Magnetoresistance and transport properties of CoFeB/MgO granular systems

Magnetoresistance and transport properties of ultrathin multilayers of CoFeB/MgO prepared by rf and dc magnetron sputtering were studied in the temperature range between 15 and 330 K. Due to the magnetic softness of CoFeB, this system offers a potentially high magnetoresistance at comparably low magnetic fields. At fixed MgO thickness of 0.7 nm, it undergoes a crossover from a granular to a multilayered structure with increasing thickness of CoFeB and metallic conduction is reached at around 0.8 nm. In the granular state, a ferromagnetic to superparamagnetic phase transition was observed at 130 K. Different charge transport phenomena were identified at different temperatures. A sharp increase in the magnetoresistance at low temperature can be attributed to higher order tunneling processes. By proper annealing procedures, an enhanced magnetoresistance of around 6% at room temperature and 14% at low temperature as well as an increase in the resistivity have been achieved. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3437278]