Antiferromagnetic Stability Research Articles

Enhancement of Magnetic Stability in Antiferromagnetic CoO Films by Adsorption of Organic Molecules.

Antiferromagnets are a class of magnetic materials of great interest in spintronic devices because of their stability and ultrafast dynamics. When interfaced with an organic molecular layer, antiferromagnetic (AF) films are expected to form a spinterface that can allow fine control of specific AF properties. In this paper, we investigate spinterface effects on CoO, an AF oxide. To access the magnetic state of the antiferromagnet, we couple it to a ferromagnetic Co film via an exchange bias (EB) effect. In this way, the formation of a spinterface is detected through changes induced on the CoO/Co EB system. We demonstrate that C60 and Gaq3 adsorption on CoO shifts its blocking temperature; in turn, an increase in both the EB fields and the coercivities is observed on the EB-coupled Co layer. Ab initio calculations for the CoO/C60 interface indicate that the molecular adsorption is responsible for a charge redistribution on the CoO layer that alters the occupation of the d orbitals of Co atoms and, to a smaller extent, the p orbitals of oxygen. As a result, the AF coupling between Co atoms in the CoO is enhanced. Considering the granular nature of CoO, a larger AF stability upon molecular adsorption is then associated with a larger number of AF grains that are stable upon reversal of the Co layer.

Origin of itinerant carriers in antiferromagnetic LaFe1−xMoxO3 studied by x-ray spectroscopies

We report on the electronic structure of doped $\mathrm{LaFe}{\mathrm{O}}_{3}$ at the crossover from an insulating-to-metallic phase transition. Comprehensive x-ray spectroscopic methodologies are used to understand core and valence electronic structure as well as crystal structure distortions associated with the electronic transition. Despite the antiferromagnetic (AFM) ordering at room temperature, we show direct evidence of itinerant carriers at the Fermi level revealed by resonant photoemission spectroscopy (RPES) at the Mo ${L}_{3}$ edge. RPES data taken at the Fe ${L}_{3}$ edge show spectral weight near the valence band edge and significant hybridization with O $2p$ states required for AFM ordering. Resonant inelastic x-ray scattering spectra taken across Fe ${L}_{2,3}$ edges show electron correlation effects ($U$) driven by Coulomb interactions of $d$ electrons as well as broad charge-transfer excitations for $x\ensuremath{\ge}0.2$ where the compound crosses over to a metallic state. Site substitution of Fe by Mo ions in the Fe-${\mathrm{O}}_{6}$ octahedra enhances the separation of the two Fe-O bonds and Fe-O-Fe bonding angles relative to the orthorhombic $\mathrm{LaFe}{\mathrm{O}}_{3}$, but no considerable distortions are present to the overall structure. Mo ions appear to be homogeneously doped, with average valency of both metal sites monotonically decreasing with increasing Mo concentration. This insulator-to-metal phase transition with AFM stability is primarily understood through intermediate interaction strengths between correlation ($U$) and bandwidth ($W$) at the Fe site, where an estimation of this ratio is given. These results highlight the important role of extrinsic carriers in stabilizing a unique phase transition that can guide future efforts in antiferromagnetic-metal spintronics.

Effect of impurity potentials on antiferromagnetism in two-dimensional Hubbard model

Recent numerical studies have shown that antiferromagnetism (AF) prevails widely near half filling in the two-dimensional Hubbard (t-t′-U) model, contrary to the experiments on cuprate superconductors. In cuprates, disorders owing to the dopants, which are usually disregarded in theory, are possibly intrinsic and considerable. Thus, as a first step, we consider the effects of point-type impurity potentials, which are randomly distributed on the lattice sites, on the stability of AF. It is found that a moderate impurity potential does not modify the stability of AF, but a sufficiently strong attractive potential V(V ≲ −U) appreciably weakens an antiferromagnetic state owing to an insufficient screening of V.

