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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We have investigated the magnetocaloric (MC) effect in a two-phase composite based on melt-spun ribbons of the intermetallic DyNi2 and TbNi2 Laves phases. The temperature dependence of the isothermal magnetic entropy change, ΔSM(T), has been calculated for the biphasic system x(DyNi2)+y(TbNi2) with 0M(T) curve with table-like shape, has been found in the temperature interval of 18-44K for the composite with x=0.4 and for values of the magnetic field change μoΔH=2 and 5 T, in good agreement with the experimental data. The refrigerant capacity, RC, reaches 221(526) J kg-1 with a temperature span δTFWHM of 32(41) K for μoΔH of 2(5) T, thus improving the values obtained for the individual RNi2 ribbons. Our findings constitute a good starting point to stimulate the search for new composites with enhanced MC properties at cryogenic temperatures. [ABSTRACT FROM AUTHOR]

The magnetic and magnetocaloric properties of a series of Fe-rich FeZrB(Cu) amorphous ribbons were investigated under magnetic field values up to μ0H of 8T. A correlation between the saturation magnetization and the maximum magnetic entropy change ∣ΔSMpeak∣ is clearly evidenced. Although these metallic glasses show relatively low ∣ΔSMpeak∣ values (from 3.6 to 4.4 J kg-1 K-1 for μ0ΔH=8T), the ΔSM(T) curve broadens upon the increase in μ0ΔH, giving rise to a large refrigerant capacity RC (above 900J kg-1 for μ0ΔH=8T). Using the universal curve method for rescaling the ΔSM(T,μ0ΔH) curves, we found a collapse of the curves around the Curie temperature. However, in the low-temperature range the curves do not match into a single one due to the existence of magnetic frustration. [ABSTRACT FROM AUTHOR]

The critical behavior of single phase Y2Fe17 melt-spun ribbons with the Th2Fe17-type crystal structure has been studied around the transition temperature (TC). This alloy undergoes a second-order ferromagnetic (FM)–paramagnetic (PM) phase transition at TC = 301 K. Various techniques, such as the modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis, were used to determine the critical exponents that were found to be β = 0.226(3), γ = 1.296(2), and δ = 6.804(5). The universality class of the critical phenomenon in the Y2Fe17 ribbons can be explained with the help of the renormalization group theory approach, in which the magnetic properties show a feature changing from three (3D)- to two-dimensional (2D) Ising model. The first-principles calculations based on density functional theory qualitatively explain the experimental results, confirming the strong correlation between lattice atoms and critical behavior in magnetic intermetallic Y2Fe17. [ABSTRACT FROM AUTHOR]

We have studied the correlation between the broadening of the isothermal magnetic entropy change and the Curie temperature (TC) distribution in nanostructured Pr2Fe17 and Nd2Fe17 alloys produced by high-energy ball-milling after milling times of 10, 20, and 40 h. The changes in the microstructure affect the Fe local environments and as a consequence the magnetic interactions, giving rise to TC distributions centered around 285K and 330K for the Pr2Fe17 and Nd2Fe17 alloys, respectively. The width of the distributions enlarges (up to 60K) as the milling-time increases, and consequently, the isothermal magnetic entropy change curves show an extended full width at half maximum. [ABSTRACT FROM AUTHOR]

A detailed analysis of the first-order reversal curve (FORC) measurements offers the possibility to separate the different magnetic interactions in multiphase or composite magnetic materials, and then to attain a precise interpretation of the role played by those interactions. In this paper, we present a modified setup based on a classical inductive magnetometer optimized for FORC measurements in soft magnetic materials, together with an improved differential method for the data analysis in order to get an accurate interpretation of the experimental results. [ABSTRACT FROM AUTHOR]

