Papers by Keyword: Magnetoresistivity MR

Abstract: Mechanical alloying (MA), super cooling process, etc. have been used to prepare amorphous phases, metastable solid solutions, nanocrystals, and so on. It is important to consolidate these powders for evaluating the physical properties, and for applications. On the other hand, shock compression can be used as an effective consolidation method for metastable material powders without recrystallization or decomposition. We had prepared metastable transition-metal system bulk alloys and compounds (Fe-Co, Fe-Cu, Fe-W, Co-Cu, Sm-Fe-N systems, etc) by using MA and shock compression. The Fe-Cu and Co-Cu metastable solid solutions showed a fit curve to the Slater-Pauling one. The Co-Cu metastable solid solution bulk alloy showed a magneto-resistance. The Fe-Co fine-grained bulk alloys show the higher coeicivity than that of molten alloy. In this paper, the preparation and magnetic properties of the metastable alloys (Fe-Co, Fe-Cu, Co-Cu systems) are reviewed, and the applications to materials science and engineering are discussed.

Abstract: Structural and magnetic properties have been investigated for the series of layered perovskite compounds LnBaCo2O5+x (Ln – rare earth metal) with different oxygen contents and thermal treatment history. Transport properties were studied at applied magnetic fields on TbBaCo2O5.5 single crystal grown by Travel Floating Zone Method (TSFZ). Oxygen isotope effect on metal-insulator transition was investigated for compounds with different rare earth cations.

Abstract: We report magnetoresistance in ferromagnet / superconductor / ferromagnet structures made of La0.7Ca0.3MnO3 and YBa2Cu3O7 in the current in plane (CIP) geometry when the temperature is fixed along the superconducting transition and the magnetic field is swept in an hysteresis loop sequence. We describe experiments changing the geometry of current versus field. We find that the shape and height of the magnetoresistance peaks is not modified no matter the field is directed parallel or perpendicular to the current. This excludes interpretations in terms of spontaneous vortices or anisotropic magnetoresistance of the ferromagnetic layers and supports the view that the magnetoresistance phenomenon originates at the spin dependent transport of quasiparticles transmitted from the ferromagnetic electrodes into the superconductor.

Abstract: We investigated the structure, magnetic and magnetoresistive properties of antiferromagnetically coupled [Fe(85Å)/Cr(tCr)]12 superlattices with the Cr layers thickness tCr = 12.4 and 13.6 Å, grown simultaneously on (100)MgO and (211)MgO substrates. It is shown that the (211)MgO substrate is appropriate for the growth of (210)Fe/Cr multilayers with a strong uniaxial in-plane anisotropy. The stepwise behavior of magnetization and magnetoresistance is revealed in the case when the magnetic field is applied along the easy axis in a film plane of (211)MgO/[(210)Fe/Cr]12 superlattices. The steps on M(H) and ΔR(H)/R dependences are caused by the flip of the magnetic moments of individual Fe layers. The qualitative information about the sequence of spin-flip transitions is extracted from the comparative analysis of magnetization and magnetoresistance data.

Abstract: Iron nitrides thermally decompose to α-Fe releasing their nitrogen above 300°C. MR effect was found out in the thin films obtained by post-annealing of the following two kinds of sputter deposited iron nitride related films. (1) α-Fe particles dispersed in AlN granular film was obtained by an annealing of Al0.31Fe0.69N sputter deposited film in hydrogen. The MR=0.82% was found out in this nitride system. (2) Fe3O4 thin films were prepared by thermal decomposition of sputter deposited iron nitride films in low oxygen partial pressure. The iron nitrides were defect rock salt type γ΄˝-FeNx (0.5≤x≤0.7) and zinc blende type γ˝-FeNy (0.8≤y≤0.9) at the sputter nitrogen gas pressure of 1Pa and 6Pa. MR ratios of the oxide films were about 2%.

