Electronic Transport Properties Of New-type Co-base Nanocluster-assembled Thin Films

Magnetic granular film,as a class of functional materials,is arrays of magnetic nanoparticles embedded into a thin film matrix,which opens a new route for potential applications in electron device due to its rich fundamental phenomena and simple preparation method.Recently,the microstructure and electrical transport properties in magnetic granular film are also one of the hottest research areas in condensed matter physics.In this thesis,a series of Co nanocluster-assembled granular films,Co/Cu and Co/Al₂O₃ nanocomposite granular films,Co/CoO nanocluster-assembled granular films with different degree of oxidation and Co-Pd nanocluster-assembled films with different Pd atomic content were prepared by using a plasma-gas-condensation cluster deposition system,which was self-designed by our research group.The effect of size distribution,cluster size,thickness,annealing temperature,Cu volume fraction,Al₂O₃ volume fraction,degree of oxidation and Pd atomic percent on the structure,magnetic property and electrical transport properties were systematically studied.Following was the main achievements:?1?The new-type Co nanocluster-assembled granular film with uniform size was prepared by using a plasma-gas-condensation cluster deposition system.The longitudinal resistivity??xx?of new-type granular films firstly decreased and then increased with increasing temperature,which could be well explained by the theory of the superposition of thermally fluctuation-induced tunneling effect?FIT?and scattering process.Additionally ?_xy^A increased with the increase of temperature.The trend of?_xy^A change with ?xx could be divided into two distinct regions and the resistivity at Tmin?the temperature corresponding to the minimum value of ?xx?was the demarcation point of two regions.The scaling relation between ?_xy^A and ps in whole temperature range could be written as ?_xy^A ? pS3.6.?2?We systematically studied the effect of cluster size?d=4.5 nm,6.5 nm,9.5 nm and 14.7 nm?on electrical transport properties based on uniform size.For all the cases,the scaling relation between ?_xy^A and ?S could be written as ?_xy^A??Sy.All of scaling exponent y were larger than 2 and the value of y decreased with decreasing cluster size.The disorder at interface and sample increased with decreasing cluster size,which leaded to the enhancement of ?_xy^A and ?s.To further study the effect of disorder at surface and interface on electrical transport properties,the effect of annealing at low temperature on electrical transport properties in Co nanocluster-assembled granular films for d=4.5 nm and 700 nm were systematically studied.At such low annealing temperature,the cluster size was kept nearly unchanging.However,the disorder at surface and interface decreased with increasing temperature,meanwhile,the additional conduction paths would be irreversibly broke down,which leaded to the enhancement of ?_xy^A and ?0 The effect of thickness on electrical transport properties in Co nanocluster-assembled granular films for d=4.5 nm were systematically studied.The result indicated that the porosity factor and surface roughness of sample increased with increasing thickness,which would lead to the enhancement of surface scattering and interface scattering,and finally would increase the ?_xy^A and ps.All of scaling exponent y were larger than 2 and the value of y decreased with the decrease of thickness.?3?A series of Co/Cu nanocomposite granular films?magnetic metal/non-magnetic metal nanocomposite granular film?were prepared by using a plasma-gas-condensation cluster deposition system by in-situ compositing of Co cluster and Cu thin film.A small amount of Cu would effectively improve the disorder at surface of Co,which would increase the surface scattering and interface scattering and finally would lead to the increase of ?_xy^A and ?0.However,as the Cu further increased and formed new conductive path,the short-circuiting effect would result in the decrease of ?_xy^A and?0.The scaling relation between ?_xy^A and ?xx0 at 5 K could be divided into two parts:in the scattering dominated region?x<18.8%?,the scaling relation between ?xyA and?_xx0 agreed well with the universal character of the 1.6 scaling relation in bad metal region;However,in the short-circuiting effect dominated region?x>18.8%?,the scaling relation was deviated from the universal scaling theory.?4?As a contrast,the Co/Al₂O₃ nanocomposite granular films?magnetic metal/insulator nanocomposite granular film?were prepared by using a plasma-gas-condensation cluster deposition system.As a small amount of Al₂O₃ was introduced,the sample was located in metallic or transition region.In this case,the longitudinal resistivity was mainly originated from FIT and scattering process.Nevertheless,the barrier height gradually increased with further increase of Al₂O₃ volume fraction.Under the circumstances,the temperature-dependent longitudinal resistivity could be described by log?xx-T-1/2 at low temperature range,which was ascribed to hopping conductivity mechanism.In addition,a series of Co/CoO nanocomposite granular films with different thickness of oxidized layer were prepared by using a plasma-gas-condensation cluster deposition system by introducing oxygen into deposition chamber.In the range of RO2 ? 0.2 sccm,the sample was located in metallic or transition region.In this case,the longitudinal resistivity could be expanded by the conductive mechanism proposed in this paper.Nevertheless,in the range of RO2 ? 0.3 sccm,the sample was located in insulating region.In this case,the temperature-dependent longitudinal resistivity could be described by log?xx-T1/2 at low temperature range.As the hopping conductivity mechanism played a dominant role in the longitudinal transport,the anomalous Hall effect become smaller.?5?Anomalous Hall effect was closely related to the strength of spin-orbit coupling of material.Pd was chosen to study the effect the spin-orbit coupling strength on electrical transport properties because of its large spin-orbit coupling strength.A series of Co-Pd nanocluster-assembled granular films were prepared by co-sputtering with Pd and Co and the influence of Pd atomic percent on electrical transport properties was systematically studied.As the Pd atomic percent increased,the sign of ?_xy^A transited from positive to negative.For x=49%,the sign of ?_xy^A transited from negative to positive with increasing temperature.