Design of a Self-Aligned, High Resolution, Low Temperature (30 Mk - 300 K) Magnetic Force Microscope
Abstract
The invention of Scanning Tunneling Microscope (STM) in 1981 opened a new avenue in atomic scale world and surface science. STM made it possible to either image atoms or manipulate them as well as measuring the local density of states of electrons on the surface. This invention also leaded invention of the another Scanning Probe Microscope (SPM) method called Atomic Force Microscopy (AFM) soon which measures the interaction forces between a sharp tip attached at the end of a lever and sample surfaces at atomic length scales. AFM could measure various forces between tip-sample like van der Waals, electrostatic, friction or magnetic forces. Measurement of these specified forces was named in the force microscopy with their force name like Friction Force Microscopy, Electrostatic Force Microscopy and Magnetic Force Microscopy (MFM) which is subject of the thesis. While STM measures only conductive specimens, AFM measures both conducting and insulating specimens. Easy sample preparation, easy to use, high resolution and affordable price of these systems have collected great attention over last two decades. Moreover, operations in various environments like low temperatures, high magnetic fields, either in vacuum or ultra-high vacuum have broadened application areas of the AFMs.
Magnetic Force Microscope (MFM) plays a crucial role for magnetic imaging down to 10 nm magnetic resolution in material science and physics. Especially, low temperature AFM/MFM has the capability of working in extreme physical conditions like down to few hundres of milliKelvin temperature range or up to few tens of Tesla magnetic field. High density magnetic recording media, superconductivity, spintronics, magnetic phase transitions, spin-glass systems, magnetic nanoparticles, topological insulators are some of the hot research topics in this area in which high resolution MFM is required for investigations.
In low temperature AFM/MFM (LT-AFM/MFM) systems, deflection of the cantilevers is measured by means of a fibre interferometer in which the light is carried into the cooling system utilizing a fibre cable. A special mechanism aligns the fibre end with respect to the cantilever for measuring the deflections. The critical thing is here that this alignment may collapse when the system is cooled down because of both the design and different thermal contractions of the materials. The developed alignment mechanisms also enlarge volume of the microscope which is crucial for fitting inner free sample space of the cooling systems.
In this study, we developed a self-aligned, low temperature atomic force / magnetic force microscope operating between 300 K and 2 K temperature ranges of liquid Helium. The OD of the microscope is less than 25.4 mm and compatible with most of the cryostat systems from various vendors. We used a specially designed alignment mechanism for eliminating tedious and time consuming alignment mechanism which utilizes alignment chip sets from Nanosensors. The alignment-free design makes the life easier for end users of the system for the whole operation ranges of the temperature. Deflection of the cantilever was measured by means of a developed Michelson type fibre interferometer. We obtained unprecedented noise floor for the interferometer that ~25 fm/√Hz at 300 K and ~12 fm/√Hz at 4 K. The shot noise was calculated to be 7.8 fm/√Hz at 4 K. This noise floor enabled us to achieve 10 nm magnetic force microscopy (MFM) resolution on the high density hard disk sample with commercial cantilevers, routinely. We showed Abrikosov vortex lattice in BSCCO(2212) single crystal at 4 K and vortices in YBCO thin film superconductor at 50 K in MFM mode. Atomic steps of both mica and HOPG samples were shown in AFM modes, too.
Liquid Helium is the most common and popular cryogen which boils at 4.2 K and if the vapor pressure is decreased, one can reach ~1.5 K temperature limit for cooling. For lowering the temperature below 1.5 K, 3He based rather complicated cryostat systems are used that they reach ~300 mK base temperature level. Furthermore, using a dilution refrigerator system, it is also possible to reach ~5 mK which uses 4He/3He mixture as a cryogen in a special way. Operating LT-AFM/MFM at these ultra low temperatures is important for material science and physics for investigating many scientific phenomena. We demonstrated two separate iv
microscope designs for a 3He system and a dilution refrigerator system. The capabilities and potentials of the microscopes are shown both in AFM modes and MFM mode at these ultra low temperatures, successfully.
In the proceeding study, the shot noise limited sensitivity of Michelson fibre interferometer was improved an order of magnitude utilizing fibre Fabry-Pérot interferometer (FFPI) which has ~1 fm/√Hz noise floor. The inaugural performance of the LT-AFM/MFM using fibre Fabry-Pérot interferometer both in AFM and MFM mode were shown between 300 K and 4 K. FFPI with this ultra noise floor would be a standalone metrology instruments for many research areas, too.
In the last part of study, we describe a novel radiation pressure based cantilever excitation method for imaging in dynamic mode atomic force microscopy (AFM) for the first time. Piezo excitation is the most common method for cantilever excitation, but it may cause spurious resonance peaks. Therefore, direct excitation of the cantilever plays a crucial role in AFM imaging. A single light beam was used both for excitation of the cantilever at the resonance and measuring the deflection of the cantilever. The laser power was modulated at the cantilever’s resonance frequency by a digital Phase Lock Loop (PLL). We typically modulate the laser beam by ~500 μW and obtained up to 1,418 Åpp oscillation amplitude in moderate vacuum levels between 4 - 300 K. We have demonstrated the performance of the radiation pressure excitation in AFM/MFM by imaging CoPt multilayers between 4-300 K and Abrikosov vortex lattice in BSCCO(2212) single crystal at 4 K for the first time.
