Force Microscopy

Researchers

Jeehoon Kim, Alex Frenzel

Instrument

We have constructed a custom variable temperature force microscope with the following features:

• Vibration isolation: floating room, floating table, internal suspension from damped springs.
• Temperature: 2K up to 340K
• Magnetic field: 5 Tesla vertical
• Scan Range: 35μm scan at room temperature;
  several mm coarse (x,y,z) motion of sample with respect to tip
• Cantilever position detection:
  fiber optic interferometer with λ=1550nm diode laser;
  position noise better than 2×10^-3 Å/√Hz
• Geometry: allows either horizontal or vertical cantilever orientation

Funding

NSF PHY 01-17795

Metal-Insulator Transition in VO₂

We are currently investigating the electric field triggered insulator-to-metal transition in vanadium dioxide (VO₂) using conducting atomic force microscopy (CAFM). We have demonstrated that a biased CAFM tip can be used to simultaneously change and measure the local conducting state of the VO₂ sample at room temperature. Additionally, we are able to visualize the transition in individual VO₂ grains by collecting spatially-resolved IV characteristics. The goals of this research are to understand the physics behind the electric field triggered MIT and to provide detailed spatial information about the transition, which will be necessary for incorporation of VO₂ into practical devices.

Collaborators

Shriram Ramanathan, Harvard University

Vortex Depinning in Iron Pnictide Superconductor NdO_1-xF_xFeAs

Fig. 5 Photo of single crystal NdO_1-xF_xFeAs sample, grown by the group of Prof. Paul Canfield, Ames Lab, Iowa.

Iron pnictide superconductors, discovered in February 2008, are a new class of high T_c superconductors that have drawn much attention, both for their role as a foil to better understand the cuprates, and for their possibly superior application potential. Key to the latter will be the understanding of vortex pinning in these materials. The two main families of iron pnictide superconductors, "1111" and "122", have different vortex pinning properties. We use our magnetic force microscope to focus on the "1111" family of materials, especially NdO_1-xF_xFeAs, which shows T_c>50K, among the highest of all the iron-pnictides. The "1111" system shows a large electrical anisotropy compared with the "122" system. This may allow exotic vortex phases such as pancake vortices and Josephson vortices. We are exploring the nature of vortex pinning by using a magnetic-tipped cantilever to manipulate vortices as a function of applied magnetic field and temperature.

Fig. 6 We use the MFM to image and manipulate a vortex in NdO_1-xF_xFeAs at 5 K with a magnetic field of 1 Gauss. (A) Image at a "safe" height where tip-sample interactions are too weak to depin vortices. (B) Lower the tip to increase the interaction and drag a vortex. (C) Re-image at a "safe" height to demonstrate the vortex motion.

Collaborators

Paul Canfield, Ames Lab, Iowa