N,N,N,N-tetradentate Macrocyclic Ligand Based Selective Fluorescent Sensor for Zinc (II)

N,N,N,N-tetradentate macrocyclic ligand (L) has been synthesized by the condensation of benzil and semicarbazide and characterized. On excitation by light of wavelength 350?nm, L exhibited a fluorescent peak at λ(max)?=?454?nm, which showed ca 6 times enhancement in intensity with a blue shift on interaction with Zn(2+). L has been found to act as a selective fluorescent sensor for zinc(2+) ion over a host of other metal ions such as- Cd(2+), Pb(2+), Hg(2+), Ca(2+), Fe(2+), Na(2+), Co(2+), Mn(2+), Cu(2+) and Ni(2+), in 1:1 CH(3)OH:H(2)O. A 1:1 complex formation between L and Zn(2+) was proved. The enhancement in the fluorescence could be explained on the basis of Photo induced electron transfer (PET) mechanism with log β?=?1.86.     

Density functional theory study on antiferromagnetic interactions in a silver (I) complex of nitronyl nitroxide

A one-dimensional (1D) silver (I) complex of nitronyl nitroxide with fairly strong antiferromagnetic interaction, in which the metal ions are diamagnetic, is investigated by means of the ab initio crystal orbital method based on the density functional theory. The calculated values of the magnetic coupling constant (J) are close to the experimental measured J value in the periodic system. The magneto-structural correlation reveals that the existence of an antiferromagnetic coupling pathway along nitronyl nitroxide units via silver (I) ion in this system. The spin population distribution also shows the existence of spin delocalization along the ONCNO–Ag–ONCNO, which affords a rational interpretation for the antiferromagnetic interaction mechanism.     

EPR study of quasi-one-dimensional complex oxides A3n + 3mA′nMn3m + nO9m + 6n (A-Sr, Ba; A′-Mg, Zn, Cu)

Exchange interactions in the quasi-one-dimensional complex oxides Sr₄MgMn₂O₉, Ba₆MgMn₄O₁₅, Ba₈MgMn₆O₂₁, and Ba₉ M ₂Mn₅O₂₁ (M-Cu, Zn, Mg) have been studied by electron paramagnetic resonance. The temperature dependence of the spectral parameters showed that the exchange interaction is antiferromagnetic and occurs mainly inside the manganese-containing groups and to a lesser extent between them.     

The formation of sharp NiO(1 0 0)-cobalt interfaces

An atomically sharp interface between an antiferromagnetic oxide and a ferromagnetic metal may be obtained by the deposition of an epitaxial oxide buffer nanolayer in between. The buffer layer consists of the oxide of the ferromagnetic metal. The concept has been demonstrated on the NiO(1 0 0)-Co system, where the inclusion of a 1-2 ML CoO(1 0 0) interlayer inhibits the interfacial redox reaction which takes place between NiO and Co metal in the absence of the buffer layer.     

Exchange interaction and spin dynamics in pentanuclear clusters, Cu3Ln2(ClCH2COO)12(H2O)8 (Ln = Nd3+, Sm3+, Pr3+)

New compounds, [Cu₃Ln₂(ClCH₂COO)₁₂(H₂O)₈]·2H₂O with Ln = Nd³⁺ (I), Sm³⁺ (II), Pr³⁺ (III), built up of pentanuclear clusters were synthesized and studied by means of X-ray analysis and electron paramagnetic resonance (EPR). X-ray data show that all compounds are isostructural and the pentanuclear clusteres may be considered as a linear system with alternating Cu(II) and Ln(III) ions: Cu(2)-L¹-Ln-L²-Cu(1)-L²-Ln-L²-Cu(2) with L¹ and L² being bridging fragments and Cu(1) and Cu(2) being structurally nonequivalent copper complexes. EPR studies demonstrate that in the temperature range of 100–293 K the signals due to only one type of the copper complexes are observed in the spectra of I–III. AtT<100 K the spectral temperature dependence is nontrivial. AtT<30 K new signals are detected in the spectra of I and II. The temperature dependence of the EPR spectra is interpreted under the assumption that the parameter of the exchange interaction Cu(2)-Ln considerably exceeds the parameter of the interaction Cu(1)-Ln. EPR spectra are calculated for the fragments of five paramagnetic centers in the frames of the model taking into account the nonequivalence of two copper complexes, short longitudinal and transverse paramagnetic relaxation times of the rare-earth ions at room temperature and the change of the relaxation rates when the temperature decreases. The results of the calculations show that it is possible to obtain information about the interactions in the system on the basis of the analysis of the temperature dependence of the EPR spectra of the central copper complex. The parameter of the isotropic part of the exchange interaction between copper and neodymium ions (for the interaction Cu(2)-Nd) is estimated as about 15 cm⁻¹. A considerable rearrangement of the spin states when the temperature changes is found for all complexes.     

