Short-range Antiferromagnetic Coupling Research Articles

The magnetic ordering transition of Li(ZnFe)As regulated via electron carriers

First-principles calculations based on the density functional theory (DFT) were performed to study the electronic structure and magnetic coupling mechanism of Li(ZnFe)As with different concentrations of electron carriers. The electron carrier was introduced by the treatment of Se nonequivalent substitution for As. The results manifested that Fe atoms were apt to short-range antiferromagnetic coupling for the Fe-3d electrons superexchange interactions. When an extra electron carrier was introduced, the antiferromagnetic stability of the co-doped system was weakened. In the case of two additional electron carriers, the magnetic ordering of the co-doped system changed from antiferromagnetic state to ferromagnetic state, which benefited from the electron carrier-mediated p-d exchange mechanism. The values of energy difference were slightly larger than zero and exhibited plateau behavior despite the variation of Fe doping distance, indicating that electron carriers promoted the co-doped system weak and long-range ferromagnetic stability.

Peierls transition, ferroelectricity, and spin-singlet formation in monolayer VOI2

Using ab initio density functional theory and single-orbital Hubbard model calculations via the density matrix renormalization group method, we systematically studied the monolayer ${\mathrm{VOI}}_{2}$ with a $3{d}^{1}$ electronic configuration. Our phonon calculations indicate that the orthorhombic $Pmm2$ FE-II phase is the most likely ground state, involving a ferroelectric (FE) distortion along the $a$ axis and V-V dimerization along the $b$ axis. Specifically, the ``pseudo Jahn-Teller'' effect caused by the coupling between empty V (${d}_{xz/yz}$ and ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$) and O $2p$ states is proposed as the mechanism that stabilizes the FE distortion from the paraelectric phase. Moreover, the half-filled metallic ${d}_{xy}$ band displays a Peierls instability along the $b$ axis, inducing a V-V dimerization. We also found very short-range antiferromagnetic coupling along the V-V chain due to the formation of nearly decoupled spin singlets in the ground state.

Ferromagnetic coupling and the effect of Fe-dt2g state on ferromagnetism in half-metallic ZnO:Fe

Based on first principle calculations using GGA + U approach, we have studied the origin of room-temperature ferromagnetism in Fe-doped ZnO. The ferromagnetic coupling and the contribution of Fe-d states on the ferromagnetism of ZnO are also studied. The p–d exchange interaction between the Fe-d and O-p states is responsible for the ferromagnetism at lower Fe concentration in ZnO; whereas at the higher concentration, the enhanced short-range antiferromagnetic coupling between Fe ions dominates over ferromagnetism. The DOS results depict that the Fe-dt2g state predominantly contributes the hybridization at the fermi level resulting the magnetism in the Fe-doped ZnO system. The total energy calculations reveal the existence of short-range ferromagnetic coupling in the system. The band structure results depict the half-metallic character of the system with spin-polarized nature.

Magnetic interactions in a proposed diluted magnetic semiconductor (Ba1−xKx)(Zn1−yMny)2P2**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0302903), the National Natural Science Foundation of China (Grant Nos. 11774422 and 11774424), the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China (Grant Nos. 14XNLQ03 and 16XNLQ01).

By using first-principles electronic structure calculations, we have studied the magnetic interactions in a proposed BaZn2P2-based diluted magnetic semiconductor (DMS). For a typical compound Ba(Zn0.944Mn0.056)2P2 with only spin doping, due to the superexchange interaction between Mn atoms and the lack of itinerant carriers, the short-range antiferromagnetic coupling dominates. Partially substituting K atoms for Ba atoms, which introduces itinerant hole carriers into the p orbitals of P atoms so as to link distant Mn moments with the spin-polarized hole carriers via the p–d hybridization between P and Mn atoms, is very crucial for the appearance of ferromagnetism in the compound. Furthermore, applying hydrostatic pressure first enhances and then decreases the ferromagnetic coupling in (Ba0.75K0.25)(Zn0.944Mn0.056)2P2 at a turning point around 15 GPa, which results from the combined effects of the pressure-induced variations of electron delocalization and p–d hybridization. Compared with the BaZn2As2-based DMS, the substitution of P for As can modulate the magnetic coupling effectively. Both the results for BaZn2P2-based and BaZn2As2-based DMSs demonstrate that the robust antiferromagnetic (AFM) coupling between the nearest Mn–Mn pairs bridged by anions is harmful to improving the performance of these II–II–V based DMS materials.

