Antiferromagnetic Superexchange Coupling Research Articles

Strain controlled antiferromagnetic insulating state along with ferroelectricity and magnetic anisotropy in Pb2CoTeO6

Controlling electronic and magnetic properties of materials by strain engineering is essential and extremely sought for high-performance spintronic devices as strain can be easily achieved experimentally. In this respect, antiferromagnetic (AFM) double perovskite oxides (DPO) have attracted a lot of attention due to ultrafast spin moments and small stray field, which is potentially desired for high-frequency and high-data storage devices. Herein, the electronic, magnetic, and ferroelectric properties of Pb 2 CoTeO 6 DPO under uniaxial ([100], [010], and [001]), biaxial ([110]), and hydrostatic ([111]) strains are investigated in the framework of first-principles calculations including Hubbard parameter U . Pristine motif emerges as AFM insulator having an indirect energy band gap of 1.88 eV due to anti-alignment of Co ions at the diagonal. The most striking feature of the present study is that system exhibits a substantial magnitude of spontaneous polarization of 32 μ C/cm 2 and magnetocrystalline anisotropy constant of 5.4 × 10 7 erg/cm 3 with an easy axis of [011]. Moreover, it is found that Co lies in a ＋ 2 state having an electronic configuration of t 2 g 3 ↑ e g 2 ↑ t 2 g 2 ↓ e g 0 ↓ with S= 1.5 and displays a spin moment of 2.71 μ B . The solidity of AFM state and robustness of the insulating nature is confirmed under the influence of strains. An indirect to direct band gap transition is established for compressively strained motifs, which is useful for optics as well as the solar cell industry. Finally, it is revealed that overall compressive and tensile strains show a negative and positive impact on the spontaneous polarization amplitude, respectively. • Anti-ferromagnetic (AFM) super-exchange coupling leads the system to AFM ground state. • System exhibits a reasonable magnitude of magnetic anisotropy energy constant. • The solidity of AFM state and robustness of the insulating nature is confirmed under the influence of strains. • A substantial amplitude of spontaneous polarization of 32 μ C/cm 2 .

Unconventional Pressure-Driven Metamagnetic Transitions in Topological van der Waals Magnets.

Activating metamagnetic transitions between ordered states in van der Waals magnets and devices bring great opportunities in spintronics. We show that external pressure, which enhances the interlayer hopping without introducing chemical disorders, triggers multiple metamagnetic transitions upon cooling in the topological van der Waals magnets Mn(Bi1-xSbx)4Te7, where the antiferromagnetic interlayer superexchange coupling competes with the ferromagnetic interlayer coupling mediated by the antisite Mn spins. The temperature-pressure phase diagrams reveal that while the ordering temperature from the paramagnetic to ordered states is almost pressure-independent, the metamagnetic transitions show nontrivial pressure and temperature dependence, even re-entrance. For these highly anisotropic magnets, we attribute the former to the ordering temperature being only weakly dependent on the intralayer parameters and the latter to the parametrically different pressure and temperature dependence of the two interlayer couplings. Our independent probing of these disparate magnetic interactions paves an avenue for efficient magnetic manipulations in van der Waals magnets.

Control of Magnetic Properties of Barium Ferrite Thin Films With Unusual Valence Fe

Thin films of BaFe1-xMxO3 (M = Hf, Zr, and Ce; 0.0 ≤ x ≤ 0.75) were fabricated using pulsed laser deposition and their magnetic properties were investigated. X-ray diffraction analysis indicated that oxygen-deficient BaFeOx (x < 3.0) with a monoclinic structure was formed when the deposition was conducted using a non-substituted target (x = 0.0). The as-grown BaFeOx films were converted into fully oxidized BaFeO3 with a perovskite structure by low-temperature oxidation in an ozone atmosphere. In contrast, the as-deposited films of Hf, Zr, and Ce-substituted films exhibited a perovskite structure, and their crystallinity did not change after low-temperature ozone annealing. The magnetic transition temperature Tc of the BaFeO3 film was 115 K, whereas the substituted BaFeO3 films showed ferromagnetic behavior even at 300 K. These results can be attributed to the weakening of the antiferromagnetic super-exchange coupling among Fe ions owing to the lattice expansion in the substituted BaFeO3. In addition, the magnetization of the films was found to increase with the decreasing ionic ratio of Fe4+/Fe3+, suggesting that the inherent carrier-induced ferromagnetic interaction is dominant in the films.

