Magnetic relaxation in two dimensional assembly of dipolar interacting nanoparticles

We perform extensive kinetic Monte Carlo simulations to investigate the magnetic relaxation characteristics in a two-dimensional (L-x x L-y) assembly of magnetic nanoparticles (MNPs) as a function of dipolar interaction strength hd, aspect ratio A(r) = L-y/L-x, and temperature T. The anisotropy axes are taken as randomly oriented in three-dimensional space. In the presence of small dipolar interaction (h(d) <= 0.3) and substantial temperature, the magnetic relaxation follows the Neel Brown model as expected. Interestingly, the dipolar interaction of enough strength is found to induce antiferromagnetic coupling in the square arrangement of MNPs (A(r) = 1.0), resulting in the fastening of magnetic relaxation with h(d). There is also a rapid increase in relaxation even with A(r) < 100 above a particular dipolar interaction strength h(d), which gets enhanced with A(r). Remarkably, magnetization relaxes slowly with h(d) for the highly anisotropic system, i.e. A(r) > 100. It is because the dipolar interaction induces ferromagnetic interaction in such a case. The temperature also affects relaxation drastically. For weak dipolar interaction, magnetization relaxes rapidly with T because of enhancement in thermal fluctuations. The effective Neel relaxation time tau(N) also depends strongly on these parameters. In the presence of strong dipolar interaction (h(d) > 0.3) and A(r) = 1.0, tau(N) decreases with h(d) for a given temperature. On the other hand, there is an increase in tau(N) with hd for huge A(r) (> 100). The comparison of relaxation behaviour and tau(N) with aligned anisotropy cases also reveals a strong dependence of these characteristics on the anisotropy axes orientation. These results are beneficial in data storage and other spintronics based applications.

Automated summary

Extensive kinetic Monte Carlo simulations were performed to investigate the magnetic relaxation characteristics in a two-dimensional assembly of magnetic nanoparticles. The results showed that the magnetic relaxation is influenced by dipolar interaction strength, aspect ratio, and temperature, with the dipolar interaction inducing antiferromagnetic or ferromagnetic coupling depending on the system properties. The findings have implications for data storage and spintronics applications.