Theoretical Study of Supermoment Relaxation in Linear Chains of Spherical Magnetic Nanoparticles

Abstract

This work presents a theoretical study of supermoment relaxation in linear chains of spherical, uniaxial, monodisperse iron (Fe) magnetic nanoparticles (MNPs) with single magnetic domains. The system is modeled within the macrospin approximation using the Stoner–Wohlfarth model [1], considering thermal fluctuations and interparticle dipolar interactions. Néel relaxation is considered, assuming that the physical rotation of the MNPs is blocked and that only the supermoments μ can reorient. The MNP chains are aligned along the z-axis, with a center-to-center distance d = D + 2w, where D = 7.5 nm is the magnetic diameter and w is the nonmagnetic coating thickness that controls the strength of dipolar interactions. Monte Carlo simulations with Metropolis dynamics and angle-restricted rotations [2] are used to study the time evolution of the longitudinal magnetization component Mz. In the absence of dipolar interactions, Mz decays due to thermal fluctuations, and the relaxation time τ follows the standard Arrhenius law [3]. When dipolar interactions are included, Mz decays more slowly and reaches a temperature-dependent nonzero value at long times. In this regime, τ deviates from Arrhenius behavior and follows a modified law characterized by an effective temperature, consistent with Vogel–Fulcher behavior [4]. Increasing the coating thickness w weakens the dipolar interactions, causing the relaxation dynamics to approach that of the non-interacting system. Conversely, increasing the chain length Np further slows the relaxation, highlighting the collective effects induced by dipolar interactions. References [1] E. C. Stoner and E. P. Wohlfarth, Phil. Trans. R. Soc. Lond. A 240, 599–642 (1948). [2] R. A. Novak et al., Phys. Rev. B (2010). [3] D. Serantes et al., J. Appl. Phys. 108, 073918 (2010). [4] S. Shtrikman and E. P. Wohlfarth, Phys. Lett. A 85, 467–470 (1981).

Keywords: Magnetic Nanoparticles; Linear chains; Relaxation