Saso Sturm

Angular Dependence of the Coercivity in Electrodeposited Co–Pt Nanostructures With a Tube–Wire Morphology

We have fabricated Co-Pt cylindrical nanostructures comprised of a pair of nanotube and nanowire segments via direct electroplating into anodic alumina (AAO) membranes. The fabrication of such nanostructures is possible due to the penetration of sputtered gold (Au) nanoparticles inside the template, which serve as nucleation spots. The current transient monitored during the deposition process allowed us to distinguish between the nanotube and nanowire formation regime during the electrodeposition. In this paper, we report on the systematic changes in the angular dependence of the coercivity behavior due to the change in the length of the nanowire segment of these nanostructures, while keeping the nanotube segment constant. Considering the range of parameters studied, we found that the tube segment always reverses its magnetization through a vortex domain wall, whereas the nanowire segment reverses its magnetization through a transverse domain wall. Our results are in good agreement with literature. The possibility to alter the magnetization reversal mode in such nanostructures provides an attractive way to control the motion of the magnetic domain walls.

Hard Magnetic Co-Pt-Based Nanotubes Produced via Direct Electroplating

Co-Pt nanostructures having a tubular geometry, a diameter of 200 nm and a length of 2 mum were produced via direct electroplating into polycarbonate-based templates. The nanotube-formation mechanism was explained using the relative rates of deposition and the diffusion of the metal ions. The as-deposited Co-Pt nanotubes are magnetically soft, with magnetocrystalline anisotropy prevailing over the shape anisotropy. The Co-Pt nanotubes were prepared in two different stoichiometries. While the Co33Pt67 nanotubes exhibit hard magnetic properties with a coercivity of 100 kA/m, a coercivity of 680 kA/m was achieved with the Co45Pt55 nanotubes. This increase in the coercivity was attributed to the transformation of the FCC Co-Pt to the L10 phase, which happens upon annealing at 700degC for 60 min. In the first case the low coercivity is attributed to the composition being slightly out of range for the transformation to the L10 phase to occur.

Magnetization-Switching Study of fcc Fe–Pd Nanowire and Nanowire Arrays Studied by In-Field Magnetic Force Microscopy

Using electrodeposition into anodic aluminum oxide (AAO) membranes, we have successfully fabricated near-equiatomic Fe48±3Pd52±3 nanowires (NWs) with the diameter of ≈200 nm and the length of ≈3.5 μm. X-ray diffraction and transmission electron microscopy/selected-area electron diffraction analyses revealed that the as-deposited NWs are isotropic and polycrystalline with an average crystal size of 5 nm and have an fcc crystal structure. Magnetic force microscopy (MFM) on a single Fe-Pd NW revealed its single-domain behavior with the easy axis of magnetization along the long axis of the NW. The magnetizationswitching behavior of a single Fe-Pd NW studied with the MFM suggested a square-shaped magnetization curve (M/Ms = 1) with HCI ≈ 3.2 kA/m. In addition, using in-field MFM techniques, the effects of dipolar interactions in an Fe-Pd array of NWs still embedded in the AAO were determined. It was found that the dipolar interactions greatly reduce the parameters of the magnetization hysteresis loop, such as the coercivity, the remanence, and the switching-field distribution of the Fe-Pd NW array.

Stabilisation of Ce-Cu-Fe amorphous alloys by addition of Al

Abstract The present work describes the formation of amorphous alloys in the (Al1−xCex)62Cu25Fe13 quaternary system (0 ≤ x ≤ 1). When the amount of Ce falls in the range 0.67 ≤ x ≤ 0.83, the alloys obtained exhibit a completely amorphous structure confirmed by powder X-ray diffraction. Otherwise, at compositions x = 0.5, 0.58, 0.92 and 1, a primary crystalline phase forms together with an amorphous matrix. The crystallisation temperature (Tx) decreases with increasing Ce content, varying from 593 K for x = 0.5–383 K for x = 1. Composition x = 0.75 is considered as the best glass former, exhibiting a large supercooled liquid region of 40 K width that precedes crystallisation. In order to form bulk amorphous alloys, ribbons with this later composition were consolidated into few millimetre thick discs using pulsed electric current sintering at different temperatures, yet preserving the amorphous structure. Meanwhile, increasing temperature above 483 K triggers crystallisation of a primary phase isostructural to AlCe3. Further increase in the temperature up to 573 K yields a higher fraction of the crystalline phase. Testing mechanical properties, using nanoindentation, revealed that both elastic modulus (E) and hardness (H) depend on the Al content, ranging from E = 85.6 ± 3.7 GPa and H = 6.2 ± 0.7 GPa for x = 0.5 down to E = 39.8 ± 1.0 GPa and H = 3.1 ± 0.2 GPa for x = 0.92.

