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Register a new userStructure and magnetism in $rm LaCoO_3$
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The temperature dependence of the hexagonal lattice parameter $c$ of single crystal $rm LaCoO_3$ (LCO) with $H=0$ and $800$Oe, as well as LCO bulk powders with $H=0$, was measured using high-resolution x-ray scattering near the transition temperature $T_oapprox 35$K. The change of $c(T)$ is well characterized by a power law in $T-T_o$ for $T>T_o$ and by a temperature independent constant for $T<T_o$ when convoluted with a Gaussian function of width $8.5$K. This behavior is discussed in the context of the unusual magnetic behavior observed in LCO as well as recent generalized gradient approximation calculations.
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With octahedrally coordinated $t_{rm 2g}$ orbitals which are active at filling $n=2$, the $rm Sr_2CrO_4$ compound exhibits rich interplay of spin-orbital physics with tetragonal distortion induced crystal field tuning by external agent such as pressure. Considering both reversed and restored crystal field regimes, collective spin-orbital excitations are investigated in the antiferromagnetic (AFM) state of a realistic three-orbital model using the generalized self consistent and fluctuation approach including spin-orbit coupling (SOC). Important effects of SOC and Coulomb interaction induced orbital mixing terms are highlighted. The behavior of the calculated energy scales of collective excitations with crystal field is in striking similarity to that of the transition temperatures with pressure as obtained from susceptibility and resistivity anomalies in high-pressure studies.
The complex magnetism and transport properties of tetragonal Ce$_3$Cu$_4$As$_4$O$_2$ were examined through neutron scattering and physical properties measurements on polycrystalline samples. The lamellar structure consists of alternating layers of $rm CeCu_4As_4$ with a single square Ce lattice and oxygen-linked Ce bi-layer $rm Ce_2O_2$. Extending along $bf c$, a tube-like Fermi surface from DFT calculations points to a quasi-two-dimensional electronic system. Peaks in the specific heat at the Ne{e}l temperature $T_{N}=24$ $rm K$, $T_{2}~=~16 $ $rm K$ and $T_{3}~=~1.9$ $ rm K$ indicate three magnetic phase transitions or distinct cross-over phenomena. For $T<T_{N}$ neutron diffraction indicates the development of ferromagnetic ab sheets for both Ce sites, with alternating polarization along $bf{c}$, a wave vector ${bf k}_{1}={bf c}^*$. For $T<T_{2}$, quasi-two-dimensional low-energy spin fluctuations with ${bf k}_{2}=frac{1}{2}{bf a}^*$ and polarized perpendicular to ${bf k}_{2}$ are suppressed. The data are consistent with quasi-two-dimensional antiferromagnetic order in the $rm CeCu_4As_4$ planes polarized along the ${bf k}_{2}$ wave vector. $T_{3}$ marks a spin-flop transition where the ${bf k}_{1}$ staggered magnetization switches to in-plane polarization. While the narrow 4f bands lie deep below the Fermi surface, there are significant transport anomalies associated with the transitions; in particular a substantial reduction in resistivity for $T<T_{N}$. At $T=100$ $ rm mK$ the ${bf k}_1$ modulated staggered moment is $0.85~mu_B$, which matches the $0.8~mu_B$ saturation magnetization achieved for H $~=~7$ $ rm T$ at $T~=~2$ $ rm K$. From low T Lorentzian fits the correlation length is in excess of 75 AA. We argue the unusual sequence of magnetic transitions results from competing interactions and anisotropies for the two Ce sites.
Inelastic neutron scattering (INS), electron spin (ESR) and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly hole-doped La_{1-x}Sr_xCoO_3, x=0.002. Both, INS and ESR low temperature spectra show intense excitations with large effective g-factors ~10-18. NMR data indicate the creation of extended magnetic clusters. From the Q-dependence of the INS magnetic intensity we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions.
The crystal and magnetic structures of stoichiometric ZnCr2Se4 have been investigated using synchrotron X-ray and neutron powder diffraction, muon spin relaxation (muSR) and inelastic neutron scattering. Synchrotron X-ray diffraction shows a spin-lattice distortion from the cubic spinel to a tetragonal I41/amd lattice below TN = 21 K, where powder neutron diffraction confirms the formation of a helical magnetic structure with magnetic moment of 3.04(3) {mu}B at 1.5 K; close to that expected for high-spin Cr3+. MuSR measurements show prominent local spin correlations that are established at temperatures considerably higher (< 100 K) than the onset of long range magnetic order. The stretched exponential nature of the relaxation in the local spin correlation regime suggests a wide distribution of depolarizing fields. Below TN, unusually fast (> 100 {mu}s-1) muon relaxation rates are suggestive of rapid site hopping of the muons in static field. Inelastic neutron scattering measurements show a gapless mode at an incommensurate propagation vector of k = (0 0 0.4648(2)) in the low temperature magnetic ordered phase that extends to 0.8 meV. The dispersion is modelled by a two parameter Hamiltonian, containing ferromagnetic nearest neighbor and antiferromagnetic next nearest neighbor interactions with a Jnnn/Jnn = -0.337.
The iron arsenide Eu3Fe2O5Fe2As2 was synthesized at 1173-1373 K in a resistance furnace and characterized by X-ray powder diffraction with Rietveld analysis: Sr3Fe2O5Cu2S2-type, I4/mmm, a = 406.40(1) pm, c = 2646.9(1) pm. Layers of edge-sharing FeAs4/4 tetrahedra are separated by perovskite-like oxide blocks. No structural transition occurs in the temperature range from 10 to 300 K. Magnetic measurements have revealed Curie-Weiss behavior with an effective magnetic moment of 7.79 muB per europium atom in agreement with the theoretical value of 7.94 muB for Eu2+. A drop in the magnetic susceptibility at 5 K indicates possible antiferromagnetic ordering. 151Eu and 57Fe Mossbauer spectroscopic measurements have confirmed a beginning cooperative magnetic phenomenon by showing significantly broadened spectra at 4.8 K compared to those at 78 K.
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