Nature Of Exchange Interaction Research Articles

ESR and magnetization studies of double perovskite Sr[formula omitted]FeNbO6

By pyrolysis of nitrate-organic mixtures, we synthesized double perovskite Sr2Fe0.6NbO5.4. We performed the study of the temperature dependencies of the EPR spectra, DC and AC magnetization, and magnetization isotherms in a wide temperature range of 4–300K. The EPR spectrum is described by the sum of two contributions below 125 K and indicates phase separation of the sample. The paramagnetic part of the inverse magnetic susceptibility is well approximated by the Curie–Weiss law with ΘCW≈−60K. The exchange integrals between iron spins, calculated from the temperature dependence of the ESR integral intensity and the Curie–Weiss temperature, coincide and are equal to J/kB≈10K. The negative sign of the Curie–Weiss temperature and the hysteresis loops indicate the antiferromagnetic canted nature of exchange interactions, which is further confirmed by the presence of a peak in the real part of the AC magnetization and its absence in the imaginary part.

Nature of Exchange Interaction in Amorphous Alloys Based on Metals of Iron Group with Metalloids

Nature of Exchange Interaction in Amorphous Alloys Based on Metals of Iron Group with Metalloids

Synthesis and study of Co doped Ni-Cd ferro-spinels by microwave assisted sol-gel auto-combustion method

Cadmium substituted Nickel Ferrite, Ni0.5CoxCd(0.5-x)Fe2O4 (x = 0.0, 0. 02) were synthesized by sol-gel auto-combustion technique with microwaves using urea as fuel. The samples were sintered at two different temperatures 600 °C and 800 °C. This paper discusses the effect of sintering temperature on varius structural, morpological, magnetic and optical properties of the syntesized samples. The structural properties such as crystallite size, lattice constant, lattice strain oxygen position parameters, bond lengths and bond angles. Phase analysis of the sample are carried out using XRD measurement. Surface morphology and qualitative and quantitative chemical composition of the samples were done by FE-SEM and energy dispersive X-ray analysis (EDX) measurements respectively. Magnetic properties such as saturation magnetization (Ms), coercivity () retentivity (Mr), squareness and were obtained from VSM measuements. The variation in the observed and measured magnetic quantities are explained on the basis of YK model.The cation distribution proposed has been discussed based on the observed experimental measured magnetic parameters and it futher discuss spin canting angle and nature of exchange interaction among the magnetic moments of octahedral and tetrahedral sites.

The First Series of Heterometallic LnIII‐VIV Complexes Based on Substituted Malonic Acid Anions: Synthesis, Structure and Magnetic Properties

The reaction of VOSO4 with K2cbdc (cbdc2– is cyclobutane‐1,1‐dicarboxylate anion) and Ln(NO3)3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Tm, Yb, Lu) yielded 17 novel heterometallic LnIII‐VIV compounds comprising stable bis‐chelate units {VIVO(cbdc)2(H2O)}. Three structural types of compounds are formed depending on the nature of the rare‐earth metal: 1D polymers {[Ln(VO)(cbdc)2(H2O)7]·0.5[VO(cbdc)2]}n (structural type I) [Ln = La (1), Ce (2)], 3D polymers {[KLn(VO)2(cbdc)4(H2O)9]·3.5H2O}n (structural type II) [Ln = La (3), Ce (4), Pr (5), Nd (6), Sm (7), Eu (8), Gd (9), Tb (10), Dy (11), Ho (12)] comprising trinuclear units {LnV2}, and octanuclear complexes {[KLn(VO)2(cbdc)4(H2O)11]·2H2O}2 (structural type III) [Ln = Y (13), Er (14), Tm (15), Yb (16), Lu (17)] containing dinuclear units {LnV}. Magnetic properties were investigated for all isolated compounds. The GdIII‐VIV compound has ferromagnetic and antiferromagnetic exchange interactions between isotropic VIV and GdIII ions. Calculations using the DFT method at the UB3LYP/6‐31G(d,p)/SDD level of theory were performed in order to explain the different nature of exchange interactions in two GdIII‐VIV pairs. The YbIII‐VIV complex displays single‐molecule magnet properties; the effective energy barrier was estimated at ΔEeff/kB = 26 K. The magnetic properties of LnIII‐VIV compounds with heavy lanthanide ions (LnIII = Er, Tm, Yb) were rationalized in terms of crystal‐field calculations for LnIII ions.