Frustrated multilayer ferromagnet-antiferromagnet structures: Beyond the scope of the exchange approximation (a review)

The frustrations of exchange interaction between ferromagnetic and antiferromagnetic layers, which arise at the uncompensated interface between the layers due to the interface roughness, have been described. The distribution of magnetic order parameters in the vicinity of the interface between the layers has been investigated, and the “layer thickness-roughness” magnetic phase diagram has been obtained in the case of the two-layer ferromagnet-antiferromagnet system and the ferromagnet-antiferromagnet-ferromagnet spin-valve system. An analysis has been performed taking into account the single-ion anisotropy energy, i.e., beyond the scope of the exchange approximation. It has been demonstrated that the number of easy axes in the layer plane, in many respects, determines the existence of an exchange shift of the hysteresis loop of the ferromagnet due to its interaction with the antiferromagnetic substrate.     

First-principles study on exchange force image of NiO(0 0 1) surface using a ferromagnetic Fe probe

Exchange force of a ferromagnetic Fe probe on antiferromagnetic NiO(0 0 1) surface has been investigated by means of a first-principles calculation. Calculated exchange force images show a clear spin image when the probe is located within 1 Å above the contact point. We can see antiferromagnetic pattern of the surface Ni atoms along the [1 1 0] direction, and asymmetric feature around surface O sites. The main contrast of Ni comes from the direct exchange interaction between the Fe probe and the surface Ni atom, while the asymmetric image possibly comes from the super exchange interaction between the Fe probe and the second layer Ni atom via the surface O. Such asymmetric feature is a key proof of the exchange force microscope image on observation.     

Evaluation of the capability of C60-fullerene to act as a magnetic coupling unit

The ability of C₆₀-fullerene to act as a magnetic coupling unit between radical centers has been evaluated using ab initio CAS(4,4) theoretical method. The results indicate that functionalized C₆₀-fullerenes have a very good potential to couple radical centers. Depending on the relative position of the two radical centers, the resulting interaction is ferro or antiferromagnetic. This suggests the use of these units as ferrocoupler or antiferrocoupler units, to obtain new families of radicals, or even polymeric structures.     

Ferromagnetic-to-antiferromagnetic transition in (Hf1−xTix)Fe2 intermetallic compounds induced by geometrical frustration of the Fe(2a) sites

The ferromagnetic-to-antiferromagnetic transition in the hexagonal (Hf_1−xTi_x)Fe₂ (0?x?1) intermetallic compounds has been investigated by ⁵⁷Fe Mössbauer spectroscopy. At 10 K, the transition occurs within rather narrow concentration limits, around x=0.55–0.65. We found that the key factor governing the unexpected quick change of the magnetic structure is the magnetic frustration of the Fe(2a) sites. The magnetic frustration is caused by the noncollinearity of the Fe(6h) magnetic sublattice. The noncollinearity arises from the rotation of the magnetic moments due to the competition between the ferromagnetic exchange interactions and the antiferromagnetic Fe(6h)–Ti–Fe(6h) interaction. In the compounds with x=0.4–0.6, the temperature transitions to the antiferromagnetic state are observed. As an example, the Hf_0.4Ti_0.6Fe₂ compound is completely antiferromagnetic above 200 K.     

EXAFS study of copper complexes with azomethinic ligand environment

A series of copper metallochelates C₂₂H₁₈CuN₄O₂ X (X = Se, S, O) as models of active centers of natural metalloproteins have been synthesized on the basis of new azomethine ligand systems. The structure of the complexes has been studied by extended X-ray absorption fine-structure spectroscopy. It is shown that, in the metallochelates with X = Se or S, one azomethine chalcogen-containing ligand undergoes tridentate interaction with copper ions, while the other ligand is an acetate group. As a result, a complex with the N₂O₂ X environment is formed, where one of the oxygen atoms of the acetate group is at a large distance from the metal ion: R = 2.56–2.68 Å. For the metallochelate with X = O, coordination of the acetate group by a copper ion is found to be absent, and only interaction with azomethine ligands having average Cu-N/O distances R = 1.96–2.04 Å is observed.     

