Papers

AVS1997 Session MI+NS-TuM: Magnetic Device Technology

Tuesday, October 21, 1997 8:20 AM in Room J3

Tuesday Morning

Start	Invited?	Item
8:20 AM		MI+NS-TuM-1 Pseudo Spin Valve MRAM B.A. Everitt, A.V. Pohm (Nonvolatile Electronics, Inc.) MRAM (magnetoresistive random access memory) has many desirable characteristics compared to other nonvolatile memory technologies including no inherent wear-out mechanism, very fast switching capability, and relatively few mask layers required for fabrication. We have investigated sub-micron memory cells patterned from asymmetric GMR sandwich materials that have 6 to 8% signal levels, exhibit sharp single-domain-like switching thresholds, and are sensed in a mode that yields a differential output signal effectively twice the intrinsic GMR of the material. The cell is designed so that the thinner of the two magnetic layers reverses freely in a read operation, while the bit state is stored in the thicker magnetic layer whose magnetization switches only upon writing the bit. Due to the high signal level and similarity in transfer characteristic to that of pinned spin valve material, we have labeled this cell structure and mode of operation "pseudo spin valve" (PSV). Experimental PSV memory cells and test structures approximately 0.15µm to 0.4µm in width were patterned and tested, and yielded GMR values of up to 7%. Dependence of the write thresholds on sense line current as well as word and bias line fields was studied and confirmed that 2D cell selection can be used in an array. Single bits exhibited sharp, single-domain-like write threshold characteristics. Using a single domain model that takes into account static torques on the two magnetization layers in a bit, as well as coupling between layers, transfer curves and switching characteristics that qualitatively match the experimental data were generated. By using the Landau-Lifshitz-Gilbert equation for single domain structures, time dependence was also studied. Results indicate that bit stability is enhanced as the thickness differential between the two magnetic layers is increased, although bit write thresholds, and consequently energy required to write the bit, are also higher.
9:00 AM		MI+NS-TuM-3 Giant Magnetoresistance of NiMnSb-Based Multilayers J.A. Caballero (University of Florida, Gainesville); J. Bass, W.C. Chiang, W.P. Pratt, Jr. (Michigan State University); F. Petroff (UMR CNRS/Thomson, LCR Thomson-CSF, France); Y.D. Park, J.R. Childress (University of Florida, Gainesville) We report on the integration of the predicted half-metallic (100% spin-polarized) ferromagnetic Heusler alloy NiMnSb into [NiMnSb/Cu]_n, [NiMnSb/Cu/NiFe]_n and [NiMnSb/Cu/Co]_n multilayer structures, and on their magnetic, microstructural, and magnetotransport properties with the current in the layer plane (CIP) and current perpendicular to the layer planes (CPP). Using a low-rate, low-pressure sputtering deposition technique, substrate temperatures as low as 250°C are sufficient to produce ultra-thin stochiometric C1_b-structured NiMnSb with the crystalline and magnetic quality of single NiMnSb films. NiMnSb layers as thin as 15Å are found to display bulk-like characteristics, and the NiFe, Co and Cu layer thicknesses were varied between 10 to 60Å. The multilayers were characterized using XRD, TEM, atomic force microscopy, SQUID magnetometry and CIP and CPP magnetoresistance (MR), using 1500Å-thick Nb strips as contacts for CPP-MR measurements. In the CIP geometry, MR ratios as high as 3% are observed for [NiMnSb/Cu] multilayers in the regime where the Cu layers are thick enough (>25Å) to decouple the NiMnSb films. CPP measurements, currently in progress, might potentially yield an enhanced effect due to the predicted spin-polarized properties of NiMnSb. The relationship between the measured magnetoresistance, NiMnSb crystalline quality, magnetic properties and interfacial roughness will be discussed.
