Dynamic response of magnetic skyrmions in multilayer heterostructures

• Main Author: Satywali, Bhartendu
• Other Authors: Christos Panagopoulos
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Science::Physics
• Online Access: https://hdl.handle.net/10356/137378
• Institution: Nanyang Technological University

Topological effects in condensed matter physics have been explored for fundamental and technological reasons in a wide variety of materials. Recently, magnetic skyrmions - circularly-shaped twisted spin textures - have aroused interest due to their enhanced stability and efficient coupling with electric currents. Consequently, they can be harnessed for data storage and logic operations. Two key requirements for skyrmion formation are (a) an absence of spatial inversion symmetry and (b) a large spin-orbit coupling in the material. Metallic multilayers featuring ferromagnet-heavy metal (FM-HM) interfaces satisfy both conditions and can hence exhibit nano-skyrmions at room temperature (RT). In this thesis, we nucleate sub-50 nm skyrmions at RT in engineered Ir/Fe/Co/Pt multilayers, where the size and order (lattice or isolated) of the skyrmions can be tuned by the relative thickness of the Fe/Co layers. We employ magnetic force microscopy (MFM) to image skyrmions at RT. A significant variation is observed in the size and shape of skyrmions in different regions of the film, suggesting local inhomogeneities in the magnetic parameters for our films. A detailed analysis of the microwave excitation modes of skyrmions is of profound importance for understanding their current and field-induced dynamics. In this thesis, we report the first experimental detection of magnetic resonance from Neel skyrmions in multilayers from 300K-100K, measured with a home-built broadband ferromagnetic resonance (FMR) probe. The field and frequency-sweep FMR measurements reveal two distinct resonant modes originating from skyrmions: a low frequency (LF) resonance exhibiting a steep proportionality to the external out-of-plane magnetic field (H), and a high frequency (HF) resonance which is less sensitive to variations in H. Combining micromagnetic simulations and analytical calculations, we identify the counterclockwise (CCW) skyrmion gyrations to be responsible for the LF mode. The HF mode is caused by the scattering of magnons from the uniformly-precessing ferromagnetic background off skyrmions. Furthermore, we present a method of tuning the skyrmion excitation frequencies by varying the thickness of the non-magnetic Pt/Ir spacer layers in our heterostructures, which modulates the dipolar coupling between the FM layers.