Design Of Magnetostructural Coupling And Magnetic Martensitic Transformations In Hexagonal MM'X And All-d-metal Heusler Shape Memory Alloys

As one of extensively researched fields of condensed matter physics and magnetic functional material science,ferromagnetic?FM?martensitic transformations?FMMTs?exhibit various physical effects,including ferromagnetic shape memory effect,magnetic field-induced strain output,magnetic entropy change and exchange bias.This dissertation is aiming at constructing wide Curie temperature window with strong magnetostructural coupling in MM'X and developing new Heusler alloys with FMMTs.I did my work systemically by use of magnetic measurement,X-ray diffraction analysis,transmission electron microscope?TEM?,differential scanning calorimetry?DSC?,strain measurement,electrical transport measurement and first principle calculations.The research content of this dissertation including two main parts as shown as follow.Applying the principle of isostructural alloying,MnNiSi was chosen as a isostructural counterpart of Mn_1-yFe_y NiGe to simultaneously raised the martensitic transformation temperature Tt and Curie temperature TC M of the martensite by high Tt and TC M of MnNiSi.Consequently,unprecedentedly wide Curie-temperature windows?CTWs?with widths of 400 K were successfully constructed in the new isostructural alloy system Mn_1-yFe_y NiGe1-xSix from 40 K to 450 K.Within the CTWs,the FM coupling and magnetization of martensite were enhanced,thus FMMTs were achieved with large magnetization change??M?across the FMMTs,based on which we observed large magnetic entropy change??Sm?from 120 K to 450 K even under a moderate magnetic field change.Compared to other typical magnetic entropy change materials,Mn_1-yFe_yNiGe1-xSix shows large ?Sm covering ultra wide temperature region,which have never been reported before.Besides,functional fatigue behaviors of Mn_1-yFe_yNiGe1-xSix were investigated and the results indicate their FMMTs possess good reversibility and robust functional stability,benefiting the multifunctional applications of the materials.The unprecedentedly CTWs provide a broad design platform of magnetostructural transitions for tunable magneto-multifunctional properties including magnetic refridgeration,large strain-based output,multi-caloric effect,functional gradient materals,energy conversion,negative expansion materals.Based on understanding on structure and phase-formation mechanism of Heusler alloys,a conclusion is drawn that covalent bonds of p-d orbital hybridization between the main-group?p-group?atoms at the D sites and transition-metal?d-group?atoms at the?A/C?sites significantly influences the phase formation and phase stability.We expect that ordered Heusler structure is formed and stabilized by d-d orbital hybridization.Based on this idea,for the first time,the low-valence Ti was introduced into the binary NiMn alloy and we developed two kinds of “all-d-metal Heusler alloy” Ni50Mn50-y Tiy and Mn50Ni50-y Tiy systems,and martensitic transformation temperature Tt of each one decreases by increasing Ti content.Experiment results show their parent phase are B2?or L21?ordered Heusler structure with antiferromagnetic?AFM?coupling.By introducing Co on Ni site in Ni50-x CoxMn25Ti25 the FM coupling was built gradually,and exchange bias was observed for x = 17.5,a critical point of FM and AFM competition.By Co substituting in Ni50-xCoxMn35Ti15 and Mn50Ni40-x Cox Ti10,the AFM coupling of parent phase are converted to FM one while the martensite keeps low magnetization state,thus FMMTs with large ?M were realized in them,indicating all-d-metal Heusler ferromagnetic shape memory alloys were developed.Introduction of Co facilitates the Mn?B?-Co?A/C?-?MnD?local FM configuration,accounting for FM coupling of parent phase,which was confirmed by first principle calculation.Magnetic multi-functionalities were obtained in all-d-metal Heusler alloys,such as ferromagnetic shape memory effect,magnetic field-induced strain output,magnetic entropy change,magnetoresistance.Especially,Mn50Ni44.5Co9.5Ti10 exhibits a large magneto-strain of 6900 ppm in an isotropic polycrystalline sample which solely originate from the large volume change of-2.54% brought by FMMT,and the sample remained a whole and with no cracks across the FMMT.The realization of all-d-metal Heusler alloys changes the stereotype that Heusler structures must contain main group element,thus broaden the space for phase formation of Heusler multifunctional alloys.This work casts light on the understanding on the phase formation,martensitic transformation,and magnetic coupling of multifunctional shape-memory ferromagnets,as well as the design of new functional materials.