Interplay between Exchange Interaction and Magnetic Shape Anisotropy of ferromagnetic nanoparticles in a non-magnetic matrix for rare-earth-free permanent magnets
Developing permanent magnets with fewer critical elements requires understanding hysteresis effects and coercivity through visualizing magnetization reversal. Here, we numerically investigate the effect of the geometry of nanoscale ferromagnetic inclusions in a paramagnetic/non-magnetic matrix to understand the key factors that maximize the magnetic energy product of such nanocomposite systems. Specifically, we have considered a matrix of 3 micron x 3 micron x40 nanometer dimension, which is a sufficiently large volume, two-dimensional representation considering that the ferromagnetic inclusions thickness is less than 3.33% of the lateral dimensions simulated. Using this approach that is representative of bulk behavior while being computationally tractable for simulation, we systematically studied the effect of the thickness of ferromagnetic strips, the separation between the ferromagnetic strips due to the nonmagnetic matrix material, and the length of these ferromagnetic strips on magnetic coercivity and remanence by simulating the hysteresis loop plots for each geometry. Furthermore, we study the underlying micromagnetic mechanism for magnetic reversal to understand the factors that could help attain the maximum magnetic energy densities for ferromagnetic nanocomposite systems in a paramagnetic/non-magnetic material matrix. In this study, we have used material parameters of an exemplary Alnico alloy system, a rare-earth-free, thermally stable nanocomposite, which could potentially replace high-strength NdFeB magnets in applications that don't require large energy products. This can stimulate further experimental work on the fabrication and large-scale manufacturing of RE-free PMs with such nanocomposite systems.
We propose a new concept of magnetic focusing for targeting and accumulation of functionalized superparamagnetic nanoparticles in living organs through composite configurations of different permanent magnets. The proposed setups fulfill two fundamental requirements for in vivo experiments: 1) reduced size of the magnets to best focusing on small areas representing the targeted organs of mice and rats and 2) maximization of the magnetic driving force acting on the magnetic nanoparticles dispersed in blood. To this aim, several configurations of permanent magnets organized with different degrees…
Magnetic and Thermal Transport Properties of Permanent Magnets with Anisotropic Grain Structure
纳米结构的永磁体对于诸如发电机和电动机的应用正获得越来越多的兴趣和重要性。由于永磁体的性能随着温度而降低，因此热管理是一个关键问题。我们研究了电流激活压力辅助致密化法制备的无稀土纳米结构SrFe 12 O 19磁体的磁和热输运性质。合成的磁体具有对齐的晶粒，使得它们的易磁化轴垂直于它们的最大表面积，以使它们的磁性能最大化。SrFe 12 O 19磁体在交叉面方向上具有细的晶粒尺寸，并且在面内方向上具有大得多的晶粒尺寸。据发现，这种微结构导致在面内方向上的大约两倍的更高的热导率，提供了有效冷却的机会。声子是这种类型的永磁体在室温附近的主要热载体。温度和方向相关的热导率测量表明，Umklapp和晶界散射是重要的面内方向，其中的特征晶粒尺寸是相对较大的，而晶界散射占主导地位的跨平面的热传输。研究的纳米/微米结构的设计策略应该很好地翻译到其他材料系统，从而具有重要的意义，纳米结构的永磁体的热管理。
Iron oxide nanoparticles have tremendous scientific and technological potential in a broad range of technologies, from energy applications to biomedicine. To improve their performance, single-crystalline and defect-free nanoparticles have thus far been aspired. However, in several recent studies defect-rich nanoparticles outperform their defect-free counterparts in magnetic hyperthermia and magnetic particle imaging. Here, an overview on the state-of-the-art of design and characterization of defects and resulting spin disorder in magnetic nanoparticles is presented with a focus on iron oxide n…
Magnetization reversal in permanent magnets occurs by the nucleation and expansion of reversed domains. Micromagnetic theory offers the possibility to localize the spots within the complex structure of the magnet where magnetization reversal starts. We compute maps of the local nucleation field in a Nd2Fe14B permanent magnet using a model order reduction approach. Considering thermal fluctuations in numerical micromagnetics we can also quantify the reduction of the coercive field due to thermal activation. However, the major reduction of the coercive field is caused by the soft magnetic grain …
