Force Microscopy


Jeehoon Kim, Alex Frenzel


photo of Force Microscope
Fig. 1 Photo of force microscope.

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

labeled photo of entire Force Microscope system
Fig. 2 Photo of force microscope setup, including vibration isolation.


NSF PHY 01-17795

Metal-Insulator Transition in VO2

CAFM geometry
Fig. 3 Geometry of the atomic force microscope setup used to measure the conductivity of VO2 as a function of voltage applied to the sample.
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.
VO2 hysteresis loop
Fig. 4 Hysteresis loop in the conductivity of VO2 as a function of applied bias, in conducting-AFM geometry.


Shriram Ramanathan, Harvard University

Vortex Depinning in Iron Pnictide Superconductor NdO1-xFxFeAs

photo of NdOFeAs sample
Fig. 5 Photo of single crystal NdO1-xFxFeAs 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 Tc 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 NdO1-xFxFeAs, which shows Tc>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.
before manipulation manipulation after manipulation
Fig. 6 We use the MFM to image and manipulate a vortex in NdO1-xFxFeAs 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.


Paul Canfield, Ames Lab, Iowa