surface, one can switch between the individual states without using any energy. In such a situation, the thermodynamic quantity called entropy adopts giant values. "It's as if you were standing on top of a high pointed mountain," explains Annie Powell. "A small change to the external parameters, for example, to the pressure, suffices for it to immediately drop steeply." Therefore, future research will examine how external pressure can be used to lead the molecule Fe10Gd10 beyond the quantum critical point.

Jürgen Schnack has been studying magnetic molecules in international collaborations for about 20 years. The goal underlying the study of magnetic molecules is to construct them so that they will fit various purposes exactly, for example, as nano data memories or refrigerants.