Structure and magnetic properties of epitaxial cobalt islands

Growth, structural and magnetic properties of cobalt island structures grown by molecular beam epitaxy on 4 nm thick island shaped Ru buffers are studied. The islands are characterized by a narrow distribution of sizes, and are separated on average by spacing of around one hundred nanometers. Growth of the islands into continuous films is found to proceed in three well defined stages, and corresponding magnetic behaviors are identified. The particles formed in the first stage of growth appear to be similar in shape and size, and their magnetic behavior suggest the presence of superparamagnetism. Particles in the second stage of growth are ferromagnetic and display a reorientation transition of their magnetization out of the film plane, consistent with that seen in patterned magnetic dot arrays. Particles coalesce in the last stage of growth and magnetic properties of continuous films are observed. Received 24 August 2001     

Impact of synthesis parameters on structural and magnetic characteristics of Co-based nanoparticles

Two different Co-based nanostructures were produced via thermolytic decomposition or reduction of proper cobalt precursors in organic solvents under vigorous stirring. The effect of synthesis parameters on the shape, size, and composition of the particles was examined. We present the differences in the structural and magnetic properties among the as-prepared sub-micron Co ‘polypod-like’ particles, which display a remarkable value for saturation magnetization (182 emu/g), and the hollow CoO nanoparticles, which exhibit weak ferromagnetic features.     

On features of the magnetic structure of spherical iron nanoparticles

The results of research into the behavior of the magnetic structure inherent to spherical ferromagnetic nanoparticles with radii of 5–30 nm are presented. The behavioral features are investigated in an external magnetic field by means of computer modeling. The hysteresis loops and formation of the vortex structure of magnetization are analyzed using particles with different sizes. The size effect of changes in the magnetization symmetry, which is analogous to phase transitions of the second kind, is established. The magnetic moments of spherical iron nanoparticles with radii of 5–30 nm are calculated. Calculations are performed by means of the Nmag micromagnetic simulation package.     

High-gradient magnetic separation of microparticles on membrane separation unit

A simple design of a magnetic separator based on a membrane made of a laser-perforated ferromagnetic foil has been proposed. The separator is primarily intended for analytical and research purposes. The developed magnetic separator of the proposed design has been tested in the separation of a composite aqueous suspension of magnetite nanoparticles adsorbed on hydroxyapatite microparticles. Separation efficiency has been determined via measuring the magnetic moment by the ferromagnetic resonance method; the suspension particle size has been found by dynamic light scattering before and after the separation process. It has been shown that all the particles with a diameter of more than 500 nm are retained during separation; the magnetization of the fraction decreases twofold after passing through the membrane.     

离子型钴铁氧体磁流体的稀土改性与机理研究

采用化学共沉淀法以柠檬酸三钠为表面改性剂制备了离子型稀土复合钴铁氧体磁流体.利用X衍射仪和透射电镜对磁粒子的组成、结构及粒径进行了分析.利用古埃磁天平和分光光度计研究了稀土离子改性对磁流体饱和磁化强度和磁感应的影响,实验结果表明:合成过程中添加稀土离子能明显降低磁性纳米粒子的粒径,制得的磁粒子均呈球形,钴铁氧体磁粒子的粒径为12～15nm,稀土钴铁氧体磁粒子的粒径为6～8nm.利用稀土改性的微观模型解释了粒径的降低.添加Dy3+能提高饱和磁化强度和磁感应,添加Y3+则能提高磁感应,却降低了饱和磁化强度.并从理论上对其改性机理进行了详细的分析.     

Ferromagnetism of zinc oxide nanograined films

The reasons for the appearance of ferromagnetic properties of zinc oxide have been reviewed. It has been shown that ferromagnetism appears only in polycrystals at a quite high density of grain boundaries. The critical size of grains is about 20 nm for pure ZnO and more than 40 μm for iron-doped zinc oxide. The solubility of manganese and cobalt in zinc oxide increases significantly with a decrease in the size of grains. The dependences of the saturation magnetization on the concentrations of cobalt, manganese, and ion are nonmonotonic. Even if the size of grains is below the critical value, the ferromagnetic properties of zinc oxide depend significantly on the texture of films and the structure of amorphous intercrystallite layers.     

