Magnetics Business & Technology - Summer 2015 - (Page 12)

FEATURE ARTICLE Spintronics on Paper: The Whys and Wherefores By Meriem Akin, Lutz Rissing Computerizing information has enabled paper to dissociate itself from being merely a passive carrier for ink. Due to its unique technical properties (such as light weight, mechanical bendability, porosity, moisture intake, chemical resistivity, thermo-mechanical stability), low cost and resource abundance, disruptive technology based on paper has emerged in the past 10 years: medical diagnosis systems, interactive displays, foldable microscopes, robust civil architecture and pulp-based computing to name a few [1-5]. At the institute of Micro-Production Technology, we research the fabrication of micro-electro-mechanical systems based on paper, and we exploit clean room fabrication processes (Figure 1) for manufacturing these systems. Currently, we research the use of paper as an interposer for magnetic coatings. In particular, we want to understand the magnetic response of a wide range of magnetic materials (ferri-, ferro and antiferromagnetic) when deposited onto paper substrates. While we use paper as a surface carrier for thin layers of magnetic materials, the integration of magnetic materials within the body of the paper is state of the art. Magnetic papers can be realized by dip coating paper sheets into monomer solutions filled with magnetic nanoparticles [6]. Also, embedding magnetic particles during the synthesis process of cellulose allows for the fabrication of super-paramagnetic papers [7]. As a matter of fact, paper with embedded magnetic stripes is in daily use for parking garage and subway tickets, etc. However, the superficial magnetic coatings of these paper-based systems are thick enough to not interplay with the paper surface. As yet, existing paper-based systems make use of binary magnetic phenomena. By depositing magnetic silicones onto paper-based cantilevers, the resulting valve can open and close to control the fluid flow in lab on a paper [8]. A further example is paper actuators that are fabricated by impregnating paper with a ferrofluid and subsequent laser machining to the desired geometry [9]. Ergo, would it be possible to insert more intricate magnetic phenomena, such as spintronic effects (anisotropic, giant or tunnel magneto-resistance) on a sheet of paper? And, how could we exploit these phenomena in engineering unorthodox paper-based devices for leisure, medical and educational use? These are some of the questions that we tackle at the Institute of Micro Production Technology. Exploratory Study We believe that the network topology of paper is decisive for the magneto-resistive response of a continuous thin deck of a magnetic material when deposited onto paper substrates (Figure 2). Due to the non-planar and stochastic orientation of the paper fibers, each local fiber region exhibits a different direction of electrical current with respect to the direction of the magnetic field. One may consider each fiber as a distinct thin and long magnetic stripe. Therefore, the magnetic response of a paper-based spintronic system is expected to be a complex superposition of local spintronic phenomena. In particular, we report on our findings with Permalloy (Py:Ni81Fe19), which classically exhibits anisotropic magneto-resistance at room temperature. We opted for sputter deposition as a dry coating technique, and we chose clean room paper (latex impregnated) as the substrate material. As a reference substrate material, we used fused silica with superior surface quality. In order to separate material behavior from surface topology, we replicated the surface 12 Magnetics Business & Technology * Summer 2015 Figure 1. Examples of clean room fabrication processes that were adapted to clean room paper substrates at the Institute of Micro Production Technology. Fabrication techniques: (a) Photolithography, (b) sputter deposition of structured seed layers and (c) electroplating, packaging techniques: (a) Wire bonding and (e) soldering on electroplated layer. Figure 2. Scanning electron micrographs of the topology of the clean room paper as coated with Py:Ni81Fe19: (a) Surface view, (b) detail of the surface and (c) cross section view. (d) Computer model of a coated paper system subject to a magnetic field. Figure 3. MOKE micrograph of Py:Ni81Fe19 on paper.

Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2015

Editor's Choice
Bonded Magnets: A Versitile Class of Permanent Magnets
Spintronics on Paper: The Whys and Wherefores
Magnets, Materials & Assemblies
Software & Design
Research & Development
Industry News
Marketplace / Advertising Index
Spontaneous Thoughts: The Rate Determining Step

Magnetics Business & Technology - Summer 2015