Exchange Interaction J1 Research Articles

Quantum critical magnetization behaviors of kagome‐ and triangular‐lattice antiferromagnets

AbstractOur previous numerical diagonalization study on the magnetization processes of the kagome‐ and triangular‐lattice antiferromagnets revealed that the former system exhibits a new field‐induced phenomenon, called a magnetization ramp, at 1/3 of the saturation magnetization, while the latter one a conventional magnetization plateau. To clarify the difference between the magnetization ramp and plateau, we consider a generalized lattice model with two exchange interactions J1 and J2, which corresponds to the kagome lattice for J2 = 0 and triangular one for J2 = J1. The present numerical diagonalization study revealed that a quantum phase transition between the magnetization ramp and plateau occurs about J2/J1 ∼ 0.1.

Two co-crystalline M(hfac)2(IPhIN)2·M(hfac)2 (M = Mn, Co) compounds with a bis(iminoylnitroxide) biradical: structure and magnetism

1-Iodo-3,5-bis(4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1H-imidazole-1′-oxyl)benzene (IPhIN = L) forms co-crystalline [M(hfac)2(IPhIN)2]·[M(hfac)2] coordination complexes (M = Mn, Co) with [Mn(hfac)2] and [Co(hfac)2]. In each system only one of the iminoylnitroxide radical moieties is coordinated as part of the [M(hfac)2(IPhIN)2] structural units; the metal ions do not coordinate the radical nitrogen atoms, but rather the nitroxide oxygen to form a cis NO–M–ON grouping in a hexacoordinate ligand sphere. Solvate water molecules in the lattice provide an important driving force for assembly of the co-crystal structures in the cobalt(II) derivative; an additional factor is the formation of halogen bonds between hfac CF3 groups and the IPhIN iodine atom. The paramagnetic susceptibility (χM) versus temperature (T) measurements showed antiferromagnetic exchange downturns at lower temperature. Based on the crystal structures of the complexes, the χMT versus T data were modelled using a paramagnetic contribution from the [M(hfac)2] fragment, an exchange interaction J1 within the L–M–L fragment, plus an intramolecular radical–radical interaction J2 within the IPhIN biradical unit. For the Mn(II) complex, a good fit was obtained for gMn = 2.14(1), grad = 2.0 (fixed), J1 = −311(9) cm−1 and J2 = 11.1(7) cm−1. A reasonable fit for the cobalt compound was obtained for gCo = 2.72(1), grad = 2.0 (fixed), J1 = −400(5) cm−1, J2 = 28.3(3) cm−1 with a generalized zero field splitting parameter D = 64(2) cm−1.

First-principles investigations on the magnetic structure of α-NaMnO2

α-NaMnO2 contains layers of high spin Mn3+ (d4) ions parallel to the ab plane, and Mn3+ layers alternate with Na+ ions layers along the c direction. To explore the interesting two-dimensional spin correlations and one-dimensional magnetic excitations, we carefully studied the electronic and magnetic properties of α-NaMnO2 via density functional theory calculations. By evaluating the intra-layer interactions J1–J4 as well as the inter-layer exchanges J5 and J6 through performing mapping analysis, we found that α-NaMnO2 displays typical two-dimensional spin correlations, which are dominated by the intra-chain interaction J1 and the inter-chain interaction J3 (J3/J1 ≈ 0.21). Interestingly, both the strong spin exchange interactions J1 and J3 come from supersuperexchange (SSE), other than superexchange (SE). Also, due to the topology of the lattices, the magnetic excitation is dominated by the intra-chain interaction J1, leading to the one-dimensional model.

Magnetism and Structure in Chains of Copper Dinuclear Paddlewheel Units

An anhydrous copper carboxylate compound of formula [Cu(trans-2-butenoate)(2)](n) has been characterized. X-ray analysis reveals a structure built by paddlewheel units bridged by pairs of Cu...O axial bonds to give infinite chains arranged in a new topological motif. Susceptibility measurements in the 10-300 K temperature range, and isothermal magnetization curves at 2, 5, 10, and 50 K with fields up to 5 T, were obtained. Electron Paramagnetic Resonance (EPR) spectra of powder samples were measured at 33.9 GHz at 300 K, and at 9.60 GHz at temperatures in the range 90 <or= T <or= 350 K. Fitting the susceptibility data to a chain model of alternating paddlewheel and [Cu(mu-O)(2)Cu] dinuclear units yielded exchange interactions J(1) = -330.6(1) cm(-1) and J(2) = 5.9(2) cm(-1). The EPR spectra reflect the usual antiferromagnetic dinuclear behavior, with zero field splitting parameters of the excited spin triplet, D = -0.329(3) cm(-1) and E approximately 0, plus a central peak not expected for isolated dinuclear units. We interpret this peak as arising from the stochastic spin dynamics of the chain introduced by exchange couplings between spins in neighbor dinuclear units, which averages out the zero field splitting. Interactions of the units with the rest of the chain acting as a spin bath give rise to a quantum transition between localized dinuclear states and states of the spin chain. This effect competes with the condensation of the antiferromagnetic dinuclear units into the singlet ground state, producing a characteristic temperature dependence of the shape of the powder EPR spectra. We interpret these features in terms of basic theories of magnetic resonance in coupled spin systems applied to the chain array of dinuclear units.

