Tuning Magnetoresistance And Magnetic Refrigeration Properties Of Rare-Earth Magnetic Materials

Magnetoresistance(MR)storage technology and magnetic cooling(MC)technology are inseparable from magnetic materials with excellent MR and MC performances.Owing to the excellent inherent magnetic properties,rare-earth(RE)based magnetic materials have become a hot candidate for MR and MC devices.However,MR and magnetocaloric effect(MCE)associated with the magnetic phase transition in RE-based materials are facing many challenges,such as unclear relationship between macroscopic MR properties and microstructures,small low-field MR(LFMR)effects,insufficient high-abundant RE materials with room temperature MCE,too high critical magnetic field and narrow working temperature range of magnetic refrigeration,etc.In addition,the current research paradigm of highperformance MR and MCE materials is based primarily on experience,lacking in depth theoretical prediction and guidance.In view of this,the main work of this doctoral dissertation starts with the micromagnetic calculation of spin transport,the calculation of the critical exponents of magnetic phase transition,the first-principles calculation of MCE performance parameters.We then investigate the mechanisms of spin-polarized transport and magnetic phase transition in RE-based magnetic materials.Furthermore,we obtain the relationship between macroscopic properties and microstructures,which gives the rational design strategies for improving MR and MCE properties.With the help of electric current annealing or high-pressure annealing techniques,MR and MCE performances are optimized efficiently on the basis of design strategies.This work realizes efficient utilization of the high-abundant RE elements of La and Ce.The main results are as follows:1.Micromagnetic calculation guided design of enhanced MR performance in lanthanum strontium manganiteConsidering that the LFMR of lanthanum strontium manganite is not strong,we investigate systematically the relationship between MR properties and microstructures of half-metallic ferromagnet La2/3Sr1/3MnO3 by using static micromagnetic models combined with the theories of spin-polarized intergrain tunneling and charge carrier hopping across a domain wall in grains,which gives strategies for enhancing LFMR.(1)With computational guidance for enhancing LFMR,we carry out a current annealing experiment.It is found that MR and its sensitivity of the current-annealed La2/3Sr1/3MnO3 are increased by 25.3%and 54.9%,respectively.This enhancement is mainly attributed to the increase of grain boundaries and the decrease of magnetic anisotropy induced by current-annealing.The experimental results are in good agreement with the theoretical prediction,which indicates that this MR formula can give guidance for further achieving excellent magnetoresistive response.(2)In order to excavate both intrinsic and extrinsic MR effect,La0.7Sr0.3MnO3 polycrystalline is post annealed under a high pressure of 6 GPa.It is observed that MR is increased by 47.2%at 1.4T.In addition,high-pressure annealing(HPA)not only produce more GBs in polycrystalline,but also significantly modifies both lattice parameters and intrinsic magnetic coupling range.According to our theoretical MR formula,the MR enhancement can be logically ascribed to the increased intergranular tunneling effect due to the increased grain boundaries by HPA and the enhanced intrinsic effect resulting from the established long-range FM interaction by HPA.2 Manipulation of magnetic refrigeration performance of the high-abundant rareearth alloys based on first-principles calculationAt present,RE-based MCE materials are overly dependent on low-abundant RE materials such as Gd,Gd5Si2Ge2,which are expensive and not conducive to commercialization.Especially,there are not enough high-abundant RE materials with MCE at room temperature and the working temperature range of most materials with large MCE is very narrow,which limits their application range.(1)In order to further excavate high-abundant RE materials with magnetic phase transition,we successfully construct ferromagnetic order and ferromagnetic-toparamagnetic phase transition by Mg doping in Pauli paramagnet of CeCo3,which contains the most abundant RE element of cerium.The origin of ferromagnetism and magnetic exchange coupling are analyzed by first-principles calculation and calculation of critical exponents of magnetic phase transition,respectively.The Curie temperature of Ce0.65Mg0.35Co3 prepared is slightly higher than room temperature,which needs to be further reduced.First-principles calculations show that the theoretical Curie temperature of Ce0.6sMg0.35Co3 decreases significantly when the lattice constant decreases after HPA.According to the predicted results,the magnetic properties are measured,which verifies that the Curie temperature of HPA sample is decreased to room temperature,and the MCE is improved.(2)LaFe11.6Si1.4 compound,containing the high-abundant RE element of lanthanum,exhibits a large magnetic entropy change.However,their operating temperature range is narrow and the critical magnetic field of itinerant-electron metamagnetic(IEM)transition is too high,which limits their application in magnetic refrigeration.First-principles calculations demonstrate that the reduced lattice constants caused by HPA can increase density of state near the Fermi energy of paramagnetic LaFe11.6Si1.4,which makes it easier to trigger IEM transition.Then we carry out the magnetic measurement,which verifies that the critical field of IEM near Curie point is reduced and the high field susceptibility is improved.More importantly,the refrigeration operation temperature window can be broadened effectively by HPA,and the sample of LaFe11.6Si1.4 annealed under 1 GPa shows an enhanced refrigeration capacity of 512 J/kg under a magnetic field change of 5 T.