The magnetic memory in computer hard drives is written to with a read-head that is separated from the magnetic bits by less than 10nm. This means that extremely localised magnetic fields can be applied, which do not affect the magnetisation states of its neighbouring bits. Such a strategy is not compatible with high-throughput magnetic reading/writing in flow channels, this is because the minimum separation between read-head and elements is on the order of microns. To apply a 'local' non-interfering magnetic field will require a large separation between the elements on the tag.
To address this, we have developed a global encoding strategy, whereby a changing magnetic field is applied to the whole tag. If each element is engineered so that it switches magnetisation direction at a particular applied field strength, then it is possible to write to each bit in turn, from hardest to softest, as demonstrated in figure 1 and described in this paper: "High Throughput Biological Analysis Using Multi-bit Magnetic Digital Planar Tags" B. Hong et al., AIP Conf. Proc. 1025 74-81
Figure 1 (left) a monotonically decreasing magnetic field is applied to sequentially switch the individual magnetic bits in order of their hardness to write code (10101), (right) focussed MOKE images of 5-bit switching following this scheme.
The concept of using multi-coercivity tags can be applied to both architectures, planar and pillar, although different methods of tweaking the magnetic properties are used in each case.
Planar Tags - Shape Anisotropy
The easiest parameter to vary in the planar tags is the shape of each magnetic bit - this is simply a matter of photomask design. In magnetic microstructures, altering the aspect ratio (length to width) changes the coercivity (the field required to flip the magnetisation), and this effect is witnessed in the video.
Click here to view movie: MOKE microscopy showing how the individual bits switch in order of hardness (increasing aspect ratio) as the applied magnetic field is increased.
We have used computer models to simulate the response of a tunnelling magnetoresistance sensor when a 5-bit planar tag passes over.
Figure 2 b) schematic of a 5 bit tag with elements of increasing size passing over a TMR sensor, and computer simulations of the sensors' response when the separation distance between tag and sensor is 5μm and the spacing between magnetic elements is a) 25μm in the (1,1,1,1,1) confirguration and c) 10μm in the (1,0,1,0,1) configuration.
Each layer in the pillar tag has the same fixed elliptical shape, so we must find different ways to coercivity-tune the elements. Fortunately, there are a number of parameters that can easily be varied in the electrodeposition setup, including (i) layer thickness, (ii) composition (iii) and grain structure. Simple 2-bit switching has been witnessed in elliptical tags with 2 magnetic layers of nominal thickness 25nm, and 50nm - as seen in Figure 3 d).
Figure 3 hysteresis loops for an array of elliptical Co elements with minor axis 5μm and major axis a) 15μm and b) 20μm - the switching field was applied along the long large axis, c) SEM image of such a tag; d) SQUID magnetometry hysteresis loop showing a 2-bit tag with magnetic layers of 25nm and 50nm switching at different applied fields.
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