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Top and bottom NiO -pinning spin valves, e.g. Ta/NiO/Co/Cu/Co/Ta and Ta/Co/Cu/Co/NiO/Ta multilayers, were investigated extensively. At the same thickness of the Cu layer, the GMR ratio for the bottom one is about 30% larger than that for the top one, which is unambiguously due to the roughness effect at the NiO/Co interface. The roughness of NiO/Co interface of the top NiO -pinning spin valve is much larger than that of the bottom one, which may be due to the deposition sequence. On the other hand, although the preferred orientation of the top NiO -pinning spin valve is more prominent than that of the bottom one, it seems not favorable to the specular reflection effect.

The magnetization of the magnetic multilayers with Fibonacci sequence was studied theoretically. By considering the Zeeman, magnetic anisotropy, bilinear and biquadratic exchange energies in the multiplayer system, the angles of the magnetization between all magnetic layers were calculated numerically by minimizing the magnetic energy. The magnetization curves and hysteresis loops were obtained. By considering the different strengths of biquadratic interaction between the magnetic layers some novel phenomena appears. The magnetization in magnetic sub-layers may rotates reversibly or irreversibly by magnetic field.

We have used an electron holography (EH) technique to directly probe the potential profile of tunnel barriers in magnetic tunnel junctions (MTJs). Barriers with under-, optimum-, or over-oxidized condition have been investigated. One important finding is that there is always slight oxidation of the top electrode because of film morphology. Sharp interfaces can be achieved in the bottom interface of optimally oxidized barrier or both interfaces in MTJs with under-oxidized barriers. We also demonstrate, theoretically and experimentally, how barrier shape affects the bias dependence and, in low barrier height case, result in inversed tunneling magnetoresistance (TMR) at high bias. The mechanism is very different from that responsible for inversed TMR in all biases. The finding leads to the possibilities of achieving better signals at high bias in real applications.

Permalloy (Py)/capping (FeMn, Ta) bilayers with interconnected network structure were deposited onto porous alumina templates with 5.0 nm thick Ta buffer layers. For the nanostructured Py/FeMn and Py/Ta bilayers the coercivity goes through a maximum value near the Py layer thickness of 17.5 nm. Despite exchange biasing, however, the coercivity of the former bilayers is smaller than that of the latter ones with the same Py layer thickness. The Ta capping layer thickness and the deposition rate of the Py layer are also found to have great influences on the coercivity of the Py layer. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

The FeCo/AlOx/Co magnetic tunnel junction was fabricated using a two-step plasma oxidation technique. The tunnelling magneto-resistance ratio of the junction as-deposited was about 8.5%, and reached 14.0% after annealing at 200°C. Transmission electron microscopy, high-resolution electron microscopy and electron holography were used to study the microstructure of the junction. The enhancement of the magneto-resistance effect after annealing could be attributed to the uniformity of the barrier layer, the improvement of FM/I interfaces, and the deoxidization of the bottom ferromagnetic electrode.

The plateaus in laser-assisted recombination and high-order harmonic generation are investigated in the frequency domain. It is found that the probability amplitude of finding an electron with a given energy is given by a generalized Bessel function, which can be represented as a coherent superposition of contributions from a few electronic quantum trajectories. This concept is illustrated by comparing the spectral density of the electron with the laser-assisted recombination spectrum. On the other hand, the plateau of high-order harmonic generation reflects the spectral density of the electron at the location of the nucleus after above-threshold ionization.

The interface structure and magnetoresistance (MR) for the single spin valve (SV) of Co/Cu/Co structure with a NiO layer at the top or under the bottom were investigated. Glancing incident x-ray reflectivity and x-ray diffuse scatter studies show that the interface roughness of NiO on Co is much larger than that of Co on NiO. The large roughness may be one of the main factors of suppressing spin reflectivity. The interface between NiO and Co becomes more flat after annealing. Different temperature dependence of MR was observed for the top and the bottom SV. We attribute the above phenomena to the competition between the roughness and the exchange effects of the NiO/Co interface.

Exchange biasing effect in Co/Co3O4(t) bilayers was carefully investigated by a vibrating sample magnetometer. It was surprising that a significant exchange bias effect appeared when the temperature varied from 90 to 220 K. In this temperature range bulk Co3O4 should be in a paramagnetic state because its Neel temperature is about 40 K. The exchange bias field (HE) of Co/Co3O4 fluctuated in the range of 110–130 Oe with various thickness of Co3O4 at 90 K and HE was zero when the temperature was above 220 K. X-ray photoelectron spectroscopy results indicated that a thin CoO layer formed near the interface, which may be due to the interdiffusion between Co and Co3O4 layers. Therefore, the exchange bias effect in Co/Co3O4 bilayers is thought to be dominated by the exchange coupling between the Co and CoO layers. Using the finite-size effect theory, we evaluated the average thickness of interfacial CoO layer to be 39 A. The fluctuation of HE in Co/Co3O4 is probably due to thickness fluctuations of the CoO. Ass...