Superexchange Paths Research Articles

Rocksalt versus layered ordering in double perovskites: A case study with La2CuSnO6 and La2CuIrO6

The nature of ordering of B and $\mathrm{B}{}^{\ensuremath{'}}$ transition metal ions in double perovskite compounds of general composition ${\mathrm{A}}_{2}\mathrm{BB}{}^{\ensuremath{'}}{\mathrm{O}}_{6}$ is an important topic, since the physical properties crucially depend on it. In the present study, considering the specific cases of ${\mathrm{La}}_{2}{\mathrm{CuSnO}}_{6}$ and ${\mathrm{La}}_{2}{\mathrm{CuIrO}}_{6}$, we carry out first-principles calculations with an aim to obtain microscopic understanding on this issue. Our study reveals the presence of Jahn-Teller distorted B ion, like ${\mathrm{Cu}}^{2+}$ helps in band energy stabilization of the layered ordering over the rocksalt ordering. However, introduction of magnetism may reverse this trend, especially in the presence of a second magnetic ion at $\mathrm{B}{}^{\ensuremath{'}}$ site, which may introduce a strong superexchange path involving B-O-$\mathrm{B}{}^{\ensuremath{'}}$, as found in the case of ${\mathrm{La}}_{2}{\mathrm{CuIrO}}_{6}$. We further find the spin-orbit coupling at Ir site drives the ${\mathrm{La}}_{2}{\mathrm{CuIrO}}_{6}$ compound to be a spin-orbit assisted Mott insulator.

Criticality-Enhanced Magnetocaloric Effect in Quantum Spin Chain Material Copper Nitrate

In this work, a systematic study of Cu(NO3)2·2.5 H2O (copper nitrate hemipentahydrate, CN), an alternating Heisenberg antiferromagnetic chain model material, is performed with multi-technique approach including thermal tensor network (TTN) simulations, first-principles calculations, as well as magnetization measurements. Employing a cutting-edge TTN method developed in the present work, we verify the couplings J = 5.13 K, α = 0.23(1) and Landé factors g∥= 2.31, g⊥ = 2.14 in CN, with which the magnetothermal properties have been fitted strikingly well. Based on first-principles calculations, we reveal explicitly the spin chain scenario in CN by displaying the calculated electron density distributions, from which the distinct superexchange paths are visualized. On top of that, we investigated the magnetocaloric effect (MCE) in CN by calculating its isentropes and magnetic Grüneisen parameter. Prominent quantum criticality-enhanced MCE was uncovered near both critical fields of intermediate strengths as 2.87 and 4.08 T, respectively. We propose that CN is potentially a very promising quantum critical coolant.

Synthesis, crystal and electronic structure of the new sodium chain sulfido cobaltates(II), Na3CoS3 and Na5[CoS2]2(Br)

Abstract The sulfido cobaltate(II) Na3CoS3 was synthesized from stoichiometric quantities of Na2S, elemental cobalt and sulfur at a maximum temperature of 1100°C. According to Na3CoS3=Na12[Co2S5(S2)][Co2S3(S2)] the orthorhombic structure of a new type (space group Cmc21, a=884.24(2), b=2177.38(5), c=1193.20(3) pm, Z=12, R1=0.0205) contains two different anions: i. dimers [Co2S5(S2)]8− of two edge-sharing [CoS4] tetrahedra with five sulfido and one η 1-disulfido ligand; ii. chains ∞ 1 [ Co 2 S 3 ( S 2 ) ] 4 − $_\infty ^1{[{\rm{C}}{{\rm{o}}_2}{{\rm{S}}_3}({{\rm{S}}_2})]^{4 - }}$ of [CoS4] tetrahedra connected via μ-sulfido (3×) besides μ-1,2-disulfido (1×) ligands. The second title compound, Na5[CoS2]2(Br), which has been likewise synthesized as a pure phase from stoichiometric quantities of Na2S, Co, S and NaBr, is isotypic to Na5[CoS2]2(S) (Na6PbO5 type; tetragonal, space group I4mm, a=914.58(7), c=625.59(5) pm; R1=0.0412). The structure contains linear chains ∞ 1 [ CoS 4 / 2 ] 2 − $_\infty ^1{[{\rm{Co}}{{\rm{S}}_{4/2}}]^{2 - }}$ running along the tetragonal c axis. In between, Br− ions are interspersed, which are coordinated by square pyramids of Na+ ions. The isotypic hydrogensulfide Na5[CoS2]2(SH) was obtained via the addition of NaSH. Several synthetic and structural arguments suggest that Na5[CoS2]2(SH) and the previously described pure sulfide are the same compound. The results of DFT band structure calculations (GGA+U, AFM spin ordering) are used to discuss and compare the chemical bonding in the new sulfido cobaltates(II) with that of the reference compound Na2[CoS2]. They also allow for obtaining insight into the superexchange path, which is responsible for the strong antiferromagnetic Co spin ordering along the chains.

