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Nowadays, Ni@C nanostructured materials are attracting a great deal of attention due to their multiple catalytic or magnetic functionalities. In this article we report on the investigation of the correlation between the microstructure and magnetic properties of Ni nanoparticles embedded in a carbon matrix. The samples were obtained following a two-step procedure that ensures protection against nanoparticle oxidation, and was carried out in the following way: (i) the synthesis of a nickel-imidazole-based metal-organic framework (MOF) by a simple method in an aqueous medium at moderate temperature (95 °C); and (ii) carbonization of the MOF at different temperatures between 400 and 600 °C to obtain a carbon-supported hybrid material, containing Ni nanoparticles with an “artichoke-like” morphology, where a Ni-FCC core is surrounded by “bracts” of Ni-HCP and Ni3C. The average size of the nanoparticle slightly changes from 7 to 10 nm as the carbonization temperature is increased, but the Ni-FCC core diameter ranges from 3 to around 6 nm. We show how the information obtained on the evolution of the magnetic behaviour with carbonization temperature, using X-ray diffraction and electron microscopy, complements each other by providing consistent structural and magnetic characteristics of the investigated Ni@C nanoparticles. In fact, this joint analysis allows us to explain the formation and transformation of different Ni-based crystalline phases along the synthesis process, including Ni3C and Ni with both hexagonal and cubic crystalline structures. The amount of conventional Ni-FCC is below 10 wt% for the sample treated at 400 °C and it can reach up to 50 wt% for that treated at 600 °C. Finally, based on our current findings we propose an explanation for understanding the magnetic properties of Ni@C, in which the Ni-FCC core spins mainly govern the magnetic coupling of the whole system.

The copper hydrogeno-arsenite tellurate material (CuAsTe) was synthesized and its magnetic properties were investigated. Thermal analyses revealed that no mass loss was recorded before 480 K. Additionally, in this current work, electrical properties based on the impedance measurements were determined. The complex impedance diagram at different temperatures demonstrated a single semicircle, implying that the response originated from a single capacitive element corresponding to the grains. The thermal evolution of the conductivity ¿dc displayed an Arrhenius type behavior and suggested that this material is an ionic protonic conductor at high temperature whereas the ¿ac conductivity was indicative that the conduction mechanism is ensured by the correlated barrier hopping (CBH) model. The variation of M¿ versus frequency proved to be a thermally activated dielectric relaxation process. Notably, we specified determined the direct optical band gap Eg = 2.3 eV in order to characterize the optical properties.

Bulk Cr2O3 is an antiferromagnetic (AFM) oxide that exhibits the magnetoelectric effect at room temperature, with neither spontaneous magnetization nor net electric polarization. These physical properties stem from a subtle competition between exchange and crystal field interactions. In this article, we exploit the symmetry breaking at the surface of Cr2O3 nanoparticles for unbalancing this delicate physical equilibrium. The emerging weak ferromagnetic signal we observe persists up to near room temperature (≈ 270 K) at which the antiferromagnetic order disappears. In addition, an exchange-bias effect, that rapidly decreases on heating from low temperature up to 30 K, is resistant to thermal disorder above 200 K. Our findings point to the possible formation of an entangled core/shell magnetic structure, where pinned uncompensated spins at the shell are randomly distributed in a low-temperature spin-glass ordering, with low net magnetic moment and an ordering temperature governed by the AFM Néel temperature.

The development of synthesis methods to access advanced materials, such as magnetic materials that combine multimetallic phosphide phases, remains a worthy research challenge. The most widely used strategies for the synthesis of magnetic transition metal phosphides (TMPs) are organometallic approaches. In this study, Fe-containing homometallic dendrimers and Fe/Co-containing heterometallic dendrimers were used to synthesize magnetic materials containing multimetallic phosphide phases. The crystalline nature of the nearly aggregated particles was indicated for both designed magnetic samples. In contrast to heterometallic samples, homometallic samples showed dendritic effects on their magnetic properties. Specifically, saturation magnetization (Ms) and coercivity (Hc) decrease as dendritic generation increases. Incorporating cobalt into the homometallic dendrimers to prepare the heterometallic dendrimers markedly increases the magnetic properties of the magnetic materials from 60 to 75 emu/g. Ferromagnetism in homometallic and heterometallic particles shows different responses to temperature changes. For example, heterometallic samples were less sensitive to temperature changes due to the presence of Co2P in contrast to the homometallic ones, which show an abrupt change in their slopes at a temperature close to 209 K, which appears to be related to the Fe2P ratios. This study presents dendrimers as a new type of precursor for the assembly of magnetic materials containing a mixture of iron- and cobalt-phosphides phases with tunable magnetism, and provides an opportunity to understand magnetism in such materials.

