Study On The Formation Mechanism And Physical Properties Of A Magnetic Thin Film System Deposited On Liquid Substrates

The microstructures and physical properties of thin films depend closely on the nature of the substrates. Since the fact that the liquid surfaces can be used as thin film substrates was discovered in 1990's, the formation mechanism, microstructures and physical properties of non-magnetic metallic thin films on liquid substrates have been studied systematically. Compared with the situation of the films on solid substrates, the interaction between the films and the liquid surfaces in the tangent direction is very small and furthermore there exists no lattice match between them, therefore this nearly free sustained film system on liquid substrates may exhibit characteristic microstructures and therefore anomalous physical properties.A nearly free sustained new iron film system, deposited on liquid substrates (i.e. silicone oil surfaces) by thermal evaporation method, has been fabricated successfully and its growth mechanism, internal stress patterns as well as low temperature magnetic properties are studied systematically in this dissertation.It is found that the growth mechanism of the iron films approximately obeys the two-stage growth model, which is similar to that of the non-magnetic metallic thin film systems on liquid substrates. The first stage involves nucleation and growth of compact atomic clusters. In the subsequent stage, the clusters diffuse and rotate randomly on the liquid surface and then ramified aggregates are shaped. Continuous iron films finally form on the liquid substrates due to the growth and connection of these ramified aggregates. In our experiment, at a fixed substrate temperature and low deposition rate, the continuous iron films are almost transparent; for higher deposition rates, the iron films exhibit a normal metallic color.In the transparent continuous iron films, band-shaped ordered patterns with macroscopical length scale and with quasi-periodic characteristics are observed firstly. The bands are composed of a large member of parallel rectangular domains with different width w but nearly uniform length L. It was found that all the bands are formed after deposition but before the samples were removed from the vacuum chamber. As the time interval△t for the samples being kept in the vacuum increases, the bands start form the film edges and then extend towards the central regions of the samples.It is proved that the formation mechanism of the bands is not related to the magnetic interactions among the iron atoms and the bands are resulted from the spontaneous organization of local materials driven by the internal stress relief in the nearly free sustained film system. During the cooling process after deposition, there exist larger internal stresses in the iron film, since the thermal expansion coefficient of the liquid substrate is much larger than that of the iron film. When the local internal stress exceeds a critical value, the iron film will break into several parts. Then the broken pieces of the iron film can move freely on the liquid surface to relieve the internal stress, which is ascribed to the interaction between the iron film and the liquid surface in the tangent direction is very small. During this period, the collision, extrusion and alternate insertion of the different pieces of the iron film result into the bands with quasi-periodic characteristic.Both the general theory of plate bulking and the experimental results show that a series of different kinds of sinusoidal (or cosinusoidal) stress patterns with different amplitudes and frequencies may exist simultaneously in the transparent iron films deposited on liquid substrates and therefore the combination of these sinusoidal stress patterns may form domain-like stress patterns with characteristic frequency spectrum. As a consequence, the relief of the domain-like stress results into the bands. It is found that the spectral exponentβ= 2.07±0.06 is a characteristic power for the bands in the transparent iron films.In the second type of continuous iron film, which exhibits a normal metallic color, the band-shaped ordered patterns described above have not been found in our experiment. The X-ray diffraction (XRD) patterns and high-resolution transmission electron microscopy (HRTEM) measurement show that this iron film system exhibit polycrystalline structure and the average size of the crystal grains is in the range of 6—10 nm. In addition, the atom force microscopy (AFM) measurement shows that, for the thinner film, the roughness of the film surface is significant with respect to its film thickness. The temperature dependent coercivity Hc (T) of the iron film system with metallic color has a marked maximum peak around the critical temperature Tcrit= 10—15 K. In our experiment, for the thinner film, the value of the maximum peak is larger. It is shown that, for the temperature T> Tcrit, the coercivity behavior Hc(T) can be considered as an inhomogeneous blocking process of magnetic moment, mainly due to the non-uniform particle (or grain) size distribution. For the temperature T< Tcrit, however, the anomalous coercivity behavior is closely related to the appearance of spin-glass like state of disorder spins at low temperature.The further experimental results show that a spin-glass like phase may occur for the disorder spins in the iron film, which is originated from the competition between the magnetic interaction and surface magnetic anisotropy. The disorder spins may exist at the grain surfaces, grain boundaries and surface oxide layers of the iron films. In our experiment, the freezing temperature of the disorder spins is Tf= 30—50 K. The exchange coupling between the freezing spins and reversible spins may cause exchange bias HE, and a maximum peak around the critical temperature rcrit= 4 K can also be observed from the temperature dependence of HE(T). In addition, the behaviors of the initial magnetization curve and thermoremanence are also greatly affected by the low temperature spin-glass like state.This dissertation is organized as following:In chapterⅠ, a brief review about the studies on the growth mechanism and the internal stresses of thin film systems on solid substrates and the research progress on magnetic thin films (or magnetic materials) is given. Then, we detail the research on the formation mechanism, microstructures and physical properties of non-magnetic metallic thin films deposited on liquid substrates.In chapterⅡ, the formation mechanism of the new iron film system deposited on liquid substrates is studied systematically.In chapterⅢ, the band-shaped ordered patterns and its formation mechanism in the transparent iron films are studied and therefore the distribution of the internal stresses of the iron film system is revealed. In chapterⅣ, we research the characteristic microstructures and anomalous low temperature magnetic properties of the iron film system with metallic color in detail.