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Abstract: Cobalt ferrite nanoparticles are a benchmark among low-to-medium energy alternatives to rare-earth permanent magnets, although their intrinsic behavior is often obscured by surface disorder, finite-size effects, and superparamagnetic relaxation. Here, we overcome these limitations by synthesizing large, highly crystalline cobalt-doped ferrite nanoparticles (≈ 25 nm), which remain blocked at room temperature and thus provide a clean platform to disentangle the fundamental role of cobalt in the spinel lattice. By systematically varying the cobalt content, we reveal a complex interplay between cation distribution, oxygen vacancy formation, and magnetic response. Structural and compositional analysis confirms predominant Co2+ occupancy at octahedral sites, accompanied by a redistribution of Fe2+/Fe3+ and non-linear oxygen vacancy generation. We find that while saturation magnetization is largely governed by defect chemistry, the coercivity and effective anisotropy are primarily controlled by cobalt incorporation and saturate at intermediate compositions. In contrast, thermomagnetic analysis reveals an anomalous evolution of magnetization at intermediate temperatures for specific cobalt contents. This behavior is consistent with a change in the anisotropy landscape, suggestive of a growing contribution from higher-order anisotropy terms, rather than a simple uniform increase in magnetocrystalline anisotropy. These results indicate that cobalt doping tunes the balance between different anisotropy contributions in a composition- and temperature-dependent manner. Overall, our findings highlight the subtle interplay between cation distribution, anisotropy landscape, and thermal stability in spinel ferrites, providing fundamental insight for the design of high-coercivity rare-earth-free nanomagnets.

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Abstract: Exchange bias fields at antiferromagnet/ferromagnet (AFM/FM) interfaces play a crucial role in the performance of spintronic devices. Despite extensive research, the physical origin of exchange bias remains incompletely understood. In this study, we conduct a detailed investigation of a prototype AFM/FM interface widely used in spintronic applications, i.e., the IrMn/CoFeB interface. High-resolution synchrotron X-ray measurements reveal the existence of uncompensated Mn spins at the interface. While most of these spins are strongly coupled to the adjacent CoFeB layer, a small fraction remains pinned to the underlying IrMn underlayer. Element-specific X-ray magnetic circular dichroism hysteresis loops show that these pinned spins can be switched by increasing the annealing magnetic field. Furthermore, micromagnetic simulations indicate that an imbalance in the quantity of antiparallel pinned spins contributes to the observed variation in exchange bias. Overall, these findings offer important insights into the microscopic mechanisms governing exchange bias and its tunability.

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Abstract: A rare example of a seven-membered heterometallic ring [CrIII6CeIIIF7(O2CtBu)14(THF)2] (MeCN)2 (1) and five eight-membered heterometallic rings, [nPr2NH2][CrIII6LnIII2F8(O2CtBu)17Lx] (2, Ln = Ce, L = HO2CtBu, x = 2, 3, Ln = Y, L = H2O, x = 1, 4, Ln = Gd, L = HO2CtBu, x = 1; 5, Ln = Tb, L = HO2CtBu, x = 1; 6, Ln = Yb, no L) have been synthesized and structurally characterized through X-ray diffraction. The structures consist of eight metals in an octagon, with Cr…Cr and Cr…Ln edges bridged by a fluoride and two carboxylates, while the Ln…Ln edges are bridged by a fluoride and three carboxylates. The magnetisation and susceptibility of these compounds were measured using SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy. The magnetic data were fitted with antiferromagnetic exchange interactions between chromium(III) ions, which can be fitted in the {Cr6Y2} complex 3 and these parameters were then used to fit the magnetic properties of the {Cr6Gd2} complex 4 adding in exchange interactions between the CrIII and GdIII The magnetisation and susceptibility below 80 K of 1 and 2 were fitted on the basis of CASSCF-SO calculations at the CeIII site, and showed a weak ferromagnetic interaction between CrIII and CeIII. For 5 and 6 the magnetisation data was fitted by subtracting the data for 3 and treating the residual data as a {Tb2} and {Yb2} dimer respectively. The EPR spectra are rich, and for 3 can be modelled as due to S = 1 and S = 2 states of the {Cr6} chain. The spectra of 1 and 2 are similar, consistent with very weak interactions between the CeIII and the {Cr6} chain, while the spectra of 5 and 6 are different to that of 3, suggesting that the low temperature spectroscopy is due to a spin system in which the LnIII ions interact with the {Cr6} chain.

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Abstract: Layered oxide materials, with their two-dimensional crystalline architectures and tunable interlayer interaction, serve as a fertile field for harnessing emergent quantum phenomena. Among these materials, metallic delafossites (e.g., PdCoO2) have emerged as a prominent system with extraordinary two-dimensional electronic properties, though their intrinsic lack of ferromagnetism has remained a fundamental constraint. Here, we report the creation of robust, bulk high-temperature ferromagnetism (𝑇𝑐>420  K) in inherently nonmagnetic PdCoO2 through controlled hydrogenation while preserving the delafossite structure. This process induces layer-selective electron doping into CoO2 layers, stabilizing Ising-type ferromagnetism with pronounced perpendicular magnetic anisotropy while preserving the material’s exceptional metallicity. Remarkably, the system self-assembles into a superlattice of alternating metallic Pd and insulating ferromagnetic hydrogenated CoO2 layers, enabling an unconventional anomalous Hall effect mediated by interlayer spin-charge coupling. These findings demonstrate that bulk ferromagnetism can be achieved in delafossite oxides while preserving their structural integrity, positioning hydrogenated delafossites as a versatile platform for exploring correlated quantum effects and designing multifunctional devices.

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Abstract: Magnetron sputtering offers a scalable route to magnetic topological insulators (MTIs) based on Cr-doped Sb2Te3. We combine a range of X-ray diffraction (XRD), reciprocal-space mapping (RSM), scanning transmission electron microscopy (STEM), scanning TEM-energy-dispersive X-ray spectroscopy (STEM-EDS), and X-ray absorption spectroscopy, and X-ray magnetic circular dichroism (XAS/XMCD) techniques to study the structure and magnetism of Cr-doped Sb2Te3 films. Symmetric 𝜃 -2𝜃 XRD and RSM establish a solubility window. Layered tetradymite order persists up to ∼10 at.-% Cr, while higher doping yields CrTe/Cr2Te3 secondary phases. STEM reveals nanocrystalline layered stacking at low Cr and loss of long-range layering at higher Cr concentrations, consistent with XRD/RSM. Magnetometry on a 6% film shows soft ferromagnetism at 5 K. XAS and XMCD at the Cr 𝐿2,3 edges exhibits a depth dependence: total electron yield (TE; surface sensitive) shows both nominal Cr2+ and Cr3+, whereas fluorescence yield (FY; bulk sensitive) shows a much higher Cr2+ weight. Sum rules applied to TEY give 𝑚𝐿=(0.20±0.04) 𝜇B /Cr, and 𝑚𝑆=(1.6±0.2) 𝜇B /Cr, whereby we note that the applied maximum field (3 T) likely underestimates 𝑚𝑆 . These results define a practical growth window and outline key parameters for MTI films.