Electronic structure and the origin of the Dzyaloshinskii-Moriya interaction in MnSi

The metallic helimagnet MnSi has been found to exhibit skyrmionic spin textures when subjected to magnetic fields at low temperatures. The Dzyaloshinskii-Moriya (DM) interaction plays a key role in stabilizing the skyrmion state. With the help of first-principles calculations, crystal field theory, and a tight-binding model we study the electronic structure and the origin of the DM interaction in the B20 phase of MnSi. The strength of the $\stackrel{P\vec}{D}$ parameter is determined by the magnitude of the spin-orbit interaction and the degree of orbital mixing, induced by the symmetry-breaking distortions in the B20 phase. Our calculations suggest strong coupling between Mn-$d$ and Si-$p$ states, which is consistent with a mixed valence ground state $|{d}^{7\ensuremath{-}x}{p}^{2+x}\ensuremath{\rangle}$ configuration. Consistent with previous calculations, we find that DFT+U leads to the experimental magnetic moment of $0.4\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$, which redistributes electrons between the majority and minority spin channels. We derive the magnetic interaction parameters $J$ and $\stackrel{P\vec}{D}$ for Mn-Si-Mn superexchange paths using Moriya's theory assuming the interaction to be mediated by ${e}_{g}$ electrons near the Fermi level. Using parameters from our calculations, we get reasonable agreement with the observations.

Role of pH value during material synthesis and grain-grain boundary contribution on the observed semiconductor to metal like conductivity transition in Ni1.5Fe1.5O4 spinel ferrite

Ni1.5Fe1.5O4 ferrite samples were synthesized by maintaining different pH values (1–12) during chemical reaction at 80 °C. The as-prepared samples were annealed at 1000 °C to form a cubic spinel structure. The heat treated samples were used for the study of electrical conductivity and dielectric properties. In this work, we understand the mechanism of unusual metal-like state in ferrite samples, characterized by negative temperature coefficient of conductivity. We have discussed various aspects, e.g., hopping mechanism through superexchange paths (Fe3+-O2−-Fe3+ and Ni2+-O2−-Ni2+), charge delocalized conduction mechanism affected by the magnetic spins order in t2g and eg electronic energy levels of B sites cations, grain size variation, relaxation of charge carriers at grains and grain boundaries of the particles, for outlining the mechanism of thermal activated charge localization (semiconductor state) and delocalization (metal-like state) effect in our samples. We have carried out a detailed analysis of conductivity spectra (Jonscher's power law fit, scaling of conductivity), impedance spectra (Cole-Cole plot by incorporating constant phase element), modulus spectra (Bergman proposed KWW function), and dielectric loss and dielectric constant spectra to extract the conductivity and relaxation contributions from grains, grain boundaries and space charge polarization in the samples.

Fabrication and magnetic properties of 4SC(NH2)2–Ni0.97Cu0.03Cl2 single crystals**Project supported by the National Natural Science Foundation of China (Grant Nos. 11404316 and 11304159), the Natural Science Foundation of Jiangsu Province, China, and the Science Foundation of Nanjing University of Posts and Telecommunications, China (Grant Nos. BK20140863 and NY213075).

