Growth And Transport Properties Of Magnetic Topological Insuiator MnBi₂Te₄ With Superconductor Composite Structure

Topological insulators（TIs）are materials with conductive surfaces and insulate bulks.One of the most significant TIs is magnetic topological insulators（MTIs）,in which magnetic elements are introduce into 3D TIs,breaking the time reversal symmetry,and open Dirac energy gap.It is critical for its unusual topological properties,such as Weyl semimetals,axion insulators and Quantum anomalous Hall effects.MnBi₂Te₄ is the first intrinsic 3D antiferromagnetic（AFM）topological insulator.It was proposed in theory at first,and then realized experimentally in recent years.One of the most commonly methods for synthesizing MnBi₂Te₄ was to use a 1:5 mixture of Bi₂Te₃ and MnTe.The main issue of this method is that it takes a long time,and excess flux be removed by centrifugation.In the area of TI-ferromagnetic heterojunction,the main research is to insert Bi₂Te₃ with different“n”values between MnBi₂Te₄layers,formulating the MnBi₂Te₄（Bi₂Te₃）_n（n=0...6）Van der Waal heterostructure;there are already some theoretical calculations of MTIs heterostructures.MnBi₂Te₄ interlaminar antiferromagnetic coupling is susceptible to external influences such as temperature,magnetic field,pressure and van der Waal heterojunction,which is macroly manifested in the transformation of T_Nfrom AFM.According to those mention above,this thesis includes following three parts:The first part is to explore the growth process of MnBi₂Te₄ single crystal by self-fluxing method.We mix in stoichiometric ratio of Mn:Bi:Te=1:2:4.The melting temperatures of the target crystal MnBi₂Te₄ and Bi₂Te₃ are close,causing Bi₂Te₃ to be ubiquitous in compounds.Therefore,it is necessary to find out the most suitable temperature window for crystal growth（600 ^oC～585 ^oC）,and then extend the crystal growth time in this temperature range.Through continuous experimental exploration,a cooling rate of 0.5 ^oC/h was adopted within the 15 ^oC window,and the single crystal quality is better with cold water quenching after three days of annealing.The crystal growth process and he experimental operation has been simplified,most importantly,the time cost has been shortened greatly。In the second part,the preparation and transport properties of MBT/YBCO composite structure were studied.After the preparation of this composite with YBCO superconductor,following phenomena related to the electromagnetic coupling between layers is analyzed:Firstly,the antiferromagnetic transition temperature T_Nof MnBi₂Te₄ begins to move to low temperature under the influence of the YBCO.If the single crystal in MBT/YBCO composite structure MnBi₂Te₄becomes thinner,the trend of T_N moving towards low temperature will be more obvious.Secondly,when the magnetic moment of MnBi₂Te₄ is AFM-CAFM phase transition at H=3 T,a resistance peak appears at low temperature 13-17K（T₁）,under low magnetic field,this resistance peak also appears at lower temperature T₂≈12 K.When the thickness of MnBi₂Te₄ is thinner,both resistance peaks are enhanced,and the AFM transformation T_N is obviously weakened;when the thickness reaches 2.058μm,antiferromagnetic transformation cannot be seen in M-T.It indicates that YBCO superconducting signal and AFM signal are coupling.YBCO reduces the magnetic moment in MnBi₂Te₄and the applied magnetic field required for AFM-FM phase transformation;meanwhile,MnBi₂Te₄ exhibits coupling effect similar to antiferromagnetic transition.In the third part,MBT/Au/YBCO and MnBi₂Te₄/Fe Se_0.4Te_0.6 composite structures were studied,among them,T_N in MBT/Au/YBCO composite structure moves to low temperature,and Au coated YBCO film weakens the antiferromagnetic coupling between MnBi₂Te₄layers,but it is not as obvious as the direct effect of YBCO on MnBi₂Te_4.The MnBi₂Te₄/Fese_0.4Te_0.6 composite structure sample,possibly because T_C of Fese_0.4Te_0.6 is less than T_Nof MnBi₂Te₄,and is much smaller than the T_C of YBCO,So compared to the YBCO superconductor,Fe Se_0.4Te_0.6 has a weak inhibition effect on AFM transition of MnBi₂Te₄.