Anisotropic magnetoresistance and spin-valve effect in all-metal mesoscopic spin-valve devices

We have fabricated all-metal lateral spin-valve devices consisting of two permalloy electrodes and an interconnecting aluminum strip. The micromagnetic behavior of the device has been imaged with a magnetic-force microscope in external magnetic fields at room temperature. During a single cooling cycle at temperatures between 2 and 120 K we have measured the anisotropic magnetoresistance of both electrodes and the magnetoresistance of the entire device. In the latter, we can clearly identify the contributions of the anisotropic magnetoresistance and the mesoscopic spin-valve effect.     

非线性响应对自旋阀磁电阻的影响

从非线性Kubo公式出发,考虑电子自旋相关体散射, 研究了自旋阀结构磁电阻效应.发现非线性响应在不同程度上影响巨磁阻效应.在零温附近,温度参数对磁电阻影响较小,而外加偏压对磁电阻的影响相对较大.     

Magnetic, resistive and magnetoresistive properties of melt spun CoCu alloys

_Co10Cu90${\mathrm{Co}}_{10}{\mathrm{Cu}}_{90}$ alloys were prepared by melt spinning in a twin roller device at 5 m/s and 23 m/s tangential wheel velocities. The low temperature (5–300 K) magnetic and electron transport properties were investigated in as cast alloys and in samples annealed for 1 h at 923 K. Magnetoresistance (MR) measurements were performed between 35 K and 295 K, with applied field up to 0.85 T. For each condition, M–H loops and M(T) curves were measured and analyzed to estimate the mean Co particle size and the blocking temperature distribution, respectively. In the as cast condition, particle sizes between 2.3 nm and 5.7 nm and mean blocking temperatures between 31 K and 247 K are found; these alloys, cooled at these relatively low rates, show appreciable room temperature MR values (0.5–1.5% for 0.85 T). A minimum resistance is found at about 30 K in alloys with compositions up to 15 at% Co.     

Morphology, magnetic, magnetoresistance and optical properties of Co-Ni-Mo alloys thin films

We present in this paper several results concerning the preparation by means of electrolysis and characterization of Co-Ni-Mo thin films. Co-Ni-Mo thin films with different molybdenum content in the range 0-25 at% Mo were prepared from a complex solution containing ions of Co, Ni and Mo, using galvanostatic control, on aluminum substrates. The effects of applied current density on the morphology, magnetic, magnetoresistance, and optical properties of the electrodeposited Co-Ni-Mo films were investigated. The applied current density significantly influenced the film composition and their magnetic properties. The increase of molybdenum content in Co-Ni films (up to 25 at% Mo) enhances the resistivity, but it reduces the magnetoresistance effect. We report the first observation of magnetoresistance as high as 8% in Co-Ni-Mo thin films.     

Cationic behavior and the related magnetic and magnetotransport properties of manganese ferrite thin films

Manganese ferrite (Mn_xFe_3−xO₄) thin films have been prepared by a sol–gel method. The samples (x≤1.25) are polycrystalline containing well-defined grains and maintain cubic spinel structure with increasing lattice constant with x. The substituting Mn ions were found to have multi-valence, +2 and +3, by X-ray photoelectron spectroscopy. The saturation magnetization of the Mn-substituted films measured by vibrating sample magnetometry was found to increase from that of Fe₃O₄ at low x and then gradually decreases as x increases further. Such magnetic behavior can be explained in terms of the dominance of Mn²⁺ species over Mn³⁺ at low x and the gradual decrease in the population ratio Mn²⁺/Mn³⁺ as x increases. The observed decrease in the coercivity with increasing x implies the increase in octahedral Mn²⁺ population. The Mn-substituted samples exhibit magnetoresistance (MR) effect the maximum intensity of which is gradually reduced with x from that of Fe₃O₄. All samples show increasing MR with increasing external field (H), while their magnetization curves start to saturate near H=2 kOe. Such increasing MR with H can be explained in terms of the tunneling of spin-polarized carriers across grain boundaries. The reduction in the MR intensity with x can be partly explained in terms of the decrease in spin-polarized carrier density associated with octahedral occupation of Mn²⁺ ions.     

