The novel properties of nanoscale layered magnetic structures depend critically on their microstructure and composition, with variations on the atomic scale leading to variations in properties. Magnetic tunnel junctions (MTJs) represent one example of such a structure. In their simplest form MTJs consist of two ferromagnetic layers on either side of a nanoscale oxide tunnel barrier. The magnitude of the spin-polarized current that tunnels across the barrier depends on the relative orientation of the magnetization in the two ferromagnetic layers. This effect -- tunnel magnetoresistance (TMR) -- is the phenomenon on which a number of technological applications are based, such as read heads in hard disk drives, and magnetoresistive random-access memory. The TMR phenomenon depends critically on the nature of the interfaces between the oxide tunnel barrier and the ferromagnetic layers on either side, as these control the magnetotransport by affecting parameters such as spin polarization, tunnel barrier height, and indeed the shape of the tunnel barrier. We have used high resolution electron microscopy (HREM), transmission electron microscopy (TEM) chemical mapping and atom probe tomography (APT) analysis to obtain high spatial resolution information about the interfaces in MTJs. We have used TEM to study not only the microstructure of the materials, but also to carry out in-situ local measurements of their transport properties, which can be directly correlated with the structure, and to carry out in-situ studies of the magnetization reversal behavior of the various layers that constitute magnetic tunnel junction structures. The results of these studies will be presented.
Argonne Physics Division Colloquium Schedule