In the zero-field-cooled exchange bias (ZEB) effect the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of external magnetic field. La1.5Sr0.5CoMnO6 stands out as presenting the largest ZEB reported so far, while for La1.5Ca0.5CoMnO6 the exchange bias field (HEB) is one order of magnitude smaller. Here we show that La1.5Ba0.5CoMnO6 also exhibits a pronounced shift of its magnetic hysteresis loop, with intermediate HEB value in respect to Ca-and Sr-doped samples. In order to figure out the microscopic mechanisms responsible for this phenomena, these compounds were investigated by means of synchrotron X-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, AC and DC magnetization, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The parent compound La2CoMnO6 was also studied for comparison, as a reference of a non-ZEB material. Our results show that the Ba-, Ca-and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states Co 2+ /Co 3+ and Mn 4+ /Mn 3+ are already present at the La2CoMnO6 parent compound, and that Ba 2+ /Ca 2+ /Sr 2+ partial substitution at La 3+ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the L-edge XAS indicate the presence of oxygen vacancies in all samples (0.05≤ δ ≤0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn. arXiv:1909.05287v1 [cond-mat.mtrl-sci]

The Fe K x-ray absorption near edge structure of BaFe(2-x)Co(x)As(2) superconductors was investigated. No appreciable alteration in shape or energy position of this edge was observed with Co substitution. This result provides experimental support to previous ab initio calculations in which the extra Co electron is concentrated at the substitute site and do not change the electronic occupation of the Fe ions. Superconductivity may emerge due to bonding modifications induced by the substitute atom that weakens the spin-density-wave ground state by reducing the Fe local moments and/or increasing the elastic energy penalty of the accompanying orthorhombic distortion.

We report investigations of the effect of electron doping in FeGa 3 via electric resistivity, specific heat and magnetic susceptibility measurements in single crystals. FeGa3 is a non-magnetic small gap semiconductor (∆ ∼ 0.3-0.4 eV). Low concentration of Co in FeGa 3 induces a crossover to a metallic-like behavior, also creating weakly coupled local moments. Electronic specific heat and resistivity suggest a mass enhancement of charge carriers. Thus, the low carrier density metal formed by doping FeGa 3 presents some physical properties that resemble heavy fermion metals.

Electron spin resonance ͑ESR͒ experiments at different fields or frequencies ͑4.1Յ Յ 34.4 GHz͒ in the Kondo lattice ͑T K Ӎ 25 K͒ YbRh 2 Si 2 single-crystal compounds confirmed the observation of a single anisotropic Dysonian resonance with g Ќc Х 3.55 and no hyperfine components for 4.2Շ T Շ 20 K. However, our studies differently reveal that ͑i͒ the ESR spectra for H Ќc show strong-field-dependent spin-lattice relaxation, ͑ii͒ a weak-field and temperature-dependent effective g value, ͑iii͒ a dramatic suppression of the ESR intensity beyond 15% of Lu doping, and ͑iv͒ a strong sample and Lu-doping ͑Յ15%͒ dependence of the ESR data. These results suggest a different scenario where the ESR signal may be associated to a coupled Yb 3+ -conduction electron resonant collective mode with a strong bottleneck and dynamiclike behavior.