Strength Of Magnetic Coupling Research Articles

Hybridization, superexchange, and competing magnetoelastic interactions in TiOBr

A crystalline sample of TiOBr is probed at room temperature by a combination of electron spectroscopies and the results are compared to theoretical embedded-cluster calculations. Resonant photoemission of the valence band confirms that the lowest binding energy feature arises from the singly occupied $\mathrm{Ti}\phantom{\rule{0.2em}{0ex}}3d$ orbital. The polarization dependence of this orbital in nonresonant photoemission is consistent with the expected dominant ${d}_{{y}^{2}\ensuremath{-}{z}^{2}}$ character. The analysis of the $\mathrm{Ti}\phantom{\rule{0.2em}{0ex}}{L}_{2,3}$ x-ray absorption spectra confirms the complete splitting of the $\mathrm{Ti}\phantom{\rule{0.2em}{0ex}}3d$ shell. X-ray absorption and resonant photoemission at the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}1s$ edge provide direct evidence for hybridization between the transition metal orbitals and the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ levels, which leads to superexchange interactions between the Ti ions. The existence of a mixing of O and Ti states and of strong superexchange interactions is supported by calculations of the ground-state electronic and magnetic properties. The calculated superexchange interchain interaction is one fifth in strength of the total magnetic coupling along the chain, and is antiferromagnetic in character. This O-mediated interchain interaction is frustrated in the room temperature phase of TiOBr and thus couples strongly to distortions of the soft lattice. The competition between the interchain magnetoelastic coupling and the spin-Peierls interaction might be at the origin of the complex $\mathrm{T}\mathrm{i}\mathrm{O}X$ phase diagram.

Spin-flop transition in uniaxial antiferromagnets: Magnetic phases, reorientation effects, and multidomain states

The classical spin-flop is the field-driven first-order reorientation transition in easy-axis antiferromagnets. A comprehensive phenomenological theory of easy-axis antiferromagnets displaying spin-flops is developed. It is shown how the hierarchy of magnetic coupling strengths in these antiferromagnets causes a strongly pronounced two-scale character in their magnetic phase structure. In contrast to the major part of the magnetic phase diagram, these antiferromagnets near the spin-flop region are described by an effective model akin to uniaxial ferromagnets. For a consistent theoretical description both higher-order anisotropy contributions and dipolar stray-fields have to be taken into account near the spin-flop. In particular, thermodynamically stable multidomain states exist in the spin-flop region, owing to the phase coexistence at this first-order transition. For this region, equilibrium spin-configurations and parameters of the multidomain states are derived as functions of the external magnetic field. The components of the magnetic susceptibility tensor are calculated for homogeneous and multidomain states in the vicinity of the spin-flop. The remarkable anomalies in these measurable quantities provide an efficient method to investigate magnetic states and to determine materials parameters in bulk and confined antiferromagnets, as well as in nanoscale synthetic antiferromagnets. The method is demonstrated for experimental data on the magnetic properties near the spin-flop region in the orthorhombic layered antiferromagnet (C_2H_5NH_3)_2CuCl_4.

First-principles and Monte Carlo combinational study on Zn 1− xCo xO diluted magnetic semiconductor

The electronic structures and magnetic properties of Zn 1− x Co x O ( x = 5.55 % , 8.33 % , 12.5 % ) are studied using first-principles calculations in combination with Monte Carlo (MC) simulation. The combinational method makes possible a complete simulation from the microscopic magnetic interaction to macroscopic magnetic behavior. The calculated results from first principles indicate that the ferromagnetic ground state is stabilized by a half-metallic electronic structure which originates from the strong hybridization between Co 3d electrons and O 2p electrons. With the magnetic coupling strengths obtained from first-principles calculations, the MC simulation predicts the ferromagnetism of Zn 1− x Co x O ( x = 5.55 % , 8.33 % , 12.5 % ) with T c = 220 , 360 , 530 K , which is consistent with the experimental facts.

