167 Matching Results

Search Results

Advanced search parameters have been applied.

Changes of the local distortions and colossal magnetoresistive properties of La(0.7)Ca(0.3)MnO(3) induced by Ti or Ga defects

Description: The magnetoresistive properties of La0.7Ca0.3MnO3 change rapidly when Ti or Ga are substituted on the Mn site for concentrations, x, from 1 to 10 percent. The samples exhibit colossal magnetoresistance (CMR) and the resistivity increases dramatically with dopant concentration. The temperature of the resistivity peak, TR, shifts rapidly to lower temperatures with increasing x and the ferromagnetic transition broadens. However, the transition temperature, Tc, is only slightly suppressed. Consequently, TR occurs well below Tc for x above 2 percent. Investigations of these materials using Mn XAFS show that changes in the local structure, parametrized by the pair-distribution width, sigma, correlate well with Tc and the sample magnetization. For a given dopant, the resistivity peak occurs when sigma{sup 2} decreases below a critical value. Both dopants produce extended defects which increases the resistivity of the nearby materials considerably. The data suggest that even at x {approx}4 percent, most of the sites are slightly distorted at low T.
Date: July 12, 2002
Creator: Bridges, F.; Cao, D.; Anderson, M.; Ramirez, A.P.; Olapinski, M.; Subramanian, M.A. et al.
Partner: UNT Libraries Government Documents Department

GMR in intermetallics

Description: Typical examples of giant magnetoresistance (GMR) effects observed in intermetallic compounds at various conditions are reviewed and some comparisons with the GMR magnetic multilayer systems are shown. Possibilities of tuning material parameters desired for applications are discussed in context of present understanding of the mechanisms responsible for GMR in this class of materials. 18 refs.
Date: April 1, 1996
Creator: Sechowsky, V.; Havela, L.; Nakotte, H. & Brueck, E.
Partner: UNT Libraries Government Documents Department

Local and average crystal structure and displacements of La{sup 11}B{sub 6} and EuB{sub 6} as a function of temperature

Description: Measurements of both the average crystal structure from Rietveld refinement of neutron powder diffraction (NPD) data and the local structure from La L{sub III}-edge x-ray-absorption fine-structure (XAFS) are presented for a La{sup 11}B{sub 6} sample as a function of temperature ({approx}10-320 K). These data are compared to XAFS results on a EuB{sub 6} sample. The single-site La and B positional distribution widths and the La-B and La-La bond length distribution widths and their temperature dependence are compared. This comparison allows an estimate of the La and B site displacements, and we find that these sublattices are only slightly correlated with each other. Moreover, while the temperature dependence of the displacement parameters of the average sites from diffraction fit an Einstein model well, the temperature dependence of the La-B bond length distribution width requires at least two vibrational frequencies, corresponding to the La and B frequencies of the individual sites. XAFS data on EuB{sub 6} indicate that the situation is the same in the Eu compound. In addition, comparisons between data taken below and above the ferromagnetic transition temperature for EuB{sub 6} place stringent limits on the lattice involvement in the associated metal-insulator transition and the ensuing large magnetoresistance effect. This lack of lattice involvement in the magnetoresistance transition is in sharp contrast to the strong lattice involvement observed in the colossal magnetoresistance lanthanum manganese perovskites.
Date: January 30, 2001
Creator: Booth, C.H.; Sarrao, J.L.; Hundley, M.F.; Cornelius, A.L.; Kwei, G.H.; Bianchi, A. et al.
Partner: UNT Libraries Government Documents Department

Role of magnetic anisotropy in spin-filter junctions

Description: We have fabricated oxide-based spin-filter junctions in which we demonstrate that magnetic anisotropy can be used to tune the transport behavior of spin-filter junctions. We have demonstrated spin-filtering behavior in La{sub 0.7}Sr{sub 0.3}MnO{sub 3}/CoCr{sub 2}O{sub 4}/Fe{sub 3}O{sub 4} and La{sub 0.7}Sr{sub 0.3}MnO{sub 3}/MnCr{sub 2}O{sub 4}/Fe{sub 3}O{sub 4} junctions where the interface anisotropy plays a significant role in determining transport behavior. Detailed studies of chemical and magnetic structure at the interfaces indicate that abrupt changes in magnetic anisotropy across the nonisostructural interface is the cause of the significant suppression of junction magnetoresistance in junctions with MnCr{sub 2}O{sub 4} barrier layers.
Date: January 10, 2011
Creator: Chopdekar, R.V.; Wong, F.; Nelson-Cheeseman, B.B.; Liberati, M.; Arenholz, E. & Suzuki, Y.
Partner: UNT Libraries Government Documents Department

