One-dimensional dispersive magnon excitation in the frustrated spin-2 chain system Ca3Co2O6

Abstract

Using inelastic neutron scattering, we have observed a quasi-one-dimensional dispersive magnetic excitation in the frustrated triangular-lattice spin-2 chain oxide Ca 3 Co 2 O 6 . At the lowest temperature (T = 1.5 K), this magnon is characterized by a large zone-center spin gap of ∼ 27 meV, which we attribute to the large single-ion anisotropy, and disperses along the chain direction with a bandwidth of ∼ 3.5 meV. In the directions orthogonal to the chains, no measurable dispersion was found. With increasing temperature, the magnon dispersion shifts towards lower energies, yet persists up to at least 150 K, indicating that the ferromagnetic intrachain correlations survive up to 6 times higher temperatures than the long-range interchain antiferromagnetic order. The magnon dispersion can be well described within the predictions of linear spin-wave theory for a system of weakly coupled ferromagnetic chains with large single-ion anisotropy, enabling the direct quantitative determination of the magnetic exchange and anisotropy parameters. PACS numbers: 75.30.Ds 75.50.Ee 78.70.Nx 75.10.PqFrustrated antiferromagnets attract much theoretical and experimental attention because of their peculiar magnetic properties. The geometry of the underlying lattice or competing interactions in these systems may give rise to a macroscopic ground-state degeneracy and thus can prevent the onset of a long-range magnetic ordering (LRO) down to the absolute zero temperature, T = 0 K. 1,2 Due to the large ground-state degeneracy, small perturbations, such as further-neighbor interaction, single-ion anisotropy, spinlattice interactions, or magnetic field, can give rise to a variety of magnetically ordered states. 3 Quasi-two-dimensional (2D) triangular-lattice antiferromagnets (TLAF) have been extensively studied as exemplar frustrated spin systems. [4][5][6][7][8] Their magnetic phase diagrams strongly depend on the exchange anisotropy. A typical 2D Heisenberg TLAF with nearest-neighbor interactions orders in the noncollinear "120 • structure" even in the extreme quantum spin-1 2 case. 9 However, the 2D Ising TLAF is known to display no LRO down to T = 0 K. 10 So far, experimental realizations of the Ising TLAF are restricted to a few systems, where ferromagnetic (FM) [11][12][13] or antiferromagnetic (AFM) 14 spin chains are arranged on a triangular lattice in the plane perpendicular to the chains. Among them, compounds with FM chains offer a rich playground to investigate a variety of exotic magnetic phases cooperatively induced by the dimensionality reduction, magnetic anisotropy, geometrical frustration, and magnetic field. 11,12 As a model system of the Ising TLAF, Ca 3 Co 2 O 6 exhibits many intriguing properties, such as field-induced magnetization steps, 15-19 time-dependent magnetic order, 20,21 magnetodielectric coupling 22 and others. [23][24][25][26][27][28] It has a rhombohedral structure (space group R3c) with a hexagonal arrangement of one-dimensional (1D) chains consisting of alternating face-sharing CoO 6 octahedra (OCT) a...

Spin correlations inCa3Co2O6: Polarized-neutron diffraction and Monte Carlo study

“…where c = 10.367Å, which also has a "long tail" at high T . Our observation of significant FM correlations well above T N is consistent with µSR data 35 and Mössbauer studies on 159 Eu-doped Ca 3 Co 2 O 6 , 36 as well as with recent inelastic neutron scattering (INS) measurements, which report that well-defined magnetic excitations persist to T ≈ 150 K. 37 We note two qualifications regarding the RMC refinements. First, since RMC refinement is a stochastic process, the values of ξ obtained from RMC represent lower bounds on the true values.…”

“…2, where the effect of finite D was not considered when fitting χT in the region 150 ≤ T ≤ 300 K. However, there is better agreement between our results and those of Ref. 30, in which the susceptibility was fitted to a model including both J 1 and D. The most informative comparison is with recently-reported INS measurements, 37 which show a weakly-dispersive spin wave propagating along c * with a large spin gap ω 0 = 27 meV (= 310 K). The dispersion was fitted using linear spin-wave theory to obtain J 1 = 4.9 K and D = ω 0 /2S = 79 K. Taking into account higher-order corrections to the spin-wave theory leads to a modified equation for the spin gap, D = ω 0 /(2S − 1), 42 which yields D = 105 K for the data of Ref.…”

Frustrated Ising chains on the triangular lattice inSr3NiIrO6

“…The unusually strong anisotropy of the nickel ion is the result of the strongly distorted local environment within the strained trigonal prism of oxygens. A similar value with opposite sign (D + J z = −7.20(2) meV) was found in Ca 2 Co 2 O 6 by inelastic neutron scattering and ab initio calculations 8,28 . Additional terms in the single chain Hamiltonian do not play an important role, regarding the spin wave excitations.…”

Exchange interactions in Ca3Co2O6probed locally by NMR

“…As far as J 1 is concerned, it is of the same order as the transverse exchange constant J ⊥ which determines the dispersion of the M = ±2 → M = ±1 transition (bringing Co ions out of the Ising subspace). The fit of a spin-wave model to INS spectra yielded J ⊥ /k B 19.5 K [27]. Thus we deduce that the anisotropy between longitudinal (J 1 ) and transverse (J ⊥ ) exchange couplings in the full Heisenberg model for S = 2 spins is of the order of 20%.…”

“…Here, however, the Ising character of the Co 3+ II pseudospins makes neutrons unable to flip them and to probe their excitation spectrum directly. A branch of quasi-one-dimensional excitations out of the Ising subspace has been recently identified around 30 meV by high-energy INS [27]. These results have been interpreted by a model of interacting S = 2 spins with easy-axis anisotropy D. Using a spin-wave approximation yields an estimate for J 1 + D, but not of J 1 and D separately.…”