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Skyrmion Lattice Detection, Tuning Fork Implementation


Figure 6: MFM images of the polished surface of a bulk sample of Fe0.5Co0.5Si. Images consist of 200 scan lines acquired using a sharp commercial cantilever (tip apex radius ~10 nm, SSS-MFMR from Nanosensors) with magnetic coating. (a) Helimagnetic phase of the sample at T = 3.2 K after zero-fi eld cooling (B = 0). (b) Metastable skyrmion- lattice phase measured at T = 3.4 K in an external magnetic fi eld B = 15 mT after fi eld cooling. Bars correspond to a length of 1 μ m. Line cuts highlight the frequency shift.


increased to 15 mT. T e sample was subsequently fi eld-cooled to base temperature again. With the persistent switch heater of the superconducting magnet enabled, the temperature stabilized at 3.4 K at the sample position. Figure 7 shows an image sequence of the coalescence of the skyrmion phase into helimagnetic phase. T e magnetic fi eld was decreased from B = 15 mT, where the metastable skyrmion lattice was observed, to B = −30 mT, where the textures coalesce into the helimagnetic structure [ 2 ].


Tuning Fork Scanning Force Microscopy measure-


ments . Magnetic force microscopy imaging may be carried out with sensors exhibiting higher force sensitivity, such as tuning forks [ 7 ]. We show shear-force tuning fork measurements


performed in our dry cryostat, which demonstrate the potential for further improvement, in particular toward the use of NV-centers in diamond-based scanning probe magnetometry [ 13 ]. T e implementation of quartz tuning forks as self-sensing probes in scanning force microscopy is especially benefi cial because of their small size, the absence of thermal eff ects induced by the optical detection, and the avoidance of unnecessary exposure of sensitive samples to light at cryogenic temperatures. To the best of our knowledge this is the fi rst time that tuning-fork shear-force microscopy measurements have been successfully reported in a dry cryostat. A tungsten tip was etched in house to reach a tip apex radius of approximately 80 nm and glued to one prong of the tuning fork [ 25 ] as shown in Figure 8a . T e viscoelastic interaction of the oscillating fork takes place through shear forces between the sample and the tip. Akiyama probes [ 26 ] are quartz resonators where a sharp silicon microcantilever is placed at the ends of the prongs, preserving the symmetry of the system. Typical values for the Akiyama spring


constant are 5 N/m. T e low noise amplitude measured between the tuning fork and the sample, with a bandwidth of 200 Hz and the feedback loop enabled, shows a normal Gaussian distribution with a standard deviation σ = 2 nm ( Figure 8b ). We demonstrate the achievement of tuning fork scanning force microscopy measurements of 20 ± 2 nm high SiO 2 patterns on Si with both Akiyama probes and in shear- force mode in our closed-cycle cryostat at T = 4 K ( Figure 9 a and 9 b, respectively). T e resolution is 200 lines per scan, acquired at 500 nm/s. Scanning in shear-force mode in a liquid-based cryostat, we measured noise amplitude better than 0.1 nm rms and, as discussed earlier, the contact mode noise measured in our dry cryostat was 65 pm.


Figure 7 : Coalescence of skyrmion-lattice phase into helimagnetic phase in Fe 0.5 Co 0.5 Si with decreasing magnetic fi eld B = 15 mT to −30 mT. Bar = 500 nm. Acquisition details as stated in Figure 6 .


2015 November • www.microscopy-today.com 15


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