Exchange bias in La0.7Sr0.3CrO3/La0.7Sr0.3MnO3/La0.7Sr0.3CrO3 heterostructures

In the recent past, heterostructures of magnetic oxide thin films have attracted a great deal of research excitement due to very interesting physical properties such as antiferromagnetic interlayer coupling, tunable exchange-bias, interfacial driven magnetic properties and high mobility electron gas across the interfaces. In this work, we report on the comprehensive magnetic properties observed from the heterostructures of (2 unit cells) La0.7Sr0.3CrO3/(8 unit cells) La0.7Sr0.3MnO3/(2 unit cells) La0.7Sr0.3CrO3, which are epitaxially deposited on SrTiO3 substrate by plasma-assisted oxide molecular beam epitaxy. Using SQUID magnetometer, the magnetic properties are studied when the magnetic field was applied both in plane and out of plane. The Curie temperature of this structure is found to be at 290 K. Most significantly, at 2 K, we observed a complete up/down shift (along magnetization axis) of hysteresis loop when the sample was cooled under a magnetic field of ± 5000 Oe in the in-plane configuration. We believe that the strong antiferromagnetic (super) exchange coupling of Mn-Cr across the two interfaces is responsible for the observed exchange bias. We will present and discuss our in-detailed experimental findings collected on this heterostructure as a function of temperature and magnetic field.

Guidelines for Selecting Interlayer Spacers in Synthetic 2D-Based Antiferromagnets from First-Principles Simulations.

Following the recent synthesis of graphene–based antiferromagnetic ultrathin heterostructures made of Co and Fe, we analyse the effect of the spacer between both ferromagnetic materials. Using density functional calculations, we carried out an exhaustive study of the geometric, electronic and magnetic properties for intercalated single Co MLs on top of Ir(111) coupled to monolayered Fe through n graphene layers (n = 1, 2, 3) or monolayered h-BN. Different local atomic arrangements have been considered to model the Moiré patterns expected in these heterostructures. The magnetic exchange interactions between both ferromagnets () are computed from explicit calculations of parallel and anti-parallel Fe/Co inter–layer alignments, and discussed in the context of recent experiments. Our analysis confirms that the robust antiferromagnetic superexchange–coupling between Fe and Co layers is mediated by the graphene spacer through the hybridization of C’s orbitals with Fe and Co’s 3d states. The hybridization is substantially suppressed for multilayered graphene spacers, for which the magnetic coupling between ferromagnets is critically reduced, suggesting the need for ultrathin (monolayer) spacers in the design of synthetic graphene-based antiferromagnets. In the case of h–BN, orbitals also mediate d(Fe/Co) coupling. However, there is a larger contribution of local ferromagnetic interactions. Magnetic anisotropy energies were also calculated using a fully relativistic description, and show out–of–plane easy axis for all the configurations, with remarkable net values in the range from 1 to 4 meV.

Thiosemiquinoid Radical-Bridged CrIII2 Complexes with Strong Magnetic Exchange Coupling.

Semiquinoid radical bridging ligands are capable of mediating exceptionally strong magnetic coupling between spin centers, a requirement for the design of high-temperature magnetic materials. We demonstrate the ability of sulfur donors to provide much stronger coupling relative to their oxygen congeners in a series of dinuclear complexes. Employing a series of chalcogen donor-based bis(bidentate) benzoquinoid bridging ligands, the series of complexes [(TPyA)2Cr2(RL4-)]2+ (OLH4 = 1,2,4,5-tetrahydroxybenzene, OSLH4 = 1,2-dithio-4,5-dihydroxybenzene, SLH4 = 1,2,4,5-tetrathiobenzene, TPyA = tris(2-pyridylmethyl)amine) was synthesized. Variable-temperature dc magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange coupling between CrIII centers in these complexes, with exchange constants of J = -2.83(3) (OL4-), -2.28(5) (OSL4-), and -1.80(2) (SL4-) cm-1. Guided by cyclic voltammetry and spectroelectrochemical measurements, chemical one-electron oxidation of these complexes gives the radical-bridged species [(TPyA)2Cr2(RL3-•)]3+. Variable-temperature dc susceptibility measurements in these complexes reveal the presence of strong antiferromagnetic metal-semiquinoid radical coupling, with exchange constants of J = -352(10) (OL3-•), - 401(8) (OSL3-•), and -487(8) (SL3-•) cm-1. These results provide the first measurement of magnetic coupling between metal ions and a thiosemiquinoid radical, and they demonstrate the value of moving from O to S donors in radical-bridged metal ions in the design of magnetic molecules and materials.

