Current-induced magnetic field detection around fine current paths by magnetic force microscopy

A commercial magnetic force microscopy (MFM), combined with a potential feedback, is described. The MFM is operated in an intermittent contact mode, and a vertical displacement of the cantilever is used for observing a topography. Magnetic force images are obtained simultaneously with the topography with this setup. At total current of 200 μA, topography, amplitude and phase difference of the magnetic force signals detected in T-junction gold wires are obtained. Approximately, the amplitude and the phase difference correspond to the peak value and the direction of the magnetic field gradient, respectively. Large gradients in the magnetic field is found to exist with counter directions which were perpendicular to the current flows around upper and lower current paths. The magnetic force signal around a left current path is noise level because the magnetic field around it is parallel to the cantilever beam. In addition, the phase difference image indicates that the directions of the magnetic field gradient are inverted between the upper and lower paths because of the opposite directions of current flow. Inverting the magnetization direction of the MFM tip, the observed phase signal is also inverted. These results show that the magnetic field gradients around current paths are successfully detected.