By Jean-Francois Roch, Université Paris-Sud and ENS Cachan
The ability to quantitatively map magnetic field distributions is of crucial importance for fundamental studies ranging from materials science to biology, and also for the development of new devices in spintronics. Recently it has clearly demonstrated that scanning NV magnetometry is an efficient technique which combines high sensitivity and nanoscale resolution (for a review, see ). This technique relies on the optical detection of the electron spin resonance associated with a single NV center in diamond attached to a AFM tip. The magnitude of the stray magnetic field above a magnetic sample can then be determined from the Zeeman shifts of the energy levels associated to this artificial atom in the solid state.
Extending this technique to cryogenic environment will open the way to investigate many magnetic phenomena occuring in complex condensed matter systems, such as superconductivity or the magnetic properties of strongly correlated systems. I will present our recent realization of a scanning magnetometer based on NV centers in a nanodiamond, in a low-temperature setup which combines atomic force microscopy and optical confocal microscopy. This scanning NV magnetometer has been applied to the imaging of magnetic domain walls in GaMnAsP, a semiconductor with a Curie temperature around 100 K which displays dilute ferromagnetism  so that spin-polarized electrical currents can reverse the magnetization direction of the magnetic domains through torque .
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