To the extent that you’re familiar with magnetostriction, you probably know that it’s what makes big transformers hum, or that it’s what tips you off if you happen to walk out of a store without paying for something. But magnetostriction has other uses, too, such as in this clever linear position sensor.

Magnetostriction is just the tendency for magnetic materials to change size or shape slightly while undergoing magnetization, thanks to the tiny magnetic domains shifting within the material while they’re aligning. [Florian B.]’s sensor uses a side effect of magnetostriction known as the Wiedenmann effect, which causes a wire to experience a twisting force if a current pulse is applied to it in a magnetic field. When the current pulse is turned off, a mechanical wave travels along the wire to a coil, creating a signal. The difference in time between sending the pulse and receiving the reflection can be used to calculate the position of the magnet along the wire.

To turn that principle into a practical linear sensor, [Florian B.] used nickel wire stretched tightly down the middle of a PVC tube. At one end is a coil of copper magnet wire, while the other end has a damper to prevent reflections. Around the tube is a ring-shaped cursor magnet, which can move up and down the tube. An exciter circuit applies the current pulse to the wire, and an oscilloscope is used to receive the signal from the wire.

This project still appears to be in the prototype phase, as evidenced by the Fischertechnik test rig. [Florian] has been working on the exciter circuit most recently, but he’s done quite a bit of work on optimizing the cursor magnet and the coil configuration, as well as designs for the signal amplifier. It’s a pretty neat project, and we’re looking forward to updates.

If you need a deeper dive into magnetostriction, [Ben Krasnow] points the way.

Although the Earth’s magnetic field is reliable enough for navigation and is also essential for blocking harmful solar emissions and for improving radio communications, it’s not a uniform strength everywhere on the planet. Much like how inconsistencies in the density of the materials of the planet can impact the local gravitational force ever so slightly, so to can slight changes impact the strength of the magnetic field from place to place. And it doesn’t take too much to measure this impact on your own, as [efeyenice983] demonstrates here.

To measure this local field strength, the first item needed is a working compass. With the compass aligned to north, a magnet is placed with its poles aligned at a right angle to the compass. The deflection angle of the needle is noted for varying distances of the magnet, and with some quick math the local field strength of the Earth’s magnetic field can be calculated based on the strength of the magnet and the amount of change of the compass needle when under its influence.

Using this method, [efeyenice983] found that the Earth’s magnetic field strength at their location was about 0.49 Gauss, which is well within 0.25 to 0.65 Gauss that is typically found on the planet’s surface. Not only does the magnetic field strength vary with location, it’s been generally decreasing in strength on average over the past century or so as well, and the poles themselves aren’t stationary either. Check out this article which shows just how much the poles have shifted over the last few decades.

It’s a well-known fact that anti-ferromagnetic materials are called that way because they cannot be magnetized, not even in the presence of a very strong external magnetic field. The randomized spin state is also linked with any vibrations (phonons) of the material, ensuring that there’s a very strong resistance to perturbations. Even so, it might be possible to at least briefly magnetize small areas through the use of THz-range lasers, as they disrupt the phonon-spin balance sufficiently to cause a number of atoms to ‘flip’, resulting in a localized magnetic structure.

The research by [Baatyr Ilyas] and colleagues was published in Nature, describing the way the 4.8 THz pulses managed to achieve this feat in FePS₃ anti-ferromagnetic material. The change in spin was verified afterwards using differently polarized laser pulses, confirming that the local structures remained intact for at least 2.5 milliseconds, confirming the concept of using an external pulse to induce phonon excitation. Additional details can be found in the supplemental information PDF for the (sadly paywalled with no ArXiv version) paper.

As promising as this sounds, the FePS₃ sample had to be cooled to 118K and kept in a vacuum chamber. The brief magnetization also doesn’t offer any immediate applications, but as a proof of concept it succinctly demonstrates the possibility of using anti-ferromagnetic materials for magnetic storage. Major benefit if such storage can be made more permanent is that it might be more stable and less susceptible to outside influences than traditional magnetic storage. Whether it can be brought out of the PoC stage into at least a viable prototype remains to be seen.