Comment on "Memory Effects in an Interacting Magnetic Nanoparticle   System" by Sun et. al, prl, 91, 167206 (2003)
  In this Comment we report a phenomenon identical to that observed in ({Y. Sun, M. B. Salamon, K. Garnier and R. S. Averback, Phys. Rev. Lett. 91, 167206 (2003)}) for systems of NiFe{$_2$}O{$_4$} particles (mean size $\approx$ 3nm) embedded in a SiO{$_2$} matrix with two different interparticle spacings 4 nm (1) and 15 nm (2), which controls the strength of the dipolar interactions. Not only do we find the memory effect to be present in the non-interacting sample (2), indeed we find it to be {\em more} prominent than in the interacting case (1). We demonstrate that this effect can be simply attributed to a superposition of relaxation times of two sets of particles. 
S. Chakravarty, A. Frydman, M. Bandyopadhyay, S. Dattagupta and S.   Sengupta