Single crystals of 4SC(NH2)2–Ni1−xCuxCl2 (x = 0.03) (Cu-DTN) containing spin S = 1/2 Cu2 + and S = 1 Ni2+ cations are synthesized by slow evaporation methods. Structural characterization demonstrates that the Cu-DTN is of a tetrahedral structure with lattice parameter c being 9.0995 Å, which is 1.32% expansion compared with that of parent material DTN due to the larger radius of the Cu ion. Direct current (DC) susceptibility measurements show that both the antiferromagnetic exchange interaction at low temperature and the large anisotropy of susceptibilities are suppressed after doping the Cu ion, which could be related to the structural distortion and the increase of the super-exchange paths in Cu-DTN.

Syntheses and coordination chemistry of perfluoroaryl-1H-tetrazoles

Attempts to prepare fluorinated 5-aryl-1H-tetrazoles by various methods were successful for (3,5-bis(trifluoromethyl)phenyl)tetrazole (BTFTH) and in part for 5-pentafluoropenyltetrazole (F5PhTetH) and 5-(tetrafluoro-4-pyridinyl)tetrazole (F4PyTetH). For BTFTH, several different methods gave satisfactory yields and purity. For the other compounds, the same methods allowed only the observation of the desired species in very small quantities in ill-defined reaction product mixtures with azidodefluorination being the dominant side reaction. Negative ionising MS proved to be superior to other MS methods showing clean elimination of mainly N2 and CF2 from the tetrazolate anions. From BTFTH, the Cu complex [(BTFT)2Cu(Py)2]n could be obtained and fully characterised, including crystal structure determination (for the complex and the ligand). Magnetic and EPR data for the polymeric [(BTFT)2Cu(Py)2]n suggest that the superexchange (SE) between the Cu2+ ions (S=½) is negligible. This can be understood by examining the crystal structure and the details of the possible SE path.

Effect of divalent Ba cation substitution with Sr on coupled 'multiglass' state in the magnetoelectric multiferroic compound Ba3NbFe3Si2O14.

(Ba/Sr)3NbFe3Si2O14 is a magneto-electric multiferroic with an incommensurate antiferromagnetic spiral magnetic structure which induces electric polarization at 26 K. Structural studies show that both the compounds have similar crystal structure down to 6 K. They exhibit a transition, TN at 26 K and 25 K respectively, as indicated by heat capacity and magnetization, into an antiferromagnetic state. Although Ba and Sr are isovalent, they exhibit very different static and dynamic magnetic behaviors. The Ba-compound exhibits a glassy behavior with critical slowing dynamics with a freezing temperature of ~35 K and a critical exponent of 3.9, a value close to the 3-D Ising model above TN, in addition to the invariant transition into an antiferromagnetic state. The Sr-compound however does not exhibit any dispersive behavior except for the invariant transition at TN. The dielectric constant reflects magnetic behavior of the two compounds: the Ba-compound has two distinct dispersive peaks while the Sr-compound has a single dispersive peak. Thus the compounds exhibit coupled ‘multiglass’ behavior. The difference in magnetic properties between the two compounds is found to be due to modifications to super exchange path angle and length as well as anti-site defects which stabilize either ferromagnetic or antiferromagnetic interactions.

Effective Hamiltonian crystal fields: Present status and applicability to magnetic interactions in polynuclear transition metal complexes

The fundamentals of the Effective Hamiltonian Crystal Field (EHCF) method, used originally to calculate intra-shell excitations in the d-shells of coordination compounds of the first row transition metals, are reviewed. The formalism of effective operators is applied to derive an explicit form of the effective operator for a dipole moment in d-shell electronic subspace, allowing us to calculate the oscillator strengths of optical d-d transitions, which are otherwise forbidden when treated in the standard EHCF approach. EHCF methodology is also extended to describing magnetic interactions of the effective spin in several open d-shells of polynuclear coordination compounds. The challenging task of improving a precision of ∼1000 cm−1 (describing the excitation energies of single d-shells by EHCF) to one of ∼100 cm−1 for the energies required to reorient spins by an order of magnitude is considered within the same paradigm as EHCF: the targeted use of McWeeny’s group function approximation and the Lowdin partition technique. These are applied to develop an effective description of a d-system. This approach is tested on a series of binuclear complexes [{(NH3)5M}2O]4+ of trivalent cations featuring oxygen super-exchange paths in order to confirm the reproducibility of the trends in the series of exchange constants values for compounds that differ in the nature of their metal ions. The results from calculations are in reasonable agreement with the available experimental data and other theoretical methods.

