Three-dimensional Antiferromagnet Research Articles

Scaling relations of three-dimensional random-exchange quantum antiferromagnets

The thermal and ground state properties of a class of three-dimensional (3D) random-exchange spin-1/2 antiferromagnets are studied using first principles quantum Monte Carlo method. Our motivation is to examine whether the newly discovered universal scaling properties, which connect the Neel temperature and the staggered magnetization density, for the clean 3D quantum dimerized Heisenberg models remain valid for the random-exchange models considered here. Remarkably, similar to the clean systems, our Monte Carlo results indicate that these scaling relations also emerge for the considered models with the introduced antiferromagnetic randomness. The scope of the validity of these scaling properties for the 3D quantum antiferromagnets is investigated as well.

Impurity-induced magnetization in a three-dimensional antiferromagnet at the quantum critical point

We consider a single impurity with spin $S$ embedded in a three-dimensional antiferromagnetic system which is close to the quantum critical point (QCP), separating magnetically ordered and disordered phases. Approaching the QCP from the disordered phase we study the spatial distribution of spin density and staggered magnetization induced by the impurity. Using two methods (self-consistent Born approximation and renormalization group) we found a power law decay of the spin density $\propto 1/r^3$, and of the staggered magnetization $\propto 1/r$ with relevant logarithmic corrections. We demonstrate that the local spin at the impurity site $r=0$ approaches to zero at the QCP. We show that in the semiclassical limit of large $S$ the problem is equivalent to the exactly solvable independent boson model. Our results demonstrate existence of spin-charge separation in the three dimensional systems in the vicinity of the QCP.

The long-range magnetic order and underlying spin model in shattuckite Cu5(SiO3)4(OH)2

The mineral shattuckite Cu5(SiO3)4(OH)2 is investigated in measurements of X-band electron spin resonance, magnetization and specific heat. The magnetic subsystem of the title compound is constituted by unique combination of well-separated brucite-like corrugated CuO2 layers and CuO2-twisted ribbons. Despite seemingly low-dimensional motives in the structure, shattuckite exhibits properties of three-dimensional antiferromagnet with Weiss/Neel temperatures ratio Θ/TN ~ 2 being weakly ferromagnetic below magnetic ordering temperature TN = 7 K. Tentatively, the spontaneous magnetization appears due to Dzyaloshinskii–Moriya interactions within the brucite-like two-dimensional patterns. The electronic structure calculations suggest strong antiferromagnetic coupling of both edge-sharing and corner-sharing CuO2 units within the layers and weak ferromagnetic coupling of edge-sharing CuO2 units within the ribbons.

Phase diagram of a three-dimensional antiferromagnet with random magnetic anisotropy.

Three-dimensional antiferromagnets with random magnetic anisotropy (RMA) that have been experimentally studied to date have competing two-dimensional and three-dimensional exchange interactions which can obscure the authentic effects of RMA. The magnetic phase diagram of Fe_{x}Ni_{1-x}F_{2} epitaxial thin films with true random single-ion anisotropy was deduced from magnetometry and neutron scattering measurements and analyzed using mean-field theory. Regions with uniaxial, oblique, and easy-plane anisotropies were identified. A RMA-induced glass region was discovered where a Griffiths-like breakdown of long-range spin order occurs.

Coexistence of localized and collective magnetism in the coupled-spin-tetrahedra systemCu4Te5O12Cl4

