High resolution magnetic force microscopy of patterned L1(0)-FePt dot arrays by nanosphere lithography

High resolution magnetic force microscopy (MFM) has been carried out on L1(0)-FePt dot arrays patterned by plasma modified nanosphere lithography. An ex situ tip magnetization reversal experiment is carried out to determine the magnetic domains and verify the imaging stability of MFM and the mutual perturbations between the magnetic tip and the sample. We have identified that the critical size for the single domain region is about 90?nm across. Comparison with MFM image simulation also suggests that the magnetizations of the triangular dots in both single and double domain states are parallel to one edge of the dots, indicating the large uniaxial magnetocrystalline anisotropy of the L1(0)-FePt phase and the need for decreasing the magnetostatic energy.     

Synthesis of super-fine L10-FePt nanoparticles with high ordering degree by two-step sintering under high magnetic field

Super-fine L10-FePt nanoparticles (NPs) with high ordering degree were successfully prepared by a modified two-step sintering method,which includes low-temperature pre-sintering,and the high magnetic field (HMF) assisted post-sintering processes.The particle size of the L10-FePt NPs was obviously refined by lowering the sintering temperature.By applying the HMF during the post-sintering process,the fine size characteristics of L10-FePt NPs were retained,and the ordering degree was significantly improved.The L10-FePt NPs with sizes of about 4.5 nm,ordering degree of 0.940,and coercivity of 22.01 kOe were obtained by this two-step sintering under a magnetic field of 12 T.The mechanism investigation of HMF enhancing the ordering degree indicates that the HMF enhances lattice distortion and magnetization energy (Zeeman energy).The enhanced lattice distortions cause high stress existing in the lattice,which can effectively promote the disordered-order transition.When the magnetic field reaches to 3 T,the Zeeman energy of the NPs is higher than the thermal disturbing energy of the NPs,and the magnetization effect is stronger.Therefore,the HMF (higher than 3 T) can obviously improve the disorder-order transition by lowering the energy barrier and accelerating the orderly diffusions of atoms.The HMF is a promising assistant to synthesize the L10-phase NPs with both of high ordering degree and super-fine size.     

Effects of Magnetic Annealing on Structure and Magnetic Properties of L10-FePt/Ag Films

The structural and magnetic properties of L1₀-FePt/Ag films were studied by X-ray diffraction and a vibrating sample magnetometer. The FeAg/Pt films were obtained by depositing FeAg thin films on thermally oxidized Si (001) substrates via magnetron sputtering and Pt layers on their surface after annealing FeAg thin films at 400 °C with and without an out-of-plane magnetic field of 10 kOe. These films were further annealed at various temperatures to obtain L1₀-FePt phase. The results indicated that the pre-annealing of FeAg thin films under 10 kOe magnetic field caused (001) orientation of Fe particles, and the deposition of Pt layer on such orientated underlayers reduced the ordering temperature of FePt in FeAg/Pt films, realizing the L1₀-FePt phase at 400 °C. The higher coercivity and ordering degree were also observed in the samples, compared with those pre-annealed without magnetic field at the same annealing condition.     

Tunable magnetic properties by interfacial manipulation of L1(0)-FePt perpendicular ultrathin film with island-like structures

Based on interfacial manipulation of the MgO single crystal substrate and non-magnetic AIN compound, a L1(0)-FePt perpendicular ultrathin film with the structure of MgO/FePt-AIN/Ta was designed, prepared, and investigated. The film is comprised of L1(0)-FePt "magnetic islands," which exhibits a perpendicular magnetic anisotropy (PMA), tunable coercivity (Hc), and interparticle exchange coupling (IEC). The MgO substrate promotes PMA of the film because of interfacial control of the FePt lattice orientation. The AIN compound is doped to increase the difference of surface energy between FePt layer and MgO substrate and to suppress the growth of FePt grains, which takes control of island growth mode of FePt atoms. The AIN compound also acts as isolator of L1(0)-FePt islands to pin the sites of FePt domains, resulting in the tunability of Hc and IEC of the films.     

