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Full-Text Articles in Engineering
Magnetic Transitions In Disordered Gdal2, D. S. Williams, Paul M. Shand, Thomas M. Pekarek, Ralph Skomski, Valeri G. Petkov, Diandra Leslie-Pelecky
Magnetic Transitions In Disordered Gdal2, D. S. Williams, Paul M. Shand, Thomas M. Pekarek, Ralph Skomski, Valeri G. Petkov, Diandra Leslie-Pelecky
Materials Research Science and Engineering Center: Faculty Publications
The role of disorder in magnetic ordering transitions is investigated using mechanically milled GdAl2. Crystalline GdAl2 is a ferromagnet while amorphous GdAl2 is a spin glass. Nanostructured GdAl2 shows a paramagnetic-to-ferromagnetic transition and glassy behavior, with the temperature and magnitude of each transition dependent on the degree and type of disorder. Disorder is parametrized by a Gaussian distribution of Curie temperatures TC with mean TC and breadth Δ TC. A nonzero coercivity is observed at temperatures more than 20 K above the highest TC of any known Gd-Al phase; however, …
Spin Blockade In Ferromagnetic Nanocontacts, M. Ye Zhuralev, Evgeny Y. Tsymbal, Sitaram Jaswal, A.V. Vedyayev, B. Dieny
Spin Blockade In Ferromagnetic Nanocontacts, M. Ye Zhuralev, Evgeny Y. Tsymbal, Sitaram Jaswal, A.V. Vedyayev, B. Dieny
Materials Research Science and Engineering Center: Faculty Publications
Using a free-electron model and a linear response theory we investigate spin-dependent electronic transport in magnetic nanocontacts in the ballistic regime of conduction. We emphasize the fact that in atomic-size ferromagnetic contacts it is possible to achieve the conductance value of e2/h, which implies a fully spin-polarized electric current. We explore some consequences of this phenomenon. In particular, we show that the presence of a nonmagnetic region in the nanocontact separating two ferromagnetic electrodes can lead to a spin blockade resulting in very large values of magnetoresistance.
Surface Structure Of Ultrathin Copolymer Films Of Ferroelectric Vinylidene Fluoride (70%) With Trifluoroethylene (30%) On Graphite, Lei Cai, Hongwei Qu, Chenxi Lu, Stephen Ducharme, Peter A. Dowben, Jiandi Zhang
Surface Structure Of Ultrathin Copolymer Films Of Ferroelectric Vinylidene Fluoride (70%) With Trifluoroethylene (30%) On Graphite, Lei Cai, Hongwei Qu, Chenxi Lu, Stephen Ducharme, Peter A. Dowben, Jiandi Zhang
Materials Research Science and Engineering Center: Faculty Publications
The structure and local structural distortions, through the polarization manipulation, of crystalline films of ferroelectric vinylidene fluoride (70%) with trifluoroethylene (30%) [P(VDF-TrFE)] copolymer on graphite were studied by scanning tunneling microscopy (STM). A quasispiral twist in C[Single Bond]C bonds with rotations about the polymer chain axis was observed by high-resolution STM, indicating a surface relaxation of the strained copolymer films. Such a relaxation behavior appears to be linked to the observed local dipole rotations accompanied by the reversal of the local polarization with biasing the STM tip. A structure model is proposed based upon the observations.
Finite-Temperature Anisotropy Of Ptco Magnets, Ralph Skomski, Arti Kashyap, David J. Sellmyer
Finite-Temperature Anisotropy Of Ptco Magnets, Ralph Skomski, Arti Kashyap, David J. Sellmyer
Materials Research Science and Engineering Center: Faculty Publications
The temperature dependence of the magnetocrystalline anisotropy of PtCo and its atomic origin are investigated by first-principle and model calculations. The Pt spin moment necessary to realize the leading 5d anisotropy contribution is due to neighboring Co atoms. At finite temperatures, Co spin disorder strongly reduces the Pt moment and the anisotropy. This is in contrast to the situation encountered in 3d and 3d–4f magnets, where the atomic magnetic moments remain largely conserved, even above the Curie temperature. A consequence of the L10 mechanism is that theK1 (T) curve of exhibits a negative curvature, in contrast to the …
Exchange Interactions And Curie Temperature Of Y–Co Compounds, Arti Kashyap, Ralph Skomski, Renat F. Sabiryanov, Sitaram Jaswal, David J. Sellmyer
Exchange Interactions And Curie Temperature Of Y–Co Compounds, Arti Kashyap, Ralph Skomski, Renat F. Sabiryanov, Sitaram Jaswal, David J. Sellmyer
Materials Research Science and Engineering Center: Faculty Publications
The Curie temperature of rare-earth Co intermetallics is investigated by self-consistent spin-polarized electronic-structure calculations on Y–Co compounds. The total exchange interaction of a given site with all other sites (Jo) is calculated by the linear-muffin-tin-orbital method using the local force theorem and employed to obtain mean-field Curie-temperature estimates. The theoretical predictions for YCo3, YCo5, and Y2Co17 are in fair agreement with the experimental Curie temperatures. For YCo5, the variation of exchange interactions (Jij) with distance is analyzed, and it is discussed how the presence of nonequivalent cobalt …