In the last decade, the renewed interest in antiferromagnetic (AF) magnetoelectric (ME) materials has been driven by the challenging multifunctionality of spintronic devices. One of the most ambitious goals is to build exchange-biased ferromagnetic/AF junctions with electric field-controlled properties. In this context, the understanding of the modifications that size reduction induces in the magnetic properties of a material being both AF and ME holds the key to control the magnetic coupling at the interface. Here, we show that the spin arrangement in chromium (III) oxide core/shell nanoparticles changes significantly as a function of the radial distance from the nanoparticle center. While the nanoparticle core retains an AF structure, magnetic moments located on a thin surface shell are in a disordered spin-glass (SG)-like state. In addition, canted spins develop at the boundary of the ME AF core. These spins, which mediate a moderate AF/SG exchange coupling at low temperature, are exchange coupled to the AF core, thus giving rise to a lower yet more robust exchange bias effect, which persists up to the Néel temperature of the AF core. [ABSTRACT FROM AUTHOR]

Among all antiferromagnetic transition metal monoxides, NiO presents the highest Néel temperature (TN ~ 525 K). In this work, the size-dependent reduction of TN in NiO nanoparticles with average diameters (D) ranging from 4 to 9 nm is investigated by neutron diffraction. The scaling law followed by TN(D) is in agreement with the Binder theory of critical phenomena in low-dimensional systems. X-ray absorption fine structure measurements link the decrease of TN to the occurrence of size effects (average undercoordination, bond relaxation and static disorder) in the nearest and next-nearest Ni coordination shells that hold the key for the maintenance of the antiferromagnetic order. [ABSTRACT FROM AUTHOR]

Co-based nanoparticles (NPs) have been extensively explored due to their prospective applications in areas as diverse as efficient water treatment (Co NPs), hydrogen generation (CoO NPs) and combustion catalysis (Co3O4 NPs). In recent years, the emergence of Co-based entities as bi-magnetic core/shell NPs has opened new avenues for their innovative use in fields ranging from energy storage and magnetic recording to biomedicine. The control and characterization of these nanomaterials thus becomes of paramount importance for targeting their foreseen applications. Here, we show that the intentional oxidation of metallic Co NPs with different sizes (3–50 nm) gives rise to a wide variety of core/shell morphologies including Co, CoO and Co3O4 phases. Bridging the information coming from high-resolution transmission electron microscopy, X-ray absorption spectroscopy and magnetic measurements gives us a self-consistent picture that describes the role played by the morphology and microstructure in the magnetism of Co and its oxides at the nanoscale. [ABSTRACT FROM AUTHOR]

Among all bi-magnetic core(transition metal)@shell(transition metal oxide) nanoparticles (NPs), Ni@NiO ones show an onset temperature for the exchange bias (EB) effect far below the Néel temperature of bulk antiferromagnetic NiO. In this framework, the role played by the magnetism of NiO at the nanoscale is investigated by comparing the microstructure and magnetic properties of NiO and Ni@NiO NPs. With the aim of bridging the two systems, the diameter of the NiO NPs (~4 nm) is chosen to be comparable to the shell thickness of Ni@NiO ones (~2 nm). The EB effect in Ni@NiO NPs is attributed to the exchange coupling between the core and the shell, with an interfacial exchange energy of Δ E ~0.06 erg cm −2 , thus comparable to previous reports on Ni/NiO interfaces both in thin film and NP morphologies. In contrast, the EB detected in NiO NPs is explained in a picture where uncompensated spins located on a magnetically disordered surface shell are exchange coupled to the antiferromagnetic core. In all the studied NPs, the variation of the EB field as a function of temperature is described according to a negative exponential law with a similar decay constant, yielding a vanishing EB effect around T ~40–50 K. In addition, the onset temperature for the EB effect in both NiO and Ni@NiO NPs seems to follow a universal dependence with the NiO crystallite size. [ABSTRACT FROM AUTHOR]

Ni(core)@NiO(shell) nanoparticles (NPs) were synthesized through the pyrolysis of an inorganic precursor taking place within the pores of an active carbon matrix at different temperatures between 673 and 1173 K, and a subsequent oxidation in air. For the lowest temperature (673 K), the smallest average size of the NPs (9 nm) and the largest percentage of NiO (82%) are observed. Upon increasing the temperature up to 1173 K, an average diameter of 23 nm is observed while the NiO percentage decreases below 20%. We found that each NP consists of a Ni core surrounded by a structurally disordered NiO shell with a constant thickness of ∼2 nm, regardless of the core size. The spins inside the NiO shell freeze into a spin glass (SG)-like state below Tf∼ 40 K. The magnetic exchange coupling between the Ni core and the NiO shell spins gives rise to the occurrence of the exchange bias (EB) effect, whose temperature dependence follows a universal exponential trend in all samples. The SG nature of the shell spins yields a vanishing EB above Tf. This is far below the Néel temperature of bulk antiferromagnetic NiO (TN∼ 523 K) that usually determines the onset of the EB effect in Ni/NiO interfaces. [ABSTRACT FROM AUTHOR]