Structure and properties of nanoporous FePt fabricated by dealloying a melt-spun Fe_(60)Pt_(20)B_(20) alloy and subsequent annealing

A nanoporous FePt alloy has been fabricated by dealloying a melt-spun Fe(60)Pt(20)B(20)alloy composed of nanoscale amorphous and face-centered-cubic FePt(fcc-FePt)phases in H2 SO4 aqueous solution.The nanoporous alloy consists of single fcc-FePt phase with an Fe/Pt atomic ratio of about 55.3/44.7,and possesses a uniform interpenetrating ligament-channel structure with average ligament and pore sizes of 27 nm and 12 nm,respectively.The nanoporous fcc-FePt alloy shows soft magnetic characteristics with a saturation magnetization of 37.9 emu/g and better electrocatalytic activity for methanol oxidation than commercial Pt/C in acidic environment.The phase transformation from disordered fcc-Fe Pt into ordered face-centered-tetragonal FePt(L10-FePt)in the nanoporous alloy has been realized after annealing at823-943 K for 600 s.The volume fraction of the L10-FePt phase in the alloy increases with the rise of annealing temperature,which results in the enhancements of coercivity and saturation magnetization from 0.14 kOe and 38.5 emu/g to 8.42 kOe and 51.4 emu/g,respectively.The ligament size of the samples is increased after annealing.     

Optimization of L10-FePt/MgO/CrRu thin films for next-generation magnetic recording media

L1₀-FePt thin films were deposited on silicon substrates with the structure of Si/CrRu/MgO/FePt. The magnetic and microstructural properties were optimized by varying the FePt sputter pressure and temperature, as well as the thicknesses of all three layers. High coercivity films greater than 1.8 T were grown when the FePt sputter pressure was at 1.33 Pa with a thickness of only 4 nm, on CrRu and MgO underlayers as thin as 10 nm and 2 nm, respectively.     

Fabrication and characterization of L1/sub 0/-ordered FePt/AlO/FeCo magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) with an L1/sub 0/-ordered FePt alloy were fabricated and characterized. As a bottom electrode, an in-plane magnetized L1/sub 0/ FePt(110) layer was grown epitaxially on an Au buffer layer. A multiple oxidation process was used for the formation of Al oxide barriers. The L1/sub 0/-FePt/AlO/FeCo MTJ prepared shows tunnel magnetoresistance of 18% and 40% at room temperature (RT) and 4.2 K, respectively. The observed RT magnetoresistance is improved, compared to the previously reported results for the L1/sub 0/-FePt MTJs by a conventional single oxidation process.     

Microstructural enhancement of high coercivity L1(0)-FePt films for next-generation magnetic recording media

The effects of substrate Ar-ion milling and Ta adhesion layer on the microstructural and magnetic properties of L1(0)-FePt films prepared on Si, SiO2, and glass substrates were investigated. It was discovered that the relatively large in-plane surface roughness of CrRu/MgO/FePt films deposited on Si substrates was due to the deformation of the CrRu layer when the composition was heated to 550 degrees C. More than an order of magnitude improvement for the in-plane surface roughness was achieved when substrate Ar-ion milling or Ta adhesion layer was incorporated into the process. While the Ta adhesion layer proved to be detrimental to the (200) growth of the CrRu layer, optimal FePt film properties with coercivity values larger than 2 Tesla and out-of-plane roughness less than 1 nm were achieved when only substrate Ar-ion milling was implemented.     

Photoluminescence efficiency and size distribution of self assembled ge dots on porous TiO2

For Ge nanodots approximately 20 nm in diameter grown by annealing a thin amorphous Ge layer deposited by molecular beam epitaxy on a mesoporous TiO2 layer on Si(001), photoluminescence (PL) was observed as a wide near-infrared band near 800 meV. Using a tight binding theoretical model, the energy-dependent PL spectrum was transformed into a dependence on dot size. The average dot size determined the peak energy of the PL band and its shape depended on the size distribution, including bandgap enlargement due to quantum confinement. Combining the dot sample PL with an established dependence of emission efficiency on dot diameter, it was possible to derive a dot size distribution and compare it with results obtained independently from atomic force microscopy.     

Effect of annealing in external magnetic field on the microstructure and magnetic properties of FePt films

We have studied the influence of annealing in an external magnetic field on the microstructure and magnetic properties of a multilayer Si/Fe(2 nm)/Fe₅₀Pt₅₀(20 nm)/Pt(2 nm) structure synthesized by means of sequential RF magnetron sputtering of the components. The magnetic field was oriented perpendicular to layers of the structure. It is established that annealing in the external magnetic field leads to the formation of predominant (001) texture in the multilayer structure with L1₀-FePt phase. Thus, a method of obtaining multilayer structures based on FePt films required for the perpendicular magnetic recording has been developed.     

