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Topological phase in magnetic materials yields a quantized contribution to the Hall effect known as the topological Hall effect, which is often caused by skyrmions, with each skyrmion creating a magnetic flux quantum h/e. The control and understanding of topological properties in nanostructured materials is the subject of immense interest for both fundamental science and technological applications, especially in spintronics. In this work, the electron-transport properties and spin structure of exchange-coupled cobalt nanoparticles with an average particle size of 13.7 nm are studied experimentally and theoretically. Magnetic and Hall-effect measurements identify topological phase transitions in the exchange-coupled cobalt nanoparticles and were used to discover a qualitatively new type of hysteresis in the topological Hall effect namely, Berry-phase hysteresis. Micromagnetic simulations reveal the origin of the topological Hall effect namely, the chiral domains, with domain-wall chirality quantified by an integer skyrmion number. These spin structures are different from the skyrmions formed due to Dzyaloshinskii Moriya interactions in B20 crystals and multilayered thin films, and caused by cooperative magnetization reversal in the exchange-coupled cobalt nanoparticles. An analytical model is developed to explain the underlying physics of Berry-phase hysteresis, which is strikingly different from the iconic magnetic hysteresis and constitutes one aspect of 21st-century reshaping of our view on nature at the borderline of physics, chemistry, mathematics, and materials science., Comment: 29 pages and 16 images

Rhombohedral Zr 2 Co 11 -based materials have shown very promising magnetic properties, but this rhombohedral phase is elusive and metastable. In this study, nanocrystalline Zr 2 Co 11 films were obtained by magnetron co-sputtering on Si (100) substrate with the in-situ annealing. Temperature dependence of magnetic domain structure and magnetic properties of nanocrystalline Zr 2 Co 11 have been investigated. Proper in-situ annealing temperature enhances the content of hard magnetic phase and improves the magnetic properties. Compared with Zr 2 Co 11 thin films grown at low temperature (823 K) and higher temperatures, thin films with in-situ annealing temperature of 1023 K have coercivity B c ( B c = μ 0 H c ) of 0.15 T and energy product BH max of 16.0 kJ/m 3 . Magnetic Force Microscopy images show the highest bright-dark magnetic domain contrast with highest root-mean-square values of the phase shift for thin film with in-situ annealing temperature of 1023 K. These findings have correlated magnetic domain structure and magnetic properties of Zr 2 Co 11 thin films, and elaborated the effect of different in-situ annealing temperatures on magnetic domain structure.

Magnets with chiral crystal structures and helical spin structures have recently attracted much attention as potential spin-electronics materials, but their relatively low magnetic-ordering temperatures are a disadvantage. While cobalt has long been recognized as an element that promotes high-temperature magnetic ordering, most Co-rich alloys are achiral and exhibit collinear rather than helimagnetic order. Crystallographically, the B20-ordered compound CoSi is an exception due to its chiral structure, but it does not exhibit any kind of magnetic order. Here, we use nonequilibrium processing to produce B20-ordered ${\mathrm{Co}}_{1+x}{\mathrm{Si}}_{1\ensuremath{-}x}$ with a maximum Co solubility of $x=0.043$. Above a critical excess-Co content (${x}_{c}=0.028$), the alloys are magnetically ordered, and for $x=0.043$, a critical temperature ${T}_{c}=328\text{ }\text{ }\mathrm{K}$ is obtained, the highest among all B20-type magnets. The crystal structure of the alloy supports spin spirals caused by Dzyaloshinskii-Moriya interactions, and from magnetic measurements we estimate that the spirals have a periodicity of about 17 nm. Our density-functional calculations explain the combination of high magnetic-ordering temperature and short periodicity in terms of a quantum phase transition where excess-cobalt spins are coupled through the host matrix.

