Giovanni Olivetti of SPINTEC will defend his PhD thesis, “Angular Momentum Transfer Between Waveforms in Axisymmetric Geometries,” on June 12th at 14:00 in the seminar room of Bâtiment A at CNRS Institut Néel, Grenoble. A reception will follow in the same building. Access to the CNRS site is restricted; attendees must send their full name to giovanni.olivetti@neel.cnrs.fr or benjamin.pigeau@neel.cnrs.fr at least 48 hours beforehand to receive an entry clearance.

The thesis explores how angular momentum is exchanged between elastic and spin excitations in magneto-mechanical systems. In axisymmetric structures, rotational symmetry enforces total angular momentum conservation, which rigorously labels eigenmodes and imposes selection rules on their coupling. Building on Noether’s theorem, the work introduces spin and orbital angular momentum contributions for generic vector fields and develops an analytical description of magneto-elastic coupling in such geometries.

Two complementary experimental platforms were realized to test this framework. The first uses chiral surface-acoustic waves coupled to a magnetic vortex texture in a patterned ferromagnetic disk. The second employs partially suspended, axisymmetric YIG microresonators that confine both low-loss mechanical motion and spin dynamics within a single structure. Together, they provide distinct routes to generate, control, and couple angular-momentum-carrying elastic and magnetic modes.

Key experimental results include the controlled excitation and optical mapping of a surface-acoustic vortex via spiral interdigitated transducers. The work also addresses stabilizing magnetic vortex textures at the center of elastic rotation in micron-scale ferromagnetic disks on anisotropic piezoelectric substrates. For the YIG microresonators, comparison with theory reveals a weak-coupling regime, and the role of weak symmetry breaking in selectively exciting otherwise degenerate counter-propagating elastic modes is discussed.

These achievements lay the physical and technological groundwork for experimentally coupling elastic and spin excitations that carry angular momentum. More broadly, they chart a path toward magneto-elastic platforms where symmetry becomes a resource for selective excitation. Partially suspended magnetic microresonators, in particular, offer a promising route to strong magnon-phonon coupling under constraints relevant for coherent functionalities. In the longer term, such systems could underpin mode-selective transducers and hybrid architectures that merge magnetic, elastic, and optical degrees of freedom, with potential applications in microwave-to-optical coherent transduction.

The jury comprises Silvia Viola Kusminskiy (RWTH Aachen) and Massimiliano Marangolo (Sorbonne Université) as referees, with Liliana Buda-Prejbeanu (Université Grenoble Alpes), Saul Vélez Centoral (Universidad Autónoma de Madrid), Philipp Pirro (RPTU Kaiserslautern-Landau), and Sebastian Goennenwein (Universität Konstanz) as examiners. Yoshichika Otani (The University of Tokyo) is an invited member. The thesis was supervised by Olivier Klein (Spintec, CEA Grenoble) and co-supervised by Benjamin Pigeau (Institut Néel, CNRS).

On June 12th at 14:00, Giovanni Olivetti (SPINTEC) will defend his PhD thesis entitled : Angular Momentum Transfer Between Waveforms in Axisymmetric Geometries

Place : CNRS Institut Néel, salle des Séminaires of Bâtiment A , the “pot de thèse” will be afterwards in the salle de Convivialité of Bâtiment A.

Access : to the CNRS site is restricted. Please contact me giovanni.olivetti@neel.cnrs.fr or benjamin.pigeau@neel.cnrs.fr at least 48h before the defense if you need an entry clearance. You just need to communicate to us your name and surname, and you will receive the entry clearance by email.

Abstract : 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 :

Garanin & Chudnovsky, Phys. Rev. B 92, 024421 (2015).

An et al., Phys. Rev. B 101, 060407 (2020).

Heyroth et al., Phys. Rev. Appl. 12, 054031 (2019).

Engelhardt et al., Phys. Rev. Appl. 18, 044059 (2022).

Jury :

Mme. SILVIA VIOLA KUSMINSKIY, RWTH Aachen University – Referee

M. MASSIMILIANO MARANGOLO, Sorbonne Université – Referee

Mme. LILIANA BUDA-PREJBEANU, Université Grenoble Alpes – Examiner

M. SAUL VÉLEZ CENTORAL, Universidad Autónoma de Madrid – Examiner

M. PHILIPP PIRRO, Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau – Examiner

M. SEBASTIAN GOENNENWEIN, Universität Konstanz – Examiner

Invited :

YOSHICHIKA OTANI, The University of Tokyo

Thesis supervisors :

OLIVIER KLEIN, Spintec, CEA Grenoble, superviser

BENJAMIN PIGEAU, Institut Néel, CNRS, Grenoble, co-superviser

The post PhD Defense – Angular Momentum Transfer Between Waveforms in Axisymmetric Geometries appeared first on Spintec.