Barium ferrite magnetic recording media

Barium ferrite particulate media have generated a lot of interest for advanced magnetic recording applications because they offer the potential to combine high recording densities with relatively low manufacturing cost. They consist of small (sub-tenth micron) plateletshaped particles with competing orthogonal anisotropies (crystalline and shape) of comparable magnitude. These anisotropies, along with the quasi-perpendicular characteristics of the barium ferrite coatings impart to them many subtle and surprising properties, requiring a careful and judicious choice of parameters for each application. The choices include the aspect ratio of the particles, their coercivity, the particle-to-binder loading, and the degree and direction of magnetic orientation. The problem areas include dispersion and orientation of the particles, overwrite characteristics of the coatings, thermal coefficients of the magnetic parameters and maintaining media coercivities at moderate levels. I this paper, we discuss the effect of the particle and coating parameters on the ensuing magnetic and recording properties of the media, and the types of choices that should be made to minimize the impact of some of the potential problems mentioned above.     

Self-organized magnetic nanowire arrays based on alumina and titania templates

Densely packed arrays of magnetic nanowires have been synthesized by electrodeposition filling of nanopores in alumina and titania membranes formed by self-assembling during anodization process. Emphasis is made on the control of the production parameters leading to ordering degree and lattice parameter of the array as well as nanowires diameter and length. Structural, morphological and magnetic properties exhibited by nanowire arrays have been studied for several nanowire compositions, different ordering degree and for different nanowire aspect ratios. The magnetic behaviour of nanowires array is governed by the balance between different energy contributions: shape anisotropy of individual nanowires, the magnetostatic interaction of dipolar origin among nanowires, and magnetocrystalline and magnetoelastic anisotropies induced by the pattern templates. These novel nanocomposites, based on ferromagnetic nanowires embedded in anodic nanoporous templates, are becoming promising candidates for technological applications such as functionalised arrays for magnetic sensing, ultrahigh density magnetic storage media or spin-based electronic devices.     

Observation of aggregates in magnetic particle lacquers

A method for preparing samples of magnetic lacquers for transmission electron microscopy is described. The samples contain only the permanent aggregates present in a suspension, and not the temporary agglomerates that are easily broken up when the suspension is sheared. The method is based on a dilution of the material studied with other magnetic particles that are dissolved after a thin film of the mixed suspension is deposited on a carbon substrate film. Using this method, it is shown that most of the chromium dioxide in a well-dispersed lacquer occurs in parallel aggregates of 2-10 particles that were never separated. The formation of new, lasting aggregates in the lacquer was found to take place in the course of days     

Alignment of SWNTs by protein-ligand interaction of functionalized magnetic particles under low magnetic fields

Carbon nanotubes (CNTs) have attracted considerable attention for applications using their superior mechanical, thermal and electrical properties. A simple method to controllably align single-walled CNTs (SWNTs) by using magnetic particles embedded with superparamagnetic iron oxide as an accelerator under the magnetic field was developed. The functionalization of SWNTs using biotin, interacted with streptavidin-coupled magnetic particles (micro-to-nano in diameter), and layer-by-layer assembly were performed for the alignment of a particular direction onto the clean silicon and the gold substrate at very low magnetic forces (0.02-0.89 T) at room temperature. The successful alignment of the SWNTs with multi-layer film was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). By changing the orientation and location of the substrates, crossed-networks of SWNTs-magnetic particle complex could easily be fabricated. We suggest that this approach, which consists of a combination of biological interaction among streptavidin-biotin and magnetite particles, should be useful for lateral orientation of individual SWNTs with controllable direction.     

Quantitative analysis of adherent cell orientation influenced by strong magnetic fields

Examines the effect of strong magnetic fields on adherent cells. Smooth muscle cells, cultured in 8- or 14-T superconducting magnets for three days, exhibited orientational order parallel to the magnetic field direction. To discuss the process and the mechanism of the orientation, the orientational characteristic of the cell culture was investigated with quantitative measurements: an orientational order parameter and the Fourier transform (FT) analysis. The orientational order parameter indicates the degree of orientation. The value of the parameter was estimated with the FT of microscopic images. The cells cultured under stronger static magnetic fields exhibited stronger ordering, while they showed weaker ordering in the control when they were cultured under a strong magnetic gradient force of 400 T/sup 2//m. The ordering was enhanced under uniform strong magnetic fields, while it was not affected or was suppressed by the strong gradient force. We suggest that the cells organize themselves to minimize their diamagnetic torsion stresses, which can be induced in the uniform magnetic fields by the membrane's diamagnetic anisotropy.     

