Magnetic Force Microscopy

One of the advantages of SPM is the ability to exploit specialty probes. Standard silicon tapping cantilevers can be coated with a magnetic alloy, typically 10-150 nm of CoCr alloy on 1-10 nm of Cr, to allow for magnetic contrast. The SPM probe tip which is in the form of a 3-sided pyramid, is magnetized using an external hard magnet. This external magnetic field aligns the domains formed on each of the sides of the pyramid, allowing the tip to act like a magnetic dipole.

The force exerted on a dipole is the dot product of the dipole moment and the gradient of the field at the surface. In magnetic force microscopy-- MFM-- these dipole magnetic forces are detected as a change in cantilever phase and amplitude a finite distance above the surface. In an MFM image, the tip is tapped on the surface to measure the surface morphology, and then re-scanned across the surface 50-150 nm above the surface to measure magnetic contrast. This re-scan above the surface is done in what is called lift mode. MFM in lift mode is just one of many interleave scans that can be performed-- a scan across the surface between morphology scans.

The left image shows the morphology of a computer hard-drive. Visible are highly oriented surface scratches. The right image shows MFM magnetic contrast in the tapping amplitude using a 100 nm lift distance with a magnetized Veeco MESP MFM probe.