▶ Fe–Pd nanotubes produced via direct template-assisted electroplating. ▶ Angular dependence measurements revealed that the magnetization reversal mechanism in
fcc Fe
50Pd
50 nanotubes proceeds via ...curling mechanism with a transition to coherent rotation. ▶ High coercivity (135
kA/m) achieved in annealed Fe
50Pd
50 nanotubes due to L1
0 ordering.
Near-equiatomic Fe–Pd-based nanotubes with diameters of 200
nm and lengths of 1
μm were directly electrodeposited from a single electrolyte into polycarbonate templates. The as-deposited Fe
50Pd
50 nanotubes were then characterized compositionally, structurally and magnetically. The as-deposited Fe
50Pd
50 tubes had an
fcc crystal structure and were magnetically soft (
H
C
≈
10
kA/m), with the easy axis of the magnetization being parallel to the axes of the tubes. Angular-dependence measurements of the coercivity, where the hysteresis loops were measured as a function of the angle (
θ) of the applied demagnetizing field, revealed a combination of magnetization reversal mechanisms, consisting of the curling mechanism, which dominates at low angles, with a transition to coherent rotation at angles ≥70°. The development of the coercivity with annealing temperature due to the L1
0 ordering was also investigated. For this purpose the as-deposited nanotubes were annealed at temperatures from 400
°C to 650
°C for 1
h in Ar
+
7% H
2 and the phase formation, the microstructure and the magnetic properties were analyzed. A maximum in the coercivity of 135
kA/m was achieved upon annealing at 550
°C.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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 Co 33 Pt 67 nanotubes exhibit hard magnetic properties with a coercivity of 100 kA/m, a coercivity of 680 kA/m was achieved with the Co 45 Pt 55 nanotubes. This increase in the coercivity was attributed to the transformation of the FCC Co-Pt to the L1 0 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 L1 0 phase to occur.
Near-equiatomic Fe-Pd-based nanotubes with diameters of 200 nm and lengths of 1 km were directly electrodeposited from a single electrolyte into polycarbonate templates. The as-deposited Fe sub(50)Pd ...sub(50) nanotubes were then characterized compositionally, structurally and magnetically. The as-deposited Fe sub(50)Pd sub(50) tubes had an fcc crystal structure and were magnetically soft (H sub()Casymptotic to 10 kA/m), with the easy axis of the magnetization being parallel to the axes of the tubes. Angular-dependence measurements of the coercivity, where the hysteresis loops were measured as a function of the angle (theta) of the applied demagnetizing field, revealed a combination of magnetization reversal mechanisms, consisting of the curling mechanism, which dominates at low angles, with a transition to coherent rotation at angles >=70. The development of the coercivity with annealing temperature due to the L1 sub(0) ordering was also investigated. For this purpose the as-deposited nanotubes were annealed at temperatures from 400 C to 650 C for 1 h in Ar + 7% H sub(2) and the phase formation, the microstructure and the magnetic properties were analyzed. A maximum in the coercivity of 135 kA/m was achieved upon annealing at 550 C.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We have examined the effects of chemically bonded organic surface layers on the behaviour of a ceramic and a metal powder. The thin monomolecular layers were found to improve the corrosion resistance ...of the sensitive Nd–Fe–B powder and make the fine alumina powder hydrophobic and, as a result, making both coated powders significantly less sensitive to environmental humidity. Our analysis of the flowability of the noncoated and coated powders revealed that the alumina powder can be classified as very cohesive—the addition of the coating even increased its cohesivity. The Nd–Fe–B powder was determined to be easy flowing, and the effect of the coating changed with the applied normal stress. The coating does, however, improve the powder-compaction properties of both powders. In addition, the beneficial effect of the coating in terms of viscosity as well as in resistance to destabilization was also observed for suspensions in organic solvents.
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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.
Magnetic micro-actuators and systems (MAGMAS) require tiny permanent magnets with dimensions of hundreds of micrometers. Such magnets need to have the highest possible energy density, which means ...Nd–Fe–B magnets are the most appropriate type. Most bottom-up fabrication techniques are either too slow or too expensive; top-down techniques involving machining tend to result in surface damage and a loss of magnetic properties. In this study, we have looked at very neodymium-rich Nd–Fe–B powders that allow us to sinter 100-μm-thick samples to full density at temperatures as low as 800
°C. These very thin magnets have coercivities of up to 1000
kA
m
−1 and are suitable for MAGMAS-type applications.
