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Magnetic nanowire arrays grown in anodized alumina templates offer a fertile ground to investigate the interplay among the exchange interaction, dipolar interaction, and magnetocrystalline anisotropy. In this study, we have successfully fabricated Co nanowire arrays with gradient diameter while maintaining the nanowire separation. Consequently, we are able to obtain nanowire arrays with dominant exchange interaction at one end with small nanowire dimeter and dominant dipolar interaction at the other with large nanowire diameter. These samples show very different magnetic properties from those observed in magnetic nanowire arrays with uniformly diameter. The easy magnetization axis, coercivity, saturation field, and reversal processes are all controlled collectively by both ends of the nanowires, which cannot be viewed as the combination of magnetic nanowire arrays with thin and thick diameters.

The magnetoresistance (MR){=[R(H) - R(0)]/R(0)} properties in ferromagnetic shape memory alloy of NiMnFeGa ribbons and single crystals, and NiFeGa ribbons have been investigated. It is found that the NiMnFeGa melt-spun ribbon exhibited GMR effect, arising from the spin-dependent scattering from magnetic inhomogeneities consisting of antiferromagnetically coupled Mn atoms in B2 structure. In the absence of these magnetic inhomogeneities, Heusler alloys seem to show a common linear MR behavior at around 0.8T(C), regardless of sample structures. This may be explained by the s-d model. At low temperatures, conventional AMR behaviors due to the spin-orbital coupling are observed. This is most likely due to the diminished MR from s-d model because of much less spin fluctuation, and is not associated with martensite phase. MR anomaly at intermediate field (rho(perpendicular to) > rho(parallel to)) is also observed in single crystal samples, which may be related to unique features of Heusler alloys. (C) 2011 Elsevier B.V. All rights reserved.

Pure and Co-doped single-phase CeO 2 crystals were synthesized by a solid-state reaction method. Samples of different oxygen vacancy concentration were studied, including (1) as-sintered crystals, (2) powders ground from the same crystal, and (3) a cold-pressed pellet from the ground powder that was unannealed and annealed at 800 °C. By analyzing the magnetic behaviors, surface/volume ratio and O vacancy concentration, the effects of oxygen vacancies on the room-temperature ferromagnetism (RT-FM) of Co-doped CeO 2 were systematically investigated. The results confirm that the RT-FM observed in Co-doped CeO 2 has a direct relationship with the oxygen vacancy concentration, and support the oxygen vacancy mediated FM mechanism.

Two series of Mn-doped Cu 2 O diluted-magnetic-semiconductor thin films were prepared by radio-frequency (RF) magnetron sputtering. One is prepared at different deposition temperature with the same Mn doping concentration; the other is deposited at the same temperature but with varying Mn concentration. They were used to find out the ferromagnetic-order zone for the Mn-doped Cu 2 O systems. Most of the samples show high (1 1 1) orientation, except low doping concentration ( 2 O lattice and there were about 1.5% cation vacancies. The grains shown in the transmission electron microscopy (TEM) images for all the samples were tiny, i.e. just 5 nm in diameter. A rough phase diagram for the ferromagnetic order existing in the Mn-doped Cu 2 O thin films was given with varying Mn doping concentration and deposition temperature.

The effects of conventional thermal annealing (CTA) and rapid recurrent thermal annealing (RRTA) on the magnetic properties of the SmCo-based films were investigated. The results show that the CTA-treated films exhibit amorphous structure with coercivity insensitively depending on annealing parameters as anticipated. As for the RRTA-treated films, single TbCu 7 -type phase structure with well crystallization is observed. The coercivity of the RRTA-treated films is strongly correlated with annealing parameters, and its optimum value is larger than that of the CTA-treated films. It is suggested that the RRTA treatment has beneficial effects on the magnetic properties of the SmCo-based films.

The effects of conventional thermal annealing (CTA), rapid thermal annealing (RTA) and rapid recurrent thermal annealing (RRTA) on coercivity of the melt-spun Sm(Co 0.6 Fe 0.27 Cu 0.1 Zr 0.03 ) 7.5 ribbons were systematically studied. The results show that the annealing parameters greatly affect the coercivity of the ribbons. The optimum coercivity is 9.8, 8.9 and 10.2 kOe for the CTA-treated, RTA-treated and RRTA-treated ribbons, respectively, indicating that the coercivity is not enhanced only by elevating the heating rate. Nevertheless, the coercivity increases to 15.1 kOe for the RRTA-treated ribbons when the cooling rate decreases to 1 °C/min.

