Probing magnetic fields on crystals of the nanomagnet Mn12-acetate by electron paramagnetic resonance.

We report on electron paramagnetic resonance (EPR) probing of magnetic fields and magnetic field gradients near the surface of a single crystal of the nanomagnet [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-Ac). As the EPR probe, we utilized a 0.7 mm x 30 microm x 30 microm fibrous needle of the organic conductor N-methylphenazinium-tetracyanoquinodimethane (NMP-TCNQ), which yields an exceptionally sharp peak, with a 0.2 G (approximately 20 microT) width. In the presence of Mn12-Ac, the probe's peak exhibits splitting on temperature lowering, which depends on the orientation of the Zeeman field relative to the axis of easy magnetization of the employed Mn12-Ac crystal. The shifted peaks yield the magnitude of the magnetic field from Mn12-Ac crystal to which the various fibers of the probe are subjected. In conjunction with electron microscopy, the shifts yield the field gradient at the crystal surface and its change with temperature. For Mn12-Ac at 10 K, the surface magnetic field was measured to be in the mT range and its gradient on the order of 50 T/m.