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Gd5Si2Ge2-based alloys can exhibit a giant magnetocaloric effect (MCE); this gives them the potential for use in cooling and refrigeration technologies. Cast alloys of this type have been reported to ...exhibit a three-phase microstructure: the main phase has a composition close to Gd5(Si1.95Ge2335); the secondary phases are Gdi(Si,Ge)1 and Gd5(Si,Ge)3, with the latter reported to have linear features in the microstructure, characteristic of a Widmanstatten pattern. In this investigation we have looked at the effect on the microstructure of Gd5Si2Ge2 resulting from a substitution of Si by Fe, according to the formula Gcl3Si2,FeGe2, where x was varied between 0 and 1. Alloys with six different compositions were prepared using the arc-melting technique. All the samples and their microstructures were observed examined in optical microscope (OM) and a field-emission-gun scanning electron microscope (FEG SEM). The microstructures were quantitatively assessed with energy-dispersive X-ray spectroscopy (EDS) and the samples were characterised using X-ray diffraction (XRD).
We have investigated the influence of delta-ferrite content in Cr-Ni-Mo stainless-steel cast alloys on the magnetic characteristics. Samples of cast alloys with mass fractions 9-11 % of Ni, 18-21 % ...of Cr and 1.8-2.5 % of Mo, with three characteristically different delta-ferrite contents, were prepared with careful selection of appropriate amounts of alloying elements and a controlled solidification procedure. The samples were then aged in the operating temperature region (290 - 350 deg C) for different periods of time (up to two years). The delta-ferrite content was determined with three different methods: i.e., empirically on the basis of chemical composition, magnetic-induction-based and metallographically. The magnetic-induction-based determination of delta-ferrite content showed that it does not change with ageing temperature and time. It confirms that only the internal ferrite structure is changed during ageing because of the spinodal decomposition. The absolute magnetic properties were determined with a hysteresisgraph and a vibrating-sampling magnetometer (VSM). The results showed that the magnetic properties depend on the chemical composition (delta-ferrite content), the ageing temperature and the time. The chemical composition has the biggest influence, but the influence of the ageing temperature and time is insignificant and the scatter of the results is relatively large. The determination of the absolute magnetic properties is a destructive method and the mechanical preparation of the samples can influence the magnetic properties. Therefore, the method is not appropriate for the in-situ observation of the kinetics of spinodal decomposition. Measurements with the VSM are more appropriate than with the hysteresisgraph because much smaller samples with any geometry can be used.
We have examined the effects of chemically bonded organic surface layers on the behaviour of a ceramic and a metal powder. The thin monomolecular layers were found to improve the corrosion resistance ...of the sensitive Nd-Fe-B powder and make the fine alumina powder hydrophobic and, as a result, making both coated powders significantly less sensitive to environmental humidity. Our analysis of the flowability of the noncoated and coated powders revealed that the alumina powder can be classified as very cohesive-the addition of the coating even increased its cohesivity. The Nd-Fe-B powder was determined to be easy flowing, and the effect of the coating changed with the applied normal stress. The coating does, however, improve the powder-compaction properties of both powders. In addition, the beneficial effect of the coating in terms of viscosity as well as in resistance to destabilization was also observed for suspensions in organic solvents.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Magnetic micro-actuators and systems require tiny permanent magnets with dimensions of hundreds of micrometres. Such magnets need to have the highest possible energy density, which means Nd-Fe-B is ...the most appropriate material. Most bottom-up fabrication techniques are either too slow or too expensive; top-down techniques involving machining tend to result in surface damage and a loss of magnetic properties. Thus, PM is an attractive route. It is shown that use of very neodymium-rich Nd-Fe-B powders makes it possible to sinter 100 μm thick specimens to full density at temperatures as low as 800°C. These very thin magnets have coercivities of up to 1000 kA m
−1
and are suitable for micro-actuator type applications.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
