Nisha Shukla

FTIR study of surfactant bonding to FePt nanoparticles

Abstract FePt nanoparticles coated with 2xa0nm thick films of surfactant have been studied as candidates for magnetic recording media. The self-assembly of these nanoparticles is influenced by the properties of their surfaces which are coated with a layer of mixed oleic acid and oleyl amine. These surfactant coated FePt nanoparticles were characterized using Fourier transform infrared spectroscopy (FTIR). The observation of both ν(COO) and ν(C=O) vibrational modes indicates that oleic acid bonds to the FePt nanoparticles in both monodentate and bidentate forms. The oleylamine bonds to the FePt nanoparticles through electron donation from the nitrogen atom of the NH2 group. The FTIR spectra indicate that there is a conversion of the alkyl chains from the oleyl form (cis-9-octadecenyl) to the elaidyl form (trans-9-octadecenyl) during the synthesis of the FePt nanoparticles. This is revealed by the presence of several vibrational absorption bands in the region of the olefinic C–H stretching modes. The presence of elaidyl groups on the FePt surfaces is very important because the structures of the oleyl groups and the elaidyl groups are quite different and are expected to pack differently around the FePt nanoparticles. This in turn will influence the self-assembly of nanoparticles on substrates.

Polyol Process Synthesis of Monodispersed FePt Nanoparticles

Monodispersed FePt nanoparticles are synthesized by reduction of iron(II) acetylacetonate and platinum(II) acetylacetonate with 1,2-hexadecanediol as the reducing reagent in the polyol process. As-prepared FePt nanoparticles are chemically disordered with fcc phase. Transmission electron microscopy (TEM) images show a self-assembled particle array with an average particle size of 3 nm and a standard deviation about 10%. The transformation from chemically disordered fcc to chemically ordered L10 phase is achieved by annealing at 650 degrees C for 30 min in Ar atmosphere where the oxygen level is less than 1 ppm. Magnetic hysteresis measurements show a coercivity of 9.0 kOe at 293K, and 16.7 kOe at 5 K for the annealed FePt nanoparticles.

Combined reactions associated with L10 ordering

Abstract Recently, self-organized magnetic arrays (SOMAs) have received a great deal of attention because of the potential of achieving magnetic data storage densities greater than 1xa0Tbit/in 2 . SOMA involves the chemical preparation of self-assembled arrays of alloyed nanoparticles (such as FePt), which can be subsequently annealed to form an L1 0 high magnetocrystalline anisotropy chemically ordered phase. However, during the anneal of the nanoparticles the system can further lower its energy by the elimination of surface area which is brought about by another solid-state reaction which is sintering. Therefore, the combined reactions of ordering and sintering must be understood in a systematic way. Previous work on L1 0 bulk alloys are reviewed with particular emphasis on the role of combined reactions such as ordering and recrystallization. A methodology is described to help in the understanding of combined reactions involving ordering and solid-state reactions concerning the elimination of crystalline defects such as dislocations, grain boundaries and external surfaces. As a first attempt, this framework based on bulk solid-state reactions is developed and further discussed for the case of SOMA nanoparticles.

Oxidation of FePt nanoparticles

Abstract Chemically disordered ∼3–7-nm-diameter FePt nanoparticles are synthesized using the airless operation technique based on decomposition of iron pentacarbonyl and reduction of platinum acetylacetonate. The particle solution is then washed and subsequently deposited onto a thermally oxidized Si substrate. Nanoparticle assemblies are formed after solvent evaporation. The samples are heat-treated using rapid thermal annealing at 650°C for 30xa0min, in an atmosphere of Ar gas with less than 1xa0ppm of O 2 . Oxidation has to be avoided to obtain the FePt L1 0 phase. Other crystalline phases such as FePt 3 (L1 2 ), magnetite Fe 3 O 4 , hematite Fe 2 O 3 and FCC Pt are obtained if oxygen is present.

Studies of switching field and thermal energy barrier distributions in a FePt nanoparticle system

Dynamic remanent hysteresis loops were measured at several time scales for a L10 ordered Fe45Pt55 nanoparticle array sample. At a fixed percentage of magnetization switched, Sharrock’s formula was applied to obtain both the thermal stability factor and the intrinsic switching field. From the magnetization dependence of the thermal stability factor, the width of the thermal energy barrier distribution was determined to be about 0.30. In comparison with the particle volume distribution width obtained from transmission electron microscopy, the energy barrier width is reduced significantly due to strong interparticle exchange interaction. The magnetization dependence of the intrinsic switching field was used to obtain the intrinsic, i.e., short time, remanent magnetization curves. The intrinsic switching field distribution width was found to be 0.34.Dynamic remanent hysteresis loops were measured at several time scales for a L10 ordered Fe45Pt55 nanoparticle array sample. At a fixed percentage of magnetization switched, Sharrock’s formula was applied to obtain both the thermal stability factor and the intrinsic switching field. From the magnetization dependence of the thermal stability factor, the width of the thermal energy barrier distribution was determined to be about 0.30. In comparison with the particle volume distribution width obtained from transmission electron microscopy, the energy barrier width is reduced significantly due to strong interparticle exchange interaction. The magnetization dependence of the intrinsic switching field was used to obtain the intrinsic, i.e., short time, remanent magnetization curves. The intrinsic switching field distribution width was found to be 0.34.

Crystallographic ordering studies of FePt nanoparticles by HREM

Abstract FePt nanoparticles with a size of 4xa0nm were prepared by the polyol process according to the reaction route described by Sun et al. The particles were annealed at 550°C and 580°C for 30xa0min in N 2 atmosphere, developing coercivities of up to 8.8xa0kOe. From conventional transmission electron microscopy the coherence lengths of the self-assembly were found to be as large as 10xa0μm in the as-prepared state and about 1xa0μm in the annealed state. The degree of sintering is zero for the 550°C annealed samples and only a small amount for the 580°C annealed samples. Ordering of the as-prepared fcc structure of the FePt nanoparticles into the L1 0 structure as a result of annealing is studied by high-resolution electron microscopy. In this investigation monodispersed nanoparticles are frequently found to undergo partial chemical ordering to the hard magnetic L1 0 phase without a change in size. Qualitative HREM observations about the amount of ordering of monodispersed nanoparticles, the low degree of sintering of the samples and large coherence length of the self-assembly together with the high coercivity developed upon annealing suggest the potential production of self-assembled ferromagnetic FePt arrays in future high-density magnetic data storage.

FePt nanoparticle hydrodynamic size and densities from the polyol process as determined by analytical ultracentrifugation

We have studied particle size distributions of FePt nanoparticles with analytical ultracentrifugation. The particles were made with the common polyol process by thermal decomposition of iron pentacarbonyl and reduction of platinum (II) acetylacetonate. The size distribution is found to be bimodal (dual) rather than a monomodal distribution as normally reported. The density values for these particles indicates that the smaller particles, ~7.4?nm diameter, have a density above 4?g?cm?3 while the larger particles, ~16?nm, have a density of ~3.2?g?cm?3 due to a higher Fe content in the lighter particles.

Nano processing for thin film disc applications

In this paper, we will discuss several of these nano-processing procedures and their merits and challenges for the fabrication of effective spin-transfer devices. Results of spin-transfer studies that have been made with these magnetic nanopillars.