Sowing Magnetic Fields

Abstract

This project aims to produce cobalt nanochains and nanowire-like structures along two perpendicular directions. Using gas-based deposition techniques based on spark-ablation, nanoparticles were deposited onto substrates which were later analyzed, either visually with a Scanning Electron Microscope or magnetically using angle-dependent remanence and coercivity measurements. Instead of the biaxial magnetic system that was expected, this work shows that depositing along two directions with our parameters seems to yield a diagonal uniaxial magnetic system. The specific qualities of the in-situ magnet used throughout the project might have caused this unexpected result. The measurements also suggest that post-annealing the nanochains purged them of their anisotropy. Future research should tune parameters such as the coverage to decrease the effect of diagonal chains on the magnetic response and image the post-annealed samples to discern what underlies their lost anisotropy.

Popular Abstract

Imagine a bar magnet made out of putty. Now stretch it along its magnetic axis. What if you didn’t stop stretching it until it was a thousand times thinner than a strand of hair, and had a diameter of tens of nanometers? What properties would such a magnet have, and how could it be useful? And is it even possible to make such a material?

Surprisingly, the answer to the last question is, well, almost. There are a range of techniques to form such super-thin magnets, or nanowires (a portmanteau of nanometer and wire) and sadly none of them use magic putty. One method is to cast them in templates with chemicals and an electric field – just like a steel plant casts steel, only on an incredibly small scale. While precise, this method also has drawbacks, including being non-continuous – each batch requires resetting the system. This takes time, and on an industrial scale could make the manufacture of these materials unprofitable. However, a new method has emerged recently that could solve this problem – gas-based deposition. The difference between gas-based and the aforementioned electro-chemical techniques is analogous to the difference between painting with spray paint and oil paint. Spray paint is practical and versatile, and though you may have more creative control with oil paints, even the world’s fastest palette-and-brush painter would struggle against the sheer gas-based torrent that spray painting allows for.

To create this mist of nanoscale particles, or nanoparticles, we apply a high voltage across two cobalt rods. This creates a lilac, eye-searing plasma spark that melts off tiny chunks of material. By selecting for the size and shape of these vaporized particles, and shepherding them with gusts of gas to a special chamber, we can create our desired nanowires. But, hang on – in the previous chemical set-up, they made the nanowires directly in-medium, and then aligned them. How is a mist of particles magically meant to turn into parallel chains? Incidentally, all we need is a magnetic field that aligns, and an electric field that attracts. Physics, in a way reminiscent of Thor and his hammer, takes care of the rest.

The physical phenomenon that makes these chains magnetically interesting goes by the name of spatial anisotropy. To understand it, let’s again imagine our bar magnet that we started with. The magnetic field lines emanate from its north pole, loop back and reenter it at its south pole. Now, stretch the bar magnet along its north-south axis, but, this time, keep in mind the field lines. As the magnet grows thinner and thinner, the magnetic field concentrates around the stretching axis. In bunching the field lines onto one axis, we are in a sense distilling and concentrating the field, reducing it until it is as strong as we want it. The end result is a magnet with a spatial anisotropy – an extreme non-uniformity that has amplified the material's existing magnetic field.

My project aims to take advantage of this effect in a new way, namely by stacking two layers of perpendicular chains. This would create a lattice of so-called nanochains that have the strange property of acting as a magnet in superposition – a system with two north and two south magnetic poles. As our mist of nanoparticles assemble into several such chains on the surface, we not only generate a material with more magnetic bang for our buck than a conventional magnet, but we also do so more efficiently – and potentially with completely new freedoms to play with. Such materials have already seen applications as surgical tools and soft robots, as paper-thin permanent magnets and in logic gates, with further developments expected in the years to come.

• author: Samuelsson, Olof
• supervisor: Rasmus Westerström; Hajar Jalili
• organization: Department of Physics; Synchrotron Radiation Research
• alternative title: A Synthesis and Study of Co Biaxial Magnetic Systems
• course: FYSK04 20251
• year: 2025
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• language: English
• id: 9204188
• date added to LUP: 2025-08-13 10:30:00
• date last changed: 2025-08-13 10:30:00