Lanping Yue

CoPt hard magnetic nanoparticle films synthesized by high temperature chemical reduction

The synthesis of hard-magnetic CoPt nanoparticle films by hydrogen reduction of Co nitride and Pt chloride mixture is reported. Thin porous alumina films are adopted as the carrier of the initial aqueous solution and of the final reducing products of CoPt nanoparticles. It is found that chemical ordering of L10–CoPt occurs above 400 °C. Partial phase transformation occurs in the alumina substrate when the treatment temperature is higher than 600 °C. The film coercivity increases with increasing annealing temperature and reaches a maximum value of 24.2 kOe when the reduction is carried out at 700 °C for 2 h.

Focused ion beam milled CoPt magnetic force microscopy tips for high resolution domain images

High-coercivity CoPt magnetic force microscope tips have been modified by focused ion beam milling to improve the resolution of magnetic domain images. The magnetic materials around the apex have been removed, leaving a 30-nm diameter magnetic particle at the tip end. Due to the smaller amount of magnetic material, the stray field from this new tip is significantly reduced, and the spatial resolution of the magnetic domain images is improved. The tip is used to obtain high-resolution domain images of a CoCrPt-SiO/sub 2//Ru perpendicular recording medium with linear recording densities from 800 to 1000 kfci. Magnetic patterns of 900 kfci, corresponding to a bit size of 28 nm, are well resolved. From the analysis of the power spectrum of the track profiles for these images, a spatial resolution as good as 11 nm under ambient conditions with a commercial magnetic force microscope is achieved.

Formation of an anisotropy lattice in Co∕Pt multilayers by direct laser interference patterning

We report on the use of direct laser interference patterning to form an “anisotropy” lattice in Co∕Pt thin film multilayers. Co∕Pt multilayers have been extensively studied and, for the compositions studied here, are characterized by strong perpendicular magnetic anisotropy in which the magnetic moment is perpendicular to the film plane. In direct laser interference patterning, two-to-four coherent laser beams from a pulsed Nd:YAG laser strike the sample surface simultaneously, and for sufficiently intense beams the sample properties are modified locally where interference maxima occur. Kerr rotation, magnetic force microscopy, and atomic force microscopy measurements after patterning by one pulse from the laser show that the films have a regular array of “dots” with in-plane magnetization in a background matrix of perpendicular magnetization. Such patterning holds promise for the study of model nanoscale magnetic systems.

Structure and magnetism of V-doped SnO2 thin films: effect of the substrate

We report on the structure and magnetism of V-doped SnO2 thin films grown by pulsed laser deposition. The structure and magnetic properties of the films strongly depend on the substrates on which the films are grown. Films grown on Al2O3 substrates are single crystalline and show anisotropic in-plane magnetic moments, while films grown on Si substrates are nanocrystalline and show no anisotropy. Compared with that of single-crystalline films, the magnetic moment of nanocrystalline films is higher at low V concentration, decreases more quickly as the V concentration increases, and is more sensitive to vacuum annealing. Our results suggest important roles for spin–orbit coupling and defects in the magnetism of diluted magnetic oxides.

On styrene-butadiene-styrene-barium ferrite nanocomposites

Abstract Magnetic investigations on a nanocomposite material obtained by spinning solutions of styrene–butadiene–styrene block copolymer containing barium ferrite nanoparticles onto Si wafers are reported. The effect of the spinning frequency on the magnetic features is discussed. It is observed that the magnetization at saturation is decreased as the spinning frequency is increased as the centrifuge force removes the magnetic nanoparticles from the solution. This is supported by the derivative of the hysteresis loops, which show two components, one with a high coercive field and another with a small coercive field. Increasing the spinning frequency increases the weight of the low coercive field component. The anisotropy in the distribution of magnetic nanoparticles, triggered eventually by the self-assembly capabilities of the matrix, is revealed by the difference between the coercive field in parallel and perpendicular configuration. It is noticed that increasing the spinning frequency enhances this difference. The effect of annealing the nanocomposite films is discussed.

