Ilg Patrick, Kröger Martin
School of Mathematical, Physical, and Computational Sciences, University of Reading, Reading RG6 6AX, UK.
Phys Chem Chem Phys. 2020 Oct 15;22(39):22244-22259. doi: 10.1039/d0cp04377j.
The intriguing properties of magnetic nanoparticles have sparked a growing number of theoretical studies as well as practical applications. Here, we provide the first comprehensive study of the influence of interactions on the two main relaxation mechanisms: internal (Néel) and Brownian relaxation. While non-interacting magnetic nanoparticles show Debye behavior with an effective relaxation time, many authors use this model also for the interacting case. Since Néel relaxation is typically a thermally activated process on times scales that are many orders of magnitude larger than the underlying micromagnetic times, we use extensive computer simulations employing a Brownian dynamics/Monte-Carlo algorithm to show that dipolar interactions lead to significant deviations from the Debye behavior. We find that Néel and Brownian relaxation can be considered as independent processes for short enough times until dipolar interactions lead to a coupling of these mechanisms, making the interpretation more difficult. We provide mean-field arguments that describe these short and long-time, effective relaxation times well for weak up to moderate interaction strengths. Our findings about the coupling of Brownian and Néel process and the effective relaxation time provide an important theoretical insight that will have also important consequences for the interpretation of magnetic susceptibility measurements and magnetorelaxometry analysis.
磁性纳米颗粒的有趣特性引发了越来越多的理论研究以及实际应用。在此，我们首次全面研究了相互作用对两种主要弛豫机制的影响：内禀（奈尔）弛豫和布朗弛豫。虽然非相互作用的磁性纳米颗粒表现出具有有效弛豫时间的德拜行为，但许多作者也将此模型用于相互作用的情况。由于奈尔弛豫通常是一个热激活过程，其时间尺度比潜在的微磁时间大许多个数量级，我们使用基于布朗动力学/蒙特卡罗算法的广泛计算机模拟来表明，偶极相互作用会导致与德拜行为有显著偏差。我们发现，在足够短的时间内，奈尔弛豫和布朗弛豫可被视为独立过程，直到偶极相互作用导致这些机制耦合，这使得解释变得更加困难。我们提供了平均场论据，对于弱到中等相互作用强度的情况，这些论据能很好地描述这些短时间和长时间的有效弛豫时间。我们关于布朗过程和奈尔过程耦合以及有效弛豫时间的发现提供了重要的理论见解，这对于磁化率测量和磁弛豫测量分析的解释也将产生重要影响。
