The development of an ultrathin magnet that operates at room temperature could lead to new applications in computing and electronics — such as high-density, compact spintronic memory devices — and new tools for the study of quantum physics.
The magnetic component of today’s memory devices is typically made of magnetic thin films. But at the atomic level, these materials are still three-dimensional — hundreds or thousands of atoms thick. For decades, researchers have searched for ways to make thinner and smaller 2D magnets and thus enable data to be stored at a much higher density. Previous achievements in the field of 2D magnetic materials have brought promising results. But these early 2D magnets lose their magnetism and become chemically unstable at room temperature.
State-of-the-art 2D magnets need very low temperatures to function but for practical reasons, a data center, for example, needs to run at room temperature. The new 2D magnet reaches the true 2D limit — it is as thin as a single atom.
The researchers synthesized the new 2D magnet — called a cobalt-doped vander Waals zinc-oxide magnet — from a solution of graphene oxide, zinc, and cobalt. A few hours of baking in a conventional lab oven transformed the mixture into a single atomic layer of zinc-oxide with a smattering of cobalt atoms sandwiched between layers of graphene. In a final step, the graphene is burned away, leaving behind just a single atomic layer of cobalt-doped zinc-oxide.
To confirm that the resulting 2D film is just one atom thick, the team conducted scanning electron microscopy experiments to identify the material’s morphology and transmission electron microscopy (TEM) imaging to probe the material atom by atom. The x-ray experiments characterized the 2D material’s magnetic parameters under high temperature.
Additional x-ray experiments verified the electronic and crystal structures of the synthesized 2D magnets. TEM was used to image the 2D material’s crystal structure and chemical composition.
The graphene-zinc-oxide system becomes weakly magnetic with a 5 to 6 percent concentration of cobalt atoms. Increasing the concentration of cobalt atoms to about 12 percent results in a very strong magnet. A concentration of cobalt atoms exceeding 15 percent shifts the 2D magnet into an exotic quantum state of “frustration,” whereby different magnetic states within the 2D system are in competition with each other. And unlike previous 2D magnets, which lose their magnetism at room temperature or above, the researchers found that the new 2D magnet not only works at room temperature but also at 100 °C (212 °F).
When a computer is commanded to save a file, that information is stored as a series of ones and zeroes in the computer’s magnetic memory such as the magnetic hard drive or a flash memory. And like all magnets, magnetic memory devices contain microscopic magnets with two poles — north and south — the orientations of which follow the direction of an external magnetic field. Data is written or encoded when these tiny magnets are flipped to the desired directions.
Zinc oxide’s free electrons could act as an intermediary that ensures the magnetic cobalt atoms in the new 2D device continue pointing in the same direction — and thus stay magnetic — even when the host (in this case, the semiconductor zinc oxide) is a nonmagnetic material.
The new material — which can be bent into almost any shape without breaking and is a million times thinner than a sheet of paper — could help advance the application of spin electronics or spintronics, a new technology that uses the orientation of an electron’s spin rather than its charge to encode data.
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