氧化镁纳米多晶的微结构和磁性

实验发现,宏观晶体是非磁性的氧化镁时,其多晶样品有弱铁磁性.本文用第一性原理电子结构方法研究了氧化镁表面、纳米颗粒和晶界的磁性.计算结果表明:绝缘的氧化镁表面可以是导电的,并且有与之相关的铁磁性;磁性表面的共同特征是在表面上有氧原子富集,包括(111)表面的纯氧原子层,(114)表面的氧原子链;其他高晶面指数表面也会有氧原子富集区域;氧化镁纳米颗粒的磁性出现在高晶面指数表面以及不同晶面交界的棱及其顶角等有氧原子富集的区域,这种由氧原子富集而形成的磁性有巡游特征.氧化镁Σ7[111]和Σ5[001]晶界的计算结果表明:在没有氧原子富集的情况下,多晶样品中晶界的磁性很弱,而在有氧原子富集的情况下,晶界磁性比较强.因此可以推断多晶样品的磁性主要出现在多晶表面、晶粒包围孔隙、微裂纹界面、晶界和其他晶体缺陷等有氧原子富集的区域.这种残余磁性可以通过热处理等结构优化过程而削弱甚至消除.     

Magnetism in structures with ferromagnetic and superconducting layers

The influence of superconductivity on ferromagnetism in the layered Ta/V/Fe_1–x V _x /V/Fe_1–x V _x /Nb/Si structures consisting of ferromagnetic and superconducting layers is studied using polarized neutron reflection and scattering. It is experimentally shown that magnetic structures with linear sizes from 5 nm to 30 μm are formed in these layered structures at low temperatures. The magnetization of the magnetic structures is suppressed by superconductivity at temperatures below the superconducting transition temperatures in the V and Nb layers. The magnetic states of the structures are shown to undergo relaxation over a wide magnetic-field range, which is caused by changes in the states of clusters, domains, and Abrikosov vortices.     

Collective states of interacting ferromagnetic nanoparticles

Discontinuous magnetic multilayers [CoFe/Al₂O₃] are studied by use of magnetometry, susceptometry and numeric simulations. Soft ferromagnetic Co₈₀Fe₂₀ nanoparticles are embedded in a diamagnetic insulating a-Al₂O₃ matrix and can be considered as homogeneously magnetized superspins exhibiting randomness of size (viz. moment), position and anisotropy. Lacking intra-particle core-surface ordering, generic freezing processes into collective states rather than individual particle blocking are encountered. With increasing particle density one observes first superspin glass and then superferromagnetic domain state behavior. The phase diagram resembles that of a dilute disordered ferromagnet. Criteria for the identification of the individual phases are given.     

Oxidation activity of Au nanoparticles on aerogel supports

We have prepared titania aerogel and titania-coated silica aerogel incorporating thiol-capped Au nanoparticles. Both composite materials showed high CO oxidation activity after they were calcined at 673 K. Compositional and morphological changes driven by calcination were evaluated with thermogravimetry and X-ray diffractometry. From the results, it was suggested that the nanoparticles transformed from a faulted to a near-regular FCC structure presumably in concert with the formation of firm contacts between the nanoparticles and the gel substrates. While the diameters of the Au particles in the titania aerogel considerably increased upon calcination, those in the titania-coated silica aerogel were almost unchanged. As a consequence, the latter composite aerogel showed higher activity for oxidation of CO.     

Magnetism induced by capping of non-magnetic ZnO nanoparticles

Magnetism of 10 nm size capped nanoparticles, NPs, of non-magnetic ZnO is analysed in terms of the surface band since, as magnetic dichroism analysis has pointed out, impurity atoms bonded to the surface act as donor or acceptor of electrons that occupy the surface states. Due to the nanometric scale of the particles the kinetic energy spectrum of the surface states can be considered as discrete. Therefore, the magnetic polarisation cannot be easily induced by pumping electrons to energy levels above the Fermi energy. It is in the Fermi level itself, generally unfilled, that develops a spontaneous magnetic moment similarly to that induced by Hund rules in unfilled atomic orbitals. It is shown, however, that the total magnetic moment of the surface originated at the unfilled Fermi level can reach values as large as 10² or 10³ Bohr magnetons.     

