Antiferromagnetic Configuration Research Articles

Proximity effects on the spin density waves in X/Cr(001) multilayers (X = Sn, V, and Mn)

We present ab initio density functional calculations of the electronic structure and magnetic properties of X2/Cr36(001) and X1/Cr37(001) multilayers, with X=Sn, V and Mn, to investigate the impact of the proximity effects of the X layers on the spin density waves of the Cr slab. We find different magnetic profiles corresponding to the spin density wave and to the layered antiferromagnetic configurations. The nature of the different magnetic solutions is discussed in terms of the different interfacial environments in the proximity of Sn, V or Mn. The magnetic behavior at the interface is discussed in connection with the electronic structure through the density of electronic states projected at the interfacial X and Cr sites. We compare the results with those previously obtained for Fe3/X1/Cr37/X1(001) multilayers to analyze the role played by the ferromagnetic iron slab.

Ground-state properties and high-pressure behavior of plutonium dioxide: Density functional theory calculations

Plutonium dioxide is of high technological importance in nuclear fuel cycle and is particularly crucial in long-term storage of Pu-based radioactive waste. Using first-principles density-functional theory, in this paper we systematically study the structural, electronic, mechanical, thermodynamic properties, and pressure-induced structural transition of ${\text{PuO}}_{2}$. To properly describe the strong correlation in $\text{Pu}\text{ }5f$ electrons, the local-density approximation $(\text{LDA})+U$ and the generalized gradient $\text{approximation}+U$ theoretical formalisms have been employed. We optimize $U$ parameter in calculating the total energy, lattice parameters, and bulk modulus at nonmagnetic, ferromagnetic, and antiferromagnetic configurations for both ground-state fluorite structure and high-pressure cotunnite structure. Best agreement with experiments is obtained by tuning the effective Hubbard parameter $U$ at around 4 eV within $\text{LDA}+U$ approach. After carefully testing the validity of the ground-state calculation, we further investigate the bonding nature, elastic constants, various moduli, Debye temperature, hardness, ideal tensile strength, and phonon dispersion for fluorite ${\text{PuO}}_{2}$. Some thermodynamic properties, e.g., Gibbs free energy, volume thermal expansion, and specific heat are also calculated. As for cotunnite phase, besides elastic constants, various moduli, and Debye temperature at 0 GPa, we have further presented our calculated electronic, structural, and magnetic properties for ${\text{PuO}}_{2}$ under pressure up to 280 GPa. A metallic transition at around 133 GPa and an isostructural transition in pressure range of 75--133 GPa are predicted. Additionally, as an illustration on the valency trend and subsequent effect on the mechanical properties, the calculated results for other actinide metal dioxides (${\text{ThO}}_{2}$, ${\text{UO}}_{2}$, and ${\text{NpO}}_{2}$) are also presented.

Magnetic properties in graphene-graphane superlattices

The magnetic properties of graphene-graphane superlattices with zigzag interfaces and separately varying widths are investigated by first-principles density functional calculations. It is demonstrated that magnetic states are energetically more stable than nonmagnetic ones due to the Stoner-type instability, and that antiferromagnetic spin configuration across the graphene strip is found to be the ground state, whereas the ferromagnetic ordering becomes metastable. The magnetic moments in the ferromagnetic states show a monotonic increase and saturation with the width of the graphene strip, independent of the graphane width due to the π-character of the spin densities. These findings point toward an alternative route for graphene-based applications without requiring physical cutting as in graphene nanoribbons.

