Spin-glass behavior in ferromagnetic Fe[Fe(CN)6]•xH2O nanoparticles

The magnetic properties of the nanoparticles of hexacyanometallate based molecular magnetic compound Fe[Fe(CN)(6)]center dot xH(2)O have been investigated using dc magnetization and the frequency dependent real part of ac susceptibility chi(')(ac) techniques. The magnetic properties of these nanoparticles with an average particle diameter similar to 50 nm have been compared with that of the bulk polycrystalline sample. A reduction in the Curie temperature as well as low coercivity and remanent magnetization have been found for the nanoparticles compared to those for the bulk polycrystalline sample. The maximum magnetization for the nanoparticles is also found to be smaller as compared to that for the bulk polycrystalline sample. For the nanoparticles, the peak temperature T-p in the chi(')(ac) vs T curve has been found to shift to a higher temperature with increasing frequency and this frequency dependence is described well by a power law. The fitting yields the relaxation time constant tau(0)=1.3x10(-13) s, the freezing temperature T-g=10.63 K, and the critical exponent z nu=5.5 +/- 0.3. For the bulk polycrystalline sample, no such shift in the ac susceptibility peak temperature has been observed. The time dependence of the thermoremanent magnetization has been observed only for the nanoparticles, which could be represented well with a stretched exponential law. The observed magnetic behavior of the nanocrystalline sample indicates the presence of disordered spins (leading to a spin-glass behavior) in these ferromagnetically ordered particles. (C) 2008 American Institute of Physics.