This thesis investigates the role of angular momentum transfer in the coupling between elastic and spin excitations in magneto-mechanical systems.
In axisymmetric geometries, rotational invariance provides the natural framework for the conservation of total angular momentum[1]. In such a case, this quantity unambiguously labels the eigenmodes of the system and constrains their coupling to an external excitation through well-defined selection rules[2]. The manuscript develops this physical picture from a general perspective based on Noether’s theorem, it introduces spin and orbital angular momentum contributions for a generic vector field and proposes an analytical description of magneto-elastic coupling in axisymmetric systems.
This framework is then applied to two complementary experimental platforms realized in the context of this doctoral work. The first combines chiral surface-acoustic excitations with a magnetic vortex texture in a patterned ferromagnetic disk. The second consists of partially suspended axisymmetric YIG microresonators, designed to support both low-loss mechanical motion and confined spin dynamics within the same structure[3]. Together, these two systems provide complementary routes to investigate how elastic and magnetic modes carrying angular momentum can be generated, controlled, and coupled in realistic devices.
In particular, this work demonstrates the controlled excitation and optical mapping of a surface-acoustic vortex by means of spiral interdigitated transducers. It further addresses the stabilization of magnetic vortex textures in micrometer-scale ferromagnetic disks on anisotropic piezoelectric substrates, as well as their positioning at the center of elastic rotation. The coupling strength in suspended axisymmetric YIG microresonators has been evaluated by comparing experimental data with theoretical modeling, indicating a weak-coupling regime. Moreover, the role of weak symmetry breaking has been discussed for the selective excitation of counter-propagating elastic modes that would otherwise be degenerate.
These developments establish several of the physical and technological conditions required to address experimentally the coupling of elastic and spin excitations carrying angular momentum. More broadly, they outline a route toward magneto-elastic platforms in which symmetry can be used as a resource for selective excitation. In this perspective, partially suspended magnetic microresonators may provide a promising route toward strong magnon-phonon coupling under geometrical and symmetry constraints directly relevant for coherent functionalities. In the longer term, such systems could serve as building blocks for mode-selective transducers and hybrid architectures combining magnetic, elastic, and optical degrees of freedom, with possible perspectives for microwave-to-optical coherent transduction[4].

References

1. Garanin & Chudnovsky, Phys. Rev. B 92, 024421 (2015). 
2. An et al., Phys. Rev. B 101, 060407 (2020). 
3. Heyroth et al., Phys. Rev. Appl. 12, 054031 (2019). 
4. Engelhardt et al., Phys. Rev. Appl. 18, 044059 (2022). ​

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