A new model questions universality in magnetism
According to Lasse Laurson, professor of computational physics and leader of this research, the result is important for theoretical physics which takes place at the interface between materials physics and statistical physics. The result opens a new line of research in which phenomena previously thought to be universal are viewed in a new light. Previous models have lacked details that have now proved relevant.
“We have developed a new way of modeling domain walls and their movement in disordered ferromagnets. By using this new model, we show that the nature of domain walls' dynamics depends on how strongly disordered the magnet is, unlike previously thought based on simpler model systems,” Laurson says.
The new model was developed by Audun Skaugen, a former postdoctoral researcher in the Complex Systems research group led by Laurson. In previous models of line-like domain walls in ferromagnetic thin films the domain walls have been modeled as elastic lines reminiscent of rubber bands, where their elastic nature tries to straighten them, while interaction with the disorder tends to make the domain walls rough. In such models, the disorder strength affects the roughness magnitude, but the statistical properties of the magnetic field driven dynamics of domain walls have been thought to be universal, i.e., independent of the disorder strength.
”Our modeling demonstrated the crucial role of certain internal degrees of freedom of the domain walls, neglected in the previous models, in determining the nature of domain wall dynamics. These internal degrees of freedom interact with the material disorder in a manner which results in disorder-dependent domain wall dynamics, Laurson says.”
The concept of universality is one of the cornerstones of physics and is important especially when studying phase transitions of matter using statistical physics. The behavior and properties of systems consisting of many interacting parts (such as atoms, molecules, etc.) are often independent of most microscopic details, i.e., the behavior is universal.
“In our work, we show that contrary to what the universality paradigm would suggest, the way in which domain walls move in disordered ferromagnets crucially depends on how strong the disorder is. The disorder in a magnet basically means dirt or other imperfections,” he adds.
The model enables larger-scale numerical simulations
The research falls within the field of magnetism. Magnetic domain walls are interfaces between magnetic domains, i.e., elementary regions within the ferromagnet magnetized in different directions from each other.
— It has been suggested that ferromagnetic nanowires could be used as memory devices in computers so that information is encoded into magnetic domains along the wire, such that the direction of the local magnetization corresponds to the bit, but the focus of our study is on other issues, Lasse Laurson clarifies.
Our research is, by its very nature, strongly fundamental research. Although direct practical applications are not sought, Laurson sees that the model developed has numerous applications in the study of domain walls.
“The model allows one to achieve up to two orders of magnitude larger system sizes in numerical simulations than in the commonly used micromagnetic simulation method, without much loss of accuracy,” he illustrates.
The study was published in the prestigious journal Physical Review Letters on 3.3.2022.
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What is a magnetic domain wall?
Magnetic domain walls are interfaces separating different magnetic domains in ferromagnets. The magnetic domains are regions in which the magnetization points in a uniform direction. More generally, domain walls are topological solitons, and in addition to magnetism they occur also, e.g., in optics and string theory.