Giant frequency dependence of dynamic freezing in nanocrystalline ferromagnetic LaCo0.5Mn0.5O3

We have investigated the magnetic properties of nanocrystalline LaCo0.5Mn0.5O3. The temperature dependence of the imaginary part of the a.c. susceptibility shows a strongly frequency dependent maximum at a temperature, Tf, which is well below the ferromagnetic transition temperature (TC ~ 230 K). The frequency dependence of Tf obeys the Arrhenius relation, f = foexp(-Ea/kBT), with physically reasonable values of fo = 109 Hz and Ea/kB = 1518 K. The frequency shift of Tf per decade of frequency is one of the highest values observed in any magnetic system, and a similarly large value is also found in LaCo0.4Mg0.1Mn0.5O3, suggesting that such behavior is intrinsic despite the apparent presence of long range ferromagnetic order.