Magnetic Materials

Thursday, April 13, 2000 8:30 AM in Room Sunrise

Start Invited? Item
8:30 AM H1-1 A Tutorial on Patterned Media: A New Paradigm for Ultra-high Density Storage
S. Schultz, M. Barbic (University of California, San Diego); J. Wong, A. Scherer (California Institute of Technology)
The present media in the disk drives used for information storage make use of continuous, thin sputtered magnetic films. Although the industry has continually made dramatic increases in bit density for the past 40 years, there are fundamental reasons for expecting that the present in-plane bit format will soon reach a maximum density of approximately 50 billion bits/square inch. We will discuss the difficulties that are faced in achieving further increases in bit density, and introduce the concept of "patterned media" whereby future bits will be stored as one of two magnetization states for individual isolated single domain magnetic particles. We will discuss the method of fabrication by nanolithography, and present data illustrating the performance that can be expected when using present commercial state-of-the-art read/write heads. Since the bit area corresponding to a storage density of 100 billion bits per square inch is only 80 nanometers square, there are practical limitations involving servo-control, tribology, data rate, and cost of manufacture which will be discussed.
9:10 AM H1-3 Magnetic Tunnel Junctions: Spin-dependent Tunneling and Magnetic Memory Elements
E.R. Nowak (University of Delaware)
Progress in preparing multilayers of thin ferromagnetic layers has led to important advances in both our understanding of spin-polarized electron transport and in applications of these materials. Magnetic tunnel junctions (MTJs), consisting of two ferromagnetic (FM) metal layers separated by a thin insulating (I) layer, can now be grown reproducibly with large “tunneling magnetoresistance” (TMR); namely, very large changes in resistances in response to an applied magnetic field. The TMR effect is associated with an asymmetry in the density of states for the two spin directions of the conduction electrons at the Fermi level in a ferromagnet. In a tunnel junction, an applied field can be used to change the relative orientation of the magnetization vectors in the two FM electrodes and, hence, affect the tunneling probability. TMR effects as large as 40% are found at room temperature in fields of only a few Oersteds. By engineering MTJ materials, useful structures can be constructed for building non-volatile magnetic memories. Since tunneling electrons come from the top few monolayers of the FM film, the success of FM-I-FM tunneling depends critically on the quality of the insulating tunnel barrier and FM-I interfaces in the trilayer structure. Indeed, the TMR exhibits both a temperature and a dc bias dependence. These effects are surprisingly significant and depend on the quality of the junction. This talk will review the evolution of magnetic tunnel junctions, and discuss the current scientific issues and the application potential in digital storage and magnetic sensor technologies. This work was done in collaboration with M. B. Weissman and S. S. P. Parkin
9:50 AM H1-5 Studies on Electrosynthesis of CuFe Alloy Films and their Electrochemical Oxidation
S. Sartale, C. Lokhande (Shivaji University, India)
Electrosynthesis of alloy films has been the subject of many meticulous studies. This is due to fact that these materials present in thin film form have several advantages relative to bulk materials such as enhancement of mechanical and magnetic properties. Presently much attention has found on the preparation and characterization of such ultrastructured materials that have giantmagnetotresistance (GMR) properties. The study of electrodeposition of CuFe alloy is particularly interesting because previous studies indicate that this alloy is promising GMR material. @paragraph@ CuFe alloy films were galvanostatically eletrodeposited from aqueous and nonaqueous (etanediol) baths using copper sulfate and ferrous sulfate as initial ingredients. The deposition potentials have determined from polarization curves. All deposition potentials have measured with respect to SCE. Effect of bath composition on composition of Cu and Fe in deposited alloy film for both aqueous and nonaqueous baths has studied using atomic absorption spectroscopy. For 16: 4 (0.1 M FeSO@sub4@ : 0.05 M CuSO@sub4@) the composition of deposit was 2 : 1. Effect of various substrates (stainless steel, brass, copper, titanium), electrolyte temperature, complexing agent (tri-sodium citrate) on deposition potentials and surface morphology has studied. For surface morphology SEM technique has used. @paragraph@ In the present investigation, the CuFe alloy films have electrochemically oxidized at room temperature using simple electrolyte (1 N KOH) cell. The effect of oxidation time and current density on the structural properties of these films has studied using XRD technique. It has found that the oxidized films show CuFe@sub2@O@sub4@ phase with hexagonal crystal structure. Surface morphology of the film has examined using SEM technique. The films are looking smooth and homogeneous.