9:20 AM		MI+NS-TuM-4 Tunneling Magnetoresistance through RF-Sputtered Alumina Barriers in Fe-Al₂O₃-Fe Junctions. Ch. Féry (University Henri Poincare - Nancy I, France); J.F. Bobo (CNRS, Stanford University); O. Lenoble, M. Piecuch (University Henri Poincare - Nancy I, France); M. Sharma, S.X. Wang (Stanford University) We have prepared iron thin films by RF magnetron sputtering, the argon pressure during the process was found to have a significant influence on their microstructure and their coercivity. Fe-Al₂O₃-Fe trilayers prepared with different coercivities of the two iron electrodes present spin-valve-like magnetic behaviors for Al₂O₃ thicknesses as thin as 9 Å. In order to measure the junction transport properties, we used contact masks with submillimetric path width. The samples were transfered to room pressure in the load-lock before each mask change. For a nominal barrier thickness of 100 Å, junctions show I(V) characteristics typical of tunneling, but with a relatively low reproducibility. The simulation of the I(V) curves yields reasonable values for the height of the barrier (1.9 eV), but low values for its width (16 Å). We attribute this to the roughness of the trilayers. Finally, the magnetoresistance measurements on these junctions, performed at low bias (10 mV), evidence for a spin-dependent tunneling of about 1% for a junction total resistance of several kiloOhms. In order to improve the reproducibility of these junctions, we have deposited plain Fe-Al₂O₃-Fe trilayers on platinum buffer layers prepared for liftoff process. Then, with junction areas ranging from 10x10 micron square up to 50x50 microns square, we obtained reproducible tunneling characteristics. The magnetoresistance of these lithographic samples is presently being investigated, as well as low temperature measurements of the first series of samples obtained with contact masks.
9:40 AM		MI+NS-TuM-5 Ferromagnet-Insulator-Ferromagnet Tunnel Junctions Using the Heusler Alloy PtMnSb F.B. Mancoff, M. Sharma, K. Bessho, J.F. Bobo, P.R. Johnson, B.M. Clemens, S.X. Wang, R.L. White (Stanford University) We have studied ferromagnet-insulator-ferromagnet tunnel junctions that include the Heusler alloy PtMnSb. The predicted 100% conduction electron spin polarization of PtMnSb should lead to a large tunneling magnetoresistance for such junctions¹. We have previously sputter deposited epitaxial PtMnSb films found to be single phase, nearly stoichiometric, and of relatively low roughness². Trilayer films of epitaxial PtMnSb/Al₂O₃/NiFe have been fabricated using rf sputtering from a Al₂O₃ target to deposit the insulating layer. Junctions were patterned by optical lithography and chemical etching. For junctions with nominal Al₂O₃ thicknesses down to 25 Å, we observe nonlinear I-V curves, indicative of tunneling behavior, as well as magnetic hysteresis loops for which the PtMnSb and NiFe layers reverse their magnetizations independently due to the difference in coercivity for these materials. The magnetoresistance properties of these junctions are also being studied at room temperature and low temperature. ¹R.A. de Groot, F.M. Mueller, P.G. van Engen, and K.H.J. Buschow, Phys. Rev. Lett. 50, 2024 (1983). ²M.C. Kautzky, F.B. Mancoff, J.-F. Bobo, P.R. Johnson, R.L. White, and B.M. Clemens, J. Appl. Phys. 81, 4026 (1997).
10:00 AM		MI+NS-TuM-6 Spin Conserved Tunneling and Large Magnetoresistance J.S. Moodera (Massachusetts Institute of Technology) The success of tunneling between two ferromagnetic(FM) layers through an insulator has shown interesting physics as well as great potential for application in digital storage industry. The tunneling current in a trilayer device, FM-I-FM where I is an ultra thin insulator(< 2 nm), is observed to be dependent on the relative orientation of the magnetization in the two FM layers. A change of over 20% in the junction magnetoresistance (JMR) is seen at room temperature in an applied magnetic field of less than 20 Oe.[1] This effect was predicted over 20 years ago, based on the spin -polarized tunneling results and the density of states argument.[2] These magnetic tunnel junctions exhibit non-volatile memory effect, leading to the possible usage of these devices as random access memory elements. Spin-valve effect has also been possible with the deposition of antiferromagnetic layer such as NiO adjacent to the FM layer, with a sensitivity factor of > 5% / Oe. The JMR effect has been observed for various combinations of FM materials and insulating layers. Ferromagnets such as NiMnSb, PtMnSb, if proven to be half-metallic, will be the ideal choice for these devices. There are still unsolved questions like the anomalous dc bias and the temperature dependence of the JMR effect. Inelastic tunneling spectroscopy show features that are unique to these FM-I-FM junctions. Starting with the basic phenomena, up to the latest will be presented and discussed. Work supported by the Office of Naval Research and the National Science Foundation. 1. J.S. Moodera et.al, Phys. Rev. Lett. 74, 3273 (1995); J. Appl. Phys. 79, 4724 (1996). 2. M. Julliere, Phys. Lett. A54, 225 (1975).
