This image shows how altering the ratio of chlorine to bromine in mixed halides allows the magnetization to vary continuously from in-plane to out-of-plane. Image: Fazel Tafti, Boston College.
This image shows how altering the ratio of chlorine to bromine in mixed halides allows the magnetization to vary continuously from in-plane to out-of-plane. Image: Fazel Tafti, Boston College.

Physicists, chemists and materials scientists have been probing the nature of layered magnetic materials for several decades, searching for clues to the properties of these materials, which are more complex than they appear.

A layered material resembles the structure of a book. From a distance it looks like a solid three-dimensional object, but when examined more closely it is actually made up of multiple, flat, two-dimensional sheets, like the pages of a book. During the past decade, scientists have pursued the ‘exfoliation’ of such layered materials, a process by which the layered material is systematically cleaved until a single atomic sheet is isolated.

A single atomic sheet of a magnetic layered material can be used to fabricate atomically flat, ultrathin magnetic devices. As an example, scientists have constructed ultrathin ‘magnetic memories’ – single atomic sheets that can store information in the directional orientation of the magnetization of their atoms.

The magnetization of a layered material is typically oriented either parallel or perpendicular to the plane of atoms. In other words, the magnetization tends to point either ‘in-plane’ or ‘out-of-plane’, indicating what is known as a magnetic anisotropy.

So far, scientists have only been aware of the in-plane or out-of-plane limits of magnetic anisotropy. In other words, the ability to control the orientation of the magnetism was defined by just the two parameters of anisotropy.

In a new paper in Advanced Materials, researchers from Boston College now report that magnetic anisotropy can actually be continuously tuned between the two limits of in-plane and out-of-plane. By varying the composition of a layered halide material, the researchers were able to point the material’s magnetization toward any direction of space instead of only in-plane or out-of-plane.

"In addition to magnetization direction, our team showed that all properties of these layered materials including light absorption, distance between the layers, and temperature of magnetic transition can be continuously controlled to any desired value," said Fazel Tafti, an assistant professor of physics at Boston College and lead author of the paper. "This is a leap of progress in tuning materials properties for optical and magnetic device industry."

To make the material, a team led by Tafti and associate professor of physics Kenneth Burch developed a ‘mixed-halide chemistry’ approach, which involves combining different halide atoms, such as chlorine or bromine, around a transition metal such as chromium. By adjusting the relative composition of chlorine to bromine, the researchers were able to adjust an internal parameter at the atomic level known as the spin-orbit coupling, which is the source of magnetic anisotropy. This tuning methodology allowed the researchers to engineer the amount of spin-orbit coupling and thus the orientation of magnetic anisotropy at an atomic level.

Tafti said that these types of magnetic layered materials could form the basis of next-generation ultrathin magnetic devices, which may replace the transistors and electric chips used today. Because of their atomic scale, these materials could help shrink the size of magnetic devices, as magnetic information can be composed on the atomically flat sheets.

"From here, we will continue to push the frontiers of magnetic layered materials by making mixed halides of transition metals other than chromium," said Tafti. "Our team demonstrated that the mixed halide chemistry is not limited to chromium and can be generalized to over 20 other transition metals. The co-leader of the project, Kenneth Burch, is trying to artificially interface different magnetic layers so the properties of one layer would affect the adjacent one. Such metamaterials can change the propagation of light in one layer based on the direction of magnetism in the neighboring layer and vice versa – a property known as the magneto-optical effect."

This story is adapted from material from Boston College, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.