Spin ladder with anisotropic ferromagnetic legs in a transverse magnetic field

We have studied the ground-state phase diagram of a two-leg spin ladder with anisotropicferromagnetic leg couplings under the influence of a symmetry-breaking transversemagnetic field by the exact diagonalization technique. In the case of antiferromagneticcoupling between legs we identified two phase transitions in the plane of magnetic fieldversus interchain coupling strength. The first corresponds to the transition from the gappedrung-singlet phase to the gapped stripe-ferromagnetic phase. The second represents thetransition from the gapped stripe-ferromagnetic phase into the fully polarizedferromagnetic phase.

Magnetic coupling in the mixture of different doped colossal magnetoresistances

Uniform bulk mixtures of micron-sized La0.7Ce0.3MnO3 and La0.7Ca0.3MnO3 colossal magnetoresistance materials have been made by standard solid-state reaction. According to the x-ray diffraction and the energy dispersion spectrum, La0.9Ce0.3MnO3 is composed of (La1−εCeε)yMnO3+δ, CeO2, and Mn–O compounds, where (La1−εCeε)yMnO3+δ behaves as a ferromagnetic material and shows no atomic interdiffusion with La0.7Ca0.3MnO3. The temperature characteristics of magnetization measurement show that the canted antiferromagnetic interaction occurs at low temperatures. The dependence of these antiferromagnetic transition temperatures on the mixing ratio can be described by the molecular mean field theory, and this reveals the strength of magnetic couplings between magnetic grains.

Supramolecular Co(II)-[2 × 2] Grids: Metamagnetic Behavior in a Single Molecule

The magnetic anisotropy of the supramolecular [2 x 2] grid [Co(II)4L4]8+, with a bis(bipyridyl)-pyrimidine-based ligand L, was investigated by single-crystal magnetization measurements at low temperatures. The magnetization curves exhibit metamagnetic-like behavior and are explained by the weak-exchange limit of a minimal spin Hamiltonian including Heisenberg exchange, easy-axis ligand fields, and the Zeeman term. It is also shown that the magnetic coupling strength can be varied by the substituent R1 in the two-position on the central pyrimidine group of the ligand L.

Radio and X-ray properties of relativistic beaming models for ultraluminous X-ray sources

We calculate the broad-band radio-X-ray spectra predicted by microblazar and microquasar models for ultraluminous X-ray sources (ULXs), exploring the possibility that their dominant power-law component is produced by a relativistic jet, even at near-Eddington mass accretion rates. We do this by first constructing a generalized disc-jet theoretical framework in which some fraction of the total accretion power, P a , is efficiently removed from the accretion disc by a magnetic torque responsible for jet formation. Thus, for different black hole masses, mass accretion rates and magnetic coupling strength, we self-consistently calculate the relative importance of the modified disc spectrum, as well as the overall jet emission due to synchrotron and Compton processes. In general, transferring accretion power to a jet makes the disc fainter and cooler than a standard disc at the same mass accretion rate; this may explain why the soft spectral component appears less prominent than the dominant power-law component in most bright ULXs. We show that the apparent X-ray luminosity and spectrum predicted by the microquasar model are consistent with the observed properties of most ULXs. We predict that the radio synchrotron jet emission is too faint to be detected at the typical threshold of radio surveys to date. This is consistent with the high rate of non-detections over detections in radio counterpart searches. Conversely, we conclude that the observed radio emission found associated with a few ULXs cannot be due to beamed synchrotron emission from a relativistic jet.

