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

Abstract

The dynamics of magnetic relaxation in a system of isolated ferrimagnetic nanoparticles depends on the ratio between the magnetic relaxation time (τ ) 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 τ (τ < 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 T H 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 < T H is used when H m is high (H m > H p ), instead of t m = T H .

Low drive field amplitude for improved image resolution in magnetic particle imaging

“…stant should change as a function of the applied field, 27,[30][31][32][33][34][35][36] and the Néel time constant can be written in a more complete form that includes a dependence on applied field. 23,25,28,34,37 In this regime of larger applied fields, numerical solutions to the Fokker-Planck equations describing the rotational motion of SPIOs further indicate that external magnetic field terms dominate over thermal terms.…”

Nanoparticle Size Effect on Some Magnetic Properties

“…In the presence of an external magnetic field applied along the easy magnetization direction of nanoparticles, the energy barrier will change (W bH ) [130], either by increasing or decreasing, depending on the sense of the magnetic field (increases when the field H is applied in the sense of magnetization M s and decreases in reverse). In Fig.…”

“…In Fig. 19 (b, c), the variation of the energy barrier for Fe 3 O 4 nanoparticles surfacted with oleic acid and dispersed in kerosene (ferrofluid) [130], having the magnetic diameter of 12.35 nm [71] and a uniaxial anisotropy constant K u = 12.2 Â 10 3 Jm À3 , is shown [19].…”

Nonlinearity of dynamic magnetization in a superparamagnetic clustered-particle suspension with regard to particle rotatability under oscillatory field

“…Under static field (Fig. 4a), θ should be zero as thermal relaxation should have an insignificant contribution to the change of magnetization direction [20]. Additionally, an oscillatory field gives rise to θ≠0 (Fig.…”