First principles calculation of electrical conductivity and giant magnetroresistance of Co{vert_bar}Cu multilayers

Description: We show that the Kubo formula can be used to calculate the nonlocal electrical conductivity of layered systems from first principles. We use the Layer Korringa Kohn Rostoker method to calculate the electronic structure and Green function of Co/Cu/Co trilayers within the local density approximation to density functional theory. This Green function is used to calculate the conductivity through the Kubo formula for both majority and minority spins and for alignment and anti-alignment of the Co moments on either side of the Cu spacer layer. This allows us to determine the giant magnetoresistance from first principles. We investigate three possibilities for the scattering in Co/Cu/Co: (1) equal electron lifetimes for Cu, majority spin Co, and minority spin Co, (2) equal electron lifetimes for majority and minority Co, weaker scattering in Cu and spin dependent interfacial scattering, (3) electron lifetimes for majority and minority spin cobalt proportional to their Fermi energy densities of states and spin dependent interfacial scattering.
Date: December 31, 1995
Creator: Butler, W.H.; Zhang, X.G.; Nicholson, D.M.C. & MacLaren, J.M.
Partner: UNT Libraries Government Documents Department

Large GMR values of sputtered Co/Cu multilayer structures with Co-Cu buffer layers

Description: We demonstrate large giant magnetoresistance (GMR) values of Co/Cu multilayers (MLs) sputtered on combined Co18{angstrom}/Cu48{angstrom} buffer layer. GMR values at room temperature reach 62% at the first antiferromagnetically (AF) coupling peak and 33% at the 2nd AF coupled peak, which are very close to those found in Co/Cu MLs sputtered on a Fe buffer layer. The large GMR effect is attributed to the superior superlattice structure of these samples, as evidenced by the x-ray reflectivity data as well as the TEM micrographs. In particular, the role of thin Co initial layer deposited beneath the Cu buffer layer on improved ML structure has been clarified from cross-sectional micrographs of high-resolution TEM.
Date: February 29, 1996
Creator: Huai, Y.; Vernon, S.P.; Stearns, D.G.; Cerjan, C. & Kania, D.R.
Partner: UNT Libraries Government Documents Department

Magnetoresistance of One-Dimensional Subbands in Tunnel-Coupled Double Quantum Wires

Description: We study the low-temperature in-plane magnetoresistance of tunnel-coupled quasi-one-dimensional quantum wires. The wires are defined by two pairs of mutually aligned split gates on opposite sides of a < 1 micron thick AlGaAs/GaAs double quantum well heterostructure, allowing independent control of their widths. In the ballistic regime, when both wires are defined and the field is perpendicular to the current, a large resistance peak at ~6 Tesla is observed with a strong gate voltage dependence. The data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands.
Date: April 27, 1999
Creator: Blount, M.A.; Lyo, S.K.; Moon, J.S.; Reno, J.L.; Simmons, J.A. & Wendt, J.R.
Partner: UNT Libraries Government Documents Department

CMOS Interface Circuits for Spin Tunneling Junction Based Magnetic Random Access Memories

