Microstructure and composition design of magnetic Ni-Mn-Sn Co-sputter deposited films

"In this work, we study the effect of the substrate temperature (ST) during sputter-deposition as well as co-sputtering deposition on the fabrication of nanostructured Ni-Mn-Sn thin films. Sputtered films show Mn losses of around 10 at.% while the average grain size () increased from 30 nm to 105 nm with the increasing of ST. Mn losses compensation is proposed by co-sputtered deposition. With such a purpose a variable electrical power was applied to the radio frequency (RF) Mn cathode. By increasing the electrical power applied to the RF Mn cathode both Mn and Ni contents approach to the targeted nominal composition Ni:Mn:Sn = 50:37:13. Elemental chemical composition analyses show that the composition varied between Ni61.5Mn26.2Sn12.3 and Ni54.6Mn30.5Sn14.9 when the applied RF-power increased from 0 W to 30 W.

Magnetic properties of the complex concentrated alloy system CoFeNi0.5Cr0.5Alx

We study the change in magnetisation with paramagnetic Al addition in the CoFeNi0.5Cr0.5–Alx (x: 0, 0.5, 1, and 1.5) complex concentrated alloy. The compositions were developed utilising the Mulliken electronegativity and d-electron/atom ratio. Spherical FeCr rich nanoprecipitates are observed for X: 1.0 and 1.5 in an AlCoNi-rich matrix. A ~ 5 × increase in magnetisation (from 22 to 96 Am2/kg) coincides with this nanoprecipitate formation—the main magnetic contribution is determined to be from FeCr nanoprecipitates. The magnetisation increase is strange as paramagnetic Al addition dilutes the ferromagnetic Fe/Co/Ni additions. In this paper we discuss the magnetic and structural characterisation of the CoFeNi0.5Cr0.5–Alx composition and attempt to relate it to the interfacial energy

Magnetostructural transition and magnetocaloric effect in MnNiGe1.05 melt-spun ribbons

"Alloy ribbons of nominal composition MnNiGe1.05 were produced using the melt-spinning technique. As-quenched (aq) polycrystalline ribbons are single-phase showing the hexagonal Ni2In-type crystal structure. After thermal annealing at 1148 K, the formation of the orthorhombic TiNiSi-type crystal structure by martensitic transformation is favored. However, XRD patterns for different temperatures indicate that the phase transition from hexagonal to orthorhombic structure is incomplete. The starting and finishing temperatures for the direct and reverse martensitic transformation for aq (annealed) samples determined by DSC were MS = 264 (268) K Mf = 235 (255) K, AS = 259 (266) K, and Af = 289 (276) K. Across this structural phase transition the annealed sample undergoes a drop in magnetization giving rise to a narrow temperature dependence of the magnetic entropy change with a peak value on heating (cooling) of 5.8 (4.8) Jkg−1K−1 for a field change of 5 T.

Investigation into the magnetic properties of CoFeNiCryCux alloys

The search for cheap, corrosion-resistant, thermally-mechanically stable functional magnetic materials, including soft magnetic and magneto-caloric materials has led to research focused on high entropy alloys (HEAs). Previous research shows that alloying elements with negative enthalpies of mixing can facilitate a second-order phase transition. On the other side of the spectrum, compositional segregation cause by positive enthalpy of mixing alloying additions (such as Cu) may also be used to tune magnetic properties. This paper studies the structural, magnetic and magneto-caloric effect of the FCC alloys CoFeNiCryCux (x = 0.0, 0.5, 1.0 and 1.5, y = 0.0, 0.8 and 1.0) to tune these properties with Cu and Cr alloying. Scanning electron microscopy of the compositions show nanoparticles forming within the grains as the Cu concentration increases. Cr addition to CoFeNiCu1.0 has a larger effect on the magnetic and magneto-caloric properties compared to the Cu addition to CoFeNiCr1.0. The addition of Cu (x = 0.5) to CoFeNiCr1.0 improved both the saturation magnetisation and Curie temperature; addition of Cr (y = 1.0) to CoFeNiCu1.0 decreased the Curie temperature by 900 K. All alloys were determined to have a second-order phase transition around their Curie temperature. The refrigerant capacity at 2 T was found to be similar to existing HEAs, although the Curie temperatures were lower than room temperature. Based on this data the CoFeNiCr0.8Cu composition was fabricated to increase the Curie temperature towards 300 K to explore these HEAs as new candidates for room temperature magneto-caloric applications. The fabricated composition showed Curie temperature, saturation magnetisation, and refrigerant capacity increasing with the small reduction in Cr content

