Single-crystal and powder neutron diffraction experiments on FePS3: search for the magnetic structure.

The long-accepted magnetic structure of FePS3 has recently been refuted through extensive powder neutron diffraction studies. Single-crystal neutron diffraction, using both quasi-Laue and monochromatic techniques, has now been conducted to reveal more information about the nature of the magnetic ordering in this compound. The magnetic unit cell was found to be twice as large as the crystallographic cell in both the α and ь directions and around three times as large along the с direction, giving a propagation vector of [1/2 1/2 0.34]. There is incomplete long-range magnetic order along the c* direction due to weak Fe-Fe interactions and large anisotropy in that direction. The variation of the spontaneous moment with temperature closely resembles that for an antiferromagnetic two-dimensional Ising model on a honeycomb lattice. © 2007, American Physical Societ

The magnon dynamics and spin exchange parameters of FePS3

The spin waves in a powdered sample of a quasi-two-dimensional antiferromagnet, FePS(3), have been measured using neutron inelastic scattering. The data could be modelled and the exchange interactions determined using a two-dimensional Heisenberg Hamiltonian with single ion anisotropy. A suitable fit to the data could only be achieved by including magnetic interactions up to the third nearest neighbour, which is consistent with the findings for other members of the MPS(3) family (M = transition metal). The best fit parameters at 6 K were J(1) = 1.49 meV, J(2) = 0.04 meV, J(3) = -0.6 meV, with an anisotropy of Delta = 3.7 meV. Measurements as a function of temperature give a coarse measure of the behaviour of the anisotropy and the nature of the phase transition. © 2012, IOP Publishing LTD

Lineshape of the singlet-triplet excitations in the dimer system Sr 3 Cr 2 O 8 to first order in the high-density 1/z expansion

The Cr 5+ ions in Sr 3 Cr 2 O 8 constitute a strongly correlated spin-1/2 dimer system. Experiments show that the collective singlet-triplet excitations in this system are well defined in the zero-temperature limit, but, when heated, the inelastic neutron scattering peaks decrease rapidly in intensity and acquire a nonzero line width. When including the fluctuations to leading order in 1/z , where z is the coordination number, the diagrammatic high-density expansion is found to offer an accurate description of the singlet-triplet excitations. The theory explains not only the temperature dependencies of the intensities and line widths, but also that strong correlation causes the lineshapes to become asymmetric at temperatures comparable to the excitation energies.© 2014, American Physical Society

Magnetic field resonantly enhanced free spins in heavily underdoped YBa2Cu3O6+x

Using neutron scattering, we investigate the effect of a magnetic field on the static and dynamic spin response in heavily underdoped superconducting YBa$_{2}$Cu$_{3}$O$_{6+x}$ (YBCO$_{6+x}$) with x=0.33 (T$_{c}$=8 K) and 0.35 (T$_{c}$=18 K). In contrast to the heavily doped and superconducting monolayer cuprates, the elastic central peak characterizing static spin correlations does not respond observably to a magnetic field which suppresses superconductivity. Instead, we find a magnetic field induced resonant enhancement of the spin fluctuations. The energy scale of the enhanced fluctuations matches the Zeeman energy within both the normal and vortex phases while the momentum dependence is the same as the zero field bilayer response. The magnitude of the enhancement is very similar in both phases with a fractional intensity change of $(I/I_{0}-1) \sim 0.1$. We suggest that the enhancement is not directly correlated with superconductivity but is the result of almost free spins located near hole rich regions.Comment: (8 pages, 5 figures, submitted to Physical Review B

Neutron scattering study of underdoped Ba(1-x)KxFe2As2 (x=0.09 and 0.17) self-flux-grown single crystals and the universality of the tricritical point

We present a combination of elastic neutron scattering measurements in zero and 14.5 T and magnetization measurements in zero and 14 T on underdoped superconducting Ba(1-x)KxFe2As2 (x=0.09 and 0.17), and the same measurements in zero field on a nonsuperconducting crystal with x = 0.09. The data suggest that the underdoped materials may not be electronic phase separated but rather have slightly inhomogeneous potassium doping. The temperature dependence of the magnetic order parameter below the transition of the sample with x = 0.09 is more gradual than that for the case of the undoped BaFe2As2, suggesting that this doping may be in the vicinity of a tricritical point. We advance therefore the hypothesis that the tricritical point is a common feature of all superconducting 122s. For the x = 0.17 sample, while T-c is suppressed from approximate to 17 to approximate to 8 K by a magnetic field of 14 T, the intensity of the magnetic Bragg peaks (1 0 3) at 1.2 K is enhanced by 10%, showing competition of superconductivity and antiferromagnetism. The intensity of the magnetic Bragg peaks (1 0 3) in the (T-c, T-N) temperature interval remain practically unchanged in 14.5 T within a 10% statistical error. The present results are discussed in the context of the existing literature. © 2012, American Physical Society