Magnetic resonance force microscopy

Magnetic resonance force microscopy has reached 90-nm resolution. MRFM maps the spins in a sample that is mounted on an ultrasensitive silicon cantilever hanging vertically over a sharp magnetic tip. If the tip’s magnetic field is highly inhomogeneous, an applied radio-frequency field will resonate with the spins in a highly localized region of the sample, and the cantilever will deflect due to the resulting forces. To achieve the nanoscale resolution, John Mamin, Dan Rugar, and their colleagues at the IBM Almaden Research Center in San Jose, California, made tips with magnetic gradients of more than a million teslas per meter and held the samples about 45 nm away. The test objects being imaged consisted of tiny islands of calcium fluoride evaporated onto the cantilever tip. The image shows a schematic (top), a simulation (middle), and the experimental result (bottom). Separations of 100 nm could be clearly resolved. Previously, the same group of physicists had used a similar setup to detect the magnetic resonance of a single unpaired electron in a sample (see Physics Today, September 2004, page 21 ). But now they are detecting much weaker nuclear spins. The nuclear advantage lies in its generality: Many materials have nuclear magnetic moments, but relatively few have unpaired electron spins. The nuclear MRFM experiment effectively explored a 650-zeptoliter volume, some 60 000 times smaller than the best conventional magnetic resonance imaging capability. Rugar thinks that their current apparatus can now detect 200 nuclear spins, which brings the group closer to their ultimate goal of imaging molecules at the single nuclear spin level. (H. J. Mamin et al., Nat. Nanotechnol. 2 , 301, 2007 .)