First-principles theoretical studies of half-metallic ferromagnetism in CrTe

Using full-potential linear augmented plane-wave method (FP-LAPW) we have studied the stability and electronic properties of the chalcogenide CrTe in three competing structures: rocksalt (RS), zinc blende (ZB), and the NiAs-type (NA) hexagonal. Although the ground state is of NA structure, RS and ZB are interesting in that these fcc-based structures, which can perhaps be grown on various semiconductor substrates, exhibit half-metallic (HM) phases above some critical values of the lattice parameter. We find that the NA structure is not HM at its equilibrium volume while both ZB and RS structures are. The RS structure is more stable than the ZB with an energy that is lower by 0.25 eV/atom. While confirming previous results on the HM phase in ZB structure, we provide hitherto unreported results on the HM RS phase, with a gap in the minority channel and a magnetic moment of $4.0\text{ }{\ensuremath{\mu}}_{B}/\text{f}\text{.u}\text{.}$ A comparison of total energies for the ferromagnetic (FM), nonmagnetic, and antiferromagnetic (AFM) configurations shows the lowest energy configuration to be FM for CrTe in all the three structures. The exchange interactions in the RS and ZB structures are studied for a wide range of the lattice parameter using the linear-response method and a mapping of the total energy to the classical Heisenberg model. These linear-response calculations are performed in the linear muffin-tin orbitals (LMTOs) basis, using the atomic sphere approximation (ASA). We have verified that the results of the electronic structure obtained via the LMTO-ASA method under local-density approximation (LDA) and $\text{LDA}+U$ schemes are in close agreement with those obtained via the more accurate FP-LAPW method. The results show that the exchange interactions in the RS structure are much more short ranged than in the ZB structure. Hence, for the RS structure the exchange interactions are also studied by using a nearest- and next-nearest-neighbor $({J}_{1}\text{\ensuremath{-}}{J}_{2})$ model and the energy differences between FM and two AFM states. These ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ model results are obtained by using both the FP-LAPW and LMTO-ASA methods and compared with the linear-response results. The calculated Curie temperatures for the RS phase are consistently higher than those for the ZB phase.

Structural and Electronic Properties of Li-Ion Battery Cathode Material FeF3

A new type of cathode materials for Li-ion batteries has been explored recently based on FeF3 in an attempt to raise the energy density and discharge voltage. In this work, the structural and electronic properties of cathode materials FeF3 for lithium ion batteries have been studied by the first-principles calculations within both the generalized gradient approximation (GGA) and GGA+U frameworks. Our results show that the antiferromagnetic configuration of FeF3 is more stable than the ferromagnetic one, which is consistent with experiments. An analysis of the electronic density of states shows that FeF3 is a Mott−Hubbard insulator with a large d−d type band gap. Additionally, a small spin polarization was found on F, consistent with a fluorin-mediated superexchange mechanism for the Fe−Fe magnetic interaction.

Structural, electronic, and thermodynamic properties of UN: Systematic density functional calculations

A systematic first-principle study is performed to calculate the lattice parameters, electronic structure, and thermodynamic properties of UN using the local-density approximation (LDA)+ U and the generalized gradient approximation (GGA)+ U formalisms. To properly describe the strong correlation in the U 5 f electrons, we optimized the U parameter in calculating the total energy, lattice parameters, and bulk modulus at the nonmagnetic (NM), ferromagnetic (FM), and antiferromagnetic (AFM) configurations. Our results show that by choosing the Hubbard U around 2 eV within the GGA+ U approach, it is promising to correctly and consistently describe the above mentioned properties of UN. The localization behavior of 5 f electrons is found to be stronger than that of UC and our electronic analysis indicates that the effective charge of UN can be represented as U 1.71+N 1.71−. As for the thermodynamic study, the phonon dispersion illustrates the stability of UN and we further predict the lattice vibration energy, thermal expansion, and specific heat by utilizing the quasiharmonic approximation. Our calculated specific heat is well consistent with experiments.

Interplay of Fe and Nd magnetism in NdFeAsO single crystals

The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at T_S = 142 K and the onset of antiferromagnetic (AFM) Fe order below T_N = 137 K. The Fe moments order AFM in the well-known stripe-like structure in the (ab) plane, but change from AFM to ferromagnetic (FM) arrangement along the c direction below T* = 15 K accompanied by the onset of Nd AFM order below T_Nd = 6 K with this same AFM configuration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and PrFeAsO.

Origins of magnetism in transition metal doped CuI

Cupric iodide is a p-type semiconductor and has a large band gap. Doping of Mn, Co, and Ni are found to make γ-CuI ferromagnetic ground state, while Cr-doped and Fe-doped CuI systems are stabilized in antiferromagnetic configurations. The origins of the magnetic ordering are demonstrated successfully by the phenomenological band coupling model based on d-d level repulsions between the dopant ions. Furthermore, using a molecular-orbital bonding model, the electronic structures of the doped CuI are well understood. According to Heisenberg model, high-TC may be expected for CuI:Mn and CuI:Ni if there are no native defects or other impurities.

