We report local studies of magnetic-field-modulated microwave reflectivity
(MFMMR) of high-T-c superconductors using a near-field scanning aperture pr
obe operating at 96 GHz. The MFMMR signal appears as a sharp peak at transi
tion temperature and is very useful in identifying superconducting phases i
n multiphase samples. We show that MFMMR is qualitatively different from su
ch a routine characterization tool as inductive technique. Simultaneous MFM
MR and inductive measurements allow us to distinguish between the surface a
nd bulk superconducting properties. The MFMMR technique may be easily exten
ded for the study of giant magnetoresistance materials. (C) 1999 American I
nstitute of Physics. [S0003- 6951(99)01838-0].