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Skyrmion Lattice Detection, Tuning Fork Implementation


Figure 4 : (a) Interferometric head for AFM measurements. (b) Contact mode noise scan histogram of the z -height values measured with a bandwidth of 200 Hz at 3.2 K. Measured amplitude of the relative displacement was 65 pm.


Magnetic force microscopy measurements . Exciting research in low-temperature solid-state physics is currently carried out by means of magnetic force and magnetic resonance force microscopy, including the imaging of currents in topological insulators [ 1 ], nanoscale spin confi gurations


(magnetic skyrmions) in chiral magnets [ 2 ], scanning diamond magnetometry using nitrogen-vacancy (NV) centers in diamonds as solid-state sensors of magnetic fi eld [ 13 – 15 ], and scanning gate microscopy [ 7 , 16 ]. T ese techniques require sensitivity to extremely small magnetic interactions with nanometer spatial resolution. T eir recent successful implementation required liquid bath cryostats. Given the scarcity of liquid helium, the implementation of scanning magnetic force microscopy inside dry cryostats represents the way forward. T e low level of vibrations achieved in our dry cryostat allowed a variety of


magnetic force microscopy measurements.


In magnetic force microscopy, tips with magnetic coatings, typically NiCr or cobalt (Co), are employed that are sensitive to variations of magnetic fi eld. T e tip is oscillated at its resonance using a dither piezo; as it is scanned across the sample, the strength of the magnetic interaction between the tip and the magnetic fi elds near the surface determines a shiſt in the oscillation frequency. Frequency shiſt s map the magnetic structure of the sample. Figure 4a shows a diagram of the cantilever-based force microscope used in this work, specifi c for applications at low temperature and high magnetic fi eld. Magnetic force microscopy is especially sensitive to the sample-tip separation, and vertical vibrations can aff ect image resolution by impairing the force sensitivity of the probe. We measured skyrmion lattices in our dry cryostat with an unprecedented signal-to-noise ratio of 20:1. Skyrmion-lattice and helimagnetic phase in single-


Figure 5: (a) Contact-mode AFM image of terraces on SrTiO3 (200 scan lines) at 3.2 K. Bar = 400 nm. Step height is 0.39 nm, corresponding to the lattice parameter of the crystal. Frame time for the acquisition was 1,680 s at a scan rate of 500 nm/s. (b) Single-line profi le showing the height of the atomically fl at terraces on SrTiO3.


14


crystal Fe 0.5 Co 0.5 Si . Magnetic skyrmions are nanoscale spin whirls. Hexagonal lattices of these whirls are observed in certain chiral magnets without inversion symmetry in a phase pocket in fi nite magnetic fi eld [ 17 , 18 , 19 ]. Perhaps most excitingly, electric currents allow these magnetic textures to move already at ultra-low current densities [ 20 , 21 ]. Combined with their size in the nanometer range and their stability arising from the topologically non-trivial winding, skyrmions promise great potential for applications in information technology [ 22 ]. In addition to the bulk chiral magnets, skyrmions are recently also studied in thin fi lms and monolayers, where they are typically driven by the surface and may be readily studied by means of SPM [ 23 , 24 ]. For this report, however, we investigated the surface of a polished single crystal of Fe 0.5 Co 0.5 Si in our dry cryostat [ 2 ]. We observed ( Figure 6a ) helimagnetic structures at T=3.2 K in zero magnetic fi eld and a skyrmion-lattice texture ( Figure 6b ) at T = 3.4 K in an externally applied fi eld B = 15 mT. T e measurements of the helimagnetic phase in Fe 0.5 Co 0.5 Si and the imaging of the skyrmion-lattice phase transition of the single crystal were carried out using a sharp tip (tip apex radius ~10 nm) with a magnetic coating (Nanosensors, SSS-MFMR). T e magnetic tip was kept at a constant height of 20–30 nm over the sample surface, with a phase-locked loop activated to monitor the cantilever resonance frequency. T e sample was heated to 60 K, and the magnetic fi eld was


www.microscopy-today.com • 2015 November


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