The boron-content dependence of magnetic domain structures and magnetic properties of nanocrystalline Zr16Co82.5−x Mo1.5B x (x = 0, 1, 2, 3, 4) melt-spun ribbons have been investigated. Compared to x = 0, the smaller average domain size with a relatively short magnetic correlation length of 120 nm and largest root-mean-square phase shift value of 0.94° are observed for x = 1. The best magnetic properties of coercivity H c = 5.4 kOe, maximum energy product (BH) max = 4.1 MGOe, and saturation polarization J s = 7.8 kG, were obtained for the ribbon with x = 1. The optimal B addition enhances the content of hard magnetic phase, promotes magnetic domain structure refinement, and increases the surface roughness, results in the enhancement of magnetic anisotropy, and thus leads to a significant increase in coercivity and energy product in this sample.

Nanopatterned arrays of discrete cobalt nanostructures showing characteristic parameter-dependent sizes are formed from continuous thin films on a carbon nanotube substrate using millisecond pulsed intense UV light. The nanoparticles exhibit ferromagnetic behavior with magnetic remanence and coercivity depending on the particle size. The end-state particle size is shown to be a function of initial thin film thickness and excitation energy and is therefore tunable. The evolutionary process from continuous thin films to a discrete morphology is thermodynamically driven by the large surface energy difference between metastable thin films and the underlying carbon nanotube substrate. Evidence of the Danielson model of the dewetting process is observed. These arrays can find applications as platforms for the self-assembly of magnetically susceptible materials, such as iron or nickel nanostructures, into a conduction matrix for applications in energy extraction from a latent heat storage device.

Permanent-magnet materials are one cornerstone of today’s technology, abundant in disk drives, motors, medical equipment, wind generators, and cars. A continuing challenge has been to reconcile high permanent-magnet performance with low raw-material costs. This work reports a Mn-Bi-Sn alloy exclusively made from inexpensive elements, exhibiting high values of Curie-temperature, magnetization, anisotropy, coercivity, and energy product. The samples are produced by field annealing of rapidly quenched Sn-containing MnBi alloys, where the improvement of the magnetic properties is caused by the substitutional occupancy of the 2c sites in the hexagonal NiAs structure by Sn. The substitution modifies the electronic structure of the compound and enhances the magnetocrystalline anisotropy, thereby improving the coercivity of the compound. The energy product reaches 114 kJ/m3 (14.3 MGOe) at room temperature and 86 kJ/m3 (10.8 MGOe) at 200 °C; this value is similar to that of the Dy-free Nd2Fe14B and exceeds that of other rare-earth-free permanent-magnet bulk alloys, as encountered in automotive applications.

A nonequilibrium rapid-quenching method has been used to fabricate NiMnIn and NiMnGa alloys that are chemically and morphologically similar but crystallographically and physically very different. NiMnGa crystallizes in a Ni2In-type hexagonal structure, whereas NiMnIn is a cubic Heusler alloy. Both alloys yield a topological Hall effect contribution corresponding to bubble-type skyrmion spin structures, but it occurs in much lower magnetic fields in NiMnIn as compared to NiMnGa. The effect is unrelated to net Dzyaloshinskii-Moriya interactions, which are absent in both alloys due to their inversion-symmetric crystal structures. Based on magnetic-force microscopy, we explain the difference between the two alloys by magnetocrystalline anisotropy and uniaxial and cubic anisotropies yielding full-fledged and reduced topological Hall effects, respectively. Since NiMnIn involves small magnetic fields (0.02–0.3 kOe) at and above room temperature, it is of potential interest in spin electronics.