The development of permanent magnets containing less or no rare-earth elements is linked to profound knowledge of the coercivity mechanism. Prerequisites for a promising permanent magnet material are a high spontaneous magnetization and a sufficiently high magnetic anisotropy. In addition to the intrinsic magnetic properties the microstructure of the magnet plays a significant role in establishing coercivity. The influence of the microstructure on coercivity, remanence, and energy density product can be understood by {using} micromagnetic simulations. With advances in computer hardware and num…
This study focuses on the potential of permanent magnets as thermoelectric converters. It is found that a SmCo$_5$-type magnet exhibits the large anomalous Ettingshausen effect (AEE) at room temperature and that its charge-to-heat current conversion coefficient is more than one order of magnitude greater than that of typical ferromagnetic metals. The large AEE is an exclusive feature of the SmCo$_5$-type magnet among various permanent magnets in practical use, which is independent of the conventional performance of magnets based on static magnetic properties. The experimental results show that…
The combination of multiple functionalities in a single material is an appealing strategy for the de-velopment of smart materials with unique features. In this work, we present the preparation of thermoresponsive magnetic nanoparticles and their one-dimensional assembly into transient micro-filaments. The material is based on 9.4 nm iron oxide nanoparticles grafted with poly(N-n-propylacrylamide) via multiphosphonic acid anchoring sites. The hybrid nanoparticles present a low critical solution temperature (LCST) transition between 21 °C and 28 °C, depending on the pH and the ionic strength. Wh…
We present an application of a high-throughput search of new rare-earth free permanent magnets focusing on 3d-5d transition metal compounds. The search involved a part of the ICSD database (international crystallographic structural database), together with tailored search criteria and electronic structure calculations of magnetic properties. Our results suggest that it possible to find candidates for rare-earth free permanent magnets using a data-mining/data-filtering approach. The most promising candidates identified here are Pt$_2$FeNi, Pt$_2$FeCu, and W$_2$FeB$_2$. We suggest these material…
Multiscale simulation is a key research tool for the quest for new permanent magnets. Starting with first principles methods, a sequence of simulation methods can be applied to calculate the maximum possible coercive field and expected energy density product of a magnet made from a novel magnetic material composition. Fe-rich magnetic phases suitable for permanent magnets can be found by adaptive genetic algorithms. The intrinsic properties computed by ab initio simulations are used as input for micromagnetic simulations of the hysteresis properties of permanent magnets with realistic structur…
Magnetic Response of NiFe2O4 nanoparticles in polymer matrix
我们报告的磁性纳米复合材料，由不同数量的NiFe 2 O 4纳米粒子在聚合物基体中的磁性能。纳米粒子表现出典型的磁化阻滞效应，这种效应对磁场、零场冷却/场冷却实验方式以及基体中粒子数量的变化敏感。具有较低的颗粒量的样品表现出向上翻转的磁化下降到5 K，而在较低的温度下，作为聚合物中的颗粒量的增加，磁化的阻塞占主导地位。我们研究这种磁性行为的核心和表面自旋的纳米粒子之间的竞争性磁有序，考虑到粒子间（偶极-偶极）相互作用对纳米粒子的磁动力学的影响。
Based on high-throughput density functional theory calculations, we investigated the effects of light interstitial H, B, C, and N atoms on the magnetic properties of cubic Heusler alloys, with the aim to design new rare-earth free permanent magnets. It is observed that the interstitial atoms induce significant tetragonal distortions, leading to 32 candidates with large ($>$ 0.4 MJ/m$^3$) uniaxial magneto-crystalline anisotropy energies (MAEs) and 10 cases with large in-plane MAEs. Detailed analysis following the the perturbation theory and chemical bonding reveals the strong MAE originates fro…
How Gold nanoparticle acquires magnetism? - Formation of large orbital moment at the interface
在本文中，我们试图找出金纳米粒子（Au-NPs）的磁性起源。我们观察到，在将金纳米粒子（Au-NPs）在Fe 3 O 4纳米粒子介质中的净磁化强度增加相比，纯Fe 3 O 4纳米粒子介质。这种磁化强度的增加可以归因于在Au/磁性颗粒界面处形成的大轨道磁矩，这表明在Au-NP中观察到的磁性是界面效应。这种界面效应得到了从正饱和磁化强度到负抗磁性贡献的突然转变的观察结果的支持，该转变是柠檬酸盐涂覆的金Au-NP上的磁场的函数。
