Enhanced magnetic anisotropy in cobalt-carbide nanoparticles

Abstract

An outstanding problem in nano-magnetism is to stabilize the magnetic order in nanoparticles at room temperatures. For ordinary ferromagnetic materials, reduction in size leads to a decrease in the magnetic anisotropy resulting in superparamagnetic relaxations at nanoscopic sizes. In this work, we demonstrate that using wet chemical synthesis, it is possible to stabilize cobalt carbide nanoparticles which have blocking temperatures exceeding 570 K even for particles with magnetic domains of 8 nm. First principles theoretical investigations show that the observed behavior is rooted in the giant magnetocrystalline anisotropies due to controlled mixing between C p-and Co d-states. V

Using graphene to control magnetic anisotropy and interaction between supported clusters

“…The enhancement is driven by the modest interaction that changes the atomic structure and couples to selected orbitals to stabilize the orientation of the magnetic moment via the intrinsic SOI. The present work complements a recent development where metal-carbon nanoparticles with size ≈8 nm have been found to have blocking temperature (TB) in excess of 570 K [5]. These nanoparticles have layers of Co atoms separated by carbon layers and our studies have shown that the large MAE is driven by the reduced mixing between C and Co states.…”

“…E(graphene), E(Co n ), and E(Co n on graphene) are the total energies of the graphene sheet, free Co n cluster, and that of the supported species, respectively. We find that Co 5 and Co 13 are bound to graphene with a BE of 1.02 and 2.24 eV, respectively, which are lower than the BE of a free Co-C dimer (3.20 eV). This indicates that the clusters are only moderately bound to graphene but enough to prevent the detachment and migration of clusters on the surface under ambient temperature.…”

“…These features emerge on a substrate that is neither magnetic nor has significant SOI. We show that Co 5 and Co 13 clusters, that have a low magnetic anisotropy of 0.9 meV (10.4 K) and 0.12 meV (1.4 K) respectively, undergo substantial enhancement of MAE when supported on graphene. The enhancement is driven by the modest interaction that changes the atomic structure and couples to selected orbitals to stabilize the orientation of the magnetic moment via the intrinsic SOI.…”

“…The principal quantity controlling the applications of magnetic clusters/nanoparticles is the magnetic anisotropy energy (MAE) [1][2][3] that determines the minimum energy for disorienting the direction of magnetization [4,5]. For small clusters containing around dozen atoms, the MAE is only of the order of few kelvins and one of the customary approaches to increase the MAE is then to deposit clusters on substrates like Pt marked by strong spin-orbit interaction (SOI) [4,6].…”

Stable magnetic order and charge induced rotation of magnetization in nano-clusters

“…However, recent studies have indicated that the transition metal based nanoparticles where the transition metal layers are separated by carbon (C) layers, exhibit large MAE. 31,32 We have also shown recently that small Co clusters diluted with C undergo significant enhancement in MAE. 33 Such enhancement of MAE in transition metal carbide (TMC) results due to the mixing of p states of C with d states of transition metal.…”