Transition Metal Thiocyanate Complexes of Picolylcyanoacetamides

A variety of transition metal complexes involving picolylcyanoacetamides (pica = NCCH2CONH-R; R = 2-picolyl- (2pica), 3-picolyl- (3pica), 4-picolyl- (4pica)) and thiocyanate have been synthesised and their solid-state structures have been determined. The complexes were all obtained from reactions between the corresponding metals salts and pica ligands with sodium thiocyanate under ambient conditions. Both 3pica and 4pica coordinate to the metal solely through the nitrogen atom of the picolyl group and form discrete tetrahedral [M(NCS)2(pica)2] (3pica; M = Mn, Zn; 4pica; M = Co) and octahedral [M(NCS)2(3pica)4] (M = Co, Fe, Ni) complexes. In addition, one-dimensional N,S-thiocyanate-bridged coordination polymers poly-[M(µ-NCS)2(pica)2] (3pica; M = Cd; 4pica; M = Co, Cd) were obtained. The ligand 2pica gave the discrete octahedral complexes [Co(NCS)2(2pica)2] and [Cd(NO3)2(2pica)2] in which 2pica chelates in a bidentate fashion through its picolyl and carbonyl groups. Magnetic susceptibility measurements on the cobalt(ii) complexes were performed and showed short-range antiferromagnetic coupling for the [Co(NCS)2(4pica)2]n 1D polymer.

Structural, electrical and magnetic characterisation of a new Aurivillius phase Bi5−xThxFe1+xTi3−xO15 (x = 1/3)

A novel Aurivillius oxide with the composition Bi5−xThxFe1+xTi3−xO15 (x=1/3) has been successfully prepared by the conventional solid-state high-temperature synthesis. X-ray diffraction confirmed that the material is strictly single-phase and has a four-layer Aurivillius structure with the orthorhombic lattice (unit cell parameters: a=5.4300(2)Å, b=5.4451(7)Å, c=41.2800(6)Å; space group A21am (36)). Temperature dependent electrical and magnetic properties of the material have been investigated and compared to those of Bi5FeTi3O15. The relative permittivity maximum and divergence of loss tangent, observed at 970K, is attributed to a ferroelectric-to-paraelectric phase transition. At low temperature, the material exhibits paramagnetic behavior with short-range antiferromagnetic coupling. Analysis of Fe K-edge XAFS spectra showed that the Fe–O distances in Bi5−xThxFe1+xTi3−xO15 (x=1/3) are shorter and the general local environment of Fe is more distorted compared to Bi5FeTi3O15. In both compounds the Fe atom is shifted from the center of the oxygen octahedron towards one of its faces, giving rise to three shorter and three longer Fe–O distances.

First-principles investigation of magnetism and ferroelectricity in Ni-doped BiCoO3

The electronic and magnetic properties of BiCo0.875Ni0.125O3 were systematically investigated within the framework of density functional theory. Calculations show that C-type antiferromagnetic structure is the ground state in Ni-doped BiCoO3. The hybridization between Bi–O, Co–O, and Ni–O plays an essential role in the C-AFM behavior. The macroscopic magnetization caused by the short-range antiferromagnetic coupling can be greatly increased by Ni doping. The total magnetic moment of BiCo0.875Ni0.125O3 is 1.02 per 2 × 2 × 2 supercell, which is considerably enhanced, compared with BiCoO3. With the GGA + U method, a ferrimagnetic insulator can be predicted for BiCo0.875Ni0.125O3. The electronic polarization of BiCo0.875Ni0.125O3 is predicted by the Berry-phase calculations, with the value of 158.9 µC cm−2. Our results clearly demonstrate that Ni doping may lead the BiCoO3 system to show ferrimagnetic properties, while maintaining its excellent ferroelectric properties.

Inverse freezing in a cluster Ising spin-glass model with antiferromagnetic interactions

Inverse freezing is analyzed in a cluster spin-glass (SG) model that considers infinite-range disordered interactions between magnetic moments of different clusters (intercluster interaction) and short-range antiferromagnetic coupling J(1) between Ising spins of the same cluster (intracluster interaction). The intercluster disorder J is treated within a mean-field theory by using a framework of one-step replica symmetry breaking. The effective model obtained by this treatment is computed by means of an exact diagonalization method. With the results we build phase diagrams of temperature T/J versus J(1)/J for several sizes of clusters n(s) (number of spins in the cluster). The phase diagrams show a second-order transition from the paramagnetic phase to the SG order at the freezing temperature T(f) when J(1)/J is small. The increase in J(1)/J can then destroy the SG phase. It decreases T(f)/J and introduces a first-order transition. In addition, inverse freezing can arise at a certain range of J(1)/J and large enough n(s). Therefore, the nontrivial frustration generated by disorder and short-range antiferromagnetic coupling can introduce inverse freezing spontaneously.

