The temperature dependence of the magnetic polarization near the Curie temperature in Ni MnGa stoichiometric alloys, directly processed from the melt in a twin-roller melt-spinning device, is investigated. The effect of the solidification rate on the Hopkinson peak detected is evaluated in samples quenched at three different tangential wheel speeds of 10, 15, and 20 m/s. The resulting microstructures were previously characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and by transmission electronmicroscopy (TEM). EDS results indicated that all the alloys have the composition Ni MnGa; at room temperature and above this temperature, a cubic L ferromagnetic ordered austenitic phase is observed. The Curie temperatures and the magnitude of the Hopkinson effect are estimated from the magnetic polarization versus temperature curves measured in a Faraday balance, in the range 300 K-400 K. As expected for samples with identical composition, the Curie temperatures remain insensitive to the processing route. At low fields (10 mT), the magnitude of the Hopkinson effect is larger in samples quenched at lower rates and it practically vanishes in all the alloys for applied fields near 100 mT.
Keywords: HOPKINSON EFFECT; Ni2MnGa; SHAPE MEMORY ALLOYS; Física de los Materiales Condensados; Ciencias Físicas; CIENCIAS NATURALES Y EXACTAS.
FePd nanowires (NWs) with different compositions have been grown into anodized aluminum oxide templates (AAO) by AC electrodeposition at room temperature. The effects of nanowire composition and morphology on the hysteresis properties of the ordered array of NWs are investigated. All the NWs are polycrystalline; the Fe-rich wires have a bcc structure while the Pd rich ones are fcc. FePd NWs are ferromagnetic and the spontaneous magnetization is found to be parallel to the nanowire long axis (out of plane easy axis) and both, the saturation polarization and the coercive field increase with the iron content.
Keywords: Alumina template; FePd nanowires; Hysteresis properties; Shape anisotropy; Otras Ciencias Físicas; Ciencias Físicas; CIENCIAS NATURALES Y EXACTAS.
Magnetic MCM-41 type mesoporous silica materials were synthetized and modified with differentiron loadings by the wet impregnation method. The evolution of iron speciation, depending on the metal loading and associated with a particular magnetic behavior was investigated by M vs. H curves, FC-ZFC curves, EPR spectroscopy and other complementary techniques such as SEM, TEM, and chemisorption of pyridine followed by FT-IR studies. A superparamagnetic contribution was larger for the lower loadings suggesting the high dispersion of very small sized iron nanospecies. However, this contribution decreased with increasing metalloading due to the growth of magnetically blocked nanoparticles (hematite) on the outer surface. Finally, a bimodal size distribution for the superparamagnetic nanospecies could be inferred; then the anisotropy constant for this phase and the corresponding nanospecies sizes were estimated.
Keywords: Fe MODIFIED MCM-41; WET IMPREGNATION; SUPERPARAMAGNETISM; ZFC-FC CURVES; BLOCKING TEMPERATURE; Nano-materiales; Nanotecnología; INGENIERÍAS Y TECNOLOGÍAS.
Magnetic MCM-41 type mesoporous silica materials were synthetized and modified with different iron loadings by the wet impregnation method. The evolution of iron speciation, depending on the metal loading and associated with a particular magnetic behavior was investigated by M vs. H curves, FC–ZFC curves, EPR spectroscopy and other complementary techniques such as SEM, TEM, and chemisorption of pyridine followed by FT-IR studies. A superparamagnetic contribution was larger for the lower loadings suggesting the high dispersion of very small sized iron nanospecies. However, this contribution decreased with increasing metal loading due to the growth of magnetically blocked nanoparticles (hematite) on the outer surface. Finally, a bimodal size distribution for the superparamagnetic nanospecies could be inferred; then the anisotropy constant for this phase and the corresponding nanospecies sizes were estimated.
Keywords: Fe-modified MCM-41; Wet impregnation; Superparamagnetism; ZFC–FC curves; Blocking temperature; Otras Ingeniería Química; Ingeniería Química; INGENIERÍAS Y TECNOLOGÍAS.
Stoichiometric Ni2MnGa alloys are processed by two rapid solidification techniques – single-roller (SR) and twin-roller (TR) melt spinning – and the resulting microstructures and magnetic properties determined. Samples processed at tangential wheel speeds of 10 m/s (V10) and 15 m/s (V15) are studied in the as-cast condition to analyze the influence of the production methods on the microstructure. Important aspects like the resulting phases, their crystallographic texture, magnetic properties, martensitic transformation temperatures and Curie temperatures are compared. In addition, the magnetization mechanism involving twin boundary motion is explored. Our results indicate that the TR method provides lower cooling rates, thicker samples, higher internal stresses and larger MnS precipitates. However, the quenching rate is mainly determined by the tangential wheel velocity. TR samples also exhibit  texture normal to the ribbon plane but in a lesser extent than SR ribbons. Martensitic transformation temperatures are higher in samples V15 (~ 150 K) than in V10 (~ 100 K), with no clear difference between the SR and TR modes. This behavior is explained by considering distinct degrees of disorder in the L21 austenite phase resulting from quenching. The hysteresis of the transformation, defined as the difference Af − MS, takes similar values in the four samples analyzed. Pre-martensitic transformation temperatures are also slightly higher in samples V15, (230 ± 3) K, than in samples V10, (222 ± 3) K, as the magnitude of the Hopkinson effect, in good agreement with a higher residual stress level in TR ribbons. In the martensitic state, all ribbons exhibit hysteresis loops characteristic of a magnetization mechanism involving twin boundary motion. The switching magnetic fields for the onset of Type I twin boundary motion result between 220 mT and 365 mT, values equivalent to twinning stresses of about 1 MPa. It is concluded that both procedures, SR and TR melt spinning, provide microstructures favoring magnetic field induced twin variant reorientation.
Keywords: MAGNETIC PROPERTIES; MARTENSITIC TRANSFORMATION; MICROSTRUCTURE; RAPID SOLIDIFICATION; SHAPE&ndash;MEMORY ALLOYS; TRANSMISSION ELECTRON MICROSCOPY; Astronomía; Ciencias Físicas; CIENCIAS NATURALES Y EXACTAS.
Tesis (Doctor en Física)--Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física, 2008.
En el presente trabajo, se estudió el comportamiento magnético de diferentes sistemas granulares nanoestructurados y los mecanismos de reversión de la magnetización operantes, relacionándolos con la naturaleza, tamaño medio de grano, relativa abundancia de las fases presentes e interacción entre ellas.
Gabriela del Valle Pozo López ; directora Silvia Patricia Silvetti.
Keywords: Magnetic properties of nanostructures; Nanoscale materials and structures; Magnetization reversal mechanisms; Nanocompuestos magnéticos; Nanoestructuras; Impregnación húmeda; Aleado mecánico; Magnetismo; Propiedades magnéticas.