Physicists Generate New Nanoscale Spin Waves

Illustration of the experiment. (Image: Dreyer et al via Nature Communications)

Alternating solid magnetic fields can be used to generate a new type of spin wave that was previously just theoretically predicted. A team of physicists from Martin Luther University Halle-Wittenberg (MLU) achieved this for the first time. They report on their work in the scientific journal Nature Communications and provide the first microscopic images of these spin waves.

Spintronics

The basic idea of spintronics is to use a unique property of electrons — spin — for various electronic applications such as data and information technology. The spin is the intrinsic angular momentum of electrons that produces a magnetic moment. Coupling these magnetic moments creates the magnetism that could ultimately be used in information processing. When these connected magnetic moments are locally excited by a magnetic field pulse, this dynamic can spread like waves throughout the material. These are referred to as spin waves or magnons.

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Spin waves.
The 30 μm × 15 μm elliptical element is excited with a large rf-amplitude at an rf-frequency of 2.4 GHz. The non-linear magnetization dynamics are analyzed at different harmonics of the driving frequency, as seen in b-g. The real and imaginary part of the uniform mode at frf normalized to its magnitude is presented in a while integer harmonics at 2 frf, 3 frf, and 4 frf are shown (with corresponding scaling factors) in d, f, and h, respectively. For large driving amplitudes, coherent half-integer multiples at 1/2 frf, 3/2 frf, 5/2 frf, and 7/2 frf become visible in a narrow field range around 7.8 mT as presented in b, c, e, and g, respectively. It depicts the experimentally obtained magnitude analyzed at a fixed field value of 7.8 mT for all shown harmonics (red). At the same time, the solid line corresponds to the frequency spectrum obtained from corresponding micromagnetic simulations (blue). (Image: Dreyer et al. via Nature Communications)

A particular type of those waves is at the heart of the work of the physicists from Halle. Usually, the non-linear excitation of magnons produces integers of the output frequency — 1,000 megahertz becomes 2,000 or 3,000, for example. Professor Georg Woltersdorf from the Institute of Physics at MLU said:

“So far, it was only theoretically predicted that non-linear processes can generate spin waves at higher half-integer multiples of the excitation frequency.”

The team has shown experimentally which conditions are needed to generate these waves and control their phase. Phase is the state of the oscillation of a wave at a certain point and time; Woltersdorf said:

“We are the first to confirm these excitations in experiments and have even been able to map them.”

According to the physicist, the waves can be generated in two stable phase states. This discovery could potentially be used in data processing applications since computers, for example, also use a binary system.

Provided by Tom Leonhardt, Martin-Luther-Universität Halle-Wittenberg [Note: Materials may be edited for content and length.]

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