Defect formation and magnetic properties of Co-doped GaN crystal and nanowire

Theoretical calculation based on density functional theory (DFT) and generalized gradient approximation (GGA) has been carried out in studying defect formation and magnetic properties of Co doped GaN crystal and nanowire (NW). Co does not exhibit site preference in GaN crystal. However, Co occupies preferably surface sites in GaN NW. Transition level of the defect is also investigated in GaN crystal. We also find that CoGa(S) in NW does not produce spin polarization and CoGa(B) produces spontaneous spin polarization. Ferromagnetic (FM) and antiferromagnetic (AFM) couplings are analyzed by six different configurations. The results show that AFM coupling is more stable than FM coupling for Co doped GaN crystal. It is also found from Co doped GaN NW calculation that the system remains FM stability for majority of the configurations. Magnetic properties in Co doped GaN crystal can be mediated by N and Ga vacancies. The FM and AFM stability can be explained by Co 3d energy level coupling.

Defect formation and magnetic properties of carbon-doped ZnO nanowires by the first principles

Theoretical calculation based on density functional theory (DFT) and local density approximation (LDA) with Hubbard parameters has been carried out in studying defect formation energy, transition energy and ferromagnetism of carbon-doped ZnO nanowires (NW). The formation and ionization characteristics of the defects [CO (B), CO (S), CZn (B), VO (B), VZn (B), IO (oct) and IZn (oct)] in ZnO NW are analyzed in the text. Ferromagnetic (FM) and antiferromagnetic (AFM) coupling between C atoms are also investigated by 9 different configurations. The FM and AFM stability are explained by the interaction of C energy level. In addition, the vacancies [VO (B) and VZn (B)] and interstitials [IO (oct) and IZn (oct)] affecting the FM coupling are also investigated. It is found that magnetic moment of C 2p can be mediated by these defects.

Influence of exchange correlation on the symmetry and properties of siderite according to density-functional theory

Density-functional theory (DFT) without spin-orbit coupling (SOC), as implemented in CASTEP, yields structural, energetic, and elastic properties of siderite $({\text{FeCO}}_{3})$ in good agreement with available data from the literature. The electronic ground state is however found ferromagnetic (FM) with a nonzero density of states at the Fermi level, whereas real siderite is antiferromagnetic (AFM) and insulating. We show in both cases how the paradox can be removed. Concerning the metallic/insulating character, a symmetry analysis of the electronic structure performed with the help of ISOTROPY shows that the absence of gap is due to an essential degeneracy of monoelectronic wave functions in the experimental $R\overline{3}c$ symmetry. The small dispersion of individual bands establishes a clear difference with the case of a real metal. Complemented by $\text{DFT}+\text{SOC}$ computations performed using ABINIT this shows the consistency of the DFT description with crystal-field theory data taken from the literature. Thus for DFT the electronic structure of siderite is characterized by correlation, but of exchange origin, as opposed to electrostatic origin. The AFM nature of the ground state is also restored by SOC. Without SOC the total energies of the FM and AFM states are very close. After introducing some modifications in a classical model of superexchange we obtain a satisfactory interpretation of our DFT results in terms of spin polarization in the iron-carbonate bonds, spin-coupling mechanisms between neighboring iron ions and finally of the observed FM over AFM stability. Hence, siderite emerges as an original model system among transition-metal compounds for the application of DFT to open-shell electronic structures.

Stabilization of antiferromagnetism inCeFe2 alloys: the effects of chemical and hydrostatic pressure

Effects of Al, Mn and Sb dopings inCeFe2 and the effect of applied pressure have been investigated. Al doping gives riseto the FM–AFM transition and a reduction in the magnetic moment andTC values,clearly indicating the growth of the AFM component. Mn and Sb dopings only cause a reduction in theTC value. It is found that, in general, external pressure enhances the antiferromagnetism inboth the pure and the doped alloys. Enhancement of the Ce 4f–Fe 3d hybridization as aresult of doping and with the external pressure may be the reason for the stabilization ofantiferromagnetism in these alloys.