GLASSY DYNAMICS OF NANOMETRIC MAGNETIC PARTICLES

When a system of small magnetic particles is not very much diluted, th e susceptibility peak arises from a delicate combination of individual blocking behind anisotropy barriers and collective freezing due to di polar interactions. We have studied a sample of 4 nm Permalloy particl es in alumina (25% concentration), and measured the frequency dependen ce of the ac susceptibility. Our scaling analysis shows that cooperati ve freezing effects do indeed contribute significantly to the blocking temperature; accounting for the dipolar interactions, the value obtai ned for individual anisotropy barriers is 10 times lower than that cru dely deduced from the blocking temperature. No clear evidence of infin ite-range correlations is found, in contrast with the case of spin gla sses. We have studied the relaxation of the zero-field-cooled magnetiz ation with similar procedures as used to characterize the slow dynamic s of spin glasses. We find that the relaxation is weakly sensitive to waiting time effects, and is more strongly influenced by the cooling t ime, as is usually quoted for structural glasses.