Tuning Of Magnetic Materials By First-principles Investigations

High spin polarization,especially full?100%?spin polarization,of charge carriers is a choice to improve properties of spintronics devices and it is important to spintronics to achieve fully-spin-polarized magnetic materials that are stable and can be easily fabricated.Full spin polarization can be theoretically realized in half-metallic materials,because one of the two spin channels is metallic and the other has a semiconductor gap.Furthermore,high-performance spintronics material also makes use of good crystalline ferromagnetic semiconductors with high Curie temperatures which naturally combine the full spin polarization with modern semiconductor technology.By first-principles calculations based on density-functional-theory?DFT?,we have investigated the crystal structural properties,electronic properties and magnetic properties of CrTe and CrO₂under the control of the biaxial stress,and the physical properties of TcO₂/TiO₂heterostructure controlled by an applied electric field.Here,we achieved high and even full spin polarization for carriers in the Ni As-type CrTe by applying tensile biaxial stress.The resulting strain is tensile in the xy plane and compressive in the z axis.With the in-plane tensile strain increasing,the electronic and magnetic properties of CrTe just changed a little,and a half-metallic ferromagnetism is achieved at an in-plane strain of 4.8%.With the spin-orbit coupling taken into account,the spin polarization is equivalent to 97.0%at the electronic transition point;and then becomes 100%at the in-plane strain of6.0%.These results make us believe that the full-spin-polarized ferromagnetism in this stable and easily-realizable hexagonal phase could be realized soon.We found that when a biaxial compressive stress is applied on rutile CrO₂,the density of states at the Fermi level decreases,there is a structural phase transition to an orthorhombic phase at the strain of-5.6%,and then appears an electronic phase transition to a semiconductor phase at-6.1%.Further analysis shows that this structural transition,accompanying the tetragonal symmetry breaking,is induced by the stress-driven distortion and rotation of the oxygen octahedron of Cr,and the half-metal-semiconductor transition originates from the enhancement of the crystal field splitting due to the structural change.Importantly,our systematic total-energy calculation indicates the ferromagnetic Curie temperature remains almost independent of the strain,near 400 K.This biaxial stress can be realized by applying biaxial pressure or growing the CrO₂epitaxially on appropriate substrates.We proposed the TcO₂uni-cell layer?of one-unit-cell thickness?on rutile TiO₂?001?substrate as a semiconductor heterostructure and use an electric field to manipulate its electronic and magnetic properties.Our study shows that the heterostructure is a narrow-gap semiconductor with an antiferromagnet-like ordering when the applied electric field is less than 0.3 V/nm,and then it transits to a half-metallic ferrimagnet with 100%spin polarization.Our further analysis indicates that the magnetization density and the electronic states near the Fermi level originate mainly from the TcO₂uni-cell layer,with the remaining minor part from the interfacial TiO₂monolayers,and the bonds and bond angles quickly converge to the corresponding values of bulk TiO₂when crossing the interface and entering the TiO₂layer.Therefore,the heterostructure is actually a 2D electron system determined by the TcO₂uni-cell layer and the TiO₂substrate.These results should be useful for investigations on realizing full?100%?spin polarization of controllable carriers as one uses in modern semiconductor technology.