Combined first-principles and thermodynamic approach toM-nitronyl nitroxide(M= Co, Mn) spin helices

Abstract

The properties of two molecular-based magnetic helices, composed of 3d metal Co and Mn ions bridged by Nitronyl Nitroxide radicals, are investigated by density functional calculations. Their peculiar and distinctive magnetic behavior is here elucidated by a thorough description of their magnetic, electronic, and anisotropy properties. Metal ions are antiferromagnetically coupled with the radicals, leading to a ferrimagnetically ordered ground state. A strong metal-radical exchange coupling is found, about 44 meV and 48 meV for Co-and Mn-helices, respectively. The latter have also relevant next-nearest-neighbor Mn-Mn antiferromagnetic interactions (of ∼ 6 meV). Co-sites are characterized by non-collinear uniaxial anisotropies, whereas Mn-sites are rather isotropic. A key result pertains to the Co-helix: the microscopic picture resulting from density-functional calculations allows us to propose a spin Hamiltonian of increased complexity with respect to the commonly employed Ising Hamiltonian, suitable for the study of finite-temperature behavior, and that seems to clarify the puzzling scenario of multiple characteristic energy scales observed in experiments.

Origin of Magnetic Relaxation Barriers in a Family of Cobalt(II)–Radical Single-Chain Magnets: Density Functional Theory and Ab Initio Calculations

“…Tight convergence criteria were selected to ensure that the results are well converged with respect to technical parameters. Scarrozza et al 18 were omitted. The two calculated model structures of 1_a and 2_a including two radicals and one Co II ion extracted from compounds 1 and 2, respectively, and the complete molecular structure of 3 are shown in Figure 2.…”

“…Picozzi et al 18 reported one cobalt-based SCM, CoPhOMe, which possesses uniaxial anisotropy with a D z of 46 K (32 cm −1 ) by DFT calculation. They deduced two equations to obtain Δ ξ .…”

“…Some good-performance Co­(II)-based SCMs (Co­(hfac) 2 ( o -CnPNN) ( n = 2, 3, 5) , and Co­(hfac) 2 ( p -CnPNN) ( n = 4, 5) , ) were researched because of their wide magnetic relaxation and coercive fields. In addition, strong metal–radical antiferromagnetic exchange coupling was found in Co­(hfac) 2 (NITPhOMe), about 44 meV (354.9 cm –1 ) for the Co II site, which is characterized by noncollinear uniaxial anisotropies. In 2018, density functional theory (DFT) calculations of the metal–organic complexes Co-(hfac) 2 (NNPhOMe) 2 and Co­(hfac) 2 (NNT) 2 demonstrate that antiferromagnetic spin–spin interactions exist, −328 K (−229.37 cm –1 ) and –1.25 K (−0.87 cm –1 ), respectively, because of the bonding overlap between Co II and the nitroxide contributions in the singly occupied molecular orbitals (SOMOs) .…”

Physical properties of a quasi-1D Ising S = ½ spin system: Ba4CoPt2O9+Δ

“…In practice however, canting phenomena are often encountered in 1D Ising compounds, leading to varying angles ±  between the easy-axes and the chain direction. It is the case for instance in the molecular compound Co(hfac)2-NITPhOMe [7] which is the archetype of the Single-Chain Magnets [8,9]; It was recently demonstrated that the noncollinearity among the local anisotropy axis compels to deeply reconsider the theoretical description of the spin dynamics in this material [10].…”

Magnetic Exchange Interaction in Nitronyl Nitroxide Radical-Based Single Crystals of 3d Metal Complexes: A Combined Experimental and Theoretical Study

“…In this paper, we report synthesis and crystal structures and magnetic properties of two nitronyl nitroxide radicals { R 1 = 4′-methoxy-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (NNPhOMe) and R 2 = 2-(2′-thienyl)-4,4,5,5-tetramethylimidazoline 3-oxide 1-oxyl (NNT)}-based four metal complexes. The theoretical analysis of magnetic properties using density functional theory (DFT), quantum chemistry, thermodynamic calculations, and so forth is commonly used for calculating the exchange interactions and exchange-coupling constant. − Different computational methods have been employed to elucidate the mechanism of spin–spin interaction, the magnitude of effective exchange integrals, and the nature of the participating spin-active MOs. , The influence of the used basis set was emphasized in some of the studies. The model systems were excerpts from various experimentally obtained nitronyl nitroxide-based complexes with transition metal ions, which contained or did not contain unpaired electrons.…”