Doping Dependence of Spin Dynamics in Electron-Doped Ba(Fe1-xCox)2As2

The spin dynamics in single crystal, electron-doped Ba(Fe1-xCox)2As2 has been investigated by inelastic neutron scattering over the full range from undoped to the overdoped regime. We observe damped magnetic fluctuations in the normal state of the optimally doped compound (x=0.06) that share a remarkable similarity with those in the paramagnetic state of the parent compound (x=0). In the overdoped superconducting compound (x=0.14), magnetic excitations show a gap-like behavior, possibly related to a topological change in the hole Fermi surface (Lifshitz transition), while the imaginary part of the spin susceptibility prominently resembles that of the overdoped cuprates. For the heavily overdoped, non-superconducting compound (x=0.24) the magnetic scattering disappears, which could be attributed to the absence of a hole Fermi-surface pocket observed by photoemission.Comment: 6 pages, 5 figures, published versio

Charge order, dynamics, and magneto-structural transition in multiferroic LuFe2_2O4_4

We investigated the series of temperature and field-driven transitions in LuFe$_2$O$_4$ by optical and M\"{o}ssbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the "order by fluctuation" mechanism for the development of charge order superstructure. Bragg splitting and large magneto optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field.Comment: 4 pages, 3 figures, PRL in prin

Competition and coexistence of antiferromagnetism and superconductivity in underdoped Ba(Fe0.953Co0.047)2As2

Neutron and x-ray diffraction studies show that the simultaneous first-order transition to an orthorhombic and antiferromagnetic (AFM) ordered state in BaFe2As2 splits into two transitions with Co doping. For Ba(Fe0.953Co0.047)2As2, a tetragonal-orthorhombic transition occurs at TS = 60 K, followed by a second-order transition to AFM order at TN = 47 K. Superconductivity (SC) occurs in the orthorhombic state below TC = 15 K and coexists with AFM. Below TC, the static Fe moment is reduced and a 4 meV spin gap develops indicating competition between coexisting SC and AFM order.Comment: 15 pages, 4 figure

Revisiting the ground state of CoAl2_2O4_4: comparison to the conventional antiferromagnet MnAl2_2O4_4

The A-site spinel material, CoAl2O4, is a physical realization of the frustrated diamond-lattice antiferromagnet, a model in which is predicted to contain unique incommensurate or `spin-spiral liquid' ground states. Our previous single-crystal neutron scattering study instead classified it as a `kinetically-inhibited' antiferromagnet, where the long ranged correlations of a collinear Neel ground state are blocked by the freezing of domain wall motion below a first-order phase transition at T* = 6.5 K. The current paper expands on our original results in several important ways. New elastic and inelastic neutron measurements are presented that show our initial conclusions are affected by neither the sample measured nor the instrument resolution, while measurements to temperatures as low as T = 250 mK limit the possible role being played by low-lying thermal excitations. Polarized diffuse neutron measurements confirm reports of short-range antiferromagnetic correlations and diffuse streaks of scattering, but major diffuse features are explained as signatures of overlapping critical correlations between neighboring Brillouin zones. Finally, and critically, this paper presents detailed elastic and inelastic measurements of magnetic correlations in a single-crystal of MnAl2O4, which acts as an unfrustrated analogue to CoAl2O4. The unfrustrated material is shown to have a classical continuous phase transition to Neel order at T_N = 39 K, with collective spinwave excitations and Lorentzian-like critical correlations which diverge at the transition. Direct comparison between the two compounds indicates that CoAl2O4 is unique, not in the nature of high-temperature diffuse correlations, but rather in the nature of the frozen state below T*. The higher level of cation inversion in the MnAl2O4 sample indicates that this novel behavior is primarily an effect of greater next-nearest-neighbor exchange.Comment: 13 pages, 8 figures, acccepted for publication in Physical Review

Unusual giant magnetostriction in the ferrimagnet Gd2/3_{2/3}Ca1/3_{1/3}MnO3_3

We report an unusual giant linear magnetostrictive effect in the ferrimagnet Gd$_{2/3}$Ca$_{1/3}$MnO$_3$ ($T_{c} \approx$80 K). Remarkably, the magnetostriction, negative at high temperature ($T \approx T_{c}$), becomes positive below 15 K when the magnetization of the Gd sublattice overcomes the magnetization of the Mn sublattice. A rather simple model where the magnetic energy competes against the elastic energy gives a good account of the observed results and confirms that Gd plays a crucial role in this unusual observation. Unlike previous works in manganites where only striction associated with 3$d$ Mn orbitals is considered, our results show that the lanthanide 4$f$ orbitals related striction can be very important too and it cannot be disregarded.Comment: 6 pages, 3 figure

Antiferromagnetic Order in MnO Spherical Nanoparticles

We have performed unpolarized and polarized neutron diffraction experiments on monodisperse 8 nm and 13 nm antiferromagnetic MnO nanoparticles. For the 8 nm sample, the antiferromagnetic transition temperature $T_N$ (114 K) is suppressed compared to the bulk material (119 K) while for the 13 nm sample $T_N$ (120 K) is comparable to the bulk. The neutron diffraction data of the nanoparticles is well described using the bulk MnO magnetic structure but with a substantially reduced average magnetic moment of 4.2$\pm$0.3 $\mu_B$/Mn for the 8 nm sample and 3.9$\pm$0.2 $\mu_B$/Mn for the 13 nm sample. An analysis of the polarized neutron data on both samples shows that in an individual MnO nanoparticle about 80$$ of Mn ions order. These results can be explained by a structure in which the monodisperse nanoparticles studied here have a core that behaves similar to the bulk with a surface layer which does not contribute significantly to the magnetic order.Comment: 7 pages, 5 figure

Quantum critical behavior in the heavy Fermion single crystal Ce(Ni0.935_{0.935}Pd0.065_{0.065})2_2Ge2_2

We have performed magnetic susceptibility, specific heat, resistivity, and inelastic neutron scattering measurements on a single crystal of the heavy Fermion compound Ce(Ni$_{0.935}$Pd$_{0.065}$)$_2$Ge$_2$, which is believed to be close to a quantum critical point (QCP) at T = 0. At lowest temperature(1.8-3.5 K), the magnetic susceptibility behaves as $\chi(T)-\chi (0)$ $\propto$ $T^{-1/6}$ with $\chi (0) = 0.032 \times 10^{-6}$ m$^3$/mole (0.0025 emu/mole). For $T<$ 1 K, the specific heat can be fit to the formula $\Delta C/T = \gamma_0 - T^{1/2}$ with $\gamma_0$ of order 700 mJ/mole-K$^2$. The resistivity behaves as $\rho = \rho_0 + AT^{3/2}$ for temperatures below 2 K. This low temperature behavior for $\gamma (T)$ and $\rho (T)$ is in accord with the SCR theory of Moriya and Takimoto\cite{Moriya}. The inelastic neutron scattering spectra show a broad peak near 1.5 meV that appears to be independent of $Q$; we interpret this as Kondo scattering with $T_K =$ 17 K. In addition, the scattering is enhanced near $Q$=(1/2, 1/2, 0) with maximum scattering at $\Delta E$ = 0.45 meV; we interpret this as scattering from antiferromagnetic fluctuations near the antiferromagnetic QCP.Comment: to be published in J. Phys: Conference Serie