Toward a More General Synthetic Route to Paramagnetic Solids Containing RCNSSS•+ Radical Cations. A Structure−Property Correlation for RCNSSS•+ (R = F5C2, Cl3C)

Reaction of Cl3CN and F5C2CN with a 1:1 mixture of S4(AsF6)2 and S8(AsF6)2 affords the paramagnetic solids Cl3CNSSSAsF6 (1CCl3AsF6) and F5C2CNSSSAsF6 (1C2F5AsF6). Isotropic electron paramagnetic resonance spectra of 1CCl3AsF6 and 1C2F5AsF6 in SO2 consist of a single line with g = 2.01675 and 2.01580, respectively. The structure of 1CCl3AsF6 contains chains of radical cations with relatively close interchain interactions. In contrast, chains are isolated in 1C2F5AsF6. The magnetic behavior of both compounds was interpreted as that of 1D Heisenberg antiferromagnetic chains (1CCl3AsF6, J = -34 cm(-1), theta = -9 cm(-1), TIP = 0.00082, rho = 0.012; 1C2F5AsF6, J = -21 cm(-1), theta = -4.2 cm(-1), TIP = 0.00092, rho = 0.065). Density functional theory calculated and experimental magnetic coupling constants were in good agreement. The correlation between intermolecular S...S contacts and the strength of magnetic couplings was established.

Effect of externally applied pressure on the magnetic behavior ofCu2Te2O5(BrxCl1−x)2

The effect of externally applied pressure on the magnetic behavior of ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}({\mathrm{Br}}_{x}{\mathrm{Cl}}_{1\ensuremath{-}x}{)}_{2}$ with $x=0$, 0.73, and 1, is investigated by a combination of magnetic susceptibility, neutron diffraction, and neutron inelastic scattering measurements. The magnetic transition temperatures of the $x=0$ and 0.73 compositions are observed to increase linearly with increasing pressure at a rate of 0.23(2) and $0.04(1)\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{kbar}$, respectively. However, the bromide shows contrasting behavior with a large suppression of the transition temperature under pressure, at a rate of $\ensuremath{-}0.95(9)\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{kbar}$. In neutron inelastic scattering measurements of ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{\mathrm{Br}}_{2}$ under pressure only a small change to the ambient pressure magnetic excitations were observed. A peak in the density of states was seen to shift from $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in ambient pressure to $\ensuremath{\sim}6\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ under an applied pressure of $11.3\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$, which was associated with an increase in the overall magnetic coupling strength.

High-field magnetization study of the [formula omitted] antiferromagnetic Heisenberg chain [formula omitted

We present a temperature-dependent high-field magnetization study of the S = 1 / 2 antiferromagnetic Heisenberg chain (AFHC) Cu ( C 6 H 5 COO ) 2 · 3 H 2 O along the magnetically characteristic axis a ″ . The data are analyzed on basis of the Bethe ansatz equations and via exact diagonalization of a linear spin chain with 20 sites. Overall, we find a very good agreement between experimental data and theoretical calculations, and extract a magnetic coupling strength J / k B along the chain of 18.8(1) K. We discuss our results in context with the possibility for a magnetocaloric effect during the magnetization process of S = 1 / 2 AFHC systems.

Effects of Mn clustering on ferromagnetism in (Ga,Mn)As

The magnetic interactions of substitutional Mn clusters in (Ga,Mn)As are investigated using density-functional total-energy supercell calculations within the projector augmented-wave method. Magnetic coupling strength values are directly calculated from the parallel and antiparallel spin alignments, and then used to estimate the Curie temperature T C . The magnetic interactions both inside the clusters and between consequent clusters are investigated. We find that the ferromagnetic coupling inside the clusters is considerably larger than the coupling between two distant clusters, and that therefore the cluster–cluster magnetic coupling strength will determine the T C of the material. Our cluster–cluster T C values agree closely with experimental ones.

Ab initio study of the magnetic coupling in oxalato-bridged dinuclear Ni(II) complexes

The magnetic coupling has been studied in various oxalato bridged dinuclear Ni(II) complexes with accurate ab initio computational schemes. We assess the dependence of the magnetic coupling between the two Ni(II) centers on the electronegativity of the atoms in the coordination sphere of the magnetic centers and on the distance between the Ni and the oxalato bridge. It is found that both have comparable effects. Furthermore, a study has been performed to establish the influence on the magnetic coupling of the counterion, the modelization of the external ligands and the symmetrization of the geometry. All three terms are found to have a very small effect on the calculated magnetic coupling strengths.

