Showing papers on "Antiferromagnetism published in 2012"
TL;DR: It is shown that antiferromagnetic nanostructures, composed of just a few Fe atoms on a surface, exhibit two magnetic states, the Néel states, that are stable for hours at low temperature.
Abstract: Control of magnetism on the atomic scale is becoming essential as data storage devices are miniaturized. We show that antiferromagnetic nanostructures, composed of just a few Fe atoms on a surface, exhibit two magnetic states, the Neel states, that are stable for hours at low temperature. For the smallest structures, we observed transitions between Neel states due to quantum tunneling of magnetization. We sensed the magnetic states of the designed structures using spin-polarized tunneling and switched between them electrically with nanosecond speed. Tailoring the properties of neighboring antiferromagnetic nanostructures enables a low-temperature demonstration of dense nonvolatile storage of information.
553 citations
TL;DR: It is demonstrated how interfacial interactions can induce a complex magnetic structure in a non-magnetic material and specifically show that exchange bias can unexpectedly emerge in heterostructures consisting of paramagnetic LaNiO3 (LNO) and ferromagnetic LaMnO 3 (LMO).
Abstract: Interfaces between insulating oxides have revealed exotic electronic and magnetic properties. It is now shown that a complex magnetic structure can emerge in an oxide superlattice, and that specific interfaces can unexpectedly exhibit exchange bias. The observations reveal the induction of antiferromagnetism in a material that is usually paramagnetic.
379 citations
TL;DR: A density-matrix renormalization group (DMRG) study of the S=1/2 Heisenberg antiferromagnet on the kagome lattice is performed to identify the conjectured spin liquid ground state, ruling out gapless, chiral, or nontopological spin liquids in favor of a topological spin liquid of quantum dimension 2.
Abstract: We perform a density-matrix renormalization group (DMRG) study of the S=1/2 Heisenberg antiferromagnet on the kagome lattice to identify the conjectured spin liquid ground state. Exploiting SU(2) spin symmetry, which allows us to keep up to 16,000 DMRG states, we consider cylinders with circumferences up to 17 lattice spacings and find a spin liquid ground state with an estimated per site energy of -0.4386(5), a spin gap of 0.13(1), very short-range decay in spin, dimer and chiral correlation functions, and finite topological entanglement γ consistent with γ=log(2)2, ruling out gapless, chiral, or nontopological spin liquids in favor of a topological spin liquid of quantum dimension 2, with strong evidence for a gapped topological Z(2) spin liquid.
370 citations
TL;DR: In this article, the authors used resonant inelastic x-ray scattering to reveal the nature of magnetic interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit entangled ground state and J(eff)=1/2 magnetic moments.
Abstract: We used resonant inelastic x-ray scattering to reveal the nature of magnetic interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit entangled ground state and J(eff)=1/2 magnetic moments. The magnon dispersion in Sr2IrO4 is well-described by an antiferromagnetic Heisenberg model with an effective spin one-half on a square lattice, which renders the low-energy effective physics of Sr2IrO4 much akin to that in superconducting cuprates. This point is further supported by the observation of exciton modes in Sr2IrO4, whose dispersion is strongly renormalized by magnons, which can be understood by analogy to hole propagation in the background of antiferromagnetically ordered spins in the cuprates.
360 citations
TL;DR: The synthesis of Li(x)( NH(2))(y)(NH(3))(1-y)Fe(2)Se(2), with lithium ions, lithium amide and ammonia acting as the spacer layer between FeSe layers, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far.
Abstract: The recent discovery of high temperature superconductivity in a layered iron arsenide has led to an intensive search to optimize the superconducting properties of iron-based superconductors by changing the chemical composition of the spacer layer that is inserted between adjacent anionic iron arsenide layers. Until now, superconductivity has only been found in compounds with a cationic spacer layer consisting of metal ions: Li+, Na+, K+, Ba2+ or a PbO-type or perovskite-type oxide layer. Electronic doping is usually necessary to control the fine balance between antiferromagnetism and superconductivity. Superconductivity has also been reported in FeSe, which contains neutral layers similar in structure to those found in the iron arsenides but without the spacer layer. Here we demonstrate the synthesis of Lix(NH2)y(NH3)1-yFe2Se2 (x ~0.6 ; y ~ 0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far and four times higher at ambient pressure than the transition temperature, Tc, of the parent Fe1.01Se. We have determined the crystal structure using neutron powder diffraction and used magnetometry and muon-spin rotation data to determine the superconducting properties. This new synthetic route opens up the possibility of further exploitation of related molecular intercalations in this and other systems in order to greatly optimize the superconducting properties in this family.