The control of chemical, morphological, textural and magnetic properties developed in magnetic activated carbons (MACs) from industrial food waste, was achieved. The process parameters involved in a sustainable and optimized one-step activation route, based on the circular economy model, played an important role and its influence was studied, offering interesting and outstanding results. Magnetite crystals were formed on the MACs surface obtained from 400 °C to 800 °C while other iron species, such as metallic iron and iron carbides, were formed only at 800° C. Acid washing favored the mineral matter removal and the presence of iron species was drastically reduced in acid-washed MACs, mainly the iron species originated at 800 °C. Water-washed MACs presented better magnetic response at high temperatures (confirmed by XRD, FTIR, Raman, Mössbauer spectroscopy and VSM characterization techniques). High BET surface area (≈ 822 m/g) and a large micropore volume (≈ 0.30 cm/g), were reached by acid washed MACs at 800 °C. The obtained MACs were fundamentally microporous, but some mesoporosity was also observed when activating agent/precursor mass ratio (1:1) is used. It can be concluded that a MACs strategic design was reached with the objective of removing specific pollutants in multiple environmental applications.

In-situ temperature XRD and mechanical spectroscopy were applied to study phase transitions in AuCu alloy from the disordered FCC state (A1). The phase transition temperature intervals were investigated upon heating and cooling using a rate of 1 K/min in order to be thoroughly determined. The following sequences of phase transitions were recorded upon continuous heating: A1->AuCuI ->AuCuII->A1 and on subsequent cooling: A1 ->A1+AuCuI+AuCuII ->AuCuI+AuCuII ->AuCuI. This transition sequences determine also temperature dependency of elastic and anelastic properties. Mechanical spectroscopy using a forced torsion pendulum shows a peak due to antiphase boundaries motion characterizing the orthorhombic phase AuCuII and a transient peak with the that appears in the tetragonal phase AuCuI together with the classical Zener peak in the A1 phase.

Phase transitions in AuCu alloy are investigated using in situ temperature XRD and mechanical spectroscopy. The measurements are carried out from room temperature up to 973 K starting from the ordered phase AuCuI. Using a temperature ramp rate of 1 K/min, the following phase transition sequence was recorded on heating: AuCuI - > AuCuI + AuCuII- > AuCuI + AuCuII + A1 - > AuCuII + A1 - > A1. The mechanical (elastic and anelastic) properties are determined by this phase transition sequence. The mechanical spectroscopy, using a forced torsion pendulum, shows a peak due to antiphase boundary movement in the orthorhombic phase AuCuII according to the temperature dependent internal friction tests. The isothermal frequency measurements show several thermally activated peaks in the phase transitions intervals.

A novel hybrid compound, based on iron, fluoride and 8-hyroxyquinoline, and formulated as (8HQN)6[Fe4F18] (QF1), has been obtained by hydrothermal method and characterized by single crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), Hirschfeld surface, IR spectroscopy, photoluminescence (PL), thermal analysis and magnetic study. (QF1) crystalizes in trigonal space group R3c with cell parameters a = 17.9239(5) Å, b = 17.9239(5) Å, c = 30.6196 (11) Å, V = 8519.1(5) Å3, and Z = 6. The crystal structure of (QF1) is built up from isolated tetramers [Fe4F18]6− surrounded by six cations 8HQN+. The cohesion between these entities is performed via NH⋯F and OH⋯F hydrogen bonds to form a three-dimensional network. Hirshfeld surface analysis was used to investigate intermolecular interactions, as well 2D finger plots were conducted to reveal the contribution of these interactions in the crystal structure quantitatively. IR spectra confirmed the presence of 8-hydroxyquinoleine molecules. The photoluminescence studies of (QF1) show a significate green emission at 2.14 eV associated with radiative recombination of excitons confined within the organic part when it was excited in UV–visible wavelength in the solid state at room temperature. Finally, the magnetic analysis shows that (QF1) has a paramagnetic behavior above 100 K.

The hydrothermal synthesis and the chemical-physical characterization of an ammonium-chromium(III)-iron(III) bis(hydrogenphosphate), Cr0.34Fe0.66NH4(HPO4)2 [CrFeNP] is reported. It was obtained in aqueous media by reaction between chromium(III) chloride, iron(III) chloride, urea, and orthophosphoric acid. The crystal structure of CrFeNP was determined from single-crystal X-ray diffraction data. It crystallizes in the triclinic system, space group P-1, and exhibits two types of different infinite tunnels along [0 1 0] and [1 0 0] directions, where the ammonium cations are located. Thermal analysis shows that the solid is stable up to ca. 600 K. The activation energy of the thermal decomposition up to 1273 K was computed by isoconversional methods. In addition, the magnetic behaviour of the material was investigated from magnetic susceptibility and magnetization measurements. CrFeNP undergoes two successive magnetic transitions at temperatures Tc = 17. 9 K and Tt = 3.1 K, which could be related to a ferri- and an antiferromagnetic magnetic phase transitions, respectively.

