Uniform Magnetic Properties for an Ultrahigh-Density Lattice of Noninteracting Co Nanostructures

Abstract

We report on the magnetic properties of two-dimensional Co nanoparticles arranged in macroscopically phase-coherent superlattices created by self-assembly on Au(788). Our particles have a density of 26 Tera=in 2 (1 Tera 10 12 ), are monodomain, and have uniaxial out-of-plane anisotropy. The distribution of the magnetic anisotropy energies has a half width at half maximum of 17%, a factor of 2 more narrow than the best results reported for superlattices of three-dimensional nanoparticles. Our data show the absence of magnetic interactions between the particles. Co=Au788 thus constitutes an ideal model system to explore the ultimate density limit of magnetic recording. DOI: 10.1103/PhysRevLett.95.157204 PACS numbers: 75.75.+a, 75.30.Gw, 81.16.Dn The bit density on magnetic hard disks has been increasing at a constant pace for many years [1]. Besides the technological challenges, we face today the question of where downscaling ends from fundamental physics. This question can be addressed by studying periodic lattices of ferromagnetic monodomain particles, where each particle stores one magnetic bit. The high density requires out-ofplane magnetic anisotropy to minimize dipolar interactions among adjacent particles. Further, to optimize the signalto-noise ratio in read or write processes, the magnetic properties have to be uniform; i.e., the particles have to be uniaxial and the distributions of moments M, and magnetic anisotropy energies (MAEs), K, have to be narrow.Chemical synthesis of self-assembled colloid particles has led to excellent size distributions with a half width at half maximum (HWHM) of 7% in diameter and 21% in volume, respectively [1,2]. However, these are accompanied by systematically much wider K distributions [3,4] partly caused by the random orientation of the particle's easy axes [4,5] causing strong dipolar interactions. Colloid particles have obvious practical advantages, one of them being the high blocking temperatures [6]. However, for the outlined reasons, the ultimate density limit could not be explored with such systems so far. Alternatively, massselected magnetic 3D clusters [7] are monodisperse and may be soft-landed onto surfaces [8], but cannot be arranged into ordered arrays of equidistant magnetic units.Self-assembly during atomic beam epitaxy on periodic strain relief patterns on single crystal surfaces has been shown to provide well ordered superlattices of 2D islands with narrow size distributions [9]. While such lattices are generally not in phase from terrace to terrace, a combination of epitaxial strain relief patterns and vicinal surfaces has been shown to lead to macroscopically phase-coherent lattices [10]. Former studies of the magnetic properties of metal islands on single crystal surfaces revealed that the spin-orbit interaction with the substrate may induce a strong enough magnetocrystalline anisotropy to yield a common out-of-plane easy magnetization axis for all islands [11][12][13][14]. Therefore we have today the tools to fabricate the required lattices of uni...

Nucleation and growth of nickel nanoclusters on graphene Moiré on Rh(111)

“…One of the promising ways to produce arrays of homogeneously distributed monodispersed NCs is to use a bottom-up approach where self-organization growth phenomena on template substrates are used. So far, regular arrays of NCs were successfully assembled using surfaces such as alumina double layers on Ni 3 Al [2,3], vicinal Au(111) surfaces [4,5], reconstructed surfaces [6,7] or h-BN nanomesh [8,9]. Recently, graphene Moiré on close-packed metal surfaces like Pt(111) [10], Rh(111) [11], Ru(0001) [12,13], and Ir(111) [14,15] has been suggested to be a good candidate for the templated growth of clusters arrays.…”

Growth of Co Nanomagnet Arrays with Enhanced Magnetic Anisotropy

“…The observed MAE distributions becomes sharper when the volume of the Co nanodot increases, which is assigned to a more homogeneous volume distribution for larger nanodots, as seen in the STM analysis. Notably, K is not observed to scale with the number of atoms N in the dot, but rather with $\sqrt{N}$, in a similar way as observed for Co nanodots on Au(788) or Pt(111) 9, 40. This can be explained by an anisotropy constant K that is mainly determined by perimeter atoms, which have lower coordination.…”

“…Considering the number of perimeter atoms deduced from the STM images (see Supporting Information), we obtain that K p is approximately (1.25 ± 0.2) meV/atom for 0.4 ML and 0.9 ML nanodot arrays, and it is slightly reduced to K p = (1.1 ± 0.2) meV/atom for the 1.3 ML array. Thus, K is notably larger in Co nanodots grown on trigons than in those grown on Pt(111), Au(788), or Au(11,12,12) [0.8–0.9 meV/atom] 3, 9, 40…”

“…Larger MAE values that are observed in nanostructures with reduced size are mainly attributed to the MC contribution produced by reduced coordination of the surface/rim atoms which avoid the quenching of their orbital moments 9, 41. Furthermore the Au atoms surrounding the Co dots have to be taken into account.…”

“…For this purpose, self‐organized growth of ferromagnetic metals has been typically performed on surfaces that exhibit spontaneous reconstructions or at vicinal surfaces. Among the most widely studied materials are Co and Fe, which grow, e.g., as atomically thick wires on vicinal Pt(997),4, 5 or as nanoclusters on Au(111) or Au(788) 6, 7, 8, 9, 10, 11. Another possibility to create templates is profiting the lattice mismatch between a substrate and a monolayer‐thick film of a different material that can give rise to the formation of Moiré patterns which in many occasions are found to work as chemically and structurally stable templates for the growth of nanostructures.…”