High-anisotropy NiAs-type MnBi films are produced by in situ annealing of Bi/Mn/Bi trilayers, [Bi/Mn/Bi]n multilayers, and subsequent magnetic field annealing. Phase components, crystallographic anisotropy, and magnetic properties of the MnxBi100−x thin films exhibits strong dependence on Mn concentration. High-purity MnBi thin films with perfect c-axis orientation are obtained by carefully controlling the Mn/Bi ratio. An energy product of 16.3 MGOe, which is the highest value reported so far, is achieved for the x = 50 film of thickness t = 100 nm. The MnBi thick film (t = 2 μm) changes from isotropic to anisotropic after magnetic field annealing. Depending on the direction of the applied field during magnetic field annealing, the MnBi thick film may have out-of-plane or in-plane anisotropy. This control of anisotropy direction enables applications of MnBi films in permanent-magnet, spintronic devices, or magnetic micro-electro-mechanical systems. In addition, the room-temperature magnetocrystalline anisotropy constant and saturation polarization of the hard magnetic MnBi phase are determined to be K = K1 + K2 = 15.0 Mergs cm−3 and Js = 8.2 kG, respectively.

The addition of Molybdenum was used to modify the nanostructure and enhance coercivity of rare-earth-free Zr2Co11-based nanocrystalline permanent magnets. The effect of Mo addition on magnetic domain structures of melt spun nanocrystalline Zr16Co84-xMox(x=0, 0.5, 1, 1.5, and 2.0) ribbons has been investigated. It was found that magnetic properties and local domain structures are strongly influenced by Mo doping. The coercivity of the samples increases with the increase in Mo content (x≤1.5). The maximum energy product(BH)maxincreases with increasingxfrom 0.5 MGOe forx=0to a maximum value of 4.2 MGOe forx=1.5. The smallest domain size with a relatively short magnetic correlation length of 128 nm and largest root-mean-square phase shiftΦrmsvalue of 0.66° are observed for thex=1.5. The optimal Mo addition promotes magnetic domain structure refinement and thus leads to a significant increase in coercivity and energy product in this sample.

In magnetic random access memory (MRAM) devices, soft magnetic thin film elements such as permalloy (Py) are used as unit cells of information. The epitaxial integration of these elements with the technologically important substrate Si (1 0 0) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni 82.5 Fe 17.5 (permalloy, Py) on Si (1 0 0) using a TiN/MgO buffer layer. Initial stages of growth are characterized by the formation of discrete islands that gradually merge into a continuous film as deposition times are extended. Interestingly, we find that the magnetic features of Py films in early stages of island coalescence are distinctly different from the films formed initially (discrete islands) and after extended deposition times (narrow distribution of equiaxed granular films). Isothermal in-plane and out-of-plane magnetic measurements performed on these transitional films show highly anisotropic magnetic behavior with an easy magnetization axis lying in the plane of the film. Importantly, when this sample is zero-field cooled, a positive exchange bias and vertical loop shift are observed, unusual for a soft ferromagnet like Py. Repeated field cycling and hysteresis loops up to the fields of 7T produced reproducible hysteresis loops indicating the existence of strongly pinned spin configurations. Classical interface related exchange bias models cannot explain the observed magnetic features of the transitional Py films. We believe that the anomalous magnetic behavior of such Py films may be explained by considering the highly irregular morphology that develops at intermediate growth times that are possibly also undergoing a transition from Bloch to Neel domain wall structures as a function of Py island size. This study broadens the current understanding of magnetic properties of Py thin layers for technological applications in magneto-electronic devices, integrated with Si (1 0 0).

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.

The magnetic domain structure of commercial alnico grades 5–7 and 9 was investigated using a magneto-optical Kerr effect (MOKE) to gain an understanding of their coercivity mechanisms at the micron to millimeter scale. In alnico 5–7, the magnetic domain structure exhibits stripes of alternating high and low induction. Magnetic domains easily cross grain boundaries if neighboring grains have a similar tilt and rotation of their crystallographic axes relative to the magnet body. In contrast for alnico 9, stripe-like magnetic domains are not observed regularly throughout the transverse section; rather, discrete localization of high- and low-induction stripe features are observed. In higher magnification MOKE experiments, i.e., ~100 μm, a zigzag-shaped magnetic domain structure was observed in both alnico 5–7 and 9. The zigzag features are four to five times smaller in size than an average grain of alnico 5–7, implying a pinning mechanism that is caused by structural elements within the grains. Discontinuous and reversible motion on a length scale of a few microns was observed for the zigzag-shaped domains for incremental changes in the applied field of ~10 Oe. Complimentary magnetic force microscopy measurements show that there are domain structures on an even smaller scale, i.e., 2 μm to 100 μm.

