Crossover behavior in dynamics of frozen ferrofluids

The imaginary part of susceptibility, chi", in frozen ferrofluids was measu red as functions of temperature, frequency, and concentration of magnetic p articles. In a very dilute region, where magnetic interaction is negligible , the relaxation time is mainly determined by Neel's relaxation in isolated particles and the frequency-dependent peak temperature in chi"(T), T-p, ob eys the Arrhenius law. In the moderate concentration range, T-p has a Vogel -Fulcher (VF) relaxation with the measuring frequency. However, for concent ration larger than a certain value, deviation from VF law occurs. Simulatio ns show that the VF relation could be attributed to Ising spin-glass-like r andom and frustrated interaction between magnetic moments of particles. We suggest that particle configuration before freezing is critical to determin e the distribution of pair exchange parameters. For high concentration, the re are more ferromagnetic than antiferromagnetic bonds. The deviation from VF law at high concentration could be due to short range correlation among spins. (C) 2000 American Institute of Physics. [S0021- 8979(00)24908-6].