Magnetism and electronic properties of LiFeAs superconducting thin filma under two-dimensional strains effect

The magnetism, band properties and electronic density of states of LiFeAs superconducting thin film with two-dimensional strain are investigated by using the first principles calculations based on density functional theory, and the influences of different strains on the characteristics of superconducting films are analyzed in detail. The results show that the magnetic ground configuration is the striped antiferromagnetic state of nostrained LiFeAs thin film, and the ground structure of this system is unchanged in the range of applied 1%−6% compressive and tensile strain. The density of states near the Fermi level is mainly from the contribution of Fe-3d orbital and a few As-4p electrons. The electron spin exchange coupling between Fe ions is realized by As ions. Furthermore, unlike the case of the nostrain and the tensile strain, with increasing the compressive strain, the localized antiparallel electron spin magnetic moments of Fe ion decrease, the density of states at the Fermi surface improves, and the itinerant electron magnetism of Fe ions increases, which all greatly suppress the antiferromagnetic properties of thin film and enhance the superconducting phase transition temperature. The superconductivity of LiFeAs thin film originates from the Cooper pairs of electrons between the hole-type and electronic-type bands near the Fermi surface through the antiferromagnetic superexchange coupling effect. Instead, the LiFeAs thin film with the tensile strain presents completely opposite properties, that is to say, the decrease of the electronic density of states in the Fermi level brings about the weakening of the metal properties and the increasing of the antiferromagnetic exchange coupling. Particularly, the band structure of hole-type near the Fermi surface disappears, and the occurrence of Cooper pairs of electrons becomes significantly reduced, resulting in the suppressed superconducting phase transition when the LiFeAs thin film is subjected to tensile strain. In addition, the change of antiferromagnetic exchange coupling and magnetic moments of Fe ions are also explained according to the variation of electronic density of states of the Fe-3d energy levels during the distortion of FeAs tetrahedrons due to compressive strain. In brief, our researches provide an effective way to improve the superconducting properties of LiFeAs thin film and may promote the relevant practical applications of iron-based superconductors in the future.

Angle resolved photoemission spectroscopy study of the spin-charge separation in the strongly correlated cuprates SrCuO2 and Sr2CuO3 with S = 0 impurities

We present the results of an Angle-Resolved Photoemission Spectroscopy study on the pristine and doped quasi-one dimensional spin chains cuprates: SrCuO2, SrCu0.99M0.01O2 with (M = Mg2+ or Zn2+), Sr2CuO3 and Sr2Cu(1-x)NixO3 with (x = 0.01 or 0.02), where the dopant is a non-magnetic impurity. Both systems are quantum critical and obey the Tomonaga-Luttinger spin liquid theory. Due to low dimensionality, the separation of the degrees of freedom of the collective excitation modes of spin and charge occurs, resulting in two distinct band dispersions ascribed to spinon and holon quasi-particles, at a binding energy of about 1.2 eV. Several experimental probes show that the finite size effect, in these strongly correlated electron compounds, resulting from chains breaking by the non-magnetic impurities, has a strong impact on the ground state of their quasi-particles excitations. On the other hand, our Angle Resolved Photoemission Spectroscopy data do not show any evolution of the spinon branch upon doping, in terms of energy position at Г. We also extract the antiferromagnetic superexchange coupling and inter-site hopping constants JAF and t.

Influence of Mn doping on structural and magnetic properties of Ti1−xMnxO2

We report here the structural and magnetic properties of Mn-doped TiO2 samples (0 ≤ Mn ≤ 0.06) prepared with a solid-state method. The cell parameters a and c increased with the increased Mn content, implying that Mn ions were substituted into the lattice of TiO2. The room-temperature M(H) loops showed ferromagnetism (FM) behavior for all samples. The magnetic moment per Mn atom decreased with the increased Mn content. However, our results demonstrate that the FM property is intrinsic to the TiO2 system and is not a result of any secondary magnetic phase or cluster formation. The magnetic properties of Ti1−xMnxO2 are discussed based on the competition between the antiferromagnetic superexchange coupling and the FM originating from interactions between Mn ions and oxygen vacancies and/or defects inserted in the TiO2 lattice.