We report high-field magnetization, electron spin resonance (ESR), and Raman scattering measurements of the coupled spin-tetrahedra system ${\mathrm{Cu}}_{4}{\mathrm{Te}}_{5}{\mathrm{O}}_{12}{\mathrm{Cl}}_{4}$ with magnetic ordering at ${T}_{N}=13.6$ K. We find thermodynamic and spectroscopic signatures for the concomitant occurrence of localized and collective magnetism. Magnetization measurements up to 60 T exhibit a spin-flop transition at ${\ensuremath{\mu}}_{0}{H}_{\mathrm{SF}}=16$ T only for $H\ensuremath{\parallel}c$ as well as periodic magnetization steps at ${\ensuremath{\mu}}_{0}H=16.5,\phantom{\rule{0.28em}{0ex}}24.8,\phantom{\rule{0.28em}{0ex}}33.8,\phantom{\rule{0.28em}{0ex}}42.3,\phantom{\rule{0.16em}{0ex}}\text{and}\phantom{\rule{0.16em}{0ex}}49.7$ T, which are independent of the crystallographic orientations. For $T>{T}_{N}$, the temperature dependence of ESR linewidth is described by a critical power law, $\ensuremath{\Delta}{B}_{pp}(T)\ensuremath{\propto}{(T\ensuremath{-}{T}_{N})}^{\ensuremath{-}0.56\ifmmode\pm\else\textpm\fi{}0.02}$. For $T<{T}_{N}$, an antiferromagnetic resonance mode is observed for $H\ensuremath{\parallel}c$, and its linewidth is given by $\ensuremath{\Delta}{B}_{pp}(T)\ensuremath{\propto}{T}^{3.13\ifmmode\pm\else\textpm\fi{}0.04}$, being close to ${T}^{4}$ expected for a classical magnet. Raman spectra show three one-magnon-like excitations superimposed on a broad two-magnon continuum. While the two higher frequency modes show an intensity variation in accordance to a three-dimensional Heisenberg antiferromagnet, the lower frequency mode clearly deviates. These results suggest that ${\mathrm{Cu}}_{4}{\mathrm{Te}}_{5}{\mathrm{O}}_{12}{\mathrm{Cl}}_{4}$ is a unique material which shows a dual character of zero-dimensional, localized and three-dimensional, collective magnetic behaviors.

High-resolution Monte Carlo study of the multicritical point in the three-dimensional XXZ Heisenberg antiferromagnet.

We use Monte Carlo simulations to study the XXZ Heisenberg antiferromagnet in a field in order to clearly determine the nature of the multicritical point. We use a hybrid sampling method with Metropolis and Wolff-cluster algorithms, along with histogram reweighting techniques. Staggered magnetization susceptibilities, Binder cumulants, and finite-size scaling are considered in an effort to detect a possible biconical phase. An analysis of the probability distribution of the magnetization allowed us to conclude that the multicritical point is bicritical and it is in the three-dimensional Heisenberg universality class.

Magnetic and dielectric properties of the PbFeBO4 single crystal

PbFeBO4 single crystals up to 4×2.5×0.5mm3 in size were grown by spontaneous crystallization from a solution in a melt and their magnetic and dielectric properties were investigated. The high Neel temperature TN=114K and the absence of the broad maximum of susceptibility above the Neel temperature suggest that the PbFeBO4 crystal is a three-dimensional antiferromagnet. The magnetic susceptibility anisotropy in the ordered state indicates that the antiferromagnetic vector is directed along the orthorhombic c axis. The anomalies of dielectric permittivities ε' and ε″ were observed in both polycrystalline and single-crystal samples within the temperature ranges where the short- and long-range magnetic orders are established. This confirms the correlation between the magnetic and electric subsystems in the crystal.

Universality of the Dispersive Spin-Resonance Mode in SuperconductingBaFe2As2

Spin fluctuations in superconducting BaFe2(As(1-x)P(x))2 (x=0.34, T(c)=29.5 K) are studied using inelastic neutron scattering. Well-defined commensurate magnetic signals are observed at (π, 0), which is consistent with the nesting vector of the Fermi surface. Antiferromagnetic (AFM) spin fluctuations in the normal state exhibit a three-dimensional character reminiscent of the AFM order in nondoped BaFe2As2. A clear spin gap is observed in the superconducting phase forming a peak whose energy is significantly dispersed along the c axis. The bandwidth of dispersion becomes larger with approaching the AFM ordered phase universally in all superconducting BaFe2As2, indicating that the dispersive feature is attributed to three-dimensional AFM correlations. The results suggest a strong relationship between the magnetism and superconductivity.

Investigation of a universal behavior between Néel temperature and staggered magnetization density for a three-dimensional quantum antiferromagnet

We simulate the three-dimensional quantum Heisenberg model with a spatially anisotropic ladder pattern using the first principles Monte Carlo method. Our motivation is to investigate quantitatively the newly established universal relation $T_N/\sqrt{c^3}$ $\propto$ ${\cal M}_s$ near the quantum critical point (QCP) associated with dimerization. Here $T_N$, $c$, and ${\cal M}_s$ are the N\'eel temperature, the spinwave velocity, and the staggered magnetization density, respectively. For all the physical quantities considered here, such as $T_N$ and ${\cal M}_s$, our Monte Carlo results agree nicely with the corresponding results determined by the series expansion method. In addition, we find it is likely that the effect of a logarithmic correction, which should be present in (3+1)-dimensions, to the relation $T_N/\sqrt{c^3}$ $\propto$ ${\cal M}_s$ near the investigated QCP only sets in significantly in the region with strong spatial anisotropy.

