Giant frequency dependence of dynamic freezing in nanocrystalline ferromagnetic LaCo_0.5Mn_0.5O₃

We have investigated the magnetic properties of nanocrystalline LaCo_0.5Mn_0.5O₃. The temperature dependence of the imaginary part of the ac susceptibility shows a strongly frequency-dependent maximum at a temperature T_f, which is well below the ferromagnetic transition temperature (T_c∼230 K). The frequency dependence of T_f obeys the Arrhenius relation, f=f_o exp(-E_a/k_BT), with physically reasonable values of f_o=10⁹ Hz and E_a/k_B=1518 K. The frequency shift of T_f per decade of frequency is one of the highest values observed in any magnetic system, and a similarly large value is also found in LaCo_0.4Mg_0.1Mn_0.5O₃, suggesting that such behavior is intrinsic despite the apparent presence of long-range ferromagnetic order.