The dynamics of magnetization relaxation in ferrofluids are studied with statistical-mechanical theory and Brownian dynamics simulations. The particle dipole moments are initially perfectly aligned, and the magnetization is equal to its saturation value. The magnetization is then allowed to decay under zero-field conditions toward its equilibrium value of zero. The time dependence is predicted by solving the Fokker-Planck equation for the one-particle orientational distribution function. Interactions between particles are included by introducing an effective magnetic field acting on a given particle and arising from all of the other particles. Two different approximations are proposed and tested against simulations: a first-order modified mean-field theory and a modified Weiss model. The theory predicts that the short-time decay is characterized by the Brownian rotation time ${\tau }_B$, independent of the interaction strength. At times much longer than ${\tau }_B$, the asymptotic decay time is predicted to grow with increasing interaction strength. These predictions are borne out by the simulations. The modified Weiss model gives the best agreement with simulation, and its range of validity is limited to moderate, but realistic, values of the dipolar coupling constant.

中文翻译：

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相互作用对铁磁流体磁化弛豫动力学的影响

利用统计力学理论和布朗动力学模拟研究了铁磁流体中磁化弛豫的动力学。粒子偶极矩最初会完美对齐，磁化强度等于其饱和值。然后允许磁化强度在零磁场条件下朝其平衡值零衰减。通过求解单粒子定向分布函数的Fokker-Planck方程来预测时间依赖性。通过引入作用在给定粒子上且由所有其他粒子产生的有效磁场，可以包括粒子之间的相互作用。提出了两种不同的近似值，并针对模拟进行了测试：一阶修正均值场理论和修正Weiss模型。${\tau }_{乙}$，与互动强度无关。有时比${\tau }_{乙}$，渐近衰减时间预计会随着相互作用强度的增加而增加。这些预测被仿真证实。修改后的Weiss模型与仿真具有最佳的一致性，其有效性范围仅限于偶极耦合常数的适度但实际的值。