3D magnetism maps reveal exotic topologies

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&ball; Physics 13, 155

A newly developed x-ray-based technique for imaging spin patterns inside 3D structures uncovers new patterns.

C. Donnelly et al., Nature (2017)

This magnetic tomography map reveals the magnetizing vortices that occur in a slice through a 5- ??m

-cobalt gadolinium diameter cylinder. The long arrows indicate two types of vortices.

Since their invention, magnetic information storage devices have declined. This downsizing has allowed technologists to put more bits into smaller and smaller gadgets. Device makers would like to increase the storage density of magnetic devices even further, but there’s a catch: They’ve almost reached the limit of miniaturization of the 2D materials they typically use. To solve this problem, the researchers plan to move to the third dimension, a change that requires new methods to monitor and characterize magnetization patterns, something Claire Donnelly of the University of Cambridge, UK, and her colleagues of the Swiss Federal Institute of Technology (ETH) in Zurich and the Paul Scherrer Institute in Switzerland are expanding. Donnelly shared his latest findings, including the discovery of a new 3D magnetic pattern, during the recent CMD2020GEFES Conference, an online meeting that hosted lectures on all aspects of condensed matter physics.

“Most [imaging] the work that has been done so far on magnetic materials has been done on planar systems or surfaces, ”says Donnelly. “The switch to 3D offers many opportunities for progress. To reach the third dimension, researchers could, for example, stack several hundred 100nm magnetic “bricks” on top of each other to create a micrometer-sized pillar.

Magnetic imaging techniques have been around for almost a century, but they typically only probe the surface of a material. For example, magnetic force microscopy captures stray magnetic fields protruding from a material, then uses them to map the pointing direction of magnetic domains. The most widely applied of these 2D techniques can only probe the magnetization of a film of material to a depth of a few hundred nanometers, which is too shallow to see what’s going on inside 3D structures. micrometric in size, explains Donnelly. The techniques she helped develop offer the possibility of probing deeper into a magnetic material.

In the first method, called magnetic tomography, a circularly polarized short-wavelength X-ray beam is focused on a point of a 3D magnetic object. Circularly polarized X-rays are sensitive to the magnetic properties of a material due to spin-orbit coupling. And when these x-rays interact with an object, their absorption depends on the orientation of the object’s spins (spin texture) and the left or right circular polarization of the x-rays. The x-ray beam is then scanned through. the object, which is rotated and tilted, allowing the group to image a series of 2D x-ray projections of the spin texture at different sample orientations. The projections are then connected to an algorithm which restores the magnetic structure of the object in 3D.

Using this technique, Donnelly and his colleagues were able to map the 3D rotational texture of a

5??m

-cobalt gadolinium diameter cylinder. In the experiment, they achieved a resolution of 100 nm, which means that they were able to distinguish on this length scale the changes in the pointing direction of neighboring magnetic domains in the cylinder. This feat was a first for visualizing 3D spin textures, explains Donnelly. Inside the cylinder, the team spotted an intricate magnetic pattern made up of vortices and anti-cortex, as well as singularities of Bloch points, which are points where magnetization disappears. The magnetic pattern surrounding the singularity has never been seen before, Donnelly says. They also recently discovered a new texture in this cylinder, what is called a magnetic vortex ring made up of a loop of vortex and antivortex pairs.

Although enabling these new discoveries, magnetic x-ray tomography has a downside: it can only be used to examine static spin textures. To probe the dynamics of magnetization, the sample must be excited and then imaged without interrupting this excitation. This requirement is “too difficult” for tomography, Donnelly says, because it requires taking two sets of data at different sample orientations, along with other factors. This question prompted her and her colleagues to develop a second technique, called magnetic laminography, which they demonstrated earlier this year.

Laminography is similar to tomography, but the axis of rotation of the sample is no longer perpendicular to the x-ray beam. This change means that the sample can be tilted so that all three spatial dimensions can be imaged. in one take. “Laminography obviates the need to measure two sets of data, and it also works best for imaging flat or extended structures, which are the sample geometries most people study,” Donnelly explains. The team showed that they could map the magnetization dynamics of a

5??m

– thick disc with a spatial resolution of 50 nm and a temporal resolution of 70 ps. Now, says Donnelly, his team can observe what happens in 3D when the material is excited by a magnetic field.

These new 3D imaging techniques could potentially allow researchers to discover even more interesting spin textures, which they could use to improve magnetic storage devices. “Magnetic textures in 3D systems can be more stable than patterns seen in planar magnetic films,” Donnelly explains, allowing materials to carry spin waves at higher speeds due to the reduced instabilities. Thus, this advancement could lead to devices with faster reading and writing capabilities. “It gives [researchers] the potential to do what [they] already do a lot faster with these materials, but there is also the potential, which is more exciting, to do completely new things.

– Katherine Wright

Katherine Wright is Associate Editor-in-Chief of Physics.


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