Non-hermiticity in spintronics: oscillation death in coupled spintronic nano-oscillators through emerging exceptional points.

The emergence of exceptional points (EPs) in the parameter space of a non-hermitian (2D) eigenvalue problem has long been interest in mathematical physics, however, only in the last decade entered the scope of experiments. In coupled systems, EPs give rise to unique physical phenomena, and enable the development of highly sensitive sensors. Here, we demonstrate at room temperature the emergence of EPs in coupled spintronic nanoscale oscillators and exploit the system's non-hermiticity. We observe amplitude death of self-oscillations and other complex dynamics, and develop a linearized non-hermitian model of the coupled spintronic system, which describes the main experimental features. The room temperature operation, and CMOS compatibility of our spintronic nanoscale oscillators means that they are ready to be employed in a variety of applications, such as field, current or rotation sensors, radiofrequeny and wireless devices, and in dedicated neuromorphic computing hardware. Furthermore, their unique and versatile properties, notably their large nonlinear behavior, open up unprecedented perspectives in experiments as well as in theory on the physics of exceptional points expanding to strongly nonlinear systems.

Nonlinear Magnetization Dynamics Driven by Strong Terahertz Fields.

We present a comprehensive experimental and numerical study of magnetization dynamics in a thin metallic film triggered by single-cycle terahertz pulses of ∼20 MV/m electric field amplitude and ∼1 ps duration. The experimental dynamics is probed using the femtosecond magneto-optical Kerr effect, and it is reproduced numerically using macrospin simulations. The magnetization dynamics can be decomposed in three distinct processes: a coherent precession of the magnetization around the terahertz magnetic field, an ultrafast demagnetization that suddenly changes the anisotropy of the film, and a uniform precession around the equilibrium effective field that is relaxed on the nanosecond time scale, consistent with a Gilbert damping process. Macrospin simulations quantitatively reproduce the observed dynamics, and allow us to predict that novel nonlinear magnetization dynamics regimes can be attained with existing tabletop terahertz sources.

Magnetization reversal driven by low dimensional chaos in a nanoscale ferromagnet.

Energy-efficient switching of magnetization is a central problem in nonvolatile magnetic storage and magnetic neuromorphic computing. In the past two decades, several efficient methods of magnetic switching were demonstrated including spin torque, magneto-electric, and microwave-assisted switching mechanisms. Here we experimentally show that low-dimensional magnetic chaos induced by alternating spin torque can strongly increase the rate of thermally-activated magnetic switching in a nanoscale ferromagnet. This mechanism exhibits a well-pronounced threshold character in spin torque amplitude and its efficiency increases with decreasing spin torque frequency. We present analytical and numerical calculations that quantitatively explain these experimental findings and reveal the key role played by low-dimensional magnetic chaos near saddle equilibria in enhancement of the switching rate. Our work unveils an important interplay between chaos and stochasticity in the energy assisted switching of magnetic nanosystems and paves the way towards improved energy efficiency of spin torque memory and logic.

High gradient, high reliability, and low wakefield accelerating structures for the FERMI FEL.

The FERMI seeded free-electron laser (FEL), located at the Elettra laboratory in Trieste, is a 4th generation light source operating in the vacuum ultraviolet to soft X-rays range. The FEL design is based on an external seeding scheme which improves the output pulse coherence and the central wavelength control and reduces the spectral bandwidth. FERMI has achieved its original energy target by producing photon energies above 300 eV from a 1.50 GeV, 600 A peak current, electron beam. However, there is a strong scientific motivation to push the energy envelop further higher to photon energy up to 600 eV to cover both the x-ray absorption edges of nitrogen K (400 eV) and oxygen K (532 eV). To achieve this goal, the electron beam energy will be increased from 1.50 GeV to 1.80 GeV and the peak beam current will be pushed towards 1 kA. This requires essentially the development of more reliable high gradient S-band accelerating structures, with low wakefields contribution up to 1 nC charge per bunch. Accordingly, in the following, we present the design of high gradient, high reliability, and low wakefield S-band accelerating structures for the upgrade program of the FERMI linac.