NONLINEAR TEMPERATURE-VARIATION OF MAGNETIC VISCOSITY IN NANOSCALE FEOOH PARTICLES

Temperature variations of the magnetization and magnetic viscosity S in 30- particles of FeOOH are reported. An average blocking tempeature Tb=47 K is determined for this system from the temperature variation of the difference susceptibility between the field-cooled and zero-field-cooled cases. The measured decrease of Tb with applied field is used to determine an average magnetic moment 7150B per particle. The relaxation data of the magnetization M(t) follow the equation M(t)/M(0)=1 -S ln(t/0), from which temperature variation of S is determined for the range 2 to 200 K. Below 7 K, S is temperature independent signifying quantum tunneling. Above 7 K, initially S increaes linearly with T, but the variation becomes increasingly nonlinear, reaching an apparent maximum at Tb=47 K. The temperature variation of S is characteristic of a phase transition at Tb since the data fits well the equation S=A(T-Tb)/Tb- with A=0.040(0.0076) and =1.10(1.29) for T>Tb(T<Tb). An average energy barrier U=2140 K and 0=5×10-12 s are also estimated from the analysis of the data. © 1995 The American Physical Society.