Coexistence of High Magnetization and Anisotropy with Non-monotonic Particle Size Effect in Ferromagnetic PrMnO₃ Nanoparticles

Instances of the coexistence of high ferromagnetic magnetization with large anisotropy are scarce in the rare-earth manganite family. In manganites, high magnetizations are compromised with small coercivity and vice versa. Using nonaqueous sol–gel techniques, the undoped PrMnO₃ nanoparticles with oxygen nonstoichiometry were rendered with exceptional ferromagnetic character. While ∼40 nm sized nanoparticles had magnetization of 84 emu/g and coercivity of 885 Oe with 50 kOe sweeping field, the bulk 2 μm sized particles showed a magnetization of 51 emu/g albeit with a higher coercivity of 2000 Oe. These parameters are so far the highest among manganite systems with similarly sized particles. The competition between the ferromagnetic and antiferromagnetic phases both at the particle core and at the grain boundaries resulted in a non-monotonous trend of magnetic properties between 20, 40, and 2 μm particles. The sudden increase of coercivity toward lower temperatures was a result of the freezing of random spins at the surface of the strongly interacting nanoparticles which also increased the magnetic anisotropy. These results are of prime significance since the coexistence of such a large magnetization with high coercivity was rarely observed in pristine or doped manganites

Surfactant-Mediated Resistance to Surface Oxidation in MnO Nanostructures

The intrinsic physical properties of nanostructures of metals and their oxides are altered when they are prone to surface oxidation in ambient atmosphere. To overcome this limitation, novel synthesis methodologies are required. In this study, solid octahedral shapes of MnO limit the inward oxygen diffusion compared to that of the MnO-nanoparticle-assembled octahedra. In addition to morphology control, which restricts the thickness of the Mn₃O₄ surface layer, the binding chemistry of the surfactants plays an essential role. For example, the Mn₃O₄ surface layer is 0.4 nm thinner with trioctylphosphine oxide than with trioctylamine as the surfactant. The nanostructures were prepared by varying the surfactants, surfactant-to-precursor molar ratio, accelerating agent, and reaction heating rate. The surface oxidation of MnO nano-octahedra was probed by Rietveld analysis of X-ray diffraction patterns and X-ray photoelectron spectroscopy and characterized by magnetic measurements, as the presence of ferrimagnetic Mn₃O₄ shell on the antiferromagnetic MnO core provides an exchange coupling at the core–shell interface. Thicker the Mn₃O₄ shell, higher is the exchange-biased hysteresis loop shift

Maneuvering the Physical Properties and Spin States To Enhance the Activity of La–Sr–Co–Fe–O Perovskite Oxide Nanoparticles in Electrochemical Water Oxidation

Perovskite oxides have attracted considerable attention as durable electrocatalysts for metal–air batteries and fuel cells due to their precedence in oxygen electrocatalysis in spite of the complexities involved with their crystal structure, spin states, and physical properties. Here we report optimization of the activity of a model perovskite system La_1–<i>x</i>Sr_<i>x</i>Co_1–<i>y</i>Fe_<i>y</i>O_3−δ (LSCF; <i>x</i> = 0.301, <i>y</i> = 0.298, and δ = 0.05–0.11) toward electrochemical water oxidation (OER) by altering the calcination temperature of the nonaqueous sol–gel synthesized nanoparticles (NPs). Our results show that improved OER activity is the result of a synergism between its morphology, surface area, electrical conductivity, and spin state of the active transition metal site. With an e_g orbital occupancy of 1.26, the interconnected ∼90 nm LSCF NPs prepared at 975 °C (LSCF-975) outperforms the other distinguishable LSCF morphologies, requiring 440 mV overpotential to achieve 10 mA/cm², a performance comparable to the best-performing perovskite oxide electrocatalysts. While the interconnected NP morphology increases the propensity of electronic conduction across crystalline grain boundaries, the morphology-tuned high spin Co³⁺ ions increases the probability of binding reaction intermediates at the available surface sites. Density functional theory based work function modeling further demonstrates that LSCF-975 is the most favorable OER catalyst among others in terms of a moderate work function and Fermi energy level facilitating the adsorption and desorption of reaction intermediates