Trasformación martensítica y efecto magnetocalórico en aleaciones Ni50Mn37Sn13 y MnCoGeBx obtenida por enfriamiento rapido.

Tesis (Doctorado en Ciencia de Materiales)"La presente tesis está orientada al estudio de la transformación martensítica (TM) y el efecto magnetocalórico de aleaciones Ni50Mn37Sn13 y Mn0.96Co1.04GeB0.02. Como aspecto complementario se incluye un capítulo en el que se estudia el protocolo térmico (PT) correcto para medir las isotermas de magnetización M(μoH) a partir de las cuales se calcula la curva de variación de entropía total ΔST(T) aplicando la relación de Maxwell en materiales con transición magneto-estructural acoplada. Éste se hizo para la transformación en calentamiento de la fase ortorrómbica-ferromagnética a la hexagonal-paramagnética en cintas de aleación MnCoGeB0.02. Se comparan las curvas ΔST(T) obtenidas a partir de las curvas M(μoH) medidas siguiendo el PT descrito por Caron y colaboradores1 (muy empleado en la literatura) y el empleado en nuestro grupo de trabajo. Se muestra que empleando este último no se sobrestima │ΔST max│, y tanto la posición del máximo como el ancho de la curva ΔST(T) coinciden con lo que indica el pico exotérmico de la curva DSC. Se estudia la relación entre el tamaño de grano promedio y la temperatura de inicio Ms de la TM en cintas de la aleación Ni50Mn37Sn13 obtenidas a diferentes velocidades. Con el decremento de desde 7.3 μm hasta 1.4 μm, Ms disminuye de 258 K a 212 K. Para este rango el modelo fenomenológico es el que mejor describe la relación entre y Ms en nuestro sistema. Este establece que con la disminución de se estabiliza la fase austenítica. Se estudia la naturaleza de la transición magneto-estructural en cintas de aleaciones Mn0.96Co1.04GeB0.02 tratadas a temperaturas entre 650 oC y 875 oC. El estudio estructural, microestrutural, calorimétrico y magnético sugieren que durante el enfriamiento en la medida que la muestra se acerca a Ms, en la fase hexagonal se forman dominios ferroelásticos cuyas paredes actúan como sitios de nucleación de la fase martensítica-ortorrómbica. Los valores de │ΔST max│para cintas con transformación magneto-estructural de esta serie son del orden de los reportados en la literatura para aleaciones en bulto de composición similar.""This thesis is focused on the study of the martensitic transformation (MT) and magnetocaloric effect in Ni50Mn37Sn13 and Mn0.96Co1.04GeB0.02 alloy ribbons. We also include a chapter related to the study of the correct thermal protocol (TP) that should be followed to measure the isothermal magnetization curves M(μoH), from which the total entropy change ΔST is calculated by using the Maxwell equation in materials with first order phase transition. We compare the ΔST(T) curves obtainedfrom the M(μoH) measurements resulting from the TP described by Caron et al2 (widely used in the literature), and the one used in our group. This comparison was conducted for the on-heating phase transformation (i.e., from the orthorhombicferromagnetic phase to the hexagonal-paramagnetic phase) of MnCoGeB0.02 alloy ribbons. It is shown that using a correct TP the │ΔST max│ is not overestimated, and both position and width of the maximum of the ΔST(T) match with the exothermic peak of the DSC curve. The relationship between the average grain size and the martensitic starting Ms phase transition is studied in Ni50Mn37Sn13 ribbon samples obtained at different quenching rates. With the decrease of from 7.3 μm to 1.4 μm, Ms decreases from 258 to 212 K. This result is explained by means of the phenomenological model, which predicts the stabilization of the austenitic phase with the reduction of the average grain size. Structural, microstructural, calorimetric, and magnetic analyses were carried out on Mn0.96Co1.04GeB0.02 alloy ribbons annealed at different temperatures betwen 650 oC and 875 oC in order to study the nature of the magnetostructural transition. The results suggest that as the sample approaches to Ms (i.e., in the on-cooling direct martensitic transformation) the formation of ferroelastic domains is favored, and, the ferroelastic domain walls act as nucleation sites of the martensitic-orthorhombic phase. │ΔST max│ values for samples of this series are similar of those reported in the literature for bulk alloys of comparable composition.

