A team of researchers has for the first time discovered magnetism in the 2-D world of monolayers, or materials that are formed by a single atomic layer.
The findings, published this week in the journal Nature, demonstrate that magnetic properties can exist even in the 2-D realm, opening a world of potential applications, as magnetic materials form the basis of technologies that play increasingly pivotal roles in our lives today.
"What we have discovered here is an isolated 2-D material with intrinsic magnetism, and the magnetism in the system is highly robust," said Xiaodong Xu, a professor of physics and of materials science and engineering at the University of Washington (UW). "We envision that new information technologies may emerge based on these new 2-D magnets."
Xu and Massachusetts Institute of Technology (MIT) physics professor Pablo Jarillo-Herrero led the international team of researchers who proved that the material, chromium triiodide, or CrI3, has magnetic properties in its monolayer form.
Other groups, including co-author Michael McGuire at the Oak Ridge National Laboratory, which is managed for the United States Department of Energy, had previously shown that CrI3, in its multilayered, 3-D, bulk crystal form, is ferromagnetic. In ferromagnetic materials, the "spins" of constituent electrons, analogous to tiny, subatomic magnets, align in the same direction even without an external magnetic field.
However, no 3-D magnetic substance had previously retained its magnetic properties when thinned down to a single atomic sheet.
"You simply cannot accurately predict what the electric, magnetic, physical or chemical properties of a 2-D monolayer crystal will be based on the behavior of its 3-D bulk counterpart," co-lead author and UW doctoral student Bevin Huang was quoted as saying in a news release from the university in U.S. Pacific Northwest.
Atoms within monolayer materials are considered "functionally" two-dimensional because the electrons can only travel within the atomic sheet, like pieces on a chessboard.
The researchers used a special type of microscopy to detect a signature in CrI3, as in other ferromagnetic materials when a beam of polarized light is reflected off the material's surface, indicative of intrinsic ferromagnetism in an isolated monolayer.
And surprisingly, in CrI3 flakes that are two layers thick, the telltale optical signature disappeared. This indicates that the electron spins are oppositely aligned to one another, a term known as anti-ferromagnetic ordering. Ferromagnetism returned in three-layer CrI3.
Yet to conduct further studies to understand why CrI3 displayed these layer-dependent magnetic phases, Xu noted that "2-D monolayers alone offer exciting opportunities to study the drastic and precise electrical control of magnetic properties, which has been a challenge to realize using their 3-D bulk crystals.
