Nature of the Spin Dynamics and1/3Magnetization Plateau in Azurite

Abstract

We present a specific heat and inelastic neutron scattering study in magnetic fields up into the 1/3 magnetization plateau phase of the diamond chain compound azurite Cu3(CO3)2(OH)2. We establish that the magnetization plateau is a dimer-monomer state, i.e., consisting of a chain of S = 1/2 monomers, which are separated by S = 0 dimers on the diamond chain backbone. The effective spin couplings Jmono/kB = 10.1(2) K and J dimer /kB = 1.8(1) K are derived from the monomer and dimer dispersions. They are associated to microscopic couplings J1/kB = 1(2) K, J2/kB = 55(5) K and a ferromagnetic J3/kB = −20(5) K, possibly as result of d z 2 orbitals in the Cu-O bonds providing the superexchange pathways.PACS numbers: 75.30. Et, 75.10.Pq, 75.45.+j Great interest has surrounded the observation of a 1/3 magnetization plateau in azurite CuThis material, famous as a painting pigment of deepblue colour, has been proposed as a realisation of the exotic diamond-chain Hamiltonian of coupled spin-1/2 moments, written aŝHere, J 2 is the magnetic coupling of the diamond backbone, while J 1 and J 3 represent the coupling of the monomers along the chain [3, 4, 5] (Fig. 3). Depending on the relative coupling strengths J 1 , J 2 , J 3 , this model affords a host of exotic phases and quantum phase transitions, including possibly M = 1/3 fractionalisation [6] or exotic dimer phases [4]. However, determining the magnetic exchange couplings in azurite has proved difficult, yielding controversial results. While a susceptibility χ study claims, subsequent numerical studies of χ dispute this claim, proposing a ferromagnetic (FM) J 3 , and thus a non-frustrated scenario [2]. The general issue underlying these starkly contrasting interpretations of the same experimental data is that of the nature of magnetic coupling in low-dimensional (low-D) quantum magnets. In azurite Cu 3 (CO 3 ) 2 (OH) 2 , the Cu 2+ ions (S = 1/2) are in a square-planar coordination on two inequivalent sites [7]. The system has a monoclinic crystal structure (space group P2 1 /c, lattice parameters a = 5.

Absence of actual plateaus in zero-temperature magnetization curves of quantum spin clusters and chains

“…also as the diamond chain is believed to be the real magnetic structure of the mineral azurite. [48][49][50][51] The lattice in depicted in Fig. 9, where the quantum spin-dimer are the vertical bonds(solid lines) while the dashed lines correspond to Ising couplings.…”

Dynamics of linarite: Observations of magnetic excitations

“…Inelastic neutron scattering (INS) is the most direct probe for investigating the magnetic excitations in low dimensional quantum magnets 35,36 . In order to refine the magnetic exchange interactions in PbCuSO 4 (OH) 2 , here we present INS data revealing the zero field excitation spectra of linarite.…”

Low-temperature heat transport of spin-gapped quantum magnets

“…With formula (13), the authors proposed two different resonant scattering mechanisms for explaining the 3 K and 17 K minimums [88,100,101]. The higher-T resonant scattering was attributed to the localized singlet-triplet excitations of spin dimers, with the resonant scattering rate [43,81] …”