Transition from ferromagnetism to superparamagnetism on the nanosecond time scale
Transition from ferromagnetism to superparamagnetism on the nanosecond time scale
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DOI：
10.1103/physrevb.65.224406
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发表时间：
2002-06-01
影响因子：
3.7
通讯作者：
Moro, E
中科院分区：
文献类型：
--
作者：
Lopez-Diaz, L;Torres, L;Moro, E
朗之万动力学已经被用来研究当减小具有面内单轴各向异性的单磁区纳米粒子的尺寸时，从铁磁性到超顺磁性转变的不同方面，观察时间在几纳秒量级。研究发现，对于这样的小粒子，磁化强度的不均匀性通常被忽略，它有利于热弛豫。定量上，由于这一贡献，发生转变的尺寸范围向更大的尺寸移动了大约1 nm。另一方面，我们还发现，与厚度相关的垂直退磁场引起了两个面内能量极小值之间的热转换。计算结果与用来描述这一效应的阿累尼乌斯定律符合得很好。发现指数前因子tau(0)与粒子厚度有很强的相关性。
Langevin dynamics has been used to study different aspects of the transition from ferromagnetism to superparamagnetism when reducing the size of thin magnetic single-domain nanoparticles with in-plane uniaxial anisotropy for observation times in the order of a few nanoseconds. It is found that nonuniformities in the magnetization, usually ignored for such small particles, favor thermal relaxation. Quantitatively, the size range over which the transition occurs is found to move roughly 1 nm towards larger sizes due to this contribution. On the other hand, we also find that the thickness-dependent perpendicular demagnetizing field induces thermal switching between the two in-plane energy minima. The results are in good agreement with the Arrhenius law, which is used to characterize this effect. A strong dependence of the preexponential factor tau(0) on the particle thickness is found.