Skyrmion Lattice Detection, Tuning Fork Implementation
Figure 2 : Vibration amplitude measured in the cryostat at T = 4 K with the cryo-cooler ON (black line) and OFF (red line). The top panel accounts for the vibration amplitude along the vertical axis; the bottom panel shows the amplitude of the vibrations along the horizontal axis. The green line represents the background level of vibrations measured independently in an isolated environment where we detect the lowest level of vibrations in our laboratory.
stainless steel tubing, hosting the microscope insert. Figure 1 shows mechanical decoupling between the pulse tube and the sample space in such a way that the vibrations from the pulse tube cryo-cooler do not infl uence vibration-sensitive experi- ments, while still ensuring a good thermal contact for suffi cient cooling power.
Measurements of vibrations . Vibration amplitudes in the system were measured using an absolute vibration detector [ 9 ]. Figure 2 shows the spectral vibrational noise density along the vertical and the horizontal axis, measured in the sample space at T = 4 K. The response of the vibration detector at low temperature was found to be consistent with its performance at room temperature. The vibration detector is inserted in the vacuum tube at the location of the scanning probe microscope to simulate experimental conditions ( Figure 1 ). Figure 2 shows measurements for each direction of displacement corresponding to the cryo-cooling system on (black line) and off (red line). The green line represents the background level of vibrations measured independently in an isolated environment where we detect the lowest level of vibrations in our laboratory. The data are shown on a logarithmic scale to visualize the amplitude spectral density. Figure 3 shows the absolute vibration amplitude between 1 Hz and 800 Hz, revealing the technical spurious noise. The rms vibration amplitude between 1 Hz and 800 Hz yields a peak value above background noise of 6 nm along the vertical axis and 3.5 nm along the horizontal axis at the pulse tube frequency f = 1.4 Hz. The peak at f = 50 Hz and its harmonics are parasitic electromagnetic pick-up of the vibration detector and do not correspond to vibrations. This is verified by comparison between the measurements with the cryo-cooling system on and off. The measured values of absolute vibration amplitude along the vertical axis in the nm range are low enough to enable SPM measurements in contact mode atomic force microscopy (AFM) and magnetic force microscopy.
2015 November •
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Figure 3 : Measurements of displacement determined by the vertical (top) and horizontal (bottom) residual vibrations in the sample space inside the closed- cycle cryostat. Amplitude is measured at base temperature with the cryo-cooler ON (black line) and the cryo-cooler OFF (red line).
Results
Contact-mode AFM measurements . For contact mode AFM an etched silicon cantilever with a silicon tip combined with optical defl ection detection was used to measure local interactions such as van der Waals or Coulomb forces. We used fi ber-based interferometric sensing [ 10 ] to detect the defl ection: a fi ber was mounted in the head of the microscope to detect the defl ection of the cantilever induced by the tip-sample interaction. A measurement of the amplitude of the relative displacement between the AFM tip and the sample was performed to evaluate the impact of the absolute vibrations of the system on the performance of the scanning probe experiments. A schematic representation of our AFM head with interferometric read-out is shown in Figure 4a . T e z -noise was measured while keeping the tip in contact with the sample surface. Vibration data describing the relative displacement between the tip and the sample for the fully enabled system were then recorded over time. Noise statistics of the tip defl ection were acquired over 10,000 points. T e sampling time was set to 5 ms, corresponding to a measurement bandwidth of 200 Hz. Figure 4b shows the noise histogram. T e result is a normal Gaussian distribution of the noise with a standard deviation σ = 65 pm rms; this value, measured with the feedback loop enabled, is comparable to the z -noise amplitude obtained in liquid systems [ 11 ]. When the feedback is turned off , the noise amplitude increases by approximately a factor of 4. To demonstrate the low vibration noise, images were taken at T = 3.2 K on terraces of height matching the lattice parameter (a = 0.39 nm) of a strontium titanate (SrTiO 3 ) commercial wafer, shallow polished at 0.1° then annealed to obtain terrace-and-step rearrangement at the surface. T e image in Figure 5a shows a contact mode scan revealing the terrace-and-step morphology of the sample surface, with steps measuring 0.4 nm. T e image was acquired in 21 ms sampling time, with a bandwidth of 47.6 Hz. T ese measurements are fully consistent with those reported of a similar commercial sample of SrTiO 3 measured at room temperature, showing 0.4 nm lattice steps without the infl uence of a cryo-cooling system [ 12 ].
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