The thickness and temperature dependence of in situ grown cobalt thin films on Cr2O3(0 0 0 1) single crystalline substrate has been studied by low energy electron microscopy (LEEM). The LEEM images indicate that growth of thin Co films (⩽5 monolayers) on chromia at 100 K tends to be continuous and flat with suppressed island growth compared to films grown on chromia at room temperature and above (to ~440 K). Low energy electron diffraction indicates that disorder builds and crystallinity of the cobalt thin film decreases with increased film thickness. Compared with cobalt thin films on Al2O3(0 0 0 1) single crystalline substrate, cobalt thin films on Cr2O3(0 0 0 1) show larger magnetic contrast in magnetic force microscopy indicating enhancement of perpendicular anisotropy induced by Cr2O3.

FeAg and FeAg/Pt films were prepared by dc magnetron sputtering at room temperature. The effects of Ag volume fraction in FeAg films and postannealing temperature and time on structure and magnetic properties of FeAg and FeAg/Pt films have been investigated. The results show that the as-deposited FeAg films are metastable. After annealing at 300°C, the phase separation of metastable FeAg films happened and the highest coercivity is obtained in Fe50Ag50/Pt film. With increasing annealing temperature, the ordering and the magnetic properties of the Fe50Ag50/Pt films were improved. When the Fe50Ag50/Pt films are annealed at 600°C for different annealing times, a long annealing time enhances the ordering of the metastable Fe50Ag50/Pt films and affects the orientation development. When the films are annealed for a long time, the grain size and the magnetic domain size also increase, which lead to an increase of correlation length due to the growth of FePt grains.

The magnetic and structural properties of atomic-scale [Fe/Pt]18 and [Fe/Pt/Ag]18 multilayer films annealed at 350 and 400°C have been investigated. The results indicate that, by introducing thin Ag addition layers, the ordering temperature of FePt with nominal 7.4-nm thickness (excluding Ag thickness) can be significantly reduced to 350°C, at which a relatively large coercivity is achieved. When annealing temperature increases to 400°C, the atomic-scale [Fe/Pt]18 and [Fe/Pt/Ag]18 multilayer films show (001) orientation. The reduced diffusion length of atoms in atomic-scale multilayer films and an enhanced atomic diffusion coefficient produced by Ag doping led to the low-temperature ordering and (001) orientation of L10 FePt phase.

The effect of Y addition and magnetic field on texture and magnetic properties of arc-melted alnico 9 magnets has been investigated. Small additions of Y (1.5 wt.%) develop a (200) texture for the arc-melted alnico 9 magnet. Such a texture is hard to form in cast samples. To achieve this goal, we set up a high-field annealing system with a maximum operation temperature of 1250 °C. This system enabled annealing in a field of 45 kOe with subsequent draw annealing for the solutionized buttons; we have been able to substantially increase remanence ratio and coercivity, from 0.70 and 1200 Oe for the Y-free alnico 9 to 0.90 and 1400 Oe for the Y-doped alnico 9, respectively. A high energy product of 7.3 MGOe has been achieved for the fully heat-treated Y-doped alnico 9. The enhancement of coercivity is believed to arise from the introduction of magnetocrystalline anisotropy from 80 nm Y2Co17-type grains, which are exchange-coupled to the main-phase alnico rods.

