Co Dimers on Hexagonal Carbon Rings Proposed as Subnanometer Magnetic Storage Bits

Abstract

It is demonstrated by means of density functional and ab-initio quantum chemical calculations, that transition metal -carbon systems have the potential to enhance the presently achievable area density of magnetic recording by three orders of magnitude. As a model system, Co 2 -benzene with a diameter of 0.5 nm is investigated. It shows a magnetic anisotropy in the order of 0.1 eV per molecule, large enough to store permanently one bit of information at temperatures considerably larger than 4 K. A similar performance can be expected, if cobalt dimers are deposited on graphene or on graphite. It is suggested that the subnanometer bits can be written by simultaneous application of a moderate magnetic and a strong electric field. PACS numbers: 31.15.es, 75.30.Gw, 75.75.+a Keywords: applications of density functional theory, magnetic anisotropy, magnetic properties of nanostructures 1 Long-term magnetic data storage requires that spontaneous magnetization reversals should occur significantly less often than once in ten years. This implies that the total magnetic anisotropy energy (MAE) of each magnetic particle should exceed 40 kT , 1 where k is the Boltzmann constant and T is the temperature. Among the elemental ferromagnets (Fe, Co, Ni, and Gd), cobalt metal shows the highest MAE, about 0.06 meV per atom in the hexagonal close packed structure. Thus, Co is the main ingredient of magnetic data storage materials at present. At room temperature, data loss due to fluctuations is avoided, if a Co grain contains not less than 40 k · 300 K/0.06 meV ≈ 15,000 atoms. In fact, the grain diameter of contemporary Co(Cr,Pt,SiO 2 ) recording media is close to 8 nm, each grain containing about 50,000 atoms and each bit being composed of some dozen grains.2 The grain size could be considerably reduced by using the intermetallic compounds FePt or CoPt with record MAE of almost 1 meV per atom in their structurally ordered L1 0 bulk phase. It is, however, hard to achieve the required ordered structure in nano-particles. 3Obviously, a further reduction of the bit size is primarily limited by the value of MAE per atom. Recent efforts to enhance this value were focused on single atoms or small clusters deposited on the surface of heavy metals. This approach combines two ideas: 4 Firstly, the magnitude of MAE is related to the size of the orbital moments. The latter are quenched for highly coordinated atoms but can be large if the coordination is low. Secondly, the magnitude of MAE is related to the strength of spin-orbit coupling which grows with atomic number. Considerable progress was achieved in this way by deposition of single Co atoms on a Pt surface, yielding a record MAE of 9 meV per Co atom. 5 Unfortunately, clusters of several Co atoms on Pt show a much smaller MAE per atom, roughly inversely proportional to the number of atoms. 5More recently, the magnetic properties of transition metal dimers came into the focus of interest. 6,7,8,9 Isolated magnetic dimers are the smallest chemical objects that possess a magnetic a...

Extremely large perpendicular magnetic anisotropy of an Fe(001) surface capped by5dtransition metal monolayers: A density functional study

“…In all 5d TM MLs, the FM states are energetically favorable, in agreement with previous work. 18,19,24,25 In these low dimensional systems, band narrowing enhances the densities of states (DOSs) at the Fermi level (E F ), thereby satisfying the Stoner criteria.…”

“…For instance, in a Co atom, a large MCA of 9.3 meV/atom (about 200 times larger than that in bulk Co) was observed in Co adsorbates on a Pt substrate. 23 Theoretical studies predict that extremely low-dimensional 4d and 5d TM systems, such as atomic dimers 24,25 and atomic chains, 18,19 can have fairly large MCAs on the order of tens of meV/atom, which are enhanced even more as the interatomic distances increase. Moreover, the small MCAs of Co and Fe are enhanced when Co monolayers (ML) on Au(111) 26 and Fe MLs on Pt(001) surfaces 27 are capped by additional Au and Pt layers, respectively.…”

Positioning of Single Co Atoms Steered by a Self-Assembled Organic Molecular Template

“…43 Employing an organic molecular template anchored on a surface may therefore prove to be a feasible route towards the construction of novel magnetic storage devices as proposed recently by theory. [23][24][25] Our work demonstrates the possibility of creating hybrid organometallic surface nanoarchitectures constructed from single atoms using an organic molecular precursor, which acts as an ordered template for atomic organization. Structures constructed in this fashion may open up new possibilities for the realization of novel nanomagnetic or spintronic concepts, whereby single atoms and their spins act as smallest functional units.…”

“…As shown in Figure 3a, the major part of the Co adsorbs monoatomically (imaged as yellow dots residing on the molecular template depicted as orange, dog-bone like protrusions), whereas defects, i.e., clusters with more than one atom, appear as white regions with irregular shape and height. So in the samples prepared in our approach Co-dimers as proposed by Xiao, et al 23,24 and Cao, et al 25 are formed only as rare events. However, with increasing coverage higher concentrations of Co-dimers may become more likely to be formed.…”

“…[23][24][25] To study the route towards the realization of these structures, in this work we employed a supramolecular engineering approach 26 to produce arrays of single magnetic atoms on top of a molecular template. Specifically, we used low-temperature scanning tunneling microscopy (STM) to investigate, first, the self-assembly N,N'-diphenyl oxalic amide molecules [27][28][29] on the Ag(111) surface and, second, the organization and bonding of Co atoms on the templated surface.…”