Mössbauer studies on yeast metallothionein

Iron-substituted yeast metallothionein, Fe(II)-yeast-MT, has been studied by Mössbauer spectroscopy. The iron in the protein is in the high-spin ferrous state. As maximum metal content, 4 Fe(II)/molecule has been determined, with the 4 metal ions forming a diamagnetic cluster due to the antiferromagnetic exchange interaction between the Fe(II) ions via bridging thiolates. In case the iron titration is less than 4 Fe(II)/apoprotein, the ions are magnetically noninteracting, with each individual Fe(II) behaving similar to Fe(II) in reduced rubredoxin.     

Localized electron states and magnetic properties at the interface of a two-dimensional graphene/MnO(001) system

The results of a density functional theory study of the band structure of two-dimensional (2D) graphene/MnO(001), new materials for spintronics, with an antiferromagnetic type of ordering are presented. The regularities of the change of the valence band electron structure in the 3D MnO → 2D MnO → 2D graphene/MnO(001) series have been studied in a comparison with X-ray photoelectron spectra. The stability of the system has been established, and the energy of chemical binding has been determined using the calculation of the structural energy of 2D graphene/MnO(001). The features of the spin state in the valence band—in particular, at the Fermi level—as well as interatomic interaction in 2D graphene/MnO(001) have been discussed in comparison with 2D and 3D MnO systems with antiferromagnetic ordering. The magnetic moment of the Mn atom in all considered systems has been estimated and compared with the experimental one. The effect of the spin polarization for the oxygen and carbon atoms has been detected. The nature of this effect has been considered.     

Influence of magnetic field on the electronic specific heat of the organic metal (BEDT-TTF)2KHg(SCN)4

Specific heat measurements of a single crystal of the organic metal (BEDT-TTF)₂KHg(SCN)₄ have been carried out at low temperatures and under a magnetic field of up to 14 T. A jump in the specific heat of about 0.1 J/mol·K, which corresponds to the antiferromagnetic phase transition, has been observed. The magnetic field is found to decrease the transition temperature at any field orientation. The strongest effect was found to take place in the field direction along the highly conducting ac plane. Zh. éksp. Teor. Fiz. 113, 1058–1063 (March 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Possible ground states of rare-earth nickelates (RNiO3, R = La,Nd, Pr): a mean-field study

The ground states of rare-earth nickelates (RNiO₃) are antiferromagnetic insulators except LaNiO₃, which is a strongly correlated metal. Rare-earth nickelates (R ≠ La) undergo a sharp transition from a high-temperature paramagnetic metal to a low-temperature antiferromagnetic insulator at finite temperature T_MI that increases with the increase in atomic number of the rare-earth ions. The magnetic and resistive transitions are coupled for NdNiO₃ and PrNiO₃, but independent for the other members of the series. Whether the antiferromagnetic and insulating ground states of nickelates are due to charge ordering, or orbital ordering, it is a matter of current dispute. The present paper intends to explain the difference in the observed ground states of RNiO₃ compounds, by calculating their band structure and density of states. The experimentally observed ground states of nickelates have been explained on the basis of mean-field calculation and the correlation effect is incorporated in the dynamical mean-field theory thereof.     

Cyclic period-3 window in antiferromagnetic potts and Ising models on recursive lattices

The magnetic properties of the antiferromagnetic Potts model with two-site interaction and the antiferromagnetic Ising model with three-site interaction on recursive lattices have been studied. A cyclic period-3 window has been revealed by the recurrence relation method in the antiferromagnetic Q-state Potts model on the Bethe lattice (at Q < 2) and in the antiferromagnetic Ising model with three-site interaction on the Husimi cactus. The Lyapunov exponents have been calculated, modulated phases and a chaotic regime in the cyclic period-3 window have been found for one-dimensional rational mappings determined the properties of these systems.     

