Susan Stoffer Sorensen and Dr. Karine Chesnel, Physics Department
The Kerr effect is a phenomenon in which as polarized visible light reflects off of a magnetized surface it experiences a small rotation in polarization. This is known as the Kerr rotation, denoted Δθ. This rotation is proportional to the magnetization of the sample, to a first-order approximation (Δθ∝M). The surface magneto-optic Kerr effect (SMOKE) is a subset of the Kerr effect which probes the magnetization of the surface of the sample. Since the samples measured by our SMOKE apparatus are as thin as 10 nm to 100 nm the surface magnetization equals the total magnetization. These samples can be used for magnetic recording or nanomedicine. The SMOKE method uses the Kerr effect to measure magnetization in the presence of an external magnetic field via a measurement of the Kerr rotation. A measurement of Kerr rotation while varying the applied magnetic field is a magnetization loop. My ORCA project was the development of a working SMOKE magnetometer, as well as the installation of an upgrade for our magnetic field.
There are several components used for the SMOKE optical setup: a laser, a 50:50 beam splitter, two linear polarizers mounted in rotating stages, two photodiodes, and a sample mount. Our setup also includes four ancillary mirrors to make beam alignment easier. Figure 1 shows a diagram of the optical components and beam path necessary for SMOKE measurements, where the mirrors have been omitted for simplicity.
During measurement, light from the laser diode passes through the first polarizer (referred to simply as the polarizer) such that the transmitted beam is completely horizontally polarized. The polarized light then passes through the beam splitter such that a portion of the light is reflected to a photodiode (PD-1) which monitors laser intensity fluctuations. This signal is used to remove fluctuations from the magnetization loop measurement. The beam then reflects off of the sample. The sample is mounted perpendicular to the applied field from the electromagnet and becomes magnetized. The reflected light experiences Kerr rotation, but because the rotation is relatively small, the light is still mostly horizontally polarized with a small vertical component. The light then passes through a second polarizer, called the analyzer. The analyzer is set vertically, almost at extinction with the polarizer. The slight offset from extinction, called the deviation angle (δ), circumvents a difficulty in determining the sign of the Kerr rotation. The analyzer therefore transmits the vertical component of the beam while blocking nearly all of the horizontal component. Since the polarization was originally completely horizontally polarized, the vertical component represents the rotated portion of the beam. A larger vertical component corresponds to a greater Kerr rotation. The intensity of the resulting signal is measured as voltage by the detecting photodiode (PD-2), which is proportional to Kerr rotation and thus is proportional to net magnetization.
To vary the magnetic field for a measurement, current is applied to an electromagnet by power supplies. Initially, we had only one power supply. This setup produced a maximum field of just under 5,000 G. In order to increase our available field, we doubled the maximum supplied current by adding three extra supplies to our system for a total of four power supplies. The resulting field is just under ±10,000 G, or ±1 T.
By taking many measurements with varying deviation (δ), polarizer, and incident angles, I have determined the ideal settings for our SMOKE apparatus. The ideal δ has a high signal-to-noise ratio and is not so large that the shape of the magnetization loop becomes distorted. I found the ideal δ range was between 10° and 16°. When the polarizer angle is moved off of either the horizontal or vertical polarizer axes the magnetization loop become slanted, and so we align our polarizer completely along the horizontal axis. Initially, our incident angle was around 20° from the sample surface, using two mirrors. Using four mirrors, we increased this angle to 68°. After this change, we were able to move our laser further from the poles of the electromagnet, eliminating the laser intensity fluctuations that were due to magnetic field interference.
The accuracy of our SMOKE measurements can be evaluated by comparing one to a measurement performed by a different magnetometer. Here, I compare a SMOKE measurement to one performed via an Extraordinary Hall Effect (EHE) magnetometer. Both loops have been normalized to a height of ±1, and therefore the features of the loop can be compared directly. This comparison is shown in Figure 2.
The matching features of the two loops leads us to an important conclusion for this project: the SMOKE is producing reliable measurements. These measurements are supported by those taken by other magnetometers. It appears that we have achieved a successfully functioning SMOKE apparatus.