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Magnetic field and magnetic field gradient near the surface of a single crystal of the nanomagnet Mn_12-acetate detected by NMP-TCNQ EPR probe

An electron paramagnetic resonance (EPR)- based method for measuring magnetic field and magnetic field gradient near magnetic molecules has been suggested. It is illustrated by application to the surface of a single crystal of the nanomagnet [Mn_12O_12(CH_3COO)_16(H_2O)_4 2CH_3COOH 4H_2O (Mn_12-Ac). As the EPR probe, a 0.7 mm x 30 micron x 30 micron fiberous needle of the organic conductor N-methylphenazinium- tetracyanoquinodimethane (NMP-TCNQ) has been employed. The probes sharp EPR peak (peak-to-peak linewidth ~20 microT) exhibits a shift as well as splitting due to increase in the magnetic moment of the Mn_12-Ac crystal on temperature lowering. The shifted peaks yield the magnitude of the additional magnetic field to which the various fibers of the probe subject. In conjunction with electron microscopy, the shifts yield the field gradient at the crystal surface, as a function of temperature. For the Mn_12-Ac crystal at 10K, the surface magnetic field was measured to be in the mT range and its gradient of the order of 50T/m.

Mn_12-acetate; EPR; NMP-TCNQ

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