Z. Śniadecki

Multifunctionality of GdPO4:Yb3+,Tb3+ nanocrystals – luminescence and magnetic behaviour

Multifunctional, luminescent and magnetic systems were synthesised by a co-precipitation method and broadly investigated. In our studies down- and visible up-conversion of Yb3+ and Tb3+ doped GdPO4 nanocrystals were observed. The synthesised nanomaterials were analysed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Spectroscopic properties were determined from excitation and emission spectra together with a subsequent analysis. The build-up characteristics of luminescence decays showed the energy transfer processes that occurred. Magnetic measurements were performed by means of vibrating sample magnetometry (VSM). These nanomaterials showed intense green emission under ultraviolet (UV), 272 nm, and near infrared (NIR), 980 nm, laser excitation. The mechanism lying behind the observed spectroscopic properties was proposed. Energy transfer between Gd3+ and Tb3+ ions, as well as cooperative energy transfer from Yb3+ to Tb3+, was found to give a dual-mode luminescence. Also, quenching processes by cross-relaxation and phonon-assisted energy transfer were observed. The presence of different lanthanide ions and the influence of the finite-size effect underlie the complex behaviour observed.

Facile non-hydrolytic synthesis of highly water dispersible, surfactant free nanoparticles of synthetic MFe2O4 (M–Mn2+, Fe2+, Co2+, Ni2+) ferrite spinel by a modified Bradley reaction

A series of the highly crystalline MFe2O4 ferrite spinel nanoparticles were synthesized via a modified Bradley reaction using microwave stimulation. Particle size was estimated using theoretical calculations from the X-ray data (Scherrer and Rietveld methods) as well as by direct experimental techniques such as TEM, DLS and NTA. The calculated average grain size for dry powders is in the range 10 to 23 nm. Hydrodynamic size was measured using DLS on non-modified, surfactant free particles of the whole MFe2O4 series. Raman spectra used for additional verification of the structure features of the produced spinel phases showed strong asymmetric behavior of the A1g mode, which was deconvoluted revealing additional components. Among all the products the lowest site inversion was found for the manganese ferrite (MnFe2O4). The oxidation of Fe3O4 leading to the formation of the Fe2O3 hematite phase induced by laser irradiation was observed. Magnetic characterization of the MFe2O4 family was carried out, showing that superparamagnetic blocking temperatures and calculated anisotropy constants K are in good agreement with the data for similar fine-particle systems.

Structural, Spectroscopic, and Magnetic Properties of Eu3+-Doped GdVO4 Nanocrystals Synthesized by a Hydrothermal Method

New interesting aspects of the spectroscopic properties, magnetism, and method of synthesis of gadolinium orthovanadates doped with Eu(3+) ions are discussed. Gd(1-x)Eu(x)VO4 (x = 0, 0.05, 0.2) bifunctional luminescent materials with complex magnetic properties were synthesized by a microwave-assisted hydrothermal method. Products were formed in situ without previous precipitation. The crystal structures and morphologies of the obtained nanomaterials were analyzed by X-ray diffraction and transmission and scanning electron microscopy. Crystallographic data were analyzed using Rietveld refinement. The products obtained were nanocrystalline with average grain sizes of 70-80 nm. The qualitative and quantitative elemental composition as well as mapping of the nanocrystals was proved using energy-dispersive X-ray spectroscopy. The spectroscopic properties of red-emitting nanophosphors were characterized by their excitation and emission spectra and luminescence decays. Magnetic measurements were performed by means of vibrating sample magnetometry. GdVO4 and Gd0.8Eu0.2VO4 exhibited paramagnetic behavior with a weak influence of antiferromagnetic couplings between rare-earth ions. In the substituted sample, an additional magnetic contribution connected with the population of low-lying excited states of europium was observed.

Amorphous states of melt-spun alloys in the system Dy–(Mn,Fe)6–(Ge,Al)6

Amorphous states have been found in rapidly quenched alloys in the pseudoternary system DyMn6−x−yFex+yGe6−xAlx (x,y=0 to 6). The amorphous state is formed in competition to the crystallization of ternary (1-6-6 rare earth-transition metal-metal) compounds. The melt-spun ribbons were investigated by x-ray diffraction, high resolution transmission electron microscopy, differential scanning calorimetry, and magnetization measurements. All amorphous samples exhibit multistep crystallization behavior without a clear indication for a glass transition below the first exothermic effect. Some of the amorphous alloys exhibit a magnetic ordering above room temperature and complex magnetic transitions similar to the properties of crystalline 1-6-6 compounds.

Magnetocaloric Effect of Amorphous Gd 65 Fe 10 Co 10 Al 10 X 5 (X = Al, Si, B) Alloys

Gd-based amorphous alloys with addition of metals and metalloids are interesting materials from the point of view of magnetic and magnetocaloric properties. Moreover, they usually possess high corrosion resistance and good mechanical properties. A series of Gd₆₅Fe₁₀Co₁₀Al₁₀X₅ (X = Al, Si, B) alloys was synthesized by melt-spinning. The amorphous structure of ribbons was confirmed by X-ray diffraction. Calorimetric curves allowed to determine crystallization temperatures, which are in the range of 260°C-303°C. The temperature dependences of the magnetic entropy changes were calculated from series of isothermal magnetization curves using Maxwell relation. Maximum value of the magnetic entropy change for the magnetic field change from 0 to 5 T is 7.1 J kg^-1 K^-1 for X = B, whereas the related refrigerant capacity (RC) is 748 J kg^-1. For Gd₆₅Fe₁₀Co₁₀Al₁₅ and Gd₆₅Fe₁₀Co₁₀Al₁₀Si₅ alloys, the maximum values of magnetic entropy change reached 6.0 and 5.9 J kg^-1 K^-1, while the obtained values for RC parameter were 700 and 698 J kg^-1, respectively.

