Preparation And Magnetic Properties Of Co-Fe-Sc Amorphous Alloys

Amorphous alloys are special alloys with disordered atomic structures and unique properties.The constituent atoms of crystalline materials are arranged in translational symmetry,which means the atomic arrangement is periodic in three dimensions.Compared with crystalline materials,long-range order is absent in amorphous materials.The metastable energy states,homogeneous composition and isotropy of amorphous alloys result in novel properties in thermodynamics,mechanism,magnetism,and chemistry（e.g.,corrosion resistance）.The amorphous alloys usually exhibit excellent soft magnetic properties.It is convenient to change the magnetic properties and atomic structures by tuning the compositon of amorphous alloys.In this paper,the object of our research is Co-Fe-Sc amorphous ternary alloys.Amorphous Co₉₀Sc₁₀ ribbons were firstly prepared by the melt-spinning method,and the magnetic properties,atomic structures and thermal stability were fully researched.Then,Co-Fe-Sc amorphous ribbons were prepared by substituting Co by Fe in Co-Sc amorphous alloys.The dependence of magnetic properties on Fe content was investigated and near room-temperature magnetocaloric effect is found in Co-Fe-Sc amorphous alloys when Co content is minority,which indicates the potential application of these amorphous alloys.The conclusions of this work are listed as below.（1）Ribbons of Co₉₀Sc₁₀ amorphous alloy were first prepared and excellent soft magnetic properties were found.It is found that the Curie temperature（>852 K）of Co₉₀Sc₁₀ amorphous alloy is higher than crystallization temperature as well as the room temperature.Meanwhile,the saturation magnetization at 300 K is 1.3 T,larger than any other reported Co-based amorphous alloys,and keep stable in a large range of temperature from 0 K to nearly 800 K,which makes this alloy to be promising on application.The RDF result shows that the average interatomic distance of Co₉₀Sc₁₀ amorphous alloy is 2.48?.The diffused distribution of amorphous structure results in the reduction of Curie temperature of Co₉₀Sc₁₀ amorphous alloy compared with crystalline Co.（2）The thermal property and crystallization kinetics of Co₉₀Sc₁₀ amorphous alloy have been investigated.The results of thermal analysis show that two crystallization peaks exist in the heating process and the two crystallization temperatures and melting point are 787 K,896K,and 1439 K,respectively.The structure relaxation appears in the amorphous matrix as the temperature is above 700 K.In the instant heating process,the apparent crystallization activation energy calculated by the Kissinger’s equation is 388 k J/mol.High crystallization temperature and crystallization activation energy indicate that Co₉₀Sc₁₀ amorphous alloy is stable at quite high temperature.In the process of isothermal crystallization,the stage crystallization activation energy of Co₉₀Sc₁₀ amorphous alloy variates with the crystallization fraction and shows a maximum value during the process.It is supposed that the ordered clusters form during the relaxation and decrease the nucleation activation energy as pre-nucleis at the initial stage of the crystallization process.（3）Co_90-xFe_xSc₁₀ amorphous ribbons were produced as Co substituted by Fe.The depencence of magnetic properties on the composition was full investigated.It is found that,among the Co_90-xFe_xSc₁₀ amorphous alloys,the largest saturation magnetization is 1.8 T for Fe₇₅Co₁₅Sc₁₀,higher than other amorphous alloys and compared to Fe-based nanocrystalline alloys;and the Curie temperature of amorphous phase of Co₈₅Fe₅Sc₁₀ is 1150 K,which is higher than any reported amorphous alloys.The Curie temperature of Co_90-xFe_xSc₁₀ amorphous alloys variates with the addition of Fe.As 0≤x≤55,the Curie temperatures are higher than the crystallization temperatures.The Curie temperature of amorphous Co₈₅Fe₅Sc₁₀ deduced by mean field theory is the maximum and the relationship between the Curie temperature and r_a/r_3dcoincide with the shape of the Bethe-Slater curve.The relationship between the saturation magnetization and the composition could be explained by the electron band theory.The relationship between the measured reduced magnetization M（T）/M（0）and the reduced temperature T/T_C could be explained in the framework of the Handrich-Kobe model which is based on the mean field theory.The curves of M（T）/M（0）-T/T_C is fitted to the Brillouin function modified by the asymmetric exchange fluctuation parameters,δ₊andδ_-.As 0≤x≤45,δ₊andδ_-is approximately zero;as 45<x≤90,δ₊andδ_-increase linearly,which indicate that the positions of Co_90-xFe_xSc₁₀ amorphous alloy shift from the top to the left on the Bethe-Slater curve.The Fe addition results in the increasing of the asymmetry of magnetic structure and weakening of average exchange interaction of amorphous alloys.The Co-Fe-Sc amorphous alloys have potential applications.（4）The Co-Fe-Sc amorphous alloys exhibit magnetocaloric effect near the room temperature when the Co content is minority.The maximum magnetic entropy change increases with the addition of Co as well as the peak temperature.When the external magnetic field change is 1.5 T,the maximum magnetic entropy change of Fe₈₃Co₇Sc₁₀ is 1.40 J/kg K and the maximum refrigerating capacity is 191.9 J/kg at Fe₈₇Co₃Sc₁₀.The values are larger than that of most reported Fe-based amorphous alloys.As the external magnetic field change increases to 2T,the refrigerating capacity increases to 265.2J/kg,approaching to the values of crystalline Gd and Gd₅Si₂Ge₂,which shows the potential applications of these alloys.