Papers

AVS2004 Session MI-FrM: Advanced Magnetic Data Storage and Thin Film Processing

Friday, November 19, 2004 8:20 AM in Room 304A

Friday Morning

Start	Invited?	Item
8:20 AM		MI-FrM-1 Magnetic Tunnel Junctions for Magnetoresistive Random Access Memory J.M. Slaughter, J. Åkerman, B. Butcher, R.W. Dave, M. DeHerrera, M. Durlam, B.N. Engel, G. Grynkewich, J. Janesky, J. Martin, S.V. Pietambaram, N.D. Rizzo, K. Smith, J.J. Sun, S. Tehrani (Freescale Semiconductor) Magnetoresistive random access memory (MRAM) employs a magnetoresistive device integrated with standard silicon-based microelectronics, resulting in a combination of qualities not found in other memory technologies. For example, MRAM is non-volatile, has unlimited read and write endurance, and has demonstrated high-speed read and write operations. Fundamentals of MRAM based on Magnetic Tunnel Junction (MTJ) devices, and recent technology developments in the areas of magnetic materials and magnetic device design is reviewed. The properties of our unique toggle-switching MRAM bit, as well as specific magnetic and electrical properties required for that bit will is discussed and compared to the conventional switching approach. The new bit cell uses a balanced synthetic-antiferromagnetic free layer and a phased write pulse sequence to provide robust switching performance with immunity from half-select disturbs. The use of this bit cell in a 4Mb MRAM circuit also is described.
9:00 AM		MI-FrM-3 Processing Challenges in the Magnetic Recording Industry J.A. Katine (Hitachi Global Storage Technologies) Following the introduction of the giant magnetoresistive sensor in 1997, the areal density of magnetic storage doubled annually for five years. This doubling meant that the critical dimension of the smallest features on the thin film recording head, the sensor trackwidth and the writer poletip width, decreased by 30 percent per year. Recently, though, there has been a dramatic slowdown in rate of increase for areal recording density, in large part due to the processing challenges associated with scaling the critical features to sub-100 nm dimensions. This talk will present an overview of the processing of magnetic recording heads, emphasizing the key challenges facing the industry. These include lithographic tooling options for printing critical dimensions that will soon be smaller than the smallest features used in IC processing. Of particular interest to the industry is the feasibility of using direct write electron beam lithography for the production of recording heads. Another obstacle to overcome will be developing etching techniques for magnetic materials that do not produce unacceptable damage at the edges of the devices. Advances are also required in developing the thin insulating gap materials with thermal and electrical properties suitable for ultrahigh areal density recording. In addition to processing challenges in the recording head, to reach areal densities approaching 1 Terabit per square inch, it is likely that the magnetic media itself will require lithographic patterning. I will outline the formidable challenges involved in producing economically viable patterned media.
10:00 AM		MI-FrM-6 Reversal Mechanism of Patterned Co/Pd Multilayer Islands G. Hu, T. Thomson (Hitachi Global Storage Technologies); C.T. Rettner (IBM Almaden Research Center); B.D. Terris (Hitachi Global Storage Technologies) Arrays of patterned Co/Pd magnetic islands with perpendicular anisotropy and sizes ranging from 30nm to 5 microns have been fabricated and characterized. Applying a field along the easy axis results in individual magnetic islands switching as a single unit, as observed by the magnetic optic Kerr effect and magnetic force microscope (MFM) measurements. The angle dependence of switching closely resembles the behavior predicted by the Stoner-Wohlfarth model with a minimum in island coercivity at 45 degrees. These results are expected for small islands which may reverse by rotation, but are surprising for the larger islands. The nominally identical continuous film exhibits a completely different behavior, where the film coercivity increases with the angle (t) between the applied field and film normal as 1/cos(t), as predicted for domain wall motion controlled reversal. These data leads to a model whereby the reversal of the larger islands is controlled by a nucleation event, followed by a rapid wall motion. The observed switching field of the island is the switching field of the small nucleation site, which reverses coherently. To test this model, we artificially introduce nucleation sites into the islands by applying an in-plane field. This results in a domain wall motion controlled angle dependent behavior in all islands capable of supporting a multi-domain ground state (size ranging from 200nm to 5 microns). The reversal behavior of the larger islands therefore depends on the initial state of the islands. In an island with no nucleation sites the reversal is governed by rotation, whereas in the same island into which nucleation sites have been pre-created, the reversal is governed by wall motion.
