Neighbor Spins Research Articles

Ordering in nematic liquid crystals from NMR cross-polarization studies

The measurement of dipolar couplings between nuclei is a convenient way of obtatining directly liquid crystalline ordering through NMR since the coupling is dependent on the average orientation of the dipolar vector in the magnetic field which also aligns the liquid crystal. However, measurement of the dipolar coupling between a pair of selected nuclei is beset with problems that require special solutions. In this article the use of cross polarization for measuring dipolar couplings in liquid crystals is illustrated. Transient oscillations observed during cross polarization provide the dipolar couplings between essentially isolated nearest neighbour spins which can be extracted for several sites simultaneously by employing two-dimensional NMR techniques. The use of the method for obtaining heteronuclear dipolar couplings and hence the order parameters of liquid crystals is presented. Several modifications to the basic experiment are considered and their utility illustrated. A method for obtaining proton–proton dipolar couplings, by utilizing cross polarization from the dipolar reservoir, is also presented.

Colossal magnetoresistance in perovskite manganite induced by localized moment of rare earth ion

The magnetic and transport properties of a single crystal of Pr 0.7Ca 0.3MnO 3 annealed by various magnetic fields were investigated by dc and ac magnetization measurements. The ordered Pr moment is almost absent in zero-field magnetization but is clearly observed when the sample was measured after annealing with a magnetic field. The Pr moment is enhanced when the annealing field is increased. Remarkably, the ordered Pr spins appear to induce colossal magnetoresistance (∼10 10%) as well as a metal–insulator (M–I) transition in Pr 0.7Ca 0.3MnO 3. This unique behavior is discussed in terms of the reorientation of Pr spins that are strongly coupled with the neighboring Mn spins.

Two ferromagnetic transitions in Pr 0.8Ca 0.2MnO 3

The magnetic and structural properties of a single crystal of Pr 0.8Ca 0.2MnO 3 have been investigated by neutron powder diffraction and bulk magnetization as a function of temperature. Two ferromagnetic (FM) transitions near 140 and 70 K, which are ascribed to the ordering of Mn and Pr spins, respectively, are clearly evidenced by both measurements. The low temperature FM transition is largely dependent upon a strong exchange coupling between the localized Pr 4f and the neighboring itinerant Mn 3d spins. Near 140 K distinctive changes occur in lattice parameters, whereas no anomalous variation develops near 70 K.

Substitution effects on ferromagnetic Mott insulator Lu 2V 2O 7

A model is proposed for the ferromagnetism of a Mott insulator Lu 2V 2O 7. In relation to this model, substitution effects of Ti and Nb atoms for V atoms in Lu 2V 2O 7 have been studied, where all ions are found to have valence of +4, i.e. S=1/2 for V 4+, S=0 for Ti 4+ and S=1/2 for Nb 4+ as a localized spin by studying their lattice parameters and Curie constants. Saturated magnetic moments in Lu 2V 2− x Nb x O 7 system are smaller than paramagnetic moments in Curie–Weiss law, indicating spin canting or anisotropy. Its effect on Curie temperature T c corresponds to the decrease of mean magnetic moments along the magnetic field, while the effect of Ti substitution on T c is explained by the decrease of number of the nearest neighbor spins. These substitution effects on T c do not contradict the present model.

Electron Spin Resonance Study on Defects in Lithium Zirconate (Li2ZrO3) Produced by 77 K γ-Irradiation

Paramagnetic defects in Li2ZrO3 produced by γ-ray irradiation at 77 K and subsequent thermal annealing have been studied using electron spin resonance (ESR). The signal at 77 K has an anisotropic g-factor of orthorhombic symmetry, g1=2.0391, g2=2.0329 and g3=2.0044, and is ascribed to a hole center O-. An aluminum-related hole center was observed by annealing at temperatures ranging from 180 K to 230 K. The signal shape can be explained in terms of the hyperfine splittings caused by a neighboring nuclear spin I=5/2 of 27Al3+ impurity. The third signal in the temperature range from 230 K to 380 K is ascribed to a hole-trapped center of unknown origin from the g-factor with an orthorhombic symmetry, g1=2.027, g2=2.012 and g3=2.0061. The fourth signal, observed by annealing above 240 K, is an unknown electron center with an isotropic symmetry g=2.0020 and linewidth of 1.55 mT.

