Antiferromagnetic Curie-Weiss Temperature Research Articles

Magnetic properties of the S = 52 anisotropic triangular chain compound Bi3FeMo2O12

Competing magnetic interactions in low-dimensional quantum magnets can lead to the exotic ground state with fractionalized excitations. Herein, we present our results on an S = 5/2 quasi-one-dimensional spin system Bi3FeMo2O12. The structure of Bi3FeMo2O12 consists of very well separated, infinite zig-zag S = 5/2 spin chains. The observation of a broad maximum around 10 K in the magnetic susceptibility suggests the presence of short-range spin correlations. Magnetic susceptibility data do not fit to S=5/2 uniform spin chain model due to the presence of 2nd nearest-neighbor coupling (J2) along with the 1st nearest-neighbor coupling J1 of the zig-zag chain. The electronic structure calculations infer that the value of J1 is comparable with J2 (J2/J1~1.1) with a negligible inter-chain interaction (J'/J ~ 0.01), implying that Bi3FeMo2O12 is a highly frustrated triangular chain system. The absence of magnetic long-range ordering down to 0.2 K is seen in the heat capacity data, despite a relatively large antiferromagnetic Curie-Weiss temperature of -40 K. The magnetic heat capacity follows nearly a linear behavior at low temperatures indicating that the S = 5/2 anisotropic triangular chain exhibits the gapless excitations.

Cubic symmetry and magnetic frustration on the fcc spin lattice in K2IrCl6

Cubic crystal structure and regular octahedral environment of Ir$^{4+}$ render antifluorite-type K$_2$IrCl$_6$ a model fcc antiferromagnet with a combination of Heisenberg and Kitaev exchange interactions. High-resolution synchrotron powder diffraction confirms cubic symmetry down to at least 20 K, with a low-energy rotary mode gradually suppressed upon cooling. Using thermodynamic and transport measurements, we estimate the activation energy of $\Delta\simeq 0.7$ eV for charge transport, the antiferromagnetic Curie-Weiss temperature of $\theta_{\rm CW}\simeq -43$ K, and the extrapolated saturation field of $H_s\simeq 87$ T. All these parameters are well reproduced \textit{ab initio} using $U_{\rm eff}=2.2$ eV as the effective Coulomb repulsion parameter. The antiferromagnetic Kitaev exchange term of $K\simeq 5$ K is about one half of the Heisenberg term $J\simeq 13$ K. While this combination removes a large part of the classical ground-state degeneracy, the selection of the unique magnetic ground state additionally requires a weak second-neighbor exchange coupling $J_2\simeq 0.2$ K. Our results suggest that K$_2$IrCl$_6$ may offer the best possible cubic conditions for Ir$^{4+}$ and demonstrates the interplay of geometrical and exchange frustration in a high-symmetry setting.

Persistent low-temperature spin dynamics in the mixed-valence iridate Ba3InIr2O9

Using thermodynamic measurements, neutron diffraction, nuclear magnetic resonance, and muon spin relaxation, we establish putative quantum spin-liquid behavior in Ba$_3$InIr$_2$O$_9$, where unpaired electrons are localized on mixed-valence Ir$_2$O$_9$ dimers with Ir$^{4.5+}$ ions. Despite the antiferromagnetic Curie-Weiss temperature on the order of 10 K, neither long-range magnetic order nor spin freezing are observed down to at least 20 mK, such that spins are short-range correlated and dynamic over nearly three decades in temperature. Quadratic power-law behavior of both the spin-lattice relaxation rate and specific heat indicates the gapless nature of the ground state. We envisage that this exotic behavior may be related to an unprecedented combination of the triangular and buckled honeycomb geometries of nearest-neighbor exchange couplings in the mixed-valence setting.

Gapped Spin-1/2 Spinon Excitations in a New Kagome Quantum Spin Liquid Compound Cu3Zn(OH)6FBr**Supported by the National Key Research and Development Program of China under Grant Nos 2016YFA0300502, 2016YFA0300503, 2016YFA0300604, 2016YF0300300 and 2016YFA0300802, the National Natural Science Foundation of China under Grant Nos 11421092, 11474330, 11574359, 11674406, 11374346 and 11674375, the National Basic Research Program of China (973 Program) under Grant

We report a new kagome quantum spin liquid candidate Cu3Zn(OH)6FBr, which does not experience any phase transition down to 50 mK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature (∼200 K). A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from 19F nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fractionalization in the fractional quantum Hall state.

