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Cobaltite-like materials having metal-insulator transitions are relevant in the consumer electronics market. In this work, we present the structural, magnetic, and electrical properties of Ba1-xNaxCoO3(with x= 0.25, 0.5, 0.75) cobaltite. All samples were synthesized using a solid-state reaction process. Their crystal structure was determined using X-ray diffraction (XRD) data by the Rietveld method, which showed that all the samples were crystallized in the orthorhombic space group C2221(N.° 20). The microstructure of the sintered samples was characterized by scanning electron microscopy (SEM). The magnetic susceptibility measurements confirmed a paramagnetic behavior for x≥ 0.5in the temperature range that was used. Likewise, a broad peak around 33 Kin the sample x = 0.25,andcharacteristic of antiferromagnetic behaviors wereobserved. Onthe other hand, resistivity contributed to determining the insulating behavior of samples where x = 0.5and x = 0.75. In contrast, at low sodium content (x = 0.25), a metal-insulator transition was observed with transition temperature near 105 K. Los materiales de tipo cobaltita que presentan transición metal-aislante tienen relevancia en el mercado de la electrónica de consumo. En este trabajo presentamos las propiedades estructurales, magnéticas y eléctricas de las cobaltitas Ba1-xNaxCoO3(con x= 0.25, 0.5, 0.75). Todas las muestras se sintetizaron mediante un proceso de reacción en estado sólido y su estructura cristalina se determinó a partir de datos de difracción de rayos X (DRX),mediante el método de Rietveld, el cual indicó que las muestras cristalizan en el grupo espacial ortorrómbico C2221(N.o20). La microestructura de las muestras sinterizadas se caracterizó mediante microscopía electrónica de barrido (MEB). Las medidas de susceptibilidad magnética confirmaron un comportamiento paramagnético para x≥ 0.5en el rango de temperatura estudiado y un pico amplio alrededor de 33 Ken la muestra x= 0.25,lo cual es característico de los comportamientos antiferromagnéticos. Las medidas de resistividad permitieron determinar el comportamiento aislante de muestras con x= 0.5y x= 0.75, mientras que a bajo contenido de sodio (x= 0.25),se observó una transición metal-aislante con temperatura de transición cercana a 105K.

International audience; Remarkable electronic transport and magnetotransport properties are found in the intermetallic compounds RCo12B6 (R = Y, Ho, and Gd). Detailed resistivity, magnetoresistance, and Hall effect measurements are reported in the temperature range between 2 and 320 K under applied magnetic fields up to 9 T. Auxiliary magnetization results and structural data are also reported. The magnetic ordering observed in these systems depends on the rare-earth atom. In the YCo12B6 compound, a ferromagnetic-type ordering is observed (TC ≈ 149 K) whereas the GdCo12B6 and HoCo12B6 compounds show ferrimagnetic-type ordering (TC ≈ 162 and 144 K, respectively) and their magnetization exhibits the compensation phenomenon. All compounds show high residual-resistance ratio (RRR) rates and resistivities typical of magnetically ordered systems, with a characteristic downward inflection at the critical temperature. The resistivity results are described in terms of the interplay between the electron-phonon interaction and a magnetic term. The magnetoresistance is consequence of a complex superposition of effects due to conduction in spin-polarized bands and to spin-disorder suppression. A strong positive contribution, which prevails at low temperatures, is described with a basis on the Campbell-Fert model that assumes conduction by two spin-polarized currents with a spin-mixing term. The usual negative effect related to suppression of spin disorder becomes the dominant contribution at intermediate-and high-temperature regimes. At low temperatures, the Hall resistivity results are also consistent with conduction by two spin-polarized bands. Moreover, a sign reversal observed in the Hall resistivity for all compounds at low temperatures indicates that the majority spin-up band is holelike, while the minority spin band is electronlike. The ordinary and anomalous Hall contributions could be separated. The intrinsic Karplus-Luttinger mechanism is the most relevant contribution to the anomalous Hall resistivity in all studied compounds. However, an additional term due to chiralities related to canting of the Co moments also plays a role in the temperature region near TC .

Non-ohmic behavior is not expected in metal–insulator granular systems in a low-field regime. There is no model to explain this behavior, even though it has been reported in several metalinsulator granular thin films (Fe-Al2O3, Co-Al2O3, and Ti-SiO2). In this paper, we show additional experimental results of Fe-SiO2 granular films and propose an explanation for the electrical properties of all above mentioned systems, based on Mott variable range hopping. The experimental results show that the localization length increases and the electrical resistance decreases with the increase of electrical potential or current. The non-ohmic behavior of the resistance and the increase of the localization length with increasing current are explained by the activation of new pathways for electrons in granular thin films that contain variable grain sizes and/or have different distances between grains.

