Wide-band complex magnetic susceptibility measurements of magnetic fluids as a function of temperature

Measurements of the complex magnetic susceptibility over the frequency and temperature ranges of 2 MHz-6GHz and 20 to -100 degrees C, respectively, are reported for the first time for a magnetic fluid. The fluid used was a colloidal suspension of magnetite particles of median diameter 9 nm in a hydrocarbon oil (isopar m). Resonance was observed and found to increase from approx 1.5 GHz to 3.3 GHz in the temperature range 20 to - 50 degrees C. The increase in resonant frequency is attributed to a decrease in thermal fluctuations with decrease in temperature. At frequencies below approximately 19 MHz, a significant drop in chi'(omega) with decrease in temperature over the temperature range 20 to -100 degrees C, is observed and is attributed to the changes in the Neel and Brownian relaxation processes. Below - 60 degrees C, the temperature at which the suspension becomes solid, Brownian relaxation ceases to exist. (C) 1999 Elsevier Science B.V. All rights reserved.