Paper MN-TuM3
Ultrasensitive, Magnet-tipped Cantilevers for Magnetic Resonance Force Microscopy

Tuesday, October 16, 2007, 8:40 am, Room 615

Magnetic resonance force microscopy (MRFM) is a technique that may one day allow us to acquire magnetic resonance images of a single molecule - an extremely exciting prospect. To date we have demonstrated a sensitivity of ~10⁵ proton spins. Achieving the attonewton force sensitivity necessary to image single proton spins requires custom-fabricating cantilevers with extreme aspect ratios and mitigating sample-induced dissipation. In MRFM the force exerted on the cantilever, per spin, is proportional to the field gradient from the cantilever’s magnetic tip. Achieving single proton sensitivity therefore also requires dramatically reducing magnet size. Unfortunately, all MRFM tips produced to date have been made by manually affixing magnets one-at-a-time to a cantilever. Even if the tips are ion-beam milled, it is difficult to see how they can be made small enough to detect a single proton. We have developed an electron-beam-lithography process for batch fabricating nanoscale tip magnets overhanging the leading edge of ultrasensitive silicon cantilevers. As proof of concept, we will present a 50-nm wide overhanging cobalt magnet fabricated by a process involving electron beam lithography and anisotropic KOH etching, as well as cantilevers with 50-600 nm wide, non-overhanging magnets. Our current goal is to integrate the separate processes of cantilever fabrication and magnet fabrication. With our designed cantilever, we expect a sensitivity of better than 10³ protons. Even through the process integration challenges are daunting, our successes so far indicate that batch fabricating cantilevers capable of detecting single proton magnetic resonance may indeed be feasible.