Hitting the light switch for magnetic manipulation

Lynn Savage, lynn.savage@photonics.com

Reversing the magnetic properties of materials is the underlying reality behind computer hard drives, audiotape and other recording media. For these purposes, changing the magnetic state of macroscale particles suffices, but efforts are under way to exploit the spins and magnetic moments of electrons, which could lead to smaller electromagnetic devices than currently exist.

Using the inorganic materials typically chosen for their ferromagnetic properties limits the amount of time an electronic spin state lasts, however. Organic chemicals have garnered interest in the nascent field of spintronics because they can hold spins longer and therefore are more suitable for use in electromagnetic solid-state devices.

Altering a magnetic field commonly is performed with other, more highly magnetic fields or with radio-frequency devices, but using photons generally is easier and would seem well-suited for spintronics. Unfortunately, little to no work has been done on matching photoinduced magnetization switching techniques with organic chemicals.

Now, Anirban Misra and his colleague Suranjan Shil of the University of North Bengal in Siliguri, India, have completed a study of the magnetic properties of several azobenzene molecules.

When exposed to UV radiation, molecules in the azobenzene family change isomeric shape, which also changes them from antiferromagnetic to ferromagnetic. Courtesy of Anirban Misra, University of North Bengal.

Using imino nitroxide, nitronyl nitroxide and verdazyl – three highly stable forms of azobenzene – the researchers delved into the molecules’ ferromagnetic properties in their natural trans isomeric forms. In the trans form, each azobenzene is antiferromagnetic, or not susceptible to magnetic fields at all. However, after exposure to UV radiation in the 340- to 380-nm range, the molecules converted into their cis forms, which proved to be ferromagnetic.

Importantly for potential solid-state data manipulation, this photoisomerization process takes place at the nanosecond scale, enabling ultraquick on-off digital data processing. In addition, the ferromagnetic cis forms of the molecules had highly polarized electron spins, whereas the trans forms did not, indicating a possible role for cis azobenzenes as spin valves in electromagnetic read heads or in magnetic field detectors.

The investigators also measured the coupling constant (J) of each organic molecule, determining that the cis form of verdazyl has the highest J, which they measured to be 79 inverse centimeters. Nitronyl nitroxide had a J of 64 cm^—1 and imino nitroxide the lowest, 15 cm^—1. According to Misra, the larger the J value, the more suitable the molecule would be for potential applications. They reported their findings in the Feb. 4, 2010, issue of the Journal of Physical Chemistry A.

The scientists are now focused on further quantifying spintronic behavior in organic molecules as well as on evaluating the properties of “magnetically interesting” organic and inorganic systems.

“This is the first report of magnetization reversal in the systems of organic origin,” Misra said. “In organic systems, spins can be preserved for longer timescales than conventional inorganic materials. This may eventually lead to the application of photoinduced spintronics, photomagnetic switches, spin valves and so on.”