We present a numerical simulation study of the exchange bias (EB) effect in nanoparticles with core/shell structure aimed to unveil the microscopic origin of some of the experimental phenomenology associated to this effect. In particular, we have focused our study on the particle size and field cooling dependence of the hysteresis loop shifts. To this end, hysteresis loops after a field cooling process have been computed by means of Monte Carlo simulations based on a model that takes into account the peculiar properties of the core, shell and interfacial regions of the particle and the EB and coercive fields have been extracted from them. The results show that, as a general trend, the EB field $h_{EB}$ decreases with increasing particle size, in agreement with some experimental observations. However, closer inspection reveals notable oscillations of $h_{EB}$ as a function of the particle radius which we show to be closely related to the net magnetization established after field cooling at the interfacial shell spins. For a particle with ferromagnetic interface coupling, we show that the magnitude and sign of $h_{EB}$ can be varied with the magnetic field applied during the cooling process.
The possibility of controlling the domain structure in spherical nanoparticles of uniaxial and multiaxial ferroelectrics using a shell with tunable dielectric properties is studied in the framework of Landau-Ginzburg-Devonshire theory. Finite element modeling and analytical calculations are performed for Sn2P2S6 and BaTiO3 nanoparticles covered with high-k polymer, temperature dependent isotropic paraelectric strontium titanate, or anisotropic liquid crystal shells with a strongly temperature dependent dielectric permittivity tensor. It appeared that the tunable paraelectric shell with a tempe…
Bimetallic nanoparticles are widely studied, for example in catalysis. However, possible restructuring in the environment of use, such as segregation or alloying, may occur. Taken individually, state-of-the-art analytical tools fail to give an overall picture of these transformations. This study combines an ensemble analysis (near-ambient-pressure X-ray photoelectron spectroscopy) with a local analysis (environmental transmission electron microscopy) to provide an in situ description of the restructuring of core--shell nickel-cobalt nanoparticles exposed to cycles of reduction and oxidation. I…
There has been renewed interest in the use of magnetic nanoparticles to convert high frequency electromagnetic energy into heat. During the last decade, numerous examples in the field of catalysis, lightweight thermoplastic composites for aeronautical and automotive engineering, and biomedical applications as well, have flourished. In a recent Letter "Ultrathin Interface Regime of Core-Shell Magnetic Nanoparticles for Effective Magnetism Tailoring", Moon et al. DOI: 10.1021/acs.nanolett.6b04016 claim to have observed a new interface spin canting phenomenon in core-shell ferrrites that pushes h…
Probing the core and shell contributions to exchange bias in Co/Co$_3$O$_4$ nanoparticles of controlled size
核/壳磁性纳米颗粒界面处的耦合被认为是导致交换偏置（EB）的原因，并且核和壳组分的相对尺寸被认为会影响相关的现象学。在这项工作中，我们已经制备了核/壳结构的纳米粒子的总平均直径约$\sim$ 27 nm和广泛的壳厚度通过控制氧化的Co纳米粒子以及分散在无定形二氧化硅主机。结构表征给出了令人信服的证据，形成的钴$_3$ O $_4$微晶相在壳周围的Co核心。场冷却磁滞回线显示非单调的依赖性的交换偏置$H_E$和强制$H_C$领域，成为最大的样品与中间壳厚度，在该晶格应变也是最大的两个阶段。我们的原子蒙特卡罗模拟的粒子具有相同的大小和组成，在实验中的结果是一致的实验观察，并使我们能够确定的变化的贡献的界面表面自旋的磁化反转引起的最大值在$H_E$和$H_C$。
The ordered structure of binary alloy nanoparticles determines their magnetic and catalytic characteristics. In the alloys after annealing, one of the components preferentially segregates on the surface to reduce surface energy. This surface segregation has been known as a factor in the construction of an ordered phase near the surface. However, the segregation-induced ordering has not been observed for nanoparticles. Here, platinum@cobalt (Pt@Co) core-shell nanoparticles were synthesized, and their structural changes after annealing at 600°C, 700°C, and 800°C for 3 hours were observed by a sc…