EFFECT OF STRAIN ON THE TRANSPORT PROPERTIES OF THE MANGANITE

The effect of strain on the resistivity and thermopower of ferromagnetic manganites has been examined based on the model that incorporates the electron-lattice interaction through the Jahn–Teller effect and an effective hopping determined by nearest neighbor spin–spin correlation of t2g electrons. The metal insulator transition temperature associated with resistivity decreases with increase in strain. In the presence of large strain the system remains in the semiconducting state. Thermopower (S) is positive and increasing function of strain and it exhibits a maximum with temperature. The temperature where maximum of S appears, shifts towards higher (lower) value in the presence of magnetic field (strain). A large magneto-thermopower that depends on strain is obtained around metal–insulator transition.

EPR spectroscopy of copper and manganese complexes encapsulated in zeolites

The structural basis for the enhanced catalytic activities of copper and manganese Schiff base complexes encapsulated in zeolite-Y is investigated by EPR spectroscopy. The study provides an unequivocal evidence for the encapsulation of complexes inside the supercages of zeolite-Y. The EPR spectroscopy distinguishes the encapsulated complexes from the “neat” and surface-adsorbed metal complexes. Neat complexes showed broad EPR spectra corresponding to nearest neighbour spin–spin interactions whereas the zeolite-encapsulated metal complexes showed well resolved metal hyperfine features similar to the spectra in dilute frozen solutions. The spin Hamiltonian parameters reveal a distorted square pyramidal geometry and an increase in the in-plane covalency of metal–ligand bond as a consequence of encapsulation. The observed changes in the molecular electronic structure are correlated to the enhanced catalytic activity of the encapsulated metal complexes.

Two-stage spin-flop transitions in the S = 1/2 antiferromagnetic spin chain BaCu(2)Si(2)O(7).

Two-stage spin-flop transitions are observed in the quasi-one-dimensional antiferromagnet BaCu(2)Si(2)O(7). A magnetic field applied along the easy axis induces a spin-flop transition at 2.0 T followed by a second transition at 4.9 T. The magnetic susceptibility indicates the presence of Dzyaloshinskii-Moriya (DM) antisymmetric interactions between the intrachain neighboring spins. We discuss a possible mechanism whereby the geometrical competition between DM and interchain interactions, as discussed for the two-dimensional antiferromagnet La(2)CuO(4), causes the two-stage spin-flop transitions.

Possible origin of large magnetostriction in manganite

A model where the e g electron of Mn-ion is coupled to lattice through the Jahn–Teller mechanism and the hopping is governed by the nearest neighbour spin–spin correlation of t 2g electrons, is considered. The thermal and magnetic field dependence of the strain is obtained treating the Coulomb and exchange interaction of e g electrons in the mean field and the spin–spin correlation of t 2g in pair approximation. Below T c, the strain is strongly suppressed by magnetic field, whereas above T c a slower decrease with quadratic field dependence is found. These results are qualitatively similar to those observed in manganites.

Spin Correlations inHo2Ti2O7: A Dipolar Spin Ice System

The pyrochlore material Ho2Ti2O7 has been suggested to show "spin ice" behavior. We present neutron scattering and specific heat results that establish unambiguously that Ho2Ti2O7 exhibits spin ice correlations at low temperature. Diffuse magnetic neutron scattering is quite well described by a nearest neighbor spin ice model and very accurately described by a dipolar spin ice model. The heat capacity is well accounted for by the sum of a dipolar spin ice contribution and an expected nuclear spin contribution, known to exist in other Ho3+ salts. These results settle the question of the nature of the low temperature spin correlations in Ho2Ti2O7 for which contradictory claims have been made.