Coupling of ferromagnetic nanoparticles through dipolar interactions

We consider two ferromagnetic nanoparticles coupled via long-range dipolar interactions. We model each particle by a three-dimensional array of classical spin vectors, with a central spin surrounded by a variable number of shells. Within each particle only ferromagnetic coupling between nearest neighbor spins is considered. The interaction between particles is of the dipolar type and the magnetic properties of the system is studied as a function of temperature and distance between the centers of the particles. We perform Monte Carlo simulations for particles with different number of shells, and the magnetic properties are calculated via two routes concerning the dipolar contribution: one assuming a mean-field like coupling between effective magnetic moments at the center of the particles, and other one, where we take into account interactions among all the pairs of spins, one in each particle. We show that the dipolar coupling between the particles enhances the critical temperature of the system relative to the case in which the particles are very far apart. The dipolar energy between the particles is smaller when the assumption of effective magnetic moment of the particles is used in the calculations.     

Magnetism manipulation in ferromagnetic/ferroelectric heterostructures by electric field induced strain

Magnetization manipulation by an electric field(E-field) in ferromagnetic/ferroelectric heterostructures has attracted increasing attention because of the potential applications in novel magnetoelectric devices and spintronic devices, due to the ultra-low power consumption of the process. In this review, we summarize the recent progress in E-field controlled magnetism in ferromagnetic/ferroelectric heterostructures with an emphasis on strain-mediated converse magnetoelectric coupling. Firstly, we briefly review the history, the underlying theory of the magnetoelectric coupling mechanism, and the current status of research. Secondly, we illustrate the competitive energy relationship and volatile magnetization switching under an E-field. We then discuss E-field modified ferroelastic domain states and recent progress in non-volatile manipulation of magnetic properties. Finally, we present the pure E-field controlled 180° in-plane magnetization reversal and both E-field and current modified 180° perpendicular magnetization reversal.     

Magnetoresistance and noncollinear structures of multilayer ferromagnetic nanoparticles

The distribution of the magnetization over multilayer particles, including three ferromagnetic layers separated by insulating spacers, is studied experimentally and theoretically. Experimental data on the magnetic state of these particles are obtained by measuring their magnetoresistance. For the case of zero applied field, it is shown that a multilayer particle with easy-plane magnetic anisotropy is a noncollinear helical state.     

Equilibrium magnetic properties of dipolar interacting ferromagnetic nanoparticles

We use Monte Carlo simulations to study the influence of dipolar interaction on the equilibrium magnetic properties of monodisperse single-domain ferromagnetic nanoparticles. Low field magnetizations simulated in zero field cooling (ZFC)/field cooling (FC) procedures and field-dependent magnetization curves above the blocking temperatures show strong dependence on the concentration and the spatial arrangement (cubic or random) of the magnetic particles. The field-dependent magnetizations can not be simply described by the T^* model at relative low temperatures due to the interplay between anisotropy and dipolar interactions, as well as the spatial arrangement effect.     

Possibility of observing chiral-symmetry effects in ferromagnetic nanoparticles

The statistics of left-and right-handed vortical magnetization states in a system of submicron-sized ferromagnetic disks is studied experimentally. Lattices of elliptic cobalt particles with a planar size of 750 × 450 nm and a thickness of 25 nm were formed using high-resolution electronic lithography. To visualize the magnetization distributions in the disks, magnetic force microscopy was used. The experimental results indicate the conservation of magnetic chiral symmetry in cobalt ferromagnetic particles of the above size and shape.     

Theoretical study of phonon spectra in ferromagnetic nanoparticles

Based on the spin-phonon model we analyze the influence of surface and size effects on the phonon properties of ferromagnetic nanoparticles. A Green's function technique in real space enables us to calculate the renormalized phonon energy and its damping depending on the temperature and the anharmonic spin-phonon interaction constants. With decreasing particle size the phonon energy can decrease or increase for different surface spin-phonon interaction constants, whereas the damping increases always. The influence of an external magnetic field is discussed, too. The theoretical results are in reasonable accordance to experimental data.     

