Slow dynamics of the magnetization in the ordered state of molecule based magnets with one-dimensional chain structure

The ac susceptibility as a function of temperature, dc magnetic field and frequency as well as the magnetization in quasistatic and pulsed magnetic fields have been measured for new molecule-based heterospin complexes [Mn(hfac)(2)BNOR] (R = H, Cl). These compounds exhibit a quasi one-dimensional chain structure, for which the interchain exchange interaction is small in comparison with the intrachain one (J/J' similar to 10(-3)). A strong frequency dependence of both the real and imaginary parts of the ac susceptibility is found at low frequencies (below 100 Hz) in both antiferromagnetic (AF) [Mn(hfac)(2)BNOH] and ferrimagnetic (FI) [Mn(hfac)(2)BNOCl] compounds within the field interval, where the magnetization process is controlled by motion of the domain walls. A number of peculiarities regarding the reversal of magnetization are observed in pulsed fields. In particular, the remanent magnetization appears in the AF compound [Mn(hfac)(2)BNOH] after application of a unidirectional pulse of field. The remanence relaxes within about 1000 ms through a two-stage thermally activated process associated with the nucleation of the AF phase within the field-induced FI phase and displacement of the domain walls separating AF and FI phases. The activation energies for these two stages are estimated to be about 1.66 and 1.80 meV respectively. The activation energy for domain wall motion in the ferrimagnetic [Mn(hfac)(2)BNOCl] compound is found to be similar to3.3 meV. The slow dynamics of the magnetization in these materials is ascribed to the complicated domain wall displacement that presumably includes the lateral motion of the intrachain wall along the separate chains because of the strong 1D character of these compounds.