temperature below which the relaxation of the magnetization becomes slow compared to the time scale of a particular investigation technique. Historically, the blocking temperature for single-molecule magnets has been defined as the temperature at which the molecule's magnetic relaxation time, τ, is 100 seconds. This definition is the current standard for comparison of single-molecule magnet properties, but otherwise is not technologically significant. There is typically a correlation between increasing an SMM's blocking temperature and energy barrier. The average blocking temperature for SMMs is 4K. Dy-metallocenium salts are the most recent SMM to achieve the highest temperature of magnetic hysteresis, greater than that of liquid nitrogen. Intramolecular magnetic exchange The magnetic coupling between the spins of the metal ions is mediated by superexchange interactions and can be described by the following isotropic Heisenberg Hamiltonian: where is the coupling constant between spin i (operator ) and spin j (operator ). For positive J the coupling is called ferromagnetic (parallel alignment of spins) and for negative J the coupling is called antiferromagnetic (antiparallel alignment of spins): a high spin ground state, a high zero-field-splitting (due to high magnetic anisotropy), and negligible magnetic interaction between molecules. The combination of these properties can lead to an energy barrier, so that at low temperatures the system can be trapped in one of the high-spin energy wells. Barrier to magnetic relaxation A single-molecule magnet can have a positive or negative magnetic moment, and the energy barrier between these two states greatly determines the molecule's relaxation time. This barrier depends on the total spin of the molecule's ground state and on its magnetic anisotropy. The latter quantity can be studied with EPR spectroscopy. Performance The performance of single-molecule magnets is typically defined by two parameters: the effective barrier to slow magnetic relaxation, Ueff, and the magnetic blocking temperature, TB. While these two variables are linked, only the latter variable, TB, directly reflects the performance of the single-molecule magnet in practical use. In contrast, Ueff, the thermal barrier to slow magnetic relaxation, only correlates to TB when the molecule's magnetic relaxation behavior is perfectly Arrhenius in nature. The table below lists representative and record 100-s magnetic blocking temperatures and Ueff values that have been reported for single-molecule magnets. Abbreviations: OAc=acetate, Cpttt=1,2,4‐tri(tert‐butyl)cyclopentadienide, CpMe5= 1,2,3,4,5-penta(methyl)cyclopentadienide, CpiPr4H= 1,2,3,4-tetra(isopropyl)cyclopentadienide, CpiPr4Me= 1,2,3,4-tetra(isopropyl)-5-(methyl)cyclopentadienide, CpiPr4Et= 1-(ethyl)-2,3,4,5-tetra(isopropyl)cyclopentadienide, CpiPr5= 1,2,3,4,5-penta(isopropyl)cyclopentadienide *indicates parameters from magnetically dilute samples Types Metal clusters Metal clusters formed the basis of the first decade-plus