MECHANICAL DETECTION OF MAGNETIC-RESONANCE

CONVENTIONAL techniques for measuring magnetic resonance involve the detection of electromagnetic signals induced in a coil or microwave cavity by the collective precession of magnetic moments (from nuclei or electrons) excited by an alternating magnetic field. In a different approach1, isolated electron spins have been detected by scanning tunnelling microscopy, with the spin precession inducing a radiofrequency modulation in the tunnelling current. Here, we describe a new and extremely sensitive method of detection, the principles of which derive from magnetic force microscopy2-5 and a recent proposal6,7 by one of us (J.A.S.). We measure the small, oscillatory magnetic force (10(-14) N) acting on a paramagnetic sample (a few grains of diphenylpicrylhydrazil, weighing <30 ng) which has been excited into magnetic resonance in the presence of an inhomogeneous magnetic field. This force is detected by optically sensing the angstrom-scale vibration of a micromechanical cantilever on which the sample is mounted. The sensitivity of this technique to the spatial distribution of the spins suggests that mechanical detection of magnetic resonance has the potential for imaging microscopic samples in three dimensions. So far, we have achieved a spatial resolution of 19 mum in one dimension.