ON THE INFLUENCE OF A UNIFORM MAGNETIC-FIELD ON THE NEEL RELAXATION OF A COLLOIDAL SUSPENSION OF NANOMETER-SIZED PARTICLES

Measurements of the complex, frequency-dependent susceptibility, chi(o mega) = chi'(omega) - i chi ''(omega), of three colloidal suspensions of magnetite in isopar M (a hydrocarbon carrier) in the frequency rang e 30 MHz to 6 GHz for 14 values of the polarizing field, H, over the r ange 0-116 kA m(-1) are presented. The effect of the polarizing field on the real and imaginary susceptibility profiles of the suspensions i s clearly demonstrated and these measurements reveal the presence of a subsidiary loss peak in the chi ''(omega) component. This effect is f ound to exist over the approximate polarizing field range 9.5-40 kA m( -1) and may be attributed to the presence of relatively strong high-fr equency relaxation modes superimposed on the Neel relaxation mode, as originally predicted by Coffey et al and later analytically explained by Garanin. The dependence of this effect on the particle concentratio n is demonstrated.