Scientists make a startling discovery about magnetic defects in topological insulators

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Defect-mediated AFM to FM path in MBT and MST. a) Projection through an in-plane direction of the MST/MBT system showing the AFM magnetic structure. b) The role of Mn antisite defects at Sb sites in the transformation of AFM into FM ground state. The mechanism is represented by orange arrows. The Mn defect on the first Sb layer from the bottom is antiferromagnetically coupled to the bottom Mn layer due to superexchange. The Mn defect on the following Sb layer above is ferromagnetically coupled to the one below, consistent with the DFT results and experimentally observed FM coupling across the vdW gap. Finally, the top Mn layers are flipped to couple antiferromagnetically with respect to the Mn defect on the bottom Sb layer due to superexchange. Thus, the Mn defects are creating a pathway for ferromagnetic coupling to manifest for the Mn layers in the MBT/MST system. Credit: Advanced material (2023). DOI: 10.1002/adma.202209951

Scientists at the Department of Energy’s Ames National Laboratory have made an intriguing discovery while conducting experiments to characterize magnetism in a material known as a dilute magnetic topological insulator into which magnetic defects are introduced. Despite the ferromagnetism of this material, the team discovered strong antiferromagnetic interactions between a few pairs of magnetic defects that play a key role in several families of magnetic topological insulators.

Topological insulators (TIs) as their name indicates, are insulators. However, due to their unique electronic band structure, they conduct electricity to the surface under the right conditions. By introducing magnetism, TIs can transmit electric currents from one point to another without any heat generation or energy loss. This quality means they have the potential to reduce the future energy footprint for computing and electricity transmission.

According to Rob McQueeney, an Ames Lab scientist and member of the research team, “Finding topological insulators is not that easy. You have to find this unique situation where the electronic bands are knotted.” He further explained that applying a magnetic field to a TI transforms the surface into a single two-dimensional insulator, while the very edges of the surface remain metallic.

An important goal is to obtain a ferromagnetic TI. Ferromagnetism is when all the magnetic moments in the material spontaneously align in the same direction. However, the team also found that TIs are susceptible to antiferromagnetic interactions when defects are introduced. Antiferromagnetism is when some of the ions spontaneously align with neighboring ions. Opposing magnetic forces decrease the overall magnetism of the material.

There are two ways scientists introduce magnetism into a TI. The first is by introducing dilute amounts of magnetic ions, 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 (MnBi2You4) and manganese-antimony-tellurium (MnSb2You4).

Since intrinsic magnetic TIs have a complete layer of magnetic ions, ideally the magnetism is not randomly distributed as in the first method.

For this project, the team focused on diluted magnetic TIs, which exhibit randomly distributed magnetic defects. “We wanted to understand magnetic interactions at the most fundamental level. We were doping our sample using small amounts of magnetic ions to try to understand how magnetic interactions occur,” said Farhan Islam, a graduate student and team member at Iowa State University. “So essentially we’re trying to understand how microscopic interactions affect the overall magnetics of the system.”

To conduct their research, the team used a specialized method called neutron scattering. This method involves passing a beam of neutrons (neutrally charged subatomic particles) through a sample of material. Data is collected by detecting where and when neutrons that have scattered from the sample hit a detector.

This type of research can only be done in 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 Oak Ridge National Laboratory.

One challenge with neutron scattering is its weak signal. The team was concerned about the study of diluted magnetism, due to the small overall number of magnetic ions. “I was very skeptical that we would see anything,” McQueeney said. “But we did. Actually, what we saw was pretty simple to observe, which was surprising.”

The team found that despite the general ferromagnetism of manganese-doped antimony telluride (Sb1.94Mn0.06You3), some isolated pairs of magnetic defects are antiferromagnetically coupled with opposite moment directions. Other magnetic pairs, especially those in different blocks of the layered structure, are ferromagnetically coupled with parallel moments. Competing magnetic forces decrease the overall magnetism of the material.

“Intrinsic magnetic TIs actually have flaws,” Islam explained. “So, for example, manganese can actually get into antimony sites where they shouldn’t be, and how manganese gets into those sites is random.”

This random mixing of the manganese site creates magnetic defects in the intrinsic magnetic TIs. The team found that the same defect interactions in dilute materials also occur in intrinsic materials (i.e. MnSb2You4). The magnetic ground state of intrinsic magnetic TIs can be either ferromagnetic or antiferromagnetic, and the team now understand how magnetic defects control this behaviour.

“We determined the interactions between defects in the dilute case and realized that these interactions are transferable to the intrinsic case,” McQueeney said. “Thus, we conclude that defects control the magnetic order for both families.”

The study is published in the journal Advanced material.

More information:
Farhan Islam et al, Role of magnetic defects in fine-tuning the ground states of topological magnetic insulators, Advanced material (2023). DOI: 10.1002/adma.202209951

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