No abstract

Low-temperature scanning tunneling spectroscopy over Co nanoislands on Cu(111) showed that the surface states of the islands vary with their size. Occupied states exhibit a sizeable downward energy shift as the island size decreases. The position of the occupied states also significantly changes across the islands. Atomic-scale simulations and ab inito calculations demonstrate that the driving force for the observed shift is related to size-dependent mesoscopic relaxations in the nanoislands.PACS numbers: 73.20.At, It was recognized quite early that metallic particles exhibit unique properties that differ significantly from their bulk counterparts [1]. Nanoislands grown on metal surfaces, in particular, have been a matter of intense research for decades in view of prospective applications in a vast variety of domains ranging from magnetoelectronics, catalysis, optoelectronics, to data storage technology. The electronic, magnetic and chemical properties of a nanoisland are governed by the size, shape and structure of the island. These, in turn, are profoundly influenced by the lattice mismatch with the metal substrate and, for heteroepitaxial systems, also by the bonding interactions in the island/substrate interface (ligand effects). Metal islands tend to adopt the lattice parameter of the underlying surface [2], and as a consequence the bond lengths between the metal atoms in the supported nanoisland are different than those in the parent metals, resulting in changes due to strain. A theoretical study on homoepitaxial double layer Cu islands on Cu(111) [3], has shown that the strain produces an inhomogeneous distribution of bond lengths over the nanoisland, the average bond length varying with island size. The nanoislands also locally distort the surface and induce a displacement pattern in the substrate which affects the diffusion of atoms and, ultimately, the growth of the nanoislands.Despite these studies, our knowledge of how strain affects the properties of a metallic nanoisland, especially the electronic states, remains very limited. The fundamental problem of the change in energy upon lattice distortion in solids has first been addressed by J. Friedel within a simple model [4], and latter on extended to various problems of lattice contractions at metal surfaces and clusters [5]. On the same lines, it has been shown for thin films that strain effects, along with ligand effects, can cause a shift of the surface d band [6,7,8], resulting in chemical properties that are significantly different from those of the pure overlayer metal. Recently, a modification of electronic states due to a local strain field induced by a nanopattern formation has been observed for Cu(100) covered with N atoms [9].In this Letter, we specifically focus on the interplay between strain-induced structural relaxations and the surface states of Co nanoislands on Cu(111). These nanoislands constitute a reference system that has been extensively investigated by Scanning Tunneling Microscopy/Spectroscopy (STM/STS) [10,11,12,13,14]. By ...

By using an N-body potential scheme constructed by fitting the interaction parameters to accurate firstprinciples calculations, we investigate the structural stability of Co atoms and clusters deposited on Cu͑100͒. We found that Co atoms and clusters prefer to be embedded inside the substrate, in a way compatible with the formation of a surface alloy observed experimentally. Enhanced stability is achieved when Co atoms are deposited on a preformed Co cluster embedded on the uppermost layer of the substrate. Co atoms deposited on Co islands are best stabilized when they concur to complete the islands, by promoting layer-by-layer growth.Ultrathin films of ferromagnetic metals have found considerable interest in recent years due to their technological applications in the area of magneto-optical and transport properties. [1][2][3] In particular the growth of Fe and Co films on Cu͑001͒, which takes place pseudomorphically on the fcc substrate, has been investigated extensively.4-10 The quality of the grown layers and of the interfaces has a strong influence on properties like giant magnetoresistance, 5 magnetic anisotropy, 6,7 and oscillatory interlayer exchange coupling. 8,9Kief and Egelhoff 10 have reported the observation of nonideal film growth, characterized by the formation of compact Co clusters and the segregation of substituted Cu on the surface. Recently, the interfacial intermixing of ultrathin Co films on a Cu͑001͒ was observed, 11 despite the fact that Co and Cu are immiscible in the bulk. 12 The intermixing in the upper layers might not only be favored kinetically, but also energetically. 13In this paper we resort to a newly developed n-body interatomic potential scheme to ascertain the energetics of atoms and clusters of Co on the Cu͑001͒. A strong tendency for a direct exchange mechanism into the Cu layer is found. Our results demonstrate that at the initial stage of monolayer growth small Co clusters are formed in the Cu surface. We investigate the mechanism of adatom-cluster interactions and show how heteroepitaxial thin film growth takes place.Our approach is based on accurate first-principles calculations of selected cluster-substrate properties, which have been employed in the fitting of the potential parameters. This results in a manageable and inexpensive scheme able to account for structural relaxation and including implicitly magnetic effects, crucial for a realistic determination of interatomic interactions in systems having a magnetic nature.The potentials are formulated in the second moment tightbinding approximation ͑TB-SMA͒.14,15 The attractive term ͑band energy͒ E B i contains the many-body interaction. The repulsive term E R i is described by pair interactions ͑Born-Mayer form͒. The cohesive energy E coh is the sum of the band energy and repulsive part:. ͑3͒ r i j is the distance between the atoms i and j. r 0 ␣␤ is the first neighbor distance in the crystalline structures of the pure metals for atom-like interactions and becomes an adjustable parameter in the case of the cross interac...

No abstract

No abstract