Studying the magnetic thin layers by analogy with the physics of soap bubbles

Physicists from the Centre for Nanosciences and Nanotechnology - C2N (CNRS/Paris-Sud University/Paris Diderot University) have studied thin films of magnetic material by applying concepts of the physics of soap bubbles. This approach, which had not been used so far, has made it possible to explain new phenomena.

There is a very interesting analogy between the physics of magnetic thin films and those of soap bubbles: in both cases, it is possible to reason in terms of interfaces, of energy associated with the surface area of these and of  a difference in pressure on both sides of an interface. In thin film physics, the soap film is replaced by the magnetic domain wall, which separates two areas in which the magnetization is uniform. And the pressure of the gas contained in the soap bubbles is replaced by the action of the applied magnetic field (B), which creates a pressure difference (worth 2MB, M being the magnetization density). These basic concepts were used by the soap bubble specialists and made it possible to explain many of the observed properties in a simple way. In the case of magnetic thin films, despite its strengths, this approach has not really been used.

Researchers from the Centre for Nanosciences and Nanotechnology - C2N (CNRS/Paris University), in collaboration with Beihang University in China and the Spintec laboratory (CEA/CNRS/Univ Grenoble Alpes), observed in CoFeB's ultra-fine films phenomena similar to those known for soap bubbles, and they could show that the analogy was needed. For starters, they found that magnetic domains of semi-circular shape were not stable in zero field. This first effect is explained by the Laplace pressure induced by the curvature of the wall, which tends to reduce the radius of the semi-circular domain it delimits. By determining the external field that must be applied to stabilize the domain, they were able to determine the interfacial tension energy associated with the wall. They were also able to observe the repulsion between two areas almost in contact: in a way totally analogous to the classic experience of soap bubbles in contact, where the big bubble "eats" the little one and the big domain crushes the little one. Their work was published in the Physical Review Applied journal.

The approach adopted here also makes it possible to explain a very common magnetic domain wall trapping phenomenon, which occurs at a sharp widening of the nanowire section. This trapping is explained by the interfacial tension force exerted at the level of the enlargement, which tends to retain the wall and must be overcome. The measurement of the field required for detrapping appears as a second method to directly measure the interfacial wall energy. This energy is a very important parameter but remains a difficult parameter to access because the experiments making it possible to measure it are likely to be skewed by formidable artefacts, that is to say artificial signals related to the experimental method which cause an analysis error. These experiments finally offer a reliable measurement method, with an understanding of simple and intuitive phenomena.

Reference:
Direct Observation of Domain-Wall Surface Tension by Deflating or Inflating a Magnetic Bubble
X. Zhang, N. Vernier, W. Zhao, H. Yu, L. Vila, Y. Zhang et D. Ravelosona
Physical Review Applied (2018)
DOI: doi:10.1103/PhysRevApplied.9.024032
-    Centre de Nanosciences et de Nanotechnologies – C2N (CNRS/Université Paris-Sud/Université Paris Diderot)
-    Fert Beijing Institute, School of Electronic and Information Engineering, Beihang University, China
-    Spintronics Unit and Component Technology -  SPINTEC (CEA/CNRS/Grenoble Alpes University)

Contact : Nicolas Vernier, Professor-researcher at Paris-Sud University - Centre de Nanosciences et de Nanotechnologies – C2N (CNRS/Université Paris-Sud/Université Paris Diderot) – nicolas.vernier @ c2n.upsaclay.fr