Fe(bpb)(CN)2]-as a Versatile Building Block for the Design of Novel Low-Dimensional Heterobimetallic Systems: Synthesis, Crystal Structures, and Magnetic Properties of Cyano-Bridged FeIII−NiIIComplexes [(bpb)2-= 1,2-Bis(pyridine-2-carboxamido)benzenate

A dicyano-containing [Fe(bpb)(CN)2]- building block has been employed for the synthesis of cyano-bridged heterometallic Ni(II)-Fe(III) complexes. The presence of steric bpb(2-) ligand around the iron ion results in the formation of low-dimensional species: five are neutral NiFe2 trimers and three are one-dimensional (1D). The structure of the 1D complexes consists of alternating [NiL]2+ and [Fe(bpb)(CN)2]- generating a cyano-bridged cationic polymeric chain and the perchlorate as the counteranion. In all complexes, the coordination geometry of the nickel ions is approximately octahedral with the cyano nitrogen atoms at the trans positions. Magnetic studies of seven complexes show the presence of ferromagnetic interaction between the metal ions through the cyano bridges. Variable temperature magnetic susceptibility investigations of the trimeric complexes yield the following J(NiFe) values (based on the spin exchange Hamiltonian H = -2J(NiFe) S(Ni) (S(Fe(1)) + S(Fe(2))): J(NiFe) = 6.40(5), 7.8(1), 8.9(2), and 6.03(4) cm(-1), respectively. The study of the magneto-structural correlation reveals that the cyanide-bridging bond angle is related to the strength of magnetic exchange coupling: the larger the Ni-N[triple bond]C bond angle, the stronger the Ni- - -Fe magnetic interaction. One 1D complex exhibits long-range antiferromagnetic ordering with T(N) = 3.5 K. Below T(N) (1.82 K), a metamagnetic behavior was observed with the critical field of approximately 6 kOe. The present research shows that the [Fe(bpb)(CN)2]- building block is a good candidate for the construction of low-dimensional magnetic materials.

Unusual criticality of [formula omitted] under pressure

We present measurements of the magnetic susceptibility χ ( T ) on Cu 2 Te 2 O 5 Br 2 under externally applied pressure. From our data we extract the pressure response of the antiferromagnetic phase transition at T 0 = 11.6 K and of the overall magnetic coupling strength. Our experiments indicate that with pressure the overall magnetic coupling strength increases by about 25% with applied pressure of only ∼ 8 kbar . In contrast, the phase transition temperature T 0 is significantly suppressed and not observable anymore at a pressure of 8.2 kbar.

Influence of metal cation order on magnetic order in manganogrunerite, (Fe,Mg,Mn)7Si8O22(OH)2, a quasi-one-dimensional chain antiferromagnet

AbstractThe magnetic susceptibility of ordered and partially disordered manganogrunerites (composition and ) has been measured between room temperature and 5 K. The influence of disordering of metal cations over the M(1,2,3) sites of the octahedral ribbon in these amphiboles is to dilute Fe2+ within the ribbon. We observe a reduction in both θp and TN as a function of both increased Mg/(Mg+Fe) and metal cation disorder, linked to a reduction in the strength of magnetic coupling between and within the one-dimensional chains of the structure.

Thickness dependence of magnetic coupling strength and thermal stability in a spin-dependent tunnel junction

The change of magnetic coupling strength between two ferromagnetic layers, separated by an insulating barrier, was investigated as a function of the barrier thickness (T B ) and thermal annealing temperatures. The magnetic junctions consist of Ta/CoFe/AlO x /NiFe/Ta layers with three different nominal thickness of T B = 1.3, 1.6, and 2.0 nm. Isothermal magnetization at room temperature revealed that, while the junction with a lower T B showed a higher magnetic coupling strength, thermal annealing at T = 225 °C increased (and diminished) the coupling strength of the junctions with T B = 1.3 and 1.6 nm (and 2.0 nm), respectively. This observation was utilized to understand consistently the magneto-resistance behavior and specific junction resistance of the junctions as a function of thermal annealing temperature. This study demonstrated that the physical properties of a magnetic tunnel junction, such as magneto-resistance ratio, specific junction resistance and their thermal stability, were substantially influenced by the insulating barrier structure as well as the interface quality between the layers.

