Magnetic transitions and spin-glass reentrance in two-dimensional [MnII(TCNE)(NCMe)2]X (X = PF6,AsF6,SbF6) molecular magnets

3d Metal Doping of Core@Shell Wüstite@ferrite Nanoparticles as a Promising Route toward Room Temperature Exchange Bias Magnets

Nanometric core@shell wüstite@ferrite (Fe_1-x O@Fe₃ O₄ ) has been extensively studied because of the emergence of exchange bias phenomena. Since their actual implementation in modern technologies is hampered by the low temperature at which bias is operating, the critical issue to be solved is to obtain exchange-coupled antiferromagnetic@ferrimagnetic nanoparticles (NPs) with ordering temperature close to 300 K by replacing the divalent iron with other transition-metal ions. Here, the effect of the combined substitution of Fe^(II)  with Co^(II)  and Ni^(II)  on the crystal structure and magnetic properties is studied. To this aim, a series of 20 nm NPs with a wüstite-based core and a ferrite shell, with tailored composition, (Co_0.3 Fe_0.7 O@Co_0.8 Fe_2.2 O₄  and Ni_0.17 Co_0.21 Fe_0.62 O@Ni_0.4 Co_0.3 Fe_2.3 O₄ ) is synthetized through a thermal-decomposition method. An extensive morphological and crystallographic characterization of the obtained NPs shows how a higher stability against the oxidation process in ambient condition is attained when divalent cation doping of the iron oxide lattice with Co^(II)  and Ni^(II)  ions is performed. The dual-doping is revealed to be an efficient way for tuning the magnetic properties of the final system, obtaining Ni-Co doped iron oxide core@shell NPs with high coercivity (and therefore, high energy product), and increased antiferromagnetic ordering transition temperature, close to room temperature.

Spin-glass-like freezing of inner and outer surface layers in hollow γ-Fe2O3 nanoparticles

Disorder among surface spins is a dominant factor in the magnetic response of magnetic nanoparticle systems. In this work, we examine time-dependent magnetization in high-quality, monodisperse hollow maghemite nanoparticles (NPs) with a 14.8 ± 0.5 nm outer diameter and enhanced surface-to-volume ratio. The nanoparticle ensemble exhibits spin-glass-like signatures in dc magnetic aging and memory protocols and ac magnetic susceptibility. The dynamics of the system slow near 50 K, and become frozen on experimental time scales below 20 K. Remanence curves indicate the development of magnetic irreversibility concurrent with the freezing of the spin dynamics. A strong exchange-bias effect and its training behavior point to highly frustrated surface spins that rearrange much more slowly than interior spins. Monte Carlo simulations of a hollow particle corroborate strongly disordered surface layers with complex energy landscapes that underlie both glass-like dynamics and magnetic irreversibility. Calculated hysteresis loops reveal that magnetic behavior is not identical at the inner and outer surfaces, with spins at the outer surface layer of the 15 nm hollow particles exhibiting a higher degree of frustration. Our combined experimental and simulated results shed light on the origin of spin-glass-like phenomena and the important role played by the surface spins in magnetic hollow nanostructures.