Exchange-coupled Fe3O4/L10-FePt bilayer films by controlled oxidation of Fe/Pt multilayer

A Fe₃O₄/L1₀-FePt bilayer thin-film magnet was fabricated via a simple one-step process by annealing Fe/Pt multilayer thin films in a N₂ gas flow. X-ray diffraction and plane-view selected-area electron diffraction results are identified with magnetite phase (Fe₃O₄) and L1₀-FePt. Cross-sectional transmission electron microscopy images show the two phases form a bilayer structure. Magnetic hysteresis loops of the bilayer show single phase behavior which is interpreted as a result of the soft Fe₃O₄ phase exchange-coupled with the hard L1₀-FePt phase, consistent with micromagnetic simulation prediction. Such bilayer structure may have potential for coercivity control in high density magnetic recording application.     

Effect of TiO2 nanopatterns on the performance of hydrogenated amorphous silicon thin-film solar cells

We investigate how TiO₂ nanopatterns formed onto ZnO:Al (AZO) films affect the performance of hydrogenated amorphous silicon (a-Si:H) solar cells. Scanning electron microscopy results show that the dome-shaped TiO₂ nanopatterns (300 nm in diameter) having a period of 500 nm are formed onto AZO films and vary from 60 to 180 nm in height. Haze factor increases with an increase in the height of the nanopatterns in the wavelength region below 530 nm. Short circuit current density also increases with an increase in the height of the nanopatterns. As the nanopatterns increases in height, the fill factor of the cells slightly increases, reaches maximum (0.64) at 100 nm, and then decreases. Measurements show that a-Si:H solar cells fabricated with 100 nm-high TiO₂ nanopatterns exhibit the highest conversion efficiency (6.34%) among the solar cells with the nanopatterns and flat AZO sample.     

Diamond nanopatterns fabricated by room-temperature nanoimprinting technology with diamond molds using polysiloxane

Fabrication of diamond nanopatterns in room-temperature (RT) nanoimprint lithography (NIL) with chemical vapor deposited (CVD) diamond molds using polysiloxane as RT-imprint resist material was investigated. The diamond molds of a convex lattice with 1 μm line-width and 5 μm pitch, and a cylinder dot with 200 nm diameter and 1 μm pitch which has a height of 1 μm using RT-NIL process were fabricated with Bi₄Ti₃O₁₂ octylate (oxide) mask in electron beam lithography technology. The maximum radio frequency (RF) oxygen plasma-etching selectivity (diamond/polysiloxane) of 19 was obtained under the conditions of RF power of 100 W, oxygen gas flow rate of 10 sccm and background gas pressure of 30 Pa. It was found that the optimum imprinting pressure and its depth obtained after the press duration of 10 min were 0.8 MPa and about 0.5 μm, respectively. The resulting diamond nanopatterns of a concave lattice with 1 μm line-width and 5 μm pitch, and a concave cylinder dot with 200 nm diameter and 1 μm pitch which have a height of 1 μm after RF oxygen plasma-etching (100W, 10 sccm, 30 Pa, 40 min) were fabricated with diamond mold RT-NIL using polysiloxane.     

Perpendicular L10-FePt/Fe and L10-FePt/Ru/Fe graded media obtained by post-annealing

L1₀-FePt/Fe and L1₀-FePt/Ru/Fe films with perpendicular orientations were fabricated by magnetron sputtering and post-annealing. In comparison to films with sharp interfaces, the graded L1₀-FePt/Fe interface was more favorable for coercivity reduction. Inserting a ruthenium (Ru) interlayer between the hard L1₀-FePt and the soft iron (Fe) layers yielded a continuous L1₀-FePt/Fe graded film, which contributed to a large reduction in coercivity. The coercivity of L1₀-FePt/Ru/Fe films can be manipulated via the Ru thickness when the soft Fe layer is thin and via the Fe thickness when the soft Fe layer is thick.     

Preparation of protein nanoarray on silicon surface by atomic force microscopy nanofabrication

Atomic force microscopy (AFM) with bias control is employed to fabricate oxidized nanopatterns on a silicon surface with a feature size as low as 50 nm. Nanopatterns made by a Pt/Ir coating probes have larger feature size than these made by the probe without, but the patterning speed is fast, 0.1 s per dot. 20 nm gold nanoparticles are immobilized on oxide nanopatterns to elucidate the dimensions of the nanoparticles on an oxide nanopattern. These patterning conditions are utilized to prepare a nanoarray for the immobilization of biotins to interact with free streptavidins. The resultant height of the biotin labeled on oxidized nanopattern is 0.93 +/- 0.1 nm and the combined height of biotin-streptavidin is 5.14 +/- 0.45 nm, as determined using the imaging functions of AFM. Based on the experimental results, a nano biochip of silicon dioxide can be utilized to monitor molecular interactions on the nanometer scale under static conditions and without the labeling of fluorescence dyes.     

