We report the synthesis, characterization, and covalent surface chemistry of "magnetomicelles", cross-linked, amphiphilic block-copolymer micelles that encapsulate superparamagnetic iron oxide nanoparticles. Because these composite nanostructures assemble spontaneously from solution by simultaneous desolvation of nanoparticle and amphiphilic poly(styrene(250)-block-acrylic acid(13)) components, explicit surface functionalization of the particles is not required, and the encapsulation method was applied to different magnetic nanoparticle sizes and compositions. TEM images of the magnetomicelles illustrated that the number of encapsulated particles could be dictated rationally by synthetic conditions. The magnetic properties of the particles were characterized by SQUID magnetometry and followed the general Langevin magnetic model for superparamagnetic materials. The micellar shells of these particles were functionalized using covalent chemistry that would not ordinarily be possible on the magnetic particle surface. As a result, this noncovalent approach provides a new route to technological applications of hydrophobic magnetic nanomaterials that lack appropriate conjugate surface chemistry.

The CoPt films with 9.7-91 at% Co and thicknesses of 15-20 nm were obtained from a new designed stable hexachloroplatinate solution at a controlled potential deposition. The effects of the substrate (Ru and Cu) and an organic additive (saccharin) on composition, crystal structure and magnetic properties of the CoPt films were studied. It was demonstrated that a Ru electrode substrate provides well-defined surface for the epitaxial growth of hcp phase, resulting in high perpendicular anisotropy. The addition of saccharin (Sacc) as an organic additive into the plating solution caused a dramatic improvement of the epitaxial growth of CoPt film on the Ru substrate. At the film thickness of interest, for bit-patterned media BPM (15-20 nm), the out-of-plane coercivity showed the highest value of 6700 Oe and the squarness M r /M s ∼1. The areal density of hard disc drives has been increasing at 25-35% per year and now it is approaching to 1 Tb/in 2 . The magnetic crystal grain volume (V) has been decreasing to several cubic nanometers which is near to the thermal instability of granular media, i.e. the superparamagnetic limit. In order to further increase the areal density thermal stability factor, K u V/kT, needs to be kept within 40-60 range. The superparamagnetic effect posses a serious challenge for continuing to increase the areal density and storage capacity of disc drives.1 In order to solve these problems the intensive research and development efforts are currently being carried out worldwide with two major concepts. The first one is to use high magnetoanisotropy (K u ) granular media, i.e. FePt or CoPt alloys, and develop a heat-assisted magnetic recording (HAMR) writer.2 The second is to use conventional perpendicular writer and develop a bit-patterned media (BPM) that effectively increase the grain volume (V).3 Both concepts need to overcome numerous technical challenges in R&D in order to manufacture recording heads with areal density >1 Tb/in 2 and stability over 10 years of data storage.The main idea of BPM technology is that each bit is stored in a single dimensionally defined magnetic switching volume, i.e. dot. Different methods-electron beam lithography (EBL), nanoimprint lithography (NIL), block co-polymer (BCP)-for the fabrication of nano-holes with the long-range ordering and diameter of sub-20 nm corresponding to the density of ∼1 Tdot/in 2 have been demonstrated. 4,5 A typical perpendicular media comprises of a multilayer structure including a substrate covered by a soft magnetic under layer (SUL), an interlayer (seed layer) and hard patterned magnetic layer (BPM). The hard magnetic layer can be a CoPt or FePt electrodeposited alloy of hcp-crystal structure with crystalline grains oriented along the c-axis (the magnetic easy axis) in the direction normal to the film. The important magnetic property at the thickness of interest in BPM (15-20 nm) is high out-of-plane coercivity (H c ) which is largely determined by magnetocrystalline anisotropy and to a lesser extent by shape anisotropy of magneti...

We report the direct preparation of monodispersed L10 phase FePt nanoparticles by controlled nucleation and growth using a gas phase aggregation source. These FePt nanoparticles became ordered during their growth in an argon gas flow. They are octahedron faceted with an average size of 5.8nm and a standard size distribution of 11%, as illustrated by transmission electron microscope. Magnetic measurements show that these FePt nanoparticles have coercivities of 8.25kOe at room temperature and 26.5kOe at 50K. This technique provides a novel approach for fabricating nanomaterials with controllable phase and shape in general.

Ferromagnetic nanoparticles are important materials for nanotechnology both in practical applications and in fundamental research. The different purposes for which they are used have posed significant challenges in their preparation. For example, the development of highly sensitive magnetoresistive (MR) sensors makes it possible to detect the binding interactions between DNA or protein molecules, which are attached by magnetic beads.[1] To achieve high selectivity and sufficient signal-to-noise ratio, oxidation-resistive ferromagnetic nanomagnets are more favorable than the superparamagnetic microbeads usually used. Nanocomposite exchangespring magnets were expected to have unusually high energy products (BH) max (magnetic flux density times magnetic field strength), which requires the fabrication of crystallographically coherent hard and soft magnetic phases with a proper exchange coupling. [2,3] This requires nanometer-scale structure and property control of both the hard and soft magnetic components. Future extremely high density magnetic storage media demand high anisotropic magnetic materials, such as L1 0 -phase FePt, to overcome the superparamagnetic limit.[4]Chemical ordering and orientation control are essential prerequisites for the popular wet-chemical approach, [5] and these are difficult to achieve by post-deposition annealing. [6] We have recently reported a vacuum technique based on the gasphase condensation principle to prepare directly ordered FePt nanoparticles. [7] Here we present a general methodology of tuning the crystal structure and magnetic properties of FePt binary alloy nanomagnets.The final crystal structure of nanoparticles is a result of their thermal history, which involves both thermodynamic and kinetic factors. The binary phase diagram of equiatomic FePt alloy shows that it has two solid-state phases: fcc disordered (face-centered cubic or A1 phase) and fct ordered (face-centered tetragonal or L1 0 phase), with fcc as the high-temperature stable phase.[8] Many of the preparation techniques are kinetically controlled, so the fcc disordered phase dominates their final products. To get the fct ordered phase post-deposition annealing is generally used. This annealing process is a first-order phase transformation process, which requires nucleation sites for the ordering to originate. In FePt thin films, grain boundaries and crystal defects can supply such starting points for phase transformation. FePt nanoparticles are more difficult to transform because they do not have conventional grain boundaries and do not have many defects either.[9] Accordingly, any post-annealing approach usually results in particle agglomeration and twin formation, which will limit their applications. An alternative, and also optimal, way to prepare nanoparticles with the desired crystal structure is to control the thermal environment for their nucleation and growth, which is the main focus of this work. There are several different ways to control the thermal environment for particle nucleation and growth by ...

Gas-phase prepared directly ordered FePt nanoparticles were shown to align with in situ magnetic fields. As shown by magnetic and x-ray diffraction data, a 3800Oe perpendicular field switched L10 FePt particles with a mean size of 6nm from in-plane arrangement in as-deposited state to out-of-plane orientation. A 5000Oe in-plane field successfully defined nanoparticles with in-plane texture. These results demonstrated the feasibility of preparing nanoparticle-based magnetic recording media and exchange-spring-type permanent magnets with desired magnetic orientation control. Only involving thermal fluctuation as the obstacle, this approach makes an ideal subject for theoretical understanding and further optimization.