A theoretical study of Fe adsorption along Bi-nanolines on the H/Si(0 0 1) surface

We have investigated the energetic stability and equilibrium geometry of the adsorption of transition metal Fe atoms near the self-organized Bi lines on hydrogen passivated Si(0 0 1) surface. Our total energy results show that there is an attractive interaction between Fe adatoms along the Bi-nanolines. For the energetically most stable configuration, the Fe adatoms are seven-fold coordinated, occupying the subsurface interstitial sites aside the Bi-nanolines. With increased coverage, Fe atoms are predicted to form two parallel lines, symmetrically on both sides of the Bi line. Within our local spin-density functional calculations, we find that for the most stable geometries the Fe adatoms exhibit an antiferromagnetic coupling.     

Growth of Au thin film on Cu-modified Si(1 1 1) surface

Miniaturizing of electronic devices requires that conductive elements maintain advanced electrical characteristics upon reducing their geometrical sizes. For gold, which is valued for its high electrical conductivity and stability against ambient conditions, creation of extra-thin films on silicon is hampered by formation of the quite complex Au/Si interface. In the present work, by forming a Si(1 1 1)5.55 × 5.55-Cu surface reconstruction prior to Au deposition we formed Au films with smoother surface morphology and higher surface conductivity compared to Au film grown on a pristine Si(1 1 1)7 × 7 surface. Scanning tunnelling microscopy and four-point probe measurements were used to characterize the growth mode of the Au film on a Si(1 1 1)5.55 × 5.55-Cu reconstruction at room temperature.     

Large vertical loop shifts in mechanically synthesized (Mn,Fe)2O3−t nanograins

Magnetic properties of zero field cooled (ZFC) and field cooled (FC) sample of (Mn,Fe)₂O_3−t nanograins have been investigated by magnetometry (up to 70 kOe) and Mössbauer spectroscopy (up to 60 kOe) in the temperature interval 4.2–300 K. Large horizontal (up to 0.8 kOe) and vertical (up to 80%) shifts of the magnetization hysteresis loops are observed in the FC regime. The obtained results are discussed in terms of exchange interaction between an antiferromagnetic core and a spin-glass-like state of the nanograins boundaries. It is shown that hysteresis loop shifts (horizontal and vertical) depend on the field cooling magnitude, an effect that can be understood by the change of the boundary magnetic structure induced by the external magnetic field. The vertical magnetization shift is described by a phenomenological model, which takes into account the magnetic interaction between the spin-glass like boundary spins and the applied field.     

Heat capacity of a five-coordinated iron(III) complex,iodobis (N,N-Dimethyldithiocarbamato) iron(III), between 0.4 and 300 K

The heat capacity of iodobis (N, N-dimethyldithiocarbamato)iron(III) has been measured between 0.4 and 300 K. Based on the heat capacity and entropy at low temperatures it was found that the present sample consists of a mixture of monomer (ca. 40%) and dinier (ca. 60%); the former brings about a λ-type phase transition from an antiferromagnetic to a paramagnetic state at T_N = (1.65 ±0.04) K while the latter exhibits a Schottky-ype anomaly due to antiferromagnetic dimeric coupling, the effect of which becomes dominant below ca. 0.7 K. The zero-field splitting parameter of a single ferric ion was estimated to be $\text{D}\text{k}\text{=}\text{D}{}_{\mathrm{D}}\text{k}\text{= 14 K}$ for the monometer and the dimer, while the dimeric coupling constant was $\text{J}{}_{\mathrm{D}}\text{k}\text{= ?0.15 K}$. The entropy at low temperatures cannot be accounted for solely by the spin manifold. Additional contribution from a tunnel-splitting of the rotational levels of four constituent methyl-groups has been discussed. In this case, the level splitting of the ground state is 2.5 J mol^?1 and the barrier height of hindering potential is 2.3 kJ mol^?1.     

First-principles studies of interlayer exchange coupling in (Ga,Co)N-based diluted magnetic semiconductor multilayers

Interlayer exchange coupling (IEC) in a series model diluted magnetic semiconductor (DMS) multilayer consisting of two magnetic (Ga, Co)N layers separated by non-doped or Mg-doped GaN non-magnetic spacers has been studied by first-principles calculations. The effects of the spacer thickness and hole doping to the IEC were studied systematically. It is observed that (1) for the GaN spacers without Mg doping, the IEC between two magnetic (Ga, Co)N layers is always ferromagnetic, which is clarified as an intrinsic character of the Ruderman–Kittle–Kasuya–Yoshida (RKKY) interaction based on a two-band model for a gapped system; and (2) for the Mg-doped GaN spacers, the IEC is tunable from ferromagnetic to antiferromagnetic by varying the spacer’s thickness and the dopant’s site.