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June 29, 2009

Cobalt atoms and carbon rings proposed as subnanometer magnetic storage bits


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 (1000 times more). As a model system, Co2-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.

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 where k is the Boltzmann constant and T is the temperature.

More recently, the magnetic properties of transition metal dimers came into the focus
of interest. Isolated magnetic dimers are the smallest chemical objects that possess a magnetic anisotropy as their energy depends on the relative orientation between dimer axis and magnetic moment. Huge MAE values of up to 100 meV per atom were predicted by density functional (DFT) calculations for the cobalt dimer.







The researchers predict that bonding of Co dimers on hexagonal carbon rings like benzene or graphene results in a perpendicular arrangement of the dimers with respect to the carbon plane and in a magnetic ground state. In this structure, a division of tasks takes place: while the Co atom closer to the carbon ring is responsible for the chemical bonding, the outer Co atom hosts the larger share of the magnetic moment. The huge magnetic anisotropy of the free dimer is preserved in this structure, since the degeneracy of the highest occupied 3d- orbital is not lifted in a hexagonal symmetry. Thus, it should be possible to circumvent the hitherto favored use of heavy metal substrates to achieve large magnetic anisotropies. On the contrary, robust and easy-to-prepare carbon-based substrates are well suited for this task. Once confirmed, the present results may constitute an important step towards a molecular magnetic storage technology.


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