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We report on the magnetic properties of CuO nanoparticles prepared by the sol-gel method. M (T) curves show a blocking temperature at [T.sub.B] = 135 K and 160 K for 13 nm and 17 nm size particles, respectively, in measuring field H = 100 Oe. The M-H data for both the samples show a reversal magnetization at 300 K, whereas below TB it exhibit symmetrical hysteresis loops with a coercive field of 200 Oe (155 Oe) and a remanence 0.306 emu/mole (0.171 emu/mole) for 13 nm (17 nm) at 5 K. Presence of superparamagnetism and net magnetic moments of CuO nanoparticles are attributed to the uncompensated [Cu.sup.2+] ions at the surface of the particles. Index Terms--Antiferromagnetism, blocking temperature, CuO nanoparticles, superparamagnetism.

The [Fe.sub.3][O.sub.4] powder was mixed with Cu-ferrite powder then sintered in an argon atmosphere at temperatures between 1000[degrees]C and 1250[degrees]C for 4 h. The Cu-ferrite content in the mixture powder was between 0 mol % and 30 mol %. The effects of Cu-ferrite content and sintering temperature on the magnetoresistance (MR) and microstructure of the sintered ferrite were investigated. It was found that the Cu ions were dispersed uniformly in all sintered samples by the analysis of Cu element mapping of the scanning electron microscopy. The maximum MR ratio of the sintered ferrite at room temperature is about 7.3 % when Cu-ferrite content was 7.28 mol% and sintering temperature was 1200[degrees]C. From the plot of log [rho] versus [T.sup.-1/2] curves and the measurement of the current versus voltage curves of the sintered samples, it can be known that the magnetoresistance mechanisms in these sintered samples are the contribution of spin-polarized tunneling effect. Index Terms--Cu-ferrite, magnetoresistance (MR), spin-dependent tunneling.

[Fe.sub.3][O.sub.4] powder was mixed with MgO powder and then sintered in an atmosphere of argon at various temperatures for 3 h to form Mg-doped [Fe.sub.3][O.sub.4] ferrites. The effects of the sintering temperature, microstructure, and Mg content on the magnetoresistance of sintered [Fe.sub.3][O.sub.4] ferrites were investigated. X-ray diffraction and chemical titration analysis of [Fe.sup2+] and [Fe.sup.3+] ions indicate that the nonstoichiometric phases of [Fe.sub.3][O.sub.4+x] and Mg[Fe.sub.2][O.sub.4-x] coexisted in the sintered sample. The temperature dependence of resistivity was examined. A plot of log p against [T.sub.-1/2] of the sintered samples exhibits a linear relationship, indicating that the dominant magnetoresistance effect is spin-dependent tunneling. The maximum magnetoresistance of the sintered sample is about 7.3% at room temperature at a magnetic field of 12 kOe. The Mg content of this sample is 0.93 at.%. Index Terms--Magnetoresistance, sintered [Fe.sub.3][O.sub.4] ferrites, spin-dependent tunneling.

The possibility of incoherent magnetization reversal process in annealed [Fe.sup.50][Co.sub.50]/Ag multilayer thin films has been studied. Small magnetoresistance was measured in as-deposited films; however, after annealing above 150 [degrees] C, the highly mobile Ag atoms form bridges between the Ag layers, and the magnetoresistance increases. For example, after annealing at 225 [degrees] C for 30 minutes, the TEM photographs show clearly the cross-section of a discontinuous multilayer structure, the coercivity is relatively small, and the magnetoresistance reaches its largest value. This is explained by the formation of elongated clusters of disk-like ferromagnetic FeCo particles, the exchange coupling between these particles is weak due to the presence of Ag layer between them. In this case, an incoherent magnetization reversal process is assumed to be dominant. For samples annealed above 250 [degrees] C, these disk-like ferromagnetic particles within a column grow larger due to diffusion. The exchange interaction increase due to the Ag atoms migrating to the bridges between the columns and decreasing the distance between disk-like ferromagnetic particles. The coercivity of the film then increases, and a coherent magnetization reversal process is assumed to become dominant. Index Terms - multilayer, magnetic, magnetoresistance.

Ba[Fe.sub.12-2x][Co.sub.x][Sn.sub.x][O.sub.19] (where x = 0 ~1.03) thin films were made by rf magnetron sputtering in Ar-[O.sub.2] mixed atmosphere. These films were then post-annealed in air between 850 [degrees] C and 1000 [degrees] C for 10 minutes, in order to get M-type phase. The contents of Co/Sn on the magnetic properties and thermal stability of the magnetic properties of the films were discussed.

The magneto-transport properties of the granular [Fe.sub.0.5][([Al.sub.20][O.sub.3]).sub.0.5] solids with various film thickness have been investigated. The temperature dependence of the resistivity shows that all samples follow a tunneling type equation [Rho](T) =[Rho]exp[([T.sub.0]/T).sup.1/[Alpha]] with [Rho] [approximately equal to] 0.0 [similar to] 1 [ohms]cm at 300K and [Alpha]=2. The results of magneto-resistance at 300K indicate that the value is about 4.5% for thick films. It increases slowly with decreasing film thickness. At about 150nm, a peak value of TMR is found, which is determined to be 5%. Then TMR drops sharply with further reducing film thickness. It becomes only 3% at t=20nm. At low temperatures, the TMR effect becomes larger and shows similar thickness dependence. However, the values of [T.sub.0] that relate to the geometry of the tunneling barrier show behavior of opposite thickness dependence. A small [T.sub.0] is associated with a large TMR effect. In this report, the correlation of the film microstructure and the TMR effect are discussed in details.

Ultrafine Gd particles in a Cu metallic matrix have been obtained by post-annealing of as-sputtered metastable [Gd.sub.x][Cu.sub.1-x] alloys at temperatures higher than 250 [degrees] C. The results of resistivity measurements show that the resistivities of as-sputtered metastable [Gd.sub.0.2][Cu.sub.0.8] and [Gd.sub.0.3][Cu.sub.0.7] alloys exhibit a resistivity minimum at low temperatures, an indication of the Kondo effect. In addition the temperature of the resistivity minimum rises with the increasing Gd content. Conversely, the resistivity of films after formation of Gd granules levels off at low temperatures. The magnetic results indicate that the metastable GdCu alloys are essentially a spin-glass at low temperature with the transition temperatures at around 3.5-5K. However, magnetization data at 80K shows that the as-sputtered samples contain some small Gd ferromagnetic clusters in the paramagnetic matrix. The magnetic particles of granular films begin to be thermally blocked at [T.sub.B] and become magnetically ordered below [T.sub.B]. Due to larger grain sizes, [T.sub.B] is much higher for x=0.3 than for 0.2. The variation of transport and magnetic properties for samples with different annealing processes are described in detail.

A fabrication process for the sputtered (Co, Zn)-doped [Gamma]-[Fe.sub.2][O.sub.3] Thin films with quite a short reduction time (10 minutes) at temperature between 350-370 [degrees] C and with oxidation time about 20 minutes at temperature between 310-330 [degrees] C has been established. It has been observed that, with a few percent of Co & Zn content, the Zn dopant has the effect of producing higher saturation magnetization and better thermal stability of coercivity, as well as better suppressing grain growth effect during the heat treatments when these films are compared to [Gamma]-[Fe.sub.2][O.sub.3] thin films doped with Co only.