Tunneling magnetoresistance in spin valves exchange biased with metallic antiferromagnet La0.45Sr0.55MnO3

We present a detailed study of tunnel magnetoresistance (TMR) in La0.45Sr0.55MnO3/La0.67Sr0.33MnO3/SrTiO3/Co spin valve structures. The nonlinear current-voltage (I-V) characteristics of the 25×25 μm2 junctions, when modeled in the framework of elastic tunneling through trapezoidal potential barrier, yield a barrier height in confirmation of SrTiO3 band gap and its thickness. The zero-bias value G(0) of the differential conductance measured directly is high at low temperatures and then drops monotonically with temperature till ≈215 K followed by a rise on increasing the temperature. We observe a lower junction resistance (negative TMR) when the magnetizations M⃗1 and M⃗2 of the two ferromagnetic layers are aligned antiparallel to each other, suggesting negative spin polarization. Moreover, the TMR shows a precipitous but asymmetric drop with bias voltage till it reaches ≈±200 mV suggesting role of inelastic excitations in reducing the TMR. The unique feature of this study is the robust exchange bias with exchange energy J≈0.13 erg cm−2 at the interface between antiferromagnetic La0.45Sr0.55MnO3 [LSMO(AF)] and ferromagnetic La0.67Sr0.33MnO3 (LSMO), which has been used to engineer coercivity contrast between the LSMO and cobalt films. The difference of coercivity (HC) between LSMO and Co increases as the temperature is lowered and manifests itself as a change in switching fields of resistance in tunnel junctions. We also show that the exchange bias shift can be engineered by different field cooling directions and by changing the relative thickness of La0.45Sr0.55MnO3 and La0.67Sr0.33MnO3.