Mn doped BiNbO4 ceramics: Thermal stability, phase transitions, magnetic properties, NEXAFS and ESR spectroscopy

Thermal stability, magnetic properties, ESR (Electron Spin Resonance) and NEXAFS (Near-Edge X-ray Absorption Fine Structure) spectra of the orthorhombic and triclinic modifications of solid solutions BiNb1-xMnxO4-δ have been studied. X-ray diffraction determined the reversibility of the phase transition from the high-temperature triclinic modification to the orthorhombic modification at 950 °C in the BiNb1-xMnxO4-δ solid solutions. The ESR spectra of the triclinic BiNb1-xMnxO4-δ revealed the sextet structure of Mn(II) ions with 8.4 mT splitting and some features at g = 3.80 and 1.47, and a broad diffuse band with g ∼ 2.2 having a sextet with 8–9 mT splitting and g = 2.0 against its background. The measurements of magnetic susceptibility allowed us to calculate the parameters of exchange interactions in dimers and the distribution of manganese atoms (II), (III) and (IV) in dependence on concentrations of the solid solutions in the orthorhombic and triclinic modifications. It was established that in the manganese-doped solid solutions of triclinic modification, the degree of aggregation and the nuclearity of the clusters were higher than in the solid solutions of orthorhombic modification. The clusters were presented in the forms of dimers and trimers of Mn(II), Mn(III) and Mn(IV) atoms with different valence atoms with antiferro- and ferromagnetic types of exchange.The solid solutions BiNb1-xMnxO4-δ as well as manganese oxides MnO, Mn2O3 and MnO2 were studied by the NEXAFS spectroscopy in order to determine the degrees of oxidation of manganese atoms. The DSC (Differential Scanning Calorimetry) curves of the samples BiNb1-xMnxO4-δ revealed the endothermic effects associated with the phase transition from the orthorhombic modification to the triclinic one at 1050 °C (x = 0.03) and 1048 °C (x = 0.06). The melting peaks were observed at 1231 °C (x = 0.03) and 1220 °C (x = 0.06), respectively. The differences in the magnetic characteristics and in the results of ESR and NEXAFs spectroscopy of the BiNb1-xMnxO4-δ of orthorhombic and triclinic modifications were due to the relative content of Mn(II), Mn(III), Mn(IV) ions and the degree of distortion of their local surroundings, composition and nature of exchange interactions in the clusters of manganese atoms.

A DFT computational study of the magnetic behaviour of cobalt dioxolene complexes of tetraazamacrocyclic ligands

Density functional theory computational modelling (UTPSSh/6-311++G(d,p)) of the structural and magnetic characteristics of the cobalt dioxolene complexes of 2,11-diaza[3.3](2,6)pyridinophane and tris(2-pyridylmethyl)amine) ligands has been performed. By means of quantum-chemical calculations it has been shown that the nature of exchange interactions between the metal ion and semiquinonate form of the redox-active ligand is determined by the structural peculiarities of tetraazamacrocyclic ligand. The structural parameters responsible for the possibility of the occurrence of spin-crossover and/or valence tautomeric rearrangements have been identified. The calculations testified that correct theoretical representation of magnetic properties of the salt-like transition metal complexes of redox-active ligands requires detailed accounting for their exterior counter-ions.

