A Caveat for Single‐Molecule Magnetism: Non‐linear Arrhenius Plots

Abstract

Single-molecule magnets (SMMs) are molecules that, below a certain temperature (TB), function as individual nanoscale magnetic particles, exhibiting magnetization hysteresis loops. As such, they represent an alternative and molecular (bottom-up) route to nanomagnetism, complementing the top-down approach to traditional magnetic nanomaterials. SMMs also exhibit fascinating quantum behavior such as quantum tunneling of the magnetization (QTM) and quantum phase interference (QPI), showing that they are truly mesoscale entities straddling the classical/quantum divide. The barrier causing slow magnetization relaxation arises from a combination of a large ground-state spin (S) and easy-axis anisotropy (negative zerofield splitting parameter, D). The most studied SMMs are the [Mn12O12 ACHTUNGTRENNUNG(O2CR)16 ACHTUNGTRENNUNG(H2O)4] family with S=10 ground states, and their derivatives, while in recent years others have been discovered. Alternating current (ac) magnetic susceptibility studies are a convenient method of assessing whether a compound might be an SMM; frequency-dependent out-of-phase (c’’M) signals are indicative of the superparamagnet-like properties of an SMM. The variation in signal position with ac frequency can then be used as a source of rate vs T kinetic data, because the c’’M peak maximum is the temperature at which the angular frequency of the oscillating field equals the rate (1/t, where t is the relaxation lifetime) of spin vector reversal. This allows construction of a ln ACHTUNGTRENNUNG(1/t) vs 1/T plot based on the Arrhenius relationship given in Equation (1), the behavior expected of a thermally-activated process over a single barrier :