MOKE on Hard-Drive Platters

ISEF Category: Materials Science

Subcategory: Electronic, Optical, and Magnetic Materials  ·  Difficulty: Intermediate  ·  Setup: School Lab  ·  Time: 1 to 2 Months

The Hook

A tiny change in magnetization can twist light by a tiny amount. That is the core trick behind the magneto-optical Kerr effect, or MOKE. You can see that twist with a laser, polarizers, and a magnetic surface. A hard-drive platter gives you a cheap, real-world test sample.

What Is It?

The magneto-optical Kerr effect happens when polarized light reflects from a magnetic surface and comes back with a small change in its polarization. Think of polarized light as a row of arrows all lined up in one direction. A magnetic film can nudge those arrows a little, and that nudge changes how much light gets through a second polarizer.

In this project, you use a DIY optical setup to detect that small change on a commercial hard-drive platter. The platter acts like a thin magnetic film on a reflective surface. When you change the tilt angle, you change the reflected beam path and the measured signal. That gives you a simple way to model how geometry affects a surface magnetization readout.

Why This Is a Good Topic

This topic gives you a real physics signal you can measure without a huge budget. You can vary one clear factor, like tilt angle or magnet orientation, and track how the optical response changes. That makes the project testable, visual, and easy to turn into graphs. It also connects to magnetic data storage, thin films, and optical sensing, which are real engineering problems.

Research Questions

• How does platter tilt angle change the Kerr rotation signal in a DIY MOKE setup?
• What is the effect of magnet orientation on the reflected light intensity after the analyzer?
• Does using different hard-drive platters change the size of the optical signal?
• To what extent does the distance between the laser and detector affect signal stability in a MOKE measurement?
• Which polarizer angle gives the clearest contrast between magnetized and non-magnetized regions?
• How does the reflectivity of the platter surface affect the signal-to-noise ratio?

Basic Materials

• Low-power red laser pointer, preferably mounted on a stand.
• Two linear polarizing filters.
• Commercial hard-drive platters from old drives.
• Small protractor or angle finder.
• Ruler or meter stick.
• Cardboard, foam board, or optical bench base.
• Tape and binder clips.
• White paper screen or phone camera for beam spotting.
• Dark room or shaded workspace.
• Smartphone camera for documentation.

Advanced Materials

• Low-power laser with stable mount.
• Two precision rotation mounts for polarizers.
• Photodiode or light sensor with data logger.
• Optical breadboard or vibration-damped table.
• Adjustable sample stage with angle readout.
• Hall-effect magnet or electromagnet for controlled magnetization.
• Gaussmeter, if available.
• Lock-in amplifier, if available.
• Mounting hardware for beam alignment.
• Safety goggles rated for the laser wavelength.

Software & Tools

• ImageJ: Measures spot position, intensity profiles, and contrast from recorded images.
• Python: Fits calibration curves and compares signal changes across tilt angles.
• Tracker: Tracks beam movement or spot shift frame by frame in videos.
• Google Sheets: Organizes trial data and makes quick graphs for first-pass analysis.
• Audacity: If you add sensor output recording, it helps inspect time-stamped signals.

Experiment Steps

1. Define whether you will measure intensity change, spot rotation, or contrast as your main signal.
2. Choose one sample geometry first, then decide how you will vary tilt without changing alignment too much.
3. Build a calibration plan that links polarizer angle or sensor output to a known reference signal.
4. Design controls that separate true magneto-optical change from plain reflection effects.
5. Plan a repeatability test so you can see whether the same sample gives the same curve on different days.
6. Decide how you will compare multiple platters or multiple regions on one platter using the same analysis method.

Common Pitfalls

• Using a bright room, which adds stray light and hides the small Kerr signal.
• Changing the laser spot location between trials, which mixes tilt effects with surface defects.
• Treating platter tilt and polarizer angle as the same variable, which makes the data hard to interpret.
• Measuring on scratched or dirty platter regions, which changes reflectivity and creates fake signal shifts.
• Forgetting a non-magnetized control, which makes it hard to prove the effect comes from magnetization and not simple reflection.

What Makes This Competitive

A strong version of this project does more than show a visible effect. You can map signal versus angle, compare multiple samples, and quantify uncertainty with repeated trials. You can also test whether one geometry gives better sensitivity than another. If you build a careful control scheme and a clean calibration curve, your work starts to look like real optical metrology.

Project Variations

• Compare different hard-drive brands or platter coatings to see whether surface finish changes Kerr signal strength.
• Test how the signal changes when you replace the laser with a different wavelength source, such as red versus green.
• Analyze the same setup with image-processing software to extract spot rotation, intensity ratio, or edge contrast as the measurement metric.

Learn More

• NIH PubMed: Search review articles on magneto-optical Kerr effect and thin-film magnetization for background physics.
• NASA NTRS: Search for optical sensing and laser alignment methods that help with setup design.
• MIT OpenCourseWare, Electromagnetism: Use course notes and lectures to review polarization, reflection, and magnetic fields.
• Journal of Applied Physics: Search for papers on MOKE measurements and thin-film magnetic characterization.
• USGS Publications Warehouse: Search for references on measurement uncertainty and experimental design in physical science projects.