Description: Magneto resistive memories (MRAM) are non-volatile memories which use magnetic instead of electrical structures to store data. These memories, apart from being non-volatile, offer a possibility to achieve densities better than DRAMs and speeds faster than SRAMs. MRAMs could potentially replace all computer memory RAM technologies in use today, leading to future applications like instan-on computers and longer battery life for pervasive devices. Such rapid development was made possible due to the recent discovery of large magnetoresistance in Spin tunneling junction devices. Spin tunneling junctions (STJ) are composite structures consisting of a thin insulating layer sandwiched between two magnetic layers. This thesis research is targeted towards these spin tunneling junction based Magnetic memories. In any memory, some kind of an interface circuit is needed to read the logic states. In this thesis, four such circuits are proposed and designed for Magnetic memories (MRAM). These circuits interface to the Spin tunneling junctions and act as sense amplifiers to read their magnetic states. The physical structure and functional characteristics of these circuits are discussed in this thesis. Mismatch effects on the circuits and proper design techniques are also presented. To demonstrate the functionality of these interface structures, test circuits were designed and fabricated in TSMC 0.35{micro} CMOS process. Also circuits to characterize the process mismatches were fabricated and tested. These results were then used in Matlab programs to aid in design process and to predict interface circuit's yields.
Date: December 31, 2002
Creator: Saripalli, Ganesh
Partner: UNT Libraries Government Documents Department

Time-resolved x-ray absorption spectroscopy of photoinduced insulator-metal transition in a colossal magnetoresistive manganite

Description: We studied the ultrafast insulator-metal transition in a manganite by means of picosecond X-ray absorption at the O K- and Mn L-edges, probing photoinduced changes in O-2p and Mn-3d electronic states near the Fermi level.
Date: August 1, 2008
Creator: Rini, M.; Tobey, R.; Wall, S.; Zhu, Y.; Tomioka, Y.; Tokura, Y. et al.
Partner: UNT Libraries Government Documents Department

Transient electronic structure of the photoinduced phase of Pr0.7Ca0.3MnO3 probed with soft x-ray pulses

Description: We use time-resolved x-ray absorption near-edge structure spectroscopy to investigate the electronic dynamics associated with the photoinduced insulator-to-metal phase transition in the colossal magnetoresistive manganite Pr{sub 0.7}Ca{sub 0.3}MnO{sub 3}. Absorption changes at the O K and Mn L edges directly monitor the evolution of the density of unoccupied states in the transient photoinduced phase. We show that the electronic structure of the photoinduced phase is remarkably similar to that of the ferromagnetic metallic phase reached in related manganites upon cooling below the Curie temperature.
Date: April 1, 2009
Creator: Rini, M.; Zhu, Y.; Wall, S.; Tobey, R. I.; Ehrke, H.; Garl, T. et al.
Partner: UNT Libraries Government Documents Department

Multidimensional Nature of Molecular Organic Conductors Revealed by Angular Magnetoresistance Oscillations

Description: Angle-dependent magnetoresistance experiments on organic conductors exhibit a wide range of angular oscillations associated with the dimensionality and symmetry of the crystal structure and electron energy dispersion. In particular, characteristics associated with 1, 2, and 3-dimensional electronic motion are separately revealed when a sample is rotated through different crystal planes in a magnetic field. Originally discovered in the TMTSF-based conductors, these effects are particularly pronounced in the related system (DMET){sub 2}I{sub 3}. Here, experimental and computational results for magnetoresistance oscillations in this material, over a wide range of magnetic field orientations, are presented in such a manner as to uniquely highlight this multidimensional behavior. The calculations employ the Boltzmann transport equation that incorporates the system's triclinic crystal structure, which allows for accurate estimates of the transfer integrals along the crystallographic axes, verifying the 1D, 2D and 3D nature of (DMET){sub 2}I{sub 3}, as well as crossovers between dimensions in the electronic behavior.
Date: September 1, 2012
Creator: Dhakal, Pashupati; Yoshino, Harukazu; Oh, Jeong-Il; Kikuchi, Koichi & Naughton, Michael J.
Partner: UNT Libraries Government Documents Department

