AVS1996 Session MI-ThM: Magnetoresistance and Exchange Coupling

Thursday, October 17, 1996 8:20 AM in Room 106A/B

Thursday Morning

Start	Invited?	Item
8:20 AM		MI-ThM-1 Photoemission Studies on the k-Space Origin of the Long Period Oscillation in Fe/Cr Multilayers D. Li, J. Pearson, S. Bader (Argonne National Laboratory); E. Vescovo, D. Huang, P. Johnson (Brookhaven National Laboratory) Fe/Cr multilayers are the prototype giant magnetoresistance (GMR) material. The microscopic origin of its 1.8 nm long-period oscillatory magnetic coupling is still being debated by theorists, while its short-period (of 2 monolayers) is universally ascribed to the nesting of the Cr Fermi surface in the <100> direction. We have carried out angle-resolved photoemission experiments at the NSLS U5 undulator beamline to investigate the k-space origin of the long-period coupling. Exceptionally well-ordered and smooth Cr layers (0.8 - 5 nm) were deposited onto an atomically flat Fe(100) whisker at 300\super o\C for epitaxial growth. We searched different candidate regions of k-space at off-normal geometry for thickness dependent Cr quantum-well (QW) states, and monitored their resultant photoemission intensity oscillations at the Fermi level (E\sub F\). In the vicinity of the d- derived lens feature of the Fermi surface, the intensity at E\sub F\ oscillates with a period of 1.7 nm as a function of Cr thickness. This indicates the d-derived QW states pass through E\sub F\ periodically. This period coincides with that for the magnetic coupling in Fe/Cr multilayers, which suggests the extremal spanning vector across the lens as the origin of the long period oscillation. The work demonstrates that angle-resolved photoemission provides a novel methodology to distinguish the k-space origin of different oscillation periodicities in the interlayer magnetic coupling. Work at ANL is supported by DOE BES-MS under W-31-109-ENG-38 and ONR N- 00014-94-F-0085; work at BNL by DOE under DE-AC02-76ch00016.
8:40 AM		MI-ThM-2 Density Functional Studies of Mixed Fe-Cr Interfaces D. Singh (Naval Research Laboratory) First principles, density functional calculations are used to investigate the magnetic structure and electronic properties of Fe/Cr stacks, including models of interfacial mixing. It is found that incorporation of mixing at the interface reduces the magnetic coupling across the interface and can change the phase in qualitative agreement with the experimental picture. The energy cost of mixing Fe and Cr at the interface is extremely low, consistent with the difficulty in growing non-intermixed Fe-Cr layers.
9:00 AM	Invited	MI-ThM-3 Exchange Coupling Across Iron Silicides in Fe/Si Multilayers A. Chaiken, R. Michel (Lawrence Livermore National Laboratory) Interlayer exchange coupling is a nearly ubiquitous occurrence in metallic multilayers where a ferromagnet is alternated with a paramagnetic spacer. Current theories of interlayer exchange coupling do a good job of explaining bilinear coupling across paramagnetic spacers, which results in a parallel or antiparallel alignment of spins in alternate ferromagnet layers. Less well understood are coupling across non-transition-metal spacer materials, and biquadratic coupling, which results in a preferred 90 degree rotation between neighboring ferromagnet layers. Both these issues arise in the study of Fe/Si multilayers, where the exchange coupling occurs across an iron silicide phase that forms during deposition. I will describe the growth of Fe/Si multilayers by ion-beam sputtering and the results of characterization by x-ray diffraction, transmission electron microscopy (TEM), electron spectroscopies, ferromagnetic resonance, polarized neutron reflectometry (PNR), and magnetization measurements. TEM images show that the iron silicide which mediates the exchange coupling is a metastable cubic phase, while element-specific electron spectroscopic measurements show that this phase is metallic. The unusual temperature dependence of the interlayer coupling in this system has been studied via PNR and SQUID magnetometry. Careful x-ray diffraction studies on epitaxial and polycrystalline films show that the unusual behavior is more pronounced in multilayers with rougher interfaces. The biquadratic interlayer coupling may therefore be morphological in origin, as has been posited by Slonczewski. This work was performed by LLNL under DOE contract number W-7405- ENG-48.
