Exceedingly Large Cubic Zeeman Terms for a High Spin Cobalt(II) Complex Originated from the Substantial Unquenched...

Area of Science:

• Quantum Chemistry
• Magnetochemistry
• Computational Materials Science

Background:

• The spin Hamiltonian (SH) is standard for modeling magnetic properties of transition metal complexes.
• Linear Zeeman terms suffice for S = 1/2, 1, and higher S systems where higher-order terms are negligible.
• Cobalt(II) complexes like CoTp2 (S=3/2) exhibit significant cubic Zeeman terms, challenging conventional models.

Purpose of the Study:

• To elucidate the physical origin of the unusually large cubic Zeeman term in CoTp2.
• To understand the conditions under which higher-order Zeeman terms become significant in high-spin systems.

Main Methods:

• Highly correlated wave function-based ab initio calculations.
• Effective Hamiltonian analysis.
• Comparison with experimental zero-field splitting and g-factors.

Main Results:

• Ab initio calculations successfully reproduced experimental magnetic properties of CoTp2.
• Efficient spin-orbit coupling within the 4E manifold introduces disproportionate orbital angular momentum.
• The total magnetic moment does not scale linearly with pseudospin operators, necessitating cubic Zeeman terms.

Conclusions:

• Cubic Zeeman terms are essential for accurately modeling magnetic moments in CoTp2 and similar high-spin complexes with orbital near-degeneracies.
• When the energy gap between orbital components is less than 2ζ, cubic Zeeman terms become comparable to linear terms, making them indispensable for S ≥ 3/2 systems.