In-situ TEM studies of magnetization reversal processes in magnetic nanostructures
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In-situ TEM studies of magnetization reversal processes in magnetic nanostructures Amanda K Petford-Long1, Thomas Bromwich2, Amit Kohn2, Victoria Jackson2, Takeshi Kasama3, Rafal Dunin-Borkowski3 and Caroline A Ross4 1 Argonne National Laboratory, 9700 S Cass Ave, Argonne, IL 60439. 2 Dept. of Materials, Univ. of Oxford, Parks Road, Oxford OX1 3PH, UK. 3 Dept. of Materials Science and Metallurgy, Univ. of Cambridge, Pembroke St, Cambridge CB2 3QZ, UK 4 Materials Science and Engineering Dept, Massachusetts Institute of Technology, Cambridge, MA ABSTRACT
One of the most widely studied types of magnetic nanostructure is that used in devices based on the giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) phenomena. In order to understand the behaviour of these materials it is important to be able to follow their magnetization reversal mechanism, and one of the techniques enabling micromagnetic studies at the sub-micron scale is transmission electron microscopy. Two techniques can be used: Lorentz transmission electron microscopy and off-axis electron holography, both of which allow the magnetic domain structure of a ferromagnetic material to be investigated dynamically in realtime with a resolution of a few nanometres. These techniques have been used in combination with in situ magnetizing experiments, to carry out qualitative and quantitative studies of magnetization reversal in a range of materials including spin-tunnel junctions, patterned thin film elements and magnetic antidot arrays. Quantitative analysis of the Lorentz TEM data has been carried out using the transport of intensity equation (TIE) approach.
INTRODUCTION Over the past few years there has been an increased effort in the development of new materials for information storage applications, often in the form of layered structures containing many thin layers. This has been motivated by an intensive demand for improvement in information storage and memory density, mainly for graphics-intensive applications. The structures that are being most widely developed for read-heads rely on the giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) effect seen as a large change in resistance in an applied magnetic field [1]. Devices that make use of the GMR effect are spinvalve (SV) structures [2], and magnetic tunnel junctions (MTJs) [3]. The limits on memory density that can be achieved using solid state memory, in addition to the requirement for low power consumption and thus for non-volatile memory means that there has also been interest in the development of magnetic memory based on the GMR and TMR effects, namely magnetoresistive random access memory (MRAM) [4]. The GMR effect results from a change in the relative orientation of the magnetization in ferromagnetic (FM) layers separated by a nonmagnetic spacer: parallel magnetization in adjacent layers gives a low resistance, whereas a high resistance is measured if the magnetization in adjacent layers is antiparallel. In SV and MTJ
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structures, the magneti
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