Scanning Probe Microscopy in an Ultra-Low Vibration Closed-Cycle Cryostat: Skyrmion Lattice Detection and Tuning Fork Implementation
Francesca Paola Quacquarelli , 1 † * Jorge Puebla , 1 †† Thomas Scheler , 1 Dieter Andres , 1 Christoph Bödefeld , 1 Balázs Sipos , 1 Claudio Dal Savio , 1 Andreas Bauer , 2 Christian Pfl eiderer , 2
Andreas Erb , 3 and Khaled Karrai 1 1 attocube systems AG , Königinstraße 11a , 80539 München , Germany 2 Physik-Department , Technische Universität München , James-Franck-Str. 1 , 85748 Garching , Germany 3 Walther Meissner Institute , Bavarian Academy of Sciences and Humanities , 85748 Garching , Germany
*
francescapaola.quacquarelli@gmail.com Introduction
Many-body phenomena such as recently imaged currents in topological insulators [ 1 ] or nanoscale spin textures in chiral magnets, so-called magnetic skyrmions [ 2 ], arise at cryogenic temperatures constraining scanning probe microscopy (SPM) to be low-temperature compatible and to operate in liquid helium based cryostats [ 3 , 4 ]. However, the high costs of helium and its scarcity have become a limiting factor, recently propelling the development of closed-cycle cryostats [ 5 , 6 ]. T ese are independent of a continuous supply of liquid helium but present a challenge to the implementation of SPM owing to their noisy mechanics. Tuning fork-based scanning gate microscopy in a pulse-tube-cooled dilution refrigerator has been recently reported [ 7 ], opening the way to SPM in dry cryostats. T is pioneering achievement required heavy intervention on the cryo-cooling system, sacrifi cing thermal contact and requiring long pre-cooling times. In this article we report on the development of an ultra-low vibration closed-cycle cryostat that allows top-loading of a variety of scanning probe microscopes with rapid turnovers. Because the microscopes do not require spring-mounting to further reduce vibrations, high-resolution confocal light microscopy imaging can be combined with scanning force microscopy. T e ultra-low amplitude of vertical vibrations measured in our system enabled the AFM resolution of 0.39 nm high atomic steps in a SrTiO 3 sample. T is low level of vibration also allowed detection of the skyrmion-lattice phase in a chiral magnet. Furthermore, it opened the way to the implementation for the fi rst time of tuning fork-based shear-force microscopy in a closed-cycle cryostat. Scanning tunneling microscopy measurements recently reported by Leiden Cryogenics BV complete the picture of SPM measurements in a closed-cycle cryostat [ 8 ]. T is article describes the cryo-cooled AFM and gives some example images.
Materials and Methods Cryo-cooling system . Our dry cryo-cooling system
( Figure 1 ) relies on conductive cooling and is based on pulse tube technology [ 6 ]. Here, a cryogenic fl uid at high pressure is pulsed longitudinally through the diff erent stages of the tube
† Now at Institut für Schlaganfall-und Demenzforschung (ISD), Klinikum der Universität München, Feodor-Lynen-Straße 17, D-81377 München, Germany †† Now at Center for Emergent Matter Science, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan
12
Figure 1 : Closed-cycle cryo-cooling system based on pulse tube technology. The system consists of three main parts: a pulse tube, a rotary valve, and a compressor. A 9 T superconducting magnet, fi xed to the low-temperature stage of the pulse tube, surrounds coaxially the sample space, and it is cooled by thermal contact. A vibration detector in its vacuum insert is top-loaded into the system to characterize vibrations in the sample space.
doi: 10.1017/S1551929515000954
www.microscopy-today.com • 2015 November
at a frequency of 1.4 Hz, reaching temperatures of approxi- mately 3 K. T e cyclic expansion and compression of the gas in the adiabatic chamber where the fl uid is stored produces a stratifi cation of temperature along the tube, so that one end is warmer than the other. T e Cryomech PT-410 2-stage pulse tube cryo-cooler is driven by a compressor, which produces 1 W of cooling power at 4.2 K. A rotary valve regulates the pressure of the gas through the pulse tube. A 9 T supercon- ducting magnet completes the system and is cooled by the pulse tube through thermal contact. Initial cooling of the whole system requires 12 hours; cooling down of the microscope insert from T=300 K to base temperature, namely 4 K, is normally achieved within 1 hour, depending on the mass of the microscope. One hour is also required to warm the system up. T e sample space is immediately accessible aſt er warming up thanks to the top-loading architecture. T e system confi g- uration enables measurement turnovers every 3 hours: sample exchange times of typically a few minutes are included in this estimate. T e SPM experiments and the characterization of the vibrations in the system take place in the 50 mm diameter sample space, consisting of air-tight, removable, thin-walled
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