Oscillatory Interlayer Exchange Coupling And Its Temperature Dependence In [Pt/Co]3/Nio/[Co/Pt]3 Multilayers With Perpendicular Anisotropy, Z. Y. Liu, Shireen Adenwalla
Oscillatory Interlayer Exchange Coupling And Its Temperature Dependence In [Pt/Co]3/Nio/[Co/Pt]3 Multilayers With Perpendicular Anisotropy, Z. Y. Liu, Shireen Adenwalla
Materials Research Science and Engineering Center: Faculty Publications
Interlayer exchange coupling that oscillates between antiferromagnetic and ferromagnetic as a function of NiO thickness has been observed in [Pt(5 Å)/Co(4 Å)]3/NiO(tNiO Å)/[Co(4 Å)/Pt(5 Å)]3 multilayers with out-of-plane anisotropy. The period of oscillation corresponds to ~2 monolayers of NiO. This oscillatory behavior is possibly attributed to the antiferromagnetic ordering in NiO. The antiferromagnetic interlayer exchange coupling for the 11 Å NiO layer shows an increase in coupling strength with increasing temperature, in agreement with the quantum interference model of Bruno for insulating spacer layers. A coexistence of exchange biasing and antiferromagnetic interlayer exchange coupling has …
Closely Linear Temperature Dependence Of Exchange Bias And Coercivity In Out-Of-Plane Exchange-Biased [Pt/Co] 3/Nio (11 Å) Multilayer, Z.Y. Liu, Shireen Adenwalla
Closely Linear Temperature Dependence Of Exchange Bias And Coercivity In Out-Of-Plane Exchange-Biased [Pt/Co] 3/Nio (11 Å) Multilayer, Z.Y. Liu, Shireen Adenwalla
Materials Research Science and Engineering Center: Faculty Publications
Strong out-of-plane exchange biasing has been observed in out-of-plane exchange biased [Pt (5 Å)/Co (4 Å)] 3/NiO (11 Å) multilayer with perpendicular easy axis. Both the exchange field HE and coercivity HC display a closely linear temperature dependence except at very low temperatures. A thin NiO layer coated on the top of a Pt/Co multilayer has a great effect on the domain pattern of the Pt/Co multilayer, which is in a more irregular configuration with much smaller domain sizes than the uncoupled Pt/Co multilayer. A simulation according to Malozemoff's random field model gives a good agreement to …
Ferromagnetic Domain-Wall Behavior During Reversal And Thermally Activated Antiferromagnetic Reversal In An Exchange-Biased Nio/Nife Bilayer, Z. Y. Liu, Shireen Adenwalla
Ferromagnetic Domain-Wall Behavior During Reversal And Thermally Activated Antiferromagnetic Reversal In An Exchange-Biased Nio/Nife Bilayer, Z. Y. Liu, Shireen Adenwalla
Materials Research Science and Engineering Center: Faculty Publications
The magnetization reversal mechanism in an exchange-biased NiO/Ni81Fe19 bilayer has been investigated using the magneto-optic Kerr effect and magnetic force microscopy imaging. The asymmetric reversal along the unidirectional axis and the two-step reversal process along the hard axis are promoted by ferromagnetic domain-wall behavior in the decreasing and increasing field branches of the as-deposited hysteresis loop, which is strongly related to the exchange coupling at the interface and the distribution of orientation of the net magnetization at the interface carried by the antiferromagnetic domains. The temperature dependence of the exchange bias and coercivity shows the thermally activated …
Atomic, Electronic, And Magnetic Properties Of Magnetic Tunnel Junctions, I.I. Oleynik, Evgeny Y. Tsymbal
Atomic, Electronic, And Magnetic Properties Of Magnetic Tunnel Junctions, I.I. Oleynik, Evgeny Y. Tsymbal
Materials Research Science and Engineering Center: Faculty Publications
We report results of first-principles density-functional studies of the atomic and electronic structure of Co/Al2O3/Co and Co/SrTiO3/Co magnetic tunnel junctions (MTJs). The atomic structure has been studied for different interface terminations and the interfacial energetics was quantified based on the work of separation. Based on energetics of cohesion, the O-terminated and TiO2-terminated interfaces were identified as the most stable structures for Co/Al2O3/Co and Co/SrTiO3/Co MTJs, respectively. The electronic structure was analyzed using local densities of states. We found that the electronic structure of the O-terminated Co/Al …
Resonant Inversion Of Tunneling Magnetoresistance, Evgeny Y. Tsymbal, Andrei Sokolov, Ildar F. Sabiryanov, Bernard Doudin
Resonant Inversion Of Tunneling Magnetoresistance, Evgeny Y. Tsymbal, Andrei Sokolov, Ildar F. Sabiryanov, Bernard Doudin
Materials Research Science and Engineering Center: Faculty Publications
Resonant tunneling via localized states in the barrier can invert magnetoresistance in magnetic tunnel junctions. Experiments performed on electrodeposited Ni/NiO/Co nanojunctions of area smaller than 0.01 μm2 show that both positive and negative values of magnetoresistance are possible. Calculations based on Landauer-Bu¨ttiker theory explain this behavior in terms of disorder-driven statistical variations in magnetoresistance with a finite probability of inversion due to resonant tunneling.
