ABSTRACT
Based on a rare-earth Gd atom with 4f electrons, through first-principles calculations, we demonstrate that a Janus 2H-GdIBr monolayer exhibits an intrinsic ferromagnetic (FM) semiconductor character with an indirect band gap of 0.75 eV, a high Curie temperature Tc of 260 K, a significant magnetic moment of 8µB per f.u. (f.u. = formula unit), in-plane magnetic anisotropy (IMA) and a large spontaneous valley polarization of 118 meV. The MAE, inter-atomic distance or angle, and Tc can be efficiently modulated by in-plane strains and charge carrier doping. Under a strain range from -5% to 5% and charge carrier doping from -0.3 e to 0.3 e per f.u., the system still retains its FM ordering and the corresponding Tc can be modulated by strains from 233 K to 281 K and by charge carrier doping from 140 K to 245 K. Interestingly, under various strains, the matrix element differences (dz2, dyz), (dx2-y2, dxy) and (px, py) of Gd atoms dominate the MAE behaviors, which originates from the competition between the contributions of the Gd-d orbitals, Gd-p orbitals, and p orbitals of halogen atoms based on the second-order perturbation theory. Inequivalent Dirac valleys are not energetically degenerate due to the time-reversal symmetry breaking in the Janus 2H-GdIBr monolayer. A considerable valley gap between the Berry curvature at the K and K' points provides an opportunity to selectively control the valley freedom and states. External tensile (compressive) strain further increases (decreases) the valley gap up to a maximum (minimum) value of 158 (37) meV, indicating that the valley polarization in the Janus 2H-GdIBr monolayer is robust to external strains. This study provides a novel paradigm and platform to design spintronic devices for next-generation quantum information technology.
ABSTRACT
The effects of Co doping on the structural, optical, magnetic and transport properties of (In0.98-xCoxSn0.02)2O3 films grown by RF-magnetron sputtering were systematically investigated by theoretical and experimental techniques. The detailed structural analyses revealed that all the (In0.98-xCoxSn0.02)2O3 films possess the cubic bixbyite structure, with the substitutional Co at the In3+ sites of the In2O3 matrix, while some of the Co atoms form Co metal clusters. Obvious room-temperature (RT) ferromagnetic behavior was observed and the saturated magnetization (Ms) first increased, then decreased with increased Co concentration, while carrier concentration nc decreased monotonically, implying that the Co metal clusters are superparamagnetic and the observed RT ferromagnetism is not mediated by the charge carriers. The Mott variable range hopping (VRH) and hard band gap hopping transport behavior dominates the conduction mechanism of the films, confirming that the carriers are strongly localized. The UV-Vis and photoluminescence (PL) measurements indicate the decreased optical band gap Eg with Co doping, and further prove that the oxygen vacancies and Co impurity band form defect complexes of donor-acceptor pairs. The density functional theoretical calculations show that the codoped Sn can change the magnetic coupling between two Co ions from antiferromagnetic to ferromagnetic by the new hybridization between the Co 3d states with the Sn induced donor band. It can be concluded that the bound magnetic polaron (BMP) based oxygen vacancies as well as the Co-O-Co ferromagnetic superexchange interaction induced by Sn codoping may be responsible for the intrinsic ferromagnetic ordering in the (In0.98-xCoxSn0.02)2O3 films. These results may provide new insight for understanding the magnetic mechanism of In2O3 based DMS systems.
ABSTRACT
The local structural, optical, magnetic and transport properties of (In0.95-xFexCu0.05)2O3 (0.06 ≤ x ≤ 0.20) films deposited by RF-magnetron sputtering have been systemically studied by different experimental techniques. Detailed structural analyses using XRD, XPS, EXAFS and full multiple-scattering ab initio theoretical calculations of Fe K-edge XANES show that the (In0.95-xFexCu0.05)2O3 films have the same cubic bixbyite structure as pure In2O3. The doped Fe ions exist at both +2 and +3 oxidation states, substituting for the In(3+) sites in the In2O3 lattice and forming a FeIn + 2VO complex with the O vacancy in the first coordination shell of Fe. However, the co-doped Cu atoms are not incorporated into the In2O3 lattice and form the Cu metal clusters due to high ionization energy. UV-Vis measurements show that the optical band gap Eg decreases monotonically with the increase of Fe concentration, implying an increasing s-pd exchange interaction in the films. All the films display intrinsic room-temperature (RT) ferromagnetism and the saturated magnetization (Ms) increases monotonically with Fe doping. The temperature dependence of the resistivity data suggests the conduction mechanism of Mott variable-range hopping (VRH) at low temperature, confirming that the carriers are localized. It can be concluded that the observed RT ferromagnetism in the films originates from the overlapping of polarons mediated by oxygen vacancies based on the bound magnetic polaron (BMP) model. The variation of the localization effect of carriers with Fe doping can obviously adjust the magnetic exchange interaction in the (In0.95-xFexCu0.05)2O3 films.
