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A relationship between boron (B) diffusion into the MgO barrier and pinhole creation in CoFeB/MgO/CoFeB-magnetic tunnel junctions (MTJs) was investigated. The diffused B in the MgO layer was identified by secondary ion mass spectrometry for the MTJs annealed at 350degC , which provide the giant magnetoresistance (TMR) ratio. The pinhole density, estimated from the statistic distribution of breakdown voltage of the TMR properties, increased as either the thickness or the B content of the CoFeB layer became thicker or higher. These experimental findings imply that the diffused B into the MgO barrier creates pinholes to short-circuit the tunnel conduction, since the amount of diffused B into the MgO barrier might be related to the total amount of the B content in the CoFeB layer. Three different techniques were found to be useful for the reduction of diffused B into the MgO barrier layer; usage of materials having boron affinity for capping layer, decrease of the total amount of B-content in CoFeB layer, and reduction of grain boundaries in the MgO barrier layer.