The synthesis and characterisation of LaCa2Fe2GaO8

The synthesis and characterisation of LaCa2Fe2GaO8 is described. Neutron powder diffraction data reveal LaCa2Fe2GaO8 adopts an anion-vacancy ordered, ‘triple layer’ structure consisting of an OOTLOOTR stacking sequence of layers of (Fe/Ga)O6 octahedra (O) and (Fe/Ga)O4 tetrahedra (T), related to that of the 4-layer brownmillerite structures (a=5.4784(1)Å, b=22.6780(4)Å, c=5.6007(1)Å, space group Pbma). The apex-linked chains of tetrahedra within structure of LaCa2Fe2GaO8 exhibit a cooperative twisting distortion which alternates in direction between adjacent layers of tetrahedra, the first time such an arrangement has been seen in a triple-layer phase. Magnetisation and neutron diffraction data reveal that LaCa2Fe2GaO8 exhibits two magnetic ordering transitions. On cooling below 550K the phase adopts a state with short range antiferromagnetic coupling within each double layer of (Fe/Ga)O6 octahedra, but only weak coupling between layers. On cooling below 470K full three-dimensional, G-type antiferromagnetic order is observed.

Strain and interface effects on the magnetic and transport properties of La0.7Ca0.3MnO3/CaO multilayers

La0.7Ca0.3MnO3/CaO (LCMO/CaO) multilayers were deposited on (100) LaAlO3 substrates by pulsed injection metal organic chemical vapour deposition. The samples were characterised by X-ray diffraction, transport and magnetization measurements. An anomalous behaviour of the electrical resistance under applied magnetic field and an exchange bias effect for the magnetization are observed for the multilayers with thinner LCMO layers. These results suggest the existence of a short range antiferromagnetic coupling at the layer interfaces.

Neutron-scattering studies of the S = 2 antiferromagnetic chain MnCl 3 (C 10 D 8 N 2 )

Quasi-elastic and inelastic neutron scattering studies of the quasi-one-dimensional S=2 antiferromagnet MnCl3(C10D8N2) are reported. The quasi-elastic measurements exhibit a broad peak at Q≈0.69 A-1, which is consistent with short-range antiferromagnetic coupling between neighboring Mn3+ ions. Inelastic experiments, at 150 mK and Q=0.70 A-1, reveal decreased magnetic scattering at energies less than 0.2 meV when compared to similar studies at 20 K. These results provide microscopic evidence for the presence of a Haldane gap and are consistent with the bulk magnetization measurements of Granroth et al.

Synthesis and Magnetization of New Room-Temperature Molecule-Based Magnets: Effect of Stoichiometry on Local Magnetic Structure by X-ray Magnetic Circular Dichroism

The synthesis and the magnetic properties of two new bimetallic chromium−vanadium ferrimagnets, compounds 1 and 2, are reported. They belong to a set of nonstoichiometric Prussian blue analogues CIyV[CrIII(CN)6]z·nH2O (C, alkali cation) with TC varying between 295 and 315 K (TC = 310 K for 1 and 315 K for 2). They are obtained through mild chemistry from the molecular precursor [CrIII(CN)6]3- and from vanadium VII salts. The synthetic conditions determine (i) the oxidation state of the vanadium (VII, VIII, or VIV), (ii) the stoichiometry, and (iii) hence the magnetic properties. The reported systems present a larger magnetization than the first reported V−Cr analogue (0.36NAβ for 1 and 0.40NAβ for 2). Furthermore, X-ray magnetic circular dichroism (XMCD) data demonstrate (i) the short-range antiferromagnetic coupling between the spins borne by the chromium(III) and vanadium ions and (ii) the reversal of the local magnetization on vanadium and chromium when changing the stoichiometry.

Synthesis and Characterization of Novel V/O/SO4Chains Incorporating 2,2‘-Bipyridine Ligands: Crystal Structure of [V2O2(OH)2(SO4)(2,2‘-bpy)2

The hydrothermal reaction of a mixture of vanadyl acetylacetonate (VO(acac)(2)), Na(2)SO(4), 2,2'-bipyridine (2,2'-bpy), and H(2)O for 48 h at 160 degrees C gives brown crystals of [V(2)O(2)(OH)(2)(SO(4))(2,2'-bpy)(2)] (1) in 70% yield. The structure of 1 consists of ribbons constructed from the infinite inorganic chains, [-{V(2)O(2)(OH)(2)}-&mgr;(2)-SO(4){V(2)O(2)(OH)(2)}-SO(4)](infinity), incorporating organic (2,2'-bipyridine) ligands. The inorganic chains are composed of the pairs of edge-sharing octahedra joined by {SO(4)} tetrahedra through octahedral-tetrahedral corner sharing. The octahedral geometry around each vanadium(IV) ion is defined by {VO(2)(OH)(2)N(2)} with each V(IV) center coordinated to a terminal oxo group, two &mgr;(2)-OH groups, two nitrogen donor atoms from a chelating 2,2'-bipyridine ligand, and an oxygen donor atom from a &mgr;(2)-SO(4)(2)(-) ligand. Crystal data for 1: monoclinic space group P2(1)/n (No. 14), a = 11.7937(2) Å, b = 12.1161(3) Å, c = 15.5763(2) Å, beta = 93.750(2) degrees, Z = 4. 1 constitutes the first example of a fully reduced vanadosulfate (V/O/SO(4)) based solid incorporating both the organic and inorganic ligands. The novel solid exhibits Curie-Weiss paramagnetism at high temperature (T > 140 K) and short-range antiferromagnetic coupling between the V(IV) centers at lower temperature.