9:40 AM		MI-ThM-5 A Manganite-Cuprate Superlattice: Study of Magnetic Coupling W. Pickett (Naval Research Laboratory) The distinctive properties of layered cuprates (high temperature superconductivity) and the "colossal magnetoresistance manganites (e.g. half-metallic ferromagnetism), and the fact that both are perovskite- based perovskites, suggests that it should be possible to make sandwiches or multilayers of these materials with novel properties. We report on first principles, local density functional studies of a thin superlattice (La\sub 2-x\Ba\sub x\CuO\sub 4\)/((La\sub 2-y\Ba\sub y\MnO\sub 4\), where x (~0.15) and y~2/3 are chosen to dope the Cu-O layers and the Mn-O layers, respectively. Alloying on the La-Ba sublattice is treated within the virtual crystal approximation. The ferromagnetic moment in the Mn-O layer produces only very minor polarization in the Cu-O layer. Various aspects of the electronic structure and perpendicular coupling will be presented and discussed, with emphasis on the differences between majority and minority channels.
10:00 AM		MI-ThM-6 Magnetic and Superconducting Properties of Ferromagnetic/Superconducting Multilayers: Ni/Nb J. Mattson, R. Osgood III, C. Potter, C. Sowers, S. Bader (Argonne National Laboratory) In this work we examine structural, magnetic and superconducting properties of the Ni/Nb ferromagnetic/superconducting multilayer system. The Ni(Nb) films are textured [111][110] and show low-angle x-ray diffraction patterns typical of smooth interfaces. We show that in Ni/Nb, oscillatory interlayer magnetic coupling is virtually absent, possibly because of the strong disorder present in the system due to the 15% lattice mismatch. The average moment of the Ni atoms in the structure drops precipitously to 20% of that of bulk Ni for d\sub Ni\ = 19 \Ao\ and d\sub Nb\ = 140 \Ao\, where d\sub Ni\ and d\sub Nb\ are the thicknesses of the Ni and Nb multilayer constituents, respectively. The Nb is not superconducting for d\sub Nb\ < 100 \Ao\ and d\sub Ni\ = 19 \Ao\. There are two regimes of magnetic behavior for the Ni: ferromagnetic and paramagnetic. The behavior of the superconducting critical temperature (T\sub c\) as a function of d\sub Nb\ in the ferromagnetic regime was measured and fitted to the Radovi\aa c\ model, where the exchange splitting of the energy bands in the ferromagnet is assumed to contribute to the pair breaking in the magnetic metal. The behavior of T\sub c\ as a function of d\sub Ni\ in the paramagnetic regime was measured and fitted to the Werthamer model. An interfacial scattering parameter was obtained from the fit in the ferromagnetic regime and applied to the paramagnetic regime. A scattering lifetime of 4.3 x 10\super -15\ seconds, similar to that reported for Nb/Gd in the paramagnetic regime, was obtained by fitting T\sub c\ of a multilayer with d\sub Nb\ = 87 \Ao\ as a function of d\sub Ni\. This work was sponsored by USDOE-BES #W-31-109-ENG-38.
10:20 AM		MI-ThM-7 Ferromagnetic/Superconducting Multilayers: Fe\sub 4\N/NbN J. Mattson (Motorola); C. Potter (Argonne National Laboratory); M. Conover (University of Illinois); C. Sowers, S. Bader (Argonne National Laboratory) The structural, magnetic and superconducting properties of series of Fe\sub 4\N/NbN multilayers are examined in the interest of studying novel coupling mechanisms in magnetic/superconducting systems. We have found that superconductivity of the NbN occurs only for layer thicknesses larger than 90 \Ao\. Ferromagnetism of Fe\sub 4\N is observed in all samples studied. Despite the presence of these transitions, no evidence is apparent for either magnetic coupling through the superconductors or superconductive coupling through the magnetic layers. The properties of the uncoupled superconductors are described within the framework of Radovic's theory\super1\ for superconducting/ferromagnetic multilayers. Although the absence of coupling between magnetic layers is not fully understood, guidelines are discussed for future searches for novel magnetic-coupling phenomena. Work supported by US DOE, BES/MS and ER-LTR contract W-31-109-ENG-38, and NSF STCS contract DMR 91-20000. 1-Z. Radovic et al., Phys. Rev. B vol. 38, p. 2388 (1988).
