The structural, magnetic, and electron transport properties of Mn55−xFexBi45 (x = 0, 2, 4, 5, 8, 11, 13, 16) films prepared by multilayer deposition and annealing using e-beam evaporation have been investigated. Fe doping has produced a significant change in the magnetic properties of the samples including the decrease in saturation magnetization and magnetocrystalline anisotropy and increase in coercivity. Although the magnetization shows a smooth decrease with increasing Fe concentration, the coercivity jumps abruptly from 8.5 kOe to 22 kOe as Fe content changes from 4% to 5%, but the change in coercivity is small as the concentration goes beyond 5%. The temperature dependence of resistivity shows that the samples with low Fe concentration (≤4%) are metallic, but the resistivity increases unexpectedly as the concentration reaches 5%, where the resistance increases with decreasing temperature below 300 K. First-principle calculations suggest that the observed magnetic properties can be understood as the consequences of competing ferromagnetic and antiferromagnetic exchange interactions between the interstitial atom and the rest of the MnBi lattice.
We report a large magnetoresistance observed in a Fe/MoS2/Fe tunnel junction, where iron electrodes are attached to the edges of MoS2 nanoribbon (i.e. the current is flowing in the plane of the MoS2 2D monolayer). Using non-equilibrium Green's functions in the framework of density functional approach, our calculations show a large magnetoresistance in Fe/MoS2/Fe junction, with the values up to 150%. The strong coupling between states of Mo atoms at the edge of the MoS2 monolayer and those at the Fe surface have a dramatic effect on the conductance property of the material as well as the MR of the Fe/MoS2/Fe tunnel junction. We conclude that the Fe electrodes polarize the spin states of MoS2 near the interface and efficiently inject carriers into MoS2. We find that the atomically-thin spacers are metallic due to a strong hybridization between the Fe and Mo states at the interface. MoS2 spacers of a larger width remain insulating. We also find that the magnetoresistance of thin MoS2 ribbons (three atomic layers wide) is negative and has a small value. Notably however, as the width of the MoS2 spacer increases, this value turns positive and increases in magnitude.
Theoretical and experimental evidence for a post-cotunnite phase transition in hafnia at high pressuresUsing first-principles density-functional theory (DFT) computations, we have predicted a new post-cotunnite (OII) phase of hafnia (HfO 2 ) at high pressures. Our computations, using the generalized gradient approximation (GGA), predict a phase transition from OII to a Fe 2 P-type structure at ~ 120 GPa (~ 140 GPa) with a slight volume collapse at the transition pressure of ~ 0.2 % (~ 0.1 %) between the two phases using the second-(third-) order Birch-Murnaghan equation of state, respectively. The prediction of the new phase is consistent with recent experiments and computations performed on similar dioxides titania (TiO 2 ) and zirconia (ZrO 2 ) at extreme pressure-temperature conditions. Importantly, our theoretical prediction for the OII → Fe 2 P transition in HfO 2 is experimentally supported by the re-analysis of Xray diffraction patterns of HfO 2 at extreme pressure-temperature (p, T) conditions. Additionally, the equation of state and hardness of the predicted phase have been computed and show that Fe 2 P-type phase while less compressible than the OII phase is nearly identical in hardness, indicating that none of the HfO 2 phases qualify as superhard.
We have studied the effects of vacancies on the structural, electronic and magnetic properties of zigzag-edged graphene nanoribbons (ZGNRs). Our calculations were carried out using an abinitio density functional pseudopotential computational method combined with the generalized gradient approximation for the exchange-correlation functional. The equilibrium geometries, electronic charge spin density distributions, electronic band structures, and magnetic moments were examined in the presence of single vacancy and double vacancies. Structural optimization showed that vacancies induce substantial structural changes in ZGNRs. We found that introducing vacancies into ZGNR changes the spatial distribution of neighbor atoms, particularly those located around the vacancies. Our calculations showed that the vacancies have significant effect on the magnetization of ZGNR. The calculations showed that the changes in the structural geometry, the electronic structure and the magnetization of ZGNR depend on the location of the vacancies with respect to the ribbon edges. These results suggest that vacancy defects can be used to modify the electronic and the magnetic properties of ZGNR.
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