Interactions and switching behavior of anisotropic magnetic dots

The magnetic properties of collections of three soft magnetic nanodots with various aspect ratios are investigated. Permalloy films are first produced by dc magnetron sputtering. Focused ion beam milling is then used to mill dots, each with different shape anisotropy. We find that each of the three dots in the system has a unique switching field, and that there is significant magnetostatic coupling. Micromagnetic simulations suggest that for dot separations of less than 50 nm there exists strong interdot interaction, leading to the possibility of controlled switching of neighboring dots. This switching behavior is of interest in magnetic information processing.

Template-mediated assembly of FePt L10 clusters under external magnetic field

FePt L10-structured clusters were synthesized by hydrogen reduction of Fe(NO3)3∙9H2O and H2PtCl6∙6H2O mixtures within the pores of alumina. They were released by dissolving the alumina matrix and capped with organic surfactants. After a series of chemical treatments, clusters with an average diameter of about 11nm were precipitated. The clusters were drop casted onto ordered nanoporous arrays. Clusters reaching the bottom of the pores formed an ordered magnetic array. The corresponding magnetic film exhibits distinct anisotropic behavior caused by the external magnetic field.

Magnetic Force Microscopy Studies of Magnetic Features and Nanostructures

The study of small magnetic features and nanostructures has attracted much attention due to interest in both technological applications and fundamental research in micromagnetism. For their characterization, a visualization technique with high lateral resolution is required. Among the wealth of techniques, magnetic force microscopy (MFM) has become a powerful tool for visualizing submicron-sized domain structures. This is mainly due to its ease of use without any specific sample preparation and the high lateral resolution of a few 10 nm. MFM is a sensitive and useful technique for direct observation of magnetic domains and their magnetic behavior, which can help elucidate properties of magnetic films and nanostructures. This chapter reviews MFM techniques and applications that demonstrate the achievement of MFM in magnetic materials and nanoscience research. The review focuses on the current MFM study involved with magnetic features and nanostructures, including magnetic interactions in nanostructured thin films and nanomagnetic patterns with special emphasis on the recent research in micromagnetism acquiring from our SPM laboratory.

Hf Doping Effect on Hard Magnetism of Nanocrystalline Zr

The effects of substituting Zr by Hf on the structural and the magnetic properties of the nanocrystalline rapidly solidified Zr18-xHfxCo82 ribbons (x = 0, 2, 4, and 6) have been studied. X-ray diffraction and thermomagnetic measurement results indicated that upon rapid solidification processing four magnetic phases occur: rhombohedral Zr2Co11, orthorhombic Zr2Co11, hcp Co, and cubic Zr6Co23 phases. Microstructure analysis results showed the reduction in the percentage of the soft-magnetic phase (Co) compared to the hard-magnetic phase (Zr2 Co11 (rhombohedral)) with the increase in the Hf concentration. All the samples under investigation have ferromagnetic nature, at 4.2 K and at room temperature. The coercive force (Hc) and the saturation magnetization are (Ms) found to linearly increases with x (x ≤ 2), then Hc slightly increases and Ms slightly decreases with increasing x. The maximum energy product (BH)max at room temperature is found to increases with increasing x reaching a maximum value for x = 4. The magnetocrystalline anisotropy parameter of these samples are calculated to be K = 1.1 MJ/m3 and independent of Hf concentration. The above results indicate that the replacement of Zr by Hf improves the hard-magnetic properties of this class of rear-earth-free nanocrystalline permanent magnet materials.

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Atomic-scale [Fe/Pt]n multilayer films with different total thickness were prepared on thermally oxidized Si (100) substrates at room temperature by monatomic layer deposition using dc-magnetron and rf-magnetron sputtering. Effects of the total thickness on (001) texture, surface morphology, and magnetic properties of the postannealed films have been investigated. It is found that the particlelike structure films with perfect (001) texture and perpendicular magnetic anisotropy are obtained with a thickness of less than or equal to 6.5 nm. After 500 °C annealing, the films with thickness of 6.5 and 11.9 nm show very smooth surface. In addition, with increasing total thickness of the films, (001) texture and perpendicular magnetic anisotropy of the annealed films deteriorate, and the films become continuous in structure. The total thickness of the films also affects the exchange-coupling interaction among FePt magnetic grains and the magnetization reversal process.