No Arabic abstract
By means of neutron scattering measurements, we have investigated spin-wave excitation in a collinear four-sublattice (4SL) magnetic ground state of a triangular lattice antiferromagnet CuFeO2, which has been of recent interest as a strongly frustrated magnet, a spin-lattice coupled system and a multiferroic. To avoid mixing of spin-wave spectrum from magnetic domains having three different orientations reflecting trigonal symmetry of the crystal structure, we have applied uniaxial pressure on [1-10] direction of a single crystal CuFeO2. By elastic neutron scattering measurements, we have found that only 10 MPa of the uniaxial pressure results in almost single domain state in the 4SL phase. We have thus performed inelastic neutron scattering measurements using the single domain sample, and have identified two distinct spin- wave branches. The dispersion relation of the upper spin-wave branch cannot be explained by the previous theoretical model [R. S. Fishman: J. Appl. Phys. 103 (2008) 07B109]. This implies the importance of the lattice degree of freedom in the spin-wave excitation in this system, because the previous calculation neglected the effect of the spin-driven lattice distortion in the 4SL phase. We have also discussed relationship between the present results and the recently discovered electromagnon excitation.
We have investigated spin-wave excitations in a magnetic-field-induced 1/5-magnetization plateau phase in a triangular lattice antiferromagnet CuFeO2 (CFO), by means of inelastic neutron scattering measurements under applied magnetic fields of up to 13.4 T. Comparing the observed spectra with the calculations in which spin-lattice coupling effects for the nearest neighbor exchange interactions are taken into account, we have determined the Hamiltonian parameters in the field-induced 1/5- plateau phase, which directly show that CFO exhibits a bond order associated with the magnetic structure in this phase.
We report a comprehensive investigation of the magnetism of the $S$ = 3/2 triangular-lattice antiferromagnet, $alpha$-CrOOH(D) (delafossites green-grey powder). The nearly Heisenberg antiferromagnetic Hamiltonian ($J_1$ $sim$ 23.5 K) with a weak single-ion anisotropy of $|D|$/$J_1$ $sim$ 4.6% is quantitatively determined by fitting to the electron spin resonance (ESR) linewidth and susceptibility measured at high temperatures. The weak single-ion anisotropy interactions, possibly along with other perturbations, e.g. next-nearest-neighbor interactions, suppress the long-range magnetic order and render the system disordered, as evidenced by both the absence of any clear magnetic reflections in neutron diffraction and the presence of the dominant paramagnetic ESR signal down to 2 K ($sim$ 0.04$J_1$$S^2$), where the magnetic entropy is almost zero. The power-law behavior of specific heat ($C_m$ $sim$ $T^{2.2}$) observed below the freezing temperature of $T_f$ = 25 K in $alpha$-CrOOH or below $T_f$ = 22 K in $alpha$-CrOOD is insensitive to the external magnetic field, and thus is consistent with the theoretical prediction of a gapless U(1) Dirac quantum spin liquid (QSL) ground state. At low temperatures, the spectral weight of the low-energy continuous spin excitations accumulates at the K points of the Brillouin zone, e.g. $|mathbf{Q}|$ = 4$pi$/(3$a$), and the putative Dirac cones are clearly visible. Our work is a first step towards the understanding of the possible Dirac QSL ground state in this triangular-lattice magnet with $S$ = 3/2.
Terahertz time-domain spectroscopy was performed to directly probe the low-energy (1-5 meV) electrodynamics of triangular lattice antiferromagnets CuFe1-xGaxO2 (x = 0.00, 0.01, and 0.035). We discovered an electromagnon (electric-field-active magnon) excitation at 2.3 meV in the paraelectric up-up-down-down collinear magnetic phase, while this electromagnon vanishes in the ferroelectric helimagnetic phase. Anti-correlation with noncollinear magnetism excludes the exchange-striction mechanism as the origin of dynamical magnetoelectric coupling, and hence evidences the observation of spin-orbit coupling mediated electromagnon in the present compound.
The anisotropic triangular lattice of the crednerite system Cu(Mn1-xCux)O2 is used as a basic model for studying the influence of spin disorder on the ground state properties of a two-dimensional frustrated antiferromagnet. Neutron diffraction measurements show that the undoped phase (x=0) undergoes a transition to antiferromagnetic long-range order that is stabilized by a frustration-relieving structural distortion. Small deviation from the stoichiometric composition alters the magnetoelastic characteristics and reduces the effective dimensionality of the magnetic lattice. Upon increasing the doping level, the interlayer coupling changes from antiferromagnetic to ferromagnetic. As the structural distortion is suppressed, the long-range magnetic order is gradually transformed into a two-dimensional order.
We report on comprehensive results identifying the ground state of a triangular-lattice structured YbZnGaO$_4$ to be spin glass, including no long-range magnetic order, prominent broad excitation continua, and absence of magnetic thermal conductivity. More crucially, from the ultralow-temperature a.c. susceptibility measurements, we unambiguously observe frequency-dependent peaks around 0.1 K, indicating the spin-glass ground state. We suggest this conclusion to hold also for its sister compound YbMgGaO$_4$, which is confirmed by the observation of spin freezing at low temperatures. We consider disorder and frustration to be the main driving force for the spin-glass phase.