300 citations
TL;DR: In this article, a spin liquid ground state in a wide region of the phase diagram, between a semi-metal (SM) and an antiferromagnetic insulator (AFMI), was found.
Abstract: A spin liquid is a novel quantum state of matter with no conventional order parameter where a finite charge gap exists even though the band theory would predict metallic behavior. Finding a stable spin liquid in two or higher spatial dimensions is one of the most challenging and debated issues in condensed matter physics. Very recently, it has been reported that a model of graphene, i.e., the Hubbard model on the honeycomb lattice, can show a spin liquid ground state in a wide region of the phase diagram, between a semi-metal (SM) and an antiferromagnetic insulator (AFMI). Here, by performing numerically exact quantum Monte Carlo simulations, we extend the previous study to much larger clusters (containing up to 2592 sites), and find, if any, a very weak evidence of this spin liquid region. Instead, our calculations strongly indicate a direct and continuous quantum phase transition between SM and AFMI.
246 citations
TL;DR: In this article, the existence of edge zero modes in spin chains with Z-n symmetry has been studied in terms of parafermions, generalizations of fermions to the Z n case.
Abstract: A sign of topological order in a gapped one-dimensional quantum chain is the existence of edge zero modes. These occur in the Z_2-invariant Ising/Majorana chain, where they can be understood using free-fermion techniques. Here I discuss their presence in spin chains with Z_n symmetry, and prove that for appropriate coupling they are exact, even in this strongly interacting system. These modes are naturally expressed in terms of parafermions, generalizations of fermions to the Z_n case. I show that parafermionic edge zero modes do not occur in the usual ferromagnetic and antiferromagnetic cases, but rather only when the interactions are chiral, so that spatial-parity and time-reversal symmetries are broken.
239 citations
TL;DR: In this article, the magnetic structure and magnetization dynamics of systems of plane frustrated Ising chain lattices are reviewed for three groups of compounds:,,, and. The available experimental data are analyzed and compared in detail.
Abstract: The magnetic structure and magnetization dynamics of systems of plane frustrated Ising chain lattices are reviewed for three groups of compounds: , , and . The available experimental data are analyzed and compared in detail. It is shown that a high-temperature magnetic phase on a triangle lattice is normally and universally a partially disordered antiferromagnetic (PDA) structure. The diversity of low-temperature phases results from weak interactions that lift the degeneracy of a 2D antiferromagnetic Ising model on the triangle lattice. Mean-field models, Monte Carlo simulation results on the static magnetization curve, and results on slow magnetization dynamics obtained with Glauber's theory are discussed in detail.
193 citations
TL;DR: Kapellasite, Cu3Zn(OH)6Cl2, a geometrically frustrated spin-1/2 kagome antiferromagnet polymorphic with herbertsmithite, is a gapless spin liquid showing unusual dynamic short-range correlations of noncoplanar cuboc2 type which persist down to 20 mK.
Abstract: Magnetic susceptibility, NMR, muon spin relaxation, and inelastic neutron scattering measurements show that kapellasite, Cu3Zn(OH)6Cl2, a geometrically frustrated spin-1/2 kagome antiferromagnet polymorphic with herbertsmithite, is a gapless spin liquid showing unusual dynamic short-range correlations of noncoplanar cuboc2 type which persist down to 20 mK. The Hamiltonian is determined from a fit of a high-temperature series expansion to bulk susceptibility data and possesses competing exchange interactions. The magnetic specific heat calculated from these exchange couplings is in good agreement with experiment. The temperature dependence of the magnetic structure factor and the muon relaxation rate are calculated in a Schwinger-boson approach and compared to experimental results.
176 citations
TL;DR: This work investigates the exchange coupling between perpendicular anisotropy Co/Pt and IrMn in-plane antiferromagnets (AFMs), as well as tunneling anisotropic magnetoresistance (TAMR) in [Pt/Co]/IrMn/AlO_{x}, and finds that TAMR gets significantly enhanced up to room temperature.