Abstract: We report on the experimental observation of large magnetoresistance in Alq3 organic light-emitting diodes (OLEDs). Very similar magnetic field effects (MFEs) of comparable magnitude are also observed in electroluminescence and photocurrent measurements. We also report on the frequency response of the magnetoresistance effect at frequencies below 100 kHz. To the best of our knowledge, the mechanism causing these MFE is currently not known.

Abstract: The new hybrid device consisting of patterned array of Co nanodots on GaMnAs channel was fabricated and demonstrated to give a new functionality to control the transport properties of GaMnAs. Magnetic state of array of Co nanodots was observed with magnetic force microscopy (MFM) observation. The magnetic state of individual Co dots can be controlled by using a MFM tip. Different distribution of magnetic state of Co nanodots in an array structure resulted in an inhomogeneous magnetic field. Magnetic field induced by the array of nanodots leads to change in the transport properties of GaMnAs. This inhomogeneous magnetic field was regarded to act as an effective potential that can trap the spin polarized carriers in the GaMnAs system with giant Zeeman splitting.

Abstract: The high sensitivity of the low temperature electrical properties of p-type pure tellurium (Te) to impurities, structural boundaries, point defects and dislocations allows to investigate the structural imperfection profiles in crystals grown under different conditions. Our interest was focused on studying the influence of grain boundaries on the electrical properties of the samples that were remelted and directionally solidified in space (µg) without a seed (W-µg), in comparison with the sample grown under the normal earth conditions (1g0) and a nanocluster sample obtained by filling with melted Te of dielectric opal matrix voids (Opal sample). The W-µg ingot of Te was prepared in the "Crystallizator" furnace under microgravity conditions aboard the "Mir" space station [1]. The concentration variation of electrically active defects and neutral defects along the samples were studied by galvanomagnetic methods (Hall effect and electrical resistivity) in a wide temperature range from 0.4 to 300 K. In these measurements, the following effects caused by the micro- and nano- crystalline structure were found: low hole mobility, high concentration of neutral defects, and anomalous positive magnetoresistance in low magnetic fields at low temperatures. Besides, the specific resistivity of the space sample was found to oscillate (up to 20%) along the length which can be correlated with the presence of a few contact points of the melt with the ampoule wall. This ingot was formed as a result of rapid homogeneous spontaneous solidification, accompanied by forming a micro-block structure. The appearance of the anomalous positive magnetoresistance was observed in the micro-block W- sample and the nanocluster Opal sample. It is a consequence of intensive hole scattering at the grain boundaries which leads to an increase of the intervalley transition probability and to a change of the spin sign of holes in a low symmetry Te crystal. According to the weak localization theory [2], the spin variation during the scattering results in a positive magnetoresistance of the sample in low magnetic fields, in contrast to bulk Te crystals.

Abstract: The layered manganese oxides LaSr2Mn2-xTxO7 (T=Fe, Cr and Ni) has been prepared. The effect of substitution of transition metal ions for Mn on magnetic and magtoresistance properties has been studied. The compound has a tetragonal symmetry and the lattice parameters a decreases with increasing the doped content but the parameter c increases with x. It indicates that the MnO6 octahedron increase the distortion with x content. With the x increase, their ferromagnetic property was depressed. The Neel temperature TN was depressed gradually from 138K to 89K, 95K and 85K at x=0.3 for T=Fe, Ni and Cr respectively. Their magnetoresistance MR is gradually enhanced for all doped system. It suggests that the introduce of disorder in B-site have an important effect on the magnetic exchange and magnetoresistance because of the weaken of the double-exchange (DE) interaction.

Abstract: Magnetoresistive switch effect (MRS effect) devices containing two gold (Au) electrodes with a gap less than 2 µm were successfully fabricated on semi-insulting GaAs substrates by wet etching method. Huge MRS effect was observed. Magnetoresistance (MR) ratio reached 1,000,000% under the magnetic filed of 1.5 T when the devices were operated just above the threshold voltage. The magnetic field sensitivity at small magnetic fields was significantly improved. MR ratio of more than 1000% was achieved at 0.03 T. A relative high MR ratio of 100,000% under the magnetic filed of 1.5 T was also achieved when the devices operating before the threshold voltage.