Researchers at the Ames National Laboratory of the Department of Energy discovered a fascinating discovery while carrying out tests to characterize magnetism in a substance called a dilute magnetic topological insulator where magnetic defects are added.
Despite the ferromagnetism of the material, the research team found powerful antiferromagnetic interactions between specific pairs of magnetic defects that are crucial to the functioning of various families of magnetic topological insulators.
Topological insulators (TIs) as their name indicates, are insulators. However, because of their unique electronic band structure, they conduct electricity on the surface under the right conditions.
TIs use magnetism to transport electrical currents from one location to another without generating heat or wasting energy. Because of this characteristic, they may help future computing and electrical transmission use use less energy.
According to Rob McQueeney, a scientist from Ames Lab and a member of the research team, “Finding topological insulators is not so easy. You have to find this unique situation where the electronic bands are knotted up.” He further explained that applying a magnetic field to a TI turns the surface into a unique two-dimensional insulator, while the very edges of the surface remain metallic.
An important goal is to obtain a ferromagnetic TI. When all of a material’s magnetic moments spontaneously align in the same direction, the result is ferromagnetism. The group also learned that when flaws are introduced, TIs are vulnerable to antiferromagnetic interactions.
When some of the ions spontaneously align with nearby ions, antiferromagnetism results. The overall magnetism of the substance is reduced by the competing magnetic forces.
There are two ways that scientists introduce magnetism into a TI. First, diluted magnetic ions are added, such as manganese-doped bismuth telluride or antimony telluride. The second is to create an intrinsic magnetic TI by inserting a layer of magnetic ions into the material, such as manganese-bismuth-tellurium (MnBi2Te4) and manganese-antimony-tellurium (MnSb2Te4).
We wanted to understand the magnetic interactions at the most fundamental level. We were doping our sample using small amounts of magnetic ions to try to understand how the magnetic interactions occur. So basically, we’re trying to understand how microscopic interactions affect the overall magnetism of the system.
Farhan Islam
Since the intrinsic magnetic TIs have a full layer of magnetic ions, ideally the magnetism is not randomly distributed the way it is in the first method.
The scientists concentrated on diluted magnetic TIs for this endeavor, which have magnetic flaws that are dispersed randomly. “We wanted to understand the magnetic interactions at the most fundamental level. We were doping our sample using small amounts of magnetic ions to try to understand how the magnetic interactions occur,” said Farhan Islam, an Iowa State University graduate student and team member. “So basically, we’re trying to understand how microscopic interactions affect the overall magnetism of the system.”
To conduct their research, the team used a specialized method called neutron scattering. This method involves passing a beam of neutrons (sub-atomic particles with a neutral charge) through a sample of material. Data is collected by noting where and when the neutrons that scattered from the sample hit a detector.
This type of research can only be done a few places in the world. Neutron scattering for this project was conducted at the Spallation Neutron Source, a Department of Energy Office of Science User Facility operated by the Oak Ridge National Laboratory.
One challenge with neutron scattering is its weak signal. The team had concerns about studying dilute magnetism, because of the small overall number of magnetic ions. “I was very skeptical that we would see anything at all,” said McQueeney. “But we did. Actually, what we saw was pretty straightforward to observe, which was surprising.”
The team discovered that despite the overall ferromagnetism of manganese doped antimony telluride (Sb1.94Mn0.06Te3), some isolated pairs of magnetic defects are coupled antiferromagnetically with opposite moment directions.
Other magnetic pairs are ferromagnetically connected with parallel moments, particularly those in various blocks of the multilayer structure. The material’s overall magnetism is reduced by the conflicting magnetic forces.
“The intrinsic magnetic TIs actually have defects in it,” Islam explained. “So for example, manganese can actually go into the sites of antimony where they’re not supposed to be and the way the manganese is going into those sites is random.”
This random manganese site-mixing creates magnetic defects in the intrinsic magnetic TIs. The scientists discovered that intrinsic materials like MnSb2Te4 have the same interactions between flaws as dilute materials. The team has now discovered how magnetic defects regulate the ferromagnetic or antiferromagnetic nature of the intrinsic magnetic TIs’ magnetic ground state.
“We determined the interactions between defects in the dilute case and realized that these interactions are transferable to the intrinsic case,” said McQueeney. “By doing so, we conclude that defects control the magnetic order for both families.”