Formation dynamics of free excitonic magnetic polarons in Cd1−xMnxTe

Formation processes of magnetic polarons from excitons were studied in Cd1−xMnxTe with the Mn composition x varying from 0.02 to 0.1 by transient photoluminescence spectroscopy. Two kinds of excitonic magnetic polarons were observed. One is the magnetic polaron composed of the free exciton, which is interacting with neighboring Mn-ion spins by the exchange interaction. The other is the magnetic polaron composed of the exciton bound to the neutral donor by aligning the surrounding Mn spins. The donor bound magnetic polarons appear for x=0.02–0.05 with the binding energy of 0.5–1.0 meV. The free excitonic magnetic polarons arise for x=0.02–0.1, where the binding energy of the free excitonic magnetic polaron varies in the range of 3.5-8.0 meV depending on the Mn composition. The stability of the free excitonic magnetic polaron state is confirmed by the appearance of the strong transient photoluminescence in the wide Mn composition range.

ARPES study of LSCO and PBCO: Electronic structure of the stripe phase and the 1/4-filled Cu-O chains

We have studied the electronic structure of the stripe phase in La 2-x Sr x CuO 4 (LSCO) and the Cu-O chains in PrBa 2 Cu 3 O 7 (Pr123) and PrBa 2 Cu 4 O 8 (Pr124) using angle-resolved photoemission spectroscopy (ARPES). In LSCO with x=0.12, the spectral feature near the Fermi level (EF) is almost flat from (π, 0) to (π, π/4), namely, along the stripe direction in LSCO. While the 1/4-filled chain in Pr123 has a band gap because of charge ordering, the metallic chain in Pr124 shows a dispersive feature which reaches EF at ~(π, π/4) and a flat feature near EF which is similar to that observed in the stripe phase of LSCO. Although Pr124 shows spectral-weight suppression near EF due to the instability toward charge ordering, the suppression is imperfect and Pr124 has a substantial spectral weight at EF probably due to the chain-chain coupling. This is in sharp contrast to LSCO with the perfect gap opening near (π, π/4). The difference between the stripe phase in LSCO and the coupled chains of Pr124 would be derived from the interaction between the stripe and the neighboring Cu spins which suppresses the charge ordering along the stripe.

Origin of spin-ice behavior in Ising pyrochlore magnets with long-range dipole interactions: an insight from mean-field theory

Recent experiments suggest that the Ising pyrochlore magnets ${\rm Ho_{2}Ti_{2}O_{7}}$ and ${\rm Dy_{2}Ti_{2}O_{7}}$ display qualitative properties of the ferromagnetic nearest neighbor spin ice model proposed by Harris {\it et al.}, Phys. Rev. Lett. {\bf 79}, 2554 (1997). The manifestation of spin ice behavior in these systems {\it despite} the energetic constraints introduced by the strength and the long range nature of dipole-dipole interactions, remains difficult to understand. We report here results from a mean field analysis that shed some light on the origin of spin ice behavior in (111) Ising pyrochlores. Specifically, we find that there exist a large frustrating effect of the dipolar interactions beyond the nearest neighbor, and that the degeneracy established by effective ferromagnetic nearest neighbor interactions is only very weakly lifted by the long range interactions. Such behavior only appears beyond a cut-off distance corresponding to $O(10^2)$ nearest neighbor. Our mean field analysis shows that truncation of dipolar interactions leads to spurious ordering phenomena that change with the truncation cut-off distance.