Abstract: We investigate the exchange coupling between perpendicular anisotropy (PMA) Co/Pt and IrMn in-plane antiferromagnets (AFMs), as well as tunneling anisotropic magnetoresistance (TAMR) in [Pt/Co]/IrMn/AlO_{x}/Pt tunnel junctions, where Co/Pt magnetization drives rotation of AFM moments with the formation of exchange-spring twisting. When coupled with a PMA ferromagnet, the AFM moments partially rotate with out-of-plane magnetic fields, in contrast with being pinned along the easy direction of IrMn for in-plane fields. Because of the superior thermal tolerance of perpendicular exchange coupling and the stability of moments in ~6 nm-thick IrMn, TAMR gets significantly enhanced up to room temperature. Their use would advance the process towards practical AFM spintronics.
169 citations
TL;DR: Based on first-principles calculations, it is found that the fully saturated silicene exhibits nonmagnetic semiconducting behavior, while half-saturation on only one side with hydrogen or bromine results in the localized and unpaired electrons of the unsaturated Si atoms, showing ferromagnetic semiconducted or half-metallic properties, respectively.
Abstract: Based on first-principles calculations, we study the structural, electronic, and magnetic properties of two-dimensional silicene saturated with hydrogen and bromine atoms. It is found that the fully saturated silicene exhibits nonmagnetic semiconducting behavior, while half-saturation on only one side with hydrogen or bromine results in the localized and unpaired electrons of the unsaturated Si atoms, showing ferromagnetic semiconducting or half-metallic properties, respectively. Total energy calculations show that the half-hydrogenated silicene exhibits a ferromagnetic order, while the half-brominated one exhibits an antiferromagnetic behavior.
TL;DR: In this article, angle-resolved photoemission spectroscopy was used to reveal the momentum distribution of the superconducting gap in FeTe1-xSex, which has the simplest structure of all Fe-based superconductors.
Abstract: We used angle-resolved photoemission spectroscopy to reveal directly the momentum distribution of the superconducting gap in FeTe1-xSex, which has the simplest structure of all Fe-based superconductors. We found isotropic superconducting gaps on all Fermi surfaces whose sizes can be fitted by a single gap function derived from a strong coupling approach, promoting local antiferromagnetic exchange interactions as a serious candidate for the pairing origin.
TL;DR: The results show that Bi substitutional sites are energetically more favorable than interstitial sites for single impurities, and the calculated magnetic coupling suggests that Cr doped Bi2Se3 is possible to be both ferromagnetic and insulating, while Fe dopedBi2 Se3 tends to be weakly antiferromagnetic.
Abstract: We theoretically investigate the possibility of establishing ferromagnetism in the topological insulator Bi2Se3 via magnetic doping of 3d transition metal elements. The formation energies, charge states, band structures, and magnetic properties of doped Bi2Se3 are studied using first-principles calculations within density functional theory. Our results show that Bi substitutional sites are energetically more favorable than interstitial sites for single impurities. Detailed electronic structure analysis reveals that Cr and Fe doped materials are still insulating in the bulk but the intrinsic band gap of Bi2Se3 is substantially reduced due to the strong hybridization between the d states of the dopants and the p states of the neighboring Se atoms. The calculated magnetic coupling suggests that Cr doped Bi2Se3 is possible to be both ferromagnetic and insulating, while Fe doped Bi2Se3 tends to be weakly antiferromagnetic. DOI: 10.1103/PhysRevLett.109.266405
TL;DR: In this article, the density functional theory DFT+U calculations are used to investigate α-MnO2, a structure containing a framework of corner and edge sharing MnO6 octahedra with tunnels in between.
TL;DR: This work has shown that antiphase coupling gives rise to plasmonic "antiferromagnetic" behavior in multiple repeated heptamer structures, supporting the propagation of low-loss magnetic plasmons in this new waveguide geometry.
