Enhancement of Exchange Bias and Perpendicular Magnetic Anisotropy in CoO/Co Multilayer Thin Films by Tuning the Alumina Template Nanohole Size.

The interest in magnetic nanostructures exhibiting perpendicular magnetic anisotropy and exchange bias (EB) effect has increased in recent years owing to their applications in a new generation of spintronic devices that combine several functionalities. We present a nanofabrication process used to induce a significant out-of-plane component of the magnetic easy axis and EB. In this study, 30 nm thick CoO/Co multilayers were deposited on nanostructured alumina templates with a broad range of pore diameters, 34 nm ≤ Dp ≤ 96 nm, maintaining the hexagonal lattice parameter at 107 nm. Increase of the exchange bias field (HEB) and the coercivity (HC) (12 times and 27 times, respectively) was observed in the nanostructured films compared to the non-patterned film. The marked dependence of HEB and HC with antidot hole diameters pinpoints an in-plane to out-of-plane changeover of the magnetic anisotropy at a nanohole diameter of ∼75 nm. Micromagnetic simulation shows the existence of antiferromagnetic layers that generate an exceptional magnetic configuration around the holes, named as antivortex-state. This configuration induces extra high-energy superdomain walls for edge-to-edge distance >27 nm and high-energy stripe magnetic domains below 27 nm, which could play an important role in the change of the magnetic easy axis towards the perpendicular direction.

The role of amorphous precursors in the crystallization of La and Nd carbonates.

Crystalline La and Nd carbonates can be formed from poorly-ordered nanoparticulate precursors, termed amorphous lanthanum carbonate (ALC) and amorphous neodymium carbonate (ANC). When reacted in air or in aqueous solutions these precursors show highly variable lifetimes and crystallization pathways. We have characterized these precursors and the crystallization pathways and products with solid-state, spectroscopic and microscopic techniques to explain the differences in crystallization mechanisms between the La and Nd systems. ALC and ANC consist of highly hydrated, 10-20 nm spherical nanoparticles with a general formula of REE2(CO3)3·5H2O (REE = La, Nd). The stabilities differ by ∼2 orders of magnitude, with ANC being far more stable than ALC. This difference is due to the Nd(3+) ion having a far higher hydration energy compared to the La(3+) ion. This, together with temperature and reaction times, leads to clear differences not only in the kinetics and mechanisms of crystallization of the amorphous precursor La- and Nd-carbonate phases but also in the resulting crystallite sizes and morphologies of the end products. All crystalline La and Nd carbonates developed spherulitic morphologies when crystallization occurred from hydrous phases in solution at temperatures above 60 °C (La system) and 95 °C (Nd system). We suggest that spherulitic growth occurs due to a rapid breakdown of the amorphous precursors and a concurrent rapid increase in supersaturation levels in the aqueous solution. The kinetic data show that the crystallization pathway for both La and Nd carbonate systems is dependent on the reaction temperature and the ionic potential of the REE(3+) ion.

Optimisation of magnetic separation: a case study for soil washing at a heavy metals polluted site.

Sandy loam soil polluted with heavy metals (As, Cu, Pb and Zn) from an ancient Mediterranean Pb mining and metallurgy site was treated by means of wet high-intensity magnetic separation to remove some of the pollutants therein. The treated fractions were chemically analysed and then subjected to magnetic characterisation, which determined the high-field specific (mass), magnetic susceptibility (κ) and the specific (mass) saturation magnetisation (σS), through isothermal remanent magnetisation (IRM) curves. From the specific values of κ and σS, a new expression to assess the performance of the magnetic separation operation was formulated and verified by comparison with the results obtained by traditional chemical analysis. The magnetic study provided valuable information for the exhaustive explanation of the operation, and the deduced mathematical expression was found to be appropriate to estimate the performance of the separation operation. From these results we determined that magnetic soil washing was effective for the treatment of the contaminated soil, concentrating the majority of the heavy metals and peaking its separation capacity at 60% of the maximum output voltage.