Polypeptide folding-mediated tuning of the optical and structural properties of gold nanoparticle assemblies

Responsive hybrid nanomaterials with well-defined properties are of significant interest for the development of biosensors with additional applications in tissue engineering and drug delivery. Here, we present a detailed characterization using UV-vis spectroscopy and small angle X-ray scattering of a hybrid material comprised of polypeptide-decorated gold nanoparticles with highly controllable assembly properties. The assembly is triggered by a folding-dependent bridging of the particles mediated by the heteroassociation of immobilized helix-loop-helix polypeptides and a complementary nonlinear polypeptide present in solution. The polypeptides are de novo designed to associate and fold into a heterotrimeric complex comprised of two disulfide-linked four-helix bundles. The particles form structured assemblies with a highly defined interparticle gap (4.8±0.4 nm) that correlates to the size of the folded polypeptides. Transitions in particle aggregation dynamics, mass-fractal dimensions and ordering, as a function of particle size and the concentration of the bridging polypeptide, are observed; these have significant effects on the optical properties of the assemblies. The assembly and ordering of the particles are highly complex processes that are affected by a large number of variables including the number of polypeptides bridging the particles and the particle mobility within the aggregates. A fundamental understanding of these processes is of paramount interest for the development of novel hybrid nanomaterials with tunable structural and optical properties and for the optimization of nanoparticle-based colorimetric biodetection strategies.     

Interface defects and formation of non-collinear magnetic ordering in Fe/Cr multilayers

Non-collinear magnetic structure of Fe/Cr multilayers was investigated within the framework of Periodic Anderson model (PAM) in mean field approximation for Coulomb repulsion on sites. Self-consistent calculations were performed for the superlattices with different step width at the interface. It is shown that due to frustration in the interface region the ground state corresponds to non-collinear orientation of magnetic moments near steps. This non-collinear ordering penetrates on a large distance from the interface both in Fe and Cr layers and leads to the non-collinear magnetic coupling between Fe layers through the Cr spacer. Angle between magnetic moments of Fe slabs depends strongly not only on the thickness of the Cr spacer but also on the interface structure at atomic scale. It is found that only very specific types of the interface defects with plural frustrations can give out-of-plane orientation of magnetic moments.     

Surface functionalization for tailoring the aggregation and magnetic behaviour of silica-coated iron oxide nanostructures

We report here a detailed structural and magnetic study of different silica nanocapsules containing uniform and highly crystalline maghemite nanoparticles. The magnetic phase consists of 5 nm triethylene glycol (TREG)- or dimercaptosuccinic acid (DMSA)-coated maghemite particles. TREG-coated nanoparticles were synthesized by thermal decomposition. In a second step, TREG ligands were exchanged by DMSA. After the ligand exchange, the ζ potential of the particles changed from -10 to -40 mV, whereas the hydrodynamic size remained constant at around 15 nm. Particles coated by TREG and DMSA were encapsulated in silica following a sol-gel procedure. The encapsulation of TREG-coated nanoparticles led to large magnetic aggregates, which were embedded in coalesced silica structures. However, DMSA-coated nanoparticles led to small magnetic clusters inserted in silica spheres of around 100 nm. The final nanostructures can be described as the result of several competing factors at play. Magnetic measurements indicate that in the TREG-coated nanoparticles the interparticle magnetic interaction scenario has not dramatically changed after the silica encapsulation, whereas in the DMSA-coated nanoparticles, the magnetic interactions were screened due to the function of the silica template. Moreover, the analysis of the AC susceptibility suggests that our systems essentially behave as cluster spin glass systems.     