Abstract: The plasmonic properties of coupled metallic nanostructures are understood through the analogy between their collective plasmon modes and the electronic orbitals of corresponding molecules. Here we expand this analogy to planar arrangements of plasmonic nanostructures whose magnetic plasmons directly resemble the delocalized orbitals of aromatic hydrocarbon molecules. The heptamer structure serves as a benzene-like building block for a family of plasmonic artificial aromatic analogs with fused ring structures. Antiphase magnetic plasmons are excited in adjacent fused heptamer units, which for a linear multiheptamer structure is a behavior controlled by the number of units in the structure. This antiphase coupling gives rise to plasmonic "antiferromagnetic" behavior in multiple repeated heptamer structures, supporting the propagation of low-loss magnetic plasmons in this new waveguide geometry.
TL;DR: In this article, the elastic properties of the iron-based superconductor Ba(Fe 1- x Co x ) 2 As 2 with eight Co concentrations were investigated and the results are similar to those of unconventional superconductors, where the properties are governed by quantum fluctuations associated with the zero-temperature critical point of long-range order.
Abstract: We investigated the elastic properties of the iron-based superconductor Ba(Fe 1- x Co x ) 2 As 2 with eight Co concentrations. The elastic constant C 66 shows a large elastic softening associated with structural phase transition. C 66 was analyzed on the basis of the localized and itinerant pictures of Fe-3d electrons, which shows a strong electron–lattice coupling and a possible mass enhancement in this system. The results are similar to those of unconventional superconductors, where the properties of the system are governed by quantum fluctuations associated with the zero-temperature critical point of long-range order, namely, the quantum critical point (QCP). In this system, the inverse of C 66 behaves just like the magnetic susceptibility in magnetic QCP systems. Although the QCPs of these existing superconductors are all ascribed to antiferromagnetism, our systematic studies on the canonical iron-based superconductor Ba(Fe 1- x Co x ) 2 As 2 have revealed that there is a signature of “structural quan...
TL;DR: The synthesis and characterization of LiZn(2)Mo(3)O(8), a geometrically frustrated antiferromagnet in which the magnetic moments are localized on small transition-metal clusters rather than individual ions is reported, demonstrating a new approach for unparalleled chemical control and tunability in the search for collective, emergent electronic states of matter.
Abstract: The emergence of complex electronic behaviour from simple ingredients has resulted in the discovery of numerous states of matter. Many examples are found in systems exhibiting geometric magnetic frustration, which prevents simultaneous satisfaction of all magnetic interactions. This frustration gives rise to complex magnetic properties such as chiral spin structures, orbitally driven magnetism, spin-ice behaviour exhibiting Dirac strings with magnetic monopoles, valence-bond solids and spin liquids. Here we report the synthesis and characterization of LiZn(2)Mo(3)O(8), a geometrically frustrated antiferromagnet in which the magnetic moments are localized on small transition-metal clusters rather than individual ions. By doing so, first-order Jahn-Teller instabilities and orbital ordering are prevented, allowing the strongly interacting magnetic clusters in LiZn(2)Mo(3)O(8) to probably give rise to an exotic condensed valence-bond ground state reminiscent of the proposed resonating valence-bond state. Our results also link magnetism on clusters to geometric magnetic frustration in extended solids, demonstrating a new approach for unparalleled chemical control and tunability in the search for collective, emergent electronic states of matter.
TL;DR: In this paper, a large change in the magnetization is observed around 370°C which is close to the Neel temperature (TN) of parent compound and another magnetic transition is also observed near 600°C.
TL;DR: Density-functional theory calculations enabled this spin-reorientation transition to be accurately reproduced for both FeRh films across the AFM-FM phase transition and show that these results are due to differences in strain.
Abstract: A spin reorientation accompanying the temperature-induced antiferromagnetic (AFM) to ferromagnetic (FM) phase transition is reported in strained epitaxial FeRh thin films. $^{57}\mathrm{Fe}$ conversion electron M\"ossbauer spectrometry showed that the Fe moments have different orientations in FeRh grown on thick single-crystalline MgO and in FeRh grown on ion-beam-assist-deposited (IBAD) MgO. It was also observed, in both samples, that the Fe moments switch orientations at the AFM to FM phase transition. Perpendicular anisotropy was evidenced in the AFM phase of the film grown on IBAD MgO and in the FM phase of that grown on regular MgO. Density-functional theory calculations enabled this spin-reorientation transition to be accurately reproduced for both FeRh films across the AFM-FM phase transition and show that these results are due to differences in strain.
