B. Idzikowski

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.

STRUCTURE, HYPERFINE INTERACTIONS, AND MAGNETIC BEHAVIOR OF AMORPHOUS AND NANOCRYSTALLINE FE80M7B12CU1 (M=MO, NB, TI) ALLOYS

57Fe Mossbauer spectrometry is used to elucidate the structural arrangement and nature of hyperfine interactions in amorphous and nanocrystalline forms of Fe80M7B12Cu1 (M=Mo, Nb, and Ti) alloys. Paramagnetic at room temperature as-quenched M=Mo amorphous alloy shows a distribution of quadrupole splitting (TC=265 K). After partial crystallization, a wide range of hyperfine interactions is observed in the residual amorphous matrix of all samples. The hyperfine field distributions corresponding to the amorphous phase and interfacial zone provide a quantitative basis for a design of topography of hyperfine interactions. Closer inspection of magnetic interactions is made by the help of unconventional radio frequency (rf) Mossbauer technique. The rf Mossbauer results show that the nanocrystalline Fe phase has a larger anisotropy than that of the parent amorphous phase. Conventional magnetic measurements, differential scanning calorimetry, x-ray diffraction, transmission electron microscopy, and scanning tunneli...

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.

Microstructure and magnetic properties of amorphous and nanocrystalline Fe80M7B12Cu1 (M - Nb, Ti or Mo) alloys

The structural and magnetic properties of melt-spun Fe 80 M 7 B 12 Cu 1 alloys (M = Nb, Ti or Mo) were examined in the amorphous as-quenched state as well as after annealing which results in a relaxed amorphous state or a nanocrystalline structure. For samples with M = Mo heat treated at temperatures below the crystallization onset, a relaxation effect connected with a decrease of the Curie temperature of the amorphous phase from Tc = 265 to 250 K was observed. As expected, annealing of these samples at higher temperatures, T a > 410°C, results in an increase of Tc due to the crystallization of BCC Fe connected with a change in the composition of the residual amorphous matrix. The coercivity of the Fe 80 M 7 B 12 Cul alloys shows a maximum with respect to the annealing temperature.

XRD and VSM study of ball-milled SrFe12O19 powder

Abstract The development of phases and the magnetic properties of nanostructural powder obtained by ball milling of SrFe 12 O 19 in vacuum and air is presented.

Mössbauer study of the magnetism and structure of amorphous and nanocrystalline Fe81−xNixZr7B12 (x=10–40) alloys

Soft magnetic amorphous and nanocrystalline Fe81−xNixZr7B12 (x=0–40) alloys with very low coercivity and improved mechanical properties, as compared with the NANOPERM alloys, were prepared by a melt-quenching technique. The nanostructure was formed by annealing amorphous precursors in the temperature range TA=440–620 °C. Formation of the nanocrystalline phase was studied by Mossbauer spectroscopy, differential scanning calorimetry, and x-ray diffraction techniques. The composition of the nanocrystalline phase strongly depends on the Ni content in the alloy. For x<30 the dominating nanocrystalline phase is the bcc Fe, similarly to the NANOPERM alloys. However, the alloy with x=40 behaves in a clearly different way. Annealing of the Fe41Ni40Zr7B12 alloy at TA=520–620 °C causes the formation of the nanograins of magnetically ordered cubic (FeNi)23B6 and FeNi phases, as identified by the Mossbauer and x-ray diffraction measurements. Annealing at temperatures exceeding 590 °C leads to the Mossbauer spectra at ...

Mössbauer study of the magnetic properties of nanocrystalline Fe80.5Nb7B12.5 alloy

The nanocrystalline body-centered-cubic (bcc)-Fe phase was formed by controlled 1 h annealing of the amorphous Fe80.5Nb7B12.5 alloy at temperatures ranging from 490 to 650 °C. The microstructure and magnetic properties of the nanocrystalline alloy were investigated by Mossbauer spectroscopy, differential scanning calorimetry, and quasistatic hysteresis loop measurements. Conventional Mossbauer spectroscopy allowed identification of phases and the determination of their relative content. The specialized radio frequency (rf)-Mossbauer technique, which employs the effects induced by the rf magnetic field (rf collapse and rf sideband effects) allowed us to distinguish the magnetically soft amorphous and nanocrystalline phase from the magnetically harder microcrystalline Fe. The rf-Mossbauer experiments performed as a function of the rf field intensity allowed determination of the anisotropy fields in each phase of the nanocrystalline alloy (amorphous matrix, nanoscale bcc-Fe grains). The measurements of the h...

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.

Structural changes in amorphous Fe80-xRxB20 (R = Y, Sm, Pr) alloys

Abstract The radio-frequency (rf) collapse of the magnetic hyperfine structure in the Mossbauer spectra is employed to study changes in the short-range order in amorphous metals via the electric quadropole hyperfine interaction. The structural changes in amorphous Fe 80- x R x B 20 (where R = Y, Sm, Pr) alloys were studied in detail as a function of x . It was found that such substitutions induce considerable changes in the short-range order. A low concentration of substituting atoms ( x x >4) lead to collective changes in the structure.