Tiny permanent moments shape microparticle dynamics

Researchers from the University of Oxford and Colorado School of Mines have shown how subtle magnetic structure within microscopic particles can strongly influence the way they move.

The team’s results highlight how even nanoscopic magnetic features can have a pronounced effect on the dynamics of much larger structures. Their study is featured on the front cover of the latest edition of Soft Matter.

Superparamagnetic microparticles are tiny polystyrene spheres – roughly 20 times smaller than the thickness of a human hair – which have thousands of magnetic nanoparticles randomly embedded inside them. Without a magnetic field, these particles undergo normal diffusion like other microscopic particles. When a magnetic field is applied, the particles become magnetised, acquiring a net magnetic moment that causes the particles to interact strongly and form chains where neighbouring particles’ magnetic moments align north-to-south.

In many previous studies, these particles are described using a relatively simple picture: their magnetic behaviour is considered to be dominated by a moment induced by the external field. At the simplest level, this induced moment remains aligned with the applied field. However, many applications of these particles use them to apply a rotational force (magnetic torque), which requires the moment to be misaligned to the external field. This behaviour suggested something more complicated than the induced-only model must be happening.

The origin of this magnetic torque has been an ongoing subject of debate in the literature. One potential ingredient that could introduce a magnetic torque is a permanent moment, i.e. a moment fixed to the body of the particle, rather than the external field. However, such moments have often been neglected in simple descriptions, because they are so much weaker than the induced contribution.

Now, a team in Oxford Chemistry has shown that these extremely weak permanent moments can have a strong impact on the orientational dynamics of particle structures.

In their new work, they studied pairs of magnetic microparticles permanently joined together to form dimers. By tracking individual dimers under a microscope, they observed in detail how their orientation changed in response to an applied magnetic field.

Under a constant field, the induced dipole model predicts that the dimers would simply align to the external field. However, the researchers observed that these dimers had two stable states, and switched between them spontaneously.

Alongside this, when the magnetic field direction was switched from up to down, they observed the dimers respond in two ways. Either the dimer did a full 180-degree flip or, at higher field strengths, a small hop (rotation of less than 45 degrees).  Which of these two responses the team observed depended on the strength of the magnetic field, like a switch, with the transition occurring over a very narrow range of field strengths.

These results could be explained if the particles possess an extremely weak permanent moment alongside the usual field-induced magnetism. Although weak compared to the induced moment, this moment can substantially alter the preferred orientations and rotational dynamics of the entire dimer. This strong impact of nanomagnetic structure on the orientational dynamics allowed the researchers to infer the strength of both permanent and induced components, which are often difficult to disentangle in standard bulk measurements.

James Tett, a DPhil student in Professor Alice Thorneywork's group at Oxford, said:

What surprised us was how strongly such a small magnetic moment could influence the motion of the whole particle structure. Whilst these particles can often be treated via a fairly simple picture, their rotational dynamics reveal that there is a much richer picture hidden inside them.

Looking ahead, the team is interested in understanding the dynamics of these systems further.

Prof Alice Thorneywork, said:

These kinds of magnetic particles find varied application in microscale devices across biophysics and soft robotics. Our work identifies and rationalises a novel dynamic response in this kind of system, which can help us to optimise particle structures for different applications.

Ultimately, similar structures with several controllable stable states could act as basic units for microscopic machines. The researchers envisage that such units could help enable more complex motion for both applied and fundamental physical insights.

The work was carried out by James Tett, Finlay Johnston, and Prof Alice Thorneywork at the University of Oxford, as well as Prof Brennan Sprinkle at the Colorado School of Mines (US). The paper, Orientational bistability and field-controlled switching of a superparamagnetic dimer, is published in Soft Matter this week, and features on the front cover of the journal.

Header image: Complex energy landscapes arising from the interplay of induced and permanent magnetic moments give rise to multiple stable dimer orientations. Image credit: Prof Brennan Sprinkle.