O presente estudo apresenta uma investigação experimental sobre as propriedades de magnetotransporte nos compostos Ba(Fe1−xMnx)2As2 com x = 0, 2,57, 5,16, 9,27 e 14,7 at % de Mn. Este sistemas apresentam ordem antiferromagnética do tipo onda de densidade de spin (Spin Density Wave - SDW). Em baixas concentrações de Mn, a transição magnética é acompanhada de uma transição estrutural do estado tetragonal e paramagnético de alta temperatura para um estado ortorrômbico e magneticamente ordenado de baixas temperaturas. Os resultados de magnetorresistência mostram efeitos de condução por duas bandas (elétrons e lacunas). Observa-se também efeitos do mecanismo de supressão da desordem de spin induzida pelo campo magnético aplicado. As medidas de resistividade Hall mostram que a contribuição Hall anômala está presente e domina em todo o regime magneticamente ordenado. O mecanismo de espalhamento por impurezas magnéticas, skew-scattering, é o principal responsável pela contribuição anômala ao efeito Hall. Outro objeto de estudo desta Tese é a família de compostos RCo12B6 (R = Y, Gd e Ho). O ordenamento magnético observado nestes compostos depende do átomo de terra rara presente. No composto YCo12B6 ocorre uma ordem do tipo ferromagnética (TC ≈ 148 K). Entretanto, os compostos GdCo12B6 e HoCo12B6 apresentam um estado do tipo ferrimagnético (TC ≈ 160 K e 144 K, respectivamente), com uma temperatura de compensação em torno de 50 K. Resultados de magnetização, calor específico, efeito magnetocalórico, resistividade, magnetorresistência e efeito Hall são apresentados e discutidos. Os resultados de resistividade e magnetorresistência indicam que a condução elétrica é polarizada em spin. A magnetorresistência é fortemente positiva em baixas temperaturas e revela efeitos de mistura das correntes dependentes de spin. A magnetorresistência é fortemente positiva em baixas temperaturas e revela efeitos de mistura das correntes dependentes de spin. Nas medidas de efeito Hall evidenciou-se a presença de bandas de elétrons e lacunas. O mecanismo de skew-scattering é a contribuição mais importante ao termo anômalo da resistividade Hall destes sistemas. Contudo, uma segunda contribuição ao efeito Hall anômalo deve ser considerada. Essa contribuição é relacionada ao efeito de quiralidade originada por canting na sub-rede de Co. Indícios da presença de frustração também são percebidos nos resultados de magnetização, magnetorresistência, efeito magnetocalórico e impedância elétrica. Neste último, também se observa o efeito Hopking nas vizinhanças da temperatura de ordem magnética. This work presents an experimental investigation on the magnetotransport properties of the compounds Ba(Fe1−xMnx)2As2, with x = 0, 2,57, 5,16, 9,27 e 14,7 at % Mn. These compounds exhibit Spin DensityWave (SDW) antiferromagnetic ordering. In low Mn concentration, a structural transition from a high-temperature paramagnetic and tetragonal phase to a low-temperature antiferromagnetic and orthorhombic phase is observed. The magnetoresistance results show contributions from the conduction by two bands (electrons and holes) and from the suppression of spin disorder induced by the applied magnetic field. Hall resistivity measurements shows that the anomalous contribution is dominant in the magnetically ordered phase. The asymmetric scattering mechanism by magnetic impurities, known as skew scattering, is responsible for the anomalous contribution to the Hall effect in the Ba(Fe1−xMnx)2As2 system. Magnetotransport studies, as well as magnetization, specific heat and magnetocaloric effect experiments were performed in the compounds RCo12B6 (R = Y, Gd and Ho). The magnetic ordering observed in these systems depends on the rare-earth atom. In the compound YCo12B6, a ferromagnetic type ordering is observed (TC ≈ 148 K). The compounds GdCo12B6 and HoCo12B6 show a magnetic order of ferrimagnetic type (TC ≈ 160 K e 144 K, respectively), with a compensation temperature around 50 K. The resistivity and magnetoresistance results indicate that the current is spin polarized. Hall resistivity measurements are consistent with two-band conduction (electrons and holes). The skew scattering mechanism is the most relevant contribution to the anomalous Hall resistivity in these compounds. However, a second anomalous contribution to the anomalous Hall effect must be considered. This term is due to canting of the Co moments that originates a chiral mechanism. Evidences of frustration are also present in the results of magnetization, magnetoresistance, magnetocaloric effect and electrical impedance. In the last, one also observes the occurrence of the Hopkinson effect around TC.