Pressure Tuning of the Spin-Orbit Coupled Ground State inSr2IrO4

X-ray absorption spectroscopy studies of the magnetic-insulating ground state of Sr2IrO4 at ambient pressure show a clear deviation from a strong spin-orbit (SO) limit J(eff)=1/2 state, a result of local exchange interactions and a nonzero tetragonal crystal field mixing SO split J(eff)=1/2, 3/2 states. X-ray magnetic circular dichroism measurements in a diamond anvil cell show a magnetic transition at a pressure of ∼17 GPa, where the "weak" ferromagnetic moment is quenched despite transport measurements showing insulating behavior to at least 40 GPa. The magnetic transition has implications for the origin of the insulating gap and the nature of exchange interactions in this SO coupled system. The expectation value of the angular part of the SO interaction, <L·S>, extrapolates to zero at ∼80-90 GPa where an increased bandwidth strongly mixes J(eff)=1/2, 3/2 states and SO interactions no longer dominate the electronic ground state of Sr2IrO4.

Crystal field and exchange interactions in rare earth transition metal salen compounds containing bipyridine, pyrazole complexes

Crystal field parameters for Pr 3+ in {[Ni(salen)Pr–(hfac) 3](H 2O)} (noted as NiPr) and {[Ni(salen)Pr(hfac) 3(pyr)]–(CHCl 3)} (noted as NiPrpyr) have been found from a fit to the thermal variation in the magnetic susceptibility of NiPr and NiPrpyr. The nature of exchange interaction in [Cu(salen)Pr(hfac)3(pyr)] (noted as CuPrpyr), {[{Cu(salen)Pr(hfac) 3} 2(pyz)](H 2O) 3} (noted as Cu 2Pr 2pyz) and {[{Cu(salen)Pr(hfac) 3} 2(bpy)]–(CHCl 3) 2} (noted as Cu 2Pr 2bpy, bpy=4,4_-bipyridine) have been found using the derived results for NiPr and NiPrpyr. All the exchange interactions give significant contribution to the thermal variation in magnetic susceptibility below 50 K. The contribution due to Pr–Cu interaction is positive while that of the Cu–Cu and Pr–Pr interactions are negative. The behaviors below 10 K for Pr–Cu and Pr–Pr are difficult to explain, and point to a possible change in structure of CuPrpyr, Cu 2Pr 2pyz and Cu 2Pr 2bpy below 10 K. The theoretical thermal variations in the magnetic specific heat of NiPr and NiPrpyr are computed and discussed.

Ab‐initio modeling of spintronic materials

AbstractHalf‐metallic ferromagnets with high Curie temperatures are important ingredients for spintronics applications. These include diluted magnetic semiconductors and other ordered ferromagnetic compounds. In this paper, we present ab‐initio density functional electronic structure calculations of several spintronic materials addressing issues on the nature of exchange interactions, electron correlation effects, influence of defects, effects of disorder and volume dependence of exchange interactions. The calculations are done by two different methods, (i) plane wave projector augmented wave and (ii) Green function based Korringa–Kohn–Rostoker‐Coherent Potential Approximation (KKR‐CPA). We also present calculations of Curie temperatures using Monte‐Carlo simulations with effective Heisenberg Hamiltonian within the framework of ab‐initio methods. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Syntheses, Structures, and Magnetic Properties of Diphenoxo-Bridged MIILnIII Complexes Derived from N,N‘-Ethylenebis(3-ethoxysalicylaldiimine) (M = Cu or Ni; Ln = Ce−Yb): Observation of Surprisingly Strong Exchange Interactions

