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One diamond + one defect = a very sensitive magnetic microscope


This equation has just been experimentally proved by a team from LPQM (CNRS & ENS Cachan) and LAC (CNRS, ENS Cachan & UPSUD), in a collaboration with our lab LPS (CNRS & UPSUD). The defect in diamond, called a NV center (a nitrogen atom coupled to an atomic vacancy in this carbon monocrystal), is like an artificial atom the luminescence of which changes strongly when excited by a resonant radiofrequency. In presence of a magnetic field, the resonance frequency changes by about 100 MHz per milli-Tesla of field applied along the N-V axis, in a linear way for fields up to 10 mT (recall the earth’s field is about 0.04 mT). These properties are known in the community and exploited in various ways, but a real magnetic microscope based on them was not yet available.

 

The instrument developed is based on an atomic force microscope, in which a cantilever terminated by a pyramid is brought in controlled contact to the sample surface. The key point is to glue a single nano-diamond (20 nm size) close to the apex of the pyramid, having beforehand checked that it contains a single NV center. The fluorescence light emitted by this defect is collected by a microscope objective, and the radiofrequency is finely scanned (around 2.9 GHz). The frequency at which the fluorescence yield drops is directly related to the magnetic field at the position of the NV defect.
 

Images obtained from a collection of magnetic nanopatterned elements prove the exquisite field sensitivity of this microscope, by revealing details hitherto unobserved. Truly enough, magnetic field microsensors exist with a good sensitivity, like micro Hall probes or micro-SQUIDs, but they average the field over a surface of 100 nm typical lateral size. Here, the field is measured over an atomic scale (0.1 nm). Noteworthy is that this microscope measures the stray field emanating from the sample. As a result, the spatial resolution of this microscope is limited by the vertical distance between object and detector, in exact similarity to the read head of a hard disk drive. Researchers are trying to reduce this distance, presently slightly below 100 nm. The remarkable field sensitivity of this microscope (and not to its vertical gradient, like in the ubiquitous magnetic force microscopy), together with its ultimate size of the detector, should allow this microscope answering pending questions about the exact nature of some magnetic structures, or revealing the orbital magnetism of microstructures.

 

The NV center magnetic field microscope. (Left) Schematic of the experiment. The NV center, attached to the tip of an atomic force microscope, is scanned above a pattermed microsquare of the soft magnetic alloy Ni80Fe20, with edge size 1 μm and 50 nm thickness. The magnetic structure consists of two magnetic domain walls along the square diagonals, with a magnetic vortex in the center. (Right) Image obtained with the NV microscope, showing the distribution of the stray magnetic field emanating from this structure (projected along the N-V axis). The rapid imaging mode developed in this work reveals the iso-field lines, with values of 0.6 and 0.9 mT here. Copyright J.-P. Tetienne – ENS Cachan.

Reference :

 

Stray-field imaging of magnetic vortices with a single diamond spin, L. Rondin et al., Nature Commun. 4:2279 (2013)

 

Contacts :

 

  • Stanislas Rohart (stanislas.rohart@u-psud.fr) Laboratoire de Physique des Solides, Université Paris-Sud et CNRS, Orsay
  • Vincent Jacques (vjacques@ens-cachan.fr) Laboratoire de Photonique Quantique et Moléculaire, ENS Cachan et CNRS, Cachan ; Laboratoire Aimé Cotton, CNRS, Université Paris-Sud et ENS Cachan, Orsay