Large Scale Periodic Magnetic Nanostructures Fabricated by Optical Interference Lithography
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Large Scale Periodic Magnetic Nanostructures Fabricated by Optical Interference Lithography
A. Carl, S. Kirsch, and E.F. Wassermann Gerhard-Mercator-Universität Duisburg, Tieftemperaturphysik, 47048 Duisburg, Germany
ABSTRACT Large scale periodic arrays of Co/Pt multilayer dots with perpendicular magnetic anisotropy are fabricated utilizing optical interference lithography with Ar+ ion lasers operating at wavelengths of 457nm and 244nm, respectively. The experimental technique allows us to fabricate dot-arrays with periodicities ranging between 125nm and 1100nm and with corresponding dot diameters between 70nm and 740nm. The dot-arrays are prepared on (100)-silicon substrates covering a total area of up to 20cm2 with a maximum dot density of about 4.1x1010dots/in2 as well as within the surface of (110)-silicon substrates. The global magnetic properties of the dot-arrays are characterized by the magneto-optical Kerr effect. The micromagnetic properties of a single Co/Pt dot are measured with quantitative magnetic force microscopy (QMFM) by using a MFM-tip, the magnetic properties of which have been calibrated within the point probe approximation with nanofabricated current carrying rings. This allows us to measure quantitatively the z-component of both the magnetization and the stray field of a Co/Pt dot on the nanometer scale.
INTRODUCTION In the past, magnetic nanostructures have attracted much attention because on the one hand they allow to investigate longstanding questions in micromagnetism [1]. On the other hand, they offer a variety of interesting applications in e.g. spin-electronics [2] or as magnetic random access memory (MRAM) devices [3]. So-called patterned media consisting of periodically arranged nano magnets are of particular interest for future high density magnetic storage media, since they could possibly replace the storage media of e.g. hard disk drives [4]. Indeed, for the currently used homogeneous magnetic media inevitable physical obstacles are expected to limit the achievable areal storage density in the near future. State-of-the-art homogeneous magnetic media consist of antiferromagnetically coupled CoCrPtTa thin films and they are capable to store information with an areal storage density of about 20-30x109bits/in2 (20-30Gbit/in2) [5]. On the microscopic scale the CoCrPtTa films consist of a granular structure in which Co-rich grains, the mean diameter of which is about 10-15nm, are well separated and magnetically exchange decoupled by virtue of Cr-segregation into the grain boundaries. An increase in areal storage density is mainly achieved by down-scaling the average grain diameter such that smaller bits can be written under the constraint that the number of grains per bit (roughly 100) is kept constant for the reason of a sufficient signal-to-noise ratio. Using homogeneous media, in the future, storage densities >100Gbit/in2 are deemed possible when taking advantage of perpendicularly magnetized media [5,6]. However, there is a finite upper limit to the storage density, imposed by the
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