Effect of pressure on the electrical resistivity and magnetism in UPdSn

Description: The electrical resistivity of a UPdSn single crystal exerted to various hydrostatic pressures was measured as a function of temperature and magnetic field. Clear anomalies in the temperature dependence of resistivity along the c-axis mark the magnetic phase transitions between paramagnetic and antiferromagnetic (AF) state at TN and the AF 1 -AF2 transition at T I .L arge negative magnetoresistance effects have been observed not only in the AF state as a result of the metamagnetic transition to canted structure ai B,, but also at temperatures far above TN. The latter result is attributed to the existence of AF correlations or short range AF ordering in the paramagnetic range. The value of TN increases with increasing applied pressure, whereas TI simultaneously decreases. It is also found that 13, decreases with increasing pressure. As a consequence, the stability range of the AF- 1 phase expands with applied pressure partially on account of the ground-state AF-2 phase.
Date: January 1, 2002
Creator: Honda, F.; Alsmadi, A. K. (Abdel Khaleq); Sechovsky, V. (Vladimir); Kamarad, J.; Nakotte, H. (Heinrich); Lacerda, A. H. (Alex H.) et al.
Partner: UNT Libraries Government Documents Department

Magnetism as a main origin for the diverse magnetotransport.

Description: Magnetotransport properties of pure Mg metal and MgB{sub 2} samples with varying amounts of unreacted Mg are systematically studied in magnetic fields up to 18 T. With increasing quantity of Mg, the inhomogeneous MgB{sub 2} samples show greatly decreased residual resistivity, enhanced residual resistance ratio (RRR) and enhanced magnetoresistance (MR), gradually approaching the transport behaviors of pure Mg metal. We use the generalized effective medium theory to show that the large RRR and MR of the inhomogeneous MgB{sub 2} samples can be quantitatively explained by a two-phase model in which the two phases are MgB{sub 2} and pure Mg.
Date: January 1, 2001
Creator: Betts, J. B. (Jonathan B.); Jaime, M. (Marcello); Lacerda, A. H. (Alex H.); Boebinger, G. S. (Gregory S.); Jung, C. U. (Chang Uk); Kim, H. J. (Hoen-Jung) et al.
Partner: UNT Libraries Government Documents Department

Interface Structure and Transport of Complex Oxide Junctions

Description: The interface structure and magnetism of hybrid magnetic tunnel junction-spin filter devices have been investigated and correlated with the transport behavior exhibited. Magnetic tunnel junctions made of theoretically predicted half-metallic electrodes (perovskite La0.7Sr0.3MnO3 and spinel Fe3O4) sandwiching a spinel NiMn2O4 tunnel barrier exhibit very high crystalline quality as observed by transmission electron microscopy. Structurally abrupt interfaces allow for the distinct magnetic switching of the electrodes as well as large junction magnetoresistance. The change in the magnetic anisotropy observed at the spinel-spinel interface supports the presence of limited interdiffusion and the creation of a magnetically soft interfacial layer, whose strong exchange coupling to the Fe3O4 electrode likely accounts for the low background magnetoresistance observed in these junctions, and the successful spin filtering when the barrier layer is ferrimagnetic.
Date: February 1, 2008
Creator: Nelson-Cheeseman, B. B.; Wong, F.; Chopdekar, R. V.; Chi, M.; Arenholz, E.; Browning, N. D. et al.
Partner: UNT Libraries Government Documents Department

Bulk Superconductivity and Disorder in Single Crystals of LaFePO

Description: We have studied the intrinsic normal and superconducting properties of the oxypnictide LaFePO. These samples exhibit bulk superconductivity and the evidence suggests that stoichiometric LaFePO is indeed superconducting, in contrast to other reports. We find that superconductivity is independent of the interplane residual resistivity {rho}{sub 0} and discuss the implications of this on the nature of the superconducting order parameter. Finally we find that, unlike T{sub c}, other properties in single-crystal LaFePO including the resistivity and magnetoresistance, can be very sensitive to disorder.
Date: February 15, 2010
Creator: Analytis, James G.; Chu, Jiun-Haw; Erickson, Ann S.; Kucharczyk, Chris; /Stanford U., Appl. Phys. Dept.; Serafin, Alessandro et al.
Partner: UNT Libraries Government Documents Department

Magnetic profiles in ferromagnetic/superconducting superlattices.