Three-Dimensional Metal Piperazinyldiphosphonate Phases with Ellipsoidal Cavities Defined by 44-Membered Rings: Crystal Structures of [M{O3PCH2NH(C2H4)2NHCH2PO3}] · H2O,M= Mn and Co

The hydrothermal reaction of a mixture of (Et4N)2MnCl4,N,N′-piperazinebis(methylenephosphonic acid), Et4NCl · H2O, and H2O in the mole ratio 1:1:5:300, adjusted to pH 5 with 40% aqueous (Bu4N)OH, at 160°C for 63 h yielded [Mn{O3PCH2NH(C2H4)2NHCH2PO3}] · H2O (1) in 60% as off-white platelets. The analogous reaction using (Et4N)2CoCl4produced [Co{O3PCH2NH(C2H4)2NHCH2PO3}] · H2O (2). The isomorphous materials 1 and 2 display three-dimensional network structures, based on binuclear units of corner-sharing metal and phosphorus tetrahedra, forming eight membered {–Co–O–P–O–}2rings. The diphosphonate groups serve to tether the binuclear units into large ellipsoidal 44-membered rings. As the piperazinyl nitrogen atoms are protonated, the organic moiety of the disphosphonate group serves as both tether and charge compensating site. Compound 1 exhibits Curie–Weiss paramagnetism with weak antiferromagnetism between Mn centers. In contrast, the susceptibility of 2 exhibits a broad maximum at low temperatures consistent with short range antiferromagnetic coupling. Crystal data: 1, C6H16MnN2O7P2, triclinicP1,a= 8.393(2)Å,b= 9.043(2)Å,c= 9.125(2)Å, α = 62.88(3)°, β = 86.36(3)°, γ = 78.96(3)°,V= 604.8(3)Å3;R= 0.056 for 1663 reflections. 2, C6H16CoN2O7P2, triclinicP1,a= 8.340(2)Å,b= 8.917(2)Å,c= 9.018(2)Å, α = 64.11(3)°, β = 86.24(3)°, γ = 78.87(3)°,V= 591.9(2)Å3;R= 0.083 for 1917 reflections.

New manganese pyrophosphates: The syntheses, crystallographic characterizations and magnetic properties of BaMnP 2O 7 and CaMnP 2O 7

The new compound BaMnP 2O 7 was obtained in two crystallographic modifications, (monoclinic- 1) and (triclinic- 2), by heating mixtures of BaCO 3, P 2O 5 and MnO 2 to 1100°C and to 1000°C for 72 h, respectively. CaMnP 2O 7 ( 3) was obtained by heating a mixture of CaCO 3, P 2O 5 and MnO 2 to 1050°C for 48 h. Both crystallographic forms of BaMnP 2O 7 and CaMnP 2O 7 ( 3) were investigated by single-crystal X-ray diffraction analysis. The high-temperature monoclinic form of BaMnP 2O 7 could not be obtained free of the low-temperature triclinic form in the bulk form. In monoclinic- 1 the manganese ions exist in a distorted MnO 6 octahedron surrounded by five closely and one remotely positioned oxygen atoms. In triclinic- 2 and - 3 forms the manganese ions are associated in pairs by the formation of Mn 2O 10 units that share one edge of two adjacent octahedra. The magnetic properties of the triclinic- 2 and - 3 forms were also investigated. The effective magnetic moments, μ eff, are 5.7 B.M. and 5.8 B.M./Mn atom for triclinic- 2 and - 3, respectively, and are consistent with a high-spin Mn 2+ ion in an octahedral environment with five unpaired electrons. The temperature-dependent magnetic measurements of 2 and 3 have revealed a combination of short-range antiferromagnetic coupling, J, between the two Mn ions within the Mn 2O 10 units and a longer range weaker antiferromagnetic coupling, J′, between the neighbouring Mn 2O 10 units, | J′/ J| = 0.18 and 0.074 for 2 and 3, respectively.