The magnetocaloric effect in Er2Fe17 near the magnetic phase transition.

Recent investigations in R2Fe17 intermetallic compounds have evidenced that these materials present a moderate magnetocaloric effect (MCE) near room temperature. A series of accurate magnetization measurements was carried out to show that the value of the demagnetizing factor has a significant influence on the absolute MCE value of Er2Fe17. In addition, the critical exponents determined from heat capacity and magnetization measurements allow us to describe the field dependence of the observed MCE around the Curie temperature.

Magnetic ionic plastic crystal: choline[FeCl4].

A novel organic ionic plastic crystal (OIPC) based on a quaternary ammonium cation and a tetrachloroferrate anion has been synthesized with the intention of combining the properties of the ionic plastic crystal and the magnetism originating from the iron incorporated in the anion. The thermal analysis of the obtained OIPC showed a solid-solid phase transition below room temperature and a high melting point above 220 °C, indicating their plastic crystalline behaviour over a wide temperature range, as well as thermal stability up to approximately 200 °C. The magnetization measurements show the presence of three-dimensional antiferromagnetic ordering below 4 K. The results from electrochemical characterization display a solid-state ionic conduction sufficiently high and stable (between 10(-2.7) and 10(-3.6) S cm(-1) from 20 to 180 °C) for electrochemical applications.

Series of 2D heterometallic coordination polymers based on ruthenium(III) oxalate building units: synthesis, structure, and catalytic and magnetic properties.

A series of 2D ruthenium-based coordination polymers with hcb-hexagonal topology, {[K(18-crown-6)]3[M(II)3(H2O)4{Ru(ox)3}3]}n (M(II) = Mn (1), Fe (2), Co (3), Cu (4), Zn (5)), has been synthesized through self-assembly reaction. All compounds are isostructural frameworks that crystallize in the monoclinic space group C2/c. The crystal packing consists of a 2D honeycomb-like anionic mixed-metal framework intercalated by [K(18-crown-6)](+) cationic template. Dehydration processes take place in the range 40-200 °C exhibiting two phase transitions. However, the spontaneous rehydration occurs at room temperature. Both hydrated and dehydrated compounds were tested as Lewis acids heterogeneous catalysts in the acetalyzation of benzaldehyde achieving high yields with the possibility to be recovered and reused. All the investigated materials do not show any long-range magnetic ordering down to 2 K. However, the Fe-based compound 2 presents a magnetic irreversibility in the ZFC-FC magnetization data below 5 K, which suggest a spin-glass-like behavior, characterized also by short-range ferromagnetic correlations. The coercive field increases as the temperature is lowered below 5 K, reaching a value of 1 kOe at 2 K. Alternating current measurements obtained at different frequencies confirm the freezing process that shows weak frequency dependence, being characteristic of a system exhibiting competing magnetic interactions.

Pressure effects on Emim[FeCl4], a magnetic ionic liquid with three-dimensional magnetic ordering.

We report a combined study using magnetization and Raman spectroscopy on the magnetic ionic liquid 1-ethyl-3-methylimidazolium tetrachloroferrate, Emim[FeCl4]. This material shows a long-range antiferromagnetic ordering below the Néel temperature T(N) ≈ 3.8 K. The effects of pressure on the magnetic properties have been studied using a miniature piston-cylinder CuBe pressure cell. This three-dimensional ordering is strongly influenced when hydrostatic pressure is applied. It is observed that low applied pressure is enough to modify the magnetic interactions, inducing a transition from antiferromagnetic to ferrimagnetic ordering. Raman spectroscopy measurements reveal important information about the existence of isolated [FeCl4](-) anions and the absence of dimeric [Fe2Cl7](-) units in the liquid and solid states. These features seem to suggest that the superexchange pathways responsible for the appearance of magnetic ordering are mediated through Fe-Cl-Cl-Fe. Furthermore, the liquid-solid phase transition exhibits a magnetic hysteresis near room temperature, which can be tuned by weak pressures.

