Nonlinear magnonics in nanostructures
(coll. Lab. A. Fert, Lab-STICC, C2N, U. Naples)
Magnetization dynamics in ferromagnetic materials is highly nonlinear. Depending on the situation, these nonlinear effects can either be beneficial or detrimental to magnonic devices, which leverage on spin-wave dynamics to process information. It is therefore important to understand and control them in nanostructures. To this aim, we have developed a two-tone spectroscopy technique, which enables to study deeply nonlinear ferromagnetic resonance in individual nanomagnets (Fig.1). We have demonstrated that the strongly out-of-equilibrium regime, which develops at large pumping power, crucially depends on the effective anisotropy of garnet nanodisks. In YIG with in-plane anisotropy, the onset of spin-wave instabilities and turbulent dynamics is pushed back at large amplitude, because the geometric confinement drastically reduces the density of spin-wave modes, thereby limiting their mutual nonlinear interactions[1]. In BiYIG with vanishing effective anisotropy, we evidenced in contrast that an auto-oscillation instability involving a few normal modes rapidly occurs (ANR project Maestro). We also started to investigate the possibility to use nonlinear spin-wave dynamics in a nanostructure to perform neuromorphic computing in the reciprocal space, the eigenmodes playing the role of neurons, and their nonlinear interactions that of synapses[2] (EU project k-NET and ANR project Marin).
[1] Li, Y., et al. Phys. Rev. X 9, 041036 (2019) 10.1103/PhysRevX.9.041036
[2] Srivastava, T., et al. Phys. Rev. Appl. 19, 064078 (2023) 10.1103/PhysRevApplied.19.064078
Figure 1: (a) Schematics of the MRFM experiment and deeply nonlinear ferromagnetic resonance pumped at large power in a YIG nanodisk (diameter 700 nm, thickness 20 nm). (b) Two-tone spectroscopy: an additional low power microwave field excites resonant peaks, corresponding to a motion of nutation in the laboratory frame.
Perspectives: We will continue to explore nonlinear magnetization dynamics in nanostructures, with an emphasis on their use for unconventional information processing. We have recently proven that parametrically excited spin-wave modes in in-plane magnetized YIG microdisks interact in a non-commutative manner, i.e., the dynamical state depends on which of the modes is first pumped. We will attempt to use this property to perform basic neuromorphic tasks such as simple signal recognition. Based on ideas from numerical models developed by collaborators, we will later investigate learning strategies for spin-wave based neural networks. In parallel, we will contribute to the moonshot project “rf spintronics for smart communications” of the PEPR SPIN, where we want to exploit the nonlinear dynamics driven in nanomagnets by microwave fields and spin transfer torques for the generation of true random numbers using chaos and for the preprocessing of complex microwave signals. For this, real-time magnetoresistive detection will be implemented after integrating the nanomagnet in magnetic tunnel junctions (collaboration with Spintec). At LNO, we will also use magnetoresistive sensors for the local detection of spin-waves by integrating them at the tip of micro-levers, in synergy between the magnonics team and the sensors team. This would provide a sensitive and versatile time-resolved detection of magnonic devices. On the excitation side, we will continue to investigate the coupling between spin-waves and acoustic waves in hybrid devices (ANR project Maxsaw, EU project Palantiri) to improve the efficiency of spin-wave excitation and to add novel functionalities to magnonics. On the longer term, there remains much fundamental physics to study in magnetization dynamics at the nanoscale, a class of highly nonlinear dynamic systems. In particular, some situations encountered in nonlinear magnonics are analogous to fluid dynamics in the presence of turbulent motion, which we will explore both in experiment and in simulation, in the hope of finding fruitful connections between these domains in collaboration with the SPHYNX group of SPEC.
All these efforts will benefit from the recent hiring of a young permanent researcher in the group (Hugo Merbouche, since 11/2023), and will be made possible by projects already in progress and the submission of new ones.
Conversion of Spin and orbital currents into charge
Several effects are at play when inter-converting spin and charge. In the bulk, the main one is the spin Hall effect but the record efficiency is obtained in Rashba 2D states at interfaces, also called the Edelstein effect. We have tried to extend these now well-known effects along two paths: the first one consists in going to very short picosecond timescales, and the second one attempts to demonstrate that spin effects can be advantageously replaced by orbital effects in some systems.
- Ultrafast spintronics [coll. Lab. Albert Fert, Synchrotron SOLEIL, IMMM, MBI] :
A significant endeavor has been focused on addressing several aspects of ultrafast spintronics. Between 2018 and 2023, this effort has involved two PhD students (T. Chirac and S. René), a postdoctoral fellow (A. Levchuk) and one dedicated ANR (JCJC SpinUP). A central part of our work has been the advancement of state-of-the-art time-resolved optical setups enabling us to explore the physics underlying the generation, propagation and detection of spin currents at picosecond and sub-picosecond timescales. Besides, a substantial amount of work has been invested in tailoring well-controlled thin layer heterostructures by pulsed laser deposition (e.g. NiO, La2/3Sr1/3MnO3) and sputtering (e.g. CoFeB, Pt, Al…) in the group, complemented by a long-standing collaboration with the LAF.