Organic Magnets

The notion of organic molecular materials showing metallic properties, such as electric conductivity or ferromagnetism, started several decades ago as a mere dream of some members of the chemical community. The goal was to create an assembly of organic molecules or macromolecules containing only light elements (C, H, N, O, S, etc.) and yet possessing the electron/hole mobility or spin alignment that is inherent in typical metals or their oxides and different from the isolated molecular materials. Organic molecular conductors initially were developed during the 1960s, but the first examples of organic molecular magnets took several more decades to be discovered, owing to the more subtle and complex structural and electronic aspects of these materials. The flurry of activity in this field can be traced to the widely held belief that even the most sophisticated properties can be rationally designed by a systematic modification of organic molecular structures. This motivation was further fueled by increased synthetic capabilities, especially for obtaining large organic molecules with suitable structures and topologies, and also by the spectacular progress of supramolecular chemistry for materials development witnessed in recent years. Also noteworthy is the pioneering work performed in the 1960s by several physical organic chemists who unraveled different ways of aligning spins within open-shell molecules (i.e., triplet diradicals, carbenes, etc.), working against nature's tendency to align them in an antiparallel manner. Magnetic interactions between unpaired electrons, located on the singly occupied molecular orbitals (SOMOs) of di- and polyradicals, or between the adjacent open-shell molecules in crystals, are a crucial issue in this evolving field. Thus, depending upon the symmetry, degeneracy,and topological characteristics of SOMOs and also on the mode of arrangement of the molecules in a crystal, the resulting interaction can align the neighboring spins parallel or antiparallel (see the introductory article by Miller and Epstein in this issue of MRS Bulletin).

Novel copper(II) complexes of “short” salen homologues. Structure and magnetic properties of the tetranuclear complex [Cu2(L2)2]2 [H2L2 = phenyl-N,N ′-bis(salicylidene)methanediamine] †

Copper(II) complexes of Schiff bases derived from the condensation of two molecules of salicylaldehyde and methanediamine (L1) or some phenyl substituted methanediamines (L2–L5) have been synthesized and characterised. The crystal structure of the phenylmethanediamine (L2) derivative has been determined. The ligand acts as bis-bidentate, bridging two copper atoms which are co-ordinated to two phenolato and two imino groups of two ligands. Two binuclear moieties [Cu2(L2)2] are held together through Cu–O(phenolato) interactions between adjacent units forming a pseudo-linear [Cu2L22]2 cluster. Variable temperature magnetic susceptibility data for this derivative reveal the presence of weak antiferromagnetic interactions (−J ≈ 2–3 cm−1) between nearest neighbour spin centres. The structural factors that may be responsible for the sign and magnitude of the exchange interactions are discussed.

EPR and Catalytic Investigation of Cu(Salen) Complexes Encapsulated in Zeolites

Zeolite-Y-encapsulated Cu(salen) and Cu(5-Cl-salen) complexes, where salen is N,N′-ethylenebis(salicylidenaminato), have been synthesized and characterized by various physicochemical techniques. “Neat” complexes showed broad EPR spectra corresponding to nearest neighbor spin–spin interactions whereas the zeolite-encapsulated metal complexes showed well-resolved metal hyperfine features similar to spectra in dilute frozen solutions indicating the encapsulation of monomeric salen complexes in zeolite cavities. Molecules adsorbed on the external surface exhibit spectra similar to “neat” complexes. The spectra arise from an unpaired electron occupying a “formal” 3dx2−y2 orbital. Calculated ground-state molecular orbital coefficients suggest an increase in the in-plane covalency of the Cu–ligand bond and depletion of electron density at copper on encapsulation thereby facilitating the attack of incoming nucleophiles like tert-butyl hydroperoxide anion at the copper atom. In a comparitive study of the catalytic activity of the “neat” and zeolite-encapsulated Cu(salen) complexes, the activity for the decomposition of H2O2, tert-butyl hydroperoxide as well as the oxidation of phenol and para-xylene increased on encapsulation. The observed changes in the molecular and electronic structure of the complexes on encapsulation are, perhaps, responsible for the enhancement in catalytic activity.