Directed magnetic field induced assembly of high magnetic moment cobalt nanowires

A directed magnetic field induced assembly technique was employed to align two phase (h.c.p. + f.c.c.) cobalt nanoparticles in a mechanically robust long wire morphology. Co nanoparticles with an average size of 4.3 nm and saturation magnetization comparable to bulk cobalt were synthesized by borohydride reduction followed by size selection and magnetic field induced assembly. The coercivity of these nanowires was higher than their nanoparticle counterpart due to shape anisotropy. The experimental coercivity values of the nanowires were lower than the predictions of the coherent rotation, fanning and curling models of coercivity due to the preponderance of superparamagnetic particles with zero coercivity.     

Magnetic and magneto-optical properties of ion-synthesized cobalt nanoparticles in silicon oxide

The magnetic and magneto-optical properties of ion-synthesized cobalt nanoparticles in the amorphous silicon oxide matrix are investigated as a function of the implantation dose. The analysis of the field dependences of the magnetization and the magneto-optical Faraday and Kerr effects demonstrates that, as the ion implantation dose increases, the superparamagnetic behavior of an ensemble of cobalt nanoparticles at room temperature gives way to a ferromagnetic response with the anisotropy characteristic of a thin magnetic film. The magnetization curves for the superparamagnetic and ferromagnetic ensembles of cobalt nanoparticles are simulated to determine their average sizes and the filling density in the irradiated layer of the silicon dioxide matrix. It is revealed that the spectral dependences of the Faraday and Kerr effects for ion-synthesized cobalt nanoparticles differ substantially from those for continuous cobalt films due to the localized excitations of free electrons in the nanoparticles.     

Effective anisotropy field variation of magnetite nanoparticles with size reduction

Size effect on the internal magnetic structure has been investigated on weakly interacting magnetite (Fe₃O₄) nanoparticles by ferromagnetic resonance experiments at 9.5 GHz as a function of temperature (4–300 K). A set of three samples with mean particle size of 2.5 nm, 5.0 nm and 13.0 nm, respectively, were prepared by chemical route with narrow size distribution (σ < 0.27). To minimize the dipolar interaction, the particles were dispersed in a liquid and a solid polymer matrix at ∼0.6% in mass. By freezing the liquid suspension with an applied external field, a textured was obtained. Thus, both random and textured suspensions were studied and compared. The ferromagnetic resonance experiments in zero-field-cooled and field-cooled conditions were carried out to study the size effect on the effective anisotropy field. The dc magnetization measurements clearly show that the internal magnetic structure was strongly affected by the particle size.     

Anisotropic ferromagnetism induced in rutile single crystals by Co implantation

The magnetic and electrical properties of Co-implanted single crystalline TiO₂ rutile are presented. For fluences of the order of 10¹⁷ cm^-2 and implantation energy of 150 keV the maximum atomic concentration of cobalt is 13 at% at a depth of 65 nm from the surface. The as implanted single crystals exhibit superparamagnetic behaviour attributed to the formation of nanosized cobalt clusters. After annealing at 1073 K an anisotropic ferromagnetic behaviour emerges with the easy magnetization axis lying in the (001) plane of rutile. The ferromagnetic behaviour is associated with oriented cobalt aggregates. Electrical conductivity of the implanted samples annealed in vacuum also exhibits anisotropic behaviour at low temperatures, but no magnetoresistive effects were detected.     