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TL;DR: It is shown that structural vortices induce magnetic ones and that, consistent with a recent experimental report, ferroelectric domain walls can carry a magnetic moment.
Abstract: Topological defects in ordered states with spontaneously broken symmetry often have unusual physical properties, such as fractional electric charge or a quantised magnetic field-flux, originating from their non-trivial topology. Coupled topological defects in systems with several coexisting orders give rise to unconventional functionalities, such as the electric-field control of magnetisation in multiferroics resulting from the coupling between the ferroelectric and ferromagnetic domain walls. Hexagonal manganites provide an extra degree of freedom: In these materials, both ferroelectricity and magnetism are coupled to an additional, non-ferroelectric structural order parameter. Here we present a theoretical study of topological defects in hexagonal manganites based on Landau theory with parameters determined from first-principles calculations. We explain the observed flip of electric polarisation at the boundaries of structural domains, the origin of the observed discrete vortices, and the clamping between ferroelectric and antiferromagnetic domain walls. We show that structural vortices induce magnetic ones and that, consistent with a recent experimental report, ferroelectric domain walls can carry a magnetic moment.
TL;DR: In this article, a direct bridge connecting antiferromagnetic exchange interactions determined in the parent compounds of these materials to the superconducting gap functions observed in the corresponding super-conducting materials is shown.
Abstract: Cuprates, ferropnictides and ferrochalcogenides are three classes of unconventional high temperature superconductors, who share similar phase diagrams in which superconductivity develops after a magnetic order is suppressed, suggesting a strong interplay between superconductivity and magnetism, although the exact picture of this interplay remains elusive. Here we show that there is a direct bridge connecting antiferromagnetic exchange interactions determined in the parent compounds of these materials to the superconducting gap functions observed in the corresponding superconducting materials: in all high temperature superconductors, the Fermi surface topology matches the form factor of the pairing symmetry favored by local magnetic exchange interactions. We suggest that this match offers a principle guide to search for new high temperature superconductors.
TL;DR: The structural, optical and magnetic properties of the Zn1−xMnxO (0, <,x,<,0.05) thin films synthesized by solgel technique have been analyzed in the light of modification of the electronic structure and disorder developed in the samples due to Mn doping as mentioned in this paper.
TL;DR: It is shown that the spin-triplet supercurrent is enhanced up to 20 times after the authors' samples are subject to a large in-plane field, because the synthetic antiferromagnet undergoes a "spin-flop" transition.
Abstract: We have observed long-range spin-triplet supercurrents in Josephson junctions containing ferromagnetic (F) materials, which are generated by noncollinear magnetizations between a central Co=Ru=Co synthetic antiferromagnet and two outer thin F layers. Here we show that the spin-triplet supercurrent is enhanced up to 20 times after our samples are subject to a large in-plane field. This occurs because the synthetic antiferromagnet undergoes a ‘‘spin-flop’’ transition, whereby the two Co layer magnetizations end up nearly perpendicular to the magnetizations of the two thin F layers. We report direct experimental evidence for the spin-flop transition from scanning electron microscopy with polarization analysis and from spin-polarized neutron reflectometry. These results represent a first step toward experimental control of spin-triplet supercurrents.
TL;DR: It is shown that joint spin-orbital excitations in an ordered phase with coexisting antiferromagnetic and alternating orbital order introduce topological constraints for the hole propagation and will thus radically modify the transport properties in doped Mott insulators where hole motion implies simultaneous spin and orbital excitations.
