Temperature dependence of transport properties of Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions

We have performed a systematic analysis of the voltage and temperature dependence of the tunneling magnetoresistance (TMR) in Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions (MTJs) with barrier thickness dMgO between 1.8 and 4 nm. All the junctions show a comparable TMR of about 300 % at low temperature with low bias voltage. Both the junction resistance and magnetoresistance decreases with inceasing temperature or bias voltage. In the bias voltage range of ±500 mV and temperature range of 13-330 K the strongest decrease of TMR with either increase in bias voltage (of about 98 %) or increase in temperature (of about 94 %) was observed for MTJ with 4.0 nm thick barrier. whereas for dMgO=1.8 nm the smallest drop of 51 % with increase in voltage and 43 % with rise of temperature was found. MTJs with 2.1 nm and 3.0 nm displayed an intermediate behaviour. This behaviour was analyzed in the framework of recently suggested models for the bias and temperature dependent transport. Especially, the influence of unpolarized and polarized hopping conductance are discussed.