From bits to bacteria: Measuring magnetic properties at the nanoscale

‘We are looking at magnetism at the nanoscale,’ postdoc researcher Irina Dolgikh explains. ‘In order to study the magnetic moment of individual particles that are much smaller than the diffraction limit, we are working on new techniques that can be used in a regular lab. Current existing techniques to do this are very complicated and expensive, and some of them require the use of synchrotron radiation facilities. This method, as pioneered by Michel Orrit, is an easy-to-use, readily accessible, and affordable alternative that offers a similar resolution.’

In fact, the setup the two postdoctoral researchers are working on can be assembled in any optics lab, her colleague Mariia Efremova adds. ‘It can also be used outside of an academic lab, for example, in an industrial setting to test devices,’ Dolgikh adds. ‘Besides the standard electronics, e.g., a lock-in amplifier, all you need are two lasers with sufficient power, some (polarization) optical components, a modulator, a magnet, an objective, and a photodiode. The experimental setup is published for anyone interested in reproducing,’ the postdocs stress.

The setup enables two types of measurements: photothermal circular dichroism (PT CD), and its extended version – photothermal magnetic circular dichroism (PT MCD). Dolgikh explains the magnetic measurement: ‘With a first laser beam, we heat the sample with alternating left- and right-circularly polarized light. If the particle under study has a magnetic moment aligned with the laser propagation direction, it absorbs these two types of polarized light differently due to the magnetic circular dichroism. This translates into a difference in heat transfer from the nanoparticle to its environment, resulting in a change in the refractive index of its direct surroundings. By probing the sample with a second laser, changes in the light intensity correspond to changes in the value of the magnetic moment of the particle.’ PT CD relies on the same physical principle, but the polarization state of the first laser is not important, and the second laser only probes the total heating effect. By the latter, the asymmetry in the shape of a single particle (for example, gold) or even a single molecule can be investigated.

Within the scope of the Gravitation project on Integrated Nanophotonics, the method can be utilized, for example, to monitor the switching of magnetic particles in spintronic memories to probe magnetism in nanoparticle chains. This can be used for the creation of logic devices, to investigate the magnetic nanoparticles for STT MRAM. ‘But it is also promising for a wide variety of other fields,’ Efremova stresses. ‘We already used the PT MCD technique to study synthetic antiferromagnetic nanoparticles, which are very important for future biomedical applications. These nanoparticles have a preferential orientation of their magnetic moment (out-of-plane). When we apply a rotating external magnetic field, the particles rotate as well to keep the energetically favorable configuration. If the particles are anchored, for example, at the cell membrane, this principle can be applied for the magneto-mechanical manipulation of cells for e.g. cancer treatment, which is a field I am personally very interested in.’

One of the main advantages of the newly developed technique is that it can be used to study the magnetic behavior of single particles, both researchers stress. ‘The standard techniques that are currently available measure the averaged behavior of particle ensembles. Innovatively, we can measure the specifics of one (magnetic) particle, with a size down to a few nanometers. Additionally, we can compare the heating behavior of small aggregates with variable particle numbers, for example, to evaluate the heating potential of various magneto-tactic bacteria in magnetic hyperthermia.’

The principle of photothermal magnetic circular dichroism was originally developed by a research group at Leiden University, led by Spinoza Award winner Michel Orrit. ‘When he retired, we brought the setup to Eindhoven for further development because we saw great potential in it,’ Efremova says. With a series of experiments, the researchers have delivered the definitive proof of concept. In a mini-review article that was recently published in Nano Letters, they also provided an overview of its possible areas of application. ‘And we are expanding the technique even further, to also include, for example, the possibility to measure in-plane magnetization, which is not detectable in the current set-up,’ Dolgikh says. ‘That will allow characterizing essentially any type of magnetic structures since their magnetic moment direction can be arbitrary.’

Albeit very promising, the technology is still in the exploration phase, the postdocs stress. ‘Now, the time has come to gear up a notch and collaborate with people within TU/e to further push the frontiers of this method and explore other application areas. We are eager to get into contact with colleagues to jointly explore the possibilities.’