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30 Nov 1998

Volume 73, Issue 22, pp. 3181-3305

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Electrodeposition of nanoscale magnetic structures

D. Hofmann, W. Schindler, and J. Kirschner

Appl. Phys. Lett. 73, 3279 (1998); http://dx.doi.org/10.1063/1.122744 (3 pages) | Cited 27 times

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Magnetic Co clusters have been electrodeposited from an aqueous electrolyte onto Au surfaces in an electrochemical scanning tunneling microscope (STM). In a two-step electrochemical process, Co is first deposited onto a Au STM tip, then completely dissolved, and locally deposited onto the substrate underneath the STM tip due to local Co2+ oversaturation, which results in a laterally varying increase of the Co/Co2+ Nernst potential at the substrate surface. Mechanical tip–sample contacts or creation of substrate defects can be excluded. The structure size is of the order of the STM tip apex diameter, and is in detail determined by the substrate potential. © 1998 American Institute of Physics.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.05.Bx Metals, semimetals, and alloys
75.50.Cc Other ferromagnetic metals and alloys
61.46.-w Structure of nanoscale materials
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.50.Tt Fine-particle systems; nanocrystalline materials
81.15.Pq Electrodeposition, electroplating
82.45.-h Electrochemistry and electrophoresis
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Fabrication and properties of heteroepitaxial magnetite (Fe3O4) tunnel junctions

X. W. Li, A. Gupta, Gang Xiao, W. Qian, and V. P. Dravid

Appl. Phys. Lett. 73, 3282 (1998); http://dx.doi.org/10.1063/1.122745 (3 pages) | Cited 102 times

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Micron-size magnetic tunnel junctions consisting of ferromagnetic Fe3O4 electrodes, with MgO as a barrier layer, have been fabricated on (100) MgO substrates. Reflection high-energy electron diffraction and transmission electron microscopy studies reveal that the Fe3O4/MgO/Fe3O4 trilayers grown by pulsed laser deposition are heteroepitaxial with abrupt interfaces. To achieve different coercivities for the top and bottom Fe3O4 layers, the trilayers are grown on MgO substrates with a CoCr2O4 buffer layer. The junctions exhibit nonlinear current–voltage characteristics and changes in junction resistance with applied field corresponding to the coercivities of the two magnetic layers. However, the observed magnetoresistance (∼0.5% at 300 K, ∼1.5% at 150 K) is much lower than would be expected for a highly spin-polarized system. Possible reasons for the reduced magnetoresistance are discussed. © 1998 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
81.15.Fg Pulsed laser ablation deposition
73.40.Rw Metal-insulator-metal structures
75.50.Dd Nonmetallic ferromagnetic materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Influence of GaAs (001) surface termination on the in-plane magnetic anisotropies of MnSb epitaxial films

H. Akinaga, S. Miyanishi, W. Van Roy, J. De Boeck, and G. Borghs

Appl. Phys. Lett. 73, 3285 (1998); http://dx.doi.org/10.1063/1.122746 (3 pages) | Cited 11 times

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We have studied the in-plane magnetic anisotropy of epitaxial MnSb (1math01) films grown on GaAs (001) by molecular beam epitaxy. The MnSb films were grown on (2×4) and (4×6) reconstructed GaAs surfaces at 250 and 50 °C. At 250 °C, the films showed a strong twofold in-plane magnetic anisotropy independent of the GaAs surface reconstruction. In contrast, at 50 °C, the in-plane anisotropy appeared only on the (2×4) reconstructed surface. The anisotropic crystallographic domain structure of the MnSb films is thought to cause the magnetic anisotropy. The anisotropic domain formation is explained by the different chemisorption of the Mn adatom on the GaAs surface as a function of the termination. © 1998 American Institute of Physics.
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75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
75.70.Ak Magnetic properties of monolayers and thin films
75.70.Rf Surface magnetism
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.35.Rh Phase transitions and critical phenomena
75.30.Gw Magnetic anisotropy
75.60.Ch Domain walls and domain structure
75.70.Kw Domain structure (including magnetic bubbles and vortices)
68.03.Fg Evaporation and condensation of liquids
68.43.Mn Adsorption kinetics
68.43.-h Chemisorption/physisorption: adsorbates on surfaces

Large tunneling magnetoresistance enhancement by thermal anneal

R. C. Sousa, J. J. Sun, V. Soares, P. P. Freitas, A. Kling, M. F. da Silva, and J. C. Soares

Appl. Phys. Lett. 73, 3288 (1998); http://dx.doi.org/10.1063/1.122747 (3 pages) | Cited 110 times

