Hugo Merbouche
Perpendicular magnetic anisotropy (PMA) has been critical for spintronic technologies. It enables the control of the magnetic energy landscape to stabilize magnetic textures, engineer faster and more reliable magnetization switching and maximize the sensitivity of magnetic sensors. PMA also strongly impacts the magnetization dynamics, and in particular nonlinear magnon-magnon interactions (m-mI). This connection has led to a quiet revolution in the magnonic community in the past few years. Understanding m-mI has become a central challenge with the prospect of efficiently generating coherent spin-waves and controlling their lifetime using spin-orbit torque (SOT) in heavy metal/ferromagnet heterostructures (HM/FM).
When electrical current flows in the HM, the SOT acts as an anti-damping torque that amplifies all thermally populated magnon modes. Because this amplification is indiscriminate, m-mI become prevalent near the critical current at which the damping is compensated, preventing the onset of auto-oscillation in extended magnetic regions or leading to the complete collapse of coherently excited waves propagating in YIG/Pt waveguides [1]. Introducing PMA in a low-damping FM insulator [2] (Bi-doped YIG), thereby nearly compensating the thin film shape anisotropy and restoring the isotropic character of the magnet, strongly decreases m-mI. It enables the emission of coherent spin-waves [2] and the formation of highly coherent magnon condensates in extended magnetic regions [3]. However, it took several more years to achieve the amplification of coherent spin-waves [4], which required efficiently decoupling the auto-oscillation from the coherent propagating spin-waves.
In the first part of my presentation, I will discuss what these experiments have taught us about m-mI, and how much remains to be understood — a point further underscored by the recent discovery that spin currents can stabilize the inverted magnetization in nearly isotropic ferromagnets [5,6]. In the second part, I will approach this open question from a different angle, considering a very simple system: a nearly isotropic BiYIG disk, magnetized out-of-plane and driven near its resonance at 5 GHz by an in-plane rf field. In this rotationally symmetric system, there is no precession ellipticity, no propagation and the magnon spectrum is discrete. However, its dynamics is deceptively complex [7], exhibiting self-modulation instability, chaos and weak-turbulence, and finely depends on the PMA. This model system is a great starting point to unravel the role of PMA, and many other parameters, in the peculiarities of nonlinear magnetization dynamics in nearly isotropic ferromagnets.
[1] M. Evelt et al., High-efficiency control of spin-wave propagation in ultra-thin yttrium iron garnet by the spin-orbit torque, Appl. Phys. Lett. 108, 172406 (2016).
[2] M. Evelt et al., Emission of Coherent Propagating Magnons by Insulator-Based Spin-Orbit-Torque Oscillators, Phys. Rev. Applied 10, 041002 (2018).
[3] B. Divinskiy et al., Evidence for spin current driven Bose-Einstein condensation of magnons, Nat Commun 12, 1 (2021).
[4] H. Merbouche, B. Divinskiy, D. Gouéré, R. Lebrun, A. El Kanj, V. Cros, P. Bortolotti, A. Anane, S. O. Demokritov, and V. E. Demidov, True amplification of spin waves in magnonic nano-waveguides, Nat Commun 15, 1560 (2024).
[5] H. Kurebayashi et al., Dynamical stability by spin transfer in nearly isotropic magnets, Nat. Mater. (2026).
[6] E. Karadza, H. Wang, N. Kercher, P. Noel, W. Legrand, R. Schlitz, and P. Gambardella, Dynamical Stabilization of Inverted Magnetization and Antimagnons by Spin Injection in an Extended Magnetic System, arXiv:2601.09569.
[7] I. Ngouagnia Yemeli et al., Self-Modulation Instability in High Power Ferromagnetic Resonance of BiYIG Nanodisks, Phys. Rev. Lett. 135, 056703 (2025).