Description: The interplay between ferromagnetism and superconductivity has been of longstanding fundamental research interest to scientists, as the competition between these generally mutually exclusive types of long-range order gives rise to a rich variety of physical phenomena. A method of studying these exciting effects is by investigating artificially layered systems, i.e. alternating deposition of superconducting and ferromagnetic thin films on a substrate, which enables a straight-forward combination of the two types of long-range order and allows the study of how they compete at the interface over nanometer length scales. While originally studies focused on low temperature superconductors interchanged with metallic ferromagnets, in recent years the scope has broadened to include superlattices of high T{sub c} superconductors and colossal magnetoresistance oxides. Creating films where both the superconducting as well as the ferromagnetic layers are complex oxide materials with similar crystal structures (Figure 1), allows the creation of epitaxial superlattices, with potentially atomically flat and ordered interfaces.
Date: February 28, 2007
Creator: te Velthuis, S. G. E.; Hoffmann, A.; Santamaria, J.; Division, Materials Science & Madrid, Univ. Complutense de
Partner: UNT Libraries Government Documents Department

Room Temperature Magnetic Barrier Layers in Magnetic Tunnel Junctions

Description: We investigate the spin transport and interfacial magnetism of magnetic tunnel junctions with highly spin polarized LSMO and Fe3O4 electrodes and a ferrimagnetic NiFe2O4 (NFO) barrier layer. The spin dependent transport can be understood in terms of magnon-assisted spin dependent tunneling where the magnons are excited in the barrier layer itself. The NFO/Fe3O4 interface displays strong magnetic coupling, while the LSMO/NFO interface exhibits clear decoupling as determined by a combination of X-ray absorption spectroscopy and X-ray magnetic circular dichroism. This decoupling allows for distinct parallel and antiparallel electrode states in this all-magnetic trilayer. The spin transport of these devices, dominated by the NFO barrier layer magnetism, leads to a symmetric bias dependence of the junction magnetoresistance at all temperatures.
Date: March 9, 2010
Creator: Nelson-Cheeseman, B. B.; Wong, F. J.; Chopdekar, R. V.; Arenholz, E. & Suzuki, Y.
Partner: UNT Libraries Government Documents Department

Non-magnetic compensation in ferromagnetic Ga1-xMnxAs and Ga1-xMnxP synthesized by ion implantation and pulsed-laser melting

Description: The electronic and magnetic effects of intentional compensation with non-magnetic donors are investigated in the ferromagnetic semiconductors Ga1-xMnxAs and Ga1-xMnxP synthesized using ion implantation and pulsed-laser melting (II-PLM). It is demonstrated that compensation with non-magnetic donors and MnI have similarqualitative effects on materials properties. With compensation TC decreases, resistivity increases, and stronger magnetoresistance and anomalous Hall effect attributed to skew scattering are observed. Ga1-xMnxAs can be controllably compensated with Te through a metal-insulator transition through which the magnetic and electrical properties vary continuously. The resistivity of insulating Ga1-xMnxAs:Te can be described by thermal activation to the mobility edge and simply-activated hopping transport. Ga1-xMnxP doped with S is insulating at all compositions but shows decreasing TC with compensation. The existence of a ferromagnetic insulating state in Ga1-xMnxAs:Te and Ga1-xMnxP:S having TCs of the same order as the uncompensated materials demonstrates that localized holes are effective at mediating ferromagnetism in ferromagnetic semiconductors through the percolation of ferromagnetic 'puddles' which at low temperatures.
Date: February 5, 2008
Creator: Scarpulla, M. A.; Stone, P. R.; Sharp, I. D.; Haller, E. E.; Dubon, O. D.; Beeman, J. W. et al.
Partner: UNT Libraries Government Documents Department

Magnetic anisotropy, coupling and transport in epitaxial Co/Cr superlattices on MgO (100) and (110) substrates