10:20 AM		MI-FrM-7 Hierarchical Self-Assembly as a Route to Future Magnetic Data Storage S.B. Darling (Argonne National Laboratory); D. Sundrani, S.J. Sibener (The University of Chicago) A novel hybrid top-down/bottom-up approach is used to hierarchically organize magnetic nanoparticles on the nanoscale. Lithographically prepared substrate channels direct the self-assembly of a high-aspect ratio diblock copolymer template resulting in nearly defect-free alignment over arbitrarily long distances.¹,² Selective wetting of the channel sidewalls by one block initiates the organization and is followed by coarsening until the entire channel volume contains aligned domains. Overfilling the channels further extends the alignment, originally nucleated on the sidewalls, both above and beyond the confined space. This approach may be useful for globally aligning domains across an entire surface with the top interface being nearly flat despite the corrugated substrate underneath. The oriented structures are surprisingly defect-tolerant, accommodating roughly 10% variations in channel width without introducing disclination or dislocation defects. The laterally alternating nanoscale structure of the diblock film is then used to template the adsorption of FePt nanoparticles. Attractive interaction of the nanocrystal capping molecules with one of the polymer blocks leads to nearly 100% selective adsorption. Hybrid hierarchical approaches such as this are promising candidates for high density storage media. ¹ D. Sundrani, S.B. Darling, S.J. Sibener, Nano Letters, 4 (2004) 273-276. ² D. Sundrani, S.B. Darling, S.J. Sibener, Langmuir, In Press.
10:40 AM		MI-FrM-8 Enhanced Magnetic Moment in Iron Nitride Thin Films R.A. Lukaszew, D. Pearson, Z. Zhang (University of Toledo) In magnetism, maximum interest is focused on iron and iron-based alloys because these materials are of greatest practical use. Iron-based alloys are most susceptible to modification by nitrogen, which can turn them from weak to strong ferromagnets. Nitrogen enters 3d metals as an interstitial provoking a dilation of the lattice. The Fe-N system exhibits several phases of technological importance including several Fe-N intestitial compounds with nitrogen ordering. The ordered iron nitrides are metastable compounds which can persist at moderate temperature because of kinetic constraints. In particular, the magnetic properties of the α"-Fe₁₆N₂ phase have been of interest for both scientists and technologists since it was first discovered to exhibit magnetization as great as 2.4 T, significantly higher than that of α-Fe. An important issue is the role of nitrogen in enhancing the iron magnetic moment. Current theories explain the enhanced magnetic moment in terms of a reduced moment on the iron sites that are the nearest-neighbours of nitrogen, and an enhanced moment on the more distant sites due to hybridisation of the 3d states of the iron that is a nearest neighbour and charge transfer from the more distant iron ¹. No polarization of the N atoms is assumed. In order to evaluate the potential of these materials for magnetic recording head as well as to understand the origin of the observed enhanced magnetic moment more detailed research is required. We will present our studies on epitaxial FeN films grown using reactive magnetron sputtering. We will show magnetic characterization of the films, performed using SQUID, MOKE and XMCD. In particular XMCD data indicates that in addition to Fe, N is also polarized in these films. Thus we believe that N polarization may be the primary reason for enhanced magnetic moment in these materials. ¹. J.M. D. Coey and P. A. I. Smith, J. Mag. Mag. Mat. 200, 405-424 (1999).