Electrical and magnetic properties of RE-doped ZnO thin films (RE = Gd, Nd)

Due to the small magnetic moments observed for 3d transition metals in ZnO [M. Diaconu, H. Schmidt, H. Hochmuth, M. Lorenz, G. Benndorf, J. Lenzner, D. Spemann, A. Setzer, K.W. Nielsen, P. Esquinazi, M. Grundmann, Thin Solid Films 486 (2005) 117], there is still space for optimizing ZnO-based diluted magnetic semiconductors for spintronics applications. Motivated by the observation of magnetic moments as high as 4000μ_B/Gd atom in GaN:Gd [S. Dhar, O. Brandt, M. Ramsteiner, V.F. Sapega, K.H. Ploog, Phys. Rev. Lett. 96 (2005) 037205], we investigated ZnO films doped with 0.01, 0.1 or 1 at.% rare earth (RE) metals. The films, with thicknesses between 20 nm and 1 μm, have been grown by pulsed laser deposition on a-plane sapphire or fused silica substrates.The homogenous incorporation of the RE ions in ZnO was investigated by combined Rutherford backscattering and particle induced X-ray emission measurements. Hall measurements revealed an unexpected dependence of the electron concentration on film thickness, proving a non-uniform distribution of electrically active defects. Magnetotransport measurements at different temperatures were performed to study the magnetoresistance and the presence of the anomalous Hall effect. Large negative magnetoresistance was obtained at 5 K, while no anomalous Hall effect was observed. These results indicate that there are no exchange interactions between the RE ions.     

Optimization of spin-valve parameters for magnetic bead trapping and manipulation

Magneto-optic Kerr effect (MOKE) and magnetoresistance (MR) measurements were used to measure the switching characteristics of spin-valve (SV) arrays currently being developed to trap and release superparamagnetic beads within a fluid medium. The effect of SV size on switching observed by MOKE showed that a 1 μm×8 μm SV element was found to have optimal switching characteristics. MR measurements on a single 1 μm×8 μm SV switched with either an external applied magnetic field or a local magnetic field generated by an integrated write wire (current density ranging from 10⁶ to 10⁷ A/cm²) confirmed the MOKE findings. The 1 μm×8 μm SV low field switching was observed to be +8 and −2 mT with two stable states at zero field; the high field switching was observed to be −18 mT. The low switching fields and the large magnetic moment of the SV trap along with our observation of minimal magnetostatic effects for dense arrays are necessary design characteristics for high-force, “switchable-magnet,” microfluidic bead trap applications.     

Structural characterization and magnetoresistance of manganates thin films and Fe-doped manganates thin films

Perovskites thin films with the composition La_0.6Ca_0.4MnO₃ doped with 20% Fe, were prepared by pulsed reactive crossed beam laser ablation, where a synchronized reaction gas pulse interacts with the ablation plume. The films were grown on various substrates and the highest colossal magnetoresistance ratio (CMR) was detected by Hall measurements for films grown on LaAlO₃ (1 0 0), which was selected as substrate for further investigations.Several growth parameters, such as substrate temperature and target to substrate distance were varied to analyze their influence on the film properties.The structure of the deposited thin films was characterized by X-ray diffraction and atomic force microscope, while Rutherford backscattering (RBS) was used to determine the film stoichiometry. The electrical properties were determined by Hall effect measurements in a magnetic field of 0.51 T.These measurements reveal that the amplitude of the CMR ratio depends strongly on the substrate and that the oxygen content influences the temperature where the transition from semiconductor to metal is observed.     

Effects of post-annealing on structural, optical and electrical properties of Al-doped ZnO thin films

Aluminum-doped zinc oxide (AZO) films were deposited at 400 °C by radio-frequency magnetron sputtering using a compound AZO target. The effects of annealing atmospheres as well as hydrogen annealing temperatures on the structural, optical and electrical properties of the AZO films were investigated. It was found that the electrical resistivity varied depending on the atmospheres while annealing in air, nitrogen and hydrogen at 300 °C, respectively. Comparing with that for the un-annealed films, the resistivity of the films annealed in hydrogen decreased from 9.8 × 10⁻⁴ Ω cm to 3.5 × 10⁻⁴ Ω cm, while that of the films annealed in air and nitrogen increased. The variations in electrical properties are ascribed to both the changes in the concentration of oxygen vacancies and adsorbed oxygen at the grain boundaries. These results were clarified by the comparatively XPS analyzing about the states of oxygen on the surface of the AZO films. There was great increase in electrical resistivity due to the damage of the surfaces, when AZO films were annealed in hydrogen with a temperature higher than 500 °C, but high average optical transmittance of 80-90% in the range of 390-1100 nm were still obtained.     

