Two-dimensional Triangular Antiferromagnet Research Articles

Magnetic phase diagram and multiple field-induced states in the intermetallic triangular-lattice antiferromagnet NdAuAl4Ge2 with Ising-like spins

Geometrical frustration and the enhancement of strong quantum fluctuations in two-dimensional triangular antiferromagnets can lead to various intriguing phenomena. Here, we studied the spin-1/2 triangular lattice antiferromagnet NdAuAl$_4$Ge$_2$. Thermodynamic and transport properties, such as magnetization and specific heat together with the resistivity measurements were performed. In zero field, two successive phase transitions were observed at $T_{\rm N1}=1.75\pm 0.02$ K and $T_{\rm N2}=0.49\pm 0.02$ K, respectively. Under magnetic field, $\rm XXZ$-type anisotropy was revealed, with the moments pointing along the easy $c$ axis. For $B\parallel c$, multiple field-induced states were observed, and the magnetic phase diagram was established based on the specific heat and magnetization data. The temperature-dependent resistivity measurements indicate that NdAuAl$_4$Ge$_2$ is a good metal. It is very likely that both the long-range RKKY interactions and the geometrical frustration play an important roles in this case.

Magnetic phases of the S=52 triangular-lattice antiferromagnet RbFe( MoO4)2 as determined by ultrasound velocity measurements

$\mathrm{Rb}\mathrm{Fe}{(\mathrm{Mo}{\mathrm{O}}_{4})}_{2}$ is one of the best physical examples of a quasi-two-dimensional easy-plane triangular-lattice antiferromagnet. Although the magnetic phase diagram of this material has been well-studied, there are discrepancies in the literature concerning the number of magnetic phases, as well as the nature of the phase transitions. By performing ultrasound velocity measurements, we have obtained a detailed phase diagram with magnetic fields parallel and perpendicular to the triangular layers. The order of the phase transitions was found to be consistent with previous theoretical studies of generic two-dimensional triangular antiferromagnets. To support our experimental findings, we present a phenomenological model that includes biquadratic and next-nearest-neighbor interplane interactions. This model is sufficient to reproduce the phase diagram at low temperatures, and the next-nearest-neighbor interactions are shown to stabilize the incommensurate phases ($\mathbb{Y}$ and $\mathbb{V}$). Furthermore, the model provides insight regarding the magnetic ordering along the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{c}$-axis in the magnetic plateau phase, suggesting that period 3 is established whenever the next-nearest-neighbor superexchange interactions are unequal. In the systems in which these interactions are expected to be equal, the model predicts degeneracy between period 2 and period 3 structures, at the classical level.

Incommensurate magnetism in K2MnS2−xSex and prospects for tunable frustration in a triangular lattice of pseudo-1D spin chains

We report the first detailed investigation of K$_2$MnS$_2$ and K$_2$MnSe$_2$ from the K$_2$MnS$_2$ structure type and their magnetic solid solution K$_2$MnS$_{2-x}$Se$_x$ and find that compounds of this structure type consist of strongly-coupled pseudo-one-dimensional antiferromagnetic chains that collectively represent a frustrated two-dimensional triangular antiferromagnet. Bulk samples of K$_2$MnS$_{2-x}$Se$_x$ with $0 \leq x \leq 2$ are characterized using X-ray diffraction, neutron diffraction, magnetization and heat capacity measurements. An incommensurate cycloid magnetic structure with a magnetic propagation vector $k = [0.58~0~1]$ is observed for all samples in K$_2$MnS$_{2-x}$Se$_x$, and the ordering is robust despite a 12\% increase in cell volume. Geometric frustration of chains results in incommensurability along $a$ and a two-step magnetic transition. The varying geometries accessible in compounds of this structure type are presented as promising avenues to tune frustration.

