Twisted nanomagnets reveal a new route to tune resonant mode

To the point:

• Twisted magnetic nanowires: Researchers at the Max Planck Institute for Chemical Physics of Solids investigated intertwined nanoscale magnetic wires arranged in double-helix geometries, similar to  DNA, revealing that they can hold magnetization dynamics.
• Direct observation of dynamics: Using advanced X-ray microscopy, they directly visualized the resonant behavior of highly coupled magnetic domain walls within these complex structures.
• Experimental challenge overcome: Probing dynamics in true 3D nanostructures has been a longstanding challenge, with most prior studies limited to simpler or larger systems. This work achieves direct observation at the nanoscale.
• Key results: The nanowires exhibit resonant dynamics with multiple oscillation modes. Simulations further show that these modes can be tuned through geometric design—rather than conventional control via external fields or voltages—highlighting geometry as a powerful new parameter for functional magnetic devices.

The simple structure of twisted wires combines the introduction of chirality into a system, with strong coupling between strands. This “double helix” geometry is ubiquitous in our world, from the smallest length scales - our own DNA - to the climbing of plants around one another in the natural world. 

But it is not only the natural world in which such a geometry is of relevance - chirality and strong inter-wire coupling is also particularly important for the world of magnetism, where these effects can lead to new structures in the magnetic order - as well as interesting dynamic effects. Indeed, in double helix nanostructures - more than one thousand times smaller than a human hair - highly coupled magnetic objects, known as domain walls, have previously been seen. However, the behaviour - how the magnetic objects in these helices move - has never been observed.

Now, researchers from the Max Planck Institute for Chemical Physics of Solids have been able to directly image the dynamic behaviour of these domain walls in nanoscale double helices using advanced x-ray microscopy at the BESSY-II synchrotron in Berlin. In this work, recently published in Advanced Materials, they reveal that these nanoscale magnetic features move, interact, and oscillate in highly controllable ways.

One of the main challenges was actually measuring such dynamics in three-dimensional nanostructures. It turns out that while there has been a growing interest in the dynamics of 3D magnetic nanostructures in recent years, experimental investigations have remained sparse, with experimental measurements being limited to larger, 3D microstructures or smaller, quasi-2.5D materials like volcanoes or flat nanowires.

Excitingly, it is not only the fact that these double helices could be measured that is noteworthy here. The researchers were able to identify the oscillatory behaviour of two domain walls in the two helices, that attract one another via magnetic fields. When they are “torn apart” they spring back together due to their strong interaction, and end up oscillating about one another, like a coupled oscillator. This behaviour is comparable to two fluid droplets connected by surface tension: when pulled apart, the restoring force drives them back together, leading to oscillatory motion about their equilibrium separation.

This oscillating behaviour has a so-called resonance -a particular frequency at which the dynamics are enhanced- that lies in the GHz regime. Remarkably, the researchers find with simulations that when the geometry of the helices is slightly modified, effectively tightening or loosening the DNA-like structure, this resonant frequency changes. Such changes in resonant frequency have previously been reliant on the application of electric or magnetic fields. Here, the prospect of geometry -controlled magnetic resonances opens new possibilities for technological applications that incorporate geometric changes to a magnetic system. Dynamic control could be achieved through the application of strain, by mechanically stretching or constraining the structure in a spring-like manner. In the future, such approaches could form the basis of, for example, fully three-dimensional neuromorphic computing technologies based on this type of magnetic phenomena, capable of mimicking the adaptative and dynamic nature of the human brain.