Description: Superlattices of Co/Cr have been epitaxially sputtered onto MgO (100) and (110) substrates coated with Cr(100) and (211) buffer layers. The Co thickness is fixed at 20{angstrom} and the Cr thickness varied from 7 to 22{Angstrom}. On the MgO(110)/Cr(211) substrates, coherent hcp-Co(1{bar 1}00)/bcc-Cr(211) superlattice structures are formed. On MgO(100)/Cr(100), XRD suggest strained hcp-Co(11{bar 2})/bcc-Cr(100) superlattices. Magnetization measurements show fourfold magnetic in- plane anisotropy for the MgO(100) orientation and twofold for the MgO(110). By utilizing a simple model based on perpendicular uniaxial anisotropies, we have concluded that the fourfold anisotropy has its origin in the second-order uniaxial Co anisotropy energy. The antiferromagnetic interlayer coupling strength exhibits a maximum value of 0.15 erg/cm{sup 2} at a Cr thickness of 13{angstrom} in the MgO(110) orientation. The MgO(100) orientation exhibits its strongest coupling of 0.55 erg/cm{sup 3} at 10{angstrom} Cr thickness. Modest GMR values no larger than 3% are observed and we find no evidence of enhanced AMR effects recently reported for Co(1{bar 1}00)/Cr(211) superlattices.
Date: November 1996
Creator: Picconatto, J. J.; Pechan, M. J. & Fullerton, E. E.
Partner: UNT Libraries Government Documents Department

Synthesis, tailored microstructures and `colossal` magnetoresistance in oxide thin films

Description: We have grown La{sub 1-x}Sr{sub x}MnO{sub 3} films, using both pulsed laser deposition and a polymeric sol-gel route. These two growth techniques result in different microstructures, but in both cases the texture (epitaxy or polycrystallinity) can be controlled by choice of substrates and growth conditions. The crystallography and microstructure of these films were studied using XRD and high- resolution TEM. The magnetic/magnetotransport properties of these films are discussed in context of their growth and microstructural parameters.
Date: September 1, 1996
Creator: Krishnan, K.M.; Modak, A.R.; Ju, H. & Bandaru, P.
Partner: UNT Libraries Government Documents Department

Two-dimensional intrinsic and extrinsic ferromagnetic behavior of layered La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} single crystals

Description: The low-field magnetization M and susceptibility {chi} are reported for the two-layered Ruddleson-Popper phase SrO(La{sub 1{minus}x}Sr{sub x}MnO{sub 3}){sub 2} for x = 0.4 (denoted La{sub 1.2}Sr{sup 1.8}Mn{sub 2}O{sub 7}) with an in-plane magnetic easy axis. As T approaches the Curie temperature (T{sub C} = 116 K) on cooling, where the metal-insulator transition occurs in zero-field, {chi} diverges as H{sup {minus}1/{delta}{prime}}, with {delta}{prime} = 21 {+-} 5. Also, near an extrinsic Curie transition attributed to {approximately} 0.1% volume fraction of intergrowths at T* = 2.47T{sub C}, M scales as (1 {minus} T/T*){sup {beta}}, with {beta} = 0.25 {+-} 0.02. These results can be understood within the context of 2D XY models, and provide a new perspective of the layered manganites.
Date: June 6, 1997
Creator: Potter, C. D.; Swiatek, M.; Bader, S. D.; Argyriou, D.N.; Mitchell, J. F.; Miller, D. J. et al.
Partner: UNT Libraries Government Documents Department

Lattice effect in perovskite and pyrochlore CMR materials

Description: Colossal magnetoresistance (CMR) in doped La manganite thin films (La{sub 1-x}M{sub x}MnO{sub 3}, where M = divalent ion, either Ca or Pb) has been shown to result in a factor of 10{sup 6} suppression of the resistance. The driving force for the CMR transition is thought to be the double-exchange interaction. Many studies of both the crystal structure and the local structure of the La{sub 1-x}M{sub x}MnO{sub 3} (M = Ca, Sr, Ba, Pb) system have now been carried out. As expected, these systems all show a strong coupling of the lattice to the CMR transition. On the other hand, neutron diffraction and x-ray absorption for the Tl{sub 2}Mn{sub 2}O{sub 7} pyrochlore, which also exhibits CMR, shows no deviations from ideal stoichiometry, mixed valency, or Jahn-Teller distortions of the MnO{sub 6} octahedron. We present results of crystallographic and local structural studies of these two important classes of CMR materials, compare the differences in structural response, and discuss the implications of these findings to our understanding of these materials.
Date: September 1997
Creator: Kwei, G. H.; Argyriou, D. N.; Lawson, A. C.; Neumeier, J. J.; Thompson, J. D.; Billinge, S. J. L. et al.
Partner: UNT Libraries Government Documents Department