Mapping of single-site magnetic anisotropy tensors in weakly coupled spin clusters by torque magnetometry

Abstract

Single-crystal torque magnetometry performed on weakly-coupled polynuclear systems provides access to a complete description of single-site anisotropy tensors. Variable-temperature, variable-field torque magnetometry was used to investigate triiron(III) complex [Fe3La(tea)2(dpm)6] (Fe3La), a lanthanum(III)-centred variant of tetrairon(III) single molecule magnets (Fe4) (H3tea = triethanolamine, Hdpm = dipivaloylmethane). Due to the presence of the diamagnetic lanthanoid, magnetic interactions among iron(III) ions (si = 5/2) are very weak (<0.1 cm(−1)) and the magnetic response of Fe3La is predominantly determined by single-site anisotropies. The local anisotropy tensors were found to have Di > 0 and to be quasi-axial with |Ei/Di| ~ 0.05. Their hard axes form an angle of approximately 70° with the threefold molecular axis, which therefore corresponds to an easy magnetic direction for the molecule. The resulting picture was supported by a High Frequency EPR investigation and by DFT calculations. Our study confirms that the array of peripheral iron(III) centres provides substantially noncollinear anisotropy contributions to the ground state of Fe4 complexes, which are of current interest in molecular magnetism and spintronics.

Experimental and Theoretical Studies on the Magnetic Anisotropy in Lanthanide(III)‐Centered Fe3Ln Propellers

“…Thus, Ln1O1 and Ln1N1 distances exhibit opposite trends upon contraction of the lanthanide ionic radius along the Tb–Tm series: as atomic number increases, a shortening of Ln1O1 bonds is accompanied by an elongation of Ln1N1 distance. Notice that the bigger La 3+ ion in Fe 3 La 19 affords La1O1=2.4826(16) Å but La1N1=2.758(3) Å, confirming that the LnN distance is not a simple function of lanthanide radius. In [Fe 3 Dy(tea) 2 (acac) 6 ], the bond lengths are similar to those of the Dy derivative herein presented, with DyO=2.344(5) and DyN=2.743(5) Å 18.…”

“…[b] Data from ref. 19. [c] Distortion of the LnO 6 polyhedron from octahedral geometry by trigonal compression/elongation.…”

“…14 In the first arrangement (propeller‐like, p ), the two oxygen donors of dpm − lie on opposite sides of the molecular plane, whereas in the second arrangement (sandwich‐like, s ) they are found on the same side. The p and s components have relative occupancies close to 40:60; by contrast, the p arrangement is preferred in Fe 3 La 19. Assuming independent coordination environments for the three iron(III) ions, the crystal is expected to contain a mixture of ppp , pps , ssp , and sss isomers, and their proportions are reported in Table S2 in the Supporting Information.…”

“…However, Ln‐centered species may be stabilized by triethanolamine (H 3 tea, see Scheme ), as shown by the low‐yield synthesis of [Fe 3 Ln(tea) 2 (acac) 6 ] (Ln=Gd, Dy; Hacac=acetylacetone) reported by Murray and co‐workers 18. By applying the same synthetic strategy used to assemble Fe 3 Cr16 and Fe 3 V17 species we soon found that [Fe 2 (μ‐OMe) 2 (dpm) 4 ] reacts with [H 3 La(tea) 2 ] to give large trigonal crystals of [Fe 3 La(tea) 2 (dpm) 6 ] ( Fe 3 La ) in high yield (Hdpm=dipivaloylmethane) 19. The whole series of “heavy”‐lanthanide(III) complexes Fe 3 Ln (Ln=Tb, Dy, Ho, Er, Tm, Yb) has now been prepared, and structurally and magnetically characterized.…”

A Pseudo‐Octahedral Cobalt(II) Complex with Bispyrazolylpyridine Ligands Acting as a Zero‐Field Single‐Molecule Magnet with Easy Axis Anisotropy

“…[36,37] The measured torque is thus given by Equation (2): [36,37] The measured torque is thus given by Equation (2):…”