Tuning Transverse Anisotropy in Co^III–Co^II–Co^III Mixed-Valence Complex toward Slow Magnetic Relaxation

Abstract

Two cobalt mixed-valence complexes with different substituents have been prepared and structurally characterized by single-crystal X-ray diffraction to alter slow magnetic relaxation by tailoring the transverse anisotropy. The trinuclear complexes [(L(1))4Co3(H2O)2](NO3)4·CH3OH·5H2O (1-NO3) and [(L(2))4Co3(H2O)2](NO3)4·6H2O (2-NO3) feature a distorted octahedral Co(II) strongly hindered in a trinuclear Co(III)-Co(II)-Co(III) mixed-valence array. Detailed magnetic studies of 1-NO3 and 2-NO3 have been conducted using direct- and alternating-current magnetic susceptibility data. In accordance with variable-field magnetic susceptibility data at low temperatures, high-field electron paramagnetic resonance (HF-EPR) spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in complexes 1-NO3 and 2-NO3. These findings indicate that the onset of the variation of distortion within complex 2-NO3 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior. Consequently, the anisotropy energy scale associated with the relaxation barrier, 5.46 cm(-1) (τo = 1.03 × 10(-5) s), is determined by the transverse E term. The results demonstrate that slow magnetic relaxation can be switched through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy.

Origin of slow magnetic relaxation in Kramers ions with non-uniaxial anisotropy

“…1b), whose up and down field-sweeping branches merge as H-0. This behaviour resembles closely that found for some other Co II ions with dominant easy-plane anisotropy [5][6][7][8][9] . Two intriguing questions that need to be addressed are, on the one hand, why do Co II spins with D40 relax so slowly at low T and, on the other, why is this relaxation only visible under finite magnetic fields.…”

“…The same or similar behaviour has thereafter been reported for a number of other Co II complexes [5][6][7][8][9] as well as for lineal Fe II complexes 10 . However, a detailed microscopic interpretation of this phenomenon is still missing.…”

The rise of 3-d single-ion magnets in molecular magnetism: towards materials from molecules?

“…Another pseudo-octahedral complex, [Co(abpt) 2 (tcm) 2 ] ( 62 ) (abpt = 4-amino-3,5-bis(2-pyridyl)-1,2,4-triazole and tcm = tricyanomethanide), has also been reported to exhibit easy plane anisotropy, with a spin reversal barrier of U eff = 85.7 K. This is the highest reported barrier thus far for this special sub-class of SIMs 69. A further example of a Co( ii ) system where the authors suggest a similar mechanism is operative is to be found in [(L 2 ) 4 Co 3 (H 2 O) 2 ](NO 3 ) 4 ( 63 ) (where L 2 is the ligand prepared from the Schiff-base condensation of pyridine-2-carboxylate and carbohydrazone) 70. The system features a single Co( ii ) ion in six-coordinate distorted octahedreal geometry, which forms the centre of a trinuclear Co III –Co II –Co III array.…”

Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism

“…The ZFS parameters of D = 20.8 and 29.8 cm -1 , E < 0.1 and < 0.65 cm -1 , and g iso factors = 2.3 and 2.4 were determined for 102 and 103, respectively, based on magnetic studies and EPR data. Their D parameter is of the same order of magnitude, but only complex 103 exhibited slow relaxing magnetization with U eff = 5.5 cm -1 under an applied dc field of 1000 Oe [206].…”