Comparative studies on the room-temperature ferrielectric and ferrimagnetic Ni3TeO6-type A2FeMoO6 compounds (A = Sc, Lu).

First-principles calculations have been carried out to study the structural, electric, and magnetic properties of Ni3TeO6-type A2FeMoO6 compounds (A = Sc, Lu). Their electric and magnetic properties behave like room-temperature ferrielectric and ferrimagnetic insulators where polarization comes from the un-cancelled antiparallel dipoles of (A(1), Fe3+) and (A(2), Mo3+) ion groups, and magnetization from un-cancelled antiparallel moments of Fe3+ and Mo3+ ions. The net polarization increases with A’s ionic radius and is 7.1 and 8.7 μCcm−2 for Sc2FeMoO6 and Lu2FeMoO6, respectively. The net magnetic moment is 2 μB per formula unit. The magnetic transition temperature is estimated well above room-temperature due to the strong antiferromagnetic superexchange coupling among Fe3+ and Mo3+ spins. The estimated paraelectric to ferrielectric transition temperature is also well above room-temperature. Moreover, strong magnetoelectric coupling is also anticipated because the magnetic ions are involved both in polarization and magnetization. The fully relaxed Ni3TeO6-type A2FeMoO6 structures are free from soft-phonon modes and correspond to stable structures. As a result, Ni3TeO6-type A2FeMoO6 compounds are possible candidates for room-temperature multiferroics with large magnetization and polarization.

A real space description of magnetic field induced melting in the charge ordered manganites: I. The clean limit

We study the melting of charge order in the half doped manganites using a model that incorporates double exchange, antiferromagnetic superexchange, and Jahn-Teller coupling between electrons and phonons. We primarily use a real space Monte Carlo technique to study the phase diagram in terms of applied field $(h)$ and temperature $(T)$, exploring the melting of charge order with increasing $h$ and its recovery on decreasing $h$. We observe hysteresis in this response, and discover that the `field melted' high conductance state can be spatially inhomogeneous even without extrinsic disorder. The hysteretic response plays out in the background of field driven equilibrium phase separation. Our results, exploring $h$, $T$, and the electronic parameter space, are backed up by analysis of simpler limiting cases and a Landau framework for the field response. This paper focuses on our results in the `clean' systems, a companion paper studies the effect of cation disorder on the melting phenomena.

Probing the links between structure and magnetism in Sr(2-x)Ca(x)FeOsO₆ double perovskites.

The synthesis, structure, and properties of the ordered double perovskites Sr2FeOsO6, Ca2FeOsO6, and SrCaFeOsO6 are reported. The latter two compounds have monoclinic P2₁/n symmetry and a(-)a(-)b(+) tilting of the octahedra, while Sr2FeOsO6 is tetragonal with I4/m symmetry and a(0)a(0)c(-) tilting. Magnetic measurements indicate and neutron powder diffraction studies confirm that Ca2FeOsO6 is a ferrimagnet with a Curie temperature of 350 K. The ferrimagnetism is retained if half of the Ca(2+) ions are replaced with larger Sr(2+) ions to form SrCaFeOsO6 (T(C) = 210 K). This substitution reduces the degree of octahedral tilting, but unlike most perovskites, the magnetic ordering temperature decreases as the Fe-O-Os bond angles approach a linear geometry. In contrast, Sr2FeOsO6 orders antiferromagnetically, as previously reported. X-ray absorption spectroscopy confirms the assignment of Fe(III) and Os(V) oxidation states for all three compounds. In these insulating double perovskites, the magnetic ground state is governed by a competition between the four-bond Fe-O-Os-O-Fe antiferromagnetic superexchange coupling of Fe(III) ions and the two-bond Fe-O-Os antiferromagnetic superexchange coupling between neighboring Fe(III) and Os(V) ions. When the Fe-O-Os bonds are linear, as they are in the c direction in Sr2FeOsO6, the four-bond coupling between Fe(III) ions prevails. The competition shifts in favor of antiferromagnetic coupling of Fe(III) and Os(V) as the Fe-O-Os bond angles bend in response to chemical pressure.

Calculation of Multipolar Exchange Interactions in Spin-Orbital Coupled Systems

A new method of computing multipolar exchange interaction in spin-orbit coupled systems is developed using multipolar tensor expansion of the density matrix in local density approximation+U electronic structure calculation. Within the mean field approximation, exchange constants can be mapped into a series of total energy calculations by the pair-flip approximation technique. The application to uranium dioxide shows an antiferromagnetic superexchange coupling in dipoles but a ferromagnetic one in quadrupoles which is very different from past studies. Further calculation of the spin-lattice interaction indicates it is of the same order with the superexchange and characterizes the overall behavior of the quadrupolar part as a competition between them.

