Active Q control may be used to modify the effective quality (Q) factor of an atomic force microscope (AFM) micro-cantilever when operating in tapping mode. The control system uses velocity feedback to obtain an effective cantilever Q factor to achieve optimal scan speed and image resolution for the imaging environment and sample type. Time delay of the cantilever displacement signal is the most common method of cantilever velocity estimation. Spill-over effects from unmodeled dynamics may degrade the closed loop system performance, possibly resulting in system instability, when time delay velocity estimation is used. A resonant controller is proposed in this work as an alternate method of velocity estimation. This new controller has guaranteed closed loop stability, is easy to tune, and may be fitted into existing commercial AFMs with minimal modification. Images of a calibration grating are obtained using this controller to demonstrate its effectiveness.

/pdf/resonant-control-of-an-atomic-force-microscope-micro-29uy8ulqk2.pdf

Resonant control of an atomic force microscope micro-cantilever for active Q control.

We have developed a low temperature, high resolution magnetic force microscope (MFM) using a quartz tuning fork that can operate in a magnetic field. A tuning fork with a spring constant of 1300 N/m mounted with a commercial MFM cantilever tip was used. We have obtained high resolution images of the stray magnetic fields exerted from grains with a spatial resolution of 15 nm and force resolution of 2 pN at 4.2 K. Tuning fork based magnetic force microscopes have the potential to be used at millikelvin temperatures due to their low power dissipation and high force sensitivity.

Low-temperature, high-resolution magnetic force microscopy using a quartz tuning fork

The atomic force microscope (AFM) [1] is a mechanical microscope capable of producing three-dimensional images of a wide variety of sample surfaces with nanometer precision in air, vacuum, or liquid environments. This article provides an overview of the AFM and its three main modes of operation, with a focus on the tapping mode of operation. The challenges associated with obtaining high-speed images with a tapping-mode AFM while minimizing image artifacts are outlined and control techniques that have been developed to overcome these challenges are reviewed.

Control Techniques for Increasing the Scan Speed and Minimizing Image Artifacts in Tapping-Mode Atomic Force Microscopy: Toward Video-Rate Nanoscale Imaging

Magnetic force microscopy (MFM) allows one to image the domain structure of ferromagnetic samples by probing the dipole forces between a magnetic probe tip and a magnetic sample. The magnetic domain structure of the sample depends on the alignment of the individual atomic magnetic moments. It is desirable to be able to image both individual atoms and domain structures with a single probe. However, the force gradients of the interactions responsible for atomic contrast and those causing domain contrast are orders of magnitude apart, ranging from up to 100 Nm−1 for atomic interactions down to 0.0001 Nm−1 for magnetic dipole interactions. Here, we show that this gap can be bridged with a qPlus sensor, with a stiffness of 1800 Nm−1 (optimized for atomic interaction), which is sensitive enough to measure millihertz frequency contrast caused by magnetic dipole–dipole interactions. Thus we have succeeded in establishing a sensing technique that performs scanning tunneling microscopy, atomic force microscopy and MFM with a single probe.

qPlus Magnetic Force Microscopy in Frequency-Modulation Mode with milli-Hertz Resolution

We introduce a versatile method to control the quality factor Q of a conducting cantilever in an atomic force microscope (AFM) via capacitive coupling to the local environment. Using this method, Q may be reversibly tuned to within ∼10% of any desired value over several orders of magnitude. A point-mass oscillator model describes the measured effect. Our simple Q control module increases the AFM functionality by allowing greater control of parameters such as scan speed and force gradient sensitivity, which we demonstrate by topographic imaging of a VO2 thin film in high vacuum.

/pdf/microcantilever-q-control-via-capacitive-coupling-ipc6tb7g6s.pdf

Microcantilever Q control via capacitive coupling

We have performed magnetic force microscopy at various temperatures utilizing piezoelectric quartz tuning forks as probes. Due to their large force constants (∼104N∕m), quartz tuning forks are intrinsically less sensitive to force gradients than conventional cantilevers. However, we demonstrate that the technique of Q-control can be used to increase their sensitivity, making their use as probes for variable temperature magnetic force microscopy a viable option.

/pdf/variable-temperature-magnetic-force-microscopy-with-2omb06s4tv.pdf

Variable temperature magnetic force microscopy with piezoelectric quartz tuning forks as probes optimized using Q-control

Critical swimming speed (Ucrit) is an important measurement of swimming performance as well as a good indicator for cardio-respiratory health. It offers a new opportunity to select fish with better fitness. Here we present results from a GWAS study on critical swimming speed and growth in Nile tilapia under conditions of recurrent hypoxia. Heritability for Ucrit was moderate (0.31±0.04). Large fish generally swim faster than smaller fish, whereas fish with better swimming performance tend to show slower growth later in life. Nine suggestive SNPs associated with swimming performance were identified under a hypoxic environment. A clear pleiotropic effect on growth traits was found for some of the SNPs associated with swimming performance, while other SNPs had only an effect on swimming performance, but not on growth. Our analyses sheds a first light on the genomic mechanisms of swimming performance and growth in Nile tilapia. 

580. QTLs controlling swimming performance and their effect on growth in Nile tilapia (Oreochromis niloticus)

Piezoelectric quartz tuning forks have been used for topographic dynamic force imaging in superfluid helium and in high magnetic fields. This has been achieved by immobilizing one tine of the tuning fork to stabilize its behavior in superfluid. Images acquired at room temperature and at 50 K are also presented. Frequency–distance curves are shown to be markedly different in superfluid than in air due to a long-range fork–sample interaction in liquid. Evidence is presented that this is due to a change in the hydrodynamic effective mass of the fork as the gap between the fork and sample is reduced. In addition, Q-control has been implemented and used to both increase and decrease the quality factors of tuning forks in both vacuum and superfluid helium.

Dynamic force microscopy in superfluid helium

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Variable temperature magnetic force microscopy with piezoelectric quartz tuning forks as probes optimized using Q-control

580. QTLs controlling swimming performance and their effect on growth in Nile tilapia (Oreochromis niloticus)

Dynamic force microscopy in superfluid helium