Design And Optimization Of ?Co,Fe?₂-Based Heusler Alloys For Spintronic Applications

The Heusler alloys are a significant class of intermetallic materials with 1:1:1?half Heusler?,2:1:1?full Heusler?and 1:1:1:1?quaternary Heusler?compositions which crystallize in A2,L2₁ and Li Mg Pd Sn-type structures.The Heusler alloys are an incredible class of materials and varied range of multi-functionalities such as halfmetallic,high-temperature ferri-ferromagnetic,shape memory alloys,topological insulators,energy technologies and magneto-caloric applications.The Heusler alloys have been vigorously investigated because of their diverse physical properties such as large magnetic moments,high spin-polarization and Curie temperature,making them attractive for the industry of spintronic such as giant magnetoresistance?GMR?and magnetic tunnel junction?MTJ?.Novel properties and a tremendous potential field of applications of Heusler alloys emerge constantly,based on their investigated electronic band structure,and a new class of Heusler alloys was predicted recently,namely topological insulators.In order to implement the Heusler alloys into spintronic devices,the half-metallicity and high spin-polarization need to be maintained in their thin-film form.This research work presents a consolidated picture of the novel alloy design and prediction of the physical properties of Heusler alloys via theoretical calculations and experimental observation.The main research contents of this work are as follows: alloy design of?Co,Fe?₂-based novel Heusler alloys,half-metallic behavior of the alloys,prediction of surface properties of thin-film Heulser alloys and doping effect of transition elements and sp-elements on?Co,Fe?₂-based Heusler alloys.Materials science is based on identifying the processing-structure-properties pattern that is related to the materials nature.The novel materials design by using the density functional theory?DFT?model is the growing trend in the scientific community.A critical innovation toward accelerating materials design has been an accurate and efficient first-principles prediction of the properties of materials with DFT.Using quantum mechanical concepts and experimental input,DFT permits one to predict properties of solids such as lattice parameters,electronic structures,magnetic moments,cohesive energies etc.To design the novel materials by DFT model,initially,we set the parameters like K-points,cutoff energy to check the convergence of the structure for magnetic and nonmagnetic states.We also choose the three types of structures with different Wyckoff positions then we investigate the minimum total energy and lattice parameter of?Co,Fe?₂-based Heusler alloys at ferromagnetic state?FM?utilizing ab-initio calculations.Half-metallic ferromagnets are applied in the form of thin-film or multilayers to the spintronic technology industry.The half-metallicity of the bulk materials may be destroyed at surfaces and interfaces due to the presence of surface states below the Fermi level.This work has focused to investigate the half-metallic behavior with 100% spin-polarization in the thin film Heusler alloys.Due to the enormous t_2g-e_g splitting of all atoms,the surface states pushed down or converge into the occupied 3d-states below the Fermi level,which open up an energy band gap in the minority states.The reduction in the atomic coordination numbers at surfaces may arise various surface effects.Due to the reduction of the number of neighboring atoms wanes the crystal field,enhancing exchange interactions at the surface atoms.The occurrence of surface states at the surfaces is triggered by the decrease of the bondingantibonding splitting due to the consequence of symmetry breaking at the surface.Moreover,the effect of doping of transition element and main-group element on?Co,Fe?₂-based Heusler alloys have studied by using GGA functional.To treat the strongly correlated electrons of the transition elements,we used the Hubbard potential?U?.We found that all the doping materials have half-metallic behavior with 100% spinpolarization,magnetic behavior which follow the Slater-Pauling rule.From mechanical properties,the considered alloys are mechanically stable with anisotropic behavior.Therefore,these dopped materials also could be promising materials for spin-devices.In this dissertation,the physical properties of?Co,Fe?₂-based Heusler alloys were investigated by using first-principles density functional theory?DFT?within generalized gradient approximation?GGA?of Perdew-Burke-Ernzerhof?PBE?.To predict the correct half-metallic gaps in Heusler alloys,the generalized gradient approximation plus Hubbard coefficient?GGA+U?technique was utilized.The physical properties of CoFeZrZ?Z=Ge,Sb Si?and CoFeHfGe alloys have studied systematically for the first time.The surface properties of?100?,?001?-terminated surfaces of CoFeHfGe,?111?-terminated surfaces of Fe₂MnP alloys and?100?,?111?-surfaces of Co₂ScSb alloys were investigated.The CoFeZrGe Heusler alloy was synthesized and characterized by an experiment.The doping effect of transition elements and main group elements on?Co,Fe?