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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]

Transformation of deuterated ethanol from the plastic crystal phase into the monoclinic one is investigated by means of a singular setup combining simultaneously dielectric spectroscopy with neutron diffraction. We postulate that a dynamic transition from plastic crystal to supercooled liquid-like configuration through a deep reorganization of the hydrogen-bonding network must take place as a previous step of the crystallization process. Once these precursor regions are formed, subsequent crystalline nucleation and growth develop with time. [ABSTRACT FROM AUTHOR]

The precise control on the combination of multiple metal atoms in the structure of metal-organic frameworks (MOFs) endowed by reticular chemistry, allows the obtaining of materials with compositions that are programmed for achieving enhanced reactivity. The present work illustrates how through the transformation of MOFs with desired arrangements of metal cations, multi-metal spinel oxides with precise compositions can be obtained, and used as catalyst precursor for the reverse water-gas shift reaction. The differences in the spinel initial composition and structure, determined by neutron powder diffraction, influence the overall catalytic activity with changes in the process of in situ formation of active, metal-oxide supported metal nanoparticles, which have been monitored and characterized with in situ X-ray diffraction and photoelectron spectroscopy studies. [ABSTRACT FROM AUTHOR]

Topochemical nitridation in ammonia at moderate temperatures of cation ordered Sr2FeWO6 produces new antiferromagnetic double perovskite oxynitrides Sr2FeWO6−xNx with 0 ＜ x≤ 1. Nitrogen introduction induces the oxidation of Fe2+ to Fe3+ and decreases TN from 38 K (x = 0) to 13 K for Sr2FeWO5N which represents the first example of a double perovskite oxynitride with both high cationic order and nitrogen content. This synthetic approach can be extended to other cation combinations expanding the possibility of new materials in the large group of double perovskites. [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]

The antiferromagnetic (AFM) transition of the normal ZnFe2O4 has been intensively investigated with results showing a lack of long-range order, spin frustrations, and a "hidden" entropy in the calorimetric properties for inversion degrees δ ≈ 0 or δ = 0. As δ drastically impacts the magnetic properties, it is logical to question how a δ value slightly different from zero can affect the magnetic properties. In this work, (Zn1-δFeδ)[ZnδFe2-δ]O4 with δ = 0.05 and δ = 0.27 have been investigated with calorimetry at different applied fields. It is shown that a δ value as small as 0.05 may affect 40% of the unit cells, which become locally ferrimagnetic (FiM) and coexists with AFM and spin disordered regions. The spin disorder disappears under an applied field of 1 T. Mossbauer spectroscopy confirms the presence of a volume fraction with a low hyperfine field that can be ascribed to these spin disordered regions. The volume fractions of the three magnetic phases estimated from entropy and hyperfine measurements are roughly coincident and correspond to approximately 1/3 for each of them. The "hidden" entropy is the zero point entropy different from 0. Consequently, the so-called "hidden" entropy can be ascribed to the frustrations of the spins at the interphase between the AFM-FiM phases due to having δ ≈ 0 instead of ideal δ = 0. [ABSTRACT FROM AUTHOR]

Recently, potential technological interest has been revealed for the production of magnetocaloric alloys using Rare-Earth intermetallics. In this work, three series of Tb x R 1 − x Cu 2 (R ≡ Gd, La, Y) alloys have been produced in bulk and nanoparticle sizes via arc melting and high energy ball milling. Rietveld refinements of the X-ray and Neutron diffraction patterns indicate that the crystalline structure in all alloys is consistent with TbCu 2 orthorhombic I m m a bulk crystalline structure. The analyses of the DC-magnetisation ( M D C ) and AC-susceptibility ( χ A C ) show that three distinct degrees of disorder have been achieved by the combination of both the Tb 3 + replacement (dilution) and the nanoscaling. These disordered states are characterised by transitions which are evident to M D C , χ A C and specific heat. There exists an evolution from the most ordered Superantiferromagnetic arrangement of the Tb 0.5 La 0.5 Cu 2 NPs with Néel temperature, T N ∼ 27 K, and freezing temperature, T f ∼ 7 K, to the less ordered weakly interacting Superparamagnetism of the Tb 0.1 Y 0.9 Cu 2 nanoparticles ( T N absent, and T B ∼ 3 K). The Super Spin Glass Tb 0.5 Gd 0.5 Cu 2 nanoparticles ( T N absent, and T f ∼ 20 K) are considered an intermediate disposition in between those two extremes, according to their enhanced random-bond contribution to frustration. [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]

Abstract: Ru-based high temperature shape memory alloys show different structures at different temperatures and composition. Equiatomic Ru–Ta and Ru–Nb show two subsequent phase transformations, but their individual influence is still unclear because of the difficulty of comparing the high temperature cubic phase, the intermediate tetragonal phase and the low temperature monoclinic phase. It is important to find a way to compare these structures in order to better understand the microstructure of these alloys and their shape memory behaviour. In order to get the necessary information the alloys have been investigated with neutron diffraction technique during in-situ heating up to about 1100 °C. This study shows the evolution of the lattice parameters and the shape of the unit cells at different temperatures for the three phases these materials exhibit. It proposes a way to compare the different structures and it gives a mathematical expression of the phase transformations. The ultimate goal of this work is to enable a better understanding of the deformation mechanisms in the unit cell and a possible anticipation of the existence of a shape memory effect in these systems. [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]