10:40 AM		MI+NS-TuM-8 Controlled Resistivity of Spin Dependent Tunnel Junctions with Al₂O₃ Barriers M.C. Tondra, D. Wang, R.S. Beech, B.A. Everitt, J.A. Taylor, J.M. Daughton (Nonvolatile Electronics) High quality spin dependent tunnel junctions having a wide range of tunnel resistivities (ρ_T) were deposited using RF diode sputtering and fabricated with a standard photolithography process, as described in an earlier publication¹. The ρ_T of the devices, at room temperature, ranged from 0.1 MΩ-µm² to 5000 MΩ-µm² and had junction magnetoresistances (JMR) of 10% - 24%. The typical structure of these devices was NiFeCo 125Å / Al₂O₃ X / CoFe 125Å, where X varied from about 16Å to 32Å. The fact that ρ_T ranged over 4 orders of magnitude for barrier thicknesses which only ranged over a factor of 2 agrees with the exponential dependence of ρ_T on tunnel barrier thickness as described in Simmons' model². The barriers were constructed using a two step process consisting of an Al deposition followed by an oxidation step. Our data suggest that for a given ρ_T , there is an optimal barrier construction of pure Al thickness and oxidation time such that the Al is ideally oxidized. The devices ranged in area from 6.25 µm² to 10⁶ µm² which corresponds to device resistances ranging from tens of Ω up to hundreds of MΩ. The SDT devices typically exhibited full output in a field of 20 Oe applied along the easy direction. ¹R.S. Beech, J. Anderson, J.M. Daughton, B.A. Everitt, and D. Wang, "Spin Dependent Tunneling Devices Fabricated Using Photolithography," IEEE Transactions on Magnetics, Vol. 32, No. 5, Sept. 1996, p. 4713. ²J.G. Simmons, "Generalized Formula for the Electric Tunnel Effect between Similar Electrodes Separated by a Thin Insulating Film," Journal of Applied Physics, Vol. 34, No. 6, June 1993, p.1793.
11:00 AM		MI+NS-TuM-9 Control of Switching in Magnetic Tunnel Junctions S. Gider, A.C. Marley, P. Kasiraj, S.S.P. Parkin (IBM Almaden Research Center) Thin magnetic films separated by an insulating tunnel barrier can display large changes in magnetoresistance (25%) over small fields (~10 Oe). The details of the switching depend on the magnetic domain structure, which is influenced by the degree of coupling between the two layers. Correlated roughness leads to a ferromagnetic interaction known as "orange peel coupling" that shifts the hysteresis loop away from zero bias. To probe the coupling, a non-magnetic spacer layer is inserted between the tunnel barrier and the free magnetic layer. The basic structure consists of a free layer of 200 Å Ni₈₁Fe₁₉, x Å Cu, 12 Å Al oxidized for 120 s, and a layer of 20 Å Co and 60 Å Ni₈₁Fe₁₉ pinned by exchange biasing. The Cu spacer layer thickness x was varied from 0 to 60 Å. The junction area of (80 micron)² is defined by contact masks as the intersection of the two magnetic layers, thus allowing for the simultaneous observation of the properties of the uncoupled layers. Magnetic reversal is studied by complementary Kerr imaging and transport measurements in order to compare the domain structure with tunneling effects in the same device. The reduction in the coupling with increasing thickness of the spacer layer may be attributed either to a decrease in the degree of correlation or a decrease in the interaction strength due to the larger separation between the layers. The magnetoresistance ratio is found to decrease exponentially with the thickness of the spacer, but the switching patterns are qualitatively similar for different thicknesses, only varying in the field at which particular patterns are formed.
11:20 AM		MI+NS-TuM-10 Hybrid Ferromagnet - Semiconductor Devices M.B. Johnson (Naval Research Laboratory) Novel magnetoelectronic devices for application as nonvolatile memory cells, logic gates, or magnetic field sensors are based on a bilayer structure comprised of a high mobility semiconducting Hall cross and a single, electrically isolated, microstructured ferromagnetic film. Prototypes have been fabricated at the micron and sub-micron size scale. The device state is determined by the magnetic state of the ferromagnetic element, and the two corresponding output states can be symmetrically bipolar or HIGH (order of tens of Ohms) and LOW (approximately zero). Fabrication involves two lithographic steps, a mesa etch of the semiconductor and a patterning of the ferromagnetic film. AFM and MFM images are used to correlate the effects of processing on the micromagnetism of the ferromagnetic component and the device output characteristics. Early prototype device sets have been fabricated using high mobility indium arsenide layers for the Hall element, but the concept is CMOS compatible. The device demonstrates inverse scalability: output levels increase as the device dimensions decrease. Issues of performance, as they relate to materials and fabrication, will be discussed.