Using magnetization measurements, we show that point defects in graphene - fluorine adatoms and irradiation defects (vacancies) - carry magnetic moments with spin 1/2. Both types of defects lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene's low-temperature magnetic properties despite the fact that maximum response we could achieve was limited to one moment per approximately 1000 carbon atoms. This limitation is explained by clustering of adatoms and, for the case of vacancies, by losing graphene's structural stability.Comment: 14 pages, 14 figure

Nanofabrication of magnetic storage media, where servo marks, discrete tracks or individual islands are defined, offer the prospect for improved performance and increased areal density. However, this increase in performance will require that new and additional processes be introduced into disk manufacturing. We review here the fundamental patterning and fabrication processes that have been proposed, along with their respective strengths and weaknesses and the potential advantages they may offer for magnetic recording. The increase in data density afforded by nanofabrication may have added significance as more conventional approaches to ever increasing density will encounter physical limitations set by the thermal stability of the recorded bits.

Monodisperse 4 nm FePt magnetic nanoparticles were synthesized by superhydride reduction of FeCl2 and Pt(acac)2 at high temperature, and thin assemblies of FePt nanoparticles with controlled thickness were formed via polymer mediated self-assembly. Adding superhydride (LiBEt3H) to the phenyl ether solution of FeCl2 and Pt(acac)2 in the presence of oleic acid, oleylamine, and 1,2-hexadecanediol at 200 °C, followed by refluxing at 263 °C, led to monodisperse 4 nm FePt nanoparticles. The initial molar ratio of the metal precursors was retained during the synthesis, and the final FePt composition of the particles was readily tuned. Alternately, adsorbing a layer of polyethylenimine (PEI) and the FePt nanoparticles onto a solid substrate resulted in nanoparticle assemblies with tunable thickness. Chemical analysis of the assemblies revealed that more iron oxide was present in the thinner assemblies annealed at lower temperature or for shorter time. Thermal annealing induced the internal particle structure change from chemically disordered fcc to chemically ordered fct and transformed the thin assembly from superparamagnetic to ferromagnetic. This controlled synthesis and assembly can be used to fabricate FePt nanoparticle-based functional devices for future nanomagnetic applications.

We demonstrate that the switching field distribution (SFD) in arrays of 50 nm to 5 microm Co/Pd elements, with perpendicular anisotropy, can be explained by a distribution of intrinsic anisotropy rather than any fabrication related effects. Further, simulations of coercivity and SFD versus element size allow the distribution of intrinsic anisotropy to be quantified in highly exchanged coupled thin films where the reversal mechanism is one of nucleation followed by rapid domain wall motion.

Magnetic nanoparticles have potential applications in high-density memory devices, but their complicated synthesis often requires high temperatures, expensive reagents, and postsynthesis annealing to achieve the desired magnetic properties. Current synthetic methods for magnetic nanoparticles often require post-synthetic modifications, suggesting that the practical application of magnetic nanoparticles will depend on the development of alternative synthetic strategies. We report a biological template to directly grow magnetic nanoparticles of desired material composition and phase under ambient conditions. A phage display methodology was adapted to identify peptide sequences that both specifically bind to the ferromagnetic L1 0 phase of FePt and control the crystallization of FePt nanoparticles using a modified arrested precipitation technique. TEM, electron diffraction, STEM, and X-ray diffraction all indicate these nanoparticles are composed of an FePt alloy with some degree of chemical ordering, and SQUID analysis shows these nanostructures are ferromagnetic at room temperature, possessing coercivities up to 1000 Oe.Since the seminal work of Murray and colleagues, 1 there has been a great deal of interest in using self-assembled films of hard-magnetic FePt nanoparticles for ultrahigh density memory devices. Nanoparticles of ferromagnetic materials are of importance due to their reduced sizes which can support only single magnetic domains, potentially leading to dramatic increases in storage density. However, the current post-synthesis annealing required to achieve the chemically ordered, high anisotropy ferromagnetic L1 0 phase leads to poor control over the spatial arrangement of nanoparticles through extensive particle aggregation. 2 Additionally, since the particles are not prepared with the desired crystallinity, it makes organizing them on a surface with their magnetic easy axis aligned difficult, limiting their technological applications. Eliminating or reducing the high temperature annealing step would greatly simplify the production of magnetic nanoparticles suitable for use in device applications. To date, the most successful approach has been to lower the L1 0 phase transformation temperature through doping of the FePt nanoparticles with silver; 3 yet annealing at temperatures >350°C is still necessary. The need to remove the annealing process from the synthesis of high anisotropy magnetic nanoparticles suggests that a radically different approach be developed.Biological organisms have evolved the ability to direct the synthesis and assembly of crystalline inorganic materials under environmentally benign conditions with control over chemical composition and phase. Examples include the use of viruses expressing material-specific peptides to nucleate semiconducting nanoparticles, 4,5 the use of porous protein crystals, 6,7 modification of the iron storage protein ferritin, 8,9 manipulation of bacteria and yeast to produce iron oxide 10 and semiconducting nanoparticles, 5,11,12 and selection of metal-sp...

We present a simple polymer-mediated process of assembling magnetic FePt nanoparticles on a solid substrate. Alternatively absorbing the PEI molecule and FePt nanoparticles on a HO-terminated solid surface leads to a smooth FePt nanoparticle assembly with controlled assembly thickness and dimension. Magnetic measurements show that the thermally annealed FePt nanoparticle assembly as thin as three nanoparticle layers is ferromagnetic. The magnetization direction of this thin FePt nanoparticle assembly is readily controlled with the laser-assisted magnetic writing. The reported process can be applied to various substrates, nanoparticles, and functional macromolecules and will be useful for future magnetic nanodevice fabrication.

A critical requirement for bit patterned media applications is the control and minimization of the switching field distribution (SFD). Here, we use the ΔH(M,ΔM) method to separate dipolar interactions due to neighbor islands from the intrinsic SFD by measuring a series of partial reversal curves of perpendicular anisotropy Co∕Pd based multilayer films deposited onto prepatterned Si substrates. For a 100-nm-period island array the dipolar broadening contributes 22% to the observed SFD. For a 45-nm-period array this value increases to 31%. These results highlight the importance of quantifying long-range dipolar interactions for determining the intrinsic SFD of patterned media.

We demonstrate that magnetic reversal in perpendicularly magnetized nanostructures is highly dependent on the nature and condition of the edges. To understand the impact of edge damage, we compare nanostructures created by ion milling to those prepared on prepatterned substrates. The size-and temperature-dependent reversal properties of 25 nm-1 m diameter nanodots show that reversal in prepatterned nanostructures is controlled by nucleation within the interior, whereas ion milling results in an edge nucleation process with an unpredicted temperature dependence of the reversal field.