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TODAY APRIL 30, 2017

Novosad's 'spin vortices' show a collective behavior in an alternating magnetic field

ARGONNE, Ill. – Scientists at the U.S. Department of Energy’s Argonne National Laboratory have used alternating magnetic fields to control the behavior of “spin vortices” trapped in small nanoscale disks made from iron and nickel that can be magnetized in two separate ways. While the majority of these nanodisks are magnetized in-plane either clockwise or counter-clockwise, a tiny region at their centers – the vortex core – is typically magnetized out of plane, either up or down. This region is called "magnetic vortex". Thus each or almost each dot has just one vortex at its center. Note that such discs, due to their small size, sometimes called "dots".

“If you were able to visualize it, it would look like a funnel,” said Argonne materials scientist Valentyn Novosad. Novosad and his collaborators wanted to find a way to control the magnetic states of pairs or even large groups of these dots that interacted with each other in a lattice.

In the resting state, the cores of the dots are randomly polarized. After applying an oscillating magnetic field to the pairs of dots, the researchers observed that the central cores began to switch back and forth repeatedly between up- and down-magnetizations. The interaction between neighbour dots is of key importance. The interaction between a pair of such vortices is so-called dipole–dipole interaction, which is of the same nature as the interaction between two magnetized pointers of two compasses placed near each other. Due to the interaction waves can propagate in arrays of magnetic vortices. The frequencies of these waves depend on the core polarities.

The magnetic field, when applied, oscillates at a particular frequency that can be tuned to match the “resonance frequency”, that is, the natural frequency of vibrations for a specific polarity combination in a pairs of neighbor dots. Each pair of dots has two resonance frequencies, corresponding to parallel (up-up or down-down) or antiparallel (up-down or down-up) magnetization states. In the parallel state, the dots' centers are magnetized in the same direction, while in the antiparallel state they are opposite.

The authors find how to controlling the core polarities (which can be "up" or "down") in an array of overlapping Ni80Fe20 dots. This is achieved by driving the system to a dynamic regime of periodic vortex reversals. Any desired core polarity combination can be achieved by adjusting the magnetic field oscillation frequency. The work might have a significant impact on other themes of magnetic research, such as magnetic storage and information-processing elements, magnonic crystals, spin–torque oscillators, and non-volatile random access memories.

About Argonne

The U.S. Department of Energy's Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation’s first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America’s scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.

Source: Argonne

Novosad's 'spin vortices' show a collective behavior in an alternating magnetic field

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