Lifetimes of Antiferromagnetic Magnons in Two and Three Dimensions: Experiment, Theory, and Numerics

A high-resolution neutron spectroscopic technique is used to measure momentum-resolved magnon lifetimes in the prototypical two- and three-dimensional antiferromagnets Rb(2)MnF(4) and MnF(2), over the full Brillouin zone and a wide range of temperatures. We rederived theories of the lifetime resulting from magnon-magnon scattering, thereby broadening their applicability beyond asymptotically small regions of wave vector and temperature. Corresponding computations, combined with a small contribution reflecting collisions with domain boundaries, yield excellent quantitative agreement with the data. Comprehensive understanding of magnon lifetimes in simple antiferromagnets provides a solid foundation for current research on more complex magnets.

Critical phenomena at the antiferromagnetic phase transition of azurite

We report on high-resolution acoustic, specific-heat and thermal expansion measurements in the vicinity of the antiferromagnetic phase transition at T_N = 1.88 K on a high-quality single crystal of the natural mineral azurite. A detailed investigation of the critical contribution to the various quantities at T_N is presented. The set of critical exponents and amplitude ratios of the singular contributions above and below the transition indicate that the system can be reasonably well described by a three-dimensional Heisenberg antiferromagnet.

Multicritical points in the three-dimensional XXZ antiferromagnet with single-ion anisotropy

The classical Heisenberg antiferromagnet with uniaxial exchange anisotropy, the XXZ model, and competing planar single-ion anisotropy in a magnetic field on a simple cubic lattice is studied with the help of extensive Monte Carlo simulations. The biconical (supersolid) phase, bordering the antiferromagnetic and spin-flop phases, is found to become thermally unstable well below the onset of the disordered, paramagnetic phase, leading to interesting multicritical points.

Magnetic Susceptibility of Quasi-two-dimensional Cuprate Antiferromagnets

Abstract Parallel magnetic susceptibility temperature dependence of the high-TC superconducting parent compound La2CuO4 is calculated in both antiferromagnetic (AFM) and paramagnetic phase. By making use of the quantum Heisenberg three-dimensional AFM model including the in-plane spin anisotropy, the calculation is performed within the framework of three different theories: Green’s function theory in random-phase approximation (RPA), linear spinwave (LSW) theory and mean-field (MF) theory. The results suggest that at low temperatures quantum spin fluctuations play an important role, while at the temperatures above the critical one short-range correlations have a great impact on the behavior of the system. This leads to the discrepancy between RPA and MF results, since the later neglects the above phenomena. Further, LSW theory expectedly agrees with RPA results only at low temperatures where the magnon interactions are negligible. Comparison to the theoretical and experimental results quoted in literature confirms that RPA method presents the most appropriate method among the applied ones, suggesting that this approach is satisfactory in the case of the parallel magnetic susceptibility, while in order to reproduce the transversal one, spin-orbit coupling must be included.

Asymmetric Thermal Line Shape Broadening in a Gapped 3D Antiferromagnet: Evidence for Strong Correlations at Finite Temperature

It is widely believed that magnetic excitations become increasingly incoherent as the temperature is raised due to random collisions which limit their lifetime. This picture is based on spin-wave calculations for gapless magnets in 2 and 3 dimensions and is observed experimentally as a symmetric Lorentzian broadening in energy. Here, we investigate a three-dimensional dimer antiferromagnet and find unexpectedly that the broadening is asymmetric-indicating that far from thermal decoherence, the excitations behave collectively like a strongly correlated gas. This result suggests that a temperature activated coherent state of quasiparticles is not confined to special cases like the highly dimerized spin-1/2 chain but is found generally in dimerized antiferromagnets of all dimensionalities and perhaps gapped magnets in general.

Excitations and spin correlations near the interface of two three-dimensional Heisenberg antiferromagnets

Magnetic excitations and spin correlations near the interface of two spin-12 Heisenberg antiferromagnets are considered using the spin-wave approximation. When the interaction between boundary spins differs essentially from exchange constants inside the antiferromagnets, quasi-two-dimensional spin waves appear in the near-boundary region. They eject bulk magnons from this region, thereby dividing the antiferromagnets into areas with different magnetic excitations. The decreased dimensionality of the near-boundary modes leads to amplified nearest-neighbor spin correlations in the interface area.

