77Se and 87Rb nuclear magnetic resonance (NMR) experiments on Rb0.74Fe1.6Se2 reveal clearly distinct spectra originating from a majority antiferromagnetic (AF) and a minority metallic-superconducting (SC) phase. The very narrow NMR line of the SC pha
se evidences the absence of Fe vacancies and any trace of AF order. The Rb content of the SC phase is deduced from intensity measurements identifying Rb0.3(1)Fe2Se2 as the actual composition of the SC fraction. The resulting estimate of 0:15 electrons/Fe brings this class of superconductors 245 family closer to the other Fe-based superconductor families.
We study spin liquid in the frustrated diamond lattice antiferromagnet CoAl2O4 by means of single crystal neutron scattering in zero and applied magnetic field. The magnetically ordered phase appearing below TN=8 K remains nonconventional down to 1.5
K. The magnetic Bragg peaks at the q=0 positions remain broad and their profiles have strong Lorentzian contribution. Additionally, they are connected by weak diffuse streaks along the <111> directions. These observations are explained within the spiral spin liquid model as short-range magnetic correlations of spirals populated at these finite temperatures, as the energy minimum around q=0 is flat and the energy of excited states with q=(111) is low. The agreement is only qualitative, leading us to suspect that microstructure effects are also important. Magnetic field significantly perturbs spin correlations. The 1.5 K static magnetic moment increases from 1.58 mB/Co at zero field to 2.08 mB/Co at 10 T, while the magnetic peaks, being still broad, acquire almost Gaussian profile. Spin excitations are rather conventional spin waves at zero field, resulting in the exchange parameters J1=0.92(1) meV, J2=0.101(2) meV and the anisotropy term D=-0.0089(2) meV for CoAl2O4. The application of a magnetic field leads to a pronounced broadening of the excitations at the zone center, which at 10 T appear gapless and nearly featureless.
Using neutron powder diffraction and Monte-Carlo simulations we show that a spin-liquid regime emerges at $all compositions in the diamond-lattice antiferromagnets Co(Al1-xCox)2O4. This spin-liquid regime induced by frustration due to the second-neig
hbour exchange coupling J2, is gradually superseded by antiferromagnetic collinear long-range order (k=0) at low temperatures. Upon substitution of Al3+ by Co3+ in the octahedral B-site the temperature range occupied by the spin-liquid regime narrows and TN increases. To explain the experimental observations we considered magnetic anisotropy D or third-neighbour exchange coupling J3 as degeneracy-breaking perturbations. We conclude that Co(Al1-xCox)2O4 is below the theoretical critical point J2/J1=1/8, and that magnetic anisotropy assists in selecting a collinear long-range ordered ground state, which becomes more stable with increasing x due to a higher efficiency of O-Co3+-O as an interaction path compared to O-Al3+-O.
We analyzed the magnetic susceptibilities of several Cr spinels using two recent models for the geometrically frustrated pyrochlore lattice, the Quantum Tetrahedral Mean Field model and a Generalized Constant Coupling model. Both models can describe
the experimental data for ACr2 O4 (with A = Zn, Mg, and Cd) satisfactorily, with the former yielding a somewhat better agreement with experiment for A = Zn, Mg. The obtained exchange constants for nearest and next-nearest neighbors are discussed.
