Structural and Magnetic Properties of Manganites Pr\u3csub\u3e1-\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3eCa\u3csub\u3e1+\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3eMnO\u3csub\u3e4\u3c/sub\u3e, Oxypnictides CeFeAsO\u3csub\u3e1-\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3eF\u3csub\u3e\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3e, and Filled Skutterudite PrOs\u3csub\u3e4\u3c/sub\u3eAs\u3csub\u3e12\u3c/sub\u3e

We present neutron scattering, with triple-axis and time-of-flight spectrometers, to study the structural and magnetic properties of the layered manganites Pr1-xCa1+xMnO4 (PCMO), and to determine the crystalline electric field (CEF) levels in iron pnictides CeFeAsO1-xFx and filled skutterudite compound PrOs4As12. For the single-layered manganites PCMO, four dopings (x=0.5, 0.45, 0.40 and 0.35) have been studied. At half-doping, the system first becomes charge- and orbital- ordered (CO/OO) near TCO = 300 K and then develops CE-type antiferromagnetic (AF) order below TN = 130 K. At temperatures TN \u3c T \u3c TCO, the appearance of short-range AF spin correlations suppresses the CO/OO induced orthorhombic strain. These results suggest that a strong spin-lattice coupling. In less hole-doped PCMO (x \u3c 0:5) compounds, the long-range, three-dimensional anti- ferromagnetic (AF) correlations at x = 0:5 are suppressed. Most remarkably, the magnetic correlations appear in the form of coexisting commensurate (CM) and incommensurate (ICM) fluctuations with strong temperature and doping dependence. The spin-wave measurement on PCMO (x = 0:5) shows good agreement with a mag- netic interaction model based on the CE-type arrangement. The magnetic excitations are dominated by ferromagnetic exchange along the zig-zag chain, while the antiferromagnetic coupling between the chains is relatively weak. The INS measurement on the x = 0:45 system reveals both symmetric and asymmetric magnetic excitations about the CM peak positions: one dispersive spin wave indicative of the x = 0:5 system; and another localized mode at about 1 meV. This strongly suggests two types of spin dynamics originated from separated phases: the CE-type magnetic phase and an additional electronic phase caused by extra electrons introduced into the CE template

Magnetic excitations and anomalous spin wave broadening in multiferroic FeV2O4

We report on the different roles of two orbital-active Fe$^{2+}$ at the A site and V$^{3+}$ at the B site in the magnetic excitations and on the anomalous spin wave broadening in FeV$_{2}$O$_{4}$. FeV$_{2}$O$_{4}$ exhibits three structural transitions and successive paramagnetic (PM)-collinear ferrimagnetic (CFI)-noncollinear ferrimagnetic (NCFI) transitions. The high-temperature tetragonal/PM -orthorhombic/CFI transition is accompanied by the appearance of an energy gap with a high magnitude in the magnetic excitations due to strong spin-orbit coupling induced anisotropy at the Fe$^{2+}$ site. While there is no measurable increase in the energy gap from the orbital ordering of V$^{3+}$ at the orthorhombic/CFI-tetragonal/NCFI transition, anomalous spin wave broadening is observed in the orthorhombic/CFI state due to V$^{3+}$ spin fluctuations at the B site. The spin wave broadening is also observed at the zone boundary without softening, which is discussed in terms of magnon-phonon coupling.Comment: 4 pages, 3 figures, including one supplemental materia

The magnetic and crystal structures of Sr2IrO4: A neutron diffraction study

We report a single-crystal neutron diffraction study of the layered $\rm Sr_2IrO_4$. This work unambiguously determines the magnetic structure of the system and reveals that the spin orientation rigidly tracks the staggered rotation of the $\rm IrO_6$ octahedra in $\rm Sr_2IrO_4$. The long-range antiferromagnetic order has a canted spin configuration with an ordered moment of 0.208(3) $\mu_B$/Ir site within the basal plane; a detailed examination of the spin canting yields 0.202(3) and 0.049(2) $\mu_B$/site for the a axis and the b axis, respectively. It is intriguing that forbidden nuclear reflections of space group $I4_1/acd$ are also observed in a wide temperature range from 4 K to 600 K, which suggests a reduced crystal structure symmetry. This neutron-scattering work provides a direct, well-refined experimental characterization of the magnetic and crystal structures that are crucial to the understanding of the unconventional magnetism exhibited in this unusual magnetic insulator.Comment: the version appeared in PR