The magnetocaloric properties of NdPrFe17 melt-spun ribbons composed of nanocrystallites surrounded by an intergranular amorphous phase have been studied. The nanocomposite shows two successive second-order magnetic phase transitions (303 and 332 K), thus giving rise to a remarkable broadening (≈84 K) of the full-width at the half-maximum of the magnetic entropy change curve, ΔSM(T), with a consequent enhancement of the refrigerant capacity RC. For a magnetic field change of 2 T, |ΔSMpeak|=2.1 J kg-1 K-1 and RC= 175 J kg-1. Therefore, the reversible magnetocaloric response together with the one-step preparation process makes these nanostructured Fe-rich alloy ribbons particularly attractive for room temperature magnetic refrigeration. [ABSTRACT FROM AUTHOR]

Abstract: We have synthesized the Dy2Fe17 alloy with an unconventional crystal structure ( space group). Neutron powder thermo-diffraction and X-ray powder diffraction under high pressure (up to 15GPa) show that the rhombohedral crystal structure is stable. Likewise, the alloy with the usual hexagonal crystal structure (P63/mmc), the rhombohedral variant of Dy2Fe17 compound, exhibits a collinear ferrimagnetic order below the Curie temperature (T C ≈363K), with antiparallel mutual alignment of the Dy and Fe sublattices. Additionally, we have observed two distinctive issues related to magneto-volume anomalies: (i) the pressure dependence of the cell volume at room temperature shows the existence of a critical pressure at which the compound is no longer ferrimagnetic; and (ii) the cell volume shows invar-like behaviour in a wide temperature range (2–290K), with a minimum at T ≈380K. The spontaneous volume magnetostriction reaches ωS =1.6×10−2 at 2K that decreases to zero at T ≈500K (≈1.4 T C), which is associated with the existence of short-range magnetic correlations. [Copyright &y& Elsevier]

Highlights: [•] The magnetic entropy change for two-ribbon (amorphous) composite materials is investigated. [•] The conditions to obtain a table-like shape of the magnetic entropy change are specified. [•] We give the essential ingredients to maximize the effective refrigerant capacity and the efficiency. [•] Our findings could be used in other magneto-caloric materials to tune the temperature range for the table-like behavior. [Copyright &y& Elsevier]

Combining different experimental techniques, investigations in hexagonal P63/mmc Er2Fe17 show remarkable magnetovolume anomalies below the Curie temperature, Tc. The spontaneous magnetostriction reaches 1.6 x 10-2 at 5 K and falls to zero well above Tc, owing to short-range magnetic correlations. Moreover, Er2Fe17 exhibits direct and inverse magnetocaloric effects (MCE) with moderate isothermal magnetic entropy △SM, and adiabatic temperature △Tad changes [△SM ~ --4.7 J(kgK)-1 and △Tad ~ 2.5 K near the Tc, and △SM ~ 1.3 J(kgK)-1 and △Tad[1 ~ --0.6 K at 40 K for △H = 80 kOe, respectively, determined from magnetization measurements]. The existence of an inverse MCE seems to be related to a crystalline electric field-level crossover in the Er sublattice and the ferrimagnetic arrangement between the magnetic moments of the Er and Fe sublattice. The main trends found experimentally for the temperature dependence of △SM and △Tad as well as for the atomic magnetic moments are qualitatively well described considering a mean-field Hamiltonian that incorporates both crystalline electric field and exchange interactions. △SM(T) and △Tad(T) curves are essentially zero at ~150 K, the temperature where the transition from direct to inverse MCE occurs. A possible interplay between the MCE and the magnetovolume anomalies is also discussed. [ABSTRACT FROM AUTHOR]