Magnetism in nanoparticles of semiconducting FeSi2

Iron disilicide has been found to exhibit superparamagnetism in nanoparticles, even though no magnetic ordering occurs in bulk. The unexpected behavior was attributed, based on magnetic studies, to chemical disorder. A lack of sextet-type signals in Mössbauer spectra supports that the observed magnetic order is confined to only a very small fraction of magnetic Fe ions. Moreover, quadrupole-splitting and isomer-shift parameters reveal a significant amount of Fe in a short-range α-FeSi₂ structure, while XRD suggests an overall β-FeSi₂ structure. Such a compositional heterogeneity is also reflected in a calorimetrically obtained spin-glass-like anomaly at low temperatures. Meanwhile, as the particle size decreases, specific heat and Sommerfeld constant are enhanced due to lattice softening and the emergence of surface charge density of states, respectively, in nanoparticles.     

Non-aggregated Pd nanoparticles deposited onto catalytic supports

Nanostructured powders have shown great promise for a variety of applications including chemical gas sensors, high surface area supports for catalysis, tribology, chemical mechanical polishing, and optoelectronics. In this report, highly dispersed Pd nanoparticles with a narrow size distribution, and mean diameter of 2±0.2 nm, were deposited at room temperature onto amorphous carbon and oxide supports (TiO₂, Al₂O₃) by pulsed-laser ablation of a Pd sputtering target. Depositions were performed in Ar at a back-fill pressure of 3 mTorr after reaching a base pressure of 10^-7 Torr. Populations of uniformly dispersed particles with an interparticle spacing of 3 to 10 nm were observed by high-resolution transmission electron microscopy with little evidence of nanoparticle aggregation. The chemical compositions of individual nanoparticles were confirmed by high spatial resolution energy-dispersive X-ray spectroscopy.     

Spin excitations in granular structures with ferromagnetic nanoparticles

Spin excitations and relaxation in a granular structure which contains metallic ferromagnetic nanoparticles in an insulating amorphous matrix are studied in the framework of the s-d exchange model. As the d system, we consider the granule spins, and the s system is represented by localized electrons in the amorphous matrix. In the one-loop approximation with respect to the s-d exchange interaction for a diagram expansion of the spin Green’s function, the spin excitation spectrum is found, which consists of spin-wave excitations in the granules and of polarized spin excitations. In polarized spin excitations, a change in the granule spin direction is accompanied by an electron transition with a spin flip between two sublevels of a split localized state in the matrix. We considered polarized spin relaxation (relaxation of the granule spins occurring by means of polarized spin excitations) determined by localized deep energy states in the matrix and the thermally activated electronic cloud of the granule. It is found that polarized spin relaxation is efficient over a wide frequency range. Estimates made for structures with cobalt granules showed that this relaxation could be observed in centimetric, millimetric, and submillimetric wavelength ranges.     

