Spin wave excitations in the pyrovanadate α−Cu2V2O7

Abstract

The pyrovanadate α-Cu2V2O7 belongs to the orthorhombic (F dd2) class of crystals with noncentrosymmetric crystal structure. Recently, the compound has been identified to be a magnetic multiferroic with a substantial electric polarization below the magnetic transition temperature TC = 35 K. Here we report the results of our inelastic neutron scattering (INS) studies on a polycrystalline sample of α-Cu2V2O7. Our INS data clearly show the existence of dispersive spin wave excitations below TC with a zone-boundary energy of 11 meV at 5 K. We have analyzed the data using linear spin wave theory, which shows good agreement between the experiment and calculation. The analysis is consistent with the third nearest neighbor exchange interaction playing a dominant role in the magnetism of the material.PACS numbers: 78.70. Nx, 75.85.+t, 75.30.Ds The family of pyrovanadates with general formula M 2 V 2 O 7 (M = Cu, Ni, Co, Mn) has attracted considerable attention due to their fascinating and diverse crystal structures, which are built out of various extended units of M-O and V-O polyhedra.1-4 They can have low-dimensional (chain, sheet, honey-comb etc.) to more complex three dimensional structures with intriguing magnetic and electronic properties.5,6 These pyrovanadates broadly crystallize in two different groups of structures: thortveitite (Sc 2 Si 2 O 7 type) or dichromate (K 2 Cr 2 O 7 type). 4Among the various members of the group, Cu 2 V 2 O 7 draws special attention both for its structural and electronic aspects. This compound can crystallize in at least four polymorphic phases (namely α, β, β ′ and γ) with different lattice symmetry (although all thortveitite type).6-8 The β phase can be described as a spin 1/2 Heisenberg system on a two dimensional (2D) honeycomb lattice .6 On the other hand, the α phase is better described by two sets of mutually perpendicular zigzag chains with strong inter-chain magnetic interactions. Here all Cu 2+ ions are equivalent with fivefold coordination to oxygen atoms forming a distorted [CuO 5 ] polyhedron. Each distorted polyhedron is linked with another two via edge sharing and together they form the two sets of mutually perpendicular zig zag chains (see Fig. 1) in the bc plane.7,9 Notably, α-Cu 2 V 2 O 7 crystallizes in the Orthorhombic F dd2 structure and it is the only member of the pyrovanadates to have a non-centrosymmetric crystal structure.The magnetic and electric properties of α-Cu 2 V 2 O 7 are equally interesting. The compound undergoes long range magnetic ordering below T C = 35 K with a spin canted structure.9-11 Recent work by our group identified the compound to be an improper multiferroic with the simul- taneous development of spontaneous electric polarization and magnetization below T C .12 Density functional theory (DFT) based calculations indicate that the magnetism in α-Cu 2 V 2 O 7 is a consequence of ferro-orbital ordering due to the unique pyramidal CuO 5 environment and the origin of the giant ferroelectric polarization is primarily due to the symmetric exc...

Magnetoelastic coupling and ferromagnetic-type in-gap spin excitations in multiferroic α-Cu2V2O7

“…This in-gap excitation branch is associated with the canted magnetic moment arising from DM interaction and it enables quantitative estimates of the DM-parameter and the effective exchange constant. The obtained value of the effective exchange constant J eff =8(3) meV is larger than the nearest neighbour exchange constants determined by neutron scattering of α-Cu 2 V 2 O 7 powder J 1 =4.67 meV and J 2 =−0.8 meV, but in good agreement with the dominant exchange J 3 =9 meV [10]. Although a similar Hamiltonian was applied for the data analysis, the anisotropy parameter obtained by INS of a single crystal sample considerably differs from our results while J eff is similar [27].…”

“…Here, J eff is the effective isotropic exchange, M FM =0.082(1)μ B the in-plane ferromagnetic moment (see figure 5), D the effective in-plane anisotropy, and g bc the g-factor in the bc-plane. Applying the constraint for the AFMR gap J DS 32 10 eff 2 = meV as detected in a recent neutron study on a powder sample from the same batch as studied here [10], and the g-value g bc ≈2, we obtain J eff =8(3) meV and D 1.6 1 = ( ) meV. The obtained value of the effective two-sublattice antiferromagnetic exchange constant is consistent with the dominant third-nearest-neighbour exchange interaction J 3 inferred from inelastic neutron data which in addition to slightly smaller nearest and next-nearest-neighbour couplings J 1 and J 2 governs the long range spin ordered phase [10].…”

“…Applying the constraint for the AFMR gap J DS 32 10 eff 2 = meV as detected in a recent neutron study on a powder sample from the same batch as studied here [10], and the g-value g bc ≈2, we obtain J eff =8(3) meV and D 1.6 1 = ( ) meV. The obtained value of the effective two-sublattice antiferromagnetic exchange constant is consistent with the dominant third-nearest-neighbour exchange interaction J 3 inferred from inelastic neutron data which in addition to slightly smaller nearest and next-nearest-neighbour couplings J 1 and J 2 governs the long range spin ordered phase [10]. Note, that the in-plane g bc -factor cannot be determined more precisely because the slope of the resonance branch is dominated by the DM interaction.…”

“…While HF-ESR measurements are susceptible to collective q = 0 spin excitations in the long range spin ordered state, i.e. antiferromagnetic resonance (AFMR) modes, the large AFM gap of ∼10 meV inferred from inelastic neutron scattering (INS) on α-Cu 2 V 2 O 7 [10] rules out the observation of AFMR modes at frequencies below 2 THz. However, the ESR spectra taken at f=61.6 GHz shown in figure 8(a) display a clear resonance peak appearing at T<20 K and in low magnetic fields 8 .…”

“…While the chains consist of edge-sharing distorted CuO 5 -polyhedra, the non-centrosymmetric orthorhombic Fdd2 structure of the α-phase permits stronger interchain interaction than the other polymorphs of Cu 2 V 2 O 7 [7][8][9]. Magnetism in α-Cu 2 V 2 O 7 is rather three-dimensional as inelastic neutron studies suggest dominant interchain exchange interaction J 3 between third-nearest-neighbours, in addition to the nearest-and next-nearest neighbour interactions J 1 and J 2 [10]. Notably, long range antiferromagnetic order evolving below T C =35 K exhibits a considerable magnetic moment arising from spin canting due to antisymmetric Dzyaloshinsii-Moriya (DM) interactions [11][12][13].…”