Barkhausen noise reveals the microstructure of materials
When a ferromagnetic sample is exposed to a slowly varying magnetic field, the sample responds through a sequence of discrete and irregular changes, or jumps, of magnetisation. This phenomenon is known as the Barkhausen effect.
The Barkhausen effect was discovered in 1919 but is still not fully understood. The results of a new study demonstrate that the majority of Barkhausen noise may, in fact, occur within the magnetic domain walls of a ferromagnetic sample.
“To demonstrate this phenomenon, we developed a new type of simulation that allowed us to explore the internal structure of domain walls and the related dynamics and observe the magnetic activity that occurs inside domain walls. We observed Barkhausen noise with a surprisingly high amplitude inside the domain walls, even higher than the noise associated with the motion of these walls,” says Associate Professor of Computational Physics Lasse Laurson.
The study is related to the Domain Wall Dynamics research project that is carried out at Helsinki Institute of Physics (HIP) and headed by Laurson.
Ferromagnetic substances are easily magnetised
The measurement of Barkhausen noise is a non-destructive testing method that sheds light on the microstructure of ferromagnetic materials. These materials contain permanent atomic magnetic dipoles that spontaneously align themselves to point in the same direction.
Ferromagnetic materials may become easily or even permanently magnetised and exhibit strong magnetic effects. Nickel, iron and cobalt are examples of ferromagnetic materials.
“For example, a non-magnetised piece of iron contains several small magnetised regions or magnetic domains so that the magnetisation of each domain is randomly aligned. When we expose the piece of iron to an external magnetic field, the walls separating the domains start moving. This motion gives rise to Barkhausen noise,” Laurson explains.
Barkhausen noise heard through a loudspeaker
Impurities in the ferromagnetic material act as obstacles to the movement of domain walls. As domain walls move past a defect, they can become momentarily pinned and then suddenly snap free. This jerky motion causes Barkhausen noise.
“This noise can be heard when the sample is connected to a loudspeaker. It is a crackling sound, as if a candy is being unwrapped. The signal can be described as a sequence of bursts of widely varying magnitude, similar to earthquakes. Hence, the Barkhausen effect is an interesting research topic for statistical physicists who examine the statistical properties of physical systems,” Laurson points out.
It is challenging to explore the Barkhausen effect taking place within the domain walls experimentally. The domain walls are very narrow and the timescales involved in these phenomena are extremely short.
“The experimental method would need to offer both ultra-high temporal and spatial resolutions,” Laurson says.
The researchers numerically simulated a film of cobalt with a thickness of 0.5 nm that was placed between two layers of platinum. The interfaces between cobalt and platinum induce a magnetic anisotropy that is perpendicular to the ferromagnetic cobalt thin film and causes the magnetic moments in regions outside the domain walls to point outward from the thin film.
Non-destructive testing allows quality control without causing damage
An in-depth understanding of the Barkhausen effect helps, for example, in the development of industrial quality control processes. There is a great demand for non-destructive testing methods in industry.
“For example, the gear wheels of a wind turbine must be as durable as possible. Barkhausen noise allows us to study the microstructure of these wheels without breaking them and reveals irregularities or other weaknesses,” Laurson says.
Text: Jaakko Kinnunen