The remarkable difference between surface and step atoms in the magnetic anisotropy of two-dimensional nanostructures

Abstract

ARTICLES 546nature materials | VOL 2 | AUGUST 2003 | www.nature.com/naturematerials E xploring the ultimate density limits of magnetic information storage, whether on computer hard disks or in MRAMs (magnetic random access memories), requires elaborate tuning of the preferred (easy) magnetization axis, of the magnetic anisotropy energy, and of the magnetic moment in the units used to store a bit. These units are single-domain particles (with diameter d < 20 nm) where the magnetic moments of all atoms are ferromagnetically aligned 1 to form the overall magnetic moment of the particle M, which is also called the macrospin. The preferred orientations of M, and the anisotropy energy barriers K separating them, are given by the delicate balance between several competing energies. These are the magnetocrystalline bulk anisotropy, its surface and step counterparts, and the shape anisotropy, or demagnetizing energy, resulting from the interaction of M with its own dipolar stray field. Unravelling the anisotropy's origin is far from trivial due to the competition between these energies 2 . This is unfortunate because the anisotropy is one of the key quantities: it defines the stability of the magnetization direction against thermal excitation,and therefore the minimum particle size for which non-volatile information storage may be achieved (at 300 K this requires K ≥ 1.2 eV). A further key parameter is the modulus of M, M defining the dipolar stray field used to read and write, but also mediating interactions between adjacent bits. These interactions are minimized for out-of-plane magnetization, and because the ultimate limit of single-particle bits may only be achieved for uniaxial systems, uniaxial out-of-plane systems are best suited to explore the ultimate density limit of magnetic recording 3,4 . Current studies attempting to identify the origin of magnetic anisotropy mainly deal with two model systems. These are colloids or three-dimensional (3D) nanoparticles, and 2D nanostructures created by molecular-beam epitaxy at single-crystal surfaces. For colloid particles, remarkable progress has been achieved in monodispersity 5 , their self-assembly into 2D superlattices 6,7 and in the accomplished anisotropy energies per constituent atom 8 . Despite their promising properties for applications,3D nanoparticles present several difficulties for tracing back the origin of anisotropy. First, although the magnetism of a single particle can be addressed 9 , it is almost impossible to study the morphology of the very same particle in conjunction with its magnetism. Second, the particles frequently have a few atomic layers of oxide at their surface, which is not ferromagnetic

First-principles studies of complex magnetism in Mn nanostructures on the Fe(001) surface

“…19,27 With the experimental techniques now available, nanowires and nanostructures may be deposited on magnetic and nonmagnetic surfaces in a controlled fashion, and their fundamental magnetic properties can be explored with the use of advanced experimental methods. [28][29][30][31][32][33][34] From the above discussions, it is quite evident that the magnetic interactions at the interface between Mn and Fe have a very rich variety depending on the geometry and local environment. Particularly, the study of the nanostructures of Mn supported on Fe is quite fascinating.…”

Local Electronic Structure and Density of Edge and Facet Atoms at Rh Nanoclusters Self-Assembled on a Graphene Template

“…In this respect, experiments have shown that low C N atoms play a pivotal role. 9,10 When the particle's size is reduced to only 10À20 Å, two main challenges are posed: (i) the formation in a controllable and reproducible manner of large-scale replicas of these individual building blocks on a suitable substrate 11 and (ii) the determination and tuning of the geometric and electronic structure of the supported nanoobjects.One of the most recent solutions to meet the first challenge is to make use of template graphene (GR) for the growth of metallic nanoclusters and for the formation of long-range-ordered superstructures. This * Address correspondence to alessandro.baraldi@elettra.trieste.it.…”

“…In this respect, experiments have shown that low C N atoms play a pivotal role. 9,10 When the particle's size is reduced to only 10À20 Å, two main challenges are posed: (i) the formation in a controllable and reproducible manner of large-scale replicas of these individual building blocks on a suitable substrate 11 and (ii) the determination and tuning of the geometric and electronic structure of the supported nanoobjects.…”

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…”