In this brief communication we applied the MZFC(T) and MFCC(T) reversible dc magnetizations to get the average superconducting kinetic energy density, kS(T,B) of YBa2Cu3O7-d, Y0.95Ca0.05Ba2Cu3O7-d and YBa1.75Sr0.25Cu3O7-d ceramic samples with the aim of study the effects of Ca and Sr doping on the kS(T,B). The MZFC(T) and MFCC(T) measurements were performed with a SQUID magnetometer from quantum design to dc magnetic fields up to 50kOe. The determination of the kS(T,B) from reversible dc magnetization is supported by virial theorem of superconductivity [kS(T,B) = - M.B]. The kS(T,B) results show an common temperature profile for all the samples which is smoothly affected by the magnetic field. On the other hand the kS(T,B) results to T ³ Tc could not be associated to the pseudogap phenomenon. The Ca doping affects more effectively the kS(T,B) behaviour then Sr doping. A possible explanation to this feature could be associated to the fact that the hole doping promoted by Ca doping depress more considerably the superconducting state and enhances the granular character of the YBa2Cu3O7-d superconductor than the chemical pressure effect promoted by Sr doping.

Este trabalho apresenta uma investigação experimental sobre as propriedades de magneto-transporte dependentes da orientação de spin de dois sistemas ferromagnéticos clássicos com anisotropias distintas: (i) o composto Heusler de momentos magnéticos localizados Pd2MnSn e (ii) ligas de Co-Fe x at% (x = 2, 4 e 6), que são ferromagnetos de bandas. O composto Heusler tem anisotropia magnética muito pequena, ao passo que nas ligas de cobalto a anisotropia é pronunciada em razão da presença de elétrons da camada-3d incompleta no nível de Fermi. Em ambos os sistemas foram realizadas medidas de resistividade elétrica, magnetorresistência e efeito Hall em diversas temperaturas entre 2K e 300K, com aplicação de campos entre 0 e 9 Tesla. Medidas de magnetização foram realizadas em apoio às experiencias de magneto-transporte. As experiências de magneto-transporte evidenciaram efeitos de correntes eletrônicas polarizadas em spin. Em particular, o termo resistivo de mistura de spin proposto por Campbell e Fert [1], que é responsável pela transferência de momentum entre as sub-bandas dependentes de spin, é importante para descrever os comportamentos da resistividade elétrica e da magnetorresistência dos sistemas estudados no limite de baixas temperaturas. Por outro lado, o processo de espalhamento por desordem de spin é dominante nas regiões de temperaturas intermediárias e altas. A medida de resistividade de Hall no Pd2MnSn em função da temperatura apresenta um mínimo em torno de T = 50 K. A análise do contribuição anômala ao efeito de Hall neste sistema é consistente com as previsões teóricas referentes aos efeitos da fase de Berry no espaço recíproco. A anisotropia espontânea da resistividade foi detalhadamente estudada no caso das ligas de Co-Fe e a forte polarização de spin relacionada ao espalhamento pelas impurezas de Fe foi evidenciado. Nestas ligas, o efeito Hall é dominado por efeitos de espalhamento assimétrico (skew scattering) produzido pelas impurezas de Fe. This work presents an experimental study of the spin-dependent magnetotransport properties in two classical ferromagnetic systems with distinct anisotropies: (i) the Heusler compound Pd2MnSn, where the magnetic moments are localized, and (ii) the Co-Fe (Fe = 2, 4 and 6 at %) alloys, that are band ferromagnets. The Heusler compound has a small magnetic anisotropy, while in the cobalt alloys the anisotropy is pronounced because of the presence of 3d electrons in the Fermi level. In both systems the electrical resistivity, magnetoresistance and Hall Effect were measured in the temperature interval between 2K and 300K under applied magnetic fields ranging between 0 and 9 Tesla. Magnetization measurements were made as a complement to the magnetotransport experiments. The magnetotransport measurements evidenced effects of spin polarized electronic currents. In particular, the spin mixing resistive term proposed by Campbell and Fert [1], to describe the momentum transfer between the spin dependent subbands plays an important role in the low temperature electrical resistivity and magnetoresistance of both systems. On the other hand, the scattering process due to spin disorder is dominant in the regions of intermediate and high temperatures. In Pd2MnSn the Hall resistivity shows a minimum around T = 50 K when plotted as a function of temperature. The analysis of the anomalous contribution to the Hall effect in this system is consistent with the theoretical predictions regarding the effects of the Berry phase in the reciprocal space. The spontaneous resistivity anisotropy was studied in detail in the case of the Co-Fe alloys. A strong spin polarization related to the electron scattering by impurities was found. In these alloys, the Hall effect is dominated by skew scattering due to Fe impurities.