Theory for Temperature Dependence of Tunnel Magnetoresistance: Crucial Role of Interfacial s-d Exchange Interaction

Large reduction of tunnel magnetoresistance (TMR) ratios with increasing temperature is a long-standing problem in the field of spintronics. We here theoretically demonstrate that the s-d exchange interaction at interfaces of magnetic tunnel junctions (MTJs) is a possible physical origin of this phenomenon. We calculate the TMR ratio at finite temperature in Fe/MgO/Fe(001) by constructing a tight-binding model with the s-d exchange interaction. It is shown that the calculated TMR ratio sharply decreases as a function of temperature, consistent with experimental results. The s-d exchange interaction propagates thermally induced spin fluctuations from d- to s-electron state, which breaks the half-metallicity in the Δ 1 state and reduces the TMR ratio significantly. We also propose a nonempirical method for estimating the coupling constant of the s-d exchange interaction J sd , which is the key parameter for the temperature dependence of the TMR ratio. The estimated values of J sd for bcc Fe 1−x Co x successfully explain the material dependence of the TMR ratio at room temperature obtained in previous experiments.