Origin of the large dispersion of magnetic properties in nanostructured oxides: Fe_xO/Fe₃O₄nanoparticles as a case study

Abstract

The intimate relationship between stoichiometry and physicochemical properties in transition-metal oxides makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between the magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides.

The influence of oxidation process on exchange bias in egg-shaped FeO/Fe3O4 core/shell nanoparticles

“…The hysteresis loops recorded at 2 K are presented on Fig.5c [19], H EB = 0.6 and 0.5 kOe (10 K, particle size 9 nm-spherical and 45 nm-cubic) [7], H EB = 3.1 kOe (10 kOe cooling field, 2 K, 32 nm cubes) [32], H EB = 0.79, 2.97 and 5.1 kOe (40 kOe cooling field, T= 5K, corresponding particle size 9, 16 and 22 nm, cubes) [33], H EB = 1.6 kOe (50 kOe cooling field, T= 10 K, particle size 23 nm, cubes) [27].…”

“…In recent studies it was found that exchange coupling in AF/FiM nanoparticles strongly depends on the core size, the shell thickness and the shape or crystal quality [7,8].…”

Direct observation of enhanced magnetism in individual size- and shape-selected 3d transition metal nanoparticles

“…The resulting dipolar stray field energy can cause a sizeable magnetic shape anisotropy, which can dominate over the other contributions [16]. Figures 6(g)-6(i) show the calculated magnetic energy barriers for three selected particle sizes (8,12, and 20 nm) as a function of the aspect ratio of prolate Fe, Co, and Ni ellipsoids, respectively. The calculations were performed as described in Ref.…”

“…These features are of great interest for novel applications, while achieving control remains challenging and requires deeper understanding of the magnetic properties at the nanoscale. Extensive efforts have been undertaken to establish simple laws to predict size-dependent properties such as the magnetic anisotropy energy [9][10][11][12]. However, experimental validation of scalable regimes has not been achieved so far, even for the common ferromagnetic 3d transition metals, Fe, Co, and Ni.…”

Probing the variability in oxidation states of magnetite nanoparticles by single-particle spectroscopy

“…S5, ESI †) in order to elucidate the existence of any changes in the internal magnetic configuration of the particles due to the presence of distinct coexisting magnetic phases. 11,25,[36][37][38] The hysteresis loops before and after FC for S1…”