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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 sputter deposited NiZn ferrite thin films were studied as a function of annealing temperature. The magnetization showed a monotonic increase with increasing annealing temperature. The coercivity shows a minimum at annealing temperature of 400 °C and shows a value of 14 Oe. Transmission electron microscopy study indicated that the grain size increases from ∼3 nm for the as-deposited case to ∼15 nm for the film annealed at 800 °C. The observed coercivity behavior could be attributed to the defects present in the films, the change in cation distribution, and the grain growth.

The magnetic properties of core/shell nanoparticles can be finely tuned through the exchange coupling at the interface, enabling large heating powers under alternating magnetic fields. However, the origin of their heating efficiency is still unclear due to the complex interplay of different heating mechanisms. Here, we show that monodisperse Fe 3 O 4 /Co x Zn 1−x Fe 2 O 4 core/shell nanoparticles can be designed to provide large heating powers for different field amplitudes and dispersion media conditions by modulating their shell composition and thickness. The fine control of the nanoparticles' effective anisotropy provided by the interface coupling between core and shell leads to values up to ∼2400 W g −1 for water colloids and ∼1000 W g −1 for immobilized particles at 80 mT and 309 kHz. A reduction in the shell thickness or Co/Zn ratio results in a transition from a viscous heating regime to a region governed by a collective behavior, characterized by chainlike formation due to interparticle interactions. These results shed light on the origin of the large heating powers of core/shell ferrites and provide an empirical guide to design highly efficient magnetic nanoheaters.

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