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)]
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.
Highlight InfoDate: March 2006
IRG2: Spin Polarization and Transmission at Nanocontacts and Interfaces