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We report changes on the magnetic behavior of the Heavy-Fermion ferromagnetic YbNi2 alloy when reducing the size of the particles to the nanometer scale by high-energy ball milling. The milling process induces a reduction on the particle size of the bulk alloy down to 10 (2) nm and a lattice strain of 1.8 (2)%, for 30 h of milling time, as calculated from the X-ray diffraction data. The ferromagnetic transition at TC = 10.5 K in the bulk alloy tends to disappear whereas other magnetic transitions arise at lower temperatures with the milling process. The AC magnetic susceptibility of the 30 h milled sample shows a frequency independent peak at 3 K, in a wide range of frequencies, consistent neither with superparamagnetic nor simple freezing of the magnetic moments, and as expected for a ferromagnetic behavior. However, from the specific heat measurements in the 30 h milled sample, a peak at 3.6 K is shifted respect to that observed in the AC magnetic susceptibility, as found in spin glasses. The results are explained by the influence of magnetic inhomogeneities and disorder of the alloys enhanced by the milling process in the ensemble of nanoparticles.

Elucidating the underlying principles behind band gap engineering is paramount for the successful implementation of semiconductors in photonic and optoelectronic devices. Recently it has been shown that the band gap of a wide and direct band gap semiconductor, such as ZnO, can be modified upon cocrystallization with amino acids, with the role of the biomolecules remaining unclear. Here, by probing and modeling the light-emitting properties of ZnO-amino acid cocrystals, we identify the amino acids' role on this band gap modulation and demonstrate their effective chirality transfer to the interband excitations in ZnO. Our 3D quantum model suggests that the strong band edge emission blue-shift in the cocrystals can be explained by a quasi-periodic distribution of amino acid potential barriers within the ZnO crystal lattice. Overall, our findings indicate that biomolecule cocrystallization can be used as a truly bio-inspired means to induce chiral quantum confinement effects in quasi-bulk semiconductors.

Resumen del trabajo presentado a la 20th International Conference on Solid Compounds of Transition Elements, celebrada del 11 al 15 de abril de 2016 en Zaragoza (España)., The discovery of the giant magnetocaloric effect in Gd5Si2Ge2 has attracted a strong interest in the study of other R5(SiGe)4-type (R= rare earth) compounds in order to understand the relationship between their structural and magnetic properties. The extraordinary sensitivity of the electronic and crystal structures to external parameters such as the application of hydrostatic pressure have led to a systematic investigation over the last years in order to comprehend the microscopic processes taking place in the 5:4 materials. Experimental observations of a Griffiths-like phase (GP) in the paramagnetic (PM) phase at a characteristic temperature TG above the long-range order temperature (either TC or TN) were reported later in these compounds. The appearance of this regime has pointed to arise from the strong interplay between structure and magnetism present in these materials. According to a recent work, this behavior has been found to be restricted to 5:4 compounds which present the monoclinic Gd5Si2Ge2-type (M) or the orthorhombic Sm5Ge4-type (O(II)) of crystal structure at room temperature. The short-range magnetic correlations observed in the M-PM and O(II)-PM phases, an essential ingredient for the existence of Griffiths-like singularities, indicated that the system retains memory of the O(I) phase (pure state) signaled by an anomaly at TG. In this work we report on a detailed study of the pressure effect on the Griffiths-like phase in several compounds of the R5(Si1-xGex)4 ( R = Tb, Gd and Dy) system stable in the M - PM and O (II) – PM phases at room temperature. The selected compounds for this study are representative of O(II)-AFM (R = Gd), O(I)-FM (R = Tb) and O(I)- FM (R = Dy) ground states, respectively presenting GP in both AFM and FM states. For all the analyzed cases, low field - dc susceptibility measurements show that the GP persists at pressures up to 10 kbar. The value of TG, associated with the onset of short-range ferromagnetic correlations, increases linearly with pressure. However, large differences in the rate of change dTG/dP have been observed among them: The rate of the Tb - based compound is an order of magnitude greater than that of the Dy-based compound. Such a different pressure dependence of the Griffiths phase in these compounds could arise from the particular character of the clusters and the competition between the different structural and magnetic phases.

The magnetic and thermal properties of TbAl2 nanosized alloys (diameters, 12 nm $\leqslant D\leqslant $ 20 nm) obtained by high-energy milling are characterised by specific heat, magnetisation and neutron scattering. The specific heat shows that the λ-anomaly at Curie temperature vanishes when the milling time reaches 300 h and its field variation shows a broad peak around 70 K disclosing a disordered magnetic state. The thermal variation of magnetization follows a Bloch process with a decrease of the stiffness constant and a faster demagnetisation with a quadratic exponent instead of the bulk ordinary ${T}^{3/2}$-dependence. The magnetic moment reduction in the nanosized alloys follows a 1/D dependence, remarking the role of disordered moment surface. The Rietveld analysis of the neutron diffraction patterns indicates a collinear ferromagnetic structure, with a reduction of the Tb-magnetic moment when decreasing the particle size. The temperature dependent overall magnetic signal of nanoparticles is derived from small-angle neutron scattering. A magnetic nanoparticle structure with an ordered ferromagnetic core and a disordered surface layer is proposed.

The structural state and static and dynamic magnetic properties of TbCu2 nanoparticles are reported produced by mechanical milling under inert atmosphere. The core magnetic structure retains the bulk antiferromagnetic arrangement. The overall interpretation is based on a superantiferromagnetic behavior which at low temperatures coexists with a canting of surface moments and mismatch of antiferromagnetic sublattices of the nanoparticles.