Abstract: We have synthesized the intermetallic YPrFe17 compound by arc-melting. X-ray and neutron powder diffraction show that the crystal structure is rhombohedral with space group (Th2Zn17-type). The investigated compound exhibits a broad isothermal magnetic entropy change ΔS M (T) associated with the ferro-to-paramagnetic phase transition (T C ≈290K). The |ΔS M | (≈2.3Jkg−1 K−1) and the relative cooling power (≈100Jkg−1) have been calculated for applied magnetic field changes up to 1.5T. A single master curve for ΔS M under different values of the magnetic field change can be obtained by a rescaling of the temperature axis. The results are compared and discussed in terms of the magneto-caloric effect in the isostructural R2Fe17 (R=Y, Pr and Nd) binary intermetallic alloys. [Copyright &y& Elsevier]

Abstract: We have synthesized three pseudo-binary (Ce,R)2Fe17 intermetallic alloys, where cerium has been partially substituted by R = Y, Pr and Dy, with the aim of tuning the Curie temperature (between 253 and 273 K) for obtaining the maximum magneto-caloric response of the alloys just below room temperature. The analysis of the x-ray powder diffraction patterns show that the three samples crystallize in the rhombohedral Th2Zn17-type crystal structure (space group ). We report the temperature dependence of the isothermal magnetic entropy change, |ΔSM |(T), the magnetic field dependence of its maximum value, |ΔSM ma x|(H), and the relative cooling power around the second-order magnetic transition for magnetic field changes μ 0ΔHmax  = 5 T. The collapse of the normalized ΔSM (θ)/ΔSM max vs. temperature (where θ is a rescaled temperature) into a single master curve allows the extrapolation of |ΔSM |(T) curves for higher magnetic field changes, and/or the estimation of the temperature dependence of the magnetic entropy for other 2:17 pseudo-binary compounds. The lower values of |ΔSM ma x| compared with those of the R2Fe17 compounds with R = Pr or Nd are explained in terms of the decrease of the saturation magnetization. The magnetic field dependence of the |ΔSM ma x| indicates that only Pr1.5Ce0.5Fe17 alloy roughly follows a mean-field behaviour. [Copyright &y& Elsevier]

We report unusual cooling field dependence of the exchange bias in oxide-coated cobaltnanoparticles 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. [ABSTRACT FROM AUTHOR]

Abstract: Fe-rich FeZrB metallic glasses exhibit magneto-caloric effect (MCE) around room temperature. Amorphous ribbons of two different compositions, Fe91Zr7B2 and Fe88Zr8B4, with respective Curie temperature values of 230 and 285 K have been studied. Although the maximum magnetic entropy change is relatively moderate ( ∼ 3 J K−1 kg−1 under an applied magnetic field change from 0 to 50 kOe), the MCE spreads over a broad temperature interval (∆T ∼ 200 K), giving rise to a large refrigerant capacity loss (RC ∼ 435 J kg−1) without any hysteresis. The Curie temperature can be easily tuned between 200 and 350 K by changing the boron content. Therefore, the MCE can be controlled over a wide temperature interval, thus making these amorphous alloys promising candidates for magnetic refrigeration near room temperature. [ABSTRACT FROM AUTHOR]

Information about the papers discussed at several meetings on neutron research is presented. Topics include the advances in energy-selective neutron imaging, neutron analysis and scientific applications of techniques of polarized neutrons. The meetings feature several persons including Rector Magnificus Karel Luyben, Egbert Balsma and Theo Rekveldt.

Abstract: Nanostructured Fe64Ni36 alloy has been obtained using high-energy ball milling for 35h of milling time, Fe64Ni36 MA-35h. The initial as-milled Fe64Ni36 MA-35h powders are inhomogeneous, showing a majority phase with a face-centred cubic (fcc) crystal structure [88(2)%] and a minority phase with body-centred cubic (bcc) crystal structure [7(2)%]. The evolution of the microstructure with temperature between 300K and 1100K has been followed by means of in situ neutron powder thermo-diffraction experiments. The room temperature values for the mean crystalline size and the mechanical-induced microstrain of the fcc phase in the as-milled sample are ∼10nm and ∼0.7%, respectively. Moreover, after heating the Fe64Ni36 MA-35h powders up to 1100K, an increase of around 65K in the Curie temperature respect to that of the commercial coarse-grained alloy of the same composition is observed. The latter together with the observed temperature dependence of the lattice parameter suggests that the Fe64Ni36 MA-35h sample subjected to the heating process exhibits invar behaviour. On heating up to 1100K thermal relaxation of the microstructure occurs giving rise to grain growth above 100nm, nearly vanishing values for the maximum strain, and the transformation of the bcc phase into the fcc one above 800K, being the latter stable in subsequent heating–cooling processes. [Copyright &y& Elsevier]