First-principles study of magnetic ground state of quantum paraelectric EuTiO3 material

Magnetic ground state of perovskite structure quantum paraelectric EuTiO3 has been known to have a planar anisotropic G-type antiferromagnet structure according to the experimental study. In this paper, based on density functional theory, first-principles computations are performed to investigate the magnetic properties and spin-exchange interaction of EuTiO3 in both of the quantum paraelectric phase and ferroelectric tetragonal phase under stress. By analyzing the energies of different magnetic structures and paths of spin exchange coupling as well as the effect of stress on the magnetic exchange paths change, it is found that when the system of EuTiO3 is free, it has a G-type antiferromagnetic structure with uniaxial anisotropic spin along [110] direction. Furthermore, in this structure, Eu 4f electron spin achieves antiferromagnetic super-exchange coupling via O 2p state at face-centered position. However, in the ferroelectric tetragonal phase structure induced by applied stress field, Eu 4f electron spin achieves ferromagnetic exchange coupling in [110] direction due to the variation of Eu-O-Eu bond angle in spin exchange path.

Structural transition and unusually strong antiferromagnetic superexchange coupling in perovskite KAgF3

High-purity samples of potassium trifluoroargentate(II), KAgF3, have been obtained via a novel synthetic pathway. This compound is found to exhibit an order-disorder phase transition around 230 K. Susceptibility measurements indicate that KAgF3 exhibits strong antiferromagnetic (AFM) coupling reminiscent of that found in copper(II) oxides.

Variation of structural, optical and magnetic properties with Co-doping in Sn1−xCoxO2 nanoparticles

Pure and Co-doped SnO2 nanoparticles with various Co concentrations of 1%, 3%, 5%, 7% and 10% have been synthesized by the coprecipitation technique. X-ray diffraction and field emission scanning electron microscope equipped with energy dispersive spectrometer were performed to analyze the crystal structure and check the possible presence of any impurity elements in the nanocrystals. Optical absorption and photoluminescence spectra were used to confirm that the Co is substituted at the Sn site in SnO2 matrix and does not form metallic clusters and other oxide phases for Sn1−xCoxO2 (x=0.00–0.10). Meanwhile, it was revealed that the energy band gap decreases and emission peak blue-shifts with the increasing Co-doping. Magnetization measurement shows that the low Co-doping (x≤0.03) develops the ferromagnetism and paramagnetism, but they are weakened by the higher Co-doping due to the formation of antiferromagnetic superexchange coupling among neighboring Co ions. The observed magnetic behaviors support the bound magnetic polarons model.

Structure and Magnetic Ordering of the Anomalous Layered (2D) Ferrimagnet [NEt4]2MnII3(CN)8 and 3D Bridged‐Layered Antiferromagnet [NEt4]MnII3(CN)7 Prussian Blue Analogues

The reaction of Mn(II) and [NEt(4)]CN leads to the isolation of solvated [NEt(4)]Mn(3)(CN)(7) (1) and [NEt(4)](2) Mn(3)(CN)(8) (2), which have hexagonal unit cells [1: R3m, a = 8.0738(1), c = 29.086(1) Å; 2: P3m1, a = 7.9992(3), c = 14.014(1) Å] rather than the face centered cubic lattice that is typical of Prussian blue structured materials. The formula units of both 1 and 2 are composed of one low- and two high-spin Mn(II) ions. Each low-spin, octahedral [Mn(II)(CN)(6)](4-) bonds to six high-spin tetrahedral Mn(II) ions through the N atoms, and each of the tetrahedral Mn(II) ions are bound to three low-spin octahedral [Mn(II)(CN)(6)](4-) moieties. For 2, the fourth cyanide on the tetrahedral Mn(II) site is C bound and is terminal. In contrast, it is orientationally disordered and bridges two tetrahedral Mn(II) centers for 1 forming an extended 3D network structure. The layers of octahedra are separated by 14.01 Å (c axis) for 2, and 9.70 Å (c/3) for 1. The [NEt(4)](+) cations and solvent are disordered and reside between the layers. Both 1 and 2 possess antiferromagnetic superexchange coupling between each low-spin (S = 1/2) octahedral Mn(II) site and two high-spin (S = 5/2) tetrahedral Mn(II) sites within a layer. Analogue 2 orders as a ferrimagnet at 27(±1) K with a coercive field and remanent magnetization of 1140 Oe and 22,800 emuOe mol(-1), respectively, and the magnetization approaches saturation of 49,800 emuOe mol(-1) at 90,000 Oe. In contrast, the bonding via bridging cyanides between the ferrimagnetic layers leads to antiferromagnetic coupling, and 3D structured 1 has a different magnetic behavior to 2. Thus, 1 is a Prussian blue analogue with an antiferromagnetic ground state [T(c) = 27 K from d(χT)/dT].

