Here, we present a study of the influence of microstructure on the magnetic properties of polycrystalline samples of the La1.5Sr0.5CoMnO6 double perovskite, with primary attention to the spontaneous exchange bias effect, a fascinating recently discovered phenomena for which some materials exhibit unidirectional magnetic anisotropy after being cooled in zero magnetic fields. By sintering La1.5Sr0.5CoMnO6 at different temperatures, we obtained samples with distinct average grain sizes, ranging from 1.54 to 6.65 mu_m. A detailed investigation of the material's structural, morphologic, electronic, and magnetic properties using X-ray powder diffraction, powder neutron diffraction, X-ray absorption near edge structure spectroscopy, scanning electron microscopy, and AC and DC magnetometry has revealed a systematic enhancement of the exchange bias effect with increasing the average grain size. This evolution is discussed in terms of changes in the material's porosity and grain morphology and its influence on the exchange couplings at the magnetic interfaces.

The electron spin polarization on half-metallic double perovskites is usually conditioned to the ordered rock-salt arrangement of the transition-metal ions along the lattice. In this work, we investigate a polycrystalline sample of the Ca1.5La0.5MnRuO6 1 . 5 La 0 . 5 MnRuO 6 compound by employing x-ray powder diffraction, high-resolution transmission electron microscopy, x-ray absorption and magnetic circular dichroism at the Mn L 2 , 3 and Ru M 2 , 3 edges, magnetometry, electrical transport, and first-principles calculations in order to show that this is a fully disordered material exhibiting near-room-temperature ferrimagnetism and half-metallic conductivity, with significant intergrain tunneling magnetoresistance. These unprecedented results are compared to those of archetypical ordered double perovskites, and discussed in terms of the Mn and Ru valences and their orbital hybridization.

Here, we present a review of the phenomenology of the spontaneous exchange bias effect, a phenomenon in which some materials exhibit unidirectional magnetic anisotropy even without the assistance of an external magnetic field applied during its cooling process. We review and discuss the most critical advances in this field of research that flourished more than a decade ago, pointing out its main features as well as its similarities and dissimilarities with the conventional exchange bias effect that has been vastly investigated since the 1950 decade. Finally, we briefly overview the obstacles to advancement in the field and discuss what we believe could be promising roads to overcome them.

Here, we report on the structural, electronic, and magnetic properties of a polycrystalline sample of the LaCaCoIrO6 double-perovskite investigated by means of synchrotron x-ray powder diffraction, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism at the Co and Ir L23 edges, magnetometry, and electrical transport. Our results indicate a configuration of nearly Co2+/Ir5+ configuration for the transition-metal ions, with spin canting within the Co antiferromagnetic superstructure responsible for the ferromagnetic-like behavior observed below 100 K. The highly insulating character of LaCaCoIrO6 and its positive magnetoresistance further suggest that this antiferromagnetic superexchange interaction occurs through an indirect hybridization between the Co eg orbitals.

The structural, electronic, and magnetic properties of La2_xBaxCoMnO6(0.25 < x < 1.0) compounds were investigated employing x-ray powder diffraction and magnetometry. The crystal structure evolves from orthorhombic Pnma to rhombohedral R3c and then to hexagonal P63/mmc space group as the Ba concentration increases. The magnetization as a function of temperature measurements revealed a ferromagnetic-like behavior for all samples, but antiferromagnetic interactions seem to also be present throughout the whole series. The competition between magnetic phases leads to the phenomenon of spontaneous exchange bias effect, observed in the zero field cooled magnetization as a function of applied magnetic field curves. The evolution of this effect along the series is discussed in terms of changes in the crystal structure.

Here we present a detailed investigation of the Co and Ir local electronic structures in La1.5A0.5CoIrO6 (A = Ba, Ca) compounds in order to unravel the orbital hybridization mechanism in these CoIr-based double perovskites. Our results of x-ray powder diffraction, ac and dc magnetization, Co and Ir L2,3-edge and Co K-edge x-ray absorption spectroscopy and x-ray magnetic circular dichroism suggest a competition between magnetic interactions. A dominant antiferromagnetic coupling is found to be responsible for the ferrimagnetic behavior observed for A = Ca below & SIM;96 K, the competing magnetic phases, and the cationic disorder in this compound giving rise to a spin-glass state at low temperatures. For the A = Ba, on the other hand, there is no evidence of long-range order down to its spin-glass transition temperature. The remarkably different magnetic properties observed between these two compounds are discussed in terms of the structural distortion that alters the strength of the Co-Ir couplings, with a relevant role played by the Co 3d eg-Ir 5d jeff = 1/2 hybridization.

