Magnetoresistance Effect Of Graphene-based Materials

Graphene has many excellent physical and chemical properties such as high carrier mobility,high mechanical strength and good chemical stability.Thanks to these excellent features,graphene are showing great potential applications in many fields.At the same time,the spin-orbit coupling and superfine interaction of graphene are weak,which makes the spin scattering weaker,and thus graphene is considered as one of the best materials to transmit spin polarization.Recent theoretical studies also predicted that in the interface between a few layers of graphene and nickel,cobalt and other ferromagnetic metals' close packing surface,there is the perfect spin filtering effect,which could improve the property of spin valve device,thus the enthusiasm for the vertical structure spin valve device based on graphene(ferromagnetic(FM)/graphene/ferromagnetic(FM))has been evoked.Now using monolayer or less layers of graphene as an intermediary or using ferromagnetic electrodes passivated with graphene,vertical structure spin valve device was prepared successfully,and the large giant magnetoresistance effect has been observed.However,to realize its commercial application,it is still necessary to further improve the properties of such devices.On the other hand,in order to accelerate the graphene application of spintronics device in the future,graphene nano material and device also need to be explored,which possess more high magnetoresistance effect,and lower production cost and higher flexibility.Based on this,in this thesis we adopt graphene synthesized by chemical vapor deposition(CVD)as an intermediary to prepare sandwich-structure spin valve.Especially,the simple and low-cost synthesis methods were applied in the preparation of ferromagnetic metal-graphene nanocomposites,then the electrical transport,spin polarization transfer behavior and novel magnetoresistance effect were explored deeply.The main achievements are as follows:1.By controlling the gas flow rate and substrate annealing time and so on a series of CVD growth condition,low pressure chemical vapor deposition(CVD)method has been used to grow high quality double-layer graphene on the copper foil substrate,as well as grow a few layers of graphene on cobalt and iron nickel alloy thin film surface.Using rosin as the supporting layer of graphene,the graphene was transferred(to the target substrate)by wet method perfectly and neatly.We transfer the graphene on copper foil to LSMO ferromagnetic electrodes by wet transferring,to adjust the LSMO electrode-organic semiconductor(C60)spin polarized injection characteristics of the interface,preparing the vertical-structure fullerene(C60)organic spin valve,and considerable magnetoresistance effect has been observed at low temperature.2.Through the improved method,the high temperature pyrolysis of a Prussian blue precursor Fe3[Co(CN)6]2 was made,and the FeCo alloy-graphene nanocomposite was prepared.By means of scanning electron microscope,high-resolution transmission electron microscope and energy dispersive X-ray element analysis characterization methods,it has been clearly demonstrated the composite material was composed of by FeCo nanocrystalline particles packed by multilayer graphene,which has core-shell structure.Increasing the temperature of calcining and the annealing of the sample in hydrogen atmosphere can significantly reduce the defect density of graphene matrix in the sample and reduce the number of layers of graphene "shell".The samples of FeCo-graphene nanocomposites prepared under different conditions were measured by low temperature magnetic properties,electrical transport and magnetic transport.It is found that the sample resistance varies with temperature following the log(?)?1 T-1/4,indicating that carrier within graphene matrix transfer by means of hopping.By controlling the preparation conditions of this nanocomposites,the adjustment of its magnetic transport properties was successfully realized.FeCo-graphene nano-composites materials prepared under 1000C and annealed by hydrogen were found to have magnetoresistance effect at room temperature as high as-9.6%,and it is observed that the magnetoresistance increased with the rising of temperature.We believe that such high magnetoresistance can be attributed to the improvement of FeCo-graphene interface quality in this magnetic nanocomposite and the efficient hopping transmission of carrier in the graphene matrix.This work provides important clues to explore new spintronic materials with high performance and low cost.3.Ni-graphene nanocomposites were synthesized by calcining[Ni1.5(N3)(nic)2(Hnic)]n precursor at high temperature.Through integrated structure characterization of the samples,it is found that the material are made with the Ni nano grain wrapped by graphene constituting core-shell structure.Increasing calcination temperature can reduce the defects of the graphene matrix and the layer number.The electrical transport measurement shows that the resistance behavior of the sample has changed greatly around 10K.The magnetic properties measurement results show that the magnetization intensity and coercivity of the samples prepared at different temperatures were getting smaller with the increasement of temperature,which is significantly different from FeCo-graphene nanocomposites.In addition,it was found that the Ni-graphene nanocomposites exhibit weak magnetoresistance effect at low temperature,but the reversal of the magnetoresistance signal occurs near 10K,i.e.the magnetoresistance changes from positive to negative.The internal reasons need to be further explored.