Characteristics of barium ferrite tape for magnetic contactduplication

The magnetic properties and electromagnetic characteristics of Ba-ferrite tape for R-DAT magnetic contact duplication (MCD) were investigated. The tapes employed Ba-ferrite particles averaging 0.06 μm in diameter with an aspect ratio between 4 and 5. Ba-ferrite tapes were prepared with perpendicular, longitudinal, and no orientation, and their MCD and ring-head recording (RHR) characteristics were compared. Several distinctive characteristics were observed in MCD, and the desirability of perpendicular orientation at short wavelengths in MCD applications is clear. Tilted orientation was also investigated, and its output level was similar to that of perpendicular orientation in MCD, while in RHR it proved superior to perpendicular orientation in the short wavelength region. The magnetization mode of the slave tape was studied by waveform analysis using the FFT method and in conjunction with the tapes' magnetic properties, such as uniaxial anisotropy energy, remanence coercivity, and squareness ratio     

The magnetic properties of ultrafine particles of Ni(OH)2

Ultrafine particles of Ni(OH)₂ were prepared by precipitation from aqueous solution. The susceptibility of the ultrafine particles shows a distinctive maximum at temperature which is rather lower than T_N of the large particles. Using the c-axis oriented ultrafine particles prepared by the sedimentation method, the magnetic susceptibility and the magnetization curve perpendicular and parallel to the c-axis of the ultrafine particles were measured. The magnetization curve showed a small hysteresis in low field and a remanent magnetization which increased with decrease of the particle size. Two-step jump was observed in the magnetization curve of the ultrafine particles when the external field was applied along the c-axis. The magnetic properties characteristic of the Ni(OH)₂ ultrafine particles are discussed.     

Self-organization of colloidal particles during drying of a droplet: Modeling and experimental study

Formation of structurized micro/nanoparticle aggregates in spray drying process is analyzed theoretically and experimentally. Colloids of mono- and bimodal particle size distribution are used as the precursors to demonstrate different patterns of particle self-organization inside the drying droplet. In case of monodisperse primary particles their self-organization in the final aggregate results in either a hollow or a full (packed) spherical structure. For primary particles with bimodal size distribution, either the layered structure of aggregates is formed (with smaller particles forming outer layer and the bigger particles captured inside) or the ordering of bigger particles on the aggregate surface is observed, depending on process parameters. Numerical investigations allow to predict and explain the conditions at which self-assembling of particles within powder aggregates takes place.     

The role of aggregation of ferrite nanoparticles on their magnetic properties

We have studied the magnetic properties of aggregates of Mn0.5Zn0.5Gd(x)Fe(2-x)O4 ferrite nanoparticles, with x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.18, 0.20. The scanning electron microscopy micrographs show significant aggregation of the nanoparticles in all samples. Zero field cooled and field cooled magnetization measurements were conducted on all samples from 400 K down to 5 K. Most zero field cooled curves were found to exhibit the usual behavior but with wide peaked regions. For some x values, the field cooled magnetization was found to increase slowly with decreasing temperature, and becomes nearly constant at low temperatures. The measurements of magnetization versus applied magnetic field were conducted on all samples at 5 K and 305 K in the field range from -15000 to 15000 Oe. At 305 K the magnetization for all samples was observed to saturate, while at 5 K the magnetization did not reach saturation for some values of x. The saturation magnetization values were suggested to be proportional to the size of particles. These results were discussed and suggested to be due to the inter-particle dipolar and exchange interactions between the particles in the aggregates, the large particle size distribution and the surface magnetization effects.     

Orientation process of acicular oxides in the magnetic tape layer

The orientation of acicular particles in the active layer of a magnetic tape is presently a usual technological process applied in magnetic tape production. The theory of this process is dealt with. A mathematical model is deduced, describing the behavior of an isolated agglomerate of acicular particles in an external magnetic field and the change of its microstructure, i.e., the orientation of acicular particles in the agglomerate. The term for the density of probability ω(α) of the angle distribution α of the longer particle axes in the orientated agglomerate is also deduced. Also the approximate relation between the orientation degree, defined by the mean quadratic deviationsof the angles α of the particular particles and by the bevel values γ of the deformed agglomerate, is shown. It appears that, besides the orientation by means of the external magnetic field, it would also be possible to orientate the magnetic suspension by other methods.     