Crystal structure and magnetic properties of Y2(Cu1−xMgx)2O5 obtained by SHS method

Abstract The single-phase polycrystalline samples of Y 2 (Cu 1− x Mg x ) 2 O 5 , x = 0.0, 0.05, 0.15 were successfully synthesized by a modified self-propagating high temperature synthesis. Effects of Mg +2 substitution for Cu +2 in metamagnetic Y 2 Cu 2 O 5 on its crystal structure and magnetic properties have been analyzed by X-ray diffraction and magnetic measurements performed within 2–300 K range. Mg doping was found to introduce small distortions in the main intradimmer superexchange paths so that ferromagnetic correlations decrease with Mg concentration. More significant impact of Cu substitution was found on the low temperature magnetism due to the breaking of infinite Cu–O chains. The formation of finite size chains introduces low temperature paramagnetic contribution and reduction in Neel temperature. Overall results give a strong indication that the antiferromagnetic ordering as well as metamagnetism persists up to the 15% of the Mg concentration.

Effective Hamiltonian Crystal Field As Applied to Magnetic Exchange Parameters in μ-Oxo-Bridged Cr(III) Dimers

The calculation of the 3d-intrashell excitations in coordination compounds by means of the Effective Hamiltonian Crystal Field (EHCF) method is generalized to their polynuclear analogues to properly describe several open d-shells and their magnetic interactions. This challenge requires improving the precision of ca. 1000 cm(-1) to ca. 100 cm(-1) characteristic for the spin-reorientation energies. The method follows the successful EHCF paradigm, namely, the concerted usage of McWeeny's group-function approximation and Löwdin's partitioning technique, for an effective description of the multicenter d-systems. The novel approach is implemented in the MagAîxTic package and validated against a series of binuclear complexes of Cr(III) featuring μ-oxygen superexchange paths. The trends in the compound series in terms of exchange constants are correctly reproduced, despite differing details of composition and structure, and the numerical results agree by order of magnitude with available experimental data and other theoretical methods.

Puzzling Polymorphism of Layered Ba(CoPO4)2

In this paper, we present the phase diagram and magnetic properties of the layered Co(2+)-based compounds of formula Ba(CoPO4)2 for which the rhombohedral γ form is well-known for its quasi-2D XY topology that is responsible for magnetization steps. The structural resolution of the new, room temperature-stable monoclinic α form shows similitude with the hydrated homologue Ba(CoPO4)2·H2O and consists of the stacking between [CoPO4](-) sheets with chain subunits. We show by means of high temperature powder XRD and thermal analyses that the α form transforms into several polymorphs also exhibiting layered architectures upon heating. Three reversible transitions at 773, 893, and 993 K were observed from DTA which allowed us to define several forms as follows: α → α' → α″ → β. The crystallographic relationships between the several polymorphs and hydrate analogue are discussed. The α' and α″ cell parameters involve a direct relationship with the α form, whereas the trigonal β phase was fully solved and found isomorphic with the compounds CaZn2P2O8 and BaAl2Si2O8. The magnetic study of the α form shows an antiferromagnetic ordering at T(N) = 17 K, with spins canting below T(N). Then, the analysis of the magnetic interactions paths occurring within the layers evidence superexchange paths and additional supersuperexchange paths between the chains. This scheme leads to a hexagonal frustrated topology responsible for the canted spin structure in a 2D-topology of anisotropic Co(2+) magnetic ions.

Copper rubidium diphosphate, Rb2Cu3(P2O7)2: synthesis, crystal structure, thermodynamic and resonant properties

