Quantifying magnetic anisotropy using X-ray and neutron diffraction

Abstract

In this work, the magnetic anisotropy in two iso-structural distorted tetrahedral Co(II) complexes, CoX 2tmtu2 [X = Cl(1) and Br(2), tmtu = tetramethylthiourea] is investigated, using a combination of polarized neutron diffraction (PND), very low-temperature high-resolution synchrotron X-ray diffraction and CASSCF/NEVPT2 ab initio calculations. Here, it was found consistently among all methods that the compounds have an easy axis of magnetization pointing nearly along the bisector of the compression angle, with minute deviations between PND and theory. Importantly, this work represents the first derivation of the atomic susceptibility tensor based on powder PND for a single-molecule magnet and the comparison thereof with ab initio calculations and high-resolution X-ray diffraction. Theoretical ab initio ligand field theory (AILFT) analysis finds the d xy orbital to be stabilized relative to the d xz and d yz orbitals, thus providing the intuitive explanation for the presence of a negative zero-field splitting parameter, D, from coupling and thus mixing of d xy and d x 2 − y 2 . Experimental d-orbital populations support this interpretation, showing in addition that the metal–ligand covalency is larger for Br-ligated 2 than for Cl-ligated 1.

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

“…From this, a similar susceptibility ellipsoid was found, Figure (bottom), as well as a similar g z direction, Figure (10.1° offset to PNPD easy axis). The found discrepancies between easy axis directions from ab initio calculations and PND/PNPD measurements are of similar magnitudes to those found by other studies. , …”

“…In contrast, the local magnetic anisotropy in paramagnetic compounds can be quantified using PND via the site susceptibility model . The method has been successfully used to determine the atomic susceptibility tensors in several SMMs. ,,,,− Recently, the applicability of the method was extended to powder samples (polarized neutron powder diffraction, PNPD), alleviating the demanding need for large single-crystals and time-consuming measurements at multiple crystal orientations. However, this remarkable progress has so far only been exploited on a single occasion to study the atomic susceptibility tensor, without independent verification from the single-crystal method.…”

“… 12 , 46 , 48 , 51 , 67 − 69 Recently, the applicability of the method was extended to powder samples (polarized neutron powder diffraction, PNPD), 50 alleviating the demanding need for large single-crystals and time-consuming measurements at multiple crystal orientations. However, this remarkable progress has so far only been exploited on a single occasion to study the atomic susceptibility tensor, 51 without independent verification from the single-crystal method. Importantly, the present work thus includes benchmarking the two methods, PNPD and single crystal PND, on the same compound.…”

“…The found discrepancies between easy axis directions from ab initio calculations and PND/PNPD measurements are of similar magnitudes to those found by other studies. 12,51 ■ CONCLUSIONS In summary, we have employed a set of sophisticated techniques, including static and dynamic magnetometry, EED based on high-quality, low-temperature synchrotron diffraction data, powder and single-crystal PND, and ab initio electronic structure analysis to decipher the origin and quantify the large magnetic anisotropy of a new Co II based SIM. This Co II SIM with two chelating diamido ligands and strongly distorted local tetrahedral geometry around Co II features a large energy separation of over 300 K between the ground and excited-state KDs and shows magnetic hysteresis up to 3.5 K. The slow relaxation dynamics are preserved upon magnetic dilution and in frozen solution, indicating that it is an intrinsic molecular property of 1.…”

“…This has been utilized in a few cases to study the correlation of magnetic anisotropy with the local coordination structure. ,− The major drawback of this method is the requirement of +1 mm 3 single crystals suitable for diffraction. However, it has recently been shown by some of us that the atomic susceptibility tensor can also be obtained reliably from powder PND measurements, ,, substantially advancing this method as it is now pertinent to nearly all materials. These experimentally observable parameters also give confidence and reliability to the results obtained from theoretical calculations that are crucial for making significant progress toward the realization of high-performance SMMs.…”