Variation Of Domain Formation In A 15 Nm Nife Layer Exchange Coupled With Nio Layers Of Different Thicknesses, Z.Y. Liu, Shireen Adenwalla
Variation Of Domain Formation In A 15 Nm Nife Layer Exchange Coupled With Nio Layers Of Different Thicknesses, Z.Y. Liu, Shireen Adenwalla
Materials Research Science and Engineering Center: Faculty Publications
The correlation between ferromagnetic domain formation and exchange bias in a series of NiFe/NiO samples with varying NiO thicknesses has been investigated using the magneto-optic Kerr effect and magnetic force microscopy. Below a critical thickness (15 nm) of NiO, the exchange bias HE is zero and ripple domains exist in the NiFe layer. Above this critical thickness, cross-tie type domain walls appear concurrently with the appearance of exchange bias. Both the number of cross-tie domain walls and the exchange bias increase with an increase in NiO thickness, reaching a maximum at 35 nm NiO, after which both show a …
Angular Dependence Of Magnetization Reversal Process In Exchange Coupled Ferromagnetic/Antiferromagnetic Bilayers, Z.Y. Liu, Shireen Adenwalla
Angular Dependence Of Magnetization Reversal Process In Exchange Coupled Ferromagnetic/Antiferromagnetic Bilayers, Z.Y. Liu, Shireen Adenwalla
Materials Research Science and Engineering Center: Faculty Publications
The angular dependence of the magnetization reversal process in the exchange biased ferromagnetic/antiferromagnetic bilayers has been investigated carefully using the Stoner–Wohlfarth rotation model. Depending strongly on the orientation of the applied field and the competition between the unidirectional and uniaxial anisotropic energies, the magnetization rotation can occur at either the same side or the different sides of the field direction for the decreasing and increasing field branches of the hysteresis loop. The calculated results and the magneto-optical Kerr effect have been used to understand the magnetization reversal process in the NiFe/NiO bilayers, which is caused mainly by the magnetization rotation.
Out-Of-Plane Exchange Coupling Between Epitaxial Ni(50 Å) And Nio(600 Å) Bilayers, Z.Y. Liu, Shireen Adenwalla
Out-Of-Plane Exchange Coupling Between Epitaxial Ni(50 Å) And Nio(600 Å) Bilayers, Z.Y. Liu, Shireen Adenwalla
Materials Research Science and Engineering Center: Faculty Publications
We have investigated the exchange coupling between an epitaxial Ni (50 Å) film with an out-of-plane magnetic easy axis and a NiO (600 Å) film by polar magneto-optic Kerr-effect measurements. The temperature dependences of exchange field HE for both as-deposited and field-cooled states exhibit the same blocking temperature TB~130 °C. The exchange field HE for the field-cooled state is lower than that for the as-deposited state. The hard-axis in-plane loop shows a much smaller value of HE. No coercivity enhancement is observed. The data are suggestive of linear coupling across the ferromagnet/antiferromagnet interface.
Bistable Memory Effect In Chromium Oxide Junctions, Andrei Sokolov, C.-S. Yang, E. Ovtchenkov, L. Yuan, Sy-Hwang Liou, Bernard Doudin
Bistable Memory Effect In Chromium Oxide Junctions, Andrei Sokolov, C.-S. Yang, E. Ovtchenkov, L. Yuan, Sy-Hwang Liou, Bernard Doudin
Materials Research Science and Engineering Center: Faculty Publications
Magnetotransport properties of granular CrO2 /Cr2O3 films made of CrO2 crystals covered by 1- 2 nm native insulating Cr2O3 are presented. Electrical properties of a limited number of grains measured in series and parallel (10 to 15 grains) reveal intergrain tunneling characteristics. At lowest temperatures, a well pronounced zero bias anomaly indicates that impurities in the junctions block the electronic flow.
Hysteresis in the IV curves are observed at intermediate temperatures on zero-field cooled samples. Changing the polarity of a short excitation pulse (100ns) of amplitude smaller than 1 V triggers a …
Substrate Dependent Bonding Of Chemisorbed 1,1’-Biphenyl-4,4’-Dimethanethiol, A. N. Caruso, R. Rajesekaran, Jody G. Redepenning, Yaroslav B. Losovyj, Peter A. Dowben
Substrate Dependent Bonding Of Chemisorbed 1,1’-Biphenyl-4,4’-Dimethanethiol, A. N. Caruso, R. Rajesekaran, Jody G. Redepenning, Yaroslav B. Losovyj, Peter A. Dowben
Materials Research Science and Engineering Center: Faculty Publications
We compare the adsorption of 1,1’-biphenyl-4,4’-dimethanethiol (BPDMT) on gold and cobalt surfaces. The molecular orbitals, identified from combined photoemission and inverse photoemission studies, exhibit shifts in binding energies with different deposition methods and substrates. These shifts indicate that this potential molecular dielectric exhibits stronger bonding to cobalt surfaces than gold surfaces.