We present a procedure to study the switching and the stability of an array of magnetic nanoparticles in the dynamical regime. The procedure leads to the criterion of multi-switching stability to be satisfied in order to have stable switching. The criterion is used to compare various magnetic-field-induced switching schemes, either present in the literature or suggested in the present work. In particular, we perform micromagnetic simulations to study the magnetization trajectories and the stability of the magnetization after switching for nanoparticles of elliptical shape. We evaluate the stability of the switching as a function of the thickness of the particles and the rise and fall times of the magnetic pulses, both at zero and room temperature. Furthermore, we investigate the role of the dipolar interaction and its influence on the various switching schemes. We find that the criterion of multi-switching stability can be satisfied at room temperature and in the presence of dipolar interactions for pulses shaped according to CMOS specifications, for switching rates in the GHz regime.
We report on the manipulation of magnetization by femtosecond laser pulses in a periodic array of cylindrical nickel nanoparticles. By performing experiments at different wavelength, we show that the excitation of collective surface plasmon resonances triggers demagnetization in zero field or magnetic switching in a small perpendicular field. Both magnetic effects are explained by plasmon-induced heating of the nickel nanoparticles to their Curie temperature. Model calculations confirm the strong correlation between the excitation of surface plasmon modes and laser-induced changes in magnetiza…
Light-matter interaction at the nanoscale in magnetic materials is a topic of intense research in view of potential applications in next-generation high-density magnetic recording. Laser-assisted switching provides a pathway for overcoming the material constraints of high-anisotropy and high-packing density media, though much about the dynamics of the switching process remains unexplored. We use ultrafast small-angle x-ray scattering at an x-ray free-electron laser to probe the magnetic switching dynamics of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femt…
Cooperative response of Pb(ZrTi)O$_3$ nanoparticles to curled electric fields
利用基于第一性原理的有效哈密顿量和有限温度蒙特卡罗模拟，研究了卷曲电场作用下零维Pb（Zr $_{0.5}$ Ti $_{0.5}$）O $_3$纳米粒子的协同响应以及涡旋开关的微观机制.我们发现，普遍接受的域共存机制是无效的环形开关。相反，偶极子显示出不寻常的集体行为，形成一个新的漩涡垂直（但不是相反）的环矩。新的和原来的涡之间的强相关性被揭示为逆转环矩的关键。对令人困惑的集体反应的微观见解进行了讨论。基于我们的发现，我们进一步描述了一种技术方法，该方法能够大幅降低涡旋切换所需的卷曲电场的大小。
National Institute of Standards and Technology（美国国家标准与技术研究院）；University of Colorado（科罗拉多大学）；Institute for Bioscience and Biotechnology Research (IBBR)（生物科学与生物技术研究所）；University of Maryland, College Park（马里兰大学帕克分校）
Dynamic phase transition properties and hysteretic behavior of a ferrimagnetic core-shell nanoparticle in the presence of a time dependent magnetic field
Magnetodielectric study in SiO2-coated Fe3O4 nanoparticle compacts
研究了表面包覆SiO_2 $_{2}$的Fe $_{3}$ O $_{4}$磁性纳米粒子的介电性能。在高温下，磁场引起的介电常数和介电损耗的变化具有相反的符号和强烈的频率依赖性，这是由非本征磁介质耦合--Maxwell-Wagner效应和磁阻效应共同作用的结果。界面缺陷的存在导致测量中出现明显的迟滞现象。另一方面，在低温下的介电和磁性的强耦合矛盾的Maxwell-Wagner模型，建议内禀磁介电耦合。我们的观测结果与最近的极化开关测量结果相一致，证实了高损耗Fe $_{3}$ O $_{4}$中存在低温多铁态。这种核壳结构为实现低损耗的实用磁电材料提供了一条新的途径。
In the context of magnetic hyperthermia, several physical parameters are used to optimize the heat generation and these include the nanoparticles concentration and the magnitude and frequency of the external AC magnetic field. Here we extend our previous work by computing nonlinear contributions to the specific absorption rate, while taking into account (weak) inter-particle dipolar interactions and DC magnetic field. In the previous work, the latter were shown to enhance the SAR in some specific geometries and setup. We find that the cubic correction to the AC susceptibility does not modify t…
The main aim of the present work is to analyse the effect of dipolar interaction strength $λ$, particle size $D$ and temperature $T$ on the hysteresis mechanism in ordered arrays of magnetic nanoparticles (MNPs) using computer simulations. The anisotropy axes of the MNPs are assumed to have random orientation to mimic the real system. In the absence of thermal fluctuations and dipolar interaction, the hysteresis follows the Stoner and Wohlfarth model irrespective of $D$, as expected. The hysteresis loop area is minimal for particle sizes $D \approx8-16$ nm at $T=300$ K and $λ=0.0$, indicating …
Antiferromagnets are robust to external electric and magnetic fields, and hence are seemingly uncontrollable. Recent studies, however, realized the electrical manipulations of antiferromagnets by virtue of the antiferromagnetic Edelstein effect. We present a general symmetry analysis of electrically switchable antiferromagnets based on group-theoretical approaches. Furthermore, we identify a direct relation between switchable antiferromagnets and the ferrotoroidic order. The concept of the ferrotoroidic order clarifies the unidirectional nature of switchable antiferromagnets and provides a cri…
Coercivity weighted Langevin magnetisation; A new approach to interpret superparamagnetic and nonsuperparamagnetic behaviour in single domain magnetic nanoparticles
Magnetic nanoparticles constitute potential nanomedicine tools based on the possibility to obtain different responses triggered by safe remote stimulus. However, such richness can be detrimental if the different performances are not accurately differentiated (and controlled). An example of this is the reorientation of magnetic nanoparticles under the influence of AC fields, which can be exploited for either magneto-mechanical actuation (MMA) at low frequencies (tens of Hz); or heat release at large ones (MHz range). While it is clear that Brownian rotation is responsible for MMA, its heating r…