Textured HCP Co nanowires with preferentially oriented (1 0 0) along the growth direction were fabricated by electrodeposition at high potential. Further increase of deposition potential results in the formation of twist wires with circumferential strain. These conclusions are unambiguously reached by combining TEM, XRD rocking curves and pole figures. The magnetic properties are determined by the combination of magnetocrystalline, shape, and stress anisotropies and magnetostatic interaction. Consequently, the magnetic easy axis can be tuned with structure and temperature, thus paving the road for magnetic nanowire array use in applications where self-biasing of magnetization is necessary.

Systematic studies on the transport property were carried out in FM (NiFe, Co, FeCo)/AlO x /NiFe magnetic tunnel junctions (MTJs) by oxidizing a wedge-shaped Al layer, resulting in over-, optimized, and under-oxidized barriers. An interesting resistance peak can be generally observed in the over-oxidized region, which was attributed to the significant change in oxidation level of the bottom electrode. This argument was supported by the barrier thickness and height fittings. Combining the studies of tunnel magnetoresistance (TMR) and the bias dependence as a function of the Al layer thickness, we can conclude that, in the MTJ exhibiting maximum TMR, the Al layer has not been fully oxidized to reach stoichiometric Al 2 O 3 , and the bottom electrode has already been slightly oxidized. A sizable TMR ratio can still be observed in under-oxidized region, even with very thick Al layer and short oxidation time, which implies that no pure metallic Al exists near the bottom ferromagnet/insulator (FM/I) interface. This is consistent with the grain boundary oxidation model for post-oxidizing the Al layer in MTJs.

In this work, we have theoretically investigated the high-frequency response of fully magnetized metallic magnetic films with magnetization perpendicular to the film surface. We have found that these magnetic films may exhibit left-handed characteristics in the frequency range between ω 0 and ω up when an incident right circularly polarized electromagnetic wave is parallel to the magnetization direction. The upper frequency limit ω up is inversely proportional to the ferromagnetic damping coefficient α . There also exists a critical value of α c , above which the left-handed properties disappear completely. The left-handed properties are manifested with a jump in transmission coefficient, which can be detected when the thickness of the film is less than hundreds of nanometers.

The exchange bias and crystalline texture of the multilayer structure (Ta/Al/seed/Fe 50 Mn 50 /Ni 81 Fe 19 /Al 2 O 3 /Ni 81 Fe 19 /Al/Ta with seed=Ni 81 Fe 19 or Ni 81 Fe 19 /Cu) has been characterized. Measurements indicate an abrupt decrease in exchange bias of the Ni 81 Fe 19 pinned layer for samples with very thin seed layers, and exchange bias as high as 325 Oe for thicker seed layers. Fluctuation of exchange bias with thickness was greatly reduced for the Ni 81 Fe 19 /Cu seed configuration. X-ray diffraction measurements demonstrate a correlation between exchange bias and strong (1 1 1) texture of FeMn. The results suggest a high sensitivity of Ni 81 Fe 19 roughness and texture on deposition conditions, and corroborate previous observations of roughness in ultrathin NiFe films.

Room temperature transport properties of two sets of CIP spin valve structures were studied. In addition to the conventional collinear four-probe technique, the dependence of the resistivity on the angle between the magnetizations of the free and pinned Py layers has been studied. We have shown that this approach allows one to separate the contributions of giant magnetoresistance, the anisotropic magnetoresistance and the nonmagnetic resistivity, which is not possible by using the conventional four-probe method.

We have investigated the AC transport properties of magnetic tunnel junctions (MTJ) in order to characterize interfacial properties. One such property is interfacial charge accumulation, which leads to a voltage drop in the electrodes of the MTJ and the measured capacitance differing from the geometrical capacitance. Through measurement of capacitance spectra, we have extracted an interfacial capacitance of 16 μF/cm 2 per interface and a screening length of 0.55 A for FeNi electrodes.