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You can use a budget microscope with laser illumination and a polarizer (prismatic crystal or even film) to obtain the linearly-polarized light. The second polarizer (analyzer) you use in the eyepiece for highlight the light beam of a particular polarization.
You can use magneto-optical effects in the Kerr geometry ('in reflection') or the Faraday effect ('in transmission' in the case of a transparent sample). Thus it is possible to visualize the pattern of domains (or domain walls in crossed polarizers). Using the video camera facilitates the process of observation of magnetization processes.
This is an optical experiment. I have a SR830 lock-in amplifier connecting to a photodiode, which measures the reflection from a sample. The intensity of a laser beam is modulated by a chopper with a reference input to the lock-in amplifier. A phase is detected in the reflection signal both from the sample and control.
In another part of my setup, the polarization of the laser beam is further modulated by a PEM. A second lock-in amplifier with a reference frequency from the PEM is used to measured the signal reflected from the sample. A relatively large phase is observed both from the sample and control. 3) Now, the properties of the sample is tuned by an application of an electric field / a magnetic field. This further increases the phase difference between the signal and reference, indicating a change in sample properties. Here I want to see how the reflection signal changes with an electric field / a magnetic field on the sample.
I was wondering I should use the same phase shift measured in #2 (at zero electric field / magnetic field). However, it results in a very large phase shift when I measure my sample even at zero field. What is the general rule of thumb for the phase shift in intensity and polarization modulation optical experiment? This said, parts of what you ask for may depend on the specific steup.
Let me assume that the comparison between sample and control is done by swapping the two samples (taking out one and inserting the other). You will probably have to make some (geometric) adjustments and the reflected beam might undergo some changes in its characteristics, e.g. Due to different degrees of surface flatness, roughness etc. Nevertheless, for the intensity measurement I would expect the same main timing (i.e. The correlation between the chopper position and the phase of the corresponding reference signal) to be relevant. I'd therefore expect very small phase differenes to occur between different samples in the intensity measurement. To get a feeling about it, do play with sample adjustment, detune slightly (e.g.
With just the reference) and see what autophase detection tells you. Do also optimize the signal by hand to judge by yourself whether the autophase adjustment is doing a good job (it generally should with an SR830:-).
Again - if you can - use an oscilloscope to see what happens and whether the results you get from the SR830 make sense in the light of what you see. You can trigger the oscilloscope by the same signal you use as the reference input to the Lock-In to get reproducible timing. The magneto-optical Kerr effect (MOKE) has proved to be a useful research tool for understanding ferromagnetism and magnetic hysteresis. The MOKE occurs when light reflected from the surface of a magnetized material experiences a change in its polarization.
Studying this rotation of polarized light provides insights into the magnetic behavior of materials that have potential in future technology to minimize bit storage and improve processing speeds. The MOKE is a simple and popular research tool; however, the diffraction limit of visible light inhibits the study of atomic scale magnetism using this method.
To overcome this limitation, Xrays from synchrotron sources around the world are employed to study magnetic properties on a smaller scale. While the use of synchrotron X-rays is much more effective and essential for the ongoing study of magnetism, providing ultrafast imaging of nanoscale dynamics, the MOKE method still serves as an efficient research tool for magnetic materials due to its low cost and simplicity. A vibrating sample magnetometer (VSM) systems are used to measure the magnetic properties of materials. The vibrating component causes a change in the magnetic field of the sample, which generates an electrical field in a coil based on Faraday’s Law of Induction. If the sample is placed within a uniform magnetic field H, a magnetization M will be induced in the sample. In a VSM, the sample is placed within suitably placed sensing coils, also held at the desired angle.
And the vibrating sample component is made to undergo sinusoidal motion, i.e., mechanically vibrated. The electromagnet activates before the testing starts so if the sample is magnetic, it will become more so the stronger the field that is produced. A magnetic field H appears around the sample and, once the vibration begins, then the magnetization of the sample can be analyzed as changes occur in relation to the timing of movement. Because magnetic flux changes induce a voltage in the sensing coils that is proportional to the magnetization of the sample. Changes in the signal are converted to values by the software to graph magnetization M versus the magnetic field H strength, often referred to as a hysteresis loop.
MO effects can be described by the MO tensor. Each component corresponds to MO effects in different directions, and generally has a different coefficient describing the strength. I've seen in many texts that people assume the coefficients to be the same for different directions of illumination. This is a pretty good approximation when dealing with messy, polycrystalline materials, but won't always be true. Also, I imagine that engineering structures to have particular geometries and broken symmetries could lead to larger effective MO constants for different directions. Use, e.g., the Stoner-Wohlfarth model to calculate an M(H) loop. If you fix it to an in-plane system you get Mx and My.
