Zigzag-Shaped Magnetic Films Used for Single-Axis Field Sensing
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“This is the most direct evidence for a collapse of a Fermi volume in any quantum critical matter,” said Steglich. “We expect this new insight to have broad implications for other strongly correlated electron systems.”
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Zigzag-Shaped Magnetic Films Used for Single-Axis Field Sensing F.C.S. da Silva and co-workers at the National Institute of Standards and Technology (NIST) in Boulder, Colo., and Gaithersburg, Md., have fabricated and simulated zigzag-shaped magnetic thinfilm elements. They chose the zigzag geometry because it can be used as a single-axis magnetic-field sensor, which is integral to nanoscale devices for data storage technologies. These devices are based on the anisotropic magnetoresistive (AMR) effect. While the AMR effect has a relatively small change in resistance, the devices compensate by their sensitivity to field changes and by very low intrinsic magnetic noise. Both of these properties arise because AMR devices are made from a single layer of magnetic material. Having control over the magnetic easy axis orientation is important for making scalable AMR sensors without complicated current- and field-biasing schemes. The critical aspect of designing such a device lies in biasing the magnetic and current flows at 45° to each other to obtain an asymmetric linear response, the researchers said. This is achieved in the zigzag elements by fabricating it in a shape in which the current flows down the center and the shape controls the local magnetic bias. To obtain a uniaxial magnetic element, a soft magnetic material is chosen with both shape and induced anisotropies to ensure that there is just one anisotropy axis. As reported in the December 13, 2004, issue of Applied Physics Letters (p. 6022; doi: 10.1063/1.1834732), these structures were fabricated using optical lithography in which a 30 nm thick Ni80Fe20 Permalloy film was sputtered onto a SiO2-coated Si wafer while a field was applied along the element’s long axis. Layers of Ta 5 nm thick were deposited before and after the Permalloy to improve adhesion, provide optimal texture for the film, and protect it from oxidation. As shown in Figure 1, the magnetic state was imaged using scanning electron microscopy with polarization analysis (SEMPA), which yields high spatial resolution and measures the magnetic direction by analyzing the spin polarization of the secondary electrons emitted during analysis. Additionally, micromagnetic simulations were performed by using the 80
Figure 1. Zigzag-shaped magnetic thinfilm structures. (a) Experimental scanning electron microscopy with polarization analysis image of a zigzag structure. (b) Simulation performed on the same geometry using object-oriented micromagnetic framework software. Reprinted with permission from Applied Physics Letters 85 (24)(2004) p. 6022. © 2004 American Institute of Physics.
object-oriented micromagnetic framework (OOMMF) software, a simulation program developed at NIST. These measurements show that it is possible to naturally bias the magnetization in magnetoresistive sensors
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