Critical specific heats of theN-vector spin models on the simple cubic and bcc lattices

We have computed through order ${\ensuremath{\beta}}^{21}$ the high-temperature expansions for the nearest neighbor spin correlation function $G(N,\ensuremath{\beta})$ of the classical N-vector model, with general N, on the simple cubic and on the body centered cubic lattices. For this model, also known in quantum field theory as the lattice $O(N)$ nonlinear $\ensuremath{\sigma}$ model, we have presented in previous papers extended expansions of the susceptibility, of its second field derivative, and of the second moment of the correlation function. Here we study the internal specific energy and the specific heat $C(N,\ensuremath{\beta}),$ obtaining updated estimates of the critical parameters and therefore a more accurate direct test of the hyperscaling relation $d\ensuremath{\nu}(N)=2\ensuremath{-}\ensuremath{\alpha}(N)$ on a range of values of the spin dimensionality N, including $N=0$ (the self-avoiding walk model), $N=1$ (the Ising spin 1/2 model), $N=2$ (the $\mathrm{XY}$ model), $N=3$ (the classical Heisenberg model). By the newly extended series we also compute the universal combination of critical amplitudes usually denoted by ${R}_{\ensuremath{\xi}}^{+}(N),$ in fair agreement with renormalization group estimates.

Dynamical phase diagram of a metamagnetic model subject to an oscillating magnetic field

We investigated the dynamical behavior of an Ising model in a square lattice subject to a time-dependent external magnetic field. In our model the exchange coupling between first neighboring spins in the horizontal direction is different from that of the vertical direction. We have employed the master equation approach for the Glauber stochastic process and used the dynamical pair approximation in order to decouple the hierarchy of equations. We have found the stationary phase diagram for this model in the plane amplitude of the oscillating field versus temperature, for different values of the frequency of the external field and of the ratio between the vertical and the horizontal couplings. Depending on these values, the phase diagram can exhibit the ferromagnetic, paramagnetic, and antiferromagnetic phases. The transition between these phases can be continuous or discontinuous, and the model may also display a tricritical behavior.

A Monte Carlo analysis of spin dynamics and Mössbauer relaxation in 2­D magnetically diluted iron oxides

The spin dynamics of two-dimensional magnetically diluted iron oxides with the K2NiF4 structure has been studied by a Monte Carlo analysis using a simple Ising model. Below the critical temperature, antiferromagnetic domains are formed in which the domain walls are ‘pinned’ by the non-magnetic atoms such that increasing dilution causes substantial interpenetration of the domains. The magnetic hysteresis observed in field-cooled and zero-field-cooled magnetic measurements on SrLaFeO4 diluted with a non-magnetic cation can be explained. Similarly, the observed Mossbauer relaxation can be simulated, and derives from differing relaxation rates for spins with few near neighbour spins, particularly those close to a domain boundary. The existence of ‘spin clusters’ which has often been assumed is not a good model. It is suggested that Ising-like behaviour in the critical region is an important aspect of these diluted Heisenberg layered systems.

Theory of excitons in the insulatingCu−O2plane

We use a local model to study the formation and the structure of the low-energy charge-transfer excitations in the insulating ${\mathrm{C}\mathrm{u}\ensuremath{-}\mathrm{O}}_{2}$ plane. The elementary excitation is a bound exciton of spin singlet, consisting of a ${\mathrm{Cu}}^{+}$ and a neighboring spin singlet of Cu-O holes. The exciton can move through the lattice freely without disturbing the antiferromagnetic spin background, in contrast to the single hole motion. There are four eigenmodes of excitons with different symmetry. The $p$-wave-like exciton has a large dispersion width. The $s$-wave-like exciton mixes with the $p$ state at finite momentum, and its dipole transition intensity is strongly anisotropic. The model is in excellent agreement with the electron energy-loss spectra in the insulating ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}.$