(out-of-plane is straight forward, too) Then you calculate the reflectivity matrix. This allows you to get the intensity at a sensor as function of field. Concerning the previous answers, I would say that one usually measures intensity, typically in form of a voltage in an amplifier circuit containing a photo diode. The voltage can be converted later into reflectivity, magnetization, or Kerr angle. That depends on your setup and what you want to study. You can have a look at Jimenez et al Rev Sci Inst 84 (2014) 053904.
And the references therein, but there are many publications on this. Quite important ones (apart from the Kerr papers of course) are: Z. 200, 664 (1999); Rev. 71, 1243 (2000). The answer is no. ´Contributions from the non-linear Kerr effect may change the magnetization loop considerably.
Shining p-polarized light can in addition result in transversal effects, i.e. Part of the signal is proportional to the magnetization component perpendicular to the reflection plane. Therefore a good starting point is to use s-polarized light, where the latter effect is avoided. Still non-linear effects result in a signal, which can be described by s=A.ml+B.(ml^2-mt^2)+C.ml.mt, with longitudinal and transversal magnetization components ml and mt. Constants A,B,C depend on the material and on the geometry. VSM always measures ml.
So even if you assume that the magnetization is homogeneous in your sample or you integrate with MOKE over a representative volume, the MOKE signal and the VSM result are not necessarily the same. 2) Another way to get the most Kerr response out of your setup is to play with the angle of incidence. Dr hyman pdf. For polar MOKE, use normal incidence.
For the others, the angle of incidence which gives you the greatest MOKE signal will depend on the index of refraction of your sample/substrate. Another idea (if you are doing longitudinal or transverse MOKE) is to angle your sample such that the angle of incidence is the Brewster's angle of your substrate. I've never tried this personally, but it was an idea my adviser came up with. The advantages of I-scan are the following: in the Z-scan method one will have different illuminated areas of the sample due to the scan of it relative to the focal plane. This could be a problem in the case of inhomogeneous samples, where a different area of the sample will be measured at each scan distance, leading to very inconsistent data; also a major advantage of this method is that the sample is no longer passing through the focus and the probability of damaging is lowered; another huge advantage came from the fact that by using this method it is easier to observe high-order effects. It depends if you want to develop your own code to calculate the FDTD or if you want to work based on existing, commercial packages. Implementing nonlinear effects in FDTD is not trivial.
There are a number of publications on this. Usually they either use specific tricks or solve Maxwell's equations in loops to emulate the material response. I have been extensively using Lumerical FDTD solutions. In this package there are functions implemented that for certain cases are quite good to solve nonlinear problems in FDTD.
However, they work best (as you might have guessed) when the nonlinearity is weak. I have personally made the experience with many people who I introduced that Lumerical has quite a good learning curve for starters. Also, I have made the experience that for simple problems the nonlinear capabilities of FDTD solutions are quite bad. But be careful with the interpretation of the results.
Nonlinear optics are not trivial for FDTD. Hi Farren,I am not sure if you've found a solution to your issue but I wanted to tell you that I measure transverse MOKE in the same way as you (by detecting intensity changes via a standard semiconductor photodiode) and that I also experience this effect quite frequently and sometimes only for certain orientations of the sample relative to the applied field. I found this paper when I first ran into this issue, this group seems to have a similar experience:had a similar experience as Andrea in that a slight tilt of the sample makes the effect vanish, so I've just attributed it to a weird optical effect (not magnetic or anything interesting).
I'm interested in non-Laser based MOKE mikroscopes using longitudinal and polar Kerr effect to visualize the domain structure. Often halogen lamps are used, todays LED's work also as a light source, heavy use of post-processing data algorithms (subtraction of current hysteresis loop step magnetization from saturation magn., normalization, etc.), dielectric coating layers to improve contrast for low moment materials like Permalloy.In the literature I have seen many variations of this method with quite differing contrast in the domain structure images.Can someone explain to me what the state of the art regarding contrast here is? A comparison of the many slightly modified systems seems tricky to me, what is really the best criterion to measure the contrast of your non-Laser MOKE quantitatively and compare to literature.
For our relatively cheap system we get good contrast for Permalloy, but what would be the way to check if we are near to the state of the art? Which sample material? Which minimal film thickness? Which sample size? Should we use the noise in the hysteresis curve deduced from the domain images when sweeping? Didn't see this often in the literature.You can also point me to some literature I missed?
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