High-field magnetization study of theS=12antiferromagnetic Heisenberg chain[PMCu(NO3)2(H2O)2]nwith a field-induced gap

We present a high-field magnetization study of the $S=\frac{1}{2}$ antiferromagnetic Heisenberg chain $[\mathrm{PM}\mathrm{Cu}({\mathrm{NO}}_{3}{)}_{2}({\mathrm{H}}_{2}\mathrm{O}{)}_{2}{]}_{n}.$ For this material, as result of the Dzyaloshinskii-Moriya interaction and a staggered g tensor, the ground state is characterized by an anisotropic field-induced spin excitation gap and a staggered magnetization. Our data reveal the qualitatively different behavior in the directions of maximum and zero spin excitation gap. The data are analyzed via exact diagonalization of a linear spin chain with up to 20 sites and on basis of the Bethe ansatz equations, respectively. For both directions we find very good agreement between experimental data and theoretical calculations. We extract the magnetic coupling strength ${J/k}_{B}$ along the chain direction to 36.3(5) K and determine the field dependence of the staggered magnetization component ${m}_{s}.$

Magnetic frustration in the stoichiometric spin-chain compoundCa3CoIrO6

The temperature dependent ac and dc magnetization and heat capacity data of Ca$_3$CoIrO$_6$, a spin-chain compound crystallizing in a K$_4$CdCl$_6$-derived rhombohedral structure, show the features due to magnetic ordering of a frustrated-type below about 30 K, however without exhibiting the signatures of the so-called "partially disordered antiferromagnetic structure" encountered in the isostructural compounds, Ca$_3$Co$_2$O$_6$ and Ca$_3$CoRhO$_6$. This class of compounds thus provides a variety for probing the consequences of magnetic frustration due to topological reasons in stoichiometric spin-chain materials, presumably arising from subtle differences in the interchain and intrachain magnetic coupling strengths. This compound presents additional interesting situations in the sense that, ac susceptibility exhibits a large frequency dependence in the vicinity of 30 K uncharacteristic of conventional spin-glasses, with this frustrated magnetic state being robust to the application of external magnetic fields.

Magnetic coupling in Co/Cr2O3/CrO2 “trilayer” films

The ferromagnetic coupling between Co and CrO2, through an insulator (Cr2O3) was characterized by in situ magneto-optic Kerr effect. By evaporating 20–60 Å Co thin films on top of epitaxial CrO2 films, a Co/Cr2O3/CrO2 trilayer system can be readily fabricated; this is possible because the native surface layer of CrO2 is Cr2O3. In situ x-ray photoemission studies show that the Co is oxidized at the interface between Co and Cr2O3, so that the system more resembles Co/CoO/Cr2O3/CrO2. The Co thickness and temperature dependence of the magnetic hysteresis loops indicate that magnetic coupling strength increases with increasing Co thickness and decreases with increasing temperature. The magnetic coupling through the insulator barrier may be related to defect states in the insulating barrier layer.

New principles in metallomesogen structure–magnetism correlations

Whereas binuclear salicylaldimine complexes and their mononuclear parents are waxlike solids with long chain N-substituents, they are liquid crystalline (mesogenic) when such substituent chains are placed on both the ligand rings and the N-atoms. The structure of the non-mesogenic binuclears is determined via X-ray crystallography, and via the strength of magnetic coupling for the mesogenic ones. In the non-mesogenic binuclears the central metal environments have a marked distortion towards tetrahedral (>36°; scale 0° for planar, 90° for tetrahedral), while in the mesogenic ones, the stronger magnetic coupling indicates that this is closer to planar (25°).