Band structure of collective modes and effective properties of binary magnonic crystals

In this paper a theoretical study of the band structure of collective modes of binary ferromagnetic systems formed by a submicrometric periodic array of cylindrical cobalt nanodots partially or completely embedded into a permalloy ferromagnetic film is performed. The binary ferromagnetic systems studied are two-dimensional periodic, but they can be regarded as three-dimensional, since the magnetization is non uniform also along the z direction due to the contrast between the saturation magnetizations of the two ferromagnetic materials along the thickness. The dynamical matrix method, a finite-difference micromagnetic approach, formulated for studying the dynamics in one-component periodic ferromagnetic systems is generalized to ferromagnetic systems composed by F ferromagnetic materials. It is then applied to investigate the spin dynamics in four periodic binary ferromagnetic systems differing each other for the volume of cobalt dots and for the relative position of cobalt dots within the primitive cell. The dispersion curves of the most representative frequency modes are calculated for each system for an in-plane applied magnetic field perpendicular to the Bloch wave vector. The dependence of the dispersion curves on the cobalt quantity and position is discussed in terms of distribution of effective “surface magnetic charges” at the interface between the two ferromagnetic materials. The metamaterial properties in the propagative regime are also studied (1) by introducing an effective magnetization and effective “surface magnetic charges” (2) by describing the metamaterial wave dispersion of the most representative mode in each system within an effective medium approximation and in the dipole-exchange regime. It is also shown that the interchange between cobalt and permalloy does not necessarily lead to an interchange of the corresponding mode dispersion. Analogously to the case of electromagnetic waves in two-dimensional photonic crystals, the degree of localization of the localized collective modes is expressed in terms of an energy concentration factor.     

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.     

Correlation between the real Co particle size distributions and the magnetic properties of GMR Cu–10Co alloy

The results of transmission electron microscopy (TEM), magnetic and magneto-resistivity investigations of the Cu–10Co (wt%) giant magneto-resistance (GMR) melt-spun ribbons are reported and discussed. To obtain different distributions of ferromagnetic Co particles in a non-magnetic Cu matrix, the alloy was aged at 550 °C for 0.5, 1, 2, 16, and 32 h. Particle size measurements were performed using quantitative TEM metallography methods. Two size classes of Co particles are identified: primary particles (P) precipitated during the melt-spinning process and the secondary particles (S) precipitated during the ageing process. The results of magnetization and coercitivity are correlated with the results of calculations based on the real Co particle distributions determined from TEM micrographs. The behavior of magnetization and coercive force in function of ageing time is explained as related with changes of a mean particle size. It is shown that the GMR effect is not influenced by Co particles distribution for the S particles with the mean size less than 10 nm, whereas for Co distributions with larger mean diameters, the GMR effect is strongly reduced.     

Magnetization reversal of elliptic Co/Si/Co nanodisks in the field of a magnetic-force microscope probe

The processes of local magnetization reversal of elliptic Co/Si/Co nanodisks under the action of a nonuniform magnetic field of a magnetic-force microscope (MFM) probe have been investigated. The specific features of the distribution of the phase MFM contrast from particles with ferromagnetic and antiferromagnetic configurations of the magnetic moments in neighboring Co layers have been discussed. It has been shown experimentally that, under the action of the probe field, there occur orientational transitions of two types: transitions from the ferromagnetic configuration to the antiferromagnetic configuration due to the reorientation of the magnetization of the upper layer and transitions in the antiferromagnetic configuration with a change in the orientation of the magnetic moment in both ferromagnetic layers. The presented results of micromagnetic simulation of the processes of transformation of the magnetization in such particles under the action of the MFM probe field explain the main regularities of the magnetization reversal processes.     

