The distribution pattern of magnetization in a sample can be very complex and it can reveal many basic properties of a sample. For example, superconducting currents in a superconductor flow in a specific way, producing a pattern of magnetization that can be used for obtaining their distribution and magnitude. Magnetic domain structure in magnetically ordered materials can provide invaluable information on magnetic anisotropy and coupling energy between the electron spins.
The distribution of magnetization on the surface of a sample can be obtained using very fine powder of magnetic particles, electron beam, X-rays and polarization rotation of the light. One of the most convenient methods employs the rotation of the polarization of light as it is reflected off the surface of the sample (Kerr effect). The degree of rotation is proportional to the local value of magnetization as the light is reflected off the sample. This method can give a very detailed magnetization structure of the sample. However, the rotation of the polarization is very weak and high degree of polarization of the incident light and high-quality analyzer are required. This method works well only for large gradients in the change of the magnetic field above the surface of the sample, otherwise the obtained contrast is very small.
Luckily, there are materials that provide a strong rotation of the polarization as light passes through them. This rotation is dependent on the magnitude of local field. Such materials can be used as magneto-optical indicators (MOI) for studying the magnetization pattern of various materials, as this magnetization pattern provides a non-homogenous magnetic field in the volume of MOI. MOI’s available in this project are made in form of films, with bottom surface of the film forming a mirror. MOI films themselves are magnetically ordered (ferrimagnets), with their magnetization oriented within the film plane. When MOI films are placed on top of a sample, their magnetization gets aligned out of the plane by inhomogenous field pattern of the sample. The rotation of the polarization of the polarized light as it passes through the film (Faraday effect) is much stronger than the Kerr effect of the sample itself and it is proportional to the local out-of-plane magnetization of MOI film.
Magnetooptical indicator films can work in two ways, in reflection or transmission mode. Both of them will be available.
a) Reflection-type magnetooptical indicator films. The light passes through the MOI film and gets reflected off its bottom mirror surface, passes through the MOI film again, is led through an analyzer and into a microscope, which reveals the pattern of sample magnetization. You will use these MOI films to study magnetization pattern on magnetic samples. They provide a superior spatial resolution, making them ideal for studying minute magnetic features of superconductors and magnetically ordered materials. Reflection-type microscope is needed to use these films.
b) Transmission- type magnetooptical indicator films. These films do not have reflective layer and light passes through them only once. Transmission microscope is needed to use these films. Their advantage are greater image contrast and greater sensitivity to magnetic field. They can also be used
themselves as a sample on which magnetic domain structure can be studied, since they display welldefined domains. In fact, the width of the magnetic domains is used as a feature that enables measuring the magnetic field strength. These are excellent samples for studying the change of magnetic domain structure, as external field changes
The reflection-type MOI films you will be using can be easily damaged by scratching and are fragile. They were made in a research lab and obtained through a collaboration with this lab. They cannot be purchased on open market. Please handle them with extreme care because we have only a few of them and they are the best type of MOI films developed to day. Never touch them with bare hands, do not drop or scratch them, put any chemicals on them or pick them up with metallic tweezers. They will break like glass if pressed hard or dropped and the MOI film layer can be easily scratched off its glassy substrate. The transmission-type films are also fragile, so handle them with great care.
Your goal in this class is to set-up a system for observing the magnetization distribution of magnetically ordered samples. To achieve this, you need to perform the following tasks:
-organize yourself as an effective team and maintain the meeting minutes
-study principles of MOI measurement
-find out what instruments and facilities are available to you to build your set-up. -gather everything that is needed, in coordination with the lab demonstrator, and build your system -test the performance of the set-up on a sample with known magnetic pattern.
-identify its strengths and limitations.
-obtain the distribution of the magnetization in at least one sample, with both, transmission and reflection-type films. You will need to swap the microscopes with another team to use both types of films, but this may prove to be an excellent opportunity for a collaborative research between the two teams.
-apply small magnetic field to the sample, which will help you derive the direction of magnetization for different volumes of your sample both by MOI and by the force on the sample (it might move)
-try to obtain magnetic pattern on the surface of sample like Fe, an iron garnet film, or similar, and explain your findings
-compare the performance of these MOI films with the more traditional ferrite garnet films that have their magnetization perpendicular to the film plane. The width of the domain walls iss used to determine the local field magnitude with such films.