Abstract: The concept of spin-orbital entanglement on superexchange bonds in transition metal oxides is introduced and explained on several examples. It is shown that spin-orbital entanglement in superexchange models destabilizes the long-range (spin and orbital) order and may lead either to a disordered spin-liquid state or to novel phases at low temperature which arise from strongly frustrated interactions. Such novel ground states cannot be described within the conventionally used mean field theory which separates spin and orbital degrees of freedom. Even in cases where the ground states are disentangled, spin-orbital entanglement occurs in excited states and may become crucial for a correct description of physical properties at finite temperature. As an important example of this behaviour we present spin-orbital entanglement in the $R$VO$_3$ perovskites, with $R$=La,Pr,...,Yb,Lu, where such finite temperature properties of these compounds can be understood only using entangled states: ($i$) thermal evolution of the optical spectral weights, ($ii$) the dependence of transition temperatures for the onset of orbital and magnetic order on the ionic radius in the phase diagram of the $R$VO$_3$ perovskites, and ($iii$) dimerization observed in the magnon spectra for the $C$-type antiferromagnetic phase of YVO$_3$. Finally, it is shown that joint spin-orbital excitations in an ordered phase with coexisting antiferromagnetic and alternating orbital order introduces topological constraints for the hole propagation and will thus radically modify transport properties in doped Mott insulators where hole motion implies simultaneous spin and orbital excitations.
TL;DR: Electronic and magnetic phase diagram of KxFe2−ySe2 system as a function of Fe valence is reported, finding a superconducting phase sandwiched between two AFM insulating phases.
Abstract: The correlation and competition between antiferromagnetism and superconductivity are one of the most fundamental issues in high temperature superconductors. Superconductivity in high temperature cuprate superconductors arises from suppressing an antiferromagnetic (AFM) Mott insulator1 while in iron-pnictide superconductors arises from AFM semimetals and can coexist with AFM orders2,3,4,5,6,7,8,9. This difference raises many intriguing debates on the relation between the two classes of high temperature superconductors. Recently, superconductivity at 32 K has been reported in iron-chalcogenide superconductors AxFe2−ySe2 (A = K, Rb and Cs)10,11,12. They have the same structure as that of iron-pnictide 122-system13,14,15. Here, we report electronic and magnetic phase diagram of KxFe2−ySe2 system as a function of Fe valence. We find a superconducting phase sandwiched between two AFM insulating phases. The two insulating phases are characterized by two distinct superstructures caused by Fe vacancy orders with modulation wave vectors of q1 = (1/5, 3/5, 0) and q2 = (1/4, 3/4, 0), respectively.
TL;DR: The canted antiferromagnetic insulating phase in ultra thin films of n<10 coincides with the occurrence of a higher symmetry structural phase with a different oxygen octahedra rotation pattern, which is an interesting candidate for an insulating tunneling barrier in room temperature spin polarized tunneling devices.
Abstract: We present a study of the thickness dependence of magnetism and electrical conductivity in ultrathin La0.67Sr0.33MnO3 films grown on SrTiO3 (110) substrates. We found a critical thickness of 10 unit cells below which the conductivity of the films disappeared and simultaneously the Curie temperature increased, indicating a magnetic insulating phase at room temperature. These samples have a Curie temperature of about 560 K with a significant saturation magnetization of 1.2±0.2μB/Mn. The canted antiferromagnetic insulating phase in ultra thin films of n<10 coincides with the occurrence of a higher symmetry structural phase with a different oxygen octahedra rotation pattern. Such a strain engineered phase is an interesting candidate for an insulating tunneling barrier in room temperature spin polarized tunneling devices.
TL;DR: In this article, powder neutron diffraction and inelastic scattering measurements of frustrated pyrochlore Nd 2 Ir 2 O 7, which exhibits a metal-insulator transition at a temperature T MI of 33 K.
Abstract: In this study, we performed powder neutron diffraction and inelastic scattering measurements of frustrated pyrochlore Nd 2 Ir 2 O 7 , which exhibits a metal–insulator transition at a temperature T MI of 33 K. The diffraction measurements revealed that the pyrochlore has an antiferromagnetic long-range structure with propagation vector q 0 of (0,0,0) and that it grows with decreasing temperature below 15 K. This structure was analyzed to be of the all-in all-out type, consisting of highly anisotropic Nd 3+ magnetic moments of magnitude 2.3±0.4µ B , where µ B is the Bohr magneton. The inelastic scattering measurements revealed that the Kramers ground doublet of Nd 3+ splits below T MI . This suggests the appearance of a static internal magnetic field at the Nd sites, which probably originates from a magnetic order consisting of Ir 4+ magnetic moments. Here, we discuss a magnetic structure model for the Ir order and the relation of the order to the metal–insulator transition in terms of frustration.