A series of heterodinuclear Cu(II)Ln(III) and Ni(II)Ln(III) complexes, [M(II)L(1)Ln(III)(NO(3))(3)] (M = Cu or Ni; Ln = Ce-Yb), with the hexadentate Schiff base compartmental ligand N,N'-ethylenebis(3-ethoxysalicylaldiimine) (H(2)L(1)) have been synthesized and characterized. The X-ray crystal structure determinations of 13 of these compounds reveal that they are all isostructural. All of these complexes crystallize with the same orthorhombic P2(1)2(1)2(1) space group with closely similar unit cell parameters. Typically, the structure consists of a diphenoxo-bridged 3d-4f dinuclear core, self-assembled to two dimensions due to the intermolecular nitrate...copper(II) or nitrate...nickel(II) semicoordination and weak C-H...O hydrogen bonds. Despite that, the metal centers of the neighboring units are well separated (the ranges of the shortest intermolecular contacts (A) are (M...M) 7.46-7.60, (Ln...Ln) 8.56-8.69, and (M...Ln) 6.12-6.20). Variable-temperature (5-300 K) magnetic susceptibility measurements of all the complexes have been made. The nature of exchange interactions in the Cu(II)Ln(III) systems has been inferred from the Deltachi(M)T versus T plots, where Deltachi(M)T is the difference between the values of chi(M)T for a Cu(II)Ln(III) system and its corresponding Ni(II)Ln(III) analogue. Ferromagnetic interactions seem to be exhibited by the Cu(II)Gd(III), Cu(II)Tb(III), Cu(II)Dy(III), Cu(II)Ho(III), Cu(II)Tm(III), and Cu(II)Yb(III) complexes, while, for the Cu(II)Er(III) complex, no definite conclusion could be reached. On the other hand, among the lower members of the series, the complexes of Ce(III), Nd(III), and Sm(III) exhibit antiferromagnetic interactions, while the Cu(II)Pr(III) and Cu(II)Eu(III) analogues behave as spin-uncorrelated systems. The observations made here vindicate the proposition of Kahn (Inorg. Chem. 1997, 36, 930). The Deltachi(M)T versus T plots also suggest that, for most of the Cu(II)Ln(III) complexes, the exchange interactions are fairly strong, which probably could be related to the small dihedral angle (ca. 4 degrees) between the CuO(2) and LnO(2) planes.

Nature of exchange interactions in stacked radicals/radical-ions of cyclic polyenes

From full configuration interaction calculations on stacked cyclic polyene radicals/radical-ions, within the Hubbard and the Pariser—Parr—Pople models, we show that the ground state is not always a triplet state as predicted by the McConnell mechanism for organic ferromagnetism. On the contrary, the (2n + 1) cyclic polyene radical stacks always possess a singlet ground state. In 2n cyclic polyene radical-ions, for small n (n < 3), the ground state is a singlet. For larger n values, the ground state appears to be a triplet. We rationalize these results based on topological arguments.

Complexes with metal-nitroxyl group coordination

This review covers research on coordination compounds of transition metals with stable nitroxyl radicals, in which the nitroxyl fragment is coordinated by the metal ion. Greatest attention is given to generalized aspects of the synthesis of compounds containing a metal-nitroxyl group bond and their magnetic properties in the solid state. Recent developments in the theoretical understanding of the magnetic properties of polycrystalline compounds of this class are discussed. The structure of these complexes and the nature of exchange interactions in them between unpaired electrons of various paramagnetic sites are examined. The review covers studies published in the period from 1980 to 1987. Earlier investigations were reviewed by Larionov [6].

Structural Analysis of Three Rivers Redware Designs

ABSTRACTStructural analysis of complete and fragmentary designs on prehistoric Three Rivers Red-on-terra-cotta and Lincoln Black-on-red bowls from south central New Mexico defines more specifically the long recognized continuity in decorative tradition between them. The resulting classification of design lays the groundwork for future research on archaeological problems to which they are pertinent, including the nature of exchange interaction within the region and relationships to other regions.

Comment on the Grest and Nagel Model in Amorphous Ferromagnetic Alloys

A clue on the nature of exchange interactions in amorphous ferromagnetic alloys is discussed in relation to the Grest and Nagel model and the experimental results of Luborsky et al.