Crystal Structure And Magnetic Phase Transitions Of Cr-alloys And Laves Phase Fe-alloys

Due to the rich physical effects that accompany the phase transition process,such as magnetocaloric effect,giant magnetoresistance effect,magnetostriction and negative thermal expansion,magnetic phase change materials have potential applications in many emerging fields which is a hot research topic in materials science and condensed matter physics.In a change of external magnetic field,temperature or pressure,the magnetic phase transition material is accompanied by changes in the crystal structure or lattice parameters.This work focuses on the preparation methods,crystal structures,magnetic properties and magnetic phase transitions of Cr-P alloys,AFe₂type Laves phase(Sc_1-xMo_x)Fe₂and(Y_1-xNb_x)Fe₂ternary alloys.The main research contents and results are as follows:1.First,Cr_1-xP_x(x=0.01,0.02,0.03,0.04,0.05,0.06,0.08,0.1)series alloys were prepared by solid-phase state sintering.For effective comparison,a series of Cr-Si alloys were prepared by arc melting method,and their crystal structure and magnetic properties were characterized.We studied the relationship between composition,structure,phase transition,magnetism,and thermal expansion in the Cr_1-xP_xand Cr_1-xSi_xseries alloys.The results of powder XRD experiments and SEM/EDX show that for the Cr_1-xP_xseries alloys,P element exists at x<0.03 in the Cr alloy matrix with a solid solvent of about 3%and Cr-P as precipitated phase(Cr₃P)at greater than 3%.Magnetic measurements and DSC results show that an antiferromagnetic-paramagnetic magnetic phase transition occurs at x=0.01and 0.02 accompanied by a large thermal hysteresis which is the characteristic of a first-order phase transition.These findings have important implications for further optimizing the magnetic structure and phase transition properties of Cr-based materials to obtain new materials for applications.2.(Sc_1-xMo_x)Fe₂(x=0.05,0.075,0.1,0.125,0.15,0.2,0.25,0.3,0.325,0.35,0.4,0.5,0.7,1)alloys were prepared by arc melting and high temperature sintering.Their crystal structure,magnetism and magnetic phase transition with the change of chemical composition were studied.X-ray diffraction results show that when x?0.4 the crystal structure is C14hexagonal single phase,while x>0.4 has phase separation of two C14structures with different lattice parameters.The results of magnetic measurements show that x?0.15 is a ferromagnet with a higher Curie temperature,with increasing Mo content gradually decreasing at Curie temperature of 120 K at x=1.For 0.35?x?0.4 alloys,abnormal changes in magnetization occur around 80 K except the ferromagnetic-paramagnetic phase transition,but a further study is required to determine whether this phenomenon is similar to that of(Sc_1-xNb_x)Fe₂alloys with negative thermal expansion effect.This findings provide new ideas for the study of magnetic and magnetic phase transition in iron-based Laves phase materials.3.(Y_1-xNb_x)Fe₂(x=0.1,0.2,0.3,0.4,0.5,0.7,0.8,0.9,1)alloys were prepared by arc melting and high temperature sintering.The influence of Nb substitutions for Y element on their crystal structures and magnetisms were systematically studied.The results of X-ray diffraction show that the crystal structure of Nb content is gradually changed from main phase of C15 cubic structure to C14 hexagonal structure,and the coexistence of two phases occur in the range of 0.1?x?0.9.A small quantity of YF₃third phases appears simultaneously in the range of 0.1?x?0.4.Compared with the as-cast alloys,the YFe₃phase of the sample with additional high temperature sintering(170h,1000?)is significantly reduced,indicating that the formation of C15cubic structure is favored in annealing condition.Magnetic measurements show that the magnetization of(Y_1-xNb_x)Fe₂series alloys decreases rapidly with increasing Nb contents.In addition,with increasing Nb contents,the magnetic order changes,from x=0.1 ferromagnet with higher Curie temperature to antiferromagnetism at x=1.This work provides new ideas for exploring new Laves phase functional materials.