Our versatile Tuning Fork-based Scanning Probe Microscopy is operational in the wide temperature range 4-300 K.

The force sensor is a Quartz Tuning Fork with high Q-factor (~10⁵ in vacuum) and high resonant frequency (~32 kHz). Probes are attached to the Tuning Fork using a Focused Ion Beam (FIB).

We have also developed an innovative FIB-based method to produce sharpened magnetic probes with a reduced stray field for high magnetic resolution (Fig 1).

Fig 1: From tuning fork to complete microscope. The microscope has a maximum scanning range of (40 μm)2 at RT and (30 μm)2 at LT. Scanning time is less than 30 minutes for the maximum range with 512 x 512 pixel resolution.

Using tuning fork AFM with a magnetic tip we can investigate the spatial distribution of Hall sensor sensitivity, allowing quantitative 2D mapping of the sensor output with a spatial resolution of 10 nm. We study the room temperature response of the Hall sensor made of epitaxially grown graphene to local electric and magnetic fields in the diffusive regime. The asymmetric pattern of the transverse voltage observed at the intersection of current and voltage arms (Fig. 2) is due to induced inhomogeneity in the current flow. The asymmetric edge pattern depends strongly on current flow direction and the tip bias potential, whereas the response in the middle of the cross is only determined by the magnetic field. We also study the spatial resolution of graphene devices by varying the height of the magnetic tip above the sensor (Fig. 3). The experimental results are quantitatively confirmed by extensive numerical modelling (in collaboration with Alessandra Manzin, INRIM) developed under the assumptions of diffusive transport regime, non-uniform orthogonal magnetic field and stationary conditions.

Fig 2: Response of a 4x5 μm size epitaxial graphene Hall cross to local electric and magnetic fields of an MFM tip at zero lift height, I=100 μA (experiment and modelling).

This technique can be used for the calibration of Hall probes to in-homogenous magnetic fields and in turn provides a convenient, traceable in-situ calibration route for the magnetised tips, such as those conventionally used in MFM.