Abstract: Using high-energy ball milling, nanostructured Pr2Fe17 powders can be obtained from their arc-melted bulk alloys. High-resolution X-ray and neutron powder diffraction experiments reveal that the Th2Zn17-type rhombohedral crystal structure is maintained, after milling for 10h, with almost unchanged values for both crystalline lattice parameters (Δa; Δc <0.05%) and vanishing mechanically induced microstrain (<0.1%). Although the mean crystalline size decreases down to 20±3nm, magnetovolume anomalies observed in pure Pr2Fe17 are still present with a significant volume decrease on heating from 5 up to 320K. After the milling, a significant increase in the magnetic anisotropy, due to the drastic reduction in crystalline size, is observed, while the value of the magnetic moment seems to be increased slightly (5%). In addition, the magnetocaloric effect of bulk and nanostructured Pr2Fe17 has been investigated. The magnetic entropy change, |ΔSM |, decreases from 6.3 to 4.5J kg−1 K−1 under an applied magnetic field μ 0 H =5T after the milling process. However, the width of the |ΔSM |(T) curve is substantially enlarged and hence the refrigerant capacity is enhanced. These findings make the iron-based nanostructured Pr2Fe17 powders interesting for applications in magnetic refrigeration at around room temperature. [Copyright &y& Elsevier]

The temperature dependence of the isothermal magnetic entropy change, ΔSM, and the magnetic field dependence of the refrigerant capacity, RC, have been investigated in a composite system xA + (1 - x)B, based on Fe87Zr6B6Cu1 (A) and Fe90Zr8B2 (B) amorphous ribbons. Under a magnetic field change of 2 T, the maximum improvement of the full-width at half maximum of ΔSM(T) curve (47% and 29%) and the RC (18% and 23%), in comparison with those of the individual alloys (A and B), is observed for x ≈ 0.5. Moreover, a flattening over 80 K in the ΔSM(T) curve around room temperature range is observed, which is a key feature for an Ericsson magnetic refrigeration cycle. [ABSTRACT FROM AUTHOR]

Scientific research using neutron scattering techniques began relatively recently in Spain. In the 1970s, Spanish scientists undertook their first neutron-scattering experiments: Initially, the topics related to materials science and/or magnetism were of interest, and only few researchers belonging to two or three different groups were involved in them. However, over the last 20 years, the number of researchers and research groups has grown significantly, as well as the scientific disciplines to which they belong, i.e. physics, chemistry, engineering, geology, biology, medicine and even archaeometry or cultural heritage). In figure 1 the eleven most active disciplines are shown, the area of the circles being proportional to the number of articles. The vertical axis represents the ratio between the average impact factor of “neutron” articles and the total Spanish articles in each discipline, while the horizontal one shows the ratio between the average numbers of citations. [ABSTRACT FROM AUTHOR]

The magnetocaloric properties of melt-spun ribbons of the Laves phase DyNi2 have been investigated. The as-quenched ribbons crystallize in a single-phase MgCu2-type crystal structure (C15; space group [formula]) exhibiting a saturation magnetization and Curie temperature of MS = 157 ± 2 A m2 kg-1 and TC = 21.5 ± 1 K, respectively. For a magnetic field change of 2 T, ribbons show a maximum value of the isothermal magnetic entropy change |ΔSMpeak| = 13.5 J kg-1 K-1, and a refrigerant capacity RC = 209 J kg-1. Both values are superior to those found for bulk polycrystalline DyNi2 alloys (25% and 49%, respectively). In particular, the RC is comparable or larger than that reported for other potential magnetic refrigerants operating at low temperatures, making DyNi2 ribbons promising materials for use in low-temperature magnetic refrigeration applications. [ABSTRACT FROM AUTHOR]