In this work, CoFe2O4/CoFe2 nanoparticles with core/shell structure were synthesized by carbothermic reduction using chitosan beads impregnated with Fe3+ and Co2+ ions. The samples were synthesized at 400, 550, 650 and 750 degrees C, during intervals of 30, 90 and 120 min and under air, N-2 and H-2 gases. To apply the carbothermic method, chitosan was thermally decomposed in presence of N(2 )gas. The structural and magnetic properties were studied using X-ray powder diffraction, transmission electron microscopy, vibrating sample magnetometry and Mossbauer spectroscopy. The results showed that the Co-Fe impregnated chitosan beads thermally treated, firstly, in air at a given temperature and afterwards treated at 400 and 550 degrees C in H-2 for 30 and 90 min led to the formation of CoFe2O4/CoFe2 nanoparticles, the samples treated at higher temperatures were single phase of CoFe2. However, the Co-Fe chitosan beads treated in N-2 for 120 min at 550, 650 and 750 degrees C favored the formation of a core/shell (CoFe2O4/CoFe2) nanoparticles with CoFe2 concentration ranging from 75.5 to 35.0 Wt %, the amount of CoFe2 increased with the increase of temperature. The highest saturation magnetization of 227.7 emu/g was achieved for the sample treated a 550 degrees C under N-2 atmosphere.

Structural, electronic and magnetic properties of polycrystalline La2−xCaxCoMnO6 (0≤x≤0.75) compounds are investigated by x-ray diffraction and magnetometry. All the samples have an orthorhombic structure and show a slight decrease in the unit cell with Ca-doping. Temperature-dependent magnetization measurements reveal a complex magnetic behavior with two ferromagnetic transitions. These transitions are ascribed to Co2+–Mn4+ and Co3+–Mn3+ couplings and suggest the presence of additional antiferromagnetic couplings in these disordered compounds. Field-dependent magnetization curves, measured after cooling the samples in a zero external magnetic field, reveal the spontaneous exchange bias (SEB) effect for the Ca-doped samples. The strengthening of the uncompensated magnetic coupling at the interfaces, caused by the increased antiferromagnetic phase, explains the increase of SEB with increasing the Ca-content.

In this work, we report the synthesis and investigation of structural, electronic, and magnetic properties of La1.5Ca0.5(Co0.5Fe0.5)IrO6. Our polycrystalline sample forms as a single-phase double perovskite in monoclinic P21/n space group. Co and Ir are most likely in bivalent and tetravalent oxidation states, respectively, while Mössbauer spectroscopy indicates that Fe is in a trivalent state. The ac and dc magnetization data suggest a ferrimagnetic behavior resulting from the presence of two antiferromagnetic sublattices at Co/Fe and Ir sites. The large coercive field HC ≃ 32 kOe observed at 10 K, comparable to that of other double perovskites of interest for hard magnets, is discussed in terms of the structural distortion and the spin and orbital magnetic moments of the transition metal ions.

Magnetic properties of A2BB'O6 (A = rare or alkaline earth ions; B,B' = transition metal ions) double perovskites are of great interest due to their potential spintronic applications. Particularly fascinating is the zero field cooled exchange bias (ZEB) effect observed for the hole doped La2-xAxCoMnO6 polycrystalline samples. In this work we synthesize La2CoMnO6, La1.5Ca0.5CoMnO6, and La1.5Sr0.5CoMnO6 single crystals by the floating zone method and study their magnetic behavior. The three materials are ferromagnetic. Surprisingly, we observe no zero or even conventional exchange bias effect for the Ca and Sr doped single crystals, in sharp contrast to polycrystalline samples. This absence indicates that the lack of grain boundaries and spin glass-like behavior, not observed in our samples, might be key ingredients for the spontaneous exchange bias phenomena seen in polycrystalline samples.

