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Researchers Show Chiral Responses Exist in a Common Magnetic Material

Impending digital data storage devices mostly rely on innovative fundamental magnetic phenomena. Better insights into these phenomena could help develop improved and more energy-efficient hard drives and memory chips.

Using a deposition machine HZDR physicists prepare certain magnetic thin films, which are then patterned lithographically into parabolas. (Image credit: HZDR/S. Floss)

At present, physicists from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the Helmholtz-Zentrum Berlin (HZB) have completed the necessary basic research for future storage devices: They used an innovative strategy of shaping magnetic thin films in curved architectures to validate the existence of chiral responses in a magnetic material that is used often.

This enables developing magnetic systems with preferred properties that are dependent on simple geometrical transformations. The researchers have now described the study in the Physical Review Letters journal.

It is well known that the left hand of a person is different from the right hand—a left glove cannot fit the right hand and vice versa. The term “chirality” is used by researchers to characterize objects that do not align with their mirror image. Specifically, chemists know very well about this property in molecules, as in left- and right-rotating lactic acid. The right-rotating variant is more easily metabolized by humans than its “mirror image.”

Chiral effects such as these are known to take place in magnetic materials, where magnetic textures also possess chiral properties: the way individual magnetic moments are arranged within the material, or, so to say, the alignment of the several tiny “compass needles” that form a magnet, could form right- and left-handed alignments. Under specific conditions, certain textures act like image and mirror image—it is not possible to make a left-handed texture to be congruent with its right-handed version.

The fascinating feature here is that “the two textures can present different magnetic behaviors,” as HZDR physicist Dr Denys Makarov indicates. “To put it simply: a right-handed texture can be more energetically preferable than a left-handed texture. Since systems in nature tend to assume their lowest possible energetic state, the right-handed state is preferred.”

Chiral effects such as these hold immense technological potential. Apart from other things, they could be useful in the future development of highly energy-efficient electronic components like non-volatile storage devices, sensors, and switches.

Magnetic Curved Architectures

Helimagnets are materials with well-defined chiral magnetic properties, due to a lack of internal magnetic symmetry. Despite the fact that they have been known for a long time, these are rather exotic materials that are difficult to produce. Moreover, helimagnets usually exhibit their unique chiral properties at low temperatures.

Dr Oleksii Volkov, Study Lead Author, Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf

This is the reason for Makarov’s team to select a different path. They built curved objects such as parabola-shaped strips by using a common magnetic material—iron-nickel alloy, also called Permalloy. They used lithography to form different parabolic strips of several micrometers from thin sheets of Permalloy.

Then, the physicists exposed the samples to a magnetic field, thereby aligning the magnetic moments in the parabola along the magnetic field. Subsequently, they performed experimental investigation of the magnetization reversal with the help of a highly sensitive analysis technique at HZB’s synchrotron. The researchers were able to demonstrate that the magnetic moments in the parabolic strip stayed in their original direction until the application of a reversed magnetic field of a specific critical value.

Surprisingly Strong Effect

This delayed response is because of the chiral effects produced by the curvature at the apex area of the parabola strips.

Theorists have predicted this unusual behavior for some time, but it was actually considered more of a theoretical trick. But now we have shown that this trick actually works in practice. We detected magnetic chiral response in a conventional soft ferromagnetic material, just through the geometric curvature of the strips we used.

Dr Florian Kronast, Helmholtz-Zentrum Berlin

During the process, the researchers stumbled upon two more surprises: On one hand, the effect was exceptionally strong, implying that it could be used to influence the magneto-electric responses of materials. On the other hand, the effect was observed in a comparatively large object: micrometer-sized parabolas that can be generated with traditional lithography. Earlier, experts had presumed that these curvature-induced chiral effects could only be detected in magnetic objects measuring nearly a dozen nanometers.

In terms of possible applications, we are looking forward to novel magnetic switches and data storage devices that utilize geometrically-induced chiral properties,” stressed Makarov. There are ideas that visualize future digital data storage in specific magnetic objects, what are called skyrmions or chiral domain walls.

The latest findings could help create such objects very easily—at ambient temperature, and using regular materials. Moreover, the newly discovered effect also open the door for innovative, highly sensitive magnetic field sensors.


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