Suppression of spin pumping with insulating layers

The damping constant of thin magnetic layers is a combination of an intrinsic part related to the material itself and an extrinsic contribution related to the spin pumping effect. Here a thermal fluctuation of the magnetization is creating a spin current that leads to a loss of angular momentum and increase in damping constant. For many applications (e.g. magnetic read heads) it is desirable to have magnetic layers with lowest damping constant as possible to reduce thermal fluctuations and noise. To reduce the spin pumping the layer next to the magnetic film should have small spin flip scattering rate and high rate of momentum scattering. Insulating layers possess these properties but are problematic to use in a CPP type sensor because of the restriction of current flow. In a sufficiently thin tunnel barrier however the RA can be very low. Here we show data on damping constant of a magnetic layer sandwiched between two tunnel barriers and examine how the extrinsic damping constant varies with the RA of the barrier. >10% reduction of damping constant can be achieved with RA values that are 10 times smaller than typically used in a magnetic read head. We also characterize the magnetic properties (Ms, Hk, magnetostriction) as a function of barrier RA. B.K. and T.M acknowledge support by NSF-CAREER Award No. 0952929.