Two spatially separated phases in semiconducting Rb0.8_{0.8}Fe1.5_{1.5}S2_2

We report neutron scattering and transport measurements on semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$, a compound isostructural and isoelectronic to the well-studied $A_{0.8}$Fe$_{y}$Se$_2 (A=$ K, Rb, Cs, Tl/K) superconducting systems. Both resistivity and DC susceptibility measurements reveal a magnetic phase transition at $T=275$ K. Neutron diffraction studies show that the 275 K transition originates from a phase with rhombic iron vacancy order which exhibits an in-plane stripe antiferromagnetic ordering below 275 K. In addition, interdigitated mesoscopically with the rhombic phase is an ubiquitous phase with $\sqrt{5}\times\sqrt{5}$ iron vacancy order. This phase has a magnetic transition at $T_N=425$ K and an iron vacancy order-disorder transition at $T_{S}=600$ K. These two different structural phases are closely similar to those observed in the isomorphous Se materials. Based on the close similarities of the in-plane antiferromagnetic structures, moments sizes, and ordering temperatures in semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$ and K$_{0.81}$Fe$_{1.58}$Se$_2$, we argue that the in-plane antiferromagnetic order arises from strong coupling between local moments. Superconductivity, previously observed in the $A_{0.8}$Fe$_{y}$Se$_{2-z}$S$_z$ system, is absent in Rb$_{0.8}$Fe$_{1.5}$S$_2$, which has a semiconducting ground state. The implied relationship between stripe/block antiferromagnetism and superconductivity in these materials as well as a strategy for further investigation is discussed in this paper.Comment: 7 pages, 5 figure

Antiferromagnetic critical pressure in URu2Si2 under hydrostatic conditions

The onset of antiferromagnetic order in URu2Si2 has been studied via neutron diffraction in a helium pressure medium, which most closely approximates hydrostatic conditions. The antiferromagnetic critical pressure is 0.80 GPa, considerably higher than values previously reported. Complementary electrical resistivity measurements imply that the hidden order-antiferromagnetic bicritical point far exceeds 1.02 GPa. Moreover, the redefined pressure-temperature phase diagram suggests that the superconducting and antiferromagnetic phase boundaries actually meet at a common critical pressure at zero temperature.Comment: 5 pgs, 4 figs; AFM ordered moment revised to 0.5 muB, added and corrected citations and reference

Effect of magnetic field on the spin resonance in FeTe(0.5)Se(0.5) as seen via inelastic neutron scattering

Inelastic neutron scattering and susceptibility measurements have been performed on the optimally-doped Fe-based superconductor FeTe(0.5)Se(0.5), which has a critical temperature, Tc of 14 K. The magnetic scattering at the stripe antiferromagnetic wave-vector Q = (0.5,0.5) exhibits a "resonance" at ~ 6 meV, where the scattering intensity increases abruptly when cooled below Tc. In a 7-T magnetic field parallel to the a-b plane, Tc is slightly reduced to ~ 12 K, based on susceptibility measurements. The resonance in the neutron scattering measurements is also affected by the field. The resonance intensity under field cooling starts to rise at a lower temperature ~ 12 K, and the low temperature intensity is also reduced from the zero-field value. Our results provide clear evidence for the intimate relationship between superconductivity and the resonance measured in magnetic excitations of Fe-based superconductors.Comment: 4 pages, 3 figure

Destruction of Neel order and appearance of superconductivity in electron-doped cuprates by oxygen annealing process

We use thermodynamic and neutron scattering measurements to study the effect of oxygen annealing on the superconductivity and magnetism in Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$. Although the transition temperature $T_c$ measured by susceptibility and superconducting coherence length increase smoothly with gradual oxygen removal from the annealing process, bulk superconductivity, marked by a specific heat anomaly at $T_c$ and the presence of a neutron magnetic resonance, only appears abruptly when $T_c$ is close to the largest value. These results suggest that the effect of oxygen annealing must be first determined in order to establish a Ce-doping dependence of antiferromagnetism and superconductivity phase diagram for electron-doped copper oxides.Comment: 5 pages, 4 figures, accepted by Phys. Rev.