Program Highlights

Transition Metal Oxides as Tunnel Barriers

Tunneling magnetoresistance (TMR) is a large change of the conductance of magnetic tunnel junctions (MTJ) in magnetic field. In this effect lies the future of crucial technologies like high-density magnetic recording and non-volatile memory. TMR is due to imbalance in spin tunneling probability or spin polarization (SP). SP is moderate and positive for all ferromagnetic materials when materials such as Al₂O₃ or MgO are used as insulators due to filtering of the d electrons from the ferromagnet. In contrast, the complex band structure of the transition metal oxides (SrTiO₃ in particular) consists of multiple rates of decay some of which have symmetry compatible with the symmetry of the d electrons in the ferromagnet (Co). Thus, tunneling of d electrons is predicted to dominate the conductance making SP of the interface negative. The large TMR, high conductance (due to small band gap of SrTiO₃), and large SP found in Co/SrTiO₃/Co MTJs are all desirable qualities for applications which suggests transition metal oxides may provide a viable alternative to MgO and Al₂O₃ barriers for device applications. [Phys. Rev. Lett. 95, 216601 (2005)]

Tsymbal Group

The image shows the three lowest decay rates in momentum space figures. The states along the axes match the symmetry of the d electrons in the electrodes.