Magnetic switching of nanostructured cobalt islands

R.D.Gomez
Laboratory for Physical Sciences, College Park, MD 20740
& Dept. of Electrical Engineering, University of Maryland, College Park, MD 20742

Welcome to this instructional site on the switching of individual cobalt islands under the application of an external magnetic field. The images are NOT simulations but are direct magnetic images taken by using magnetic force microscopy (MFM)[1]. The animation consists of 35 images, taken successively at progressively increasing field from zero to 1350 Oe. In the parlance of magnetics, this represents the ascending branch of the magnetization loop. The intrinsic coercivity is large and it is highly unlikely that the probe's magnetic field on its own will cause magnetization switching or perturbation. The field is parallel to the field plane and along the horizontal axis. However, if the applied field is near the coercivity of the island, the tip can induce profound changes. The samples were furnished by R. New, R. Pease and R.L. White of the Dept. of Electrical Engineering, Stanford University. The films were patterned using e-beam lithography, sputtered on silicon substrates and with a 10 nm chromium underlayer.

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The bright and dark areas can be regarded at the north and south poles of these nano-magnets. islands with a single distinct bright/dark pair are known as single domains, while those that have more complex structures are commonly referred to as "multidomains". The islands have dimensions of 200nm x 400nm x 20nm. As such, the magnetization lies predominantly on the film plane. Furthermore, because the islands are twice as long as they are wide, the "easy" axis (or the preferred axis where the magnetization would orient) is more or less parallel to the long dimension (horizontal). At H=0, most of the islands are oriented nominally in N-S direction (dark-bright) while a few are oriented in the opposite sense. The apparent dispersion of the easy axis about the horizontal axis is due to the crystalline anisotropy of the grains the make up the individual islands. This has been predicted by theory. [1]. Islands which are nominally aligned with the field exhibit gradual rotation of the moment. (Best seen is topmost left island.) Single domain islands switch at specific field. The fact that the switching event is observed is because the tip induces the event. Typically, the reversal occurs when the tip is roughly on the middle of the island. Multidomain islands show multistep switching, i.e., different subdomains switch at different fields. In this case, each island can be seen as having a square hysteresis loop, with well defined coercivity transition fields. As an aggregate however, the transition field broadens due to the variance in the switching fields of the individual islands. The normal notion of "domain wall motion" does not apply to this system.
(1) R.M.H. New, R.F.W. Pease, R.L. White, J. Appl. Phys. 79, 5851 (1996).
For in depth reading, see: R.D. Gomez, M.C. Shih, R.M.H. New, R.F.W. Pease and R.L. White,"Switching characteristics of sub-micron cobalt islands", J. Appl. Phys. 80, 6441-6446 (1996); R.L. White, Data Storage Magazine Sept. 1997.
Also checkout Permalloy Animation to see domain wall motion.