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

The dynamics of magnetic relaxation in a system of isolated ferrimagnetic nanoparticles depends on the ratio between the magnetic relaxation time (tau) and the measurement time (t(m)), which is usually considered to be equal to the period (T-H) of the external alternating magnetic field (t(m) = T-H). When t(m) approaches tau (tau < t(m)), the magnetic moments cannot relax completely, thus leading to a deviation from the superparamagnetic behaviour (SPM), and a magnetic remanence of the system when the deviation is large. An external magnetic field (H) can significantly change the dynamics of the relaxation, especially when its amplitude (H-m) is high. This paper shows that there is a limit field (threshold field (H-p)) that depends on the anisotropy field of the nanoparticle, its magnetic volume and on the temperature; beyond this field, the magnetic moments cannot pass the potential barrier and they remain blocked. It will be shown that under these conditions the measurement time can no longer be considered to be t(m) = T-H, but is a measurement time t(mH) < T-H that in addition to TH will also depend on H-p and H-m. When the amplitude of the alternating magnetic field is lower than the value of the threshold field (H-m < H-p), the measurement time is reduced to the period of the magnetic field. The theory proposed for a system of aligned nanoparticles has been verified experimentally in the case of a ferrofluid-type system. The result obtained brings in important corrections for determining the magnetic volume of the nanoparticles or the magnetic anisotropy constant if the condition t(m) = t(mH) < TH is used when Hrn is high (H-m > H-p), instead of t(m) = T-H.