Tunable magnetic properties of arrays of Fe(110) nanowires grown on kinetically-grooved W(110) self-organized templates

We report a detailed magnetic study of a new type of self-organized nanowires disclosed briefly previously [B. Borca et al., Appl. Phys. Lett. 90, 142507 (2007)]. The templates, prepared on sapphire wafers in a kinetically-limited regime, consist of uniaxially-grooved W(110) surfaces, with a lateral period here tuned to 15nm. Fe deposition leads to the formation of (110) 7 nm-wide wires located at the bottom of the grooves. The effect of capping layers (Mo, Pd, Au, Al) and underlayers (Mo, W) on the magnetic anisotropy of the wires was studied. Significant discrepancies with figures known for thin flat films are evidenced and discussed in terms of step anisotropy and strain-dependent surface anisotropy. Demagnetizing coeffcients of cylinders with a triangular isosceles cross-section have also been calculated, to estimate the contribution of dipolar anisotropy. Finally, the dependence of magnetic anisotropy with the interface element was used to tune the blocking temperature of the wires, here from 50K to 200 K

Nanometers-thick self-organized Fe stripes: bridging the gap between surfaces and magnetic materials

We have fabricated 5nm-high Fe(110) stripes by self-organized (SO) growth on a slightly vicinal R(110)/Al2O3(11-20) surface, with R=Mo, W. Remanence, coercivity and domain patterns were observed at room temperature (RT). This contrasts with conventional SO epitaxial systems, that are superparamagnetic or even non-magnetic at RT due to their flatness. Our process should help to overcome superparamagnetism without compromise on the lateral size if SO systems are ever to be used in applications

X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism investigation of size effects on field-induced N\'eel-cap reversal

X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism is used to investigate the influence of an applied magnetic field on N\'eel caps (i.e., surface terminations of asymmetric Bloch walls). Self-assembled micron-sized Fe(110) dots displaying a moderate distribution of size and aspect ratios serve as model objects. Investigations of remanent states after application of an applied field along the direction of N\'eel-cap magnetization give clear evidence for the magnetization reversal of the N\'eel caps around 120 mT, with a $\pm$20 mT dispersion. No clear correlation could be found between the value of the reversal field and geometrical features of the dots

Phase diagram of magnetic domain walls in spin valve nano-stripes

We investigate numerically the transverse versus vortex phase diagram of head-to-head domain walls in Co/Cu/Py spin valve nano-stripes (Py: Permalloy), in which the Co layer is mostly single domain while the Py layer hosts the domain wall. The range of stability of the transverse wall is shifted towards larger thickness compared to single Py layers, due to a magnetostatic screening effect between the two layers. An approached analytical scaling law is derived, which reproduces faithfully the phase diagram.Comment: 4 page

Non-universality of artificial frustrated spin systems

Magnetic frustration effects in artificial kagome arrays of nanomagnets with out-of-plane magnetization are investigated using Magnetic Force Microscopy and Monte Carlo simulations. Experimental and theoretical results are compared to those found for the artificial kagome spin ice, in which the nanomagnets have in-plane magnetization. In contrast with what has been recently reported, we demonstrate that long range (i.e. beyond nearest-neighbors) dipolar interactions between the nanomagnets cannot be neglected when describing the magnetic configurations observed after demagnetizing the arrays using a field protocol. As a consequence, there are clear limits to any universality in the behavior of these two artificial frustrated spin systems. We provide arguments to explain why these two systems show striking similarities at first sight in the development of pairwise spin correlations.Comment: 7 pages, 6 figure

High-Temperature Activated AB2 Nanopowders for Metal Hydride Hydrogen Compression

A reliable process for compressing hydrogen and for removing all contaminants is that of the metal hydride thermal compression. The use of metal hydride technology in hydrogen compression applications though, requires thorough structural characterization of the alloys and investigation of their sorption properties. The samples have been synthesized by induction - levitation melting and characterized by Rietveld analysis of the X-Ray diffraction (XRD) patterns. Volumetric PCI (Pressure-Composition Isotherm) measurements have been conducted at 20, 60 and 90 oC, in order to investigate the maximum pressure that can be reached from the selected alloys using water of 90oC. Experimental evidence shows that the maximum hydrogen uptake is low since all the alloys are consisted of Laves phases, but it is of minor importance if they have fast kinetics, given a constant volumetric hydrogen flow. Hysteresis is almost absent while all the alloys release nearly all the absorbed hydrogen during desorption. Due to hardware restrictions, the maximum hydrogen pressure for the measurements was limited at 100 bars. Practically, the maximum pressure that can be reached from the last alloy is more than 150 bars.Comment: 9 figures. arXiv admin note: text overlap with arXiv:1207.354

Negative magnetocaloric effect in Fe 1-x Rh x compounds

International audienceOn increasing temperature, the Fe1-xRhx alloys present a transition from an antiferromagnetic to a ferromagnetic state, which induces a negative magnetocaloric effect (MCE). The magnetocaloric effect, in particular of the Fe0.49Rh0.51 alloy, was studied by direct measurements and accordingto specific heat measurements. Here, we report the recent results obtained on the annealed Fe0.48Rh0.52 compound we prepared by arc melting. The isothermal entropy change ΔS allowing an estimation of the MCE was determined from magnetic measurements. The ΔS experimentally found is larger than that reported for the annealed Fe0.49Rh0.51 sample. However a significant difference occurs between transition temperature of the annealed sample Fe0.48Rh0.52 and that reported in literature. This result shows that magnetic and magnetocaloric properties in this kind of materials are very sensitive to the technique of samples preparation. Besides, the magnetocaloric effect close the ferromagnetic – paramagnetic transition in Fe0.48Rh0.52 is also discussed