Magnetic ordering effects in the phonon spectra of orthorhombicRMnO3 compounds

The phonon properties of orthorhombic manganitesRMnO3 are studied using a Green’s function technique taking into account anharmonicspin–phonon and phonon–phonon interaction terms. The strong spin–phonon interactionleads to anomalies in the phonon energy and the damping around the magnetic phasetransition. The phonon spectrum is discussed for different exchange interactionJ1 and spin–phononinteraction Rsp constants. Independence on the sign of Rsp we obtain softening or hardening of the phonon modes withdecreasing temperature below the phase transition temperatureTN. This is associated with phonon modulation of the exchange interaction. It is shown thatthe phonon energy and the phonon damping depend on the radius of the rare earth ionrR and on the ion doping. The influence of the external magnetic field is discussed,too.

Magnetic properties of small clusters as function of the geometry

Abstract The magnetic properties of the ground state of one- and two-dimensional small clusters (Na=16 sites) are studied solving exactly the Heisenberg Hamiltonian. We are interested in studying the magnetic behavior as a function of the lattice geometry. The total spin S shows an interesting behavior as a function of the ferromagnetic (antiferromagnetic) exchange interaction J1>0 (J2

Magnetization Plateaus of Frustrated Kondo Necklace Model

The magnetization plateaus of frustrated Kondo necklace model have been studied with use of the bosonization technique and level spectroscopy method. It is shown that the model may show three phases (dimer plateau, Néel plateau and Tomonaga-Luttinger liquid phases) due to competitions among Ising anisotropy Δ, the nearest and next nearest neighbor exchange interactions J1, J2. It is shown from bosonization technique that quantum transition between the dimer and Néel plateaus is Gaussian type, while the transition from one of these plateau phases to Tomonaga-Luttinger phase is Brezinskii-Kosterlitz-Thouless type. The phase diagram is constructed on the Δ-J2/J1 plane with use of level spectroscopy method, under the condition that the system has a half of the saturation magnetization.

Surface critical behaviour near the uniaxial Lifshitz point of the axial next-nearest-neighbour Ising model

The semi-infinite axial next-nearest-neighbour Ising (ANNNI) model in the disordered phase is treated within a molecular-field approximation, and the singularities of various response functions characterizing the critical behaviour at the surface are obtained. In previous work (Binder K and Frisch H L 1999 Eur. Phys. J. B 10 71) the axis where a nearest-neighbour ferromagnetic (J1) and next-nearest-neighbour antiferromagnetic (J2) exchange compete was chosen perpendicular to the surface plane. In the present work we consider an orientation of this axis parallel to the surface, allowing also for different values of these exchange interactions (j1,j2) in the surface plane. We derive the conditions for the occurrence of a surface transition, where the surface plane (at a temperature where the bulk is still disordered) orders either ferromagnetically or into a modulated structure. At the ordinary transition of the surface we obtain the mean-field values of the surface critical exponents, including the Lifshitz point of the bulk, where L1 = 1/2, L11 = -1/2, L = 0, L = 3. These exponents differ from their counterparts for the case where the axis of competing interactions is oriented perpendicular to the surface, and thus it is shown that for a uniaxial Lifshitz point the surface critical behaviour depends on the surface orientation.

Etude des proprietes thermodynamiques d'un groupement tetranucleaire. Application au compose Na 3RuO 4

The exact eigenvalues spectrum of the spin hamiltonian H= −2∑(i, j) JijSiSj have been calculated for a tetranuclear cluster formed by four spins 3/2 at the vertices of a lozenge. Two isotrope exchange interactions J1 and J2 are able to explain the thermodynamic properties (magnetic susceptibility, entropy, specific heat). A ground state transition from singlet to triplet state occurs when the J2J1 ratio reaches the value 43. The magnetic susceptibility data of Na3RuO4 fit well with the theoretical values proposed for J1K= (−19,5 K) and J2/k (−22,5 K).

Spin-Wave Dispersion in KMnF3

The spin-wave dispersion curve of antiferromagnetic KMnF3 at 4.2°K has been measured by means of neutron inelastic scattering. With incident neutron energies of 105° and 165°K, propagation was observed out to the zone boundary in the pseudocubic (110) plane. Least-squares fitting of the spin-wave energies to an expression involving the nearest and next-nearest-neighbor exchange interactions J1 and J2 and an anisotropy field HA yielded the results J1/k=3.80°±0.04°K, J2/k=0.11°±0.02°K and HA=3.1±0.4 kOe.

Magnetic Properties of CuF2

We have measured the moment of a pressed powder sample of monoclinic CuF2 from 4.2° to 300°K in fields to 25 kOe. χ(T) indicates an AFM transition at TN=69°K, and analysis gives ḡ≈2.3 and Θ≈−200°K. TN and Θ are consistent with magnetic ordering of the first kind with AFM exchange interactions J1(corner-bc)≈34°K and J2≈22°K. Weak ferromagnetism of anisotropic exchange origin is expected on the grounds of symmetry; the measured ferromagnetic moment is σ0≈1 G cm3/mole, corresponding to a canting angle φ≈0.01°. Assuming magnetic dipolar anisotropy and estimating principal axes and values for the anisotropic g tensor, we find the spin axis to be about 2° from c and 99° from a in the ac plane with anisotropy field HA∼2000 Oe, and the weak moment to be parallel to the monoclinic axis b with canting field HDM∼1000 Oe. Pseudodipolar anisotropy may be expected to change the orientation, but not the order of magnitude of these fields. The uniform mode AFM resonances are not degenerate in general, but both would be expected to occur at v/c∼9 cm−1 using these values.