Study Of The Growth And The Magnetic Properties Of Magnetic Topological Insulator Single Crystals

Topological insulator （TI） with ferromagnetism （FM） is promising forthe observation of Anomalous Quantum Hall Effect （AQHE） and otherspintronic applications.This thesis focuses on the single crystal growth andmagnetic properties of FM TI. On the one hand, from the viewpoint of crystalgrowth, the growth habit and defects of single crystals were studied mainlythrough the observation of the morphology, on the other hand, the origin ofFM in TIs were clarified by physical properties measurement.Besides, because of the unique superheating character of REBCO thinfilms, it is used widely as the seeds of growing various REBCO singlecrystals, bulks and liquid epitaxial thick films. Thin film thermal stability iskey factor in inducing growth of material, which makes the correspondingstudy important for industrial applications and fundamental scientificresearch.The thesis contents the following four parts: 1. High quality doped TI single crystals are the basis for the research onrelated physical properties. We studied the crystal growth of pure and Mndoped Bi2-xMnxTe3single crystals. From various morphologies observation,we find the crystals grown from melt are superior to the self-flux methodwhich easily induces defects such as vacancies and flux inclusions.Optimizing the growth conditions will eliminate the defects and improvecrystal quality to some extent. In Mn-doped crystals, though STM resultsshowed Mn substituted for Bi as we expected, most of Mn were excluded andformed Mn-rich clusters. The study of the morphology helps us to reveal themechanism of the growth under different conditions and will guide ourpreparation of high quality single crystals ultimately.2. Cr-doped Sb₂Te₃is a potential candidate for observation of AQHEand other spintronic applications because of its strong out of plane easy axis.In order to realize spin-related quantum phenomena including AQHE in TI,ferromagnetism should come from the intrinsic magnetic TI. Bulk carrierdensity and magnetic properties of Sb_1.95-xBi_xCr_0.05Te3single crystals werestudied. We found that the carrier density can be tuned by Bi substitution.Both the Curie temperature and magnetic moments reach their minimum atx=0.3, which are consistent with the carrier density. The strongly relatedcorrelation among the three values indicates the origin of ferromagnetism in Cr-doped Sb₂Te₃is not CrxTeynano-clusters or nano-crysatls. All the resultssuggest that Cr-Sb₂Te₃is intrinsic magnetic topological insulator which isvery important for spin-related applications.3. The origin of FM in Fe-doped Bi₂Se₃remains controversial till now.Also from the perspective of carrier tuning magnetism in intrinsic dilutedmagnetic semiconductors, we studied the electronic structure and magneticproperties of iron-doped topological insulator Bi_1.84-xFe_0.16Ca_xSe₃singlecrystals. By co-doping Fe and Ca atoms, ferromagnetic bulk states withdifferent carrier density （from n-type to p-type） were obtained. Different fromCr-Sb₂Te₃, the M-T curves of all samples show three magnetic transitions.Effective magnetic moments for each Fe atom were estimated as small asabout0.07B, and the magnetism in the samples had no direct relation withcarrier density. Separation of magnetic Fe_xSe_yphase and diamagnetic Bi₂Se₃mother phase was observed in all samples. Our results suggest that the bulkferromagnetism in Fe-doped Bi₂Se₃is not intrinsic and regardless of carrierdensity.4. Varied Influencing factors on thermal stability of RE123thin filmshave been studied before, among which, the substrate effect and thetemperature coefficient of solubility （t.c.s.） of different RE elements inBa-Cu-O liquid is the most important. In order to find their potential correlation, peritectic melting evolutions of different α/sub films wereinvestigated in-situ by high temperature optical microscopy. It is found thatthe substrate and the t.c.s. of RE element in the liquid work simultaneously insuperheating capability. Distinctively, thermal stability of Y123is moresensitive to the substrate variation than that of Sm123. Moreover, for givenα/substrate film structures, combining experimental data of the capability ofsuperheating （ΔT_c^s） and the t.c.s. decided relation between supersaturation （σ）and degree of superheating （ΔT^s）（σ-ΔT^scurve）, comprehensive correlationamong the lattice bonding of substrate, the t.c.s. and the ΔT_c^sis constructed.The results derived from this work extend the understanding on superheatingphenomenon of thin films in peritectic melting to more fundamental andtechnological fields.