The resonance decay function method in the determination of the pre-factor of the Néel relaxation time of single-domain nanoparticles

Abstract In its simple form, the relaxation time of the Neel relaxation process of the magnetic moment of single-domain particles is given by τ N =τ 0 N exp( σ ), σ being the ratio of anisotropy energy to thermal energy. The pre-factor, τ 0 N , is normally given a value of 10 −9 s, but values ranging from 10 −8 to 10 −12 s have been reported in literature. Here, by means of the field and frequency dependence of the complex magnetic susceptibility, χ ( ω , H ) =χ ′( ω , H ) −iχ ″( ω , H ), of a magnetic fluid sample, in the MHz–GHz range, in conjunction with the determination of the sample decay function, b ( t ), the pre-factor τ 0 N is determined. b ( t ) is readily obtained through the inverse Fourier transformation relationship, which exists between b ( t ) and χ ″( ω ).