Introduction
The present experiment aims at providing insight in the behaviour of magnetic domains in a solid under the influence of an external magnetic field. The set-up uses a simple optical microscope operating in transmission mode viewing a magnetic thin film sample inside a Cu wire coil and sealed in between two polarizing sheets. Features such as magnetic hysteresis, the Barkhausen effect, magnetic bubbles etc. can be observed or measured but also aspects of image processing and polarization are part of this experiment.
Theoretical Background
When magnetic spins line up below the Curie temperature due to the isotropic exchange energy, magnetic domains form producing a net magnetic moment M. The direction of magnetization of this magnetic moment within a selected domain depends on the crystallography of the material and follows so-called axes of easy magnetization. The latter is governed by the so-called anisotropy energy. Thus, the combination of exchange and anisotropy energies determines in which direction the net magnetization in a given domain will point. The actual shape of the pattern (parallel, angled, serpentine, …) will depend on the material. Interfaces separating domains magnetized in different directions are called Bloch walls.
When an external magnetic field is applied, the Bloch walls will move and domains with a magnetization parallel with the external field will grow at the expense of the other domains due to their lower contribution to the total energy.
Experimentation Method
Magnetic domains can be visualized using the effect of the magnetization on the polarization direction of plane polarized light, the so-called Faraday effect, which can be applied in a transparent mode when thin films of the magnetic material can be grown.
The first goal of the experiment is to try to focus the microscope by adjusting the object position in a stepwise manner until a serpentine dark and pale image is obtained. In the second part of the experiment, a direct current through the magnetizing coil can be varied in order to induce a magnetic field perpendicular to the film. Also the direction (+/-) of the electric current can be changed. As a result of this externally induced magnetic field, the magnetic domains will reorder (grow and shrink) and new images can be obtained. By carefully adjusting the current, the so-called Barkhausen effect can sometimes be observed, which is a jerky movement of domain walls. Occasionally magnetic bubbles can be observed close to the complete single magnetic domain state of the film.
By obtaining different images under different external field conditions, different parameters can be measured, such as width ratio of sequential parallel domains over a line trace or surface ratio of both domain types in a chosen area. These can be studied producing a hysteresis curve as a function of the applied magnetic field (the latter must be calculated using the measured current and known coil configuration).
Motivation to perform this experiment remotely
More and more microscopes become remotely operated under various circumstances. These can be due to the high cost of the instrument, like in the case of advanced electron microscopes with a resolution below the Angstrom, microscopes in special conditions like space or medical environments etc. Working with a remotely controlled microscope and learning how to focus, change illumination, deal with time-lapses, take pictures, etc., can thus be a very valuable and additional experience next to other lab practices.
Setup
Short description
A ferrimagnetic garnet (FMG) is used as thin film (8 – 12 μm). The easy directions of the magnetic domains are perpendicular to the film normal. As a result of the Faraday effect, the magnetic domains will show up as dark or bright regions when seen between a polarizer and analyzer couple oriented in the right way. The FMG plus polarizer couple is sealed inside the unit together with a magnetizing coil and viewed in a conventional optical microscope with a magnification of 320x.
Schematic view
Remote Interface
Variables that can be controlled remotely
- focus of the microscope (via height of the sample)
- illumination of the sample (brightness & contrast)
- electric current through coil
- speed when changing electric current through coil
Variables that can be measured remotely
- focus of the microscope (via height of the sample)
- illumination of the sample (brightness & contrast)
- electric current through coil
What is visualized remotely?
- images
- buttons for microscope operation
- electric current
Educational
Learning Objectives
- remote control of a microscope
- physics of magnetic domains
- basic image processing
Target Group
- Bachelor students in physics
- Bachelor students in chemistry
- Bachelor students in engineering science
- Bachelor students in materials science
- also suitable as demo in high school physics
Professional Relevance
- A technique to visualize magnetic domains and to measure structural parameters of the domain configuration