Multi-field Modulated Phase Transition And Related Properties In Magneto-lattice Coupled Mateirals Sy

Magneto-lattice coupled materials systems are usually multi-functional materials with ferroic-caloric effect?magnetocaloric,barocaloric,elastocaloric?,negative thermal expansion et.al.As these function of the materials are closely related with its magnetostructural transition process.The study of internal and external fileds tuning of the magneto-structural transition and further the related properties are meaningful and essential for promoting the application of the materials.In this thesis,Ni₂In type hexagonal MM'X,Ni₂Mn_1-xIn_x Heusler and La(Fe_1-xSi_x)₁₃ alloys has been fabricated through deposition of films,cold-pressing and melt-spinning technique.The influence of internal chemical atomic disorder,and residual stress introduced during the fabrication process,external field such as magnetic field,hydrostatic pressure and electric field induced strain on the phase transition,magnetism and the related properties of these materials systems has been systematically researched.The main results are shown as follows:1.Fabricating low-dimension films and realizing the magnetic refrigeration in nano-scale is one of the new developing direction of magnetic refrigeration.In this article,the deposition of Mn-Co-Ge-In films using the pulsed laser deposition technique has been explored.The study shows that the films grown retain the first order nature,however,the residual stress field and grain size effect broaden the transition of the alloy films.Compared to the magnetic caloric effect that has been reported in meta-magnetic films,the present Mn-Co-Ge-In films exhibit comparable magnetic entropy change.Besides,the films deposited on?0001?-Al₂O₃,?001?-SrTiO₃,?001?-LaAlO₃ substrates denote tunable thermal expansion behavior with particle size of the films.Specifically,Mn-Co-Ge-In/Al₂O₃ shows negative thermal expansion coefficient of-6.56?10^-6/K?270 K³90 K?while Mn-Co-Ge-In/LaAlO₃ denotes ultra-low thermal expansion of-2?10^-7/K?290 K³90 K?.The growth of films could thoroughly avoid the brittleness nature of the alloy and provide research foundations for the growth of MnCoGe based alloys films.The method gives a new train of thought on the fabrication of films with required thermal expansion coefficient,by utilizing the sensitiveness of the magneto-structural transition to the strain and defects,one could realize the quantified modulation of negative thermal expansion.2.By fabricating MnCoGe based alloys through the cold-pressing and melt-spinning method,the residual strain modulated transition behavior and magnetocaloric effect has been systematically studied.The Mn_1-xCoGe ribbons has been fabricated through melt-spinning technique,the influence of grain size,grain boundaries defects and residual stress field on the phase transition and magnetocaloric effect of the alloy has been researched.Melt-spinning process could stabilize the hexagonal structure and thoroughly suppress the martensitic transition of the alloy.Annealing at 850?for 15min could promote the diffusion of atoms to its equilibrium sate,release the residual stress,refine grains,thus induce martensitic phase in the alloy and resume magneto-structural transition.Accompanying the magneto-structural transformation,the melt spun Mn_0.99CoGe and Mn_0.98CoGe ribbons denote magnetic entropy change of 19 J kg^-1K^-1 and 18 J kg^-1K^-1.As a potential magnetic refrigerant,MnCoGe based alloys has bad mechanical properties and is difficult to shape up.Otherwise,the magnetic refrigerant usually require thin slices with thickness lower than 1 mm or spheres with diameters of 200⁴00?m.Based on which,we have fabricated Mn-Co-Ge-In thin slice with thickness of 0.5 mm and diameter of 0.5 cm through cold pressing under the pressure as high as⁵ GPa.The residual stress in the pressed thin slice result to the broadening of the magnetostructural transition of the alloy.Characterization of magnetocaloric effect denote that the parent alloy particles,thin slices pressed under 3GPa and 5 GPa own magnetic entropy change of 30 J kg^-1K^-1,8 J kg^-1K^-1,and 4 J kg^-1K^-1,meanwhile,the refrigerating temperature range are 10 K,54 K and 73 K respectively.3.The transition of the alloy is not only related to the valence electron of the alloy,chemical atomic order and grain size also have great influence on the transition process and the related properties of the alloy.Up to now,however,the atomic order and grain size effect are not well understood and the reported result are not uniform.The reason could be ascribed to the fact that the grain size and atomic order always mutually changes,which inhibit the distinguishing of the effect of the two parameters.In this work,the grain size and atomic order