11:00 AM		MI-FrM-9 Direct Observation of Nano-Oxide Formation in Spin Valve Multilayers A.T. McCallum, S.E. Russek (NIST) The addition of thin oxide layers, that specularity reflect electrons, to spin valve trilayers has been shown to increase the giant magnetoresistance of these structures. Usually the specularity of an oxide is deduced by comparing the resistance and magnetoresistance of samples with and without nano-oxide layers. These comparisons are clouded by sample to sample variations and the fact that adding an oxide changes the growth mode of material deposited on top of the oxide. In-situ conductance measurements allow direct observation of the specularity increase as the oxide forms. One key advantage of this measurement is that on one sample the effects of a range of oxygen exposures are measured. The Co free layer of bottom pinned spin valves was oxidized and the conductance and magnetoconductance were measured. These measurements show that the CoO_x layers become specular within ~5 monolayers of oxygen exposure. Conductance measurements during the oxidation of relatively thick layers of Co show a specularity increase of at least 0.10. Subsequent oxidation does not change the specularity of the nano-oxide. RHEED patterns taken during the oxidation show the appearance of a new face centered cubic, fcc, 111 lattice with a lattice constant consistent with CoO. The region between these two lattices is probably the region that determines the amount of specularity at that interface with the nano-oxide. A second set of bottom pinned spin valves were made with a 1 nm thick Co_1-xFe_x cap on the free layer. This material was then exposed to oxygen. One nm should be about the amount of metal oxidized. The more Fe that was included in the free layer cap the less the specularity increased with oxygen exposure. The sample with a Co cap had a increase in the magnetoconductance of 0.00020 Ω^-1 during oxidation. The sample with a pure Fe cap had a decrease in magnetoconductance of 0.00005 Ω^-1 with oxidation.
11:20 AM		MI-FrM-10 Novel Green Plasma Etch Chemistries for Magnetic Metals. A.S. Orland, A.A. Dyachenko, R. Blumenthal (Auburn University) Chlorine, an enviromental menace, is traditionally used to etch magnetic metals in manufacturing of semiconductor devices. In this work, the etching of magnetic metals with hydrogen plasmas containing environmentally friendly gases such as carbon monoxide, carbon dioxide and cyclopentadiene (Cp) is investigated by means of supersonic pulse, plasma sampling mass spectrometry. Previous results, in our group,¹ have indicated that the etch rates of nickel are significantly enhanced in CO/H₂ plasmas and CO₂/H₂ plasmas at the same time that formate and oxalate signals appear in the mass spectrum. It is further assumed that these species serve as new primary etchants resulting in the formation of volatile metal formates and oxalates. The etch rates and chemical mechanisms responsible for the etching in CO/H₂, CO₂/H₂ and Cp/H₂ plasmas will be presented. ¹ A.Orland, Ph.D. thesis, Auburn University, 2003.
11:40 AM		MI-FrM-11 Characterization of FePt-based Magnetic Nanocomposite Thin Films Prepared by Pulsed Filtered Vacuum Arc Deposition Y.W. Lai, M.F. Chiah, N. Ke, Q. Li, W.Y. Cheung, S.P. Wong (Chinese University of Hong Kong) We have prepared FePt-X (X = C, Cu or Ag) nanocomposite thin films of various compositions consisting of FePt grains embedding in carbon, copper or silver matrices using a pulsed filtered vacuum arc deposition technique. In addition to usual co-deposition processes, another process was adopted where multilayers of the three elemental components with an appropriate design of thickness and sequence were first deposited followed by a rapid thermal annealing (RTA) in an argon atmosphere. Characterization of these films was performed using Rutherford backscattering spectrometry, x-ray diffraction, transmission electron microscopy, and vibrating sample magnetometry. The dependence of the structure and magnetic properties, such as the phase and size of the magnetic grains and the coercivity of these films, on the deposition parameters, annealing conditions and the matrix materials were studied in details. Both x-ray diffraction and transmission electron microscopy analyses confirmed the formation of L1₀ phase FePt nano-grains after appropriate annealing. For example, for the film with a particular composition of Fe₄₃Pt₄₅Cu₁₂, the L1₀ phase formation was observed after the RTA process at 400°C, and the film exhibited a coercivity of 6.5 kOe. Compared to films prepared by usual co-deposition process, those films prepared using the multilayer deposition approach see a significant lowering in the ordering temperature at which the L1₀ phase started to form. The degree of lowering in the ordering temperature was seen to depend on the species of the matrix material. It is believed that different mechanisms are responsible for such ordering temperature lowering in films of different matrices. This work is supported in part by the Research Grants Council of Hong Kong SAR (Ref. Number: CUHK4216/00E).