Magnetic anisotropies in single and multilayered thin films grownby bowed-substrate sputtering

Thin-film structures composed of nearly nonmagnetostrictive single-layer Co₇₆Fe₄B₂₀ or magnetostrictive Fe₈₀B₂₀ and Co₇₅Si₁₅B₁₀ amorphous layers have been deposited on bowed glass substrates using the RF-sputtering technique. The fabrication procedure induces a postdeposition compressive stress in the thin-film structure when the sample is retrieved from an arching device in the sputtering chamber. This results in an induced magneto-elastic anisotropy that governs the magnetic easy axis of the film, depending on the sign of the magnetostriction constant of each layer. Particular attention is paid here to heterogeneous structures made of bi- or multilayers with magnetic easy axis oriented in a different direction in each layer. Bulk magnetic properties were evaluated from hysteresis loops and thermomagnetization measured by vibrating sample magnetometry (VSM) and quantum interference device (SQUID) magnetometry. Magnetic domain walls and out-of-plane magnetized domains were observed by a Kerr imaging system and magnetic force microscopy. The combination of microstructure and strains induced in the layers determines the orientation of the observed magnetic anisotropies, which vary from high in-plane anisotropies up to out-of-plane orientations for selected films. The results, which provide reassurance that effective anisotropies are induced in each of the layers, are discussed in terms of the interactions between magnetic phases with different induced easy magnetization axes     

Transient magnetic birefringence for determining magnetic nanoparticle diameters in dense, highly light scattering media

The increasing use of biofunctionalized magnetic nanoparticles in biomedical applications calls for further development of characterization tools that allow for determining the interactions of the nanoparticles with the biological medium in situ. In cell-incubating conditions, for example, nanoparticles may aggregate and serum proteins adsorb on the particles, altering the nanoparticles' performance and their interaction with cell membranes. In this work we show that the aggregation of spherical magnetite nanoparticles can be detected with high sensitivity in dense, highly light scattering media by making use of magnetically induced birefringence. Moreover, the hydrodynamic particle diameter distribution of anisometric nanoparticle aggregates can be determined directly in these media by monitoring the relaxation time of the magnetically induced birefringence. As a proof of concept, we performed measurements on nanoparticles included in an agarose gel, which scatters light in a similar way as a more complex biological medium but where particle-matrix interactions are weak. Magnetite nanoparticles were separated by agarose gel electrophoresis and the hydrodynamic diameter distribution was determined in situ. For the different particle functionalizations and agarose concentrations tested, we could show that gel electrophoresis did not yield a complete separation of monomers and small aggregates, and that the electrophoretic mobility of the aggregates decreased linearly with the hydrodynamic diameter. Furthermore, the rotational particle diffusion was not clearly affected by nanoparticle-gel interactions. The possibility to detect nanoparticle aggregates and their hydrodynamic diameters in complex scattering media like cell tissue makes transient magnetic birefringence an interesting technique for biological applications.     

On the texture of γFe2O3particles in magnetically and mechanically oriented magnetic tape

The degree of preferred orientation in magnetic recording tape was investigated by an X-ray pole figure technique and by measuring magnetic properties. Acicular γFe2O3particles dispersed in PVA film were oriented by mechanical stretching and magnetic orientation. The pole figures of these samples indicated the texture with [110] to the orientating direction. The degree of orientation in stretched film was much higher than in magnetically oriented film. Further, it was observed that the particles tend to align uniaxially in mechanical orientation, and biaxially in magnetic orientation.     

