Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userFirst-principles study of the inversion thermodynamics and electronic structure of Fe$M_2X_4$ (thio)spinels ($M=$ Cr, Mn, Co, Ni; $X=$ O, S)
118
0
0.0
(
0
)
Added by
David Santos-Carballal
Publication date
2015
fields
Physics
and research's language is
English
Authors
David Santos-Carballal
Ask ChatGPT about the research
No Arabic abstract
Fe$M_2X_4$ spinels, where $M$ is a transition metal and $X$ is oxygen or sulfur, are candidate materials for spin filters, one of the key devices in spintronics. We present here a computational study of the inversion thermodynamics and the electronic structure of these (thio)spinels for $M=$ Cr, Mn, Co, Ni, using calculations based on the density functional theory with on-site Hubbard corrections (DFT+$U$). The analysis of the configurational free energies shows that different behaviour is expected for the equilibrium cation distributions in these structures: FeCr$_2X_4$ and FeMn$_2$S$_4$ are fully normal, FeNi$_2X_4$ and FeCo$_2$S$_4$ are intermediate, and FeCo$_2$O$_4$ and FeMn$_2$O$_4$ are fully inverted. We have analyzed the role played by the size of the ions and by the crystal field stabilization effects in determining the equilibrium inversion degree. We also discuss how the electronic and magnetic structure of these spinels is modified by the degree of inversion, assuming that this could be varied from the equilibrium value. We have obtained electronic densities of states for the completely normal and completely inverse cation distribution of each compound. FeCr$_2X_4$, FeMn$_2X_4$, FeCo$_2$O$_4$ and FeNi$_2$O$_4$ are half-metals in the ferrimagnetic state when Fe is in tetrahedral positions. When $M$ is filling the tetrahedral positions, the Cr-containing compounds and FeMn$_2$O$_4$ are half-metallic systems, while the Co and Ni spinels are insulators. The Co and Ni sulfide counterparts are metallic for any inversion degree together with the inverse FeMn$_2$S$_4$. Our calculations suggest that the spin filtering properties of the Fe$M_2X_4$ (thio)spinels could be modified via the control of the cation distribution through variations in the synthesis conditions.
rate research
Read More
The electronic structure of Li-doped Ni$_{1-x}$Fe$_x$O has been investigated using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The Ni $2p$ core-level PES and XAS spectra were not changed by Li doping. In contrast, the Fe$^{3+}$ intensity increased with Li doping relative to the Fe$^{2+}$ intensity. However, the increase of Fe$^{3+}$ is only $sim 5%$ of the doped Li content, suggesting that most of the doped holes enter the O $2p$ and/or the charge-transferred configuration Ni $3d^8underline{L}$. The Fe 3d partial density of states and the host valence-band emission near valence-band maximum increased with Li content, consistent with the increase of electrical conductivity. Based on these findings, percolation of bound magnetic polarons is proposed as an origin of the ferromagnetic behavior.
Substantial amounts of the transition metals Mn, Fe, Co, and Ni can be substituted for Li in single crystalline Li$_2$(Li$_{1-x}T_x$)N. Isothermal and temperature-dependent magnetization measurements reveal local magnetic moments with magnitudes significantly exceeding the spin-only value. The additional contributions stem from unquenched orbital moments that lead to rare-earth-like behavior of the magnetic properties. Accordingly, extremely large magnetic anisotropies have been found. Most notably, the magnetic anisotropy alternates as easy-plane $rightarrow$ easy-axis $rightarrow$ easy-plane $rightarrow$ easy-axis when progressing from $T$ = Mn $rightarrow$ Fe $rightarrow$ Co $rightarrow$ Ni. This behavior can be understood based on a perturbation approach in an analytical, single-ion model. The calculated magnetic anisotropies show a surprisingly good agreement with the experiment and capture the basic features observed for the different transition metals.
Using an elastic neutron scattering technique under a pulsed magnetic field up to 30 T, we determined the magnetic structure in the half-magnetization plateau phase in the spinel CdCr$_2$O$_4$. The magnetic structure has a cubic $P4_3$32 symmetry, which is the same as that observed in HgCr$_2$O$_4$. This suggests that there is a universal field induced spin-lattice coupling mechanism at work in the Cr-based spinels.
In view of the recent experimental predictions of a weak structural transition in CoV$_{2}$O$_{4}$ we explore the possible orbital order states in its low temperature tetragonal phases from first principles density functional theory calculations. We observe that the tetragonal phase with I4$_1/amd$ symmetry is associated with an orbital order involving complex orbitals with a reasonably large orbital moment at Vanadium sites while in the phase with I4$_1/a$ symmetry, the real orbitals with quenched orbital moment constitute the orbital order. Further, to study the competition between orbital order and electron itinerancy we considered Mn$_{0.5}$Co$_{0.5}$V$_{2}$O$_{4}$ as one of the parent compounds, CoV$_{2}$O$_{4}$, lies near itinerant limit while the other, MnV$_{2}$O$_{4}$, lies deep inside the orbitally ordered insulating regime. Orbital order and electron transport have been investigated using first principles density functional theory and Boltzmann transport theory in CoV$_{2}$O$_{4}$, MnV$_{2}$O$_{4}$ and Mn$_{0.5}$Co$_{0.5}$V$_{2}$O$_{4}$. Our results show that as we go from MnV$_{2}$O$_{4}$ to CoV$_{2}$O$_{4}$ there is enhancement in the electrons itinerancy while the nature of orbital order remains unchanged.
First-principle calculations with different exchange-correlation functionals, including LDA, PBE and vdW-DF functional in form of optB88-vdW, have been performed to investigate the electronic and elastic properties of two dimensional transition metal dichalcogenides(TMDCs) with the formula of MX$_2$(M=Mo,W; X=O,S,Se,Te) in both monolayer and bilayer structures. The calculated band structures show a direct band gap for monolayer TMDCs at the K point except for MoO$_2$ and WO$_2$. When the monolayers are stacked into bilayer, the reduced indirect band gaps are found except for bilayer WTe$_2$, in which direct gap is still present at the K point. The calculated in-plane Young moduli are comparable to graphene, which promises the possible application of TMDCs in future flexible and stretchable electronic devices. We also evaluated the performance of different functionals including LDA, PBE, and optB88-vdW in describing elastic moduli of TMDCs and found that LDA seems to be the most qualified method. Moreover, our calculations suggest that the Young moduli for bilayers are insensitive to stacking orders and the mechanical coupling between monolayers seems to be negligible.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
