|Not All Nanodisk Magnetic Vortices Are Created Equally|
Magnetic vortices – hurricanes of magnetism only a few atoms across – have generated intense interest in the high-tech community because of their potential application in nonvolatile random access memory (RAM) data storage systems. A team of researchers led by Peter Fischer and Mi-Young Im of the Center for X-Ray Optics (CXRO) worked in collaboration with scientists in Japan to discover that, contrary to what was previously believed, the formation of magnetic vortices in ferromagnetic nanodisks is an asymmetric phenomenon. This breaking of symmetry could lead to failure in a data storage device during its initialization process. These new findings indicate that the road to magnetic vortex RAM might be more difficult to navigate than previously supposed, but there might be unexpected rewards as well.
Magnetic vortex states are generated in ferromagnetic nanodisks because the spin of electrons, which gives rise to magnetic moments, must follow the shape of the disk to ensure closure of magnetic flux lines. This results in the curling of the in-plane magnetization flux lines. At the center of these curling flux lines is a needlelike core, an “eye-of-the-hurricane” that points either up or down with respect to the surface plane of the nanodisk. The magnetization of a ferromagnetic nanodisk therefore has two components, the up or down polarity of the core and the chirality (rotation) of the in-plane magnetization, which can be either clockwise or counter-clockwise. It has been proposed that these four independent orientations can be used to store binary data in novel nonvolatile storage devices.
Prior to this study, the formation of magnetic vortex states was assumed to occur with perfect symmetry because the energy states of the four orientations were equivalent; however, this is not the case. Although this finding might hinder the application of magnetic vortices in storage devices, the nonsymmetric behavior could be used as a biasing effect in applications of magnetic vortices to sensor or logic devices.
The key to the discovery of the asymmetric formation process of magnetic vortices was the research team’s ability to simultaneously observe both chirality and polarity in a large array of nanodisks while previous studies focused on either the chirality or polarity in a single disk. This study demonstrates how mesoscale phenomena (which encompass complexity and functionality over various length scales) can be significantly different from those on the nanoscale.
The simultaneous observation of mesoscale behavior with nanoscale accuracy was accomplished using the XM-1 soft x-ray microscope at ALS Beamline 6.1.2. XM-1 enables full-field magnetic transmission soft x-ray microscopy with spatial resolution down to 20 nanometers, thanks to high quality x-ray optics provided by CXRO researchers. Being able to capture multiple disks in a single image at very short exposure times allowed for the accumulation of significant statistics quickly.
Researchers fashioned nanodisks from permalloy, a nickel and iron alloy whose magnetic properties are well known. Using electron-beam lithography researchers patterned large arrays of disks, each with a radius of 500 nanometers and a thickness of 100 nanometers. The arrays were deposited on silicon nitride membranes to allow for sufficient transmission of soft x-rays and were then imaged in XM-1.
Rigorous 3D micromagnetic simulations for the generation process of vortex states supported the interpretation that the asymmetry originates both from an intrinsic interaction, which breaks the symmetry, as well as from more extrinsic factors, such as the roughness of the sample surface. This study provides an important knowledge base for future research into this phenomenon and its potential applications.
Research conducted by: K. Yamada and T. Ono (Kyoto University), T. Sato and Y. Nakatani (University of Electro-Communications at Chofu), S. Kasai (Japan’s National Institute for Materials Science) and M.-Y. Im and P. Fischer (CXRO, Berkeley Lab).
Funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Operation of the ALS is supported by DOE BES.
Publication about this research: M.-Y. Im, P. Fischer, K. Yamada, T. Sato, S. Kasai, Y. Nakatani, and T. Ono, “Symmetry breaking in the formation of magnetic vortex states in a permalloy nanodisk,” Nat. Comm. 3, 983 (2012).
ALS Science Highlight # 255