Magnetically induced ferroelectricity inCu2MnSnS4andCu2MnSnSe4

We investigate magnetically-induced ferroelectricity in Cu2MnSnS4 by means of Landau theory of phase transitions and of ab initio density functional theory. As expected from the Landau approach, ab initio calculations show that a non-zero ferroelectric polarization P along the y direction is induced by the peculiar antiferromagnetic configuration of Mn spins occurring in Cu2MnSnS4. The comparison between P, calculated either via density-functional-theory or according to Landau approach, clearly shows that ferroelectricity is mainly driven by Heisenberg-exchange terms and only to a minor extent by relativistic terms. At variance with previous examples of collinear antiferromagnets with magnetically-induced ferroelectricity (such as AFM-E HoMnO3), the ionic displacements occurring upon magnetic ordering are very small, so that the exchange-striction mechanism (i.e. displacement of ions so as to minimize the magnetic coupling energy) is not effective here. Rather, the microscopic mechanism at the basis of polarization has mostly an electronic origin. In this framework, we propose the small magnetic moment at Cu sites induced by neighboring Mn magnetic moments to play a relevant role in inducing P. Finally, we investigate the effect of the anion by comparing Cu2MnSnSe4 and Cu2MnSnS4: Se-4p states, more delocalized compared to S-3p states, are able to better mediate the Mn-Mn interaction, in turn leading to a higher ferroelectric polarization in the Se-based compound.

Theoretical specific heat from spin wave in comparison with experimental results in Fe-oxide superconductors

Based on the magnetic order of Fe-oxide superconductors from experiments we apply the Heisenberg interaction model to analyze systematically the antiferromagnetic configuration with a spin-density-wave by a rigorous mathematical–physical derivation and to suggest analytic expressions for the internal energy and specific heat under long-wave conditions. The theoretical values show good agreement with experiments; the theoretical and experimental results are discussed in terms of the phonon contribution to the specific heat, so that the spin-density-wave theory can properly describe the physical characteristics of Fe-oxide superconductors.

Magnetic Properties and Relaxation of Vanadium Monolayer on Pd(001) Surface

We investigated the magnetism of vanadium monolayers on a Pd(001) surface. The electronic structure and the magnetic properties of the V/Pd(001) system were determined with the use of the full-potential linearized augmented plane-wave method within the general gradient approximation. Three magnetic configurations were studied: non-, ferro-, and antiferromagnetic. From the total energy calculations, we found that the V/Pd(001) system is the most stable in the antiferromagnetic configuration. The importance of relaxation on the magnetic properties of the systems was also studied. It was found that the Pd(001) surface covered with a V monolayer undergoes considerable relaxation in which the spacing between Pd layers increases in all three magnetic configurations. Contrary to the Pd interlayer spacing, the distance between the V overlayer and the topmost Pd layer is reduced. The interlayer spacing between the V overlayer and the Pd surface layer is the largest for the antiferromagnetic configuration. In the relaxed antiferromagnetic structure, the magnitude of the calculated magnetic moments on the V atoms was <TEX>$1.31\;{\mu}_B$</TEX>. The presence of the vanadium monolayer does not affect the paramagnetic properties of the Pd(001) surface.

3d 및 4d 전이금속과 Pd가 c(2×2) 합금을 이룬 단층의 자성에 대한 제일원리 연구

Pd과 3d(V, Ti) 및 4d(Mo, Ru, Rh) 전이금속이 c(<TEX>$2{\times}2$</TEX>) 합금을 이루고 있는 단층의 자성을 제일원리적 FLAPW 에너지띠 방법을 이용하여 연구하였다. 이들 합금과 비교를 위하여 Pd과 V, Ti 및 Mo, Ru, Rh 원자로만 이루어진 단층의 전자구조도 계산하였다. 3d 원소인 V와 Ti 만으로 이루어진 단층의 경우, V은 반강자성, Ti의 경우는 강자성상태가 안정적이었으나, 이들이 Pd와 c(<TEX>$2{\times}2$</TEX>) 합금을 이루었을 경우 모두 자기모멘트가 반대 방향을 가지는 준강자성 상태가 안정적이었다. 반면에 4d 원자인 Mo, Ru, Rh이 Pd와 c(<TEX>$2{\times}2$</TEX>) 합금을 이룰 경우에는 자기모멘트들이 같은 방향을 가졌다. 자기모멘트 값을 보면, Ru이나 Rh의 경우 순수단층이나 Pd과 합금을 이룬 경우 그리 큰 차이를 보이지 않았으나, Mo의 경우 <TEX>$0.02\;{\mu}_B$</TEX>에서 <TEX>$2.98\;{\mu}_B$</TEX>로 급격히 증가하였다. 합금을 이루는 두 원소사이의 전하이동은 전기음성도에 따르게 됨을 알았다. We investigated the electronic structure and magnetism of the (3d, 4d)-Pd alloyed c(<TEX>$2{\times}2$</TEX>) monolayer systems, by use of the FLAPW band method. For comparison, pure 3d- and 4d-transition metal monolayers are also considered. We found that the antiferromagnetic configuration of pure V monolayers is sustained in the V-Pd alloy system, while the Ti-Pd alloy system is changed to antiferromagnetic configuration from the ferromagnetic state in pure Ti monolayer. The 4d TM (Mo, Ru, Rh)-Pd monolayers are found to be stable in ferromagnetic configurations. The magnetic moments of Ru and Rh atoms in Ru-Pd and Rh-Pd systems are almost same with those of pure Ru and Rh monolayers, while the magnetic moment of Mo atom is increased to <TEX>$2.98\;{\mu}_B$</TEX> in Mo-Pd alloyed system from the value of Mo monolayer, <TEX>$0.02\;{\mu}_B$</TEX>.

