May 8 2019
Magnetic skyrmions are minute entities found in magnetic materials. These entities include localized twists in the direction of magnetization of the medium.
Magnetic measurement images showing how the number of skyrmions in a nanomaterial varies with magnetic field strength. These results were used to prove the influence of skyrmions on the Hall resistivity, a phenomenon called the Topological Hall Effect. (Image credit: M. Raju)
Each of these entities is highly stable since its removal requires the magnetization direction of the material to be untwisted, quite similar to a knot on a string that can only be untied by pulling the remaining portion of the string out of the knot.
Magnetic skyrmions are potential candidates for futuristic magnetic storage devices due to their stability and very small size—they have widths of 50 nm or less and occupy only a fraction of the area occupied by magnetic bits in existing hard disks. Therefore, scientists have been actively looking for materials that can consist of magnetic skyrmions, as well as to analyze their magnetic and electrical properties.
A group of researchers from Singapore and Israel has recently reported a significant advancement in gaining insights into the behavior of magnetic skyrmions. For the first time, they have demonstrated that the occurrence of magnetic skyrmions is unequivocally connected to a phenomenon called the topological Hall effect, which illustrates the way electric currents are deflected by a skyrmion’s emergent magnetic field. The study was published in the Nature Communications journal in March 2019.
The researchers analyzed a synthetic nanomaterial optimized for accommodating magnetic skyrmions, formed of consecutive layers of iron, iridium, platinum, and cobalt, each with a thickness of 1 nm or less. In 2017, the same nanomaterial had offered the first-ever evidence for the topological Hall effect at room temperature, noticed by the research team of Christos Panagopoulos at Nanyang Technological University, Singapore (NTU Singapore), who also headed the current study.
Professor Panagopoulos and his colleagues demonstrated that the Hall resistivity of the nanomaterial—the ratio of transverse voltage to electric current when a magnetic field is present—manifested anomalies that were challenging to explain except by the effect of magnetic skyrmions.
The interesting thing about the way skyrmions influence the Hall resistivity is that it depends on how the magnetization twists around each skyrmion. Mathematically, such twists are called ‘topological’ features, which is why the physical phenomenon is referred to as the ‘topological Hall effect’.
Christos Panagopoulos, NTU Singapore
However, certain aspects of the 2017 experiments were difficult to account for. The data appeared to point out that the anomalies in the Hall resistivity were 100 times greater than theoretical estimations based on the topological Hall effect. In order to come up with a definite connection, it was essential to carefully match the electrical measurements with direct observations of magnetic skyrmions.
In order to achieve this, the Panagopoulos team joined hands with the laboratory of Ophir Auslaender at Technion, the Israel Institute of Technology. With the help of an advanced low-temperature magnetic force microscope, the Auslaender team acquired highly accurate images of the skyrmions in the nanomaterial. Specifically, they discovered that some “wormlike” magnetization patterns were created by multiple skyrmions joined together.
The collaboration combined magnetic imaging and electrical Hall measurements, thereby managing to considerably narrow the disparity between theory and experiment.
The first thing we realized was that the number of magnetic skyrmions had been underestimated by a factor of ten. Digging deeper, we were able to show that the number of magnetic skyrmions is directly proportional to the topological Hall resistivity. This provides conclusive evidence that the skyrmions are responsible, not some other unaccounted-for phenomenon.
M. Raju, Research Fellow, NTU Singapore
Raju is also one of the lead authors of the study
In spite of this progress, Professor Panagopoulos said that the topological Hall resistivity is still higher compared to what the theory predicts, and proposes that the remaining discrepancy could be a matter of theoretical restrictions.
The topological Hall effect concept is based on assumptions, such as adiabaticity, that are theoretically convenient but may not be accurate for real materials. With the aid of these improved experimental methods, we are building a more sophisticated understanding of how electrical charges interact with magnetic spin in these important and technologically-promising materials.
Christos Panagopoulos, Professor, NTU Singapore