Magnon-phonon coupling and two-magnon continuum in the two-dimensional triangular antiferromagnetCuCrO2

CuCrO2 is a manifestation of a two-dimensional triangular antiferromagnet which exhibits an incommensurate noncollinear magnetic structure similar to a classical 120 degree ordering. Using the inelastic neutron scattering technique, direct evidence of a magnon-phonon coupling in CuCrO2 is revealed via the mixed magnon-phonon character of the excitation mode at 12.5 meV as well as a minimum at the zone boundary. A simple model Hamiltonian that incorporates an exchange striction type magnon-phonon coupling reproduces the observed features accurately. Also, continuum excitations originating from the interaction between quasiparticles are observed with strong intensity at the zone boundary. These features of the magnetic excitations are key to an understanding of the emergent excitations in noncollinear antiferromagnetic compounds.

Frustration-induced magic number clusters of colloidal magnetic particles

We report the formation of stable two-dimensional clusters consisting of long-range-interacting colloidal particles with predefined magnetic moments. The symmetry and arrangement of the particles within the cluster are imposed by the magnetic frustration. By satisfying the criteria of stability, a series of magic number clusters is formed. The magic clusters are close packed and have compensating magnetic moments and chirality. Thus, the system can be regarded as a classical mesoscopic model for spin arrangements in two-dimensional triangular antiferromagnets, although the exact nature of the interactions between the macroscopic magnetic moments is different.

Field-Induced Phase Transitions Driven by Quantum Fluctuation inS=1/2 Anisotropic Triangular Antiferromagnet Cs2CuBr4

The field induced magnetic phase transitions of Cs 2 CuBr 4 were investigated by means of magnetization process and neutron scattering experiments. Cs 2 CuBr 4 should be characterized as S = 1/2 two-dimensional triangular antiferromagnet. Below the ordering temperature T N = 1.4 K, the Spin structure is the helical incommensurate structure almost within the triangular lattice plane. In the field direction within the triangular lattice plane, Cs 2 CuBr 4 exhibits the magnetization plateaux at one-third and two-thirds of the saturation magnetization. The spin structure in the one-third plateau phase is found to be almost collinear up-up-down structure which should be stabilized by quantum fluctuation as predicted by the theoretical studies.

Magnetic Frustration and Anisotropy Effects in Giant Magnetic Molecule Mo72Fe30

A large, almost spherical magnetic molecule of Mo 72 Fe 30 is studied theoretically. The Fe 3+ ions having S =5/2 are located at the vertices of icosidodecahedron, which consists of 32 faces (20 triangles and 12 pentagons). This system is frustrated so that it has some similarity with the two-dimensional triangular antiferromagnet, but because of the pentagons it is different from the latter. The presence of pentagons leads to more degeneracy in the classical ground state, which can be lifted by fluctuations and magnetic anisotropy. In this paper we study both effects on the magnetic susceptibility and the specific heat, using mainly the Monte Carlo method. As for the anisotropy energy, it is pointed out that because of the low symmetry of the nearest-neighbor Fe–Fe bond the Dzyaloshinsky–Moriya (DM) term must be present. It is concluded from the results that the anisotropy is playing an important role in many experimental results.

Magnetization and Spin Correlation of Two-Dimensional Triangular Antiferromagnet LuFe2O4

Magnetization, neutron diffraction and Mössbauer measurements have been performed on a two dimensional (2D) double-layered triangular antiferromagnet LuFe 3+ Fe 2+ O 4 single crystal. Two magnetic rods (0, 1, l ) and (1/3, 1/3, l ) with broad widths across them reveal that 2D magnetic ordering is not of long range and no 3D one occurs down to 4.2 K in contrast to an ordinary magnetic system. Thermoremanent magnetization measurements suggest that the system consists of various size ferrimagnetic clusters. The characteristic profile of l -scan in (0, 1, l ) can be realized by taking account of correlation among clusters for intra double-layers and inter double-layers. The temperature variations of intensity in magnetic scattering can be explained qualitatively by assuming unequal exchange interactions among three sublattice.