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.

Automated summary

Slow magnetic relaxation is a common feature in magnetic materials, with the relaxation time determined by high spin reversal barriers and molecular spin. Recent experimental data has shown slow magnetic relaxation in systems with positive, small or negligible D. Some materials exhibit multiple relaxation channels.