Magnetic orientation and magnetic anisotropy in paramagnetic layered oxides containing rare-earth ions

The magnetic anisotropies and easy axes of magnetization at room temperature were determined, and the effects of rare-earth (RE) ions were clarified for RE-based cuprates, RE-doped bismuth-based cuprates and RE-doped Bi-based cobaltite regarding the grain orientation by magnetic field. The easy axis, determined from the powder orientation in a static field of 10 T, depended qualitatively on the type of RE ion for all three systems. On the other hand, the magnetization measurement of the c-axis oriented powders, aligned in static or rotating fields, revealed that the type of RE ion strongly affected not only the directions of the easy axis but also the absolute value of magnetic anisotropy, and an appropriate choice of RE ion is required to minimize the magnetic field used for grain orientation. We also studied the possibility of triaxial grain orientation in high-critical-temperature superconductors by a modulated oval magnetic field. In particular, triaxial orientation was attempted in a high-oxygen-pressure phase of orthorhombic RE-based cuprates Y₂Ba₄Cu₇O_y. Although the experiment was performed in epoxy resin, which is not practical, in-plane alignment within 3° was achieved.     

Magnetic orientation and magnetic anisotropy in paramagnetic layered oxides containing rare-earth ions

Abstract

The magnetic anisotropies and easy axes of magnetization at room temperature were determined, and the effects of rare-earth (RE) ions were clarified for RE-based cuprates, RE-doped bismuth-based cuprates and RE-doped Bi-based cobaltite regarding the grain orientation by magnetic field. The easy axis, determined from the powder orientation in a static field of 10 T, depended qualitatively on the type of RE ion for all three systems. On the other hand, the magnetization measurement of the c-axis oriented powders, aligned in static or rotating fields, revealed that the type of RE ion strongly affected not only the directions of the easy axis but also the absolute value of magnetic anisotropy, and an appropriate choice of RE ion is required to minimize the magnetic field used for grain orientation. We also studied the possibility of triaxial grain orientation in high-critical-temperature superconductors by a modulated oval magnetic field. In particular, triaxial orientation was attempted in a high-oxygen-pressure phase of orthorhombic RE-based cuprates Y₂Ba₄Cu₇O_y. Although the experiment was performed in epoxy resin, which is not practical, in-plane alignment within 3° was achieved.     

The effect of polymer coatings on proton transverse relaxivities of aqueous suspensions of magnetic nanoparticles

Iron oxide magnetic nanoparticles are good candidates for magnetic resonance imaging (MRI) contrast agents due to their high magnetic susceptibilities. Here we investigate 19 polyether-coated magnetite nanoparticle systems comprising three series. All systems were synthesized from the same batch of magnetite nanoparticles. A different polyether was used for each series. Each series comprised systems with systematically varied polyether loadings per particle. A highly significant (p < 0.0001) linear correlation (r = 0.956) was found between the proton relaxivity and the intensity-weighted average diameter measured by dynamic light scattering in the 19 particle systems studied. The intensity-weighted average diameter measured by dynamic light scattering is sensitive to small number fractions of larger particles/aggregates. We conclude that the primary effect leading to differences in proton relaxivity between systems arises from the small degree of aggregation within the samples, which appears to be determined by the nature of the polymer and, for one system, the degree of polymer loading of the particles. For the polyether coatings used in this study, any changes in relaxivity from differences in water exclusion or diffusion rates caused by the polymer are minor in comparison with the changes in relaxivity resulting from variations in the degree of aggregation.     

The DORF effect: Magnetization ripple in particulate media

In 1966, Pearce [1] introduced without analysis the DORF Effect (DisOrientation by Reverse Field) in liquid dispersions of acicular magnetic particles. Bate and Dunn [2] modified orientation magnets to circumvent the effect and produce recording tapes with improved orientation. This paper describes studies of the DORF Effect and analyzes the behavior of particle groups in liquid media when a reverse field is applied. Magnetization reversal occurs by particle group rotation through a symmetric fanning mechanism which shares features with the magnetization ripple phenomenon [3,4] in continuous metal films. The critical field for particle disorientation follows a dipole-dipole interaction relation like that discussed by Jacobs and Bean [5]. For gamma ferric oxide, critical fields are distributed over the range from 20 to 100 Oe. Rotating sample magnetometer data show the DORF Effect is a destruction of uniaxial anisotropy without creation of biaxial or other anisotropies. Hysteresis work in liquid samples is almost totally due to friction between moving particles.