By means of ab-initio calculations, we have investigated the chemisorption paroperties of ethanol onto segregating binary nanoalloys. We select nanostructures with icosahedral shape of 55 atoms with a Pt outermost layer over a M core with M=Ag,Pd,Ni. With respect to nanofilms with equivalent composition, there is an increse of the ethanol binding energy. This is not merely due to observed shortening of the Pt-O distance but depends on the nanoparticle distortion after ethanol adsorption. This geometrical distortion within the nanoparticle can be interpreted as a radial breathing, which is sens…
Using the Landau-Ginzburg-Devonshire approach, we study screening-induced phase transitions in core-shell ferroic nanoparticles for three different shapes: an oblate disk, a sphere, and a prolate needle. The nanoparticle is made of a ferroic CuInP2S6 core and covered by a "tunable" screening shell made of a phase-change material with a conductivity that varies as the material changes between semiconductor and metallic phases. We reveal a critical influence of the shell screening length on the phase transitions and spontaneous polarization of the nanoparticle core. Since the tunable screening s…
Bimetallic nanoparticles (BNPs) exhibit diverse morphologies such as core-shell, Janus, onion-like, quasi-Janus, and homogeneous structures. Although extensive effort has been directed towards understanding the equilibrium configurations of BNPs, kinetic mechanisms involved in their development have not been explored systematically. Since these systems often contain a miscibility gap, experimental studies have alluded to spinodal decomposition (SD) as a likely mechanism for the formation of such structures. We present a novel phase-field model for confined (embedded)systems to study SD-induced…
Heat transfer from irradiated metallic nanoparticles is relevant to a broad array of applications ranging from water desalination to photoacoustics. The efficacy of such processes relies on the ability of these nanoparticles to absorb the pulsed illuminating light and to quickly transfer energy to the environment. Here we show that compared to homogeneous gold nanoparticles having the same size, gold-silica core-shell nanoparticles enable heat transfers to liquid water that are faster. We reach this conclusion by considering both analytical and numerical calculations. The key factor explaining…
We report on atom probe tomography analyses of Pd and Pd@Au nanoparticles embedded in a Ni matrix and the effects of local evaporation field variations on the atom probe data. In order to assess the integrity of the reconstructed atom maps, we performed numerical simulations of the field evaporation processes and compared the simulated datasets with experimentally acquired data. The distortions seen in the atom maps for both Pd and Pd@Au nanoparticles could be mostly ascribed to local variations in chemical composition and elemental evaporation fields. The evaporation field values for Pd and N…
In this work we investigate novel spherical core-shell nanoparticles with band inversion. The core and the embedding medium are normal semiconductors while the shell material is assumed to be a topological insulator. The envelope functions are found to satisfy a Dirac-like equation that can be solved in a closed form. The core-shell nanoparticle supports midgap bound states located at both interfaces due to band inversion. These states are robust since they are topologically protected. The energy spectrum presents mirror symmetry due to the chiral symmetry of the Dirac-like Hamiltonian. As a m…
Magnetic particle hyperthermia, in which colloidal nanostructures are exposed to an alternating magnetic field, is a promising approach to cancer therapy. Unfortunately, the clinical efficacy of hyperthermia has not yet been optimized. Consequently, routes to improve magnetic particle hyperthermia such as designing hybrid structures comprised from different phase materials are actively pursued. Here we demonstrate enhanced hyperthermia efficiency in relative large spherical Fe/Fe-oxide core/shell nanoparticles through the manipulation of interactions between the core and shell phases. Experime…
Nanoscale distribution of magnetic anisotropies in bimagnetic soft core-hard shell MnFe$_2$O$_4$@CoFe$_2$O$_4$ nanoparticles
