Scientists at the Department of Energy’s Ames National Laboratory made an interesting discovery while conducting experiments to characterize magnetism in a material known as a dilute magnetic topological insulator with magnetic defects embedded in it. Despite the ferromagnetism of this material, the team found strong antiferromagnetic interactions between some pairs of magnetic defects, which play a key role in several families of magnetic topological insulators.
Topological insulators (TIs) are, as their name suggests, insulators. However, because of their unique electronic band structure, they conduct electricity at the surface under the right conditions. By introducing magnetism, TI can transmit electricity from one point to another without heat or energy loss. This quality means they have the potential to reduce future energy footprints for computing and transporting electricity.
According to Ames Lab scientist and research team member Rob McQueeny, “Topological insulators are not easy to find. You have to find this special case where the electron strips are knotted.” He further explained to TI that applying a magnetic field turns the surface into a unique two-dimensional insulator, while the edges of the surface remain metallic.
An important goal is to obtain ferromagnetic TI. Ferromagnetism is the self-alignment of all magnetic moments in a material. However, the team also found that TI is susceptible to antiferromagnetic interactions when defects occur. Antiferromagnetism is the self-alignment of some ions with neighboring ions. Opposing magnetic forces reduce the overall magnetism of the material.
There are two ways scientists can introduce magnetism into TI. The first is to inject a dilute amount of magnetic ions such as bismuth telluride or antimony telluride with manganese. The second is to create a proprietary magnetic TI by introducing a layer of magnetic ions such as manganese-bismuth-tellurium (MnBi) into the material.2The4) and manganese-antimony-tellurium (MnSb2The4). Since the intrinsically magnetic TI has a full layer of magnetic ions, ideally the magnetism is not randomly distributed as in the first method.
For this project, the team focused on dilute magnetic TIs that have randomly distributed magnetic defects. “We wanted to understand magnetic interactions at their most fundamental level. “We doped our sample with small amounts of magnetic ions to understand how magnetic interactions occur,” said Farhan Islam, a graduate student at Iowa State University and a member of the team. “Basically, we’re trying to understand how microscopic interactions affect the overall magnetism of the system.”
To conduct research, the team used a special technique called neutron scattering. This method involves passing a beam of neutrons (subatomic particles with a neutral charge) through a sample of material. Data is collected by noting where and when neutrons scattered from the sample hit the detector. Such research can only be done in a few places in the world. Neutron spalling for this project was conducted at the Spallation Neutron Source, Office of Energy, Science Users Facility, operated by Oak Ridge National Laboratory.
One difficulty with neutron scattering is its weak signal. The team raised concerns about studying dilute magnetism because of the low total number of magnetic ions. “I seriously doubted we were going to see anything,” McQueeny said. “But we did. In fact, what we saw was so easy to observe that it was surprising.”
The team discovered that manganese-doped antimony telluride (Sb) despite its overall ferromagnetism.1.94Mn0.06The3), some isolated pairs of magnetic defects are in antiferromagnetic coupling with opposite torque directions. Other magnetic pairs, especially those in different blocks of a layered structure, are ferromagnetically coupled with parallel moments. Competing magnetic forces reduce the overall magnetism of the material.
“The internal magnetic TI has problems,” Islam explained. “For example, manganese can actually end up in antimony sites where it shouldn’t, and the way manganese ends up in those places is random.”
This accidental mixing of manganese causes magnetic defects in the internal magnetic TI. The team found that the same interactions between defects in dilute materials also occur in intrinsic materials (i.e. MnSb2The4). The magnetic ground state of an intrinsically magnetic TI can be either ferromagnetic or antiferromagnetic, and the team now understands how magnetic defects control this behavior.
“We discovered interactions between defects in the dilute state and realized that these interactions can be transferred to the intrinsic state,” McQueeny said. “Thus, we conclude that defects control the magnetic order of both families.”
This study Farhan Islam, Yongbin Lee, Daniel M. “The role of magnetic defects in ground-state tuning of magnetic topological insulators” by Pajerowski, JinSu Oh, Wei Tian, Lin Zhou, Jiaqiang Yang, Liqing is discussed further. Ke, Robert J. McQueeny and David Waknin and published in Advanced Materials.
Ames National Laboratory is a National Science Laboratory operated by Iowa State University of the US Department of Energy. Ames Laboratory develops innovative materials, technologies and energy solutions. We use our expertise, unique capabilities and interdisciplinary collaboration to solve global problems.
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