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omega_minus

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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.