采用不同探测深度的元素选择性磁谱组合，测量了核@壳MnFe $_2$ O $_4$ @CoFe $_2$ O $_4$ 7.0 nm颗粒中磁各向异性的纳米级分布。由于这张照片是无法通过任何其他技术，涌现的磁性被揭示出来。在整个纳米尖晶石中，矫顽场不是恒定的。非常薄的（0.5 nm）CoFe $_2$ O $_4$硬壳对另外非常软的MnFe $_2$ O $_4$核施加强磁各向异性：在MnFe $_2$ O $_4$芯内部测量到大的结晶度梯度，接近界面区域的值较低，而内芯呈现出相当大的矫顽磁场（0.54T）和非常高的剩余磁化（饱和时磁化的90%）。
Engineering the structure of core-shell colloidal semiconductor nanoparticles (CSNPs) is attractive due to the potential to enhance photo-induced charge transfer (PICT) and induce favourable optical and electronic properties. Nonetheless, the sensitivity of telluride CSNPs to high temperatures makes it challenging to precisely modulate their surface crystallinity. Herein, we have developed an efficient strategy for synthesising telluride CSNPs with thin amorphous shells using aqueous cation exchange (ACE). By changing the synthesis temperature in the range 40 to 110C, the crystallinity of the …
Magnetic properties of dense nanoparticle arrays with core/shell morphology
我们计算了有序阵列的复合磁性纳米粒子的铁磁（FM）的核心和反铁磁（AFM）壳，位于三角形晶格和耦合通过静磁力的磁滞。每个纳米颗粒由一对交换耦合（J）的各向异性自旋（Meiklejohn-Bean模型）描述。使用Metropolis Monte Carlo算法获得磁化磁滞回线。对于硬磁纳米颗粒，我们发现随着偶极耦合强度的增加，矫顽磁力减小，而交换偏置场则表现出非单调行为，这是由于随机各向异性和颗粒间偶极相互作用之间的竞争造成的。讨论了通过增加堆积密度来提高交换偏置场的可能性。
We present molecular dynamics (MD) simulations of radiation damage in pure Fe nanoparticles (NP) and bimetallic FeCu core-shell nanoparticles (CSNP). The CSNP includes a perfect body centered cubic (bcc) Fe core coated with a face-centered cubic (fcc) Cu shell. Irradiation with Fe Primary Knock-on Atoms (PKA) with energies between 1 and 7 keV leads to point defects, without clustering beyond divacancies and very few slightly larger vacancy clusters, and without interstitial clusters, unlike what happens in bulk at the same PKA energies. The Fe-Cu interface and shell can act as a defect sink, a…
While most of the experimental studies are dealing with partially oxidized metallic nanosolids, their behaviour is not well understood theoretically. To this end we utilized molecular dynamics simulation and charge-optimized many-body potential to probe melting of Cu nanoparticles and nanowires with and without a monolayer of Cu$_2$O as a shell. It is shown that partially oxidized nanosolids present a different melting behaviour than the widely accepted picture of melting, and especially different than surface melting. (i) For both types of nanosolids we observed inward diffusion of $\sim$40\%…
Nonmagnetic impurities and roughness effects on the finite temperature magnetic properties of core-shell spherical nanoparticles
受最近研究[V. Dimitriadis et al. Phys. Rev. B \textbf{92}，064420（2015）]的启发，我们研究了具有核-壳结构的球形纳米颗粒的有限温度磁性，包括淬灭的（i）表面和（ii）界面杂质（静态孔）以及（iii）粗糙化界面效应。粒子核由铁磁自旋组成，并且它被铁磁壳包围。通过基于改进的Metropolis算法的蒙特卡罗模拟，我们实现了经典的海森堡哈密顿纳米粒子。特别注意也被致力于阐明这些系统的热和磁相变功能上的颗粒尺寸的影响。对于表面层不完整的纳米颗粒，发现较大的颗粒具有较低的补偿点，该补偿点随着空位量的增加而逐渐减小，并在某一临界值处消失。考虑到纳米粒子的稀释界面，我们的蒙特卡罗模拟结果表明，在无序谱中存在一个区域，补偿温度随稀释参数的减小而线性降低。对于具有粗糙界面的纳米颗粒，观察到粗糙度对补偿点和临界温度的变化都不起任何显著作用。然而，低温饱和磁化的核和壳界面区域敏感地依赖于粗糙度参数。
One photon up-conversion photoluminescence is an optical phenomenon whereby the thermal energy of a fluorescent material increases the energy of an emitted photon compared with the energy of the photon that was absorbed. When this occurs with near unity efficiency, the emitting material undergoes a net decrease in temperature--so called optical cooling. Because the up-conversion mechanism is thermally activated, the yield of up-converted photoluminescence is also a reporter of the temperature of the emitter. Taking advantage of this optical signature, cesium lead trihalide nanocrystals are sho…
Bimetallic plasmonic nanoparticles enable tuning of the optical response and chemical stability by variation of the composition. The present numerical simulation study compares Ag-Au alloy, Ag@Au core-shell, and Au@Ag core-shell bimetallic plasmonic nanoparticles of both spherical and anisotropic (nanotriangle and nanorods) shapes. By studying both spherical and anisotropic (with LSPR in the near-infrared region) shapes, cases with and without interband transitions of Au can be decoupled. Explicit comparisons are facilitated by numerical models supported by careful validation and examination o…
