Imagine a magnet unlike any other, one with a unique arrangement that has baffled scientists and remained elusive in the natural world. This is exactly what researchers from Nanjing University, in collaboration with Los Alamos National Laboratory (LANL), have achieved. They've created an artificial nanomagnet that exhibits a strange, yet fascinating magnetic phase known as "ferrotoroidicity."
In simple terms, think of this new magnet as a series of magnetic moments arranged in a nice circle, where each moment points towards the next, forming closed loops. This special magnet, predicted by theorists but never before observed in nature, breaks both space and time symmetries. Magnets like these are not just scientific curiosities; they have significant implications for technology.
This groundbreaking work, led by Yong-Lei Wang's group at Nanjing University with theoretical support from Cristiano Nisoli of LANL, marks a pivotal moment in magnetic research. For the first time, scientists have not only predicted but directly observed and controlled this extraordinary magnetic state and its transformations.
This work is published in Nature Nanotechnology (https://doi.org/10.1038/s41565-024-01666-6). Dr. Wen-Cheng Yue and Zixiong Yuan are the co-first authors, and the corresponding authors are Dr. Sining Dong, Prof. Huabing Wang, Prof. Cristiano Nisoli, and Prof. Yong-Lei Wang.
The abstract of the article is as following:
Ferrotoroidicity—the fourth form of primary ferroic order—breaks both space and time-inversion symmetry. So far, direct observation of ferrotoroidicity in natural materials remains elusive, which impedes the exploration of ferrotoroidic phase transitions. Here we overcome the limitations of natural materials using an artificial nanomagnet system that can be characterized at the constituent level and at different effective temperatures. We design a nanomagnet array as to realize a direct-kagome spin ice. This artificial spin ice exhibits robust toroidal moments and a quasi-degenerate ground state with two distinct low-temperature toroidal phases: ferrotoroidicity and paratoroidicity. Using magnetic force microscopy and Monte Carlo simulation, we demonstrate a phase transition between ferrotoroidicity and paratoroidicity, along with a cross-over to a non-toroidal paramagnetic phase. Our quasi-degenerate artificial spin ice in a direct-kagome structure provides a model system for the investigation of magnetic states and phase transitions that are inaccessible in natural materials.
Source: School of Electronic Science and Engineering
Correspondent: Wang Yonglei
