| Transition metal dichalcogenides(TMD)materials have attracted attention since the early 1960s,.Transition metal disulfide has attracted the attention and research of many scientific researchers due to its simple structure and abundant physical properties in the past decades.It has important significance for basic research and has potential application value.After graphene was mechanically stripped by researchers in 2004,since low-dimensional materials exhibited distinctly different characteristics from bulk materials,they have attracted people to pay more attention.Therefore,the transition metal dichalcogenides,with inter-layer van der Waals forces coupling,have received more research from researchers.And among these materials,two-dimensional magnetic materials can be widely used in the field of spintronics,and magnetic mechanism and device research under two-dimensional limits have attracted many researchers to carry out a lot of research work.Therefore,the studies for the size dependences of magnetic materials have been considered intensively in experiments and theories,and the research enthusiasm continues to rise.In particular,recent realization of the creative work on antiferromagnetic ordering in monolayer FePS3 and layer-dependent ferromagnetism in a van der Waals crystal monolayer CrI3 and bilayer Cr2Ge2Te6 has further stimulated the enthusiasm of the study.In this paper,we measured the transport properties and magnetic properties of several transition metal compounds(Mn1/3TaS2,Nb3SiTe6,Ca3Ru2O7),and studied the size effect about them.At the same time,we prepared nano-sheet samples of transition metal oxide Ca3Ru2O7 and studied the size effect.This paper is divided into the following six chapters:1.IntroductionThis chapter mainly summarizes the work related to this article,including the structure of transition metal dichalcogenides,the nature of intercalated transition metal dichalcogenides,etc.,as well as the structure,magnetic properties,transport and phase diagrams of transition metal oxide Ca3Ru2O7.2.Experimental TechnologyIn this chapter we introduce some of the experimental methods which used to prepare and characterization samples used in this paper.For example,the samples were characterized by X-ray diffraction spectrum(XRD)and Energy Dispersive X-Ray Spectroscopy(EDS),and the nanosheet samples were prepared by scanning electron microscope(SEM),EBL and FIB technologies.The transport properties and magnetic properties of the samples were tested by using PPMS and MPMS.3.Physical Properties of Transition Metal dichalcogenides Mn1/3TaS2In this chapter,we synthesized the Mn1/3TaS2 single crystal and measured its magnetic properties and electrical transport properties.We found that there is a significant magnetocrystalline anisotropy in Mn1/3TaS2,and the easy axis is not a specific axis,but is in the ab plane.In the magnetic measurement,We observed the field cooling(FC)and zero field cooling(ZFC)curve bifurcation phenomenon.Through the measurement of AC magnetic susceptibility and data fitting analysis of single crystal,it was found that Mn1/3TaS2 single crystal produced a cluster spin glass state at low temperature.The unsaturated behavior of the Hall data indicates the existence of a disordered state in the single crystal.In the magnetoresistance measurement,when the magnetic field is applied to the magnetic hard axis(c axis),there is a small positive magnetoresistance effect in the low temperature and low field.This is similar to the magnetic resistance in the MnSi nanowires.We speculate that this positive magnetoresistance behavior may be related to the phase change of the magnetic structure caused by the applied magnetic field.4.Size effect of transition metal dichaclogendis Mn1/3TaS2In this chapter,we prepared a series of nanoplates with different thicknesses by using mechanical stripping and EBL technology,and measured their electrical transport properties.We found that the Curie temperature Tc of nanoplates is related to thickness by comparing their data with bulk samples.The Curie temperature Tc decreases with decreasing thickness and disappears in the 8 nm nanosheets,which clearly shows that thinning of the thickness suppresses the generation of ferromagnetic order.Through the measurement of magnetoresistance,it is found that the magnetic anisotropy begins to weaken when the thickness is 20 nm,and disappears completely at 8 nm when compared with the bulk or thick nanosheets.The Hall data symbols for the 20 nm thick nanosheet samples were opposite to the Hall data symbols for the bulk samples and the 148 nm thick nanosheet samples when measuring Hall data for three different thickness samples.These thickness-varying transport results indicate that in this ferromagnetic Mn1/3TaS2 two-dimensional material,the thickness effectively controls its magnetic properties,and when the thickness is less than 20 nm,the ferromagnetic to paramagnetic transition occurs.5.Size effect of transition metal dichaclogenides Nb3SiTe6In this chapter,we prepared five samples of different thickness Nb3SiTe6 nanosheets and measured their electrical transport properties systematically.Based on the temperature dependence of the resistance,we find that the resistivity increases monotonously with the decrease of the thickness of nanosheets.And when the thickness of the nanosheet dropped to 6 nm,the resistance produced an upturn at a low temperature.In the measurement of magnetoresistance,we found that as the thickness of the nanosheet decreases,the value of the magnetoresistance in the nanosheet increases,and the carrier type in Nb3SiTe6 is a hole by measuring Hall data,and the carrier mobility increases with decreasing temperature.These results clearly show that the thickness change causes a change in the transport in Nb3SiTe6.6.Size Effect of Transition Metal Oxide Ca3Ru2O7 In this chapter,we prepared two thickness nanosheet samples(200 nm and 120 nm)and systematically measured their transport properties.By comparing the data with the bulk sample from the previous article,we found that when the applied magnetic field parallel to the ab plane,the magnetic transition magnetic field Hc in the nanosheets is significantly higher than in the bulk sample.This change in magnetization corresponds to the transition from the AFM-a state in the material to the forced FM state in the applied magnetic field.This indicates that the reduction in the thickness of the nanoplatelets makes it harder for the in-plane magnetic moments to be flipped by polarization in the applied magnetic field.In the measurement of magnetoresistance,we also found that the magnetoresistive linear uplift behavior of the bulk material and the 200 nm sample above the magnetic transition magnetic field disappeared when the thickness of the nanosheet decreased to 120 nm.The linear up-warping of this magnetoresistance is considered to be related to the degree of freedom of the t2g orbit,which means that as the thickness of the nanoplatelet is thinned,the t2g orbital freedom in the material is changed,resulting in a change in magnetostriction.The magnetic resistance no longer exhibits a linear increase.Through zero-field and field-added resistance measurements,we found that a characteristic peak appears at 10 K in the in-plane resistance curve of the nanosheet sample and this peak does not exist in a bulk material sample at zero field,and this characteristic peak near 10 K appears at the high field in the bulk sample,and its position does not move with the increase of the magnetic field.Combined with our data,we speculate that the characteristic peak around 10 K may be related to the degree of freedom of the lattice or the orbit,and the applied magnetic field changes the degree of freedom of the lattice or orbit.In the field RT measurement,the characteristic peak around 30 K gradually disappears as the magnetic field increases.Combined with our Hall data,we speculate that this characteristic peak may be related to the carrier type.All the results showed that the change in thickness caused a change in the properties of Ca3Ru2O7. |
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