Magnetoresistance of drop-cast film of cobalt-substituted magnetite nanocrystals.

An oleic acid-coated Fe2.7Co0.3O4 nanocrystal (NC) self-assembled film was fabricated via drop casting of colloidal particles onto a three-terminal electrode/MgO substrate. The film exhibited a large coercivity (1620 Oe) and bifurcation of the zero-field-cooled and field-cooled magnetizations at 300 K. At 10 K, the film exhibited both a Coulomb blockade due to single electron charging as well as a magnetoresistance of ∼-80% due to spin-dependent electron tunneling. At 300 K, the film also showed a magnetoresistance of ∼-80% due to hopping of spin-polarized electrons. Enhanced magnetic coupling between adjacent NCs and the large coercivity resulted in a large spin-polarized current flow even at 300 K.

Large, negative magnetoresistance in an oleic acid-coated Fe(3)O(4) nanocrystal self-assembled film.

An oleic acid-coated Fe3O4 nanocrystal self-assembled film was fabricated via drop casting of colloidal particles on a SiO2/Si substrate. The film exhibited bifurcation of the zero-field-cooled and field-cooled magnetizations around 250 K. The nonlinear current-voltage (I-V) characteristics between the source and drain electrodes in both zero and non-zero magnetic fields (H) were observed above and below the bifurcation temperature. A large negative magnetoresistance (MR ≈ -60%) was achieved at 200 K and H = 1 T. Even at 295 K and 0.2 T, the negative MR (∼ -50%) persisted. A Fowler-Nordheim plot and power-law scaling of the I-V characteristics revealed that the current flows through two-dimensional (2D) percolated electron tunneling paths. The enlargement of MR can be attributed to spin-dependent electron tunneling between magnetically coupled Fe3O4 nanocrystals self-assembled in 2D ordered arrays.

Magnetic and magnetoelectric properties of self-assembled Fe2.5Mn0.5O4 nanocrystals.

We report magnetoresistance of -40%, corresponding to 80% spin polarization, at magnetic field of 0.5 T and 200 K for oleic acid-coated Fe(2.5)Mn(0.5)O(4) nanocrystals (FMO NCs) self-assembled on a SiO(2)/Si substrate by drop casting fabrication. The FMO NCs exhibited spin glass transition around 150 K and nonlinear current-voltage (I-V) characteristics. Fowler-Nordheim plot of the I-V characteristics indicated that electrons tunnel directly barriers between the FMO NCs. Transmission electron microscopy revealed that the FMO NCs are elongated hexagon in shape with size of ∼15 × 20 nm. The FMO NCs self-assembled in two-dimension hexagonal networks of collinear ferromagnetic moments. The [111] easy magnetization axis of each FMO NC was parallel to each other in the hexagonal arrays. Geometrically frustrated lattice of collinear ferromagnetic moments supports both a low and a high intergranular tunneling conductance for the self-assembled FMO NCs without and with magnetic fields, respectively.

Effects of hydrogen in working gas on valence States of oxygen in sputter-deposited indium tin oxide thin films.

X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy-elastic recoil detection analysis (RBS-ERDA) revealed that hydrogen in working gas for dc-plasma sputter deposition resided in indium tin oxide (ITO) films and generated the O(-) state seen as the suboxide-like O 1s peak in XPS. Growth of the suboxide-like O 1s peak was parallel with an increase of the resided hydrogen quantified by RBS-ERDA. The first-principles band structure calculation revealed that the electronic structure of In(2)O(3) crystal was realized typically for the most conductive as-deposited film grown in the gas containing hydrogen of 1%. The as-deposited film grown in the gas containing hydrogen of more than 1% exhibited rather high density but low mobility of carriers and showed the electronic structure above 4 eV originated from the O(-) state due to the resided hydrogen in addition to that of the most conducting one. Both well preserved In(2)O(3) band structure and proper concentration of the O(2-) vacancy are indispensable for achieving the highest conductivity; however, the O(-) state lowers efficiency of the carrier doping using the O(2-) vacancy in the lattice and increases density of the ionized scattering center for the carriers.

Magnetoresistance and microstructure of magnetite nanocrystals dispersed in indium-tin oxide thin films.

Epitaxial indium-tin oxide (ITO) thin films were fabricated on a yttria-stabilized zirconia (YSZ) substrate by pulsed-laser deposition using magnetite (Fe(3)O(4)) nanoparticle dispersed ITO powders as a target. Magnetoresistance of the film at a field of 1 T was 39% at 45 K, and it stayed at 3% above 225 K. The film demonstrated cooling hysteresis in the temperature dependence of direct-current magnetization. Transmission electron microscopy revealed that phase-separated Fe(3)O(4) nanocrystals with widths of approximately 40-150 nm and heights of approximately 10-25 nm precipitated and grew epitaxially on the substrate in the film. Both the Fe(3)O(4)(111) and ITO(001) planes were parallel to the YSZ(001) plane. The Fe(3)O(4)(11-2) and -(1-10) planes were parallel to the ITO(100) and -(010) planes, respectively, and the planes connected smoothly at the grain boundary. The contour map of the electron density for the Fe(3)O(4)(111) plane by the first-principles electronic structure computation was similar to that for the ITO(001) plane. The [111]-oriented Fe(3)O(4) nanocrystals played the role of spin aligner for charge carriers of the epitaxial ITO film.

Transmission electron microscopy and electron diffraction study of the short-range ordering structure of alpha-LiFeO2.

The basic structure of alpha-LiFeO2, lithium iron oxide, is a cubic NaCl-type structure with a lattice constant of 0.42 nm; some short-range ordering characterized by octahedral clusters exists. The local structure of the short-range ordering was investigated by transmission electron microscopy and electron diffraction. A new short-range ordering structure was found in local areas. The local structure has a cubic lattice with a doubled lattice constant. The occupation factors of cations on Wyckoff sites 4(a) and 4(b) are different from those on 24(d) sites, but the stoichiometric composition in cubic clusters is the same as the macroscopic composition. The number of pairs in which iron cations exist in nearest-neighbor sites and next nearest-neighbor sites is reduced in the structure. This means that a magnetic interaction between the iron cations is reduced by cation ordering even without spin ordering at room temperature.