Tunneling Magnetoresistance (TMR)

Tunneling magnetoresistance is a quantum mechanical effect which occurs when two ferromagnets are separated by a few atomic layers of insulator. The conductance of such a tunneling junction can vary dramatically depending on whether the ferromagnets are alligned in parrallel or antiparallel. The effect is termed "tunneling magnetoresistance" (TMR), and the ratio of conductances $($G_P-G_AP)/G_AP is called the tunneling magnetoresistance ratio.

TMR in Fe/MgO/Fe

Early tunneling junctions were based on amorphous Alumina barriers and produced TMR ratios of the order of 50%. A breakthrough came in 2001 when a new fully crystalline junction with MgO barrier was proposed PRB 63, 220403(R) (2001) . In this publication we predicted that the TMR ratio of an Fe/MgO/Fe junction is very high, and grows as the thickness of the MgO barrier increases. It exceeds 1000% with just a few atomic layers of MgO. Very high TMR ratios in Fe/MgO/Fe were confirmed experimentally by the Yuasa and Parkin groups three years later (see Yuassa et. al. Nat. Mater. 3, 868 2004, and Parkin et. al. Nat. Mater. 3, 862 2004).

The experimental observations of Yuassa et. al. differed from those predicted in the epitaxial junction in that the TMR ratios saturated with MgO thickness at around 250%. In Phys. Rev. B 74 140404 (2006) we show that a modest ammount of interfacial impurities in the Fe/MgO/Fe tunneling junction easily accounts for this effect and leads to spectacular agreement with experiment.

More recently better growth techniques have lead to much higher quality interfaces and TMR ratios as high as 1000% have been observed experimentally. MgO based tunneling junctions are already being used as the latest technology for hard disc reading heads.

TMR in other systems

In Phys. Rev B 71 220402 (2005) we demonstrate that resonant tunneling occurs when metallic spacers are inserted on both sides of the insulator, such as in an epitaxial Fe/Au/MgO/Au/Fe(001). Such resonant tunneling can lead to very high TMR ratios. Additionally the TMR is found to oscillate as a function of the metalic spacer thickness, with periods dictated by its Fermi surface.

In JMMM 272 E1467 (2004) and Phys. Rev. B 68 174421 (2003) we examine oscillations of TMR in tunneling junctions containing a non-magnetic spacer as a function of the spacer thickness. we also study the effect of interface impurities on these oscillations.

In PRB 56, 11810 (1997) and PRB 60, 1117 (1999) we developed a theory of the tunneling magnetoresistance across a vacuum gap.

Some review articles on TMR