A new compound, Rb2Cu3(P2O7)2, has been obtained from the melt in the Rb–Cu–P–O system. Its monoclinic crystal structure was determined by single-crystal X-ray diffraction: space group P21/c, Z = 2, a = 7.7119(8) Å, b = 10.5245(9) Å, c = 7.8034(9) Å, β = 103.862(5)° at 293 K, R = 0.030. The copper ions show coordination number (CN) 6 (4+2, distorted tetragonal bipyramidal). Trimers of [CuO6] polyhedra sharing cis-edges form together with diphosphate groups of two tetrahedra [P2O7] a microporous 3D framework with channels open along the c direction. The rubidium ions positioned in the channels show CN 10. The new phase is isotypic to Cs2Cu3(P2O7)2. The regular changes in cell dimensions in the row Cs2Cu3(P2O7)2 → Rb2Cu3(P2O7)2 are caused by the compression of channel volumes due to decrease of the Cu–O–P angles in the framework windows. An electron spin resonance study indicates appearance of short range magnetic correlations below ∼120 K, long range magnetic order takes place at TN = 9.2 K as follows from magnetization and specific heat measurements. First principles calculations of the magnetic exchanges indicate that the effective Cu–Cu hopping interactions corresponding to super–super-exchange paths involving P atoms are much stronger than those within the edge-sharing Cu2–Cu1–Cu2 trimer units.

Lattice and spin excitations in multiferroich-YbMnO3

Lattice and spin excitations have been studied by Raman scattering in hexagonal YbMnO3 single crystals. The temperature dependences of the phonon modes show that the E2 mode at 256 cm-1 related to the displacement of Mn and O ions in a-b plane is coupled to the spin order. The A1 phonon mode at 678 cm-1 presents a soft mode behavior at the Neel temperature. Connected to the motion of the apical oxygen ions along the c direction, this mode controls directly the Mn-Mn interactions between adjacent Mn planes and the superexchange path. Crystal field and magnon mode excitations have been identified. The temperature investigation of the spin excitations shows that the spin structure is strongly influence by the Yb-Mn interaction. Under a magnetic field along the c axis, we have investigated the magnetic reordering and its impact on the spin excitations.

Novel alkaline earth copper germanates with ferro and antiferromagnetic S=1/2 chains

Two new alkaline earth copper(II) germanates were hydrothermally synthesized: CaCuGeO4·H2O (1) and BaCu2Ge3O9·H2O (2), and their structures determined by single crystal X-ray diffraction. Compound (1) crystallizes in space group P21/c with a=5.1320(2)Å, b=16.1637(5)Å, c=5.4818(2)Å, β=102.609(2)°, V=443.76(3)Å3 and Z=4. This copper germanate contains layers of composition [CuGeO4]∞2− comprising CuO4 square planes and GeO4 tetrahedra with calcium and water molecules in the inter-layer space. Compound (2) crystallizes in the Cmcm space group with a=5.5593(3)Å, b=10.8606(9)Å, c=13.5409(8)Å, V=817.56(9)Å3 and Z=4. This structure contains GeO6 and CuO6 octahedra as well as GeO4 tetrahedra, forming a three-dimensional network of interconnecting six-membered ring channels. The magnetic susceptibility for both samples can be interpreted as S=1/2 chains, in agreement with the copper topology observed in the crystal structure. The susceptibility of (1) exhibits a Bonner–Fisher type behavior, resulting from antiferromagnetic intra-chain interactions without three-dimensional ordering down to 5K—the lowest measured temperature. This observation, together with the absence of super-exchange paths between the copper chains, make this system particularly promising for the study of low dimensional magnetism. The magnetic properties of (2) show a very weak ferromagnetic near-neighbor interaction along the chain. In this compound a peak the χT plot seems to indicate the onset of interchain antiferromagentic correlations. However, no ordering temperature is detected in the susceptibility data.

Ionothermal Synthesis of the First Luminescent Open‐Framework Manganese Borophosphate with Switchable Magnetic Properties

AbstractA new open‐framework manganese borophosphate, KMnBP2O7(OH)2, has been prepared under ionothermal conditions in the ionic liquid 1‐butyl‐1‐methylpyrrolidinium bromide. Single‐crystal X‐ray diffraction analysis showed that KMnBP2O7(OH)2 [monoclinic, P21/c, a = 6.659(1), b = 12.049(2), c = 9.790(2) Å, β = 109.12(3)°, V = 742.2(3) Å3, and Z = 4] has an unprecedented open‐framework structure. {MnO5(OH)} octahedra and {BO3(OH)} and {PO3(OH)}/{PO4} tetrahedra link to form a 3D framework with distorted eight‐membered‐ring channels along the crystallographic [001] direction. The negative charges of the inorganic framework are balanced by K+ ions located in the channels. At room temperature, KMnBP2O7(OH)2 shows a bright orange‐red luminescence that redshifts with decreasing temperature. The luminescence lifetimes were determined to be 0.17 ms at room temperature and 3.49 ms at 77 K. Below 3 K, d5 Mn2+ of two neighboring, edge‐sharing {MnO5OH}2 units couple antiferromagnetically, and the magnetic state can be best described as a singlet spin system. These singlets continuously turn into triplets when increasing the magnetic field up to 7 T at 2 K due to relatively weak spin–spin coupling through two identical 98.92(6)° Mn–O–Mn superexchange paths, turning the system into a metastable ferromagnetic state.

Does B3LYP correctly describe magnetism of manganese complexes with various oxidation numbers and various structural motifs?

We assessed the applicability and basis set dependency of the B3LYP functional to investigate magnetic interactions of Mn complexes. For the purpose, we constructed a test set consisting of 16 Mn complexes with various oxidation states and structural motifs. The B3LYP results correctly reproduced magnetism and magneto–redox correlation of the standard μ-oxo motifs with superexchange paths, while it does not work for weak magnetic complexes. We also showed that a modest basis set yields results similar to those of triple-zeta plus diffuse-and-polarization functions. This basis set is expected to be a standard basis set for investigating magnetism of manganese complexes.

Symmetrical interfacial reconstruction and magnetism in La0.7Ca0.3MnO3/YBa2Cu3O7/La0.7Ca0.3MnO3heterostructures

We have analyzed the interface structure and composition of La${}_{0.7}$Ca${}_{0.3}$MnO${}_{3}$/YBa${}_{2}$Cu${}_{3}$O${}_{7}$/La${}_{0.7}$Ca${}_{0.3}$MnO${}_{3}$ trilayers by combined polarized neutron reflectometry, aberration-corrected microscopy, and atomic column resolution electron-energy-loss spectroscopy and x-ray absorption with polarization analysis. We find the same stacking sequence at both top and bottom cuprate interfaces. X-ray magnetic circular dichroism experiments show that both cuprate interfaces are magnetic with a magnetic moment induced in Cu atoms as expected from symmetric Mn-O-Cu superexchange paths. These results supply a solid footing for the applicability of recent theories explaining the interplay between magnetism and superconductivity in this system in terms of the induced Cu spin polarization at both interfaces [J. Salafranca and S. Okamoto, Phys. Rev. Lett. 105, 256804 (2010)].

CaCu2(SeO3)2Cl2: Spin-12Heisenberg chain compound with complex frustrated interchain couplings

We report the crystal structure, magnetization measurements, and band-structure calculations for the spin-1/2 quantum magnet CaCu2(SeO3)2Cl2. The magnetic behavior of this compound is well reproduced by a uniform spin-1/2 chain model with the nearest-neighbor exchange of about 133 K. Due to the peculiar crystal structure, spin chains run in the direction almost perpendicular to the structural chains. We find an exotic regime of frustrated interchain couplings owing to two inequivalent exchanges of 10 K each. Peculiar superexchange paths grant an opportunity to investigate bond-randomness effects under partial Cl-Br substitution.

Long-range superexchange in Cu2A2O7(A=P, As, V) as a key element of the microscopic magnetic model

A microscopic magnetic model for alpha-Cu2P2O7 is evaluated in a combined theoretical and experimental study. Despite a dominant intradimer coupling J1, sizable interdimer couplings enforce long-range magnetic ordering at T_N=27 K. The spin model for alpha-Cu2P2O7 is compared to the models of the isostructural beta-Cu2V2O7 and alpha-Cu2As2O7 systems. As a surprise, coupled dimers in alpha-Cu2P2O7 and alternating chains in alpha-Cu2As2O7 contrast with a honeycomb lattice in beta-Cu2V2O7. We find that the qualitative difference in the coupling regime of these isostructural compounds is governed by the nature of AO4 side groups: d-elements (A = V) hybridize with nearby O atoms forming a Cu-O-A-O-Cu superexchange path, while for p-elements (A = P, As) the superexchange is realized via O-O edges of the tetrahedron. Implications for a broad range of systems are discussed.