Rotational dynamics of mono-domain magnetic nanoparticles

[omega_minus]

Hi,

I have a question about the rotation of a single-domain magnetic nanoparticle that is suspended in a ferrofluid immersed in an external field. Specifically, I am trying to work out the path that a normal vector on the surface of the sphere traces out in time.

There are 2 ways the magnetization vector inside the particle could align itself with the external field. The first is by the magnetization vector rotating relative to the crystal axes as described by the Landau-Lifschitz-Gilbert equation (LLG) (basically damped Larmor precession). This vector would describe a decaying spiral around the effective field. It would not simply align itself in the field like a compass needle.

The 2nd way to align itself with the external field would be if the physical particle rotated in the carrier liquid assuming the magnetization vector is locked relative to the crystal axes (as in very high anisotropy for example).

My exact question then is this:

If the reorienting of the particle is done solely through physical rotation, does the particle precess or does it simply rotate like a compass needle?

I have read several papers on the Brownian rotation of particles in ferrofluids but many attribute the rotation to thermal noise (like actual Brownian rotation). I am more interested in the effect of the magnetic field on the torque than thermal noise but no one seems to address this directly.

I think my confusion is rooted in the fact that the torque on the magnetization vector is such that it undergoes precession via LLG , and this is (I think) the only torque that acts on the single-domain particle (via its assumed high anisotropy) and so should cause the particle to "warble" (precess). But I can't prove it and although the frequency is very high and damping very quick, I have never seen an extended magnetic object precess in the macroscopic world. So I'm not sure if precession is an internal mechanism only. But even if it gets "averaged out" on macroscopic objects, my particle is mono domain and 10nm in diameter...

Thanks in advance for any insights. Let me know if I can clarify anything above.

 

[eljose79]

Hi there,

Thank you for your question. I am a scientist who specializes in the study of magnetic nanoparticles, and I would be happy to provide some insights on this topic.

First of all, you are correct that there are two main ways in which a single-domain magnetic nanoparticle can align itself with an external magnetic field. The first is through the rotation of the magnetization vector relative to the crystal axes, as described by the Landau-Lifschitz-Gilbert (LLG) equation. This results in a decaying spiral motion of the magnetization vector around the effective field.

The second way is through the physical rotation of the particle itself, assuming that the magnetization vector is locked relative to the crystal axes. This is typically seen in particles with very high anisotropy, where the magnetization vector is strongly aligned with the crystal axes.

Now, to answer your question about whether the particle precesses or simply rotates like a compass needle in this scenario, the answer is that it depends on the specific conditions and parameters of the system. In general, the particle will experience both precession and rotation, but the relative contributions of each may vary.

For example, if the particle is in a high viscosity medium, such as a ferrofluid, and the external field is not very strong, then the precession may be very slow and the particle will mainly rotate like a compass needle. On the other hand, if the particle is in a low viscosity medium and the external field is strong, then the precession may dominate and the particle will exhibit more of a spiraling motion.

In terms of the effect of the magnetic field on the torque, it is important to consider that there are multiple torques acting on the particle, including the torque from the external field and the torque from the LLG equation. The exact balance between these torques will determine the behavior of the particle.

In terms of the precession of extended magnetic objects in the macroscopic world, it is true that it is not commonly observed due to the high frequency and quick damping. However, as you mentioned, with a single-domain particle that is only 10nm in diameter, the precession may be more easily observed.

I hope this helps to clarify your understanding. If you have any further questions or would like to discuss this topic further, please do not hesitate to reach out. Best of luck with your research!