A novel thermochemical process based on a single step was optimized to obtain magnetic activated carbons from an industrial biomass waste. Anhydrous iron chloride was used as an activating agent and mixed directly with the chestnut shell waste. The effect of the activation temperature (220-800 °C) on the chemical, morphological, textural, and magnetic properties of the materials was studied. The results demonstrated the presence of different iron compounds depending on the activation temperature set as well as their influence on morphological and textural development of the magnetic activated carbons (BET specific surface area, S up to 568 m g, total pore volume, V, up to 0.294 cm g vs 1 m g and 0.007 cm g, respectively, for the raw biomass waste). The techniques employed, especially Mössbauer spectroscopy, showed relative contributions of the different iron compounds (magnetite, maghemite, metallic iron, and so on) in the materials. The higher activation temperature (800 °C) favored the formation of metal Fe and iron carbide. Additionally, the magnetic properties measured by vibrating sample magnetometry confirmed the coexistence of different ferromagnetic phases with the remanent magnetization, M, (up to 3.88 emu/g) and coercivity, H, (up to 140 Oe), being larger as the activation temperature increases. A higher activation temperature favored the development and evolution toward other iron compounds, while at low temperature, 220 °C, the presence of these compounds were null, and their behavior resembled the results obtained for the original biomass waste.

Crystalline powder aggregates of a new nickel coordination polymer with 2-carboxyethylphosphonic acid (HPPAH) and 4,4′-bipyridine, Ni(HPPA)(4,4′-bipy)(HO), has been obtained by hydrothermal synthesis, and their crystal structure has been ab initio determined from synchrotron powder data. The compound shows a layer structure in bc plane, with Ni atoms in two different environments. The loss of the coordinated water by heating leads to the apparition of an anhydrous phase, which remains stable when cooling down to room temperature. Synthesized material presents a paramagnetic behaviour with weak antiferromagnetic interactions and zero-field splitting at low temperature (T < 50 K).

Single-phase Cr doped zinc ferrites have been synthesized by hydrothermal method. The crystal structure, microstructure, crystallite size and distribution, composition and magnetic properties of the synthesized nanoparticles were determined using X-ray powder diffraction, electron microscopies and vibrating sample magnetometry. Powders are composed of ultrafine spherical nanoparticles with a highly homogeneous elemental composition. The nanoparticles have partially inversed cubic spinel structure with average crystallite size below 10 nm. Superparamagnetic behavior is observed at high temperature while ferrimagnetic ordering and surface spin canting is reported at 5 K. Substitution of Fe with Cr ions controls both structural and magnetic parameters.

NiO nanoparticles (NPs) with a nominal size range of 2–10 nm, synthesized via hightemperature pyrolysis of a nickel nitrate, have been extensively investigated using neutron diffraction and magnetic (ac and dc) measurements. The magnetic behavior of the NPs changes noticeably when their diameter decreases below 4 nm. For NPs larger than or equal to this size, Rietveld analysis of the room temperature neutron diffraction patterns reveals that there is a reduction in the expected magnetic moment per Ni2+ ion with respect to bulk NiO, which is linked to the existence of a magnetically disordered shell at the NP surface. The presence of two peaks in the temperature dependence of both the dc magnetization after zero-field-cooling and the real part of the ac magnetic susceptibility is explained in terms of a core (antiferromagnetic, AFM)/shell (spin glass, SG) morphology. The high-temperature peak (T > 50 K) is associated with collective blocking of the uncompensated magnetic moments inside the AFM core. The low-temperature peak (T < 10 K) is a signature of a SG-like freezing of the surface Ni2+ spins. In addition, an exchange bias (EB) effect emerges due to the core/shell magnetic coupling. The cooling field and temperature dependences of the EB effect and the coercive field are discussed in terms of the core size and the effective magnetic anisotropy of the NPs. However, NiO NPs of 2 nm in size no longer show AFM order and the Ni2+ magnetic moments freeze into a SG-like state below T ∼ 3 K, with no evidence of EB effect.

©2004 The American Physical Society The work was partially supported by the Research Grants No. MAT2002-04178-C04 and No. MAT2000-1047 and by EU under IHP programme. We also thank the ILL and the CRG-D1B, together with ISIS for the allocation of neutron beam time, and the SCT at the University of Oviedo for the high resolution XRD facility. D.M.-B. thanks Spanish MCyT for additional support., Both Cu and Fe metals, with face-centered cubic (fcc) crystal structures, are nonmagnetic. However, the substitution of Cu atoms by Fe in the fcc-Cu lattice leads to the formation of a random solid solution and the appearance of ferromagnetic order, with a value of the magnetic moment per Fe atom in a fcc environment even above 2µ_ß. This striking behavior is closely related to magnetovolume effects (Invar), which we have detected by means of lattice thermal expansion and magnetization measurements in FeCu alloys., EU under IHP programme, MEC, Depto. de Física de Materiales, Fac. de Ciencias Físicas, TRUE, pub