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Register a new userSpin gap in a quasi-1D S=1/2 antiferromagnet K2CuSO4Cl2
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Electron spin resonance experiments in the quasi-1D S=1/2 antiferromagnet K$_2$CuSO$_4$Cl$_2$ reveal opening of a gap in absence of magnetic ordering, as well as an anisotropic shift of the resonance magnetic field. These features of magnetic excitation spectrum are explained by a crossover between a gapped spinon-type doublet ESR formed in a 1D antiferromagnet with uniform Dzyaloshinskii-Moriya interaction and a Larmor-type resonance of a quasi-1D Heisenberg system
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Magnetic excitations of the quasi-1D S=1/2 Heisenberg antiferromagnet (HAF) Cs2CuCl4 have been measured as a function of magnetic field using neutron scattering. For T<0.62 K and B=0 T the weak inter-chain coupling produces 3D incommensurate ordering. Fields greater than Bc =1.66 T, but less than the field (~8 T) required to fully align the spins, are observed to decouple the chains, and the system enters a disordered intermediate-field phase (IFP). The IFP excitations are in agreement with the predictions of Muller et al. for the 1D S=1/2 HAF, and Talstra and Haldane for the related 1/r^2 chain (the Haldane-Shastry model). This behaviour is inconsistent with linear spin-wave theory.
We report on magnetic resonance studies within the magnetically ordered phase of the quasi-1D antiferromagnet LiCuVO_4. Our studies reveal a spin reorientational transition at a magnetic field H_c1 ~ 25 kOe applied within the crystallographical (ab)-plane in addition to the recently observed one at H_c2 ~75 kOe [ M.G. Banks et al., cond-mat/0608554 (2006)]. Spectra of the antiferromagnetic resonance (AFMR) along low-frequency branches can be described in the frame of a macroscopic theory of exchange-rigid planar magnetic structures. These data allow to obtain the anisotropy of the exchange interaction together with a constant of the uniaxial anisotropy. Spectra of 7Li nuclear magnetic resonance (NMR) show that, within the magnetically ordered phase of LiCuVO_4 in the low-field range H < H_c1, a planar spiral spin structure is realized with the spins lying in the (ab)-plane in agreement with neutron scattering studies of B.J. Gibson et al. [Physica B Vol. 350, 253 (2004)]. Based on NMR spectra simulations, the transition at H_c1 can well be described as a spin-flop transition, where the spin plane of the magnetically ordered structure rotates to be perpendicular to the direction of the applied magnetic field. For H > H_c2 ~ 75 kOe, our NMR spectra simulations show that the magnetically ordered structure exhibits a modulation of the spin projections along the direction of the applied magnetic field H.
We investigated the crystal structure of Rb$_2$Cu$_3$SnF$_{12}$ and its magnetic properties using single crystals. This compound is composed of Kagome layers of corner-sharing CuF$_{6}$ octahedra with a 2a x 2a enlarged cell as compared with the proper Kagome layer. Rb$_2$Cu$_3$SnF$_{12}$ is magnetically described as an $S$=1/2 modified Kagome antiferromagnet with four kinds of neighboring exchange interaction. From magnetic susceptibility and high-field magnetization measurements, it was found that the ground state is a disordered singlet with the spin gap, as predicted from a recent theory. Exact diagonalization for a 12-site Kagome cluster was performed to analyze the magnetic susceptibility, and individual exchange interactions were evaluated.
Cu(pz)2(ClO4)2 (with pz denoting pyrazine, C4H4N2) is among the best realizations of a two-dimensional spin-1/2 square-lattice antiferromagnet. Below T_N = 4.21 K, its weak interlayer couplings induce a 3D magnetic order, strongly influenced by external magnetic fields and/or hydrostatic pressure. Previous work, focusing on the [H, T] phase diagram, identified a spin-flop transition, resulting in a field-tunable bicritical point. However, the influence of external pressure has not been investigated yet. Here we explore the extended [p, H, T] phase diagram of Cu(pz)2(ClO4)2 under pressures up to 12 kbar and magnetic fields up to 7.1 T, via magnetometry and 35Cl nuclear magnetic resonance (NMR) measurements. The application of magnetic fields enhances T_XY , the crossover temperature from the Heisenberg to the XY model, thus pointing to an enhancement of the effective anisotropy. The applied pressure has an opposite effect [dT_N/dp = 0.050(8) K/kbar], as it modifies marginally the interlayer couplings, but likely changes more significantly the orbital reorientation and the square-lattice deformation. This results in a remodeling of the effective Hamiltonian, whereby the field and pressure effects compensate each other. Finally, by comparing the experimental data with numerical simulations we estimate T_BKT, the temperature of the Berezinskii-Kosterlitz-Thouless topological transition and argue why it is inaccessible in our case.
Hexagonal antiferromagnets Cs$_2$Cu$_3$MF$_{12}$ (M = Zr, Hf and Sn) have uniform Kagome lattices of Cu$^{2+}$ with S = 1/2, whereas Rb$_2$Cu$_3$SnF$_{12}$ has a 2a by 2a enlarged cell as compared with the uniform Kagome lattice. The crystal data of Cs$_2$Cu$_3$SnF$_{12}$ synthesized first in the present work are reported. We performed magnetic susceptibility measurements on this family of Kagome antiferromagnet using single crystals. In the Cs$_2$Cu$_3$MF$_{12}$ systems, structural phase transitions were observed at $T_t = 225$ K, 172 K and 185 K for M = Zr, Hf and Sn, respectively. The magnetic susceptibilities observed for $T > T_t$ are almost perfectly described using theoretical results obtained by exact diagonalization for the 24-site Kagome cluster with $J/k_B = 244$ K, 266 K and 240 K, respectively. Magnetic ordering accompanied by the weak ferromagnetic moment occurs at $T_N = 23.5$ K, 24.5 K and 20.0 K, respectively. The origins of the weak ferromagnetic moment should be ascribed to the lattice distortion that breaks the hexagonal symmetry of the exchange network for $T < T_t$ and the Dzyaloshinsky-Moriya interaction. Rb$_2$Cu$_3$SnF$_{12}$ is magnetically described as a modified Kagome antiferromagnet with four types of neighboring exchange interaction. Neither structural nor magnetic phase transition was observed in Rb$_2$Cu$_3$SnF$_{12}$. Its magnetic ground state was found to be a spin singlet with a triplet gap. Using exact diagonalization for a 12-site Kagome cluster, we analyzed the magnetic susceptibility and evaluated individual exchange interactions. The causes leading to the different ground states in Cs$_2$Cu$_3$SnF$_{12}$ and Rb$_2$Cu$_3$SnF$_{12}$ are discussed.
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