La1.5Sr0.5CoMn0.5Fe0.5O6 (LSCMFO) compound was prepared by solid state reaction and its structural, electronic and magnetic properties were investigated. The material forms in rhombohedral [Formula: see text] structure, and the presence of distinct magnetic interactions leads to the formation of a Griffiths phase above its FM transition temperature (150 K), possibly related to the nucleation of small short-ranged ferromagnetic clusters. At low temperatures, a spin glass-like phase emerges and the system exhibits both the conventional and the spontaneous exchange bias (EB) effects. These results resemble those reported for La1.5Sr0.5CoMnO6 but are discrepant to those found when Fe partially substitutes Co in La1.5Sr0.5(Co1-x Fe x )MnO6, for which the EB effect is observed in a much broader temperature range. The unidirectional anisotropy observed for LSCMFO is discussed and compared with those of resembling double-perovskite compounds, being plausibly explained in terms of its structural and electronic properties.

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.

La$_{1.5}$Ba$_{0.5}$CoMnO$_{6}$ is a re-entrant cluster glass material exhibiting a robust negative exchange bias (EB) effect even after being cooled from an unmagnetized state down to low temperature in zero magnetic field. Here we thoroughly investigate this phenomena by performing magnetization as a function of applied field [$M(H)$] measurements at several different temperatures and maximum applied magnetic fields ($H_m$). The spontaneous EB (SEB) effect is observed below 20 K, and shows a maximum value for $H_m$ = 75 kOe. The effect is greatly enhanced when the $M(H)$ curves are measured after the system is cooled in the presence of a magnetic field. The asymmetry of the $M(H)$ curves here investigated can be well described by a recently proposed model based on the unconventional relaxation of the SG-like moments during the hysteresis cycle.

The delicate balance between spin-orbit coupling, Coulomb repulsion, and crystalline electric field interactions observed in Ir-based oxides is usually manifested as exotic magnetic behavior. Here we investigate the evolution of the exchange coupling between Co and Ir for partial La substitution by Ca in ${\mathrm{La}}_{2}{\mathrm{CoIrO}}_{6}$. A great advantage of the use of ${\mathrm{Ca}}^{2+}$ as a replacement for ${\mathrm{La}}^{3+}$ is the similarity of their ionic radii. Thus, the observed magnetic changes can more easily be associated with electronic variations. A thorough investigation of the structural, electronic, and magnetic properties of the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{CoIrO}}_{6}$ system was carried out by means of synchrotron x-ray powder diffraction, muon spin rotation and relaxation $(\ensuremath{\mu}\mathrm{SR})$, AC and DC magnetization, x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism, Raman spectroscopy, electrical resistivity, and dielectric permittivity. Our XAS results show that up to 25% Ca substitution at the La site results in the emergence of ${\mathrm{Co}}^{3+}$, possibly in a high-spin state, while the introduction of a larger amount of Ca leads to an increase in the Ir valence. The competing magnetic interactions resulting from the mixed valences lead to the coexistence of a magnetically ordered and an emerging spin-glass (SG) state for the doped samples. Our $\ensuremath{\mu}\mathrm{SR}$ results indicate that for ${\mathrm{La}}_{2}{\mathrm{CoIrO}}_{6}$ a nearly constant fraction of a paramagnetic (PM) phase persists down to low temperatures, possibly related to the presence of a small amount of ${\mathrm{Ir}}^{3+}$ and to the antisite disorder at Co/Ir sites. For doped compounds the PM phase freezes below 30 K, but there is still some dynamics associated with the SG. The dielectric data obtained for the parent compound and the one with 25% Ca doping indicate a possible magnetodielectric effect, which is discussed in terms of the electron hopping between the transition-metal ions, the antisite disorder at Co/Ir sites, and the distorted crystalline structure.

In this work we propose an alternative model to explain the spontaneous exchange bias (SEB) effect observed in spin glass (SG)-like systems. As in a previously proposed model [1], it is based on the unconventional dynamics of the SG-like moments at the magnetic hysteresis cycle. However, using a reliable estimate of the amount of SG-spins that are relaxing during the cycle, the new model can correctly describe the changes in the SEB observed for measurements performed at different temperatures and different maximum applied fields.