Universal Néel temperature in three-dimensional quantum antiferromagnets

We study three-dimensional dimerized $S=1/2$ Heisenberg antiferromagnets, using quantum Monte Carlo simulations of systems with three different dimerization patterns. We propose a way to relate the N\'eel temperature ${T}_{N}$ to the staggered moment ${m}_{s}$ of the ground state. Mean-field arguments suggest ${T}_{N}\ensuremath{\propto}{m}_{s}$ close to a quantum-critical point. We find an almost perfect universality (including the prefactor) if ${T}_{N}$ is normalized by a proper lattice-scale energy. We show that the temperature ${T}^{*}$ at which the magnetic susceptibility has a maximum is a good choice, i.e., ${T}_{N}/{T}^{*}$ versus ${m}_{s}$ is a universal function (also beyond the linear regime). These results are useful for analyzing experiments on systems where the spin couplings are not known precisely, e.g., ${\mathrm{TlCuCl}}_{3}$.

Width of the longitudinal magnon in the vicinity of the O(3) quantum critical point

We consider a three-dimensional quantum antiferromagnet in the vicinity of a quantum critical point separating the magnetically ordered and the magnetically disordered phases. A specific example is TlCuCl${}_{3}$ where the quantum phase transition can be driven by hydrostatic pressure and/or by external magnetic field. As expected, two transverse and one longitudinal magnetic excitations have been observed in the pressure-driven magnetically ordered phase. According to the experimental data, the longitudinal magnon has a substantial width, which has not been understood and has remained a puzzle. In the present work, we explain the mechanism for the width, calculate the width and relate the value of the width with the parameters of the Bose condensate of magnons observed in the same compound. The method of an effective quantum field theory is employed in the work.

A frustrated three-dimensional antiferromagnet: stacked J1–J2 layers

We study a frustrated 3D antiferromagnet of stacked J1–J2 layers. The intermediate ‘quantum spin liquid’ phase, present in the 2D case, narrows with increasing interlayer coupling and vanishes at a triple point. Beyond this, there is a direct first-order transition from Néel to columnar order. Possible applications to real materials are discussed.

Crystal structure and magnetic properties of 6H-SrMnO3

The 6$H$-SrMnO${}_{3}$ polymorph was prepared using a high-pressure and high-temperature technique at 6 GPa and 1773 K. Its crystal structure was refined using synchrotron x-ray powder diffraction data (space group $P$6${}_{3}$/mmc (no. 194), $Z$ $=$ 6, $a$ $=$ 5.42892(1) \AA{}, $c$ $=$ 13.40252(3) \AA{}). Magnetic and transport properties of 6$H$-SrMnO${}_{3}$ were investigated using direct current magnetization, specific heat, and resistivity measurements. The antiferromagnetic N\'eel temperature ${T}_{\mathrm{N}}$ of 6$H$-SrMnO${}_{3}$ is 235 K; this value is close to that of cubic-SrMnO${}_{3}$ (${T}_{\mathrm{N}}$ $=$ 240 K) and 4$H$-SrMnO${}_{3}$ (${T}_{\mathrm{N}}$ $=$ 280 K). 6$H$-SrMnO${}_{3}$ and 4$H$-SrMnO${}_{3}$ have the same building blocks of face-sharing MnO${}_{6}$ octahedra. Magnetic susceptibility of 6$H$-SrMnO${}_{3}$ exhibits behavior typical for classical three-dimensional antiferromagnets with a maximum near ${T}_{\mathrm{N}}$---in contrast to those of cubic-SrMnO${}_{3}$ and 4$H$-SrMnO${}_{3}$, where broad maxima are observed far above ${T}_{\mathrm{N}}$. Isothermal magnetization curves of 6$H$-SrMnO${}_{3}$ (at 2 K) show an upturn deviation from the linear behavior above 10 kOe, indicating a possible spin--flop magnetic phase transition.

Fermionic Spin Excitations in Two- and Three-Dimensional Antiferromagnets

Spin excitations in an ordered Heisenberg magnet are magnons-bosons with spin 1. That may change when frustration and quantum fluctuations suppress the order and restore the spin-rotation symmetry. We show that spin excitations in the S=1/2 Heisenberg antiferromagnet on a kagome lattice are spinons-fermions with spin 1/2. In the ground state the system can be described as a collection of small, heavy pairs of spinons with spin 0. A magnetic excitation of lowest energy amounts to breaking up a pair into two spinons at a cost of 0.06 J.