ATOMS · Advanced Tools to Observe Magnetic and dynamical properties of Skyrmions and vortices down to the atomic scale

Advanced Tools to Observe Magnetic and dynamical properties of Skyrmions and vortices down to the atomic scale

Spin polarized scanning tunnelling microscopy (SP-STM) is a powerful tool to study the local spin structure of magnetic surface with a lateral resolution down to atomic level. The time resolution of conventional STM is limited by the bandwidth of its transimpedance amplifier which is used to convert tunnelling current in the pA/nA range to a manageable voltage. Typical bandwidth are around several kHz which is much too low to track magnetization precession dynamics of single nano-objects. In contrast magnetic resonance techniques allow to probe high frequency magnetization dynamics but of macroscopic sample. The ATOMS project aimed at the experimental development of a new technique combining SP-STM and magnetic resonance. This experimental technique enables the measurement of magnetization dynamics down to the atomic scale. The basic idea of the experiments is inspired from the spin torque diode effect [Nature 438, 339 (2005)]. A continuous radio frequency (rf) voltage is mixed to the bias voltage of the STM. If there is a magnetization precession under the spin-polarized STM tip, the tunnelling conductance will be modulated at the frequency of the magnetization precession and the tunnelling current is rectified. This rectified current, measurable by conventional STM transimpedance amplifiers corresponds to a ferromagnetic resonance signal in the sample. In order to well transmit the high frequency voltage to the tunnelling junction of the STM, a new rf transmission line has been implemented to the microscope dedicated to this project. Because of damping and reflection in the cable, the rf voltage transmitted through the cable reaching the the tunnelling junction of an STM strongly depends on frequency. For a reliable understanding of rf excited sample, the transmission of the setup needed to be determined in detail. The exact knowledge of the transmission characteristic of the setup allows to pre-adjust the rf amplitude at the continuous wave source in order to keep a constant signal amplitude at the tunnelling junction. The method develop during the fellowship has been published [Applied Physics Letter, 107, 093101 (2015)]. This allowed to boost the 100 MHz bandwidth of the STM to 3 GHz. In order to raise the possibility of successful measurement, a good proof of concept sample has been chosen for a first study. Magnetic vortex gyration is a good candidate for several reasons: (1) Its gyration mode is in the accessible frequency range. (2) It is a well-known system and its dynamical properties are easy to simulate in order to predict the resonance frequency, the rf current required for the excitation, the amplitude and line-shape of the signal we aim to measure. A complete micromagnetic simulation study of the vortex gyration mode excited by a locally injected spin polarized rf current has been carry out. This simulation work allowed to reproduce the real experiments and to get a quantitative knowledge of the signal expected to be measure. It tuned out that the rf current required to excite the gyration mode are extremely high: several hundreds of nA. These high rf current are experimentally accessible only under extreme tunnelling condition. This made the experiments unstable and explains why no unambiguous magnetic resonance features have been observed in this system so far. Even thought the experimental study of the dynamical properties of magnetic vortices has not been successful, the micromagnetic simulation study realized on this system comfort the idea of studying the gyration mode of magnetic skyrmion. These magnetic objects are extremely mobile under very low spin polarized current density. The rf current required to excite the gyration mode in these object are several order of magnitude smaller than the one required for magnetic vortices. This makes the experiments easier. In the purpose of carrying out magnetic resonance experiments on skyrmions, two spin structures was investigated: (1) Fe (1 ML)/Ir(111) where a skyrmionic spin structure has been reported in the literature [Nature Physics, 7, 713 (2011)]. (2) Co (1 ML)/Ru(0001). Investigation of the magnetism of this structure has revealed at zero magnetic field a chiral spin spiral has a ground state. This spin spiral is stabilized by the Dzyaloshinskii Moriya interaction (Figure 1.a attached file). Application of a small out of plane magnetic field (150 mT) allows to stabilize isolated skyrmion in a ferromagnetic background (Figure 1.b – 1.c – 1-d attached file). This study constitute the first experimental observation of a chiral non collinear spin structure stabilized at an interface between a 4d metal (Ruthenium) and a 3d ferromagnetic element (Cobalt). Fully relativistic DFT calculation performed in collaboration with Arthur Ernst (Max Planck Intitute – Halle) are in progress. This theoretical work should allow to explain the non collinear magnetism in this system. Dynamical SP-STM experiments have been carried out on the system Fe (1 ML)/Ir(111). They revealed a resonance signal at 615 MHz (Figure 2.b attached file). The spatial dependence of the signal (amplitude and line-shape) is directly correlated to the spin structure and evolve with magnetic field (Figure 2.c – 2.d attached file). This feature is of magnetic resonance origin. It can either correspond to a gyration mode in the skyrmionic spin structure or to coherent oscillation of a domain wall in the skyrmion lattice. In order to get a good understanding of the origin of this magnetic resonance signal, a theoretical description of the dynamical properties of this system is under progress in collaboration with R. Hertel (Institut de Physique et Chimie des matériaux de Strasbourg) is in progress. In sight of their innovative physical properties (high mobility under very low spin polarized current density and dynamical process at low frequency), skyrmion are the focus for technological advance in magnetic storage and radiofrequency devices. Already, skyrmion based spin transfer nano-oscillators have been proposed [New Journal of Physics, 17, 023061 (2015)]. The new experiments develop during the fellowship is an ideal experimental tool to study magnetic precession in such small magnetic objects as skyrmions. In the actual socio-economic context, the expected final results of this project should bring a major contribution to the field.

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