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Register a new userFinite size effects and magnetic order in the spin-1/2 honeycomb lattice compound InCu{2/3}V{1/3}O{3}
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High field electron spin resonance, nuclear magnetic resonance and magnetization studies addressing the ground state of the quasi two-dimensional spin-1/2 honeycomb lattice compound InCu{2/3}V{1/3}O{3} are reported. Uncorrelated finite size structural domains occurring in the honeycomb planes are expected to inhibit long range magnetic order. Surprisingly, ESR data reveal the development of two collinear antiferromagnetic (AFM) sublattices below ~ 20 K whereas NMR results show the presence of the staggered internal field. Magnetization data evidence a spin reorientation transition at ~ 5.7 T. Quantum Monte-Carlo calculations show that switching on the coupling between the honeycomb spin planes in a finite size cluster yields a Neel-like AFM spin structure with a substantial staggered magnetization at finite temperatures. This may explain the occurrence of a robust AFM state in InCu{2/3}V{1/3}O{3} despite an unfavorable effect of structural disorder.
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We report a combined $^{115}$In NQR, $^{51}$V NMR and $mu$SR spectroscopic study of the low-temperature magnetic properties of InCu$_{2/3}$V$_{1/3}$O$_3$, a quasi-two dimensional (2D) compound comprising in the spin sector a honeycomb lattice of antiferromagnetically coupled spins $S=1/2$ associated with Cu$^{2+}$ ions. Despite substantial experimental and theoretical efforts, the ground state of this material was has not been ultimately identified. In particular, two characteristic temperatures of about $sim 40$ K and $sim 20$ K manifesting themselves as anomalies in different magnetic measurements are discussed controversially. A combined analysis of the experimental data complemented with theoretical calculations of exchange constants enabled us to identify below 39 K an ``intermediate quasi-2D static spin state. This spin state is characterized by a staggered magnetization with a temperature evolution that agrees with the predictions for the 2D XY model. We observe that this state gradually transforms at 15 K into a fully developed 3D antiferromagnetic Neel state. We ascribe such an extended quasi-2D static regime to an effective magnetic decoupling of the honeycomb planes due to a strong frustration of the interlayer exchange interactions which inhibits long-range spin-spin correlations across the planes. Interestingly, we find indications of the topological Berezinsky-Kosterlitz-Thouless transition in the quasi-2D static state of the honeycomb spin-1/2 planes of InCu$_{2/3}$V$_{1/3}$O$_3$.
PbFe$_{1/2}$Ta$_{1/2}$O$_{3}$ (PFT) belongs to the family of PbB$_{x}$B$_{1-x}$O$_{3}$ which have inherent chemical disorder at the B-site. Due to this disorder, a complex magnetic phase diagram is expected in the material. In this paper, we report experimental results of magnetic properties in PFT through macroscopic characterization, neutron scattering and M{o}ssbauer spectroscopy techniques. With these results we show for the first time that PbFe$_{1/2}$Ta$_{1/2}$O$_{3}$ behaves very similar to PbFe$_{1/2}$Nb$_{1/2}$O$_{3}$, i.e, it undergoes AF transition at 153 K and has a spinglass transition at 10 K, below which the antiferromagnetism coexists with spinglass. We suggest that the mechanism which is responsible for such a non-trivial ground state can be explained by a speromagnet-like spin arrangement similar to the one proposed for PbFe$_{1/2}$Nb$_{1/2}$O$_{3}$.
Less common ligand coordination of transition-metal centers is often associated with peculiar valence-shell electron configurations and outstanding physical properties. One example is the Fe$^+$ ion with linear coordination, actively investigated in the research area of single-molecule magnetism. Here we address the nature of 3$d^9$ states for Cu$^{2+}$ ions sitting in the center of trigonal bipyramidal ligand cages in the quasi-two-dimensional honeycomb compound InCu$_{2/3}$V$_{1/3}$O$_3$, whose unusual magnetic properties were intensively studied in the recent past. In particular, we discuss the interplay of structural effects, electron correlations, and spin-orbit couplings in this material. A relevant computational finding is a different sequence of the Cu ($xz$, $yz$) and ($xy$, $x^2!-!y^2$) levels as compared to existing electronic-structure models, which has implications for the interpretation of various excitation spectra. Spin-orbit interactions, both first- and second-order, turn out to be stronger than previously assumed, suggesting that rather rich single-ion magnetic properties can be in principle achieved also for the 3$d^9$ configuration by properly adjusting the sequence of crystal-field states for such less usual ligand coordination.
Novel magnetic ordering on the honeycomb lattice due to emergent weak anisotropic interactions generated by the mixing between the $J=1/2$ sector and the magnetically inactive 3/2 sector is investigated in a three-orbital interacting electron model in the absence of Hunds coupling. Self-consistent determination of magnetic order yields anisotropic N{e}el and zigzag orders for different parameter regimes, highlighting the effect of the emergent single-ion anisotropy. Study of magnon excitations shows extremely small magnon energy scale compared to the hopping energy scale, and enhancement of anisotropy effects for smaller spin-orbit coupling. These results account for several features of the honeycomb lattice compounds such as $rm Na_2 Ir O_3$ and $rm Ru Cl_3$, where the leading order anisotropic interactions within the magnetically active $J=1/2$ sector are completely quenched due to the edge-sharing octahedra.
Inelastic neutron scattering study has been performed in an S=3/2 bilayer honeycomb lattice compound Bi3Mn4O12(NO3) at ambient and high magnetic fields. Relatively broad and monotonically dispersive magnetic excitations were observed at ambient field, where no long range magnetic order exists. In the magnetic field-induced long-range ordered state at 10 T, the magnetic dispersions become slightly more intense, albeit still broad as in the disordered state, and two excitation gaps, probably originating from an easy-plane magnetic anisotropy and intrabilayer interactions, develop. Analyzing the magnetic dispersions using the linear spin-wave theory, we estimated the intraplane and intrabilayer magnetic interactions, which are almost consistent with those determined by ab initio density functional theory calculations [M. Alaei et al., Phys. Rev. B 96, 140404(R) (2017)], except the 3rd and 4th neighbor intrabilayer interactions. Most importantly, as predicted by the theory, there is no significant frustration in the honeycomb plane but frustrating intrabilayer interactions probably give rise to the disordered ground state.
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