₂-based Heusler alloys have investigated to find the half-metallic behavior and magnetic properties.The phase stability,spin-polarized electronic structure,magnetic,mechanical and thermal properties of Co-based quaternary Heusler alloys were studied by utilizing DFT.The investigated results exhibited that the equilibrium lattice parameter well agrees with available theoretical data.The computed negative cohesive energy showed the stability of all the materials.The CoFeZrGe,CoFeZrSb,CoFeZrSi and CoFeHfGe alloys showed half-metallic nature with 100% spin-polarization at equilibrium lattice parameters with band gap of 0.43,0.70,0.59 eV and 0.37 eV for GGA and an improved band gap of 0.86,1.04,1.08 eV and 0.81 eV for GGA+U respectively.The electronic structures were significantly affected by the Coulomb interaction of 3d and 4d electrons of transition elements Co,Fe,Zr and Hf in CoFeZrZ and CoFeHfGe alloys.The calculated total magnetic moments are 1.01 ?B/f.u.,2?B/f.u.,1?B/f.u.and 1?B/f.u.for CoFeZrZ?Z = Ge,Sb and Si?and CoFeHfGe alloys respectively and agree well with the Slater-Pauling rule?SP?which connects the electronic properties directly to the magnetic properties.The main contribution for the total magnetic moments for CoFeZrZ and CoFeHfGe is by the transition elements Fe and Co and there is less contribution by sp-elements like Ge,Sb and Si.The predicted total and atomic magnetic moments of CoFeZrZ and CoFeHfGe alloys were steady within the GGA and GGA+U techniques.The calculated elastic constants revealed that the CoFeZrZ and CoFeHfGe alloys are mechanically stable and ductile nature.The order of stiffness of the materials is CoFeZrSi > CoFeZrSb > CoFeHfGe > CoFeZrGe.The investigated values of anisotropy?A?of these composites are higher than unity.Therefore the materials are anisotropic.The Debye temperature and heat capacity of Co-based quaternary Heusler alloys were successfully investigated.Half-metallicity and spin-polarization are the crucial properties of spindependent devices and Heusler alloys could be the candidate for spintronic devices.The half-metallic nature with 100% spin-polarization at the surfaces needs to be conserved to implement the Heusler alloys in spintronic devices.Therefore we studied the half-metallicity and magnetism in surfaces of?Co,Fe?₂-based Heusler alloys.The atomic relaxation of?100?,?001?-slabs of CoFeHfGe,?111?-slabs of Fe₂MnP alloy and?100?,?111?-slabs of Co₂ScSb alloy and surface energy of?100?,?001?-slabs of CoFeHfGe alloy were studied.For?100?and?001?-slabs of CoFeHfGe alloy,the halfmetallicity is destroyed and HfGe?100?-slabs has the highest spin-polarization 90.72%.Fortunately,the half-metallicity is preserved in P?111?-slab of Fe₂MnP Heulser alloy and ScSb?100?,Sc?111?-surfaces of Co₂ScSb alloy which can be used in the giant magnetoresistance?GMR?and magnetoresistive random-access memory?MRAM?devices.However the half-metallicity is destroyed in Fe?111?,Mn?111?,Co Co?100?,Co?111?and Sb?111?-surfaces due the surface states emerged at the Fermi level.We also observed a precise substantial surface reconstruction in CoFe?100?,CoFe?001?,Fe?111?and Mn?111?-slabs while the P?111?-slab presents a huge surface reconstruction due to the atomic partial density of states?APDOS?at the Fermi level.The atomic magnetic moments in the CoFe?100?-slab tend to increase with the depth of the slab.The atomic magnetic moments of the HfGe?100?-slab in L1,L4 and L9 layer accord well with those of the corresponding atoms in bulk Co Fe HFGe structure.The Hf atom in layers showed ferromagnetic behavior with other atoms in all the layers agreeing with the corresponding bulk.The computed surface magnetic moments of Fe?111?,Mn?111?and P?111?-slabs are extremely close to the bulk values which enables the conclusion that the GGA technique is reliable for the study of the surface magnetic properties.The P atom in all terminations has negligible atomic magnetic moments due to the lack of d-orbitals in the P atom.We also investigated the atomic magnetic moments of?100?,?111?-surfaces of Co₂ScSb alloy and found that the atomic moments of the S9 layer for five surfaces are precisely close to the corresponding bulk values.Finally,the transition element and sp-element are doped in Co₂V_1-xZr_xGa and CoFeZrSi_1-xGe_x?x = 0.00,0.25,0.50,0.75,1.00?Heusler alloys.It is found that the alloys considered exhibit half-metallic behavior with 100% spin-polarization in the spin-down channel.The energy band gaps increase as the component of x in Co₂V_1-xZr_xGa alloys increases and decrease as the component of x in CoFeZrSi_1-xGe_x alloys.The total magnetic moments decrease with increasing concentration in Co₂V_1-xZr_xGa,in agreement with the Slater-Pauling rule.The calculated magnetic moments of CoFeZrSi_1-xGe_x are 0.99,1.06,1.02,1.00 and 1.0?B/f.u..The elastic constants,bulk modulus,shear modulus,Poisson's ratio,anisotropy and Pugh ratio?B/G?are predicted and discussed.The analysis of elastic moduli indicates that the compounds are mechanically stable with ductile nature.The anisotropy has fundamental importance in materials science and it may be responsible for the micro-cracks in the materials.The studied compounds have anisotropy behavior.Moreover,the dielectric functions,optical conductivity,reflectivity and absorption coefficient of CoFeZrSi 1-xGex compounds are predicted by using complex dielectric functions.This work establishes that,for all the studied materials,we observed half-metallic behavior with 100% spin-polarization and large magnetic moments in agreement with the SP rule.Thorough analysis from the density of states and atomic magnetic moments indicates that the 3d orbital of transition elements is responsible for persuading magnetic moments in full and quaternary Heusler alloys.The half-metallic behavior is observed in the surfaces of the alloys considered.The study of mechanical properties exhibited the stability of the materials.The fundamental understanding and results obtained from this thesis will help to design the Heusler based devices fo r the application of spintronics.