Magnetic anisotropies of Ho(III) and Dy(III) single-molecule magnets experimentally determined via polarized neutron diffraction

Multi-Technique Experimental Benchmarking of the Local Magnetic Anisotropy of a Cobalt(II) Single-Ion Magnet

A comprehensive understanding of the ligand field and its influence on the degeneracy and population of d-orbitals in a specific coordination environment are crucial for the rational design and enhancement of magnetic anisotropy of single-ion magnets (SIMs). Herein, we report the synthesis and comprehensive magnetic characterization of a highly anisotropic Co^II SIM, [L₂Co](TBA)₂ (L is an N,N'-chelating oxanilido ligand), that is stable under ambient conditions. Dynamic magnetization measurements show that this SIM exhibits a large energy barrier to spin reversal U _eff > 300 K and magnetic blocking up to 3.5 K, and the property is retained in a frozen solution. Low-temperature single-crystal synchrotron X-ray diffraction used to determine the experimental electron density gave access to Co d-orbital populations and a derived U _eff, 261 cm^-1, when the coupling between the d _{x ² - y ²} and d_xy orbitals is taken into account, in very good agreement with ab initio calculations and superconducting quantum interference device results. Powder and single-crystal polarized neutron diffraction (PNPD, PND) have been used to quantify the magnetic anisotropy via the atomic susceptibility tensor, revealing that the easy axis of magnetization is pointing along the N-Co-N' bisectors of the N,N'-chelating ligands (3.4° offset), close to the molecular axis, in good agreement with complete active space self-consistent field/N-electron valence perturbation theory to second order ab initio calculations. This study provides benchmarking for two methods, PNPD and single-crystal PND, on the same 3d SIM, and key benchmarking for current theoretical methods to determine local magnetic anisotropy parameters.

Experimental and theoretical investigations on three DyIII4 single molecule magnets: structural and magneto-structural correlations

The work in this report describes the syntheses, crystal structures, dc/ac magnetic behaviour, and theoretical calculations (both ab initio CASSCF and DFT) of three defect dicubane/planar butterfly type tetradysprosium(III) compounds of compositions [Dy^III₄L₄(μ₃-OH)₂(carboxylate)₂(dmf)₂] (carboxylate = formate (1), acetate (2), propionate (3)), where H₂L = 2-(2-hydroxy-3-ethoxybenzylideneamino)phenol. In the butterfly type structures, two Dy^III centres (Dy_b) occupy the body positions while two other (Dy_w) units occupy the wing positions. SHAPE analyses reveal that the coordination geometries of the Dy_b and Dy_w centres, both octacoordinated, are triangular dodecahedron (TDD) and square antiprism (SAPR), respectively. Variable-temperature magnetic susceptibility measurements give an indication of weak antiferromagnetic interactions and variable-field magnetization measurements reveal strong anisotropy in all the three compounds. The variable-temperature/frequency in-phase/out-of-phase AC susceptibility data reveal that all these three compounds are SMMs with two relaxation channels under zero dc field; slow relaxation (SR) and fast relaxation (FR) processes could be assigned to the SAPR (Dy_w) and TDD (Dy_b) metal centres, respectively. The simulated U_eff and τ₀ values are: 49.0 cm^-1 and 1.76 × 10^-7 s for 1, 30.3 cm^-1 and 1.51 × 10^-8 s for 2 and 23.4 cm^-1 and 9.64 × 10^-7 s for 3. Furthermore, ab initio CASSCF/RASSI-SO/SINGLE_ANISO calculations reveal that the ground state of Dy^III centres are axial in nature with a dominating contribution from m_J = |±15/2>. The magnetization relaxation occurs via the first excited KD resulting in the large computed blocking barrier of Dy_w (SAPR) centres compared to that of the Dy_b (TDD) centres which corroborates the experimental measurements. The exchange parameters obtained from DFT calculations are generally in line with those obtained from the fitting of χ_MT vs. T in POLY_ANISO calculations. Interesting structural and magneto-structural correlations have been found, which are the major outcomes of this investigation.