Annealing effect on low frequency noise in MgO-based magnetic tunnel junctions

The annealing temperature (T-a) dependence of the low frequency noise in exchange-biased magnetic tunnel junctions (MTJs) with an MgO barrier and CoFeB electrodes has been investigated for 200 < T-a < 425 degrees C. A 1/f noise spectrum is observed for the parallel and antiparallel states, where the Hooge parameter (alpha) for the parallel state remains unchanged with T-a after the CoFeB electrodes is fully crystallized, while alpha for the antiparallel state increases over the range of T-a from 275 to 425 degrees C. The 1/f noise for the antiparallel state is largely of magnetic origin, and it can be attributed to magnetization fluctuations of ferromagnetic electrodes due to the loss of the exchange bias, whereas there is little magnetic noise in the parallel state. An annealing temperature of 325 degrees C yields an optimized signal-to-noise ratio, as the CoFeB electrodes become fully crystallized. The bias dependence of alpha is discussed based on the Glazman-Matveev model, which considers both elastic and inelastic tunneling.