Much work on mesoscopic structures in TFM has concerned magnetic rings, typically a few microns across and less than thirty nanometres in thickness. These structures are interesting for both fundamental and applications-driven studies, as their magnetic reversal behaviour combines that of the magnetic dot (high rotational symmetry) and the magnetic nanowire (confined domain wall motion). In TFM, rings made from both single magnetic layers (such as Co, Fe, and NiFe) and spin-valve structures (two magnetic layers separated by a conducting spacer such as Cu) have been characterised by coplanar waveguide measurements, magneto-optical techniques, and magnetoresistance methods. Figure 1 shows a chip containing seven spin-valve rings (both elliptical and circular), which was fabricated in MIT via three steps of both e-beam lithography and metal deposition. Projects being carried out at present include fabrication of tunnel barrier devices for spin injection/detection in both vertical and lateral geometries, and we also aim to develop a facility to characterise spin accumulation by both magnetoresistance and Kerr effect imaging at low temperatures.
Scanning electron micrographs showing (a) a chip connecting the spin-valve rings to a magnetoresistance measurement setup via contact pads, (b) the seven elliptical and circular rings to be tested, and (c) one of the elliptical rings. The layer structure of the rings was NiFe (5nm)/Cu (4nm)/Co (7nm). Thanks to Dr. F.J. Castano and Prof. C.A. Ross of the Dept. of Materials Science & Engineering, MIT.
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