Quantum spin dynamics

Abstract:

The heterobimetallic complex [Cu(II)Mn(III)(L)(2)(py)(4)](ClO(4))·EtOH (1) built using the pro-ligand 2,2'-biphenol (LH(2)), contains a rare example of a Jahn-Teller compressed Mn(III) centre. Dc magnetic susceptibility measurements on 1 reveal a strong antiferromagnetic exchange between the Cu(II) and Mn(III) ions mediated through the phenolate O-atoms (J = -33.4 cm(-1)), with magnetisation measurements at low temperatures and high fields suggesting significant anisotropy. Simulations of high-field and high frequency powder EPR data suggest a single-ion anisotropy D(Mn(III)) = +4.45 cm(-1). DFT calculations also yield an antiferromagnetic exchange for 1, though the magnitude is overestimated (J(DFT) = -71 cm(-1)). Calculations reveal that the antiferromagnetic interaction essentially stems from the Mn(d(x(2)-y(2)))-Cu(d(x(2)-y(2))) interaction. The computed single-ion anisotropy and cluster anisotropy also correlates well with experiment. A larger cluster anisotropy for the S = 3/2 state compared to the single-ion anisotropy of Mn(III) is rationalised on the basis of orbital mixing and various contributions that arise due to the spin-orbit interaction.

Abstract:

The model compounds (NBu(4))(2)[ReCl(4)(CN)(2)] (1), (DMF)(4)ZnReCl(4)(CN)(2) (2), and [(PY5Me(2))(2)Mn(2)ReCl(4)(CN)(2)](PF(6))(2) (3) have been synthesized to probe the origin of the magnetic anisotropy barrier in the one-dimensional coordination solid (DMF)(4)MnReCl(4)(CN)(2) (4). High-field electron paramagnetic resonance spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in compounds 1-3. These findings indicate that the onset of one-dimensional spin correlations within the chain compound 4 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior. Within this picture, it is the transverse E term associated with the Re(IV) centers that determines the easy axis and the anisotropy energy scale associated with the relaxation barrier. The results demonstrate for the first time that slow magnetic relaxation can be achieved through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy, thus providing a new direction in the design of single-molecule and single-chain magnets.

Abstract:

A pseudotetrahedral cobalt(II) complex with a positive axial zero-field splitting parameter of D = 12.7 cm(-1), as determined by high-field EPR spectroscopy, is shown to exhibit slow magnetic relaxation under an applied dc field.