Research highlights
1. Quantum magnonics
Scientists couple magnetization to superconductivity for quantum discoveries
"Researchers at Argonne have demonstrated an on-chip quantum circuit and realized strong coupling between a superconducting resonator and a magnetic device. The results introduce a new platform for investigating on-chip quantum magnonics and quantum information processing. "
Source: Press Release | Argonne National Laboratory, Sep. 2019
Strongly coupled magnon-photon hybrid system in a superconducting circuit
We demonstrate strong magnon-photon coupling of a thin-film permalloy device fabricated on a superconducting resonator. Our results suggest the combination of superconducting resonator and metallic ferromagnets can be a promising platform for investigating on-chip quantum magnonics and spintronics, and brings new potential for coherent manipulation and long-distance propagation of spins.
See: Yi Li, T. Polakovic, Y.-L. Wang, J. Xu, S. Lendinez, Z. Zhang, J. Ding, T. Khaire, H. Saglam, R. Divan, J. Pearson, W.-K. Kwok, Z. Xiao, V. Novosad, A. Hoffmann and W. Zhang, Strong magnon-photon coupling in ferromagnet-superconducting resonator thin-film devices, Physical Review Letters, 121, 107701 (2019) (Editors' Suggestion, Featured in Physics)
Strongly coupled magnon-magnon hybrid system
We demonstrate strong magnon-magnon coupling in a exchange-coupled magnetic bilayer system. In addition, we identify a dampinglike torque from spin pumping, which can be utilized to suppress the decoherence of the magnon-magnon hybrid mode. Our results reveal new insight for tuning the coherence in magnon-magnon hybrid dynamics and are important for magnon-based coherent information processing.
Yi Li, W. Cao, V. P. Amin, Z. Zhang, J. Gibbons, J. Sklenar, J. Pearson, P. M. Haney, M. D. Stiles, W. E. Bailey, V. Novosad, A. Hoffmann, and W. Zhang, “Coherent spin pumping in a strongly coupled magnon-magnon hybrid system” Physical Review Letters, 124, 117202 (2020)
2. Magnetic nano-devices
Probing the coupling of two Spin-Torque Nano-Oscillators (STNOs)
The auto-oscillation dynamics of two STNOs are studied. With finite coupling, they can synchronize. However due to the complexity of synchronization, the coupling strength are never quantified. Here we use an external rf source (rf field in this work) to probe the coupling. A remote frequency pulling is measured (see the Fig.) when the rf field synchronizes the other STNO. Fitting to an analytical formula that we have developed, the coupling strength is extracted and agrees well with theoretical calculations.
See: Yi Li, X. de Milly, F. Abreu Araujo, O. Klein, V. Cros, J. Grollier and G. de Loubens, Probing phase coupling between two spin-torque nano-oscillators with external source, Phys. Rev. Lett., 2017
Selective control of two synchronized STNOs
The ability to manipulate synchronization is important for applications of coupled oscillator networks, such as mimicking neural networks. In this work we demonstrate selective control of two strongly coupled vortex STNOs by using an external microwave field. Surprisingly, the control of individual STNO states is not sensitive to their strong coupling. This provides a device-selective, coupling-insensitive and channel-sharing polarity switching technique, which can be an important technical requirement in small and densely packed STNO networks.
See: Yi Li, X. de Milly, O. Klein, V. Cros, J. Grollier and G. de Loubens, Selective control of vortex polarities by microwave field in two robustly synchronized spin-torque nano-oscillators, Appl. Phys. Lett., 2018
Nutation dynamics in a single YIG nanodisc
In a single YIG nanodisc, we can drive its magnetization precession up to a cone angle of 90 degrees. In this strongly nonlinear regime, we successfully observe a brand new excitation: the nutation mode of a macrospin magnetization, which are measured sensitively using Magnetic Resonance Force Microscopy (MRFM)
See: Yi Li, V. V. Naletov, O. Klein, J. L. Prieto, M. Muñoz, V. Cros, P. Bortolotti, A. Anane, C. Serpico and G. De Loubens, Nutation spectroscopy of a nanomagnet driven into deeply nonlinear ferromagnetic resonance, arXiv: 1903.05411 (accepted in Physical Review X)
Phase-sensitive spin-torque magnetometry by optical detection
We have achieved optical detection of spin-precession phase in ferromagnetic resonance, which can be used to determine the spin-orbit torque from heavy metals. This technique allows both spatial- and phase-resolved detection of magnetization dynamics with tabletop optical measurements, which is potential for characterizing local spin-orbit coupling in quantum materials and complex circuits for coherent spin-information processing.
See: Yi Li, H. Saglam, Z. Zhang, R. Bidthanapally, Y. Xiong, J. Pearson, V. Novosad, H. Qu, G. Srinivasan, A. Hoffmann, and W. Zhang, Simultaneous Optical and Electrical Spin-Torque Magnetometry with Phase-sensitive Detection of Spin Precession, Phys. Rev. Applied, 11, 034047 (2019)
3. Fundamentals in magnetization dynamics
Damping enhancement at large spatial variations (large-k spin wave)
Standing spin wave (SSW) modes confined along the thickness direction of FM thin films (FM=NiFe, CoFeB, Co). An additional Gilbert damping term is found in SSW modes which is associated to intralayer spin pumping.
See: Yi Li, W. E. Bailey, Wave-Number-Dependent Gilbert Damping in Metallic Ferromagnets, Phys. Rev. Lett., 2016
For an artificial parallel of above see: H.-Z. Yang, Y. Li and W. E. Bailey, Large spin pumping effect in antisymmetric precession of Ni79Fe21/Ru/Ni79Fe21, Appl. Phys. Lett., 2016
Nonlinearity at high frequencies (up to 334 GHz)
Ferromagnetic resonance up to 334 GHz, 13 Tesla is measured on NiFe and Co thin films. Compared with low-frequency extrapolation, negative shifts of both resonance field (H_res) and linewidth (ΔH) are found. This new dynamical term, called the inertial term, is associated to the retardation of the Gilbert damping torque.
See: Yi Li, A-L. Barra, S. Auffret, U. Ebels and W. E. Bailey, Inertial terms to magnetization dynamics in ferromagnetic thin films, Phys. Rev. B (Rapid Communications), 2015
Giant damping anisotropy in epitaxial metallic ferromagnets (Co50Fe50)
We report a giant Gilbert damping anisotropy in epitaxial Co50Fe50 thin film with a maximum-minimum damping ratio of 400 %, determined by broadband spin-torque as well as inductive ferromagnetic resonance. We conclude that the origin of this damping anisotropy is the variation of the spin orbit coupling for different magnetization orientations in the cubic lattice, which is further corroborate from the magnitude of the anisotropic magnetoresistance in Co50Fe50.
See: Yi Li, F. Zeng, S. S.-L. Zhang, H. Shin, H. Saglam, V. Karakas, O. Ozatay, J. Pearson, O. Heinonen, Y. Wu, A. Hoffmann, W. Zhang, Giant anisotropy of Gilbert damping in epitaxial CoFe films, Phys. Rev. Lett., 2019 (see also Phys.org highlight)