The cyano-bridged trinuclear compound, (NEt4)[Mn 2(salmen)2(MeOH)2Fe(CN)6] (1) (salmen2-= rac-N,N′-(1-methylethylene)bis(salicylideneiminate)) , reported previously by Miyasaka et al. (ref 19d) has been reinvestigated using combined ac and dc susceptibility measurements. The strong frequency dependence of the ac susceptibility and the slow relaxation of the magnetization show that 1 behaves as a single-molecule magnet with an ST = 9/2 spin ground state. Its relaxation time (τ) follows an Arrhenius law with τ0 = 2.5 × 10-7 s and Δeff/ kB = 14 K. Moreover, below 0.3 K, τ saturates around 470 s, indicating that quantum tunneling of the magnetization becomes the dominant process of relaxation. (NEt4)[Mn2 (5-MeOsalen) 2Fe(CN)6] (2) (5-MeOsalen2- = N,N′-ethylenebis(5-methoxysalicylideneiminate)) is a heterometallic one-dimensional assembly made of the trinuclear [MnIII(SB)-NC-Fe III-CN-MnIII(SB)] (SB is a salen-type Schiff-base ligand) motif similar to 1. Compound 2 has two types of bridges, a cyano bridge (-NC-) and a biphenolate bridge (-(O)2-), connecting MnIII and FeIII ions and the two MnIII ions, respectively. Both bridges mediate ferromagnetic interactions, as shown by modeling the magnetic susceptibility above 10 K with gav = 2.03, JMn-Fe/k B = +6.5 K, and J′/kB = +0.07 K, where J is the exchange coupling between the trimer units. The dc magnetic measurements of a single crystal using micro-SQUID and Hall-probe magnetometers revealed a uniaxial anisotropy (DT/kB = -0.94 K) with an easy axis lying along the chain direction. Frequency dependence of the ac susceptibility and time dependence of the dc magnetization have been performed to study the slow relaxation of the magnetization. A mean relaxation time has been found, and its temperature dependence has been studied. Above 1.4 K, both magnetic susceptibility and relaxation time are in agreement with the dynamics described in the 1960s by R. J. Glauber for one-dimensional systems with ferromagnetically coupled Ising spins (τ0 = 3.7 × 10-10 s and Δ1/kB = 31 K). As expected, at lower temperatures below 1.4 K, the relaxation process is dominated by the finite-size chain effects (τ′0 = 3 × 10-8 s and Δ2/kB = 25 K). The detailed analysis of this single-chain magnet behavior and its two regimes is consistent with magnetic parameters independently estimated (J and DT) and allows the determination of the average chain length of 60 nm (or 44 trimer units). This work illustrates nicely a new strategy to design single-chain magnets by coupling ferromagnetically single-molecule magnets in one dimension.
ASJC Scopus subject areas
- Colloid and Surface Chemistry