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The effects of Hf-layer insertion in the AlOx tunnel barrier on tunnel magnetoresistance (TMR) are studied, and we have modified the oxide barrier by forming Hf??Al-oxide materials in the middle of Al oxide. Junctions with Hf-inserted barriers show higher TMR ratio and weaker temperature dependence than those with the conventional Al-oxide barrier. From the analysis of temperature dependence, it was concluded that the enhancement of TMR by the introduction of an Hf layer was due to the reduction of defects in the barrier.