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  Physics Express 2011, 1: 25
  Research Article
Origin of magnetic anomalies in the liquid, mixed and frozen states of ferrofluids
  M.B. Morales, S. Pal, N.A. Frey, M.H. Phan, P. Mukherjee, H. Srikanth  
Center for Integrated Functional Materials and Department of Physics, University of South Florida, Tampa, FL 33620, USA
  A comparative study of ferrofluids, with particle blocking and carrier fluid freezing temperatures being close or far apart from each other, yields evidence for distinctly different behaviors seen in the magnetic response in the liquid, frozen and mixed states. We demonstrate that the physical origins of relaxation peaks in the complex susceptibility or the spin-glass-like cusps in the zero-field-cooled (ZFC) magnetization are associated with the particle blocking and carrier liquid freezing effects. Quantitative fits of the frequency dependent AC susceptibility to the Vogel-Fulcher model, τ = τoexp[Ea/k(T-To)], clearly delineates the difference in glassy nature in the frozen and mixed states. The blocking effect of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing spin-glass-like slow dynamics. The coexistence of Néel and Brown relaxations in the mixed and frozen states is observed. A correlation between the blocking temperature of magnetic nanoparticles and the freezing temperature of the solvent in ferrofluids is established.
  Magnetic nanoparticles; Ferrofluids; Interparticle interaction; Magnetic relaxation; Biomedicine  

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