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 spontaneous exchange bias (SEB) effect is a remarkable phenomenon recently observed in some reentrant spin-glass materials. Here, we investigate the SEB in La1.5(Sr0.5−xBax)CoMnO6 double-perovskites, a system with multifarious magnetic phases for which a notable increase in the exchange bias field is observed for intermediate Sr/Ba concentrations. The Ba to Sr substitution leads to the enhancement of the crystal lattice, which is accompanied by the increase in both the effective magnetic moment (μeff) and the antiferromagnetic (AFM) transition temperature that is observed below the ferromagnetic ordering. Such an increase is likely related to the increased fraction of Co3+ in the high spin configuration, leading to the enhancement of the Co3+–O–Mn4+ AFM phase and the reduction in the uncompensation of the AFM coupling between Co and Mn. The combined effect of the increased μeff and AFM phase plausibly explains the changes in the SEB effect.

We investigate the thermal-driven charge density wave (CDW) transition of two cubic superconducting intermetallic systems Lu(Pt 1−x Pd x) 2 In and (Sr 1−x Ca x) 3 Ir 4 Sn 13 by means of x-ray diffraction technique. A detailed analysis of the CDW modulation superlattice peaks as function of temperature is performed for both systems as the CDW transition temperature T CDW is suppressed to zero by an nonthermal control parameter. Our results indicate an interesting crossover of the classical thermal-driven CDW order parameter critical exponent from a three-dimensional universality class to a mean-field tendency, as T CDW vanishes. Such behavior might be associated with the presence of quantum fluctuations which influences the classical second-order phase transition, strongly suggesting the presence of a quantum critical point (QCP) at T CDW = 0. This also provides experimental evidence that the effective dimensionality exceeds its upper critical dimension due to a quantum phase transition.