The effect of the external magnetic field on the thermal relaxation of magnetization insystems of aligned nanoparticles

Abstract

The dynamics of magnetic relaxation in a system of isolated ferrimagneticnanoparticles depends on the ratio between the magnetic relaxation time(τ) and themeasurement time (tm), which is usually considered to be equal to the period(TH) of the externalalternating magnetic field (tm = TH). When tm approaches τ (τ<tm), the magnetic moments cannot relax completely, thus leading to a deviationfrom the superparamagnetic behaviour (SPM), and a magnetic remanenceof the system when the deviation is large. An external magnetic field(H) can significantly change the dynamics of the relaxation, especially when its amplitude(Hm) is high. This paper shows that there is a limit field (threshold field(Hp)) that depends on the anisotropy field of the nanoparticle, its magnetic volumeand on the temperature; beyond this field, the magnetic moments cannot passthe potential barrier and they remain blocked. It will be shown that underthese conditions the measurement time can no longer be considered to betm = TH, but is ameasurement time tmH<TH that in addition to TH will also depend on Hp and Hm. When the amplitude of the alternating magnetic field is lower than the value of the threshold field(Hm<Hp), the measurement time is reduced to the period of the magnetic field. The theory proposedfor a system of aligned nanoparticles has been verified experimentally in the case of aferrofluid-type system. The result obtained brings in important corrections for determining themagnetic volume of the nanoparticles or the magnetic anisotropy constant if the conditiontm = tmH<TH is usedwhen Hm is high (Hm>Hp),instead of tm = TH.