Magnetic Properties Of Low-dimensional Nanostructured Systems

Preparation of new low-dimensional nanometer material with controllable structure and property is an important research direction in nanometer technology. Through controlling the morphology, crystal structure and the size of nanometer system to improve the performance and fabricate nanodevices with special functions is research hotspot for the nanoscale science. Low dimensional system of nanometer materials, in which magnetic configuration often show novel phenomena, are widely used in the new generation vertical magnetic recording and spintronic devices. With the device of miniaturization, the size of magnetic device unit becomes smaller and smaller, it is very significant to study magnetic properties of low-dimensional nanometer system. In this work, one dimensional nanostructured nanowire arrays and two dimensional nanostructured oxide thin films were proposed, and their structure and magnetism were investigated. The main contents are as follows:1. The structure and magnetism of one dimensional nanowire arraysNickel ferrite doped with manganese Ni₁-xMn_xFe₂O₄ (x = 0.0-0.75) nanowire arrays were synthesized using the sol–gel technique and with further oxidization. The Ms values of the samples increase initially with the addition of Mn up to 50 at.%, and then decrease sharply with further added of Mn content. The variational magnetization is related to the special microstructure of nanowire arrays and the position of metal cations in the crystal lattice. The saturation magnetization of NiFe₂O₄ nanowire arrays is lower than that of bulk ones. The decrease in particle size causes the dramatic increase of population of surface atoms, which leads to the noncollinear magnetic structure becoming predominant. The increase in the proportion of noncollinear structure leads to the decrease in saturation magnetization of the nanowire samples. But the magnetizations of doped samples are greater than that of block material.Co-doped ZnO nanowires were successfully prepared by post-annealing Co–Zn alloy nanowires electrodeposited in AAO templates. The influence from annealing temperature on coercivity and the remanent magnetization of samples were studied. The samples annealed at 500?C show stronger magnetic properties: Hc = 925Oe and squareness Mr/Ms = 69%. However, samples annealed at higher temperature show weak magnetization, which may be due to the partial of cobalt oxidation to cobalt oxides at higher annealing temperatures.2. The magnetic characterization for two dimensional diluted magnetic semiconductor thin films. In the present work,in studies of the small magnetization values and the presence of significant uncertainty in the background subtraction process in diluted magnetic semiconductor thin films, for glass, Si and sapphire, base on PPMS-6000, an improved magnetic correction method was proposed. The magnetic moment and coercivity of the samples were corrected by the improved method, and the maximum fitting error that due to the substrates was calculated. The influence on sample signal from nonlinearity near M-H curve zero was analyzed. The accuracy and rationale of the improved method are discussed and compared with the traditional method.Al doped ZnO thin films were prepared on a glass substrate using argon oxygen ratios of 6:1, 4:1, 2:1 and 1:1 by reactive magnetron co-sputtering. After deposition, the films were annealed in vacuum at 200℃, 300℃, 400℃and 500℃for 30 min. The raw data were corrected by the improved magnetic correction method. The ferromagnetism in Al-ZnO films was not observed at room temperature. The Al/ZnO/Al composite films were deposited on a glass substrate using magnetron sputtering. Then, the films were annealed in vacuum at 200℃and 500℃for 30 min, respectively. And then the films annealed in vacuum were annealed in air at the same temperature for 30 min. We found that the samples annealed in air showed room temperature ferromagnetism, which can be attributed to electron charge transfer taking place between Al and Zn, which lead to the change of their structure. The Fe doped TiO₂ film showed room temperature ferromagnetism, which may be attributed to the substitution of Fe ions into the Ti sites.3. The magnetic and electric properties for two dimensional BiFeO3 composite films.NiFe₂O₄-BiFeO3 thin films were prepared by sol-gel spin coating. X-ray diffraction analysis shows that the perovskite BiFeO3 and the spinel structured NiFe₂O₄ phases formed separately. The leakage current density (J) in the films decreases and the remnant polarizations (Mr) increases with the increasing NFO. We conjecture reasonably that magnetoelectric coupling existed in the composite films by calculating the magnetic moment of NFO. Comparing to BFO film, leakage current density (J) in 0.25NFO-0.75BFO film was found to be nearly two orders of magnitude lower, and the Mr and saturation magnetization (Ms) reached maximum, were 2.3μC/cm² and 70.2emu/cm³, respectively.BiFe1- xMnxO3 thin films were prepared by sol-gel spin coating. Above the as-deposited BFMO film, TiO₂ layer was deposited using magnetron sputtering, Then the TiO₂/BiFe1 - xMnxO3 composite film was formed. Not only does the TiO₂ layer suppress the formation of oxygen deficiencies, but as a Schottky barrier, it can improve the insulation of the BFMO films. Moreover, no impurity phase was observed in any of the films since the TiO₂ layers suppressed volatilization of Bi ions. As a result, the leakage current density was greatly reduced. Compared with BFMO films, the leakage current density of the TiO₂/BFMO thin films were found to be lower by nearly two orders of magnitude, and the remnant polarizations were increased by nearly ten times.Enhanced ferromagnetism was observed in the films with x = 0.10. The remanent magnetization Mr and coercive field Hc were 0.8emu/cm³ and 80Oe, respectively. The source of magnetism in composite films may be related to two sides. On the one hand, with an increasing Mn/Fe ratio, the spiral spin modulation of BFO was disrupted, which results in canting sublattice, and the films show weak ferromagnetism; on the other hand, the size of grains in BFO decreases with increasing Mn, then the magnetism in BFO enhanced.