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Register a new userSpiral magnetic ordering of the Eu moments in EuNi$_{2}$As$_{2}$
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The ground-state magnetic structure of EuNi$_{2}$As$_{2}$ was investigated by single-crystal neutron diffraction. At base temperature, the Eu$^{2+}$ moments are found to form an incommensurate antiferromagnetic spiral-like structure with a magnetic propagation vector of $mathit{k}$ = (0, 0, 0.92). They align ferromagnetically in the $mathit{ab}$ plane with the moment size of 6.75(6) $mu_{B}$, but rotate spirally by 165.6(1){deg} around the $mathit{c}$ axis from layer to layer. The magnetic order parameter in the critical region close to the ordering temperature, $mathit{T_{N}}$ = 15 K, shows critical behavior with a critical exponent of $beta_{Eu}$ = 0.34(1), consistent with the three-dimensional Heisenberg model. Moreover, within the experimental uncertainty, our neutron data is consistent with a model in which the Ni sublattice is not magnetically ordered.
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The magnetic ground state of the Eu$^{2+}$ moments in a series of Eu(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals grown from the Sn flux has been investigated in detail by neutron diffraction measurements. Combined with the results from the macroscopic properties (resistivity, magnetic susceptibility and specific heat) measurements, a phase diagram describing how the Eu magnetic order evolves with Co doping in Eu(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ is established. The ground-state magnetic structure of the Eu$^{2+}$ spins is found to develop from the A-type antiferromagnetic (AFM) order in the parent compound, via the A-type canted AFM structure with some net ferromagnetic (FM) moment component along the crystallographic $mathit{c}$ direction at intermediate Co doping levels, finally to the pure FM order at relatively high Co doping levels. The ordering temperature of Eu declines linearly at first, reaches the minimum value of 16.5(2) K around $mathit{x}$ = 0.100(4), and then reverses upwards with further Co doping. The doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu$^{2+}$ moments, which is mediated by the conduction $mathit{d}$ electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the Eu$^{2+}$ and Fe$^{2+}$ moments, might be responsible for the change of the magnetic ground state and the ordering temperature of the Eu sublattice. In addition, for Eu(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals with 0.10 $leqslant$ $mathit{x}$ $leqslant$ 0.18, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is expected to account for the compromise between the two competing phenomena.
We have successfully synthesized single crystals of EuNi$_5$As$_3$ using a flux method and we present a comprehensive study of the physical properties using magnetic susceptibility, specific heat, electrical resistivity, thermoelectric power and x-ray absorption spectroscopy (XAS) measurements. EuNi$_5$As$_3$ undergoes two close antiferromagnetic transitions at respective temperatures of $T_{N1}$ = 7.2 K and $T_{N2}$ = 6.4 K, which are associated with the Eu$^{2+}$ moments. Both transitions are suppressed upon applying a field and we map the temperature-field phase diagrams for fields applied parallel and perpendicular to the easy $a$ axis. XAS measurements reveal that the Eu is strongly divalent, with very little temperature dependence, indicating the localized Eu$^{2+}$ nature of EuNi$_5$As$_3$, with a lack of evidence for heavy fermion behavior.
Using complementary polarized and unpolarized single-crystal neutron diffraction, we have investigated the temperature-dependent magnetic structures of Eu$_{0.5}$Ca$_{0.5}$Fe$_{2}$As$_{2}$. Upon 50 % dilution of the Eu sites with isovalent Ca$^{2+}$, the Eu sublattice is found to be still long-range ordered below $mathit{T_{Eu}}$ = 10 K, in the A-typed antiferromagnetic (AFM) structure. The moment size of Eu$^{2+}$ spins is estimated to be as large as 6.74(4) $mu_{B}$ at 2.5 K. The Fe sublattice undergoes a spin-density-wave transition at $mathit{T_{SDW}}$ = 192(2) K and displays an in-plane AFM structure above $mathit{T_{Eu}}$. However, at 2.5 K, the Fe$^{2+}$ moments are found to be ordered in a canted AFM structure with a canting angle of 14(4){deg} out of the $mathit{ab}$ plane. The spin reorientation of Fe below the AFM ordering temperature of Eu provides a direct evidence of a strong interplay between the two magnetic sublattices in Eu$_{0.5}$Ca$_{0.5}$Fe$_{2}$As$_{2}$.
Using polarized neutron diffraction and x-ray resonant magnetic scattering (XRMS) techniques, multiple phase transitions were revealed in an underdoped, non-superconducting Eu(Fe$_{1-x}$Ir$_{x}$)$_{2}$As$_{2}$ ($mathit{x}$ = 0.06) single crystal. Compared with the parent compound EuFe$_{2}$As$_{2}$, the tetragonal-to-orthorhombic structural phase transition and the antiferromagnetic order of the Fe$^{2+}$ moments are significantly suppressed to $mathit{T_{S}}$ = 111 (2) K and $mathit{T_{N,Fe}}$= 85 (2) K by 6% Ir doping, respectively. In addition, the Eu$^{2+}$ spins order within the $mathit{ab}$ plane in the A-type antiferromagnetic structure similar to the parent compound. However, the order temperature is evidently suppressed to $mathit{T_{N,Eu}}$= 16.0 (5) K by Ir doping. Most strikingly, the XRMS measurements at the Ir $mathit{L_{3}}$ edge demonstrates that the Ir 5$mathit{d}$ states are also magnetically polarized, with the same propagation vector as the magnetic order of Fe. With $mathit{T_{N,Ir}}$ = 12.0 (5) K, they feature a much lower onset temperature compared with $mathit{T_{N,Fe}}$. Our observation suggests that the magnetism of the Eu sublattice has a considerable effect on the magnetic nature of the 5$mathit{d}$ Ir dopant atoms and there exists a possible interplay between the localized Eu$^{2+}$ moments and the conduction $mathit{d}$-electrons on the FeAs layers.
We present a systematic study on the physical properties of EuFe$_{2-x}$Ni$_{x}$As$_{2}$ (0$leq$emph{x}$leq$0.2) by electrical resistivity, magnetic susceptibility and thermopower measurements. The undoped compound EuFe$_{2}$As$_{2}$ undergoes a spin-density-wave (SDW) transition associated with Fe moments at 195 K, followed by antiferromagnetic (AFM) ordering of Eu$^{2+}$ moments at 20 K. Ni doping at the Fe site simultaneously suppresses the SDW transition and AFM ordering of Eu$^{2+}$ moments. For $xgeq$0.06, the magnetic ordering of Eu$^{2+}$ moments evolves from antiferromagnetic to ferromagnetic (FM). The SDW transition is completely suppressed for $xgeq$0.16, however, no superconducting transition was observed down to 2 K. The possible origins of the AFM-to-FM transition and the absence of superconductivity in EuFe$_{2-x}$Ni$_{x}$As$_{2}$ system are discussed.
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