Sub-Micron Size Magnetic Multilayers with Perpendicular Current Flow (CPP)

In these multilayers the maximum resistance change occurs when the relative orientations of the magnetizations in adjacent ferromagnetic layers vary from antiparallel (AP) to parallel (P). Since these two orientations can serve as memory states, there is keen interest in applying giant magnetoresistance to magnetic random-access memories (MRAM). One proposed MRAM consists of a toroidal shaped multilayer of sub-micron diameter, with the shape constraining the magnetization to be circular, either clockwise or counterclockwise. A low CPP current can 'read' the state of the memory unit via its resistance, and a high CPP current density can switch the relative orientations of magnetizations. The figure shows such a sub-micron structure made with a [Co/Cu] multilayer with the Cu thickness chosen to produce an approximate AP state in low field. At 4.2K, we have achieved a CPP magnetoresistance of ~ 25%, comparable to that in large samples. We have also seen high CPP current densities modify the relative orientations of the Co layer magnetizations. "This behavior is consistent with a Stoner-Wohlfarth analysis assuming that the flipping occurs by uniform rotation of the magnetization in the plane of the sample.

R_CPP vs H for an MSU toroidal-shaped [Co(1.5nm)/Cu(2.1nm)]₂₀ multilayer.

Reference:
"Fabrication and Magnetic Characterization of Single Domain Co₉₁Fe₉ Nanostructures", C.E. Moreau, J.A. Caballero, R. Loloee, W.P. Pratt, Jr., and N.O. Birge, J. Appl. Phys. 87, 6316 (2000).