Describing spin ice, emergence of magnetic monopoles.

Figure 1: In this diagram [1], the terahedrons have four sites, each containg an ion with a magnetic dipole moment, which is oriented either toward or away from the centre of the tetrahedron. Each tetrahedron can be described as an emergent magnetic monopole, if the divergence of its B-field is non-zero.

What Do I Mean by Magnetic Monopoles?

When hearing the phrase ‘Magnetic Monopole Fluid’, you might first thinks of a fluid comprised of theoretical elementary particles with a single magnetic pole, which would be in disagreement with Maxwell’s equations, and would explain charge quantization. What you may not think of is a fluid of emergent magnetic monopoles, which have been shown to exist in certain materials, called ‘spin ice’ [3]. In these materials, which have a crystal structure characterised by corner-sharing tetrahedrons, the magnetic dipole moment (spin) of each ion, which occupy sites on the tetrahedrons, points either toward or away from the centre of each of the tetrahedrons of which it is a constituent (Figure 1). In this fashion, the lowest energy state of tetrahedron has two ions with spin ‘out’, and two with spin ‘in’. If we look at slightly excited states, we see they instead have 3 ‘in’ and 1 ‘out’ or the opposite, states which have non-zero divergence, and can be described as having positive and negative magnetic charge respectively, this charge indicates monopoles.

Dynamics

 

There are many interesting properties to these monopoles, one of which is their dynamics. These dynamics describe the motion of the emergent magnetic monopoles due to the flipping of spins, and cannot be descibed properly using the standard model of spin ice dynamics (SM), where spin-flips which create monopoles are forbidden. Instead, a model which takes into account local transverse field distributions [1] is used (bSM), which results in two different spontaneous spin-flip timescales. In looking at the monopole current generated by applied magnetic field, a very interesting dichotomy of timescales (descriptions of current decay) can be seen [2], with both a longer polarization timescale, describing the diffusion of the magnetic charge, and a reconfiguration timescale, which is only described by the bSM model. The reason that this dichotomy is so interesting is that it emerges not from interactions, or geometric constraints, but from spin configurations on an atomic scale [2].

Magnetic Monopole Applications

The applications of these emergent magnetic monopoles are related to furthering spintronics (as binary information carriers), superinsulating materials, and sensors. The potential impacts are thus potentially massive, in both classical and quantum computing, and all of technology, as well as the furthering of our physics knowledge. Another reason to be excited about this subject in Ireland, as that one of the groups doing considerable work in this subject is here, in UCC.

References

[1] Hallén, J. N., Grigera, S. A., Tennant, D. A., Castelnovo, C., & Moessner, R. (2022). Dynamical fractal and anomalous noise in a clean magnetic crystal. https://doi.org/10.17863/CAM.91960

[2] Hsu, Chun-Chih & Takahashi, Hiroto & Jerzembeck, Fabian & Dasini, Jahnatta & Carroll, Chaia & Dusad, Ritika & Ward, Jonathan & Dawson, Catherine & Sharma, Sudarshan & Luke, Graeme & Blundell, Stephen & Castelnovo, Claudio & Hallén, Jonathan & Moessner, Roderich & Davis, Jc. (2024). Dichotomous dynamics of magnetic monopole fluids. Proceedings of the National Academy of Sciences. 121. 10.1073/pnas.2320384121.

[3] Udagawa, Masafumi & Jaubert, Ludovic. (2021). Spin Ice. 10.1007/978-3-030-70860-3.

0 replies

Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply