Dominant Magnetic Interactions Research Articles

Ferromagnetic Order from p-Electrons in Rubidium Oxide

Magnetic dioxygen molecules can be used as building blocks of model systems to investigate spin-polarization that arises from unpaired p-electrons, the scientific potential of which is evidenced by phenomena such as spin-polarized transport in graphene. In solid elemental oxygen and all of the known ionic salts comprised of magnetic dioxygen anions and alkali metal cations, the dominant magnetic interactions are antiferromagnetic. We have induced novel ferromagnetic interactions by introducing oxygen deficiency in rubidium superoxide (RbO(2)). The anion vacancies in the resulting phase with composition RbO(1.72) provide greater structural flexibility compared to RbO(2) and facilitate a Jahn-Teller-driven order-disorder transition involving the anion orientations at similar to 230 K, below which their axes become confined to a plane. This reorganization gives rise to short-range ferromagnetic ordering below similar to 50 K. A ferromagnetic cluster-glass state then forms below similar to 20 K, embedded in an antiferromagnetic matrix that orders at similar to 5 K. We attribute this inhomogeneous magnetic order to either subtly different anion geometries within different structural nanodomains or to the presence of clusters in which double exchange takes place between the anions, which are mixed-valence in nature. We thus demonstrate that nonstoichiometry can be employed as a new route to induce ferromagnetism in alkali metal oxides.

Effects of Fe doping and Fe–N-codoping on magnetic properties of SnO 2 prepared by chemical co-precipitation

The effects of Fe-doping and Fe–N-codoping on the magnetic properties of SnO 2, prepared by chemical co-precipitation technique, are investigated in details. We found that the paramagnetism is the dominant magnetic interaction in Fe doped SnO 2. A weak antiferromagnetic coupling between Fe 2+ ions is also confirmed through Zero field-cooled (ZFC) and field-cooled (FC) magnetization studies. On the other hand, hystersis behavior is observed for Fe–N-codoped SnO 2 samples with coercivity H c∼420 and 352 Oe for x=0.05 and 0.10, respectively. As no other secondary or impurity phase is detected by XRD study and the presence of N is confirmed by EDX analysis, this observed ferromagnetism is originated due to the substitution of N in Sn 1− x Fe x O 2. N doping at the oxygen site can be regarded as defect and introduces a hole in this system. As a result, a hole-induced ferromagnetism might be the origin of the observed ferromagnetism in Fe–N-codoped SnO 2 samples.

Ferromagnetically Coupled Shastry-Sutherland Quantum Spin Singlets in(CuCl)LaNb2O7

A thorough crystal structure determination at very low temperature of (CuCl)LaNb₂O₇, originally proposed as a spin-1/2 square-lattice antiferromagnet, is reported thanks to the use of single-crystal x-ray diffraction and powder neutron diffraction. State-of-the-art calculations (maximum entropy method) reveal that (CuCl)LaNb₂O₇ is orthorhombic with Pbam symmetry. First-principles calculations demonstrate that the dominant magnetic interactions are antiferromagnetic between fourth nearest neighbors with a Cu-Cl-Cl-Cu exchange path, which lead to the formation of spin singlets. The two strongest interactions between the singlets are ferromagnetic, which makes (CuCl)LaNb₂O₇ the first system of ferromagnetically coupled Shastry-Sutherland quantum spin singlets.

Metal Monophosphonates M{(2-C5H4NO)CH2PO3}(H2O)2 (M = Co, Ni, Mn, Cd): Synthesis, Structure, and Magnetism

Four isostructural metal monophosphonates, M{(2-C(5)H(4)NO)CH(2)PO(3)}(H(2)O)(2) with M = Co (1), Ni (2), Mn (3), and Cd (4), were synthesized and structurally characterized. These compounds show a double-chain structure in which the M(2)(μ-O)(2) dimers are connected by O-P-O bridges. The magnetic responses of 1-3 were investigated over a wide range of magnetic fields (up to 10 T) and temperatures (down to 50 mK). Except for 4, which is weakly diamagnetic from 2 K to room temperature, the dominant magnetic interactions are antiferromagnetic. Isothermal magnetic field sweeps at 50 mK provide signatures in the magnetic responses that are associated with antiferromagnetic to field-induced fully polarized (magnetically saturated) transitions. Analysis of the magnetic data indicates that 1 and 2 form magnetic dimer-like clusters with weak dimer-dimer interactions present. Contrastingly, the magnetic interactions present in 3 are significantly weaker, so a definitive description of the magnetism of this compound is elusive.

Synthesis and characterization of La 1+ xSr 2− xCoMnO 7− δ ( x=0,0.2; δ=0,1)

The n=2 Ruddlesden–Popper phases LaSr 2CoMnO 7 and La 1.2Sr 1.8CoMnO 7 have been synthesized by a sol–gel method. The O6-type phases LaSr 2CoMnO 6 and La 1.2Sr 1.8CoMnO 6 were produced by reduction of the O7 phases under a hydrogen atmosphere. The materials crystallize in the tetragonal I4/ mmm space group with no evidence of long-range cation order in the neutron and electron diffraction data. Oxygen vacancies in the reduced materials are located primarily at the common apex of the double perovskite layers giving rise to square pyramidal coordination around cobalt and manganese ions. The oxidation states Co 3+/Mn 4+ and Co 2+/Mn 3+ predominate in the as-prepared and reduced materials, respectively. The materials are spin glasses at low temperature and the dominant magnetic interactions change from ferro- to antiferromagnetic following reduction.

Grafting Derivatives of Mn6 Single-Molecule Magnets with High Anisotropy Energy Barrier on Au(111) Surface

We study the magnetic properties of two new functionalized single-molecule magnets belonging to the Mn 6 family (general formula [Mn (III)6O2(R-sao)6(O2C-th)2L(4-6)], where R=H (1) or Et (2), HO2C-th=3-thiophene carboxylic acid, L=EtOH, H2O and saoH2 is salicylaldoxime) and their grafting on the Au(111) surface. Complex 1 exhibits spin ground-state S=4, as the result of ferromagnetic coupling between the two antiferromagnetic Mn (III) 3 triangles, while slight structural changes in complex 2, switch the dominant magnetic exchange interactions from anti- to ferromagnetic, enhancing the spin ground-state to S=12 and, consequently, the effective energy barrier for the relaxation of magnetization. Direct-current and alternating-current magnetic susceptibility measurements show that the functionalized complexes preserve the main magnetic properties of the corresponding not-functionalized Mn 6 clusters (i.e., total spin value and magnetic behavior as a function of temperature), though a reduction of the anisotropy barrier is observed in complex 2. For both complexes, the -O2C-th functionalization allows the direct grafting on Au(111) surface by liquid-phase deposition. X-ray photoemission spectroscopy demonstrates that the stoichiometry of the molecular cores is preserved after grafting. Scanning tunneling microscopy (STM) reveals a sub-monolayer distribution of isolated clusters with a slightly higher coverage for complex 1. The cluster stability in the STM images and the S-2p energy positions demonstrate, for both derivatives, the strength of the grafting with the gold surface.

Crystal Structure and Physical Properties of Fe1.5Pb5.5In10S22

A new compound, Fe2+(1.5)Pb2+(5.5)In3+(10)S2-(22), was found in the Fe-Pb-In-S system. The crystals had a shiny metallic gray luster and were obtained in a barlike shape. The melting and recrystallizing points are 1098 and 1090 K, respectively. The crystal structure determined by X-ray diffraction is P2/m, a = 14.558(1) A, b = 3.8556(3) A, c = 15.558(1) A, beta = 96.876(1) degrees , V = 867.0(1) A(3), Z = 1, Dc = 5.893 g/cm(3), and R1 = 3.78%, which is an isostructure with Sn(5.5)In(11)S(22). In the structure, lead atoms exist at three crystallographically independent sites, and one of them, Pb1, is occupied by a quarter of indium atoms. All of the Fe ions are randomly distributed at six indium sites. This compound is a semiconductor with a band gap, E(a) = 0.95 eV. The dominant magnetic interaction is antiferromagnetic, but magnetic orderings are not observed down to 2 K.

Optical properties of bulk Zn 1− xCo xO magnetic semiconductors

Polycrystalline Zn 1− x Co x O ( x=0, 0.02, 0.05, 0.10 and 0.15) oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in open air. X-ray diffraction analyses using Rietveld refinement indicate that a stoichiometric single phase with a wurtzite-like structure was found in Zn 1− x Co x O samples with x up to 0.10. The elemental mapping using energy dispersive X-ray spectroscopic analyses presents a uniform distribution of Co. Optical transmittance measurements show that several extra absorption bands appear in the Co-doped ZnO, which is due to the transitions between the crystal-field-split 3d levels of tetrahedral Co 2+ substituting Zn 2+ ions. Raman measurements show that limited host lattice defects are induced by Co doping. Magnetization measurements reveal that the Co-doped ZnO samples are paramagnetic due to the absence of free carriers and in low temperature the dominant magnetic interaction is nearest-neighbor antiferromagnetic.

Magnetic interactions of (μ-pyrazolato)-bridged copper(II) complexes determined by solid-state MAS NMR

We investigated electron spin densities of pyrazolato-bridged complexes [Cu(pz)2]n (1) and [Cu2(pz)2(NO3)(H2O)(phen)2]NO3 (2) (Hpz = pyrazole, phen = 1,10-phenanthroline) using solid-state high-resolution NMR to elucidate the magnetic interaction paths with the help of molecular orbital theory. We prepared deuterated analogue of these complexes, 1-d6 and 2-d6, to measure temperature dependence of 2H and 13C NMR shifts between 190 and 350 K. The hyperfine coupling constants (HFCCs) and electron spin densities were determined from the slopes of the shifts as a function of the magnetic susceptibilities. The derived spin densities were all positive, which indicates the dominant magnetic interaction paths of these complexes are not π but σ orbitals of the pyrazolate ligand. The NMR results reasonably agreed with those of density functional theory (DFT) calculations for molecular models of 1 and 2.

Magnetic properties of Co-doped ZnO nanoparticles prepared by vaporization-condensation in a solar reactor

We report on the magnetic properties of Co-doped (5% and 10% molar) ZnO nanoparticles prepared by the vaporization-condensation method in a solar reactor. X-ray diffraction patterns do not show evidence of Co segregation or any other phase different from wurtzite-type ZnO. High-resolution electron microscopy shows no traces of Co clusters or any other secondary phase. Samples prepared at 70–100Torr inside glass reactors are paramagnetic and the dominant magnetic interactions are antiferromagnetic in nature. Increasing the Co content promotes a reinforcement of the antiferromagnetic interactions. A reduction of the high-temperature effective paramagnetic moment is also observed.

Leading interactions in theβ−SrV6O15compound

The present study shows that the electronic structure of the $\beta$-$AV\_6O\_{15}$ family of compounds ($A = Sr, Ca, Na ...$) is based on weakly interacting two-leg ladders, in contrast with the zig-zag chain model one could expect from their crystal structure. Spin dimer analysis, based on extended H\"{u}ckel tight-binding calculations, was performed to determine the structure of the dominant transfer and magnetic interactions in the room temperature $\beta$-$SrV\_6O\_{15}$ phase. Two different two-legs ladders, associated with different charge/spin orders are proposed to describe these one-dimensional $\beta$-type systems. The antiferromagnetic ladders are packed in an 'IPN' geometry and coupled to each other through weak antiferromagnetic interactions. This arrangement of the dominant interactions explains the otherwise surprising similarities of the optical conductivity and Raman spectra for the one-dimensional $\beta$-type phases and the two-dimensional $\alpha$-type ones such as the well-known $\alpha^\prime$-$NaV\_2O\_5$ system.

Hybrid density functional theory study of vanadium monoxide

First-principles calculations of nondefective VO in the $\mathrm{Fm}3\ifmmode\bar\else\textasciimacron\fi{}m$ structure based on a range of single-particle Hamiltonians containing varying amounts of exact exchange indicate that the ground electronic state is that of a ${d}^{3}$ high spin, antiferromagnetic (AF), Mott-Hubbard insulator with an ${\mathrm{AF}}_{1}$ spin alignment of the local cation moments. This description remains essentially unchanged down to 10% exact exchange, and only for the pure density functional theory (DFT) potential is the ${\mathrm{AF}}_{1}$ phase found to be metallic. Strong spin-lattice interaction is predicted with differences in lattice constant of up to 1.6% between ${\mathrm{AF}}_{1}$ and FM (ferromagnetic) order. The ${\mathrm{AF}}_{1}$ lattice constant is predicted to be $\ensuremath{\sim}4.37\AA{},$ which is roughly 7% greater than the reported lattice constants for the defective material. The bulk modulus is comparable to those of CaO, MnO, and NiO. A mapping of total energies onto an Ising spin Hamiltonian containing both direct and superexchange interactions reveals the dominant magnetic interaction to be the direct coupling of antiferromagnetically aligned nearest-neighbor cation spins, which leads to the stability of the ${\mathrm{AF}}_{1}$ phase. Direct coupling energies are found in the range $\ensuremath{-}11.1$ to $\ensuremath{-}44.3\mathrm{meV}$ as the proportion of exact exchange is reduced, leading to an estimated critical disorder temperature in the range 300--450 K. However, the limitations of such mapping are exposed by a consideration of the relative stabilities of the ${\mathrm{AF}}_{1}$ and ${\mathrm{AF}}_{3}$ alignments. Orbital projected densities of states reveal filled to unfilled gaps which depend strongly on the proportion of exact exchange and for the B3LYP potential (20% exact exchange) are $\ensuremath{\sim}2.5\mathrm{eV}$ for the spin-forbidden $xy(\ensuremath{\uparrow})\ensuremath{\rightarrow}xy(\ensuremath{\downarrow})$ excitation, $\ensuremath{\sim}3.0\mathrm{eV}$ for $xy(\ensuremath{\uparrow})\ensuremath{\rightarrow}{z}^{2}(\ensuremath{\uparrow}),$ and $\ensuremath{\sim}3.5\mathrm{eV}$ for $\stackrel{\ensuremath{\rightarrow}}{V}\mathrm{O}$ charge transfer. Variationally stable, highly local crystal-field excited states ranging in energy from $\ensuremath{\sim}0.6$ to $\ensuremath{\sim}2.7\mathrm{eV}$ are predicted for exchange-correlation potentials down to 30% exact exchange and from comparisons with the corresponding band excitation estimates of $\ensuremath{\sim}1$ to $\ensuremath{\sim}2\mathrm{eV}$ are obtained for the localization energy of Frenkel excitons. From a mapping of the excited crystal-field energies onto a Kanamori Hamiltonian, values are obtained for the lattice Racah B and C parameters and the $d$-orbital averaged exchange and crystal-field energies. A comparison of mapped and directly calculated energies of the two-electron excitation $(xz,yz)\ensuremath{\rightarrow}{(z}^{2}{,x}^{2}\ensuremath{-}{y}^{2})$ confirms the validity of Kanamori mapping, notably in the limit of exact exchange.

Magnetism in polycrystalline cobalt-substituted zinc oxide

We present results of the preparation (by precursor decomposition), characterization (with x-ray diffraction, transmission electron microscopy, and dc magnetization measurements), and detailed computation (using density functional theory) of polycrystalline ZnO with up to 15% substitution of the zinc sites by divalent cobalt. The experimental results indicate the dominant magnetic interaction in well-characterized stoichiometric phases ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{O},$ $x=0.05,$ 0.10, 0.15, is nearest-neighbor antiferromagnetic. Our computations suggest robust ferromagnetism will only occur in the presence of additional hole doping.

Electronic, Magnetic, and Magnetoresistance Properties of the n=2 Ruddlesden–Popper Phases Sr3Fe2−xCoxO7−δ (0.25≤x≤1.75)

A series of oxygen-deficient n=2 Ruddlesden–Popper phases, Sr3Fe2−xCoxO7−δ (0.25≤x≤1.75), were prepared by solid-state reactions. Temperature-dependent susceptibility and field-dependent magnetization data indicate that for x≥0.25 the dominant magnetic interactions are ferromagnetic. The onset of strong ferromagnetic interactions is evident at ∼200 K, and a transition to a cluster-glass state is observed for all compositions below ∼45 K. The temperature variation of resistivity for all the compounds shows variable-range hopping behavior with two different localization energy scales: one for T<40 K and another for T>80 K. Large negative magnetoresistance (the largest MR ∼−65% for x=0.25) is observed for all phases. The magnetic susceptibility, Mössbauer and X-ray absorption near-edge spectroscopy data indicate that the formal oxidation state of Fe is close to 4+. The key role of d delocalization in the Sr3Fe2−xCoxO7−δ system is compared to the Sr3Fe2−xMnxO7−δ series, where d localization dominates the properties.

Paramagnetic susceptibility and ferromagnetism in Sr2FeMoO6 perovskites

In this article, we report a detailed study of the paramagnetic susceptibility χ of Sr2FeMoO6 ceramic samples prepared under various conditions. We show that χ−1(T,H) has a rather unconventional behavior in the 800 K&amp;gt;T&amp;gt;TC(≈400 K) temperature range, namely, a field-dependent χ−1(T) slope which is also sensitive to the synthesis conditions. Detailed analysis and comparison of samples allow us to conclude that this behavior is nonintrinsic but rather reflects the sample quality. We quantitatively show that small amounts of ferromagnetic impurities (&amp;lt;0.1%) can account for the observed behavior. In spite of this, we demonstrate that the dominant magnetic interactions in this system are ferromagnetic, thus strongly supporting a double-exchange mechanism for magnetic interactions.

Magnetic and transport properties of III–V diluted magnetic semiconductor Ga1−xCrxAs

Magnetic and transport properties of Cr-based III–V diluted magnetic semiconductor Ga1−xCrxAs Cr concentrations up to x=0.10 have been investigated. For all the films, no long-range magnetic order was observed down to 2 K. A sign of paramagnetic Curie temperature is positive, indicating that the dominant magnetic interaction between Cr atoms is ferromagnetic. The effective number of Bohr magneton is estimated to be 5.1±0.4, which is close to that of Cr2+ ion. The magnetization curve shows superparamagnetic behavior at low temperatures. The radius of the local spin order in x=0.034 is estimated to be 1.5–2.0 nm at T=5 K and it decreases with increasing temperature. From Hall effect measurements at room temperature, the magnitude of mobility is estimated to be less than 0.5 cm2/V s. This low mobility strongly suggests that the hopping conductivity is dominant in Ga1−xCrxAs films.

Magnetic-field-induced structural transitions in a ferrofluid emulsion.

A ferrofluid emulsion, subjected to a slowly increasing magnetic field, exhibits a complicated structural behavior: a gas of Brownian particles changes to columnar solid structures due to induced dipole interaction. Two transition (intermediate) structural regimes are observed: (i) randomly distributed chains and particles and (ii) distinct thin columns and randomly distributed chains and particles. Three structural transition magnetic fields are found, one marking each structural transition, from the initial to the final structural regime. A structural diagram of the structural transition magnetic fields, H(C), versus particle volume fractions, straight phi, is constructed experimentally. Theoretical models of scaling calculations, based upon the dominant magnetic interaction in each structural regime, give the three structural transition magnetic-field relations as H(C1) proportional to straight phi(-1/2), H(C2) proportional to straight phi(-1/4), and H(C3) proportional to (straight phi(gamma)/G2)exp(piG/straight phi((gamma/2))), where gamma=0.39 and G=0.29 for our sample. The final end shape of columns and the relative position between columns show that the end-end repulsion between chains is important in the structural formation.

Absorption Edge and Magnetic Behaviour of a New Hexagonal System

Single crystals of Cd1-xMnxInGaS4 with 0.2 ≤ x ≤ 0.9 were grown for the first time by the chemical transport method. The energy gap from the absorption spectra shows a concentration dependence between Eg=2.29 eV for x = 0.4 and Eg=2.12 eV for x = 0. 9.The temperature dependence of the optical energy gap is well represented by the Varshni equation.The temperature dependence of the inverse susceptibility in the high temperature range (80-300K) displayed a Curie-Weiss behaviour for all x values. The paramagnetic Curie temperatures were all negative indicating that the dominant magnetic interactions between the Mn ions are antiferromagnetic.

A weakly coupled molecular spin-ladder (BEDT-TTF)Zn(SCN) 3

The title compound, (BEDT-TTF)Zn(SCN) 3, has two uniform-stacks of BEDT-TTF +. The extended-Hückel calculation reveals that the dominant magnetic interaction is in the stack direction, while it is about 1/10 between the stacks. Thus the title compound is a weakly-coupled spin-ladder. The existence of spin-gap (340 K) is for the first time confirmed for the weak-coupling limit, by measuring static magnetization. The tight-binding band calculation reveals that, if a metallic state is realized by a carrier-doping, it will be stable against penodic lattice distortions.

Cluster-variation-method simulations of theM1−xMx′Pt3(M,M′=Mn,Fe,Co)magnetic phase diagrams with competing magnetic interactions

The alloying of isomorphic L12 ferromagnetic CoPt3, MnPt3, and antiferromagnetic FePt3 yields pseudobinary ~M 12xMx 8Pt3! compounds whose magnetic phase diagrams display paramagnetic, ferromagnetic, antiferromagnetic, spin glass, and reentrant spin glass regions, depending on concentration and temperature. We show that it is possible to reproduce many features of these phase diagrams, namely, the Curie and Neel temperatures, using a simple Ising Hamiltonian, treated in the pair approximation of a square cluster-variation method, with only three dominant magnetic interactions between 3d elements, the magnetic role of Pt being neglected. The homoatomic interactions ~JMM , JM8M8! are given by the magnetic transitions of the binary compounds, whereas the heteroatomic JMM8 interaction is either parametrized or fitted and left free to vary with concentration. @S0163-1829~97!01225-3#