Antiferromagnetically coupled magnetic thin films are promising candidates for the design of new magnetic storage and logic devices. The ability to control the interlayer thickness, therefore the magnetic reversal response, of exchange-coupled magnetic layers is of paramount importance in nanotechnology, especially in magnetic sensing element design and applications. In this work, magnetic force microscopy (MFM) with improved sensitivity and high spatial resolution probes was used to obtain a more detailed view of magnetization reversal behavior and domain evolution in the indirect exchange-coupled trilayer system: Co/Ru/Co. The effect of the variable Ru interlayer thickness on the exchange coupling and thus the magnetic domain structure during the ferromagnetic (FM)/antiferromagnetic (AF) coupling transition in Co/Ru/Co films is well demonstrated. The MFM images display a distinct signature of AF coupling for the films with Ru thickness of 0.4 nm. MFM has proven to be an effective tool for detecting FM/AF interlayer coupling and exploring magnetic domain structures in exchange-coupled layered thin films.

The addition of Molybdenum was used to modify the nanostructure and enhance coercivity of rare-earth-free Zr2Co11-based nanocrystalline permanent magnets. The effect of Mo addition on magnetic domain structures of melt spun nanocrystalline Zr16Co84-xMox (x=0, 0.5, 1, 1.5, and 2.0) ribbons has been investigated. It was found that magnetic properties and local domain structures are strongly influenced by Mo doping. The coercivity of the samples increases with the increase in Mo content (x≤1.5). The maximum energy product (BH)max increases with increasing x from 0.5 MGOe for x=0 to a maximum value of 4.2 MGOe for x=1.5. The smallest domain size with a relatively short magnetic correlation length of 128 nm and largest root-mean-square phase shift Φrms value of 0.66° are observed for the x=1.5. The optimal Mo addition promotes magnetic domain structure refinement and thus leads to a significant increase in coercivity and energy product in this sample.

FePt nanoclusters have been implanted onto polyimide films and subjected to thermal annealing in order to obtain a special magnetic phase (L10) dispersed within the polymer. SQUID measurements quantified the magnetic features of the as-prepared and annealed hybrid films. As-implanted FePt nanoparticles in polyimide films exhibited a blocking temperature of 70 ± 5 K. Thermal annealing in zero and 10 kOe applied magnetic field increased the magnetic anisotropy and coercivity of the samples. Wide Angle X-Ray Scattering confirmed the presence of FePt and L10phase. All samples (as deposited and annealed) exhibited electron spin resonance spectra consisting of two overlapping lines. The broad line was a ferromagnetic resonance originating from FePt nanoparticles. Its angular dependence indicated the magnetic anisotropy of FePt nanoparticles. SEM micrographs suggest a negligible coalescence of FePt nanoparticles, supporting that the enhancement of the magnetic properties is a consequence of the improvement of the L10structure. The narrow ESR line was assigned to nonmagnetic (paramagnetic) impurities within the samples consistent with graphite-like structures generated by the local degradation of the polymer during implantation and annealing. Raman spectroscopy confirmed the formation of graphitic structures in annealed KHN and in KHN-FePt.

In this paper, we report on the production of Fe cluster/FePt matrix nanocomposite permanent magnets. Monodispersed Fe clusters with sizes below 10 nm were formed by gas aggregation techniques. These Fe clusters were imbedded in an FePt matrix by alternate deposition from two sources. Specimens with a range of Fe cluster phase content from 0 to 30 vol% were produced by controlling deposition times from each source. As-deposited FePt formed in the A1 structure; thus, post-deposition heat treatment was necessary to form the hard magnetic L10 FePt compound. A single-step heat treatment at 600 °C for 10 min leads to nanocomposite structures with excellent magnetic properties. The coercivity decreased with increasing Fe cluster content, while the energy product initially increased, reaching a maximum of almost 18 MGOe, and then decreased at higher Fe cluster content. Secondary heat treatment at 500 °C significantly improved the magnetic properties when compared with the single-step heat treatment at 600 °C. Increased coercivity and remanence was observed, resulting in energy products of 21 MGOe. The energy products are close to 70 percent greater than expected for uncoupled systems.

