Crystal Growth Of MnBi₂Te₄ And Study On Electric Transport Properties Of Composite Structures MnBi₂Te₄/La_0.7Sr_0.3MnO₃

MnBi₂Te₄ is the first intrinsic antiferromagnetic topological insulator（AFM TI）,which is coupled by van der Waals force and is a material of intra-layer ferromagnetic and interlayer antiferromagnetic exchange interaction.MnBi₂Te₄ is predicted to achieve the quantum Anomalous Hall effect（QAHE）at higher temperatures.In recent years,the study of topological properties of MnBi₂Te₄ crystals has become a hot research direction.The first step to study its special properties is to grow crystals.The growth temperature range and change rate need to be precisely controlled.However,in a large number of studies,the growth process of MnBi₂Te₄ crystals takes a long time.The ratio of Mn:Bi in the initial material is an important factor affecting the product.By applying a high magnetic field,the antiferromagnetic（AFM）state of MnBi₂Te₄ can be completely transformed into ferromagnetic（FM）state,and the combination of magnetic material with MnBi₂Te₄ may also introduce magnetic sequence and change the physical properties of MnBi₂Te₄.Based on the above considerations,the paper is divided into the following two parts:In the first part,the growth process of MnBi₂Te₄ crystals was optimized:rapid cooling at 900～600^oC followed by slow cooling at 600～582 ^oC（0.6 ^oC/h）to grow large MnBi₂Te₄crystals.Then the effect of Mn:Bi ratio on crystal products was studied systematically.By studying the crystal structure and composition of the product,it is found that with the increase of Mnconcentration,the composition of the product gradually changes from Mndoping Bi₂Te₃ to forming MnBi₂Te₄.Whenα>0.2（Mn:Bi:Te=α:2:3+α）,MnBi₂Te₄crystals are likely to form.Finally,the thermoelectric properties of the quenched bulk mixture samples were studied,and the optimal thermoelectric value（ZT）was about 0.01.In the second part,MnBi₂Te₄ crystal is bonded to La_0.7Sr_0.3MnO₃（LSMO）substrate to form a composite structure MnBi₂Te₄/La_0.7Sr_0.3MnO₃（MBT/LSMO）.There are three bonding modes:Silicone rubber bonding（10%,7%）,epoxy bonding（7%）,LSMO polycrystalline ceramic bonding（epoxy glue point coated on both ends of the chip）.The structure,morphology and electric transport characteristics of the composite samples were studied.In the cross-section scanning images of the composite samples,MnBi₂Te₄ crystal bonded by silicone rubber and epoxy bond is closely attached to the substrate LSMO,with almost no gap in the middle.However,the composite sample with polycrystalline ceramics has small gaps.In the electric transport test,it is found that the transition temperature（T_N）of the composite samples decreases with the decrease of MnBi₂Te₄ crystal thickness.At 3 T,two resistivity peaks appear at low temperature for silica gel bonded and epoxy bonded samples,and the two transition temperatures decrease with the decrease of crystal thickness.These phenomena all reflect the magnetic proximity effect of LSMO substrate on MnBi₂Te₄and are related to the thickness of MnBi₂Te₄.In particular,the MBT/LSMO-3 of polycrystalline ceramics with zero field has a resistivity peak at 18.2 K,and two additional resistivity peaks at 14-18 K at 1 T,which may be due to the strong ferromagnetism of LSMO polycrystalline ceramics.MnBi₂Te₄ near LSMO was induced to show another magnetic sequence transformation.At high field（5 T）,MBT/LSMO-3 bonded by 10%silicone rubber and MBT/LSMO-1 and MBT/LSMO-2 bonded by polycrystalline ceramic showed no obvious antiferromagnetic transition.At last,the effects of these compounding methods on MnBi₂Te₄ are summarized.