A method of manufacturing an MRAM device, the method including forming a first magnetic layer on a substrate; forming a first tunnel barrier layer on the first magnetic layer such that the first tunnel barrier layer includes a first metal oxide, the first metal oxide being formed by oxidizing a first metal layer at a first temperature; forming a second tunnel barrier layer on the first tunnel barrier layer such that the second tunnel barrier layer includes a second metal oxide, the second metal oxide being formed by oxidizing a second metal layer at a second temperature that is greater than the first temperature; and forming a second magnetic layer on the second tunnel barrier layer.