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We illuminate how the tunnel magnetoresistance (TMR) of MgO-based magnetic tunnel junctions (MTJs) is affected by the structure, materials, and fabrication processes. First, we demonstrate a possibility to control detrimental diffusions by separating a step for obtaining a grain-to-grain epitaxy in CoFeB/MgO/CoFeB layers from a step for achieving a high exchange-bias field in the pinned layer. A high TMR and large exchange-bias field can be obtained simultaneously by circumventing Mn diffusion and minimizing Ru diffusion during the annealing process at high temperature. Second, we show that the MTJs consisting of CoFeB/ MgO/ CoFeB/ Ru/ ferromagnet (FM), where FM is Co, Ni, NiFe, CoFe, or CoFeB, can provide a reasonably high TMR and decent thermal stability, presumably useful for the memory applications.