Spin and structural transitions in AlFeO 3 and FeAlO 3 perovskite and post-perovskite

We extend the field of investigation of the perovskite to post-perovskite phase transition from the Earth's lowermost mantle with a study of the double substitution Mg + Si = Al + Fe. For this we use an advanced formalism within the density functional theory, the planar augmented wavefunction method, to investigate the perovskite and post-perovskite structures containing only Al and Fe 3+ as cations. We look in particular at their relative stability and at their magnetic properties. We distinguish two crystallographic cases: AlFeO 3 and FeAlO 3, corresponding respectively to two ordered cases: one with Fe and then one with Al in octahedral coordination. For each case we investigate several spin configurations. We observe that up to 90 GPa the FeAlO 3 perovskite structure, i.e. with Fe 3+ in the interoctahedral space, with antiferromagnetic configuration and large local magnetic moment is the most stable one. Beyond 90 GPa the post-perovskite structure of AlFeO 3, i.e. with Fe 3+ in octahedral coordination, with antiferromagnetic configuration and small local magnetic moment is the most stable one. The perovskite to post-perovskite phase transition at 90 GPa is associated with a site exchange that triggers a partial collapse of the magnetic moment. The local magnetic moment vanishes beyond 150 GPa in post-perovskite. Our calculations suggest that the presence of Al + Fe 3+ in perovskite/post-perovskite renders the phase transition sluggish, induces a large density jump at the transition and contributes into maintaining a residual magnetic spin down to the base of the mantle.

Field-induced spin reorientation and giant spin-lattice coupling inEuFe2As2

We have studied a EuFe2As2 single crystal by neutron diffraction under magnetic fields up to 3.5 T and temperatures down to 2 K. A field induced spin reorientation is observed in the presence of a magnetic field along both the a and c axes, respectively. Above critical field, the ground state antiferromagnetic configuration of Eu$^{2+}$ moments transforms into a ferromagnetic structure with moments along the applied field direction. The magnetic phase diagram for Eu magnetic sublattice in EuFe2As2 is presented. A considerable strain ($\sim$0.9%) is induced by the magnetic field, caused by the realignment of the twinning structure. Furthermore, the realignment of the twinning structure is found to be reversible with the rebound of magnetic field, which suggested the existence of magnetic shape-memory effect. The Eu moment ordering exhibits close relationship with the twinning structure. We argue that the Zeeman energy in combined with magnetic anisotropy energy is responsible for the observed spin-lattice coupling.

Magnetoelectric coupling in zigzag graphene nanoribbons

Zigzag graphene nanoribbons can have magnetic ground states with ferromagnetic, antiferromagnetic, or canted configurations, depending on carrier density. We show that an electric field directed across the ribbon alters the magnetic state, favoring antiferromagnetic configurations. This property can be used to prepare ribbons with a prescribed spin-orientation on a given edge.

The Half-Metallicity of Zigzag Graphene Nanoribbons with Asymmetric Edge Terminations

The spin-polarized electronic structure and half-metallicity of zigzag graphene nanoribbons (ZGNRs) with asymmetric edge terminations are investigated by using first-principles calculations. It is found that compared with symmetric hydrogen-terminated counterparts, such ZGNRs maintain a spin-polarized ground state with the anti-ferromagnetic configuration at opposite edges, but their energy bands are no longer spin degenerate. In particular, the energy gap of one spin orientation decreases remarkably. Consequently, the ground state of such ZGNRs is very close to half-metallic state, and thus a smaller critical electric field is required for the systems to achieve the half-metallic state. Moreover, two kinds of studied ZGNRs present massless Dirac-fermion band structure when they behave like half-metals.

