Laser irradiation switches the magnetic and structural properties of a molecular complex
Researchers from the RIKEN SPring-8 Center in Harima and from the universities of Tsukuba and Tokyo have demonstrated that laser light of a single wavelength can induce reversible transformations in the molecular bond, leading to changes in a material’s magnetic properties.
Reversible magnetic transformation by laser light is a promising path towards optical telecommunications applications, as “this process can be used as a fast and efficient optical switching technique,” explains Kenichi Kato from the RIKEN team. One candidate material for use in such applications is the Fe(phen)2(NCS)2 complex (Fig. 1 - Click on link below). This compound shows no magnetism at low temperatures, but is magnetic above 176 K.
In their study, published in the journal Applied Physics Letters (1), the researchers demonstrate that illumination with laser light of the material in its low-temperature phase can induce an almost complete transformation from the non-magnetic to the magnetic state. As long as the laser light remains on, the magnetic phase is stable for more than eleven hours without any signs of deterioration. After the light is turned off, however, the relaxation speed is of the order of 10 seconds, which is several hours faster than previously observed at low temperatures. The team also found the magnitude of the switching can be controlled via the intensity of the laser light.
To elucidate the origin of the switching process, the researchers studied the material’s crystal structure at the SPring-8 synchrotron x-ray light source. They showed that the molecular bonds between the iron and nitrogen atoms are important for the magnetism of the compound. In the crystal structure, the iron atom is surrounded by six nitrogen atoms, which form an octahedron—a structure formed by two pyramids attached to each other at their respective bases. The experiments show that the iron–nitrogen bonds in the octahedra are modified in the ‘on’ state under laser illumination. This peculiar change is the origin of the reversible magnetic properties of the complex.
To improve the switching properties of the molecules, the details and dynamics of this re-arrangement need to be understood. “Our next target is to see the switching process as moving pictures of the electron distribution using the high-time resolution possible with SPring-8,” says Kato. When complete, these experiments could lead to the design of improved molecules for even faster switching and for operation temperatures closer to room temperature.
Reference
1. Kato, K., Takata, M., Moritomo, Y., Nakamoto, A. & Kojima, N. On-off optical switching of the magnetic and structural properties in a spin-crossover complex. Applied Physics Letters 90, 201902 (2007).