are tuned through changing the spinning speed combining post annealing.The atomic order of the obtained ribbons are quantified through the method raised by Webster,combining the magnetic characterization,the influence of the atomic order and grain size on the martensitic transition were ascertained.The results denote that increasing spinning speed could reduce the atomic order and raise the martensitic transformation temperature.The atomic order for B2phase of 15 m/s,30 m/s and 45 m/s ribbons are 0.80,0.75 and 0.59,while the transition temperature are 220 K,238 K and 254 K respectively.The annealing process could increase atomic order,meanwhile,the grain size increased from 3?m to 8?m for the15 m/s ribbons.The comprehensive influence shows that,for Ni-Mn-In,the reduction of atomic order results to the increase of martensitic transition temperature,and plays a more important effect when the grain size is small,meanwhile,the martensitic transition temperature increases with the grain size.Further research shows that the atomic order could effectively modulate the magneto-structural transition temperature and further the magnetic refrigerating temperature range of the alloy,the three kind of ribbons fabricated has refrigerating temperature range covering 195 K to 244 K.Besides,we have for the first time researched atomic order modulation effect of exchange bias in the alloy system,the result denote that reducing the atomic order could increase the exchange bias field,and the exchange bias field are 153 Oe,174 Oe and226 Oe for the three kind of ribbons respectively.Finally,the hydrostatic pressure influence has been researched,the result denote that the hydrostatic pressure could stabilize the martensitic phase,strengthen the ainti-ferromagnetic coupling,which raises the martensitic transition temperature and strengthen the exchange bias effect.The work ascertained the internal parameters such as atomic order,grain size and external pressure influence on the transition,magnetocaloric effect and exchange bias effect.The tuning of atomic order on the spontaneous and conventional exchange bias effect has been comprehensively studied for the first time.The result denote that the atomic order could modulate the transition and further the magnetic refrigerating temperature range and exchange bias effect,which is meaningful for promoting the application of the alloy.4.La(Fe_1-xSi_x)₁₃ alloy system has itinerant magnetism and considerate magnetovolume effect,meaning that the transition and magnetism are sensitive to the chemical and external field such as hydrostatic pressure,this makes it an ideal ferromagnetic material for the study of magnetoelectric effect and electric-magnetic dual field tuning of magnetism and magnetocaloric effect.Choosing La(Fe_0.94Co_0.06)_11.8Si_1.2 alloy with magnetic transition residing near room temperature,and fabricated La?Fe,Co,Si?₁₃/PMN-PT artificial multiferroic structure,we have studied the magneto-electric effect of La?Fe,Co,Si?₁₃ alloy.The strain effect induced by the piezoelectric PMN-PT single crystal shows the ability to modulate the magnetization of the La?Fe,Co,Si?₁₃ thin slice bonded on it through the modulation of the energy band structure.Around the room temperature of 288 K and with a magnetic field of 0.01 T applied,an electric filed of 12 kV/cm could decrease the magnetization of La?Fe,Co,Si?₁₃ by 5.1%.Furthermore,the artificial multi-ferroic structure owns an electric field induced reversible and permanent magnetic memory effect.5.Nd₂Fe₁₄B is a typical permanent magnetic material,in which,researching the intrinsic magnetic properties of the alloy could help to guide the optimization of the permanent magnetic properties of the alloy.Through the introduction of Ta buffer layer,the permanent magnetic Nd₂Fe₁₄B films with textured structure has been successfully grown on the?011?-PMN-PT single crystal.The influence of buffer layer,film thickness,substrates,microstructure influence on the permanent magnetic properties and spin reorientation has been studied.Furthermore,the modulation of electric field induced strain on the spin reorientation and magnetism of the deposited Nd₂Fe₁₄B films has been studied,and a new mechanism for the magneto-electric effect has been raised.Below the spin reorientation temperature of Nd₂Fe₁₄B,the dynamic strain field could influence the structure distortion of Nd₂Fe₁₄B and tuning the crystal filed coefficient,further the magnetic anisotropy of the alloy,as the spin reorientation and magnetism is intimately related to the magnetic anisotropy,the magnetization of the films can thus been modulated.When the temperature is lowered down to 30 K,with magnetic field of 2 T applied,an electric field of 6 kV/cm could decrease the magnetization of Nd₂Fe₁₄B films by 30%.