10:40 AM	Invited	MI-ThM-8 Surface and Interface Issues in Magnetoelectronic Devices G. Prinz (Naval Research Laboratory) It is now possible to build electronic devices whose operation is based upon the spin-polarized character of the carriers. The high ordering temperature of the magnetic metals used in these devices permits operation at room temperature, and the devices themselves are readily fabricated by standard lithographic procedures for integration into current semiconductor integrated circuit technology. The basic physical mechanisms, which arise from the spin polarized electronic structure, will be briefly reviewed and a number of examples of device applications will be presented from the areas of sensors, computer memory and computer logic. Prospects for transitions from research into the market will be discussed.
11:20 AM		MI-ThM-10 Size Effects in Submicrometer Spin Valve Magnetoresistive Random Access Memory Devices S. Russek, R. Cross, J. Oti (National Institute of Standards & Technology); Y. Kim (Quantum Peripherals Colorado, Inc.) Spin valve devices for use in magnetoresistive random access memory (MRAM) cells have been fabricated with line widths down to 400 nm. NiFe-Co-Cu-Co-NiFe-FeMn multilayer films were patterned to form devices with varying line widths and aspect ratios. Below 1 \mu\m line widths the MRAM switching fields and switching field asymmetries become very sensitive to the size and geometry of the device. As the device size is reduced the MRAM response becomes more ideal as the magnetic layers behave more like single domain particles. However, the switching fields and switching field asymmetries can become quite large at small dimensions when magnetostatic effects dominate device response. For a 500 nm wide, 5 /mu/m long device with 7.5 nm NiFe-3.0 nm Cu- 7.5 nm NiFe, the parallel and antiparallel switching fields are -11 Oe and 32 Oe respectively. The switching fields can be varied by changing the thickness of the free, pinned, and Cu layers; the aspect ratio of the device; and the interlayer exchange coupling. The experimental data is compared to numerical and micromagnetic models of the MRAM devices. Methods to engineer small devices with symmetric switching fields of the size appropriate to VLSI applications are identified.
11:40 AM		MI-ThM-11 Patterning of Cu, Co, Fe and Ag for Magnetic Nanostructures K. Jung, Y. Park, J. Caballero, J. Childress, S. Pearton (University of Florida, Gainesville); F. Ren (Bell Laboratories) Wet and dry etching of thin metallic multilayer structures is necessary for development of sensitive magnetic field sensors and memory devices based on spin-valve GMR elements. While it is well established that Cu, Co, Fe are soluble in HNO/sub 3/ and H/sub 3/PO/sub 4/ at room temperature, little effort has been made to investigate selective wet and dry etch chemistries. For example we find Ag is not etched in H/sub 2/SO/sub 4/, HCl or H/sub 3/PO/sub 4/ under conditions where etch rates for the other metals are in the range of 2,000-60,000A/min and the rate-limiting step is reaction at the metal surface. Electron Cyclotron Resonance SF/sub 6//Ar plasmas provide etch selectivities of />=/5:1 for Ag over Cu, Co and Fe, while lower selectivities are obtained with CH/sub 4/H//sub 2//Ar. Cl/sub 2/-based plasma chemistries leave significant metal-chlorine surface residues, which can be removed in-situ by low ion energy H/sub 2/ or Ar plasma treatments that eliminate corrosion problems. Cu etch rates in excess of 3,000A/min at 25/sub o/C can be obtained in ECR Cl/sub 2//Ar discharges because the high ion flux prevents formation of a CuCl/sub X/-rich selvedge layer, which normally only is volatile for etch temperatures />=/220/super o/C in conventional reactive ion etch systems. Photoresist masks suffer severe reticulation under ECR conditions, at least for microwave powers >400W, and SiO/sub 2/ or SiN/sub 4/ offer much better etch resistance.