Magnetic Rings: Influence of Asymmetry on Switching Mechanism
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Magnetic rings: Influence of asymmetry on switching mechanism
Anand Subra Mani, Dwarakanath Geerpuram, Vidhya Shankar Baskaran, Guadalupe Rodriguez and Vitali Metlushko Department of Electrical and Computer Engineering, University of Illinois at Chicago, 851 S Morgan, Chicago, IL 60607 USA
ABSTRACT We found that the introduction of well-controlled asymmetry provides precise control of the switching mechanism and leads to improved switching characteristics in magnetic nano-rings.
INTRODUCTION Nanoscale magnetic structures have attracted a lot of interest over the last years due to their potential use in memory devices. Magnetic Random Access Memory (MRAM) offer high density of data storage, fast data access rates and low power consumption. The non-volatility of MRAM is a big advantage over the currently used DRAM and SRAM. The important requirement during the operation of MRAM is the uniformity and reliability in performance i.e. repeatable magnetic states for data storage and controllable direction of magnetization in the data storing layers. The shape of the storage layer of MRAM plays an important role in the formation of the two stable magnetic states (1 and 0) and determines the switching mechanism between them. A new design was proposed by Zhu et al. [1], which used the clockwise and counterclockwise directions of magnetization in circular rings to store data. Recently, S.P Li et al. [2] and Rothman et al. [3] found two pairs of different stable states at remanence in the circular rings. The formation of the two states depended upon the thickness of the ring and the ratio of the inner and outer diameters. For wide rings with large ring width/smaller inner diameter and large film thickness, the “vortex” states were formed at remanence. Here, the field lines encircle the center. For the narrow rings, two polarized “onion” states were found at remanence, where the magnetizations move around the center in opposite directions and form head-to-head and tailto-tail domain walls. The most important consideration in these designs is to control the switching process reliably and also to minimize the stray field effects from neighboring elements during selective writing. The switching in these rings occurs by the nucleation and annihilation of domain walls. Proposed improvements for the control of the switching process include variation of shape of the element and also the introduction of a physical domain wall pinning locations. For circular magnetic dot, Schneider et al. [4] demonstrated that by making one edge of the dot flat the switching process and the vortex formation can be controlled. For circular rings, Klaui et al. [5] included a physical domain wall pinning feature in the form of a notch in rings. He demonstrated that due to the different pinning of the two domain walls present in the onion state, rings switch
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from the onion state into the vortex state with predictable chirality. Recently, Saitoh et al. [6] proposed a new design by changing the inner hole as an ellipse and
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