Abstract
The remanent domain structures of composite element magnetic barcodes have been imaged using photo-emission electron microscopy with contrast from x-ray magnetic circular dichroism (XMCD-PEEM) and analysed with reference to the results of micromagnetic simulations. The magnetisation configuration at the end of wide strips is found to be perpendicular to the majority magnetisation direction. This transitions to an incomplete rotation for nominal strip widths below 300 nm and is found to affect the mechanics of magnetisation reversal for nominal strip widths below 200 nm, owing to a difference in magnetisation orientation when an external magnetic field is applied that is just smaller than the magnetic coercivity of the structures and a corresponding change in reversal dynamics. This change in domain structure as strip width decreases is consistent with both the influence of shape anisotropy and with measurements of magnetic hysteresis. The magnetisation reversal characteristics of composite element structures are found to be dependent on the relative magnetisation configurations of neighbouring strips, which in turn are found to vary stochastically upon the application and removal of a magnetic field along the easy axis of the structure. It is found that the application of a canted field is necessary to ensure sharp, consistent magnetisation reversal of bits when writing a binary code. These results confirm that either improved lithography of narrower strips or non-rectangular elements would be necessary to further increase the number of individually programmable bits in a barcode.
Highlights
Magnetic barcodes have been identified as versatile, non-volatile magnetic memories that have potential applications in multiplexed molecular identification and biological and chemical assays [1]
A silicon wafer is prepared by cleaning in solvents and Ti/Au is grown on the surface by thermal evaporation. This Au layer prevents charge build-up during both the electron beam lithography and the XMCD-PEEM measurements—precise lithography and measurement are more critical in these structures than promotion of a large magnetic grain size
The width at which the magnetisation configuration at the tips of the strips changes between a remanent domain fully perpendicular to the magnetic easy axis and one at an angle to both the easy and hard axis in these simulations does not coincide with the strip width at which we found a change in power law relationship in measurements of magnetic hysteresis [4]
Summary
Magnetic barcodes have been identified as versatile, non-volatile magnetic memories that have potential applications in multiplexed molecular identification and biological and chemical assays [1]. The relationship between magnetic strip width and coercivity was found to have two regions that could be described by power laws, with a crossover between the two regions for widths 150–200 nm [4]. It was hypothesised that the change in power law relation resulted from a change in magnetisation configuration structure and, a change in magnetic reversal mechanism. In order to produce devices that perform predictably and consistently when writing a code, it is necessary to have a good understanding of the magnetisation reversal mechanism: in particular, the effect of the magnetisation structure, both at remanence and with an external magnetic field applied, on their magnetic coercivity and hysteresis
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