Single-Spin MRFM Sensitivity at IBM

The presence of a single electron's spin has been detected by a magnetic resonance force microscope (MRFM), a device which brings together two exquisite sensing technologies---magnetic resonance imaging (MRI) and atomic force microscopy (AFM).

The ultimate goal of MRFM is to map the interior of a material sample, such as a complicated semiconductor structure or a bio-molecule, at atomic-scale resolution. To do this the MRFM uses a very frail cantilever, 85 microns long and 150 nm thick, with a tiny magnetic tip, plus a nearby radio-frequency coil to create a bowl-shaped resonance zone.

Any magnetic particle---such as a single electron or even the nucleus of a hydrogen atom (a proton)---that comes into the zone can interact magnetically with the cantilever, whose oscillation frequency is altered in a detectable way by the presence of the spin. Spin is a quantum parameter; a particle with spin will undergo interactions with other magnetic objects. Classically speaking, a particle with spin will behave like a tiny bar magnet.

An MRFM scan differs from an MRI scan in that "scanning" in MRI uses very sophisticated techniques by which a signal is obtained from all different 3D regions (voxels) simultaneously. MRFM, by contrast, is more of a point-wise scan, followed by an image reconstruction procedure.

MRFM isn't just a form of microscopy (telling you where the molecules are) but in the case of nuclear spins, is also a form of spectroscopy that can, in principle, identify certain chemical elements, at least those whose nuclei are magnetic.

Since MRFM made its debut more than a decade ago the sensitivity of the device has improved by a factor of ten million, but it can't yet detect single nuclei. The intrinsic magnetic strength (the "magnetic moment") of a single nucleus is just too weak, about 650 times weaker than an electron's magnetic moment.

To detect individual nuclear spins and thereby achieve spatial resolution at the atomic scale, a further improvement in sensitivity by a factor of about a thousand will be necessary. Currently, conventional MRI forms images from nuclear spins, but needs a trillion or more to get a strong enough signal.

Now, for the first time, an MRFM has mustered sufficient sensitivity to detect the spin of a single electron amid a sample where most of the electrons in the atoms are paired up (and thus rendered nonmagnetic). In the 15 July issue of Nature, Dan Rugar and his colleagues at IBM Almaden (San Jose) report on an MRFM device which uses a slender cantilever operating at a temperature of 1.6 K.

The precision of the setup and the chilly conditions permit single electrons in a silicon dioxide sample to be located. The associated spatial resolution, at least in one of the three dimensions, is a mere 25 nm. (A few months ago an MRFM result with something like a million-electron-spin resolution was reported; see Update 680.)

In terms of imaging sharpness the new IBM device is about 40 times better than the best conventional MRI available. Not only is MRFM a potentially splendid imaging device, but it may also play a part in future quantum information devices owing to MRFM's ability to manipulate and read the quantum state of individual spins. (See IBM's MRFM website.)