La Construcción del Conocimiento como Proceso Activo en la Enseñanza

This paper is related with the historical development of pedagogical sciences in active education. Basically is showed the hypothesis that a better education is this one where the knowledge is build by the student, having these an active position in his own education, decreasing the passivity or listening and memorizing professors conferences. It wasn't until the ends of XIX century, when pedagogic was considered an independent science of knowledge, before, from ancient, we found ideas respect the teaching method as a part of the predominant philosophy in every region and epoch, and really closed to the interesting of religion and state.Este trabajo intenta realizar un recorrido en la historia de la pedagogía, básicamente resaltando aquellos intentos que con mayor o menor resultados planteaban la hipótesis de que una mejor educación es aquella en la que el conocimiento se construye por el estudiante, al tener este una posición activa en su propia educación, disminuyendo la pasividad de escuchar y memorizar las conferencias de los profesores. No es sino hasta finales del siglo XIX cuando la pedagogía se considera una rama independiente del saber, anteriormente, desde la antigüedad, encontramos ideas respecto al método de enseñanza como parte de la filosofía predominante en cada región y época, y ampliamente sujeta a los intereses de la religión y el estado

Study of dual-phase functionalisation of NiCoFeCr-Alx multicomponent alloys for the enhancement of magnetic properties and magneto-caloric effect

We have studied the effect of phase separation on the magnetic and magneto-caloric properties of the CoFeNi0.5Cr0.5-Alx (x= 0.0, 1.0 and, 1.5) system. Results show that a collaborative behaviour amongst FeCr-rich segregated nanoparticles (NPs) increases the saturation magnetisation (Ms) whilst the Curie temperature (Tc) is controlled by the amount of added Al. With a strong ferromagnetic coupling between segregated FeCr-NPs, the CoFeNi0.5Cr0.5-Al1.0 sample shows the highest Ms (100 Am2kg-1) with an increase of 61% over the Al-free CoFeNi0.5Cr0.5 sample. It is argued that as the ferromagnetic interaction increases in a degenerated super-spin-glass like state of the NPs the field induced phase transition is broadened whilst the magnetic entropy decreases. In turn, the CoFeNi0.5Cr0.5-Al1.0 sample shows the highest refrigerant capacity (17.1 Jkg-1 at μ0ΔH = 1.0 T), and the smallest measured magnetic entropy change (ΔSmpeak = 0.22 Jkg-1K-1). We found that the enhanced magnetic and refrigerant capacity by mean of phase separation and NPs clustering are amongst the highest reported for the multi-component alloys being investigated for energy applications in the high temperature range

On the structural, microstructural and magnetic properties evolution of Ni0.5FeCoAlCrx alloys

The multicomponent alloy CoFeNiAlCr has shown great promise as a soft magnetic material due to its small coercive field and high saturation magnetisation at room temperature. By changing the ratio of the components, the Curie temperature can be tuned, along with improvements in both the coercive field and saturation magnetisation. The alloy system is interesting as the ratio of Al to Cr determines whether a single solid phase or a dual phase alloy is formed. This work has investigated the effects of Cr addition on the structural and magnetic properties of CoFeNi0.5AlCrx. It discusses the evolution of the NPs-matrix segregations and its constituents’ phases