TL;DR: In this article, the authors present an in-depth density functional theory study of the spin-resolved electronic structure of (monoatomic) vacancies in graphene and bilayer graphene and use two different methodologies: supercell calculations with siesta code and cluster-embedded calculations with the alacant package.
Abstract: The observation of intrinsic magnetic order in graphene and graphene-based materials relies on the formation of magnetic moments and a sufficiently strong mutual interaction. Vacancies are arguably considered the primary source of magnetic moments. Here we present an in-depth density functional theory study of the spin-resolved electronic structure of (monoatomic) vacancies in graphene and bilayer graphene. We use two different methodologies: supercell calculations with the siesta code and cluster-embedded calculations with the alacant package. Our results are conclusive: The vacancy-induced extended $\ensuremath{\pi}$ magnetic moments, which present long-range interactions and are capable of magnetic ordering, vanish at any experimentally relevant vacancy concentration. This holds for $\ensuremath{\sigma}$-bond passivated and unpassivated reconstructed vacancies, although, for the unpassivated ones, the disappearance of the $\ensuremath{\pi}$ magnetic moments is accompanied by a very large magnetic susceptibility. Only for the unlikely case of a full $\ensuremath{\sigma}$-bond passivation, preventing the reconstruction of the vacancy, a full value of 1 ${\ensuremath{\mu}}_{B}$ for the $\ensuremath{\pi}$ extended magnetic moment is recovered for both monolayer and bilayer cases. Our results put on hold claims of vacancy-induced ferromagnetic or antiferromagnetic order in graphene-based systems, while still leaving the door open to $\ensuremath{\sigma}$-type paramagnetism.
TL;DR: In this article, the effects of hole doping on a Mott insulator on the honeycomb lattice where spins interact via direction-dependent Kitaev couplings and weak antiferromagnetic Heisenberg couplings was studied.
Abstract: We study the effects of doping a Mott insulator on the honeycomb lattice where spins interact via direction-dependent Kitaev couplings ${J}_{\mathrm{K}}$, and weak antiferromagnetic Heisenberg couplings $J$. This model is known to have a spin-liquid ground state and may potentially be realized in correlated insulators with strong spin -orbit coupling. The effect of hole doping is studied within a $t$-$J$-${J}_{\mathrm{K}}$ model, treated using the SU(2) slave boson formalism, which correctly captures the parent spin liquid. We find superconductor ground states with spin triplet pairing that spontaneously break time-reversal symmetry. Interestingly, the pairing is qualitatively different at low and high dopings, and undergoes a first-order transition with doping. At high dopings, it is smoothly connected to a paired state of electrons propagating with the underlying free particle dispersion. However, at low dopings the dispersion is strongly influenced by the magnetic exchange, and is entirely different from the free-particle band structure. Here the superconductivity is fully gapped and topological, analogous to spin polarized electrons with ${p}_{x}+i{p}_{y}$ pairing. These results may be relevant to honeycomb lattice iridates such as ${A}_{2}$IrO${}_{3}$ ($A$ = Li or Na) on doping.
TL;DR: In this paper, the authors present ab initio calculations of magnetic exchange parameters of stoichiometric Heusler compound Ni${}_{2}$MnSn and a few nonstoichiometric Ni-Mn-Sn alloys, which are compared to those obtained from a mean field approximation using the Heisenberg model.
Abstract: We present ab initio calculations of magnetic exchange parameters of stoichiometric Heusler compound Ni${}_{2}$MnSn and a few nonstoichiometric Ni${}_{2}$Mn${}_{1+x}$Sn${}_{1\ensuremath{-}x}$ cases. Use of the exchange parameters in subsequent Monte Carlo simulations allows us to evaluate the magnetization curves as a function of temperature and composition as well as the critical temperatures of the magnetic phase transitions. The latter are compared to those obtained from a mean-field approximation using the Heisenberg model. We find that the variation of the experimental Curie temperatures of nonstoichiometric alloys can be explained theoretically if we assume that the main impact of disorder is the intermixing of manganese and tin on their corresponding sublattices and the simultaneous appearance of strong antiferromagnetic trends which originate from the nearest-neighbor Mn-Mn interactions on different sublattices. The Curie temperatures of the Ni-Mn-Sn alloys which have been obtained from the Monte Carlo simulations are in qualitative agreement with the experimental transition temperatures.