Size Effects On Transport Properties Of Cluster-assembled Films

Nanomaterials have attracted extensive attention due to their unique physical and chemical properties,and the related researches have greatly enriched the study content of modern science.The discovery of novel phenomena in nanosystems not only helps to understand the microscopic world and find new physical laws,but also greatly promotes the development of new nanodevices.As a new level of matter structure,the clusters provide a new way to explore new functional materials.At the same time,the ultra-sensitive size-dependent behavior of clusters also provides sufficient opportunities for finding and designing nanomaterials with special properties.The cluster-assembled films prepared by the low-energy cluster beam deposition（LECBD）technique can well preserve the original structure and properties of the clusters.More importantly,the very narrow cluster size distribution provides a great help for studying the size-dependent behavior of films and for finding characteristic sizes of sharp transitions in physical properties.Based on this technique,some cluster-assembled films were prepared through purposeful structure design and material selection,and the effect of cluster size on the electrical transport properties of films was systematically investigated.And the special attention is paid to finding the phenomenon of sudden change of physical properties at the characteristic size so as to improve and deal with some problems in the field of spintronics.At the same time,the ANSYS finite element simulation and the OOMMF micromagnetic simulation are used to verify and analyze the experiment and theory.The main contents are as follows:1.Cluster-assembled Fe/Fe₃O₄ core-shell nanostructured films were prepared by LECBD technique,and the degree of oxidation of core-shell clusters was effectively controlled by adjusting the flight distance of Fe clusters.A novel switchable metal-insulator transition（SMIT）behavior has been discovered in the range of characteristic core-occupation ratio.The feature of SMIT is that the film quickly switches from the metal state to the insulation state and then backs to the metal state,thus showing the rapid switching behavior of the sign of temperature coefficient of resistivity（TCR）.This characteristic is attributed to the fast synchronous switching behavior of current conduction channels between the Fe core and the Fe₃O₄ shell.The cluster-assembled films with characteristic core-occupation ratios not only retain the metallic properties of Fe,but also do not mask the Verwey transition of Fe₃O₄,thus realizing the synergistic effect at the nanoscale.In this thesis,the ANSYS finite element simulation is used to show the switching details of the current conduction channel between core and shell,and the effective medium theory is used to verify the core-occupation ratio range with SMIT characteristic.This study provides an idea and method for developing new SMIT characteristic.2.Early theories suggested that granular films with strong surface scattering effect have the anomalous Hall effect（AHE）with the opposite sign to their parent material,and the theoretical and applied research of the sign reversal of the AHE will be greatly promoted if this behavior can be observed.In view of this,we prepared a series of cluster-assembled Ni₈₀Fe₂₀ nanostructured films with different particle sizes by adjusting the deposition distance.Since both the high surface-to-volume ratio of the monodisperse clusters and the loose structure of the cluster-assembled films can greatly improve the surface effect,the AHE of the films shows the behavior of sign reversal below the characteristic size of 16.17 nm with the decrease of cluster size.The fitting results based on the scaling law confirm that the sign reversal is attributed to the switch of dominant position between the bulk and surface scattering mechanisms.And the accuracy of the characteristic size is further verified by the experimental results of magnetoresistance and magnetism.Meanwhile,the magnetic properties of the films were deeply explored using OOMMF micromagnetic simulations.This work provides an effective avenue for regulating the AHE by surface engineering.3.Magneto-sensitive sensors that can detect very small angle changes have a wide range of application prospects.And the anisotropic magnetoresistance（AMR）effect provides a new opportunity for the development of magneto-sensitive sensors due to its unique angle-sensitive properties,but the current progress is still not satisfactory.Therefore,cluster-assembled Ni₈₀Fe₂₀ films with characteristics of the confined space were prepared by LECBD technique.The experimental results show that the magnetoresistance（MR）curves of the films exhibit a significant jump behavior when the cluster size decreases below 9.08 nm.However,the original jump behavior disappears when the magnetic field direction deviates from the z-axis of the film by0.2°,and the reverse jump behavior appears when the deviation reaches 0.5°.More importantly,the switching of the jump trend is accompanied by significant resistance switching behavior.Therefore,Ni₈₀Fe₂₀ films assembled by extremely small-sized clusters have double induction to the deviation of the angle.Theoretical analysis reveals that the change of jump trend originates from the switching of the dominant effect between the classical anisotropic magnetoresistance and the domain wall magnetoresistance.Finally,the ANSYS finite element simulation and the OOMMF micromagnetic simulation are used to provide a good theoretical basis for the appearance and change of resistance jump behavior.This work provides an ideal candidate material for the development of next generation magneto-sensitive angle sensor.Clusters represent prototype systems for basic and applied research,so it is very meaningful to explore their new characteristics.The above studies have deepened the understanding of the electrical transport properties in cluster-assembled nanostructured films,which shows the potential and charm of clusters in various research directions,and can promote the study of basic theory and functional devices of cluster physics.