Spin-dependent tunneling and Coulomb blockade in ferromagnetic nanoparticles

In this paper we review studies on spin-dependent transport in systems containing ferromagnetic nanoparticles. In a tunnel junction with a nanometer-scale-island, the charging effect leads to an electric current blockade phenomenon in which a single electron charge plays a significant role in electron transport, resulting in single-electron tunneling (SET) properties such as Coulomb blockade and Coulomb staircase. In a tunnel junction with a ferromagnetic nano-island and electrode, it was expected that the interplay of spin-dependent tunneling (SDT) and SET, i.e., spin-dependent single-electron tunneling (SD-SET), would give rise to remarkable tunnel magnetoresistance (TMR) phenomena. We investigated magnetotransport properties in both sequential tunneling and cotunneling regimes of SET and found the enhancement and oscillation of TMR. The self-assembled ferromagnetic nanoparticles we have employed in this study consisted of a Co–Al–O granular film with cobalt nanoparticles embedded in an Al–O insulating matrix. A _Co36Al22O42${\mathrm{Co}}_{36}{\mathrm{Al}}_{22}{\mathrm{O}}_{42}$ film prepared by a reactive sputtering method produced a TMR ratio reaching 10% and superparamagnetic behavior at room temperature. The TMR ratio exhibited an anomalous increase at low temperatures but no indication of change with bias voltage. In Section 4, we show that the anomalous increase of the MR provided evidence for higher-order tunneling (cotunneling) between large granules through intervening small granules. We emphasize that the existence of higher-order tunneling is a natural consequence of the granular structure, since broad distribution of granule size is an intrinsic property of granular systems. In Section 5, we concentrate on SD-SET properties in sequential tunneling regimes. We fabricated two types of device structures with Co–Al–O film using focused ion-beam milling or electron-beam lithography techniques. One had a granular nanobridge structure: point-shaped electrodes separated by a very narrow lateral gap filled with the Co–Al–O granular film. The other had a current-perpendicular-to-plane (CPP) geometry structure: a thin Co–Al–O granular film sandwiched by ferromagnetic electrodes with the current flowing in the direction perpendicular to the film plane through a few Co particles. We found the enhancement and oscillation of TMR due to spin-dependent SET in sequential tunneling regimes. In Section 6, we report experimental evidence of a spin accumulation effect in Co nanoparticles leading to the oscillation of TMR with alternate sign changes. Furthermore, we discovered that the spin relaxation time in the nanoparticles is unprecedentedly enhanced up to the order of more than hundreds of nanoseconds, compared to that evaluated from the spin-diffusion length of ferromagnetic layers in previous CPP-GMR studies, i.e., the order of tens of picoseconds.     

Magnetization reversal of ferromagnetic nanoparticles under inhomogeneous magnetic field

We investigated remagnetization processes in ferromagnetic nanoparticles under inhomogeneous magnetic field induced by the tip of magnetic force microscope (MFM) in both theoretical and empirical ways. Systematic MFM observations were carried out on arrays of submicron-sized elliptical ferromagnetic particles of Co and FeCr with different sizes and periods. It clearly reveals the distribution of remanent magnetization and processes of local remagnetization of individual ferromagnetic particles. Modeling of remagnetization processes in ferromagnetic nanoparticles under magnetic field induced by MFM probe was performed on the base of Landau–Lifshitz–Gilbert equation for magnetization. MFM-induced inhomogeneous magnetic field is very effective to control the magnetic state of individual ferromagnetic nanoparticles as well as to create different distribution of magnetic field in array of ferromagnetic nanoparticles.     

Direct observation of ferromagnetic spin polarization in gold nanoparticles

We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using x-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms as estimated by XMCD shows a good agreement with results obtained by conventional magnetometry. This evidences intrinsic spin polarization in nanosized gold.     

Laser morphological manipulation of gold nanoparticles periodically arranged on solid supports

Laser morphological manipulation has been performed on gold nanoparticles periodically arranged on solid supports by nanosphere lithography. It is shown that third harmonic Nd:YAG pulse laser irradiation (15 mJ/cm² per pulse) leads to an evolution from the initial polycrystalline, triangular shaped particles to spherical single crystalline particles. These morphological changes are accompanied by a significant change in the optical absorption properties of the array. This study demonstrates that laser irradiation is an excellent technique to control the properties of nanostructured materials on solid supports. Further, it is suggested that the selected area treatment possible with the laser is a valuable new technique for designing functional nanomaterials for applications such as high-density data storage devices. PACS 61.46.+w; 42.79.Vb; 52.38.Bv; 78.67.Hc     

Magnetism of magnetite nanoparticles: Effects of finite size and coating

How the size of magnetite nanopaticles affects different magnetic properties of the material (magnetic moment, Curie temperature, blocking temperature, and so on) is examined experimentally. Both native particles and particles coated with bioresorbed layers are studied. Experimental results are compared to results obtained via numerical simulations in the classical Heisenberg model with the help of the Monte Carlo method.