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 an 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 an intermediate H-EB value with respect to Ca- and Sr-doped samples. To figure out the microscopic mechanisms responsible for this phenomenon, 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 Co2+/Co3+ and Mn4+/Mn3+ are already present at the La2CoMnO6 parent compound, and that Ba2+/Ca2+/Sr2+ partial substitution at the La3+ 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 <= delta <= 0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and they indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.

La1.5Ca0.5CoIrO6 is a re-entrant spin-glass (SG) that exhibits antiferromagnetic and ferromagnetic couplings at T∼95 and ∼86 K, respectively, and at T∼25 K a SG phase emerges. In this work we investigated the effect of hydrostatic pressure (P) on La1.5Ca0.5CoIrO6 magnetic properties. By means of magnetization as a function of temperature measurements, carried under different applied pressures and/or magnetic fields, we have found that the freezing temperature of the SG phase exhibits an initial increase followed by a decrease with increasing P, and that the maximum P = 7.9 kbar was not sufficient to prevent the formation of the frozen state. Since the ordering temperatures of the antiferromagnetic and ferromagnetic phases are also affected by P, we discuss the results here found in terms of changes on the balance between the magnetic phases.

The zero-field-cooled exchange bias (ZEB) effect is a remarkable phenomenon recently reported for some reentrant spin glass-like compounds. In this work, the time-evolution of magnetization is thoroughly investigated for two ZEB materials in order to figure out the role played by the spin glass-like phase on such effect. La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ and La$_{1.5}$Ca$_{0.5}$CoMnO$_{6}$ were chosen as representative samples of ZEB systems, since the former compound presents the largest ZEB reported so far, while the second has a much smaller effect, despite being structurally/chemically similar. Comprehensive magnetic measurements were carried on both samples, and the results are discussed in terms of the amount and time-evolution of the spin glass-like phase under the influence of a varying field. We also propose a phenomenological model, based on the pinning of spin glass-like moments and on the dynamics of their magnetic relaxation, to explain the asymmetry observed in the hysteresis loops. The good agreement between the simulated and experimental results confirms our hypothesis that the spin glass-like phase is key to the ZEB effect.

La1.5Ca0.5CoIrO6 is a re-entrant spin-glass double-perovskite that exhibits a non-negligible spontaneous exchange bias effect at low temperatures. When performing magnetization as a function of applied field [M(H)] measurements after cooling the sample in the presence of a magnetic field, the exchange bias is greatly enhanced. In this work we report a detailed investigation of the influence of the maximum applied field (Hm) on the exchange bias effect for M(H) curves measured with and without the presence of an applied field on cooling. In both cases the shift in the hysteresis loops decreases for increasing Hm. We have also investigated the influence of the cooling field on the exchange bias effect. By increasing the applied field on cooling there is an initial increase of the exchange bias, followed by a decrease of the effect for larger cooling fields. A detailed study of the magnetic evolution of the system with temperature showed that despite the enhancement of the coercive field observed at temperatures above the freezing of the spin-glass phase, there is no trace of exchange bias effect at these temperatures. We discuss our results in terms of the pinning of the spins at the magnetic interfaces.

In this work we report the synthesis and structural, electronic and magnetic properties of La1.5Ca0.5CoMnO6 double-perovskite. This is a re-entrant spin cluster material which exhibits a non-negligible negative exchange bias effect when it is cooled in zero magnetic field from an unmagnetized state down to low temperature. X-ray powder diffraction, X-ray photoelectron spectroscopy and magnetometry results indicate mixed valence state at Co site, leading to competing magnetic phases and uncompensated spins at the magnetic interfaces. We compare the results for this Ca-doped material with those reported for the resemblant compound La1.5Sr0.5CoMnO6, and discuss the much smaller spontaneous exchange bias effect observed for the former in terms of its structural and magnetic particularities. For La1.5Ca0.5CoMnO6, when successive magnetization loops are carried, the spontaneous exchange bias field inverts its sign from negative to positive from the first to the second measurement. We discuss this behavior based on the disorder at the magnetic interfaces, related to the presence of a glassy phase. This compound also exhibits a large conventional exchange bias, for which there is no sign inversion of the exchange bias field for consecutive cycles.