Phase diagram and isotope effect in (PrEu)_0.7Ca_0.3CoO_3 cobaltites exhibiting spin-state transitions

We present the study of magnetization, thermal expansion, specific heat, resistivity, and a.c. susceptibility of (Pr$_{1-y}$Eu$_y$)$_{0.7}$Ca$_{0.3}$CoO$_3$ cobaltites. The measurements were performed on ceramic samples with $y = 0.12 - 0.26$ and $y = 1$. Based on these results, we construct the phase diagram, including magnetic and spin-state transitions. The transition from the low- to intermediate-spin state is observed for the samples with $y > 0.18$, whereas for a lower Eu-doping level, there are no spin-state transitions, but a crossover between the ferromagnetic and paramagnetic states occurs. The effect of oxygen isotope substitution along with Eu doping on the magnetic/spin state is discussed. The oxygen-isotope substitution ($^{16}$O by $^{18}$O) is found to shift both the magnetic and spin-state phase boundaries to lower Eu concentrations. The isotope effect on the spin-state transition temperature ($y > 0.18$) is rather strong, but it is much weaker for the transition to a ferromagnetic state ($y < 0.18$). The ferromagnetic ordering in the low-Eu doped samples is shown to be promoted by the Co$^{4+}$ ions, which favor the formation of the intermediate-spin state of neighboring Co$^{3+}$ ions.Comment: 13 pages, including 11 figures, to be published in Phys. Rev.

Metal-insulator transition in manganites: mixture of oxygen isotopes versus magnetic field

We have investigated the effect of oxygen isotope substitution on the metal-insulator transition temperature and the resistivity of the narrow band manganite (La0.25Pr0.75)0.7Ca0.3MnO3 in a constant magnetic field. A set of 16 samples having different mixtures of 16O, 17O and 18O isotopes with average mass varying from 16.0 to 17.8 a.m.u. was studied. We have found that the magnetoresistance and the isotope effect can be linked together with a single parameter - effective magnetic field, which decreases linearly with an increase of average oxygen mass with a slope of -2 T/a.m.u. The applicability of the small polaron model is discussed.Comment: Submitted to Moscow International Symposium on Magnetism MISM'0

Angle Dependence of the Transverse Thermal Conductivity in YBa2_2Cu3_3O7_7 single crystals: Doppler Effect vs. Andreev scattering

We have measured the transverse thermal conductivity $\kappa_{xy}$ of twinned and untwinned YBa$_2$Cu$_3$O$_7$ single crystals as a function of angle $\theta$ between the magnetic field applied parallel to the CuO$_2$ planes and the heat current direction, at different magnetic fields and temperatures. For both crystals we observed a clear twofold variation in the field-angle dependence of $\kappa_{xy}(\theta) = - \kappa^0_{xy}(T,B) \sin(2\theta)$. We have found that the oscillation amplitude $\kappa^0_{xy}$ depends on temperature and magnetic field. Our results show that $\kappa^0_{xy} = a B \ln(1/(bB))$ with the temperature- and sample-dependent parameters $a$ and $b$. We discuss our results in terms of Andreev scattering of quasiparticles by vortices and a recently proposed theory based on the Doppler shift in the quasiparticle spectrum.Comment: 5 pages, 4 figure

Magnetically intercalated multilayer silicene

Silicene, a Si-based analogue of graphene, is predicted to exhibit topological electronic phases with exotic properties capable to revolutionize electronics. In particular, the silicene structure is highly advantageous for spintronics. However, lack of synthetic routes to free-standing and magnetically functionalized silicene compounds prevents experimental corroboration of the predictions. Here we synthesize EuSi2, multilayer silicene intercalated with inherently magnetic Eu atoms, on SrSi2/Si(001) templates. The resulting films are formed by crystallites of two mutually orthogonal orientations. The structure is firmly established with electron diffraction, X-ray diffraction and electron microscopy. The compound EuSi2 exhibits non-trivial magnetic and transport properties. The data are compared with those for EuSi2 films grown on SrSi2/Si(111) templates

Effect of Eu Doping and Partial Oxygen Isotope Substitution on Magnetic Phase Transitions in (Pr1-yEuy)(0.7)Ca0.3CoO3 Cobaltites

We study experimentally and theoretically the effect of Eu doping and partial oxygen isotope substitution on the transport and magnetic characteristics and spin-state transitions in (Pr1-yEuy)(0.7)Ca0.3CoO3 cobaltites. The Eu doping level y is chosen in the range of the phase diagram near the crossover between the ferromagnetic and spin-state transitions (0.10 < y < 0.20). We prepared a series of samples with different degrees of enrichment by the heavy oxygen isotope O-18, namely, with 90, 67, 43, 17, and 0% of O-18. Based on the measurements of the ac magnetic susceptibility chi(T) and electrical resistivity rho(T), we analyze the evolution of the sample properties with a change of the Eu and O-18 content. It is demonstrated that the effect of increasing the O-18 content on the system is similar to that of increasing the Eu content. The band structure calculations of the energy gap between t(2g) and e(g) bands including the renormalization of this gap due to the electron-phonon interaction reveals the physical mechanisms underlying this similarity

Magnetically intercalated multilayer silicene

Silicene, a Si-based analogue of graphene, is predicted to exhibit topological electronic phases with exotic properties capable to revolutionize electronics. In particular, the silicene structure is highly advantageous for spintronics. However, lack of synthetic routes to free-standing and magnetically functionalized silicene compounds prevents experimental corroboration of the predictions. Here we synthesize EuSi2, multilayer silicene intercalated with inherently magnetic Eu atoms, on SrSi2/Si(001) templates. The resulting films are formed by crystallites of two mutually orthogonal orientations. The structure is firmly established with electron diffraction, X-ray diffraction and electron microscopy. The compound EuSi2 exhibits non-trivial magnetic and transport properties. The data are compared with those for EuSi2 films grown on SrSi2/Si(111) templates