Electronic structure and magnetic anisotropy for nickel-based molecular magnets

Abstract

Recent magnetic measurements on tetra-nickel molecular magnets [Ni(hmp)(ROH)Cl]4, where R=CH3, CH2CH3, or (CH2)2C(CH3)3 and hmp − is the monoanion of 2-hydroxymethylpyridine, revealed a strong exchange bias prior to the external magnetic field reversal as well as anomalies in electron paramagnetic resonance peaks at low temperatures. To understand the exchange bias and observed anomalies, we calculate the electronic structure and magnetic properties for the Ni4 molecules with the three different ligands, employing density-functional theory. Considering the optimized structure with possible collinear spin configurations, we determine a total spin of the lowest-energy state to be S = 0, which does not agree with experiment. We also calculate magnetic anisotropy barriers for all three types of Ni4 molecules to be in the range of 4-6 K. which has a total ground-state spin of S = 10 and magnetic anisotropy barrier of 65 K. 2,3 Magnetic hysteresis loop measurements on Mn 12 -acetate showed quantum tunneling of magnetic moment through the magnetic anisotropy barrier when the external magnetic field was applied along the easy axis. 3

Exchange coupling and contribution of induced orbital angular momentum of low-spinFe3+ions to magnetic anisotropy in cyanide-bridgedFe2Park1, Holmes2 2006Phys. Rev. BSelf Cite28060View full textAdd to dashboardCite

“…However, for the Mn 2+ , Co 2+ , and Ni 2+ ions, the ligand fields are not so strong that high-spin states are preferred for the ground state. ͑Refer to Table I for nominal spin and orbital angular momenta for the metal ions.͒ This feature is corroborated in the calculated projected density of states ͑DOS͒ onto 43 although experimental data suggested an S = 4 ferromagnetic ground state. 44 In DFT, it is assumed that a single Slater's determinant is a good approximation to a ground state.…”

Strongly Correlated Electrons in the[Ni(hmp)(ROH)X]4Single Molecule Magnet: A<

“…Not only were the early theoretical attempts unable to reproduce the correct ground state, but the resulting coupling constants were also found to be antiferromagnetic and orders of magnitude higher than the experimental values [17]. It was also found in the calculation that the spin density is not quite localized around the nickel atoms, as expected.…”

Self-Interaction Corrections Within the Fermi-Orbital-Based Formalism

“…In systems composed of lighter elements, such as metaloxide-based molecular magnets, DFT often qualitatively provides correct electronic structures and magnetic anisotropies but the size of the inter-ionic exchange parameters, and therefore spin-excitations, are overestimated within DFT (Postnikov et al, 2006), due to slight delocalization of the d-electrons. However, calculation on a class of Ni 4 O 4 molecular magnets has proven that standard DFT approaches can lead to qualitatively incorrect results (Cao et al, 2008;Park et al, 2005). Inclusion of a Hubbard U treatment, a semiempirical cousin of SIC, has been shown to correctly predict the spin-ordering (Cao et al, 2008) in systems where DFT fails.…”