Magnetic properties of electron-doped La0.23Ca0.77MnO3 nanoparticles

Magnetic properties of electron-doped La_0.23Ca_0.77MnO₃ manganite nanoparticles, with average size of 12 and 60?nm, prepared by the glycine?Cnitrate method, have been investigated in the temperature range 5?C300?K and magnetic fields up to 90?kOe. It is suggested that weak ferromagnetic moment results from ferromagnetic shells of the basically antiferromagnetic nanoparticles and from domains of frustrated disordered phase in the core. Assumption of two distinct sources of ferromagnetism is supported by the appearance of two independent ferromagnetic contributions in the fit of the T ^3/2 Bloch law to spontaneous magnetization. The ferromagnetic components, which are more pronounced in smaller particles, occupy only a small fraction of the nanoparticle volume and the antiferromagnetic ground state remains stable. It is found that the magnetic hysteresis loops following field cooled processes, display size-dependent horizontal and vertical shifts, namely, exhibiting exchange bias effect. Time-dependent magnetization dynamics demonstrating two relaxation rates were observed at constant magnetic fields upon cooling to T?<?100?K.     

Comparison of frequency,field, and time domain ferromagnetic resonance methods

We present vector network analyzer ferromagnetic resonance measurements of epitaxial Fe films having a thickness of 16 monolayers. Our objective is to test the reliability of this novel frequency domain technique with respect to frequency and damping. For this purpose we compare vector network analyzer ferromagnetic resonance to pulsed inductive microwave magnetometry, time resolved magneto-optic Kerr effect (both methods in the time domain), and conventional ferromagnetic resonance (measured in the field domain) in terms of position and width of the ferromagnetic resonance. In addition, we compare the various techniques with respect to the signal to noise ratio of the raw data. All data is obtained using the same well characterized ultrathin magnetic Fe/GaAs (0 0 1) film. Finally, we demonstrate the potential of the vector network analyzer ferromagnetic resonance technique for the investigation of nano-structured magnetic elements having nonuniform magnetization configuration. The absorption spectrum of Permalloy disks with a diameter of 200 nm and a thickness of 15 nm shows up to eight distinct resonance peaks. The spatial structure of the corresponding modes was derived from numerical calculations and reveals that azimuthal modes up to the fifth order have been observed inductively.     

Electromagnetic and magnetic properties of magnetophotonic crystal based on opal matrix with Co and CoO nanoparticles

Magnetic properties of magnetophotonic crystals based on opal matrices have been studied as well as their electromagnetic properties in millimeter waveband. The particles of cobalt oxide are embedded into the inter-sphere voids of the matrix. After annealing in hydrogen the cobalt oxide particles transform to metallic cobalt. It has been shown that if antiferromagnetic cobalt oxide remains besides ferromagnetic cobalt, the low-temperature magnetic hysteresis loop is shifted along the field axis. Magnetic field influences essentially on the microwave transmission and reflection coefficients only after annealing in hydrogen that is if the ferromagnetic phase presents in the sample. The spectra of magnetic resonance and antiresonance are studied.     

Femtosecond magneto-optical Kerr microscopy

We have developed a magneto-optical Kerr microscope that allows us to measure the ultrafast magnetization dynamics of ferromagnetic nanostructures. The magneto-optical signal can be recorded in a confocal reflection geometry with an accurate selection of the polarization. The magnetization dynamics is obtained from pump-probe measurements using frequency nondegenerate collinear pump and probe beams with a temporal resolution of 180 fs. Both probe and pump beams are focused to their diffraction limit, leading to an overall spatial resolution of 600 nm. The efficiency of the apparatus is tested by investigating the magnetization dynamics of individual CoPt(3) disks with a submicrometer diameter and a thickness of 15 nm.     

Nanoparticles of: Synthesis and superparamagnetic properties

By using oil in water micelles, cobalt ferrite particles having an average diameter around 3 nm were synthetised. These nanoparticles are characterized by the presence of cation vacancies and no Fe(II) is observed, as it has been described in literature previously. Chemical interfacial treatment allows to coat the particles with citrate derivatives. The magnetic properties of uncoated and coated particles strongly diluted in a polymer substrate are compared by magnetization measurements and ⁵⁷Fe M?ssbauer spectroscopy. The anisotropy constant is shown to be independent of coating, whereas the magnetization is found to be larger in the uncoated particles. Received 3 February 1998