Collective properties of magnetic mesospins
- Location: Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala
- Doctoral student: Östman, Erik
- About the dissertation
- Organiser: Materialfysik
- Contact person: Östman, Erik
Mesoscopic spin systems consist of an ensemble of lithographically patterned nanomagnetic elements - mesospins. The interactions between the mesospins, can be designed at will by altering their lateral arrangement, enabling the study of collective magnetic order in a wide range of systems.
The spin dimensionality of a mesospin is controlled by its shape and form. Thin elongated elements are Ising-like, with only two possible magnetization directions. Disc shaped elements can be single domain and behave XY-like, with a magnetization direction free to rotate in the plane of the disc. Larger disc sizes result in magnetic vortices. Tuning the material parameters of the elements enables mesospin dynamics at and below room temperature. Combining all of the above, the magnetic state of a lattice is then defined by the mesospins lateral arrangement, their spin dimensionality, and the temperature.
In this Thesis we investigate the magnetic order and dynamic properties in a series of different configurations, where the nano-magnetic elements are in the vortex state, Ising-like mesospins or of mixed mesospin dimensionality. Chains of Ising-mesospins were investigated and shown to be successfully described by the Ising model. A lossless transition between the magnetic vortex state and the collinear state, was found in square arrays of magnetic discs. In a more complicated interaction regime, square artificial spin ice, the dynamical range of the Ising-like mesospins in the lattice was probed, in terms of magnetization relaxation studies.
Utilizing the configurational freedom in mesoscopic spin systems, together with the possibility to alter the spin dimensionality of the elements, it is possible to create a lattice with no naturally occurring analogue. In such a lattice, where XY mesospins were added to square artificial spin ice, it was found that the degeneracy of the square ice model was restored. Furthermore, using a reciprocal space analysis tool, the magnetic spin structure factor, the system was shown to possess the characteristic features of a Coulomb spin liquid with strong local correlations and absence of long range order. Increasing the interaction between the elements, results in an emergent magnetic order on a large length-scale.