Magnetization dynamics in racetrack memory

Magnetization Dynamics in Racetrack Memory Various devices have been proposed which use magnetic domain walls (DWs) in nano-sized magnetic structures to perform logic operations or store information. In particularin ‘Racetrack memory’ bits of information represented by DWs are shifted in amagnetic wire to be stored. For these memory and logic devices to be successful, greatcontrol of DW motion is of vital importance. In cooperation with IBM’s Almaden research laboratory a pump-probe Kerr magneto-optical scanning microscope has been developed. In order to control DW injection,motion and reset, magnetic fields have to be applied locally on the nanowire. For this aspecial Damascene CMOS chip has been fabricated at the 200 mm wafer facility atIBM Microelectronics Research Laboratory (MRL). Probing of the local magnetizationis done with a focused pulsed laser spot of 400 nm diameter where the polarizationrotation caused by the Kerr effect is measured after reflection. In order to achieveoptimal focusing a perpendicular incident laser beam is focused with a high numericalaperture objective. Synchronized ‘pumping’ in this scheme is achieved by successively:1 injecting a DW; 2 propagate the DW down the nanowire with either current throughor an applied field pulse over the nanowire; 3 and finally resetting the whole nanowireto its original magnetization by applying a large field together with the injection of anopposite magnetic domain. With this setup field and current induced DW motion isstudied in permalloy nanowires ranging in width from 200 to 700 nm and thickness of20 nm. For control of DWs in Racetrack memory it is important to understand the differentmechanism for driving a DW already in motion (dynamic) and driving a DW that iscurrently at rest (static). The propagation field, the minimum field below which no DWmotion takes place, is measured for both dynamic DWs and static DWs. It is found thatStatic DWs require a much higher field than DWs already in motion. A model is buildwhere this effect is related to the wire roughness, successfully describing the existenceof a propagation field, the difference between both propagation fields and a specificeffect related to the method of injection.