In a material as thin as a single layer of atoms, MIT physicists identified an exotic “multiferroic” state. Their discovery is the first to show that multiferroic characteristics can exist in a material that is perfectly two-dimensional.
The findings, which were reported in Nature, pave the door for smaller, faster, and more efficient data storage devices made of ultrathin multiferroic bits, as well as other novel nanoscale architectures.
“Two-dimensional materials are like LEGOs you put one on top of another to make something different from either piece alone,” says study author Nuh Gedik, professor of physics at MIT. “Now we have a new LEGO piece: a monolayer multiferroic, which can be stacked with other materials to induce interesting properties.”
MIT contributors include main author Qian Song, Connor Occhialini, Emre Egeçen, Batyr Ilyas, and Riccardo Comin, the Class of 1947 Career Development Associate Professor of Physics, as well as collaborators in Italy and Japan and at Arizona State University.
Curiously coupled
“Ferroic” refers to the collective switching of any property in a material’s electrons, such as charge orientation or magnetic spin, by an external field in materials science. Materials can exist in a variety of ferroic states. Ferromagnets, for example, are materials in which electron spins align collectively in the direction of a magnetic field, similar to how flowers rotate with the sun.
If using electric fields, the process of writing bits would be much faster because fields can be created in a circuit within a fraction of a nanosecond potentially hundreds of times faster than with electrical current.
Riccardo Comin
Ferroelectrics, too, are made up of electron charges that align themselves with an electric field. Materials are either ferroelectric or ferromagnetic in most circumstances. They rarely embody both states at the same time.
“That combination is very rare,” Comin says. “Even if one took the entire periodic table and put no boundary on the combination of elements, there are not many of these multiferroic materials that can be produced.”
In recent years, however, scientists have created materials in the lab that have multiferroic characteristics, acting as both ferroelectrics and ferromagnets in a strangely linked manner. The magnetic spins of electrons, for example, can be altered by both a magnetic and an electric field.
The potential for this linked, multiferroic state to enhance magnetic data storage systems is particularly exciting. Data is written onto a fast rotating disk imprinted with small domains of magnetic material in traditional magnetic hard drives.
A tiny tip suspended over the disk provides a magnetic field that can collectively alter the electron spins of a domain in one direction or the other to represent either a “0” or a “1,” the basic “bits” that encode data.
The magnetic field at the tip is usually created by an electrical current, which needs a lot of energy, some of which can be lost as heat. Electrical currents have a limit to how rapidly they can generate a magnetic field and swap magnetic bits, in addition to overheating a hard disk.
Physicists such as Comin and Gedik believe that if these magnetic bits were constructed of a multiferroic material, they could be switched utilizing faster and more energy-efficient electric fields instead of current-induced magnetic fields.
“If using electric fields, the process of writing bits would be much faster because fields can be created in a circuit within a fraction of a nanosecond potentially hundreds of times faster than with electrical current,” Comin says.
Size has been a significant barrier to device integration. Physicists have only discovered multiferroic characteristics in rather large samples of three-dimensional materials, which are too large to be used in nanoscale memory bits. No one has been able to create a two-dimensional multiferroic material that is perfectly two-dimensional.
“All known examples of multiferroics are in 3D, and there was a fundamental question: Can these states exist in 2D, in a single atomic sheet?” Comin says.
Ferroic flakes
The scientists turned to nickel iodide (NiI2), a synthetic material that is known to be multiferroic in bulk form, to find a solution.
“In our case, it was a dual challenge, to try to make nickel iodide into a 2D form and to measure it to see if it retained multiferroic properties,” Comin says.
Other two-dimensional materials, such as graphene, can be created by simply exfoliating layers from bulk graphite, but nickel iodide is more difficult to work with. In order to synthesize the material in 2D form, the researchers needed a novel method. The team, led by Song, used an epitaxial growth process, in which thin atomic sheets of material are “grown” on another foundation material.
Song and his colleagues employed hexagonal boron nitride as the bulk foundation in their experiment, which they heated in a furnace. They sprayed nickel and iodide powders over the material, which settled into flawless, atom-thin nickel iodide flakes on the boron nitride.
Gedik and Comin used optical techniques developed in their respective labs to examine the material’s magnetic and electrical response to test each flake’s multiferroic capabilities.
“The wavelength of light we use is around half a micron, so we can zoom in on a small region of this flake and study its properties with great precision,” Comin explains.
The researchers froze the 2D flakes to temperatures as low as 20 kelvins, where the material previously showed multiferroic characteristics in 3D form. They next conducted independent optical tests to investigate the material’s magnetic and electrical properties, respectively. The material was discovered to be both ferromagnetic and ferroelectric at a temperature of roughly 20 K.
The team’s investigations show that nickel iodide in its two-dimensional form is multiferroic. Furthermore, the research is the first to show that multiferroic order may exist in two dimensions, which are excellent for creating nanoscale multiferroic memory bits.
“We now have a material that’s multiferroic in 2D. Before, we didn’t know what to work with if we wanted to make a nanoscale multiferroic device. Now we do. And we are starting to make these devices in our lab now,” Comin says. “We want to use electric fields to control magnetism, to see how fast we can switch multiferroic bits, and how we can miniaturize these devices. That’s the roadmap, and now we’re much closer.”
The Department of Energy, as well as the National Science Foundation and the Gordon and Betty Moore Foundation, contributed to this study.
