Phase diagram of Eu magnetic ordering in Sn-flux-grown Eu(Fe1−x_{1-x}Cox_{x})2_{2}As2_{2} single crystals

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.Comment: 10 pages, 9 figure

Elaboration of the structural and physical characteristics: Ni-doped ZnO bulk samples prepared by solid state reactions

Ni-doped ZnO (Zn1-xNixO, with 0.25 ? × ? 0.50) diluted magnetic semiconductors were prepared by the solid state reaction method. We have studied the structural properties of the samples by using the X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Energy Dispersive X-Ray spectroscopy (EDX) techniques. The SEM and AFM results clearly demonstrate that Ni2+ ions integrate into the ZnO structure without any problem. The grains of the samples are very well connected to each other and tightly packed, and vary in size from 0.2µm to 2µm. From the XRD and EDX spectra of the samples, it has been concluded that the doping causes no change in the hexagonal wurtzite structure of ZnO. However, the XRD indicated that three additional peaks, related to the (102),(012) and (108) planes appeared for the doped samples. Furthermore, an additional NiO-associated diffraction peak appears for the highest concentration of Ni, i.e., for x=0.50 of Ni2+ doping, which indicates an upper limit for Ni concentration. The estimated crystal sizes from the XRD results vary from 4.38 A• to 9.73 A•. The lattice parameter a and c of Ni-doped ZnO are slightly smaller and higher than that of pure ZnO, respectively. These observations may be due to the slightly different ionic sizes of Zn2+ and Ni2+ ions

Spin reorientation of the Fe moments in Eu0.5Ca0.5Fe2As2 : Evidence for strong interplay of Eu and Fe magnetism

Using complementary polarized and unpolarized single-crystal neutron diffraction, we have investigated the temperature-dependent magnetic structures of Eu0.5Ca0.5Fe2As2. Upon 50% dilution of the Eu sites with isovalent Ca2+, the Eu sublattice is found to be still long-range ordered below TEu=10 K, in the A-type antiferromagnetic (AFM) structure. The moment size of Eu2+ spins is estimated to be as large as 6.74(4)μB at 2.5 K. The Fe sublattice undergoes a spin-density-wave transition at TSDW=192(2) K and displays an in-plane AFM structure above TEu. However, at 2.5 K, the Fe2+ moments are found to be ordered in a canted AFM structure with a canting angle of 14(4)∘ out of the ab plane. The spin reorientation of Fe below the AFM ordering temperature of Eu provides direct evidence of a strong interplay between the two magnetic sublattices in Eu0.5Ca0.5Fe2As2

Magnetism in Eu(Fe1-xIrx)2As2 iron pnictides studied by complementary scattering methods

Abstract: Doped EuFe2As2 is a unique representative of the 122 type family of iron based superconductors due to the two magnetic sublattices and the strong coupling between spin-, lattice- and charge degrees of freedom. In present work, the magnetic ground states of the localized Eu2+ spins in underdoped, nonsuperconducting Eu(Fe0.94Ir0.06)2As2 and optimally doped, superconducting Eu(Fe0.88Ir0.12)2As2 (TSC = 22 K) were investigated by polarized and unpolarized single-crystal neutron diffraction measurements, respectively. The Eu2+ spins in the underdoped (x = 0.06) crystal were found to order within the ab plane in the A-type antiferromagnetic (AFM) structure. However, the Eu2+ spins in the optimally doped (x = 0.12) crystal were found to be ferromagnetically aligned along the c direction with an ordered moment of 7.0(1) μB at 2.5 K, coexisting with the bulk superconductivity. In addition, compared with the parent compound EuFe2As2, which shows the AFM order of the Fe2+ moments below TN,Fe = 190 K, TN,Fe was significantly suppressed to 85(2) K and completely suppressed, by 6% and 12% Ir doping, respectively. Most strikingly, the x-ray resonant magnetic scattering measurements on the underdoped (x = 0.06) crystal revealed the magnetic polarization of the 5d Ir dopant atoms and suggested a possible interplay between the localized Eu2+ moments and the conduction d electrons on the FeAs layers