Unexpected behavior of single ion magnets

Slow magnetic relaxation, as a prerequisite of the single-ion magnetism, is a rather common feature identified for hundreds of the first-row transition metal complexes. A paradigm is generally accepted that the relaxation time tau is dominated by high barrier to spin reversal U-eff that is proportional to the absolute value of the axial zero-field splitting parameter D, and the square of the molecular spin S. U-eff = vertical bar D vertical bar S-2 and U-eff = vertical bar D vertical bar(S-2 - 1/4) for non-Kramers and Kramers ions, respectively. Recent experimental data shows that the slow magnetic relaxation close to one second occurs also in systems with positive, small or negligible D or in systems with S = 1/2 and/or orbital degeneracy where the D-parameter is undefined. Moreover, numerous systems exhibit two or three relaxation channels: low-frequency (LF) around 1 Hz, high-frequency (HF) above 500 Hz, and intermediate-frequency (IF) in between. An external DC magnetic field massively supports the low-frequency relaxation channel with the mole fraction x(LF) > 0.5 that coexists with the HF relaxing species. Unlike the expectations, some complexes exhibit a reciprocating thermal behavior when the low-temperature relaxation time for the HF channel on cooling is shortened. (C) 2020 Elsevier B.V. All rights reserved.