Probing the magnetism of nanostructures buried in metallic surfaces and their possible utilization

AbstractWe discuss the possibility to probe magnetic properties of nanostructures buried beneath a metallic surface by means of local probe techniques. We prove, that those properties can be deduced from the spin‐resolved local density of states (LDOS) above the surface. In‐plane polarization maps in vacuum above the surface are shown to allow one to simultaneously detect electronic, magnetic, and even geometric properties of subsurface structures. It is argued, that the coupling of buried nanostructures to each other can be deduced from the symmetry of the polarization map. To underline the importance of studying buried nanostructures several possible applications of buried magnetic impurities are pointed out. The exchange coupling of an adatom to a nanostructure or a monolayer (ML) across a paramagnetic spacer is shown to oscillate with the thickness of the latter. This could provide one with reliable means to stabilize the spin of a magnetic adatom in either a ferromagnetic or an antiferromagnetic configuration with respect to the magnetic orientation of the monolayer or nanostructure. The possibility to tailor the exchange coupling between single magnetic impurities on a surface through the adjustment of the overlayer thickness and the interatomic separation in a dimer, is discussed.

Origin of antiferromagnetism in CoO: A density functional theory study

We have investigated the origin of antiferromagnetism of CoO in the rocksalt structure using spin-polarized density functional theory calculations. We find that in the rocksalt structure, the superexchange interaction between the occupied and unoccupied eg states plays the dominant role, which leads to an antiferromagnetic ground state, but the system also has a strong direct exchange interaction between the partially occupied minority spin t2g states that leads to the unusual situation that the ferromagnetic phase is more stable than most antiferromagnetic configurations.

High-pressure phase transitions and compressibility of wolframite-type tungstates

This paper reports an investigation on the phase diagram and compressibility of wolframite-type tungstates by means of x-ray powder diffraction and absorption in a diamond-anvil cell and ab initio calculations. X-ray diffraction experiments show that monoclinic wolframite-type MgWO4 suffers at least two phase transitions, the first one being to a triclinic polymorph with a structure similar to that of CuWO4 and FeMoO4-II. The onset of each transition is detected at 17.1 and 31 GPa. In ZnWO4 the onset of the monoclinic-triclinic transition has been also found at 16.7 GPa. This transition does not involve any change in the atomic coordination as confirmed by x-ray absorption measurements. These findings are supported by density-functional theory calculations, which predict the occurrence of additional transitions upon further compression. Calculations have been also performed for wolframite-type MnWO4, which is found to have an antiferromagnetic configuration. In addition, our study reveals details of the local-atomic compression in MgWO4 and ZnWO4. In particular, below the transition pressure the ZnO6 and equivalent polyhedra tend to become more regular, whereas, the WO6 octahedra remain almost unchanged. Fitting the pressure-volume data we obtained the equation of state for the low-pressure phase of MgWO4 and ZnWO4. These and previous results on MnWO4 and CdWO4 are compared with the calculations. The compressibility of wolframite-type tungstates is also systematically discussed. Finally Raman spectroscopy measurements and lattice dynamics calculations are presented for MgWO4.

First-principles study of magnetic properties ofL10-ordered MnPt and FePt alloys

Using first-principles methods, we study the magnetic and electronic properties of three different spin configurations of the $\text{L}{1}_{0}$ phase of FePt and MnPt alloys. It is found that MnPt and FePt may be approximately considered as magnetic antipodes with opposite ferromagnetic (FM)---antiferromagnetic (AFM) and in-plane/out-of-plane magnetocrystalline anisotropy (MCA) relationships. In MnPt, the most stable phase is the AFM configuration with AFM chessboard spin coupling in the (001) plane, FM spin coupling between (001) planes, and all spin directions aligned in the (001) plane. Whereas in FePt, the most stable is the FM configuration with all spin directions aligned perpendicular to (001) plane. The out-of-plane MCA of MnPt is more than an order of magnitude less $(\ensuremath{\sim}0.1\text{ }\text{meV})$ than that of FePt $(\ensuremath{\sim}2.9\text{ }\text{meV})$ in their corresponding magnetic ground states. Our calculations indicate that an AFM state can be achieved in FePt by a small variation in tetragonality ratio (from 0.98 to 0.94). A pseudogap is observed at the Fermi energy for MnPt and just below the Fermi energy for FePt for the chessboard AFM model. This pseudogap may explain the ground-state magnetic configuration of MnPt.