Recent studies have addressed issues such as subtle differences in spin/charge conversion mechanisms at ultrashort timescales[1] (coll IMMM), the role of ultrafast spin currents in the dynamics of meandered magnetic textures[2] (coll. LAF, SOLEIL) or the element specific ultrafast quench in magnetic multilayers[3] (coll. MBI). Finally, antiferromagnets and their terahertz dynamics are one of the pillars of ultrafast spintronics. After a simulation study[4], our current effort is in the experimental study of ultrafast transfer mechanisms of spin angular momentum to antiferromagnetic THz magnons (PEPR SPIN).
- Orbital currents to charge conversion (coll. Uni. Genève and GMT-SPEC) :
The spin orbit interaction plays an important role in magnetism and for many quantum materials. It is the cornerstone of the physics of many electronic phases emerging at interfaces because of broken inversion symmetry. In that context, it is known that the ‘spin Rashba’ effect locks electronic spins with momentum to produce large spin galvanic effects, i.e. spin to charge conversion. In the past five years, it has been argued that these spin-based effects could in fact be a spin-orbit coupling consequence of a potentially larger orbital effect. This field of research has potential to boost spintronic devices to larger outputs. One of the best systems to harvest orbital effects has been identified to be the electronic state present at the LaAlO3/SrTiO3 interface (samples from the University of Geneva). Using a detailed analysis of the in-plane anisotropy and gate voltage dependence of the angular momentum conversion into charge, along with a comparison to calculations carried out in the GMT group, we have been able to demonstrate that indeed the conversion is dominated by orbital effects[5]. This provides evidence that the Rashba splitting in this system stems from orbital textures in k space. These results open the door to a broader use of pure orbital angular momentum effects and confirm the potential of the orbital degree of freedom for information storage and processing.
[1] Levchuk, A. et al. Appl. Phys. Lett. 123, 012407 (2023).
[2] Léveillé, C. et al. Nat. Commun. 13, 1412 (2022).
[3] Von Korff Schmising, C. et al. Phys. Rev. Res. 5, 013147 (2023).
[4] Chirac, T. et al., Phys. Rev. B 102, 134415 (2020).
[5] Anas El Hamdi et al., Nature Physics 19, nᵒ 12, 1855 (2023).
Figure 2: a) Schematics of a CoFeB based THz emitter pumped by a 800 nm femtosecond laser pulse b) & c) Wavelength dependence of emitted THz signals for inverse spin Hall and inverse Edelstein effects (From Levchuk et al.).
Perspectives: We will continue to explore nonlinear magnetization dynamics in nanostructures, with an emphasis on their use for unconventional information processing. We have recently proven that parametrically excited spin-wave modes in in-plane magnetized YIG microdisks interact in a non-commutative manner, i.e., the dynamical state depends on which of the modes is first pumped. We will attempt to use this property to perform basic neuromorphic tasks such as simple signal recognition. Based on ideas from numerical models developed by collaborators, we will later investigate learning strategies for spin-wave based neural networks. In parallel, we will contribute to the moonshot project “rf spintronics for smart communications” of the PEPR SPIN, where we want to exploit the nonlinear dynamics driven in nanomagnets by microwave fields and spin transfer torques for the generation of true random numbers using chaos and for the preprocessing of complex microwave signals. For this, real-time magnetoresistive detection will be implemented after integrating the nanomagnet in magnetic tunnel junctions (collaboration with Spintec). At LNO, we will also use magnetoresistive sensors for the local detection of spin-waves by integrating them at the tip of micro-levers, in synergy between the magnonics team and the sensors team. This would provide a sensitive and versatile time-resolved detection of magnonic devices. On the excitation side, we will continue to investigate the coupling between spin-waves and acoustic waves in hybrid devices (ANR project Maxsaw, EU project Palantiri) to improve the efficiency of spin-wave excitation and to add novel functionalities to magnonics. On the longer term, there remains much fundamental physics to study in magnetization dynamics at the nanoscale, a class of highly nonlinear dynamic systems. In particular, some situations encountered in nonlinear magnonics are analogous to fluid dynamics in the presence of turbulent motion, which we will explore both in experiment and in simulation, in the hope of finding fruitful connections between these domains in collaboration with the SPHYNX group of SPEC.
All these efforts will benefit from the recent hiring of a young permanent researcher in the group (Hugo Merbouche, since 11/2023), and will be made possible by projects already in progress and the submission of new ones.