Fast pulse sequence switches magnetization

All-optical switching has always previously been obtained with low repetition rate pulse sequences. In their recent paper, the Smadici group asked: can switching be realized by directly applying the fast pulse sequence of a TiS oscillator? Farzaneh Hoveyda, Erich Hohenstein & Serban Smadici explain their approach to this question in their own words below.

The answer to the above question, in short, is “yes”. Examination of images shows both that this switching occurs by domain wall motion, and the importance of heat accumulation.

This may bring to mind experiments in magnetic bubble materials, but with the difference that the domains are nucleated by the laser, and the large coercive field keeps the domains stable without an external field. TiS lasers are beginning to be applied in manufacturing processes on a large scale, and obtaining switching directly from an oscillator simplifies potential practical implementations.

Magnetization switching

All-optical switching (AOS) between thermal demagnetization (TD) stripes. Adapted from J. Phys. Condens. Matter 29 225801 © IOP Publishing

The unique properties of these lasers have often expanded the frontiers of science in unpredictable ways. In particular, ultrafast magnetism evolved over the last twenty years from impossibility to a promising research field. Magnetization was expected to respond in the stately precession of a classical macrospin with periods of hundreds of picoseconds (1 ps = 10,-12 s) to a very short transient disturbance. To the delight of everyone, the magnetization could be made to change within a few hundred femtoseconds (1 fs = 10-15 s). As it turned out, a classical macrospin fragments into a quantum state with an ultrafast time-evolution when a large amount of energy is inserted very quickly into the material by the light pulse.

All-optical switching to a single-domain final state (in contrast to the above demagnetization) in ferrimagnetic and ferromagnetic materials provided further challenges. Novel quantum models explained the switching in ferrimagnets. However, switching in ferromagnets cannot be similarly obtained, since ferrimagnetic models rely on specific exchange interactions between the two magnetic sub-lattices.

This suggested that we could make a contribution to this dynamic topic. Our results demonstrate switching in ferromagnetic materials under new experimental conditions. Can the observed large temperature variations be useful? We are searching for a better understanding of the transient state obtained after excitation with time-resolved measurements of magnetization and temperature.

About the authors

Smadici group

From left to right: Erich Hohenstein, Farzaneh Hoveyda & Serban Smadici

Farzaneh Hoveyda and Erich Hohenstein are completing a PhD and MS, respectively, at the University of Louisville. Serban Smadici graduated from Columbia University in 2005 with expertise in ultrafast science, and joined the University of Louisville in 2013 after working at Brookhaven National Laboratory and University of Illinois at Urbana-Champaign on X-ray scattering at the National Synchrotron Light Source.


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Categories: Journal of Physics: Condensed Matter, JPhys+

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