Single crystalline magnetic FeCo nanostars were prepared using an organometallic approach under mild conditions. The fine tuning of the experimental conditions allowed the direct synthesis of these nano-octopods with body-centered cubic (bcc) structure through a one-pot reaction, contrarily to the seed-mediated growth classically used. The FeCo nanostars consist of 8 tetrahedrons exposing {311} facets, as revealed by high resolution transmission electron Microscope (HRTEM) imaging and electron tomography (ET), and exhibit a high magnetization comparable with the bulk one (Ms = 235 A.m².kg -1 ). Complex 3D spin configurations resulting from the competition between dipolar and exchange interactions are revealed by electron holography. This spin structures are stabilized by the high aspect ratio tetrahedral branches of the nanostars, as confirmed by micromagnetic simulations. This illustrates how magnetic properties can be significantly tuned by nanoscale shape control.
Epitaxial resolution of a mixture of nanostructures through growth on a crystallographically oriented surface.
Analysis of first-order reversal curves (FORCs) is a powerful tool to probe irreversible switching events in nanomagnet assemblies. As in essence switching events are related to the intrinsic properties of the constituents and their interactions, resulting FORC diagrams contain much information that can be crosslinked and complex to deconvoluate. In order to quantify the relevant parameters that drive the FORC diagrams of arrays of perpendicularly magnetized nanomagnets, we present step-by-step simulations of assemblies of hysterons to determine the specific signatures related to different known inputs. While we explored the consequences of dipolar interactions using either mean field or magnetostatic approaches, we completed by taking the hysteron switching-field distribution (SFD) as either normal or log-normal. We demonstrated that the transition between FORC diagrams composed of an isolated interaction field distribution (IFD) and a wishbone shape operates via the SFD deviation, 𝜎 𝐻 𝑠𝑤 , in the presence of a weakly dispersed interaction field. In the presence of a magnetostatic interaction field, the IFD profile is peaked and a coercive field distribution (CFD) sums to the IFD as 𝜎 𝐻 𝑠𝑤 increases. A transition between IFD+CFD and wishbone shapes is clearly demonstrated as a function of the interaction field deviation 𝜎 𝐻 𝑖𝑛𝑡 . In addition, we demonstrate that whatever the considered cases, 𝜎 𝐻 𝑠𝑤 can be quantitatively extracted from the FORC diagrams within an error inferior to 10%. These findings are of interest for dipolar coupled perpendicularly magnetized nanomagnets, as in assemblies of magnetic nanowires and nanopillars, as well as bit patterned media.
The organization of nano-objects on macroscopic surfaces is a key challenge for the technological improvement and implementation of nanotechnologies. For achieving operational functions, it is required to assemble nano-objects as controllable building blocks in highly ordered superstructures. Here we demonstrate the growth and self-organization of metallic nanowires on surfaces into hexagonal superlattices with tunable characteristics lengths depending of the used stabilizing surfactants. Starting from a reacting mixture containing a Pt(111) substrate, a Co organo-metallic precursor, an amine and an acid dissolved in a solvent, we quantify the structural evolution of superlattices of vertical single-crystalline Co nanowires on Pt using a combined analysis of small angle neutron scattering, transmission and scanning electron microscopies. It is shown the concerted steps of growth and self-organization of the nanowires into a spontaneous 2D hexagonal lattice on Pt, at intervals starting from a few hours of reaction to a highly ordered superlattice at longer times. Furthermore, it is shown that apart from long-chain acid and long chain aliphatic amine pairs used as stabilizers, the combination of a long chain aliphatic and a short aromatic ligand in the synthesis can be also employed for the nanowire superlattices development. Interestingly the possibility to employ different pairs allows quantitative modulation of the nanowire arrays, such as the interwire distance and the packing fraction.
Ultradense arrays of magnetic nanoelements present considerable interest for extending areal densities in magnetic recording media, provided that they display high switching fields and corresponding low standard deviations. Here, we report the switching field distribution of bottom–up synthesized single-crystalline vertical Co nanowires self-organized in 2D hexagonal superlattices. The combined shape and Co hexagonal compact magnetocrystalline anisotropies in individual nanowires of diameter as small as 6 nm define a robust perpendicular magnetic anisotropy despite important interactions in superlattices of 10 × 1012 NWs/in2. Using quantitative analysis of temperature-dependent first-order reversal curves, we capture the switching field distribution in this dipolar-coupled perpendicularly magnetized nanomagnets. First, the interwire dipolar interactions are treated separately and show a dominant mean field character with temperature independent amplitudes that scale with the nanowire packing fraction. Then, the intrinsic switching field distribution, namely, independent of interwire interactions, is determined as a function of temperature in the 5–300 K range. The mean value and deviation are both found to be driven by the intrawire dipolar interaction and the temperature-dependent uniaxial magnetocrystalline anisotropy, but of smaller amplitudes than those expected from bulk behavior. With coercive fields ranging between 0.3 and 0.8 T, the switching field deviations relative to coercivity reach 20%, which is a moderate value regarding pitch arrays as small as 8 nm.
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