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

Abstract

Magnetic nanoparticles are critical building blocks for future technologies ranging from nanomedicine to spintronics. Many related applications require nanoparticles with tailored magnetic properties. However, despite significant efforts undertaken towards this goal, a broad and poorly understood dispersion of magnetic properties is reported, even within monodisperse samples of the canonical ferromagnetic 3d transition metals. We address this issue by investigating the magnetism of a large number of size-and shape-selected, individual nanoparticles of Fe, Co, and Ni using a unique set of complementary characterization techniques. At room temperature, only superparamagnetic behavior is observed in our experiments for all Ni nanoparticles within the investigated sizes, which range from 8 to 20 nm. However, Fe and Co nanoparticles can exist in two distinct magnetic states at any size in this range: (i) a superparamagnetic state, as expected from the bulk and surface anisotropies known for the respective materials and as observed for Ni, and (ii) a state with unexpected stable magnetization at room temperature. This striking state is assigned to significant modifications of the magnetic properties arising from metastable lattice defects in the core of the nanoparticles, as concluded by calculations and atomic structural characterization. Also related with the structural defects, we find that the magnetic state of Fe and Co nanoparticles can be tuned by thermal treatment enabling one to tailor their magnetic properties for applications. This paper demonstrates the importance of complementary single particle investigations for a better understanding of nanoparticle magnetism and for full exploration of their potential for applications.

Unraveling Finite Size Effects on Magnetic Properties of Cobalt Nanoparticles

“…From recent DFT calculations it is clear that up to 309 atoms the icosahedral shape is favored for Co nanoparticles. 18 On the other hand, at much higher size (diameter ≈ 10 nm), the experiments show clearly the Wulff shape (truncated octahedron or TO in the following) of Co nanoparticles, 33 as expected for close-packed transition metals. In this context, we will compare the main results for the icosahedra to the case of a TO morphology for a higher size (405 atoms, ≈ 2 nm of diameter).…”

Tuning the Size, Shape and Density of γ′-GayFe4−yN Nanocrystals Embedded in GaN

“…It is expected that the in-plane anisotropy, required for data-storage applications, is enhanced for the elongated disc-shaped NCs. Therefore, prospective studies of the magnetic moments of the single NCs by employing X-ray photoelectron-emission spectroscopy combined with X-ray circular dichroism [26] as a function of the size, shape and density adjusted according to the findings here reported are likely to represent the next step towards the full control and on-demand tunability of planar arrays of γ'-Ga y Fe 4−y N NCs embedded in GaN.…”

Internal Structure and Magnetic Properties in Cobalt Ferrite Nanoparticles: Influence of the Synthesis Method

“…At low temperature, M R / M S reaches a maximum of 0.77 for TD9 and a minimum of 0.47 for AC10, as reported in Table 2 together with the main parameters of the magnetic analysis. Multiple factors have been proposed to explain the variation in the magnetic properties of NPs with analogous composition and size, and its origin is still under debate. In the following sections we will explore the interplay between several factors responsible for the main magnetic features evidenced in Figure .…”

“…However, in spite of those advances, a broad variation of the NPs' magnetic properties is usually found, even for NPs with similar size and composition . Such broad dispersion has been typically ascribed to the presence of size and shape distributions, stoichiometry gradients, interparticle interactions, and structural defects, which depend strongly on the synthesis procedure and whose contribution is difficult to identify .…”