Bloch walls

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What are Bloch walls?

Bloch walls are the separating interface between so-called Weiss domains that occur in ferromagnets. A Bloch wall is therefore the boundary between two areas of parallel aligned elementary magnets. Bloch walls are named after the Swiss-born physicist Felix Bloch. Bloch walls are energetic barriers with a magnetic field that is stronger than in the surrounding area. This is why the Bloch walls can be made visible in an experiment with the aid of extremely fine ferromagnetic particles.

Table of Contents

In ferromagnetic materials, the electron spins are aligned in parallel groups. The parallel groups, i.e. the areas within which the spins are aligned in parallel, are known as Weiss domains. The area boundaries between these Weiss domains are the so-called Bloch walls.

An external magnetic field causes many small domains (in a non-magnetised ferromagnetic material, the domains are usually a few tenths of a millimetre in size) to merge into larger domains, and the magnetisation of the solid material becomes measurable on the outside. During this process, the Bloch walls shift. A sudden change in the alignment of all electron spins in a Weiss domain will occur Barkhausen jump).

Marked dividing lines between Weiss domains, so-called Bloch walls

Bloch walls are area boundaries between Weiss domains. Magnetic flux densities with different directions converge here. When a solution of very fine ferromagnetic particles (colloidal suspension of ferromagnetic dust) is placed on a solid object that is also ferromagnetic, the magnetic forces cause the particles to collect along the intersecting line of the Bloch walls with the outer surface of the material that is in contact with the suspension, forming so-called 'Bitter stripes' that can be observed directly under the microscope.

Bloch walls are not real "walls" within the solid object. Instead, they represent an energetic "wall". They separate two energetically different areas from each other. The magnetic fields of the electron spins, which are aligned differently in two Weiss domains, converge here and form a region of high magnetic flux density. If a real separating interface of the ferromagnetic material, i.e. a boundary surface of the solid object, intersects such a Bloch wall, the field lines emerge from the material along the Bloch wall.

Making Bloch walls visible

A beautiful method for making the Bloch walls visible was developed by the American physicist Francis Bitter. He coated a ferromagnetic object with a suspension of colloidal ferromagnetic particles. The ferromagnetic particles then settle along the Bloch walls due to the high magnetic field strength and form an image of the intersecting line between the Bloch wall and the boundary surface of the solid object. The resulting stripes are also called 'Bitter stripes' after Francis Bitter.

Author:
Dr Franz-Josef Schmitt

Dr Franz-Josef Schmitt is a physicist and academic director of the advanced practicum in physics at Martin Luther University Halle-Wittenberg. He worked at the Technical University from 2011-2019, heading various teaching projects and the chemistry project laboratory. His research focus is time-resolved fluorescence spectroscopy in biologically active macromolecules. He is also the Managing Director of Sensoik Technologies GmbH.

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