Correlation Between Local Structure And Magnetic Properties Of Several Typical Magnetic Materials

Metal magnetic materials are widely used for energy storage and conversion,playing an important role on the development of national economy and defense technology.Their theoretical performance is determined by chemical composition and crystal structure of the ferromagnetic phase(intrinsic factors),but the practical magnetic performance is determined by defects such as grain boundary and domain boundary(external factors).Consequently,how to approach the theoretical limitation through both intrinsic and external modifications and how to get novel properties by defect controlling are important scientific issues in this field.In the present work,we selected the rare earth permanent magnet Nd-Fe-B with the strongest magnetic performance and magnetoelastic materials with magnetic-field-induced-strain(MFIS),and systemically investigated the "Correlation between Local Structure and Magnetic Properties",aiming to improve the macroscopic magnetic performance and/or to explore novel functions.1)Based on revealing the correlation between grain boundary(GB)and coercivity of slightly off-stoichmetric Nd2Fe14B magnet,we restructured the GB through introducing low melting point Dy-containing alloy and enhanced the coercivity efficiently.It has been a long puzzle that the obtained coercivity in Nd-Fe-B sintered magnets is only-20-30%of the theoretical value(known as Brown's paradox),which stimulates worldwide efforts to minimize this gap.As the structural and intrinsic magnetic properties of the matrix 2:14:1 phase(?97vol.%)are determined,the influence of the grain boundary phase(-3vol.%)on the coercivity is of significant importance.Consequently,we firstly investigated the correlation between the GB phase and coercivity of the slightly off-stoichiometric Nd2Fe14B sintered magnet through controlling the sintering process.It was found that at lower sintering temperatures,full densification cannot be reached even prolonging the sintering time,and the coupling between adjacent 2:14:1 phase grains results in a low coercivity.At higher sintering temperatures,the 2:14:1 phase grains grow up quickly,also deteriorating the coercivity.At an optimum sintering temperature of 1050? the magnet reaches a full densification with most 2:14:1 phase grains of 4-5?m,showing a high coercivity of 16.4 kOe.The formation of smooth and continuous grain boundary RE-rich phase reduces the magnetic interactions and the grain refinement hinders nucleation of reversed domains at low fields,respectively.Then,low melting point Dy69Ni31(at.%)powders are introduced into the slightly off-stoichiometric Nd2Fe14B(Pr,Nd)12.36FebalB6.09(at.%)magnet to restructure the grain boundary.The Dy69Ni31 additive provides extra rare-earth that improves the liquid-phase sintering with the formation of more continuous and thicker intergranular GBs,accompanied by the diffusion of Dy towards surface region of the 2:14:1 phase grains.A significant increment in coercivity of 6.25 kOe is realized with a slight reduction in remanence by-3.0%per unit at.%Dy.Further work reveals that the composition,distribution and structure of the GB phase can be optimized through changing the annealing time.It not only results in further increment in coercivity but also improves the corrosion resistance efficiently.Consequently,the grain boundary restructuring approach combines advantages of the modification of intergranular GBs and the formation of a magnetically hardening shell surrounding 2:14:1 phase grains,which can be a promising approach to fabricate low rare-earth and high performance Nd-Fe-B magnets for mass production.2)Defects can suppress the first-order structural transformation but not the second-order magnetic transition of magnetic shape memory alloy,resulting in novel magnetic properties.Defects-induced local symmetry breaking has led to unusual properties in nonferromagnetic ferroelastic materials upon suppressing their martensitic transformation.Thus,it is of interest to discover additional properties by local symmetry breaking in one important class of the ferroelastic materials,i.e.,the ferromagnetic shape memory alloys.For the case of ferromagnetic shape memory alloys,we have found that the existence of defect can forbid the first class of martensitic transition,but cannot forbid the second class of magnetic transition.In this thesis,local symmetry breaking including both tetragonal nano-inclusions and anti-phase boundaries(APBs),suppresses martensitic transformation of a body-centered-cubic Fe50Mn23Ga27 alloy however,does not affect the magnetic ordering.Lower ordering degree at APBs and local stress fields generated by the lattice expansion of tetragonal nanoparticles hinder the formation of long-range ordered martensites.The half-metal-like conducting behavior upon suppressing martensitic transformation extends the regime of ferromagnetic shape memory materials and may lead to potential applications in spintronic devices.3)Local martensitic transformation has been found in the new-type magnetostrictive material,Fe-Ga alloy,providing evidence to understand the over ten-fold magnetostriction enhancement by soluting nonmagnetic Ga into BCC Fe.The mechanically strong and malleable Fe100-xGax alloys(known as Galfenol)have attracted intensive attention due to the combination of large magnetostriction,low switching field and high TC,which are promising for applications in sensors,actuators and transducers.The origin of its large magnetostriction,however has prompted much controversy since its discovery in 1999.Previous investigations have ascribed the maximum magnetostriction at x?27 to shear modulus softening,which indicates a premartensitic transformation or local symmetry breaking.Spontaneous martensitic transformation,however,has been thought to be forbidden for such a system that contains high concentration of defects.Here we found that an ordering-treated Fe72,4Ga27.6 alloy transforms into six-layer modulated monoclinic(6M)martensites locally after cryogenic treating.The D03 ordering is promoted and stabilized by performing further aging for 219 h at 973 K after solution-treating at 1373 K.Verified by electron diffraction,the lattice parameters of local 6M martensites are a = 4.88 A,b=5.85 A,c = 14.70 A,? =95.78°.In combination with the magnetometry measurements,the local martensitic transformation exhibits non-thermoelastic feature.Such findings may add important insight into understanding the microstructural origin of the extraordinarily enhanced magnetostriction.