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Spin tunnel junctions with tunneling magnetoresistance of 36.5%±0.5%, resistance-area product of 35±6 kΩ×μm2, and junction area between 6 and 75 μm2 were fabricated. The barrier height is 2.5±0.3 eV and the barrier thickness is 7.7±0.3 Å. Large tunneling magnetoresistance (TMR) values are obtained by vacuum anneal (at temperatures from 100 to 240 °C for over 5 h) of junctions prepared with as-deposited TMR of 21%±1.7%, and an as-deposited resistance-area product of 25±6 kΩ×μm2. Two regimes occur during anneal. The first one occurs for anneals up to 200 °C where TMR and junction resistance increase, but the barrier parameters are unaltered. The second occurs above 200 °C, where TMR increases faster, together with an increase in barrier height. At 240 °C, TMR starts to decrease. Rutherford backscattering analysis indicates an asymmetry in the oxygen distribution in the as-deposited barrier. The oxygen distribution becomes homogeneous for anneals above 150 °C. © 1998 American Institute of Physics.
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73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
73.40.Gk Tunneling
81.40.Gh Other heat and thermomechanical treatments

Giant magnetoresistance in a low-temperature GaAs/MnAs nanoscale ferromagnet hybrid structure

P. J. Wellmann, J. M. Garcia, J.-L. Feng, and P. M. Petroff

Appl. Phys. Lett. 73, 3291 (1998); http://dx.doi.org/10.1063/1.122748 (3 pages) | Cited 27 times

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We report the observation of a giant magnetoresistance effect in a low-temperature (LT-)GaAs/MnAs nanoscale ferromagnet hybrid structure. The MnAs nanomagnets are formed by ion implantation of Mn into LT GaAs and subsequent annealing. We have studied the magnetotransport using a vertically biased p+-GaAs/LT-GaAs:MnAs/p+-GaAs structure. A negative magnetoresistance (Δρ/ρ = [ρ(B)−ρ(0)]/ρ(0)) of up to −80% (B = 7 T) is observed at low temperatures (T<20 K), which changes its sign from negative to positive between T = 15 K and T = 20 K. The value of the positive magnetoresistance decreases with increasing temperature from +115% (20 K) to +1.4% (300 K). The magnetoresistance variations with B and T are correlated with the nanomagnet spacing in the structure. © 1998 American Institute of Physics.
Show PACS
75.47.De Giant magnetoresistance
75.50.Pp Magnetic semiconductors
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
75.50.Dd Nonmetallic ferromagnetic materials
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
72.20.My Galvanomagnetic and other magnetotransport effects
61.72.up Other materials
61.80.Jh Ion radiation effects
81.40.Gh Other heat and thermomechanical treatments
81.40.Rs Electrical and magnetic properties related to treatment conditions
75.50.Kj Amorphous and quasicrystalline magnetic materials
81.07.-b Nanoscale materials and structures: fabrication and characterization
75.50.Tt Fine-particle systems; nanocrystalline materials

Three-dimensional strain states and crystallographic domain structures of epitaxial colossal magnetoresistive La0.8Ca0.2MnO3 thin films

R. A. Rao, D. Lavric, T. K. Nath, C. B. Eom, L. Wu, and F. Tsui

Appl. Phys. Lett. 73, 3294 (1998); http://dx.doi.org/10.1063/1.122749 (3 pages) | Cited 133 times

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The evolution of three-dimensional strain states and crystallographic domain structures of epitaxial colossal magnetoresistive La0.8Ca0.2MnO3 films have been studied as a function of film thickness and lattice mismatch with two types of (001) substrates, SrTiO3 and LaAlO3. In-plane and out-of-plane lattice parameters and strain states of the films were measured directly using normal and grazing incidence x-ray diffraction techniques. The unit cell volume of the films is not conserved, and it exhibits a substrate-dependent variation with film thickness. Films grown on SrTiO3 substrates with thickness up to ∼250 Å are strained coherently with a pure (001)T orientation normal to the surface. In contrast, films as thin as 100 Å grown on LaAlO3 show partial relaxation with a (110)T texture. While thinner films have smoother surfaces and higher crystalline quality, strain relaxation in thicker films leads to mixed (001)T and (110)T textures, mosaic spread, and surface roughening. The magnetic and electrical transport properties, particularly Curie and peak resistivity temperatures, also show systematic variations with respect to film thickness. © 1998 American Institute of Physics.
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68.55.-a Thin film structure and morphology
75.47.De Giant magnetoresistance
73.61.At Metal and metallic alloys
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
72.15.Gd Galvanomagnetic and other magnetotransport effects
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
61.66.Fn Inorganic compounds
68.35.B- Structure of clean surfaces (and surface reconstruction)
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
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