Magnetic switching fields in square monolayer and bilayer nanodots

The increasing density in recording media has augmented the interest in magnetic structures with dimensions in the range of nanometers. One of the interesting nanoscale devices for these kind of applications are the square magnetic nanodots. However a very large integration of this structures in a 2D array leads to interactions between different dots due to the existence of strong magnetic poles at the dot borders. For certain dimensions they may also become unstable due to the high magnetostatic energy and may fall into circular magnetisation distributions, usually referred to as vortex. Magnetostatically coupled bilayers exhibit a higher stability in parallel magnetisation distributions due to a good magnetic flux closure between magnetic layers. Parallel magnetisation structures appear to be stable in bilayer nanodots for a wider range of dimensions rather than in monolayers. Therefore, the higher stability of the magnetisation in this structures as well as the reduction of the magnetic poles at the dot borders make this kind of structures interesting from the high density recording point of view. In this paper it is studied the different stable configurations for square magnetic dots in materials with two magnetic layers with dimensions suitable for a good magnetostatic coupling between them. Results are compared with dots in materials with one single magnetic layer with similar dimensions. Magnetic fields necessary to switch between the different stable configurations as well as the switching mechanisms are also studied.     

Ordered, self-organized cobalt nanodots in Co-diamond-like carbon thin films

A formation process for ordered, self-organized cobalt (Co) nanodots in diamond-like carbon (DLC) thin films deposited by magnetron sputtering in a plasma-assisted Ar/CH4 discharge is presented. episilon-Co dots -5 nm in diameter, separated by 1-2 nm DLC boundaries and arranged in hexagonal arrays were produced on Si substrates. The formation mechanism relies on a self-organization process which is based on surface energy minimization and local magnetic field interaction. The proposed plasma-assisted process presents a controlled and cost-effective bottom-up nanofabrication approach for the production of well-ordered magnetic nanodots based on self-organization.     

Magnetic films on nanoperforated templates: a route towards percolated perpendicular media

We present a study on the magnetization reversal in Co/Pt multilayer films with an out-of-plane easy axis of magnetization deposited onto substrates with densely distributed perforations with an average period as small as 34?nm. Deposition of magnetic Co/Pt multilayers onto the nanoperforated surface results in an array of magnetic nanodots surrounded by a continuous magnetic film. Following the evolution of the magnetic domain pattern in the system, we suggest that domain walls are pinned on structural inhomogeneities given by the underlying nanoperforated template. Furthermore, a series of micromagnetic simulations was performed in order to understand the modification of the pinning strength of domain walls due to the magnetic interaction between nanodots and the surrounding film. The results of the simulations show that magnetic exchange coupling between the nanodots and the surrounding film strongly influences the pinning behavior of the magnetic domain walls which can be optimized to provide maximal pinning.     

FePt纳米薄膜研究进展

由于FePt在超高密度磁存储材料方面的广阔应用前景及其局限性,研究人员对FePT薄膜进行了大量的研究及改性.根据国外近期在此领域的研究现状,综述了单相、复相及掺杂FePt薄膜的制备以及对结构和性质的影响.复相或掺杂主要是通过结构的改变来降低L10晶相转变温度和FePt颗粒的大小,通过其耦合作用来影响FePt薄膜的磁学性能,使其成为超高密度存储器材料.     

Superheating water by CW excitation of gold nanodots

A temperature-dependent photoluminescent thin film of Al(0.94)Ga(0.06)N doped with Er(3+) is used to measure the temperature of lithographically prepared gold nanodots. The gold nanodots and thin film are excited simultaneously with a continuous wave (CW) Nd:YAG 532 nm laser. The gold nanodot is submersed under water, and the dot is subsequently heated. The water immediately surrounding the nanodot is superheated beyond the boiling point up to the spinodal decomposition temperature at 594 ± 17 K. The spinodal decomposition has been confirmed with the observation of critical opalescence. We characterize the laser scattering that occurs in unison with spinodal decomposition due to an increased coherence length associated with the liquid-liquid transition.