Cobalt single-molecule magnet

Abstract

The first cobalt molecule to function as a single-molecule magnet, [Co 4 (hmp) 4 (MeOH) 4 Cl 4 ], where hmpis the anion of hydroxymethylpyridine, is reported.The core of the molecule consists of four Co(II) cations and four hmpoxygen atoms ions at the corners of a cube. Variable-field and variable-temperature magnetization data have been analyzed to establish that the molecule has a S=6 ground state with considerable negative magnetoanisotropy. Single-ion zero-field interactions (DS z 2 ) at each cobalt ion are the origin of the negative magnetoanisotropy. A single-crystal of the compound was studied by means of a micro-SQUID magnetometer in the range of 0.040-1.0K.Hysteresis was found in the magnetization versus magnetic field response of this single crystal. It is concluded that this is the first cobalt molecule to function as a singlemolecule magnet.

Electronic structure and magnetic anisotropy of the [Co4(hmp)4(CH3OH)4Cl4] molecule

“…This observation opens up an exciting possibility of altering the MAE by manipulating the pyridine-pyridine separations, in particular, by the application of pressure. It may be mentioned here that Yang et al [2] have established that the magnetization barrier is ∼ 100-200 K. The large discrepancy between the theoretical and experimental values further supports the possibility that the MAE may exhibit significant pressure dependencies. So crystal packing effects may play an important role here.…”

“…Observation of such behavior in a molecular magnet is greatly facilitated by a reasonably high net spin and a large magnetic anisotropy energy (MAE). A very recent experimental report of a single molecule magnet consisting of Co 4 (hmp) 4 (CH 3 OH) 4 Cl 4 where hmp-is the deprotonated hydroxymethylpyridine, suggests that this molecule is quite promising since the magnetic anistropy energy per transition metal atom is high ( 25-50K) [2] compared to other magnetic molecules Fe 8 -tacn [3], Mn 12 -acetate [4,5,6,7] where it is ( 3-6K). The reported ferromagnetic ordering of the Co 4 molecule also differs qualitatively from the ferrimagnetic spin ordering observed in the Mn 12 and Fe 8 .…”

“…One possible way of achieving an overall easy axis is through the orthogonal hard axis alignment model in which each cobalt exhibits a local easy plane and the intersection of easy planes between the lower and upper Co atoms determines an easy axis [2]. Another way is for each atom to exhibit a local easy axis along the global uniaxis.…”

“…Density functional theory [8,9] based allelectron, spin-polarized calculations were carried out with the NRLMOL code [10,11] within the generalized gradient approximation to the exchange-correlation functional [12]. In the experimentally obtained Co 4 cluster [2], the four cobalt atoms in the molecule are bonded to organic hydroxy-methyl-pyridine(hmp) ligands ( Fig. 1(a)).…”

Neutron scattering and magnetic observables forS=1∕2spin clusters and molecular magnets

“…We show the predictions of Eqs. (37,43) for in-plane scattering, with momentum transfer q = π/a. Since this material has two strong bonds and a weak dimer (α ≈ 9) [14], |λ should be the ground state, and the |λ → |ρ and |λ → |σ transitions shown in the figure should both be observable (These are expected at 4.2 meV and 7.0 meV respectively, given the parameters of Luban et al) The very different angular distributions predicted for the scattered neutrons show that it should be straightforward to distinguish between these transitions in an inelastic neutron scattering experiment, given a single crystal of this or a similar trimer material.…”