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.

We fabricated exchange-coupled composite films with perpendicular anisotropy using nanograms of L10 FePt as a magnetically hard base layer and FePt : SiO2 as a relatively soft phase. The degree of anisotropy and nanostructure in the soft layer was controlled by varying the deposition temperature. Growth of the low-anisotropy layer followed the particulate structure of the base layer at deposition temperatures of 390°C or greater. Perpendicular coercivities were strongly dependent on both the anisotropy and thickness of the softer layer.

Cis ferromagnetic, with a room-temperature coercivity of 70 Oe and a magnetization of 0.8 emu/g at 5.5 T,although the hysteresis loop is not saturated. The theoretical prediction that interacting locally nickel-richregions may be responsible for ferromagnetic behavior is supported by the observation of magnetically glassybehavior at low magnetic fields.I. INTRODUCTION

The effect of Hf on the permanent magnetism of nanocrystalline Zr18-xHfxCo82 ribbons (x = 0, 2, 4, and 6) was investigated by magnetic properties measurement and magnetic force microscopy (MFM). Emphasis is on the local magnetic domain structures in polycrystalline rapidly solidified Zr18-xHfxCo82 ribbons for four different samples with small fractions of Hf dopants (x ≤ 6). The investigation of the magnetic properties of the Zr18-xHfxCo82 ribbons revealed that all the samples under investigation are ferromagnetic at room temperature, and the corresponding MFM images show bright and dark contrast patterns with up-down magnetic domain structures. It is found that the saturation magnetization and the coercivity depend on Hf doping concentration x in the samples. For a sample with Hf concentration x = 4, the maximum energy product (BH)max value is 3.7 MGOe. The short magnetic correlation length of 131 nm and smallest root-mean-square phase shift value of 0.680 were observed for x = 4, which suggests the refinement of the magnetic domain structure due to weak intergranular exchange coupling in this sample. The above results indicate that suitable Hf addition is helpful for the magnetic domain structure refinement, the coecivity enhancement, and the energy-product improvement of this class of rare-earth-free nanocrystalline permanent-magnet materials.

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.

Structural inhomogeneity on length scales comparable to magnetic interaction lengths can produce complex magnetic behavior. Crystalline GdAl2 is a ferromagnet with a Curie temperature of 170 K, while amorphous GdAl2 thin films exhibit classic spin-glass behavior with a freezing temperature of 16 K. Nanostructured GdAl2, made by mechanically milling initially crystalline GdAl2, exhibits ferromagnetic and spin-glass-like transitions; however, the spin-glass-like transition occurs at a higher temperature than the freezing temperature of amorphous GdAl2 thin films. Curie–Weiss analysis suggests that the paramagnetic-to-ferromagnetic transition is due to the ferromagnetic ordering of small GdAl2 clusters and that the glassy transition is most likely due to spin-glass-like ordering of a surface/interface phase.

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.

A special structure of nanostructured titatium (Ti) thin film prepared by ionbeam sputtering deposition technique has been found by means of X-ray diffractometry and transmission electron microscopy. The results show that the Ti film has a new f.c.c. structure with lattice parameter a = 0.402 nm, which does not exist in the bulk Ti target. The crystalline structure of the samples with grain size less than 5 nm is stable at room temperature, and the formation of this abnormal structure in nanometer-size crystalline Ti thin film is discussed.

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.

We report on a mechanism of dynamic dipolar coupling in magnetic superlattices via long-wavelength nonevanescent fields. In the spin excitation spectra of our heterophase stripe structures, such interactions mediate a $\mathrm{\text{singlet}}\ensuremath{\leftrightarrow}\mathrm{\text{doublet}}$ crossover in the frequency regime driven by the orientation of an external static field. This crossover is a new feature observed in collective behavior of superlattices, though there is some analogy of this phenomenon with birefringence taking place in optical superlattices. We envision applying the collective effects described here in microwave photonic devices.

