Exchange-coupled suppression of vortex formation in permalloy nanodot chain arrays

Abstract

The effect of interdot exchange coupling on the magnetization reversal processes in nanodots has been investigated in a study on permalloy dot arrays with dot diameters of 300 nm and thicknesses of 40 nm. The dots, fabricated using e-beam lithography and ion beam deposition, are exchange coupled via 50 nm long permalloy bridges of widths ranging from 0 to 60 nm. Chains of five collinear coupled dots form the unit cell of the array structure. Magneto-optical Kerr effect hysteresis loops are reported with comparison to simulations. With field applied along the coupling direction, vortex nucleation occurs sequentially from the outer to the inner dots of the chain. Compared with uncoupled dots, nucleation is suppressed by the interdot exchange coupling resulting in highly correlated magnetization patterns. Buildup of exchange energy in the inner dots ultimately drives the bridge magnetization to switch, followed by abrupt, simultaneous vortex nucleation in these dots. Along the perpendicular-to-the-coupling direction, the exchange interaction has little effect and magnetic reversal is almost identical to arrays of isolated dots of the same geometry.