Co nanoparticles inserted into a porous carbon amorphous matrix: the role of cooling field and temperature on the exchange bias effect.

We report unusual cooling field dependence of the exchange bias in oxide-coated cobalt nanoparticles embedded within the nanopores of a carbon matrix. The size-distribution of the nanoparticles and the exchange bias coupling observed up to about 200 K between the Co-oxide shell (∼3-4 nm) and the ferromagnetic Co-cores (∼4-6 nm) are the key to understand the magnetic properties of this system. The estimated values of the effective anisotropy constant and saturation magnetization obtained from the fit of the zero-field cooling and field cooling magnetization vs. temperature curves agree quite well with those of the bulk fcc-Co.

On the crystal structure and thermal decomposition of ammonium-iron(III) bis(hydrogenphosphate).

NH(4)Fe(HPO(4))(2) and its deuterated form have been synthesized as monophasic polycrystalline materials. Their crystal structures, including hydrogen positions, were determined by Rietveld refinement and Fourier synthesis using constant-wavelength neutron powder diffraction data. In addition, the thermal decomposition of NH(4)Fe(HPO(4))(2) was found to give mixtures of Fe(4)(P(2)O(7))(3) and Fe(PO(3))(3)via NH(4)FeP(2)O(7) formation, the crystal structure of which has also been refined from X-ray powder diffraction data.

Antiferro-quadrupolar structures in UPd3 inferred from x-ray resonant Bragg diffraction.

A systematic analysis of resonant x-ray Bragg diffraction data for UPd(3), with signal enhancement at the U M(IV) edge, including possible structural phase transitions leads to a new determination of the space groups of the material in the phases between T(0)=7.8 K and T(+1)=6.9 K, as P 222(1), and between T(-1)=6.7 K and T(2)=4.4 K, as P2(1). In addition, the quadrupolar order parameters, {Q(ab)}, inferred from diffraction data for the phase between T(-1) and T(2), are {Q(xz)} and {Q(yz)} at the (103) Bragg reflection and {Q(xy)} at the (104) reflection.

Nanocrystalline Nd2Fe17 synthesized by high-energy ball milling: crystal structure, microstructure and magnetic properties.

Nanocrystalline Nd(2)Fe(17) powders have been obtained by means of high-energy ball milling from nearly single-phase bulk alloys produced by arc melting and high temperature homogenization annealing. The rhombohedral Th(2)Zn(17)-type crystal structure of the bulk alloy remains unaltered after the milling process, with almost unchanged values for the cell parameters. However, the severe mechanical processing induces drastic microstructural changes. A decrease of the mean crystalline size down to around 10 nm is observed, giving rise to a considerable augmentation of the disordered inter-grain boundaries. This modification of the microstructure affects the magnetic behaviour of the milled powders, although the magnetic structure remains collinear ferromagnetic. While a unique ferro-to-paramagnetic transition temperature, T(C) = 339 ± 2 K, is observed in the bulk alloy, the nanocrystalline samples exhibit a more likely distribution of T(C) values. The latter seems to be responsible for the significant broadening of the temperature range in which magneto-caloric effect is observed, and the lowering of the maximum value of the magnetic entropy change.

Long-range magnetic ordering in magnetic ionic liquid: Emim[FeCl4].

Up to now most of the magnetic ionic liquids containing tetrachloroferrate ion FeCl(4) have evidenced a paramagnetic temperature dependence of the magnetic susceptibility, with only small deviations from the Curie law at low temperatures. However, we report on the physical properties of 1-ethyl-3-methylimidazolium tetrachloroferrate Emim[FeCl(4)], that clearly shows a long-range antiferromagnetic ordering below the Néel temperature T(N)≈3.8 K. In addition, the field dependence of the magnetization measured at 2 K is characterized by a linear behaviour up to around 40 kOe, while above this field the magnetization becomes saturated with a value of 4.3 µ(B)/Fe, which is near the expected fully saturated value of 5 µ(B)/Fe for an Fe(3+) ion.