WOS:000237682900052 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.

Interlayer exchange coupling has been studied in a series of ${[\mathrm{Pt}({t}_{\mathrm{Pt}}\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})∕\mathrm{Co}(4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})]}_{n}∕\mathrm{NiO}({t}_{\mathrm{NiO}})∕{[\mathrm{Co}(4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})∕\mathrm{Pt}({t}_{\mathrm{Pt}}\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})]}_{n}$ multilayers with perpendicular anisotropy. The coupling oscillates between antiferromagnetic and ferromagnetic as a function of ${t}_{\mathrm{NiO}}$ with a period of $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, and the oscillatory behavior is related to the antiferromagnetic ordering of the $\mathrm{NiO}$ layer. This interlayer coupling between two $\mathrm{Co}∕\mathrm{Pt}$ multilayers is shown to occur domain by domain by magnetic force microscopy imaging. For the strongest antiferromagnetic coupling at ${t}_{\mathrm{NiO}}=11\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, an oscillation with a period of $\ensuremath{\sim}6\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ is superposed onto the exponential decay of the coupling strength as a function of ${t}_{\mathrm{Pt}}$. The exponential decay with ${t}_{\mathrm{Pt}}$ is ascribed to the exponential decay of the coupling between the $\mathrm{Co}$ layers across the $\mathrm{Pt}$ layers in each $\mathrm{Co}∕\mathrm{Pt}$ multilayer, and the superposed oscillatory behavior can be attributed to multiple reflections of electron waves at the $\mathrm{Co}∕\mathrm{Pt}$ interfaces and their interference. A linear dependence of the antiferromagnetic coupling strength on $1∕n$, (where $n$ is the number of repeats), is suggestive of a surface interaction for this interlayer coupling.

The effect of disorder on the ferromagnetic transition is investigated in mechanically milled $\mathrm{Gd}{\mathrm{Al}}_{2}$. $\mathrm{Gd}{\mathrm{Al}}_{2}$ is a ferromagnet when crystalline and a spin glass when amorphous. Mechanical milling progressively disorders the alloy, allowing observation of the change from ferromagnetic to a disordered magnetic state. X-ray diffraction and pair-distribution-function analysis are used to determine the grain size, lattice parameter, and mean-squared atomic displacements. The magnetization as a function of temperature is described by a Gaussian distribution of Curie temperatures. The mean Curie temperature decreases with decreasing lattice parameter, where lattice parameter serves as a measure of defect concentration. Two different rates of change with lattice parameter are observed: the first, slower rate occurs while grain size is changing, and the grain size is constant in the second regime. The breadth of the Curie temperature distribution is linear with lattice parameter. Milled and annealed $\mathrm{Gd}{\mathrm{Al}}_{2}$ shows that the mean Curie temperature has the same general dependence on lattice parameter as milled, but unannealed $\mathrm{Gd}{\mathrm{Al}}_{2}$, while the breadth of the Curie temperature distribution is different in annealed samples compared to the unannealed materials.

Nonlinear optical properties of Ge nanocrystallites embedded in SiO/sub 2/ (Ge-SiO/sub 2/) thin films prepared by the ion-beam sputtering technique have been studied by the Z-scan technique. The large values of nonlinear absorption coefficient and nonlinear refractive index of the samples were estimated respectively. Thus, the third-order optical nonlinear susceptibility /spl chi//sup 3 /obtained for Ge-SiO/sub 2/ thin film specimens is several orders of magnitude larger than that of the bulk Ge crystals. The results indicate that Ge-SiO/sub 2/ thin films showing enhanced optical nonlinearity have bright prospects as suitable nonlinear optical materials for optical function devices.

