The study of organic magnets on metallic substrates is being pursued for applications such as information storage. Now, researchers Keio University1 in Japan demonstrate how the magnetization arising at the interface of non-magnetic molecules on a gold surface can be reversibly switched by optical excitation.

The interaction between organic thin films and gold substrates has recently been shown to lead to intriguing functionalities. For example, although gold is diamagnetic, the bonds between the gold surface and sulfur molecules induce magnetism at the gold surface. Along the Au-S bonds electrons are transferred from Au to S, which leads to ferromagnetism at the Au sites.

Here, the researchers studied the properties of an azobenzene disulfide monolayer on a gold substrate, and found the presence of a strong magnetism at the interface. The magnetization was highly anisotropic—being stronger perpendicular to the surface than parallel to it. However, compared with the observation of similar effects in other molecules, this approach enables the control of magnetism by optical illumination.

Fig. 1: The photoisomerization of organic molecules on a gold substrate can reversibly alter the magnetization at the interface through an electron transfer process.

Magnetism decreased when the organic film was illuminated with UV light. This process was reversible and magnetization was switched back to its original state by visible light. The origin of this switching effect is a photoisomerization process—a structural change of the molecules in response to light (Fig. 1). This leads to changes in the electronic structure of molecules, causing a reversal of the electron flow across the Au-S bond, which weakens the magnetism.

The photoisomerization process is entirely reversible, and therefore can be used to deliberately switch the magnetization at the interface. “This represents a novel strategy that could form the basis for developing magneto-optical memory and photo-switching devices,” says Yasuaki Einaga, who led the research team.

Moreover, Einaga is certain of the broader impact of this approach. “This strategy has the potential for developing a new class of photo-functional materials,” he says. Not only could this influence of the photoswitching on the electronic states of inorganic materials be used to further enhance the switching effect by using improved nanoscale structures for a greater switching effect, but also to perhaps control other electronic properties than magnetism.