Surface morphology, structure, magnetic and electrical transport properties of reactive sputtered polycrystalline Ti1−xFexN films

Polycrystalline Ti1−xFexN films with different Fe atomic fractions (x) were fabricated by reactive facing-target sputtering. The lattice orientation changes from (200) to (111) with the increase of x, which makes the surface morphology evolve from spherical, triangular-pyramid-like islands to random-leafs-like ones. The films are ferromagnetic with a Curie temperature of higher than 305K, and the saturation magnetization (Ms) is very small. Obvious asymmetric M–H curves are observed at low temperatures, and the shift of M–H curves decreases with the increasing temperature because of the relaxation of the pinned moments at low temperatures. All of the samples show semiconducting-like behavior with a mechanism of variable-range hopping at low-temperature range. Magnetoresistance (MR) is very small. The small Ms and MR can be explained by the facts that the interaction between Fe3+–N3−–Fe3+ is antiferromagnetic superexchange coupling, and no double exchange exists in Fe3+–N3−–Ti3+, which is different from that in the Cr-doped TiN films.

Room temperature ferromagnetism behavior of Sn1−xMnxO2 powders

In this paper, we have investigated Mn-doped SnO2 powder samples prepared by solid-state reaction method. X-ray diffraction showed a single phase polycrystalline rutile structure. The atomic content of Mn ranged from ∼0.8 to 5at%. Room temperature M–H loops showed a ferromagnetic behavior for all samples. The ferromagnetic Sn0.987Mn0.013O2 showed a coercivity Hc=545Oe, which is among the highest reported for dilute magnetic semiconductors. The magnetic moment per Mn atom was estimated to be about 2.54μB of the Sn0.9921Mn0.0079O2 sample. The average magnetic moment per Mn atom sharply decreases with increasing Mn content, while the effective fraction of the Mn ions contributing to the magnetization decreases. The magnetic properties of the Sn1−xMnxO2 are discussed based on the competition between the antiferromagnetic superexchange coupling and the F-center exchange coupling mechanism, in which both oxygen vacancies and magnetic ions are involved.

First-principles study of the interaction between paramagneticV4+centers through formally magnetically inactiveVO4tetrahedra in the quasi-one-dimensional spin systemsSr2V3O9andBa2V3O9

We report a first-principles density-functional theory study of the origin of the magnetic properties of the quasi-one-dimensional spin systems ${\text{Sr}}_{2}{\text{V}}_{3}{\text{O}}_{9}$ and ${\text{Ba}}_{2}{\text{V}}_{3}{\text{O}}_{9}$. The calculated coupling constants are in very good agreement with experimental data and provide a basis to understand the correlation between structural features and magnetic coupling constants. In ${\text{Sr}}_{2}{\text{V}}_{3}{\text{O}}_{9}$, the predominant coupling is antiferromagnetic and is indeed mediated by one of the two different types of ${\text{VO}}_{4}$ tetrahedra. However, there is also a weaker ferromagnetic interaction along the direction of the octahedral chains, which is mediated by the corner-sharing oxygen atoms through a spin-polarization mechanism. The second type of tetrahedra apparently do not contribute to the magnetic coupling. In ${\text{Ba}}_{2}{\text{V}}_{3}{\text{O}}_{9}$, the antiferromagnetic coupling is dominated by a very substantial superexchange interaction of the vanadium ${d}_{xy}$ orbitals through the tetrahedra sharing only one oxygen atom with the octahedral chain. The second type of tetrahedra and the oxygen atoms nonshared with the tetrahedra provide ferromagnetic interactions which only partially counteract the antiferromagnetic superexchange coupling. Interchain disorder is predicted to have a very small influence on the coupling constants.