Theory of paramagnetic scattering in highly frustrated magnets with long-range dipole-dipole interactions: The case of theTb2Ti2O7pyrochlore antiferromagnet

Highly frustrated antiferromagnets composed of magnetic rare-earth moments are currently attracting much experimental and theoretical interest. Rare-earth ions generally have small exchange interactions and large magnetic moments. This makes it necessary to understand in detail the role of long-range magnetic dipole-dipole interactions in these systems, in particular in the context of spin-spin correlations that develop in the paramagnetic phase, but are often unable to condense into a conventional long-range magnetic ordered phase. This scenario is most dramatically emphasized in the frustrated pyrochlore antiferromagnet material Tb2Ti207 which does not order down to 50 mK despite an antiferromagnetic Curie-Weiss temperature Tcw ~ -20 K. In this paper we report results from mean-field theory calculations of the paramagnetic elastic neutron-scattering in highly frustrated magnetic systems with long-range dipole-dipole interactions, focusing on the Tb2Ti207 system. Modeling Tb2Ti207 as an antiferromagnetic <111> Ising pyrochlore, we find that the mean-field paramagnetic scattering is inconsistent with the experimentally observed results. Through simple symmetry arguments we demonstrate that the observed paramagnetic correlations in Tb2Ti207 are precluded from being generated by any spin Hamiltonian that considers only Ising spins, but are qualitatively consistent with Heisenberg-like moments. Explicit calculations of the paramagnetic scattering pattern for both <111> Ising and Heisenberg models, which include finite single-ion anisotropy, support these claims. We offer suggestions for reconciling the need to restore spin isotropy with the Ising like structure suggested by the single-ion properties of Tb3+.

Magnetic properties of Zn doped La 1.85Sr 0.15(Cu 1− xZn x)O 4 in the normal state

Series of Zn doped La 1.85Sr 0.15(Cu 1− x Zn x ) polycrystalline samples were prepared and their static magnetisation measured between 2 K and 300 K for fields up to 5 T. A careful analysis allows the subtraction of the temperature-dependent susceptibility from the bulk ( x = 0). The resulting Curie-Weiss behaviour is consistent with a Zn induced magnetic moment of 1.06±0.15 μ B/Zn distributed among the four nearest Cu neighbours to the Zn site. An antiferromagnetic Curie-Weiss temperature of 22–27 K slowly varying with x is obtained. We estimate the exchange constant for magnetic pair breaking by the Zn induced magnetic moments to be J Zn/LSCO = 0.16±0.05 eV within a classical isotropic s wave model.

Magnetic studies of Cu-based organometallic monomers and polymers

We report here magnetic studies of Cu-based organometallic monomers and polymers. Magnetic susceptibility measurements at high temperature, on monomers having different side groups, show antiferromagnetic Curie–Weiss behavior consistent with a single spin-1/2 per Cu atom and an antiferromagnetic Curie–Weiss temperature θ≊1 K. The low-temperature magnetization remains linear to μBH/kBT ≥ 1, suggestive of 1D antiferromagnetic Heisenberg interactions. A sharp decrease in the effective spin per Cu atom is observed in the polymerized organometallic system, with the number of spins decreasing with the degree of polymerization.

Electron paramagnetic resonance and relaxation of amorphous silicon below 1 K

Amorphous silicon, generated within crystalline Si by 28Si + ion implantation, exhibits an electron spin relaxation rate which varies with temperature as T 2.37 between 0.3 and 4.2 K. These results exclude the current model of a phonon-limited, direct relaxation mechanism in a-Si. A relaxation process, consistent with the known temperature variation, is outlined. EPR signal strengths, relative to a known paramagnet at temperatures near 1.2 and 0.4 K, put limits on an antiferromagnetic Curie-Weiss temperature of 0⩽θ⩽0.06 K.