Structure and properties of ZnSe nanocomposite thin films

The structure and electrophysical and optical properties of semiconductor ZnSe nanocomposite thin films are studied. These films are obtained by discrete thermal evaporation in an ultrahigh vacuum. ZnSe films are synthesized in various structural states in the condensation temperature range 2–200°C. The optical spectra of these films are studied in the visible region.     

Structure and physical-mechanical properties of nanostructured thin films

The structure and mechanical properties of nanostructured thin films based on carbides, nitrides, and borides of transition metals are described. The mechanisms of localized deformation of the films during indentation are compared. It is shown that the tendency of a material to form shear bands during deformation can be predicted using the parameter H³/E², which describes the resistance of the material to plastic deformation. The columnar structure of the films is found to play an important role during deformation, which proceeds via slipping of columnar structural elements along the direction of an applied load.     

Structure and photoluminescence properties of ZnS films grown on porous Si substrates

ZnS films were deposited on porous silicon (PS) substrates with different porosities. With the increase of PS substrate porosity, the XRD diffraction peak intensity decreases and the surface morphology of the ZnS films becomes rougher. Voids appear in the films, due to the increased roughness of PS structure. The photoluminescence (PL) spectra of the samples before and after deposition of ZnS were measured to study the effect of substrate porosity on the luminescence properties of ZnS/PS composites. As-prepared PS substrates emit strong red light. The red PL peak of PS after deposition of ZnS shows an obvious blueshift. As PS substrate porosity increases, the trend of blueshift increases. A green emission at about 550 nm was also observed when the porosity of PS increased, which is ascribed to the defect-center luminescence of ZnS. The effect of annealing time on the structural and luminescence properties of ZnS/PS composites were also studied. With the increase of annealing time, the XRD diffraction peak intensity and the self-activated luminescence intensity of ZnS increase, and, the surface morphology of the ZnS films becomes smooth and compact. However, the red emission intensity of PS decreases, which was associated with a redshift. White light emission was obtained by combining the luminescence of ZnS with the luminescence of PS.     

Structure and magneto-optical properties of fine-grain garnet thin films

The structure and magneto-optical properties of fine-grain garnet thin films crystallized by the rapid recurrent thermal annealing (RRTA) method have been studied. The RRAT method has been used to crystallize BiGaDyIG garnet single-layer or BiGaDyIG/AI double-layer films and to get nanometer grain size (about 30–50 nm), which results in a large Faraday rotation angle, a smoother surface and fewer voids in the films. Meanwhile we have discovered that the Faraday rotation angle increases with the number of recurrences during the rapid annealing and quenching. With the more recurrent annealing one can not only get a strong Faraday effect, but it suppresses the appearance of DyFeO₃ phase in garnet films, which has been explained very well. By applying the new method, the as-deposited films have been succesfully crystallized to the (BiDy)₃(FeGa)₅O₁₂ garnet phase. They exhibited excellent magneto-optical properties with a coercivity of about 1500 Oe and effective Faraday rotation angle of 1.5°. The composition, magnetic and magneto-optical properties of the crystallized garnet films have been examined.     

Microstructural evolution of tungsten oxide thin films

Tungsten oxide thin films are of great interest due to their promising applications in various optoelectronic thin film devices. We have investigated the microstructural evolution of tungsten oxide thin films grown by DC magnetron sputtering on silicon substrate. The structural characterization and surface morphology were carried out using X-ray diffraction and Scanning Electron Microscopy (SEM). The as deposited films were amorphous, where as, the films annealed above 400 °C were crystalline. In order to explain the microstructural changes due to annealing, we have proposed a “instability wheel” model for the evolution of the microstructure. This model explains the transformation of mater into various geometries within them selves, followed by external perturbation.     