Magnetic interactions and in vitro study of biocompatible hydrocaffeic acid-stabilized Fe–Pt clusters as MRI contrast agents

A detailed magnetic study of separated Fe–Pt NPs and Fe–Pt clusters was performed to predict their optimal size and morphology for the maximum saturation magnetization, a factor that is known to influence the performance of a magnetic-resonance-imaging (MRI) contrast agent. Excellent stability and biocompatibility of the nanoparticle suspension was achieved using a novel coating based on hydrocaffeic acid (HCA), which was confirmed with a detailed Fourier-transform infrared spectroscopy (FTIR) study. An in vitro study on a human-bladder papillary urothelial neoplasm RT4 cell line confirmed that HCA-Fe–Pt nanoparticles showed no cytotoxicity, even at a very high concentration (550 μg Fe–Pt per mL), with no delayed cytotoxic effect being detected. This indicates that the HCA coating provides excellent biocompatibility of the nanoparticles, which is a prerequisite for the material to be used as a safe contrast agent for MRI. The cellular uptake and internalization mechanism were studied using ICP-MS and TEM analyses. Furthermore, it was shown that even a very low concentration of Fe–Pt nanoparticles (<10 μg mL−1) in the cells is enough to decrease the T2 relaxation times by 70%. In terms of the MRI imaging, this means a large improvement in the contrast, even at a low nanoparticle concentration and an easier visualization of the tissues containing nanoparticles, proving that HCA-coated Fe–Pt nanoparticles have the potential to be used as an efficient and safe MRI contrast agent.

Microstructures and Magnetic Properties of Electrodeposited Co-Pt Nanowires With Diameters Below 100 nm

We have investigated the effect of the microstructure on the magnetic properties of Co-Pt nanowires (NWs) with diameters of 15, 50, and 80 nm. These Co-Pt NWs were fabricated by using polycarbonate membranes with different pore diameters via direct electrodeposition. A detailed transmission-electron-microscopy analysis revealed that the Co-Pt NWs transform from a polycrystalline to a single-crystalline-like structure along the growth direction of the NWs. The selected-area electron-diffraction investigation of the Co-Pt NWs with 50 nm diameters revealed a fcc-dominant crystal structure, while the 15 and 80 nm NWs were composed of an intermixture of fcc and hcp phases. This investigation allows us to understand the magnetic hysteresis loops of the Co-Pt NW arrays. Furthermore, the magnetic domains of the individual Co-Pt NWs were studied with magnetic force microscope (MFM), and the MFM contrasts for the 50 nm and 80 nm diameter NWs are interpreted as consisting of z-vortices.

Structural and compositional properties of Sm-Fe-Ta magnetic nanospheres prepared by pulsed-laser deposition at 157 nm in a N2 atmosphere

Sm-Fe-Ta intermetallic alloys in their bulk form can exhibit hard magnetic properties after absorbing nitrogen at the interstitial sites within the 2:17 rhombohedral structure [1]. Lately, these properties have become attractive for the expanding field of nanotechnology, and much effort has been put into the synthesis of various nanostructures that will potentially serve as the building blocks in future electronic devices, such as circulators, magnetostrictive heads and nano-electro-mechanical systems (NEMS). One of the most promising routes for the synthesis of nanostructured Sm-Fe-Ta intermetallic alloys in the form of nanosized spheres appears to be pulsed-laser deposition (PLD), performed in a nitrogen background pressure using a target with the composition Sm13.8Fe82.2Ta4.0 [2].