Measurements of DC susceptibility, AC susceptibility, and AC susceptibility with an applied DC bias field were performed on mechanically milled GdAl2. A paramagnetic phase exists above a temperature T ≈ 140 K. However, there are significant deviations from the Curie-Weiss Law in this temperature regime, suggesting multiple magnetic components. Fits to the high temperature data show that two Curie-Weiss terms represent the data quite well. Below 140 K one of these magnetic components becomes ferromagnetic as indicated by a shoulder in the AC susceptibility and DC susceptibility data. This ferromagnetic component is suppressed by the application of sufficiently strong DC bias field. Accompanying this shoulder is a peak at lower temperatures (T < 50 K), which suggests the existence of another component that is magnetically glassy in nature. The two-component behavior of mechanically milled GdAl2 can be explained in terms of the nanostructure of the material, which consists of nanometer-sized grains and a disordered interphase.

Mechanical milling provides a unique means of studying the influence of grain size and disorder on the magnetic properties of nanostructured alloys. This paper compares the role of milling in the nanostructure evolution of two ferromagnets – SmCo5 and GdAl2 – and the subsequent impact of nanostructure on magnetic properties and phase transitions. The ferromagnetic properties of SmCo5 are enhanced by short (< 2 hours) milling times, producing up to an eight-fold increase in coercivity and high remanence ratios. The coercivity increase is attributed to defect formation and strain. Additional milling increases the disorder and produces a mix of ferromagnetic and antiferromagnetic interactions that form a magnetically glassy phase. GdAl2, which changes from ferromagnetic in its crystalline form to spin-glass-like in its amorphous form, is a model system for studying the dependence of magnetically glassy behavior on grain size and disorder. Nanostructured GdAl2 with a mean grain size of 8 nm shows a combination of ferromagnetic and magnetically glassy behavior, in contrast to previous studies of nanostructured GdAl2 with a grain size of 20 nm that show only spin-glass-like behavior.

We have found that near-band-edge cathodoluminescence (CL) emission decreases with time for some nominally undoped GaN samples. The rate of intensity decrease depends on the incident beam current. It also depends on the size of the area being scanned, which is determined by the magnification used, although the electron beam voltage and current are held constant and similar regions of GaN are being examined. Faster decrease with time occurs with higher beam currents and higher magnifications. The reduced luminescence efficiency persists over at least 24 hour beam-off periods. The dependence of CL intensity on beam current and on scanned area may be a result of different levels of local charging, and structural modifications caused by this charging, for different beam current and magnification settings.

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.

Ultrathin Co/Ru/Co trilayers are investigated experimentally by magnetization curves and magnetic-force microscopy (MFM). Emphasis is on the domain-wall fine structure of antiphase domain walls in the films. The trilayers are produced by sputtering and consist of two Co layers of equal thickness (5 nm), exchange-coupled through a Ru layer of variable thickness. The sign and magnitude of the interlayer exchange are tuned by the thickness of the Ru interlayer. The exchange and its distribution are investigated by measurements of the static magnetization curves. For a Ru thickness of 0.4 nm, the exchange is predominantly antiferromagnetic and the MFM images show fairly immobile domain walls. Micromagnetic model calculations yield immobile antiphase domain walls whose thickness decreases with increasing magnetic field but is typically of the order of 100 nm in agreement with experiment.

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.

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.

A strong photoluminescence (PL) with a peak position of ∼420 nm (2.95 eV) under exciting radiation of λ=300 nm, and two new luminescence peaks at ∼420 and 470 nm, under λ=633 nm excitation at room temperature are observed, evidently for Ge nanocrystallites embedded in SiO2 thin films (Ge–SiO2) prepared by the ion-beam sputtering technique. The studies reveal that the nanometer-size crystalline Ge have a type of face-centered-cubic structure and characteristics of direct optical transition. It is suggested that the visible PL of Ge–SiO2 thin films is not only related to the quantum size effect due to three-dimensional confinement of electrons and holes, but also closely associated with the particular microstructure of the Ge nanocrystallites.