Mictomagnetic behavior of structurally disordered melt-spun DyMn6−xGe6−xFexAlx (0 ≤ x ≤ 6) alloys

The presence of magnetic clusters with different chemical and/or geometric order (e.g., regions with various Fe atoms content) in some of the melt-spun DyMn6−xGe6−xFexAlx (0 ≤ x ≤ 6) alloys of amorphous structure is indicated. This statement is based on macroscopic magnetic measurements M(T), M(H), ac susceptibility, as well as those of microscopic 57Fe Mossbauer spectrometry. With decreasing Fe content, an increase in the hyperfine field strength Bhyp as well as that in the volume of the magnetic fraction in the alloy is observed. The Mossbauer effect investigations on the DyMn3Ge3Fe3Al3 alloy suggest the presence of magnetic fluctuations because the magnetic component increases after applying an external field. ac susceptibility measurements confirm the presence of spin dynamics. There is a cusp in the temperature dependence of the real part of magnetic susceptibility χ′(T) with a significant shift in freezing temperature Tf measured at different frequencies. The Vogel-Fulcher law and critical slowing-d...

Induced magnetic ordering in alloyed compounds based on Pauli paramagnet YCo2

Intermetallic YCo2 compound is a Pauli exchange-enhanced paramagnet. Structural and magnetic properties melt-spun YCo2 pure and alloyed with Nb or Ti are presented. The samples crystallize in MgCu2-type phase with lattice constant a changing from 7.223 A for YCo2, through 7.213 A for Y0.9Nb0.1Co2 to 7.192 A for Y0.9Ti0.1Co2, where Y atoms are replaced by Nb or Ti atoms. Nanocrystalline phases can be produced by appropriate cooling rates for the solidification process. By the synthesis process free volumes, vacancies, and alloyed atoms are introduced into the YCo2 intermetallic. Ab-initio calculations have been performed to investigate the effects of substitution on the spin-split electronic band structure in the ordered YCo2. A ferrimagnetic ground state is found in the alloyed systems with substitution on the Y-site which is energetically favorable compared to point defects on Co-sites. However, the experimentally found increased magnetic ordering in alloyed YCo2 appears to be based on microstructure eff...

Mechanism of amorphous state formation, crystalline structure, and hyperfine interactions in DyMn6−xGe6Fex(0≤x≤6) alloys

Metallic glass formation is observed in rapidly quenched quaternary DyMn6−xGe6Fex (0≤x≤6) alloys. The easy formation of amorphous states competes with the nucleation of ternary 1:6:6 rare earth-transition metal-metal compounds DyMn6Ge6 and DyFe6Ge6. The ribbon shaped samples were quenched and investigated by x-ray diffraction, differential scanning calorimetry, and F57e Mossbauer spectrometry. Melt-spun alloys from the series of DyMn6−xGe6Fex with x=0, 2≤x≤3, and x=6 do not display an amorphous state but a crystalline chemically disordered structure similar to that of TbCu7- or TbFe6Sn6-type (space group P6/mmm). Amorphous samples exhibit two crystallization steps but there is no clear evidence for a glass transition effect in the calorimetric data. The Kissinger analysis was performed to calculate the effective activation energy Ea, which is equal to 345±20 kJ/mol for amorphous DyMn5.5Ge6Fe0.5 alloy. Mossbauer spectra at 77 and 300 K consist of either magnetic sextets or quadrupolar doublets for high and...

Activation Energies of Crystallization in Amorphous RMn_{4.5}Ge_{4.5}Fe_{1.5}Al_{1.5} (R = La, Y, Dy) Alloys

The effective activation energies, characteristic crystallization temperatures and enthalpies of amorphous RMn4.5Ge4.5Fe1.5Al1.5 (R = Y, La, Dy) alloys produced using melt-spinning technique were investigated by differential scanning calorimetry. X-ray diffraction measurements were performed for as-quenched and annealed samples. The crystalline structure of annealed YMn4.5Ge4.5Fe1.5Al1.5 and DyMn4.5Ge4.5Fe1.5Al1.5 alloys was determined as orthorhombic TbFe6Sn6-type with Cmcm (63) space group. The alloy with Y appears as a more useful non-magnetic analogue for DyMn4.5Ge4.5Fe1.5Al1.5 than the La-based alloy. The differential scanning calorimetry curves for Dyand Y-based alloys also exhibit similar thermal behavior. The effective activation energies Ea were determined using the Kissinger approach and high values up to 778 ± 74 kJ/mol for La-based sample were established. The comparison of Y-, La-, and Dy-based alloys suggests improvement of thermal stability with the increase in rare-earth element atomic radius in the glassy RMn4.5Ge4.5Fe1.5Al1.5 systems.