  The low-frequency and shot noises in spin-valve CoFeB/MgO/CoFeB magnetic
tunneling junctions were studied at low temperature. The measured 1/f noise
around the magnetic hysteresis loops of the free layer indicates that the main
origin of the 1/f noise is the magnetic fluctuation, which is discussed in
terms of a fluctuation-dissipation relation. Random telegraph noise (RTN) is
observed to be symmetrically enhanced in the hysteresis loop with regard to the
two magnetic configurations. We found that this enhancement is caused by the
fluctuation between two magnetic states in the free layer. Although the 1/f
noise is almost independent of the magnetic configuration, the RTN is enhanced
in the antiparallel configuration. These findings indicate the presence of
spin-dependent activation of RTN. Shot noise reveals the spin-dependent
coherent tunneling process via a crystalline MgO barrier.
