Current-induced magnetic skyrmions oscillator

Abstract

Spin transfer nano-oscillators (STNOs) are nanoscale devices which are promising candidates for onchip microwave signal sources. For application purposes, they are expected to be nano-sized, to have broad working frequency, narrow spectral linewidth, high output power and low power consumption. In this paper, we demonstrate by micromagnetic simulation that magnetic skyrmions, topologically stable nanoscale magnetization configurations, can be excited into oscillation by a spin-polarized current. Thus, we propose a new kind of STNO using magnetic skyrmions. It is found that the working frequency of this oscillator can range from nearly 0 Hz to gigahertz. The linewidth can be smaller than 1 MHz. Furthermore, this device can work at a current density magnitude as small as 10 8 A m −2 , and it is also expected to improve the output power. Our studies may contribute to the development of skyrmion-based microwave generators. IntroductionSkyrmions are topologically protected objects with particle-like properties that play an important role in many different contexts, such as liquid crystals [1], quantum Hall magnets [2], Bose-Einstein condensates [3], etc. Recently, with the development of observation technology, particularly in the domain of neutron scattering [4], spin-polarized scanning tunneling microscopy (STM) [5], Lorentz force microscopy [6-8], and electron holography [9], skyrmions have been observed in bulk ferromagnetic crystals, thin films and nanowires. The spin texture of magnetic skyrmions is a stable configuration that, in most systems, results from a balance between the ferromagnetic exchange coupling, the Zeeman energy from the applied field and the chiral interaction, known as the Dzyaloshinskii-Moriya interaction (DMI) [10][11][12]. The DMI is induced because of the lack of, or breaking of, inversion symmetry in the magnetic structure, either due to the non-centrosymmetric crystal lattice or to the interfaces between different materials [8].Magnetic skyrmions were originally discovered in bulk ferromagnets lacking inversion symmetry, such as MnSi [13], FeGe [7,14], Fe 0.5 Co 0.5 Si [15] and other B20 transition metal compounds [16]. Then they were observed in thin films and nanowires of similar materials [9,17,18], and recently, in the multiferroic insulator Cu 2 OSeO 3 [19]. In addition, a more stable two-dimensional skyrmion crystal has been created artificially by nanopatterning [20] and a spontaneous skyrmion ground state has been created in Co/Ru/Co multilayer nanodisks without the DMI (the competition of the exchange energy, demagnetization energy and uniaxial anisotropy energy acts similar to the DMI) by a numerical approach [21]. Meanwhile, an effective method was reported to nucleate or annihilate isolated skyrmions experimentally by using STM at one monolayer of Fe grown in Ir(111) [22].It was recently realized that the magnetic skyrmions not only have mathematical beauty but can also be used as spintronic devices. Recent research has demonstrated that magnetic skyrmions have great potentia...

Gyrotropic Skyrmion Modes in Ultrathin Magnetic Circular Dots

“…2a,b). The gyrofrequency (11) is determined by the exchange stiffness A, intrinsic DMI parameter D, the perpendicular anisotropy constant K, as well as the dot saturation magnetization and radius R. The recent simulations by Zhang et al 33 of the gyrotropic dynamics of the skyrmion in a free layer of circular nanopillar are in qualitative agreement with Eq. (11).…”

“…Additional reason for that are perspective applications of the RT isolated skyrmions in nanoscale devices 33 . Isolated skyrmions, conditions required for their existence in patterned films [11][12][13][14]23,34,35 , and the dynamics of isolated skyrmions in magnetic films and nanodots have been simulated 29,33,36,37 and experimentally explored 38 . Lin et al 29 simulated dynamics of isolated skyrmion in an infinite film and found one gyrotropic mode, several breathing and non-symmetric skyrmion shape modes in the area of the skyrmion stability.…”

Collective dynamical skyrmion excitations in a magnonic crystal

“…27 The dynamics of coupled skyrmions 28 and collective excitation 29 were considered as well. The spintronic application of skyrmion-based spin transfer nano-oscillator (STNO) was proposed 30 . In this Letter, we numerically study collective skyrmion dynamics in the chain of the magnetic nanodots with skyrmion texture.…”

A microwave field-driven transistor-like skyrmionic device with the microwave current-assisted skyrmion creation

“…Indeed, the magnetic skyrmion as information carrier attracts increasing interest for developing the nextgeneration data storage devices due to its remarkable stability, extremely small size, and low-current depinning property [21][22][23][24][25][26][27][28][29][30] . Concurrently, many other applications of the magnetic skyrmions have been proposed and demonstrated, such as skyrmion-based logic devices 31 , oscillators 32,33 , and electronic devices [34][35][36] .…”