Effects of rapid thermal annealing on structural, magnetic and optical properties of Ni-doped ZnO thin films

XPS depth profiles were used to investigate the effects of rapid thermal annealing under varying conditions on the structural, magnetic and optical properties of Ni-doped ZnO thin films. Oxidization of metallic Ni from its metallic state to two-valence oxidation state occurred in the film annealed in air at 600 °C, while reduction of Ni²⁺ from its two-valence oxidation state to metallic state occurred in the film annealed in Ar at 600 and 800 °C. In addition, there appeared to be significant diffusion of Ni from the bottom to the top surface of the film during annealing in Ar at 800 °C. Both as-deposited and annealed thin films displayed obvious room temperature ferromagnetism (RTFM) which was from metallic Ni, Ni²⁺ or both with two distinct mechanisms. Furthermore, a significant improvement in saturation magnetization (M_s) in the films was observed after annealing in air (M_s = 0.036 μ_B/Ni) or Ar (M_s = 0.033 μ_B/Ni) at 600 °C compared to that in as-deposited film (M_s = 0.017 μ_B/Ni). An even higher M_s value was observed in the film annealed in Ar at 800 °C (M_s = 0.055 μ_B/Ni) compared to that at 600 °C mainly due to the diffusion of Ni. The ultraviolet emission of the Ni-doped ZnO thin film was restored during annealing in Ar at 800 °C, which was also attributed to the diffusion of Ni.     

Structure and electrical properties of thin films of copper selenide

Electron-diffraction and electron-microscope methods were used to investigate the structure of Cu₂Se films of close to stoichiometric composition. It is shown that in polycrystalline and single-crystal films of thickness >400Å at room temperature, the tetragonal modification is stable, which at temperatures above 400°K is transformed into the cubical modification. In thinner films d<400 Å the cubical modification of copper selenide is stable at room temperature. A sharp peak is observed at 400°K on the temperature dependence of the resistance; this is connected with the phase transition. At room temperature, copper selenide is a degenerate p-type semiconductor with carrier concentration 5 · 10²²–8 · 10²⁰ cm^–3, depending on the thickness of the film.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 90–94, August, 1973.     

Composition influence on the microstructures and magnetic properties of FePt thin films

The Fex Pt 100x (10nm) (x=31-51) thin films are fabricated on Si (100) substrates by using magnetron sputtering. The highly ordered L1 0 FePt phase is obtained after post-annealing at 700℃in Fe 47 Pt 53 thin film. The sample shows good perpendicular anisotropy with a square loop and a linear loop in the out-of-plane and the in-plane direction, respectively. The variations of the magnetic domains are investigated in the films when the content value of Fe changes from 31% to 51%.     

Annealing effects on electrical and optical properties of ZnO thin films synthesized by the electrochemical method

Microstructural and electrical properties of potentiostatically electrodeposited ZnO thin films from an aqueous bath were investigated after annealing at different temperatures in Ar and 5% H₂/Ar atmospheres. It is confirmed that the bandgap energy of ZnO thin films decreased with annealing from 3.42 to 3.27-3.29 eV by calculating the wavelength of the absorption region. The annealing at temperatures as low as 200 °C decreased the sheet resistance of ZnO thin films because of the extinction of Zn(OH)₂ in the atmosphere. In addition, the sheet resistance of ZnO thin films decreased by annealing in a 5% H₂ atmosphere, which caused an increase of carrier concentration by hydrogen reduction.     

Magnetic and magnetoresistive properties of Fe10Ni90 films

We study the effect of the substrate temperature and the temperature and conditions of thermomagnetic treatment on the magnetic and electrical properties of Fe10Ni90 films deposited on single-crystal silicon substrates by ion-beam sputtering. The laws found to govern these properties are associated with the transformations of the microstructure of the films. The maximum magnetoresistance ratio is shown to occur in films deposited on slightly heated substrates and vacuum-annealed at about 500C. The coercive force, anisotropy field in the plane of the films, the density of bonds at domain walls, and the magnetoresistance ratio are determined as functions of the thickness.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 9–13, January, 1991.We thank E. I. Teitel and N. N. Schchegoleva for measuring the microstructure parameters of the films.