Bubbles have a rich history as transducers in particle-physics experiments. In a solid-state analogue, we use bubble domains in nanomagnetic films to measure magnetic nanoparticles. This technique can determine the magnetic orientation of a single nanoparticle in a fraction of a second, and generate a full hysteresis loop in a few seconds, which is much faster than any other reported technique. We achieve this unprecedented speed by tuning the nanomagnetic properties of the films, including the Dzyaloshinskii-Moriya interaction, in the first application of topological protection from the skyrm…
We present a data set of calculated Curie temperatures for the main-phase compounds of rare-earth permanent magnets. We employ ab initio electronic structure calculations for the itinerant ferromagnetism and an effective spin model for the finite-temperature magnetism. Curie temperatures are derived on the basis of a classical Heisenberg model mapped via Liechtenstein's formula for atomic-pair-wise exchange couplings. Relative trends with respect to the species of rare-earth elements in calculated Curie temperatures for R2Fe14B are in agreement with experimental trends. Quantitative comparison…
Prospects for light-rare-earth-based permanent magnet compound R$_{2}$Fe$_{14}$B (R=La$_{1-x}$Ce$_{x}$ with $0 \le x\le 1$) are inspected from first principles referring to the latest experimental data. Ce-rich 2:14:1 compounds come with good structure stability, reasonably good combination of magnetization and magnetic anisotropy, while a drawback lies in the low Curie temperature that is only 120~K above the room temperature at the Ce$_2$Fe$_{14}$B limit. Best compromise is inspected on the basis of ab initio data for (La$_{1-x}$Ce$_{x}$)$_2$Fe$_{14}$B referring to the magnetic properties of…
In this work, a successful synthesis of magnetic cobalt ferrite (CoFe2O4) nanoparticles is presented. The synthesized CoFe2O4 nanoparticles have a spherical shape and highly monodisperse in the selected solvent. The effect of different reaction conditions such as temperature, reaction time and varying capping agents on the phase and morphology is studied. Scanning transmission electron microscopy showed that the size of these nanoparticles can be controlled by varying reaction conditions. Both X-ray diffraction and energy dispersive X-ray spectroscopy corroborate the formation of CoFe2O4 spine…
Influence of a transverse static magnetic field on the magnetic hyperthermia properties and high-frequency hysteresis loops of ferromagnetic FeCo nanoparticles
Enhanced magnetism, memory and aging in Gold-Iron oxide nanoparticle composites
在这份报告中，我们提出了系统的磁性研究纯氧化铁纳米粒子和金氧化铁纳米复合材料，增加金颗粒尺寸/含量。对于这些样品的磁性研究，我们测量了：（1）零场冷却（ZFC）和场冷却（FC）磁化，（2）交流磁化率，（3）在各种温度下磁化与场的关系，（4）在固定温度下对于各种等待时间tw的热剩磁磁化弛豫（TRM）和零场冷却磁化弛豫（ZFCM）以研究老化效应，（5）磁化记忆效应;（6）交换偏压随冷场的变化。详细的磁性测量分析表明，纯Fe 3 O 4纳米粒子样品的行为像一个超顺磁性和掺入金（Au）纳米粒子的纳米复合材料系统慢慢演变从超顺磁性超自旋玻璃态。随着Au纳米颗粒尺寸/含量的增加，记忆和老化效应增强。在这项研究中，最重要的观察是与金纳米粒子的掺入的磁化强度的增强。这种增强作用随着纳米复合材料中Au含量的增加而增加。我们已经解释了由于在Au/磁性颗粒界面处形成大的轨道磁矩而导致这种磁化强度增强的原因。
Relevance of $4f$-$3d$ exchange to finite-temperature magnetism of rare-earth permanent magnets: an ab-initio-based spin model approach for NdFe$_{12}$N
提出了稀土永磁化合物的经典自旋模型。我们的目标化合物NdFe $_{12}$ N是一种超越当今冠军磁体化合物Nd $_{2}$ Fe $_{14}$ B的材料，其固有磁性具有更简单的晶体结构。计算的磁化强度和各向异性场的温度依赖性与最新的实验结果在领先的顺序同意。在我们的数字控制下，我们提出，工程$5d$ -电子介导的间接交换耦合之间的$4f$ -电子在Nd和$3d$ -电子从Fe将最关键的帮助，以提高材料的效用在操作温度范围内。
We demonstrate the use of model order reduction and neural networks for estimating the hysteresis properties of nanocrystalline permanent magnets from microstructure. With a data-driven approach, we learn the demagnetization curve from data-sets created by grain growth and micromagnetic simulations. We show that the granular structure of a magnet can be encoded within a low-dimensional latent space. Latent codes are constructed using a variational autoencoder. The mapping of structure code to hysteresis properties is a multi-target regression problem. We apply deep neural network and use param…
Today, spinel ferrite nanoparticles are widely used in the medical field for cancer treatment, drug delivery, and magnetic resonance imaging contrast agent due to their superior magnetic properties and biocompatibility. Due to the strong dependence of magnetic properties on morphology and other structural aspects, the magnetic properties of these nanoparticles depend on their manufacturing process. One of the goals of the researchers is to improve the magnetic properties of these nanoparticles by substituting elements such as manganese, cobalt, zinc, nickel, magnesium and iron. In this researc…
