Magnetic flux and magnetic domain structure in fabricated MRAM bits were observed by an electron holography technique under the residual magnetic field <0.2 mT at the specimen position. The small residual filed condition without the objective lens switched off increases the detection limit of a magnetic field in the electron holography. Here we report a detailed analysis of the magnetic microstructure in MRAM bits performed by extracting the magnetic information alone from a reconstructed phase image. The MRAM bits fabricated with a 12 nm CoFeBx-free layer revealed non-uniform distributions in the magnetic flux and the magnetic domain structure contrary to the MRAM bits with a 6 nm CoFeBx-free layer. Lorentz microscopy study of the MRAM bits with the application of external magnetic fields allows us to quantitatively evaluate the expected magnetic switching field of the MRAM bits which should be an important factor to achieve a high-density MRAMs.
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