10:30 AM H1-7 Layered Magnetic Materials for Magnetoresistive RAM
J.M. Slaughter, E.Y. Chen, S. Tehrani (Motorola Labs, Physical Sciences Research Laboratories)
Magnetoresistive Random Access Memory (MRAM) is based on magnetic memory elements integrated with CMOS. Key attributes of MRAM technology are nonvolatility and unlimited read and write endurance. A leading candidate material for MRAM storage elements is the Magnetic Tunnel Junction (MTJ), a thin-film stack that includes magnetic electrodes separated by an insulating spacer layer. In an MTJ-based element the current flows perpendicular to the layers, tunneling through the thin dielectric spacer layer, typically aluminum oxide. The resistance of the memory bit is either low or high depending on the relative polarization, parallel or anti-parallel, of the magnetic electrodes. A typical MTJ stack also includes a mechanism to pin the polarization of one of the magnetic layers in a fixed direction. The direction of polarization of the unpinned magnetic layer (free layer) switches between parallel and anti-parallel, providing the two resistance states used for information storage. The switching field is produced by current flow through wires patterned below and above the memory element. The memory element itself is patterned to submicron dimensions. Desirable properties of MTJ material, which we have demonstrated, include its high magnetoresistance ratio (MR), in the 30% to 40% range, which results in a large signal, and its relatively high resistance, necessary for high-speed read operations with low operating voltage. @paragraph@The thickness of the critical layers, including the tunnel barrier, fixed layer and free layer, is typically 15 to 40 Angstrom. Although thickness uniformity and repeatability of all the layers is important, the most sensitive layer in the MTJ stack is the AlOx tunnel barrier. The tunnel barrier is very thin, ? 20 Å, and the resistance of the element is exponentially dependent on the barrier thickness. Uniformity of the MR and the absolute resistance of the cell are critical, since the absolute value of the MTJ resistance is compared with a reference cell during read mode. This sets strict requirements on the AlOx process uniformity since the effect of thickness variation on resistance is amplified by the exponential dependence on thickness. Results are presented for material with AlOx barriers that are pinhole free, very smooth, and extremely uniform over a wafer (1-sigma < 1%). Another important layer to control is the free layer. In patterned bits, the thickness of the free layer is directly related to the field, or current, required for switching the bit. Thin layers are desirable because they result in low switching fields for low power write operations. However, there are fundamental limits to how thin the layer can be made while continuing to function effectively. Ion beam deposition and magnetron sputtering are the preferred methods for depositing the layers. The AlOx tunnel barrier is formed by depositing an Al layer, with thickness < 15 Angstrom, and then oxidizing it with an O2 plasma to form the AlOx. @paragraph@This presentation will describe the formation and properties of MTJ material appropriate for MRAM, with emphasis on the trends and limits associated with the tunnel barrier and the free layer. Results are presented for MTJ material with very uniform resistance and uniform high MR over 150 mm wafers. @paragraph@This work was partially supported by DARPA.
11:10 AM H1-9 Deposition of Magnetic Films for Thin Film Magnetic Recording Heads
J.C.S. Kools, K. Rook, A. Devayasaham, I. Wagner (Veeco Process Equipment)
In this paper, we will give an overview of vacuum deposition of magnetic thin films for the use in thin film magnetic heads. These magnetic films can be divided in three different classes? Multilayered Giant Magnetoresistive (GMR) materials are used to sense the magnetic flux from the media. Typical stacks ("spin-valves") are build of 10 or more individual layers with individual layer thicknesses down to a few Angstroms. These layers consist of ferromagnetic materials such as Ni@sub 81@Fe@sub 19@, Co@sub 90@Fe@sub 10@, non magnetic materials such as Ta, Ru, Cu and antiferromagnetic materials such as NiMn or PtMn. Relatively thin (a few hundreds of Angstroms) hard magnetic films are applied as magnetic stabilization of the GMR material. Such films are typically CoCrPt alloys deposited on a Cr seedlayer. Relatively thick (0.1- 2 µm) soft magnetic films are used as plating base and soft magnetic shields. Such films are materials such as NiFe alloys of various composition, amorphous Co-based alloys, and nanocrystalline Fe-based alloys. These films are being deposited using Ion Beam Deposition (IBD) and Physical Vapor Deposition (PVD) techniques. We will discuss these respective applications from the point of view of device manufacturing in an industrial context. Optimization of the deposition process involves control of the material properties and geometry of the deposited film. The materials used in these thin film heads have traditionally been more complex in terms of desired microstructure, contamination level, magnetic and electrical properties than the films as used in traditional microelectronics. Recently, as lateral dimensions are rapidly approaching the deep submicron level, control of the geometry of the deposited film becomes increasingly important.