The impact of B substitution in Ni50Mn35In15-xBx Heusler alloys with x = (0, 0.5, 0.75, 1, 1.1 1.5, and 2) on the structural, magnetic, transport, and parameters of magnetocaloric effect has been studied by means of room temperature XRD-diffraction, differential scanning calorimetry (DSC), and thermomagnetic measurements (in a magnetic field up to 5 T and temperature interval 5-400 K). Direct adiabatic temperature (ΔTAD) measurements have been carried out for an applied magnetic field change (ΔH) of 1.8 T. The partial substitution of In by B in Ni50Mn35In15-xBx Heusler alloys induced a non-linear temperature shift of the magnetostructural transition while Curie temperature (TC) was found to be nearly constant (TC ~ 320 K) for all compounds. The transition temperatures (T-x) phase diagram has been constructed for H = 0.005 T. The MCE parameters were found to be larger or comparable to parameters observed in other MCE materials, such as Ni50Mn34.8In14.2B and Ni50Mn35In14X (X=In, Al, and Ge) Heusler alloys. It has been demonstrated that the martensitic transformation temperature and the corresponding ∆SM can be tuned through a slight variation in composition of B in NiMnInB alloys. A magnetoresistance associated with martensitic transformation was found to be -60% for x = 0.75 at T = 240 K for a magnetic field change of 5 T. The maximum absolute value of ΔTAD = 2.5 K was observed at the magnetostructural transition for Ni50Mn35In14.5B0.5. The roles of the magnetic and structural changes on the transition temperatures are discussed.

The magnetocaloric and thermomagnetic properties of the Ni1-xCrxMnGe1.05 (for x = 0, 0.035, 0.070, 0.105, 0.110, 0.115, and 0.120) system have been studied by X-ray diffraction, differential scanning calorimetry (DSC), resistivity and magnetization measurements. A change in crystal structure from orthorhombic to hexagonal was observed in the XRD data with an increase in chromium concentrations. The values of the cell parameters and volume of the unit cell for hexagonal phase were determined. It was found that the partial substitution of Cr for Ni in Ni1-xCrxMnGe1.05 results in a first order magnetostructural transition from antiferromagnetic to ferromagnetic (FM) at TM of about132 K, 100 K, and 110 K for x= 0.105, 0.115, and 0.120, respectively. A FM to paramagnetic second order transition has been observed at TC around 200K. A magnetic entropy change of = 4.5 J/kg K, 5.6 J/Kg K, and 5.06 J/Kg K was observed in the vicinity of TC for x = 0.105, 0.115, and 0.120 respectively at ΔH = 5T. The values of the latent heat and corresponding total entropy changes have been determined from Differential Scanning Calorimetry (DSC) measurements. Magnetoresistance values of about -5% were measured near TC for x =0.105. The maximum value of refrigeration capacity (RC) and relative cooling power (RCP) was found to be 155 J/Kg and 175 J/Kg respectively for x = 0.120. A concentration-dependent (T-x) phase diagram of transition temperatures has been constructed using the magnetic and DSC data. The structural, magnetic and magnetocaloric properties of GdNi2Mnx system (for x = 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.5) have been studied by x-ray diffraction and magnetization measurements. A mixture of the Laves phase C15 and a phase with rhombohedral structure PuNi3- type (space group R m) was observed in the XRD data. A second order magnetic phase transition from ferromagnetic (FM) to paramagnetic (PM) was found, characterized by a long-range exchange interaction as predicted by mean field theory. The maximum value of magnetic entropy changes, -∆SM, near TC for ∆H = 5T, was found to be 3.1 J/KgK, 2.8 J/KgK, 2.9 J/KgK, and 2.5 J/Kg K for x = 0.8, 1.2, 1.4, and 1.5 respectively. In spite of the low values of ΔSM, the RC and RCP value was found to be 176 J/Kg and 220 J/Kg for the GdNi2Mn0.8 compound, respectively.

Le manuscrit présente des travaux relatifs à la caractérisation structurale, électrique et magnétique des matériaux manganites poly-cristallin, les manganites qui présentent un intérêt industriel croissant compte tenu de ses nombreux domaines d'application et sa complexité du point de vu fondamentale; ces matériaux sont des systèmes dont les électrons sont fortement corrélés par la présence de plusieurs interactions en compétition. Les principaux objectifs de la thèse étaient de mieux comprendre l'effet de la substitution aléatoire sur le site A de la structure double pérovskite (famille Ruddlesden – Popper A2MnO4 dérivée de pérovskite AMnO3), principalement sur les propriétés magnétique. Cette étude a été effectuée sur toute la gamme de température, étude des phénomènes critique au pont de transition et l'étude de la susceptibilité magnétique a haut température. Enfin, une étude d'autres matériaux présentant des propriétés cristallographiques et physique très intéressantes de type RMX5. Les résultats ainsi obtenu permettent des progrès significatifs dans la compréhension de ces matériaux. / The manuscript presents the work on the structural, electrical and magnetic materials manganites poly-crystalline manganites that have a growing industrial interest due to its many applications and the complexity of fundamental point of view, these materials are systems which electrons are strongly correlated with the presence of several competing interactions. The main objectives of the thesis were to better understand the effect of random substitution on the A site of the perovskite structure twice (family Ruddlesden - Popper A2MnO4 derived from perovskite AMnO3), mainly on the magnetic properties. This study was conducted over the entire temperature range, ie, the study of phenomena Spin Glass at very low temperatures, the study of critical phenomena at the transition point and the study of the magnetic susceptibility at high temperature. Finally, a study of other materials having physical and crystallographic properties very interesting type RMX5.

Magnetic refrigeration based on the magnetocaloric effect (MCE) is a promising alternative to conventional gas compression based cooling techniques. Understanding impacts of reduced dimensionality on the magnetocaloric response of a material such as Gadolinium (Gd) or its alloys is essential in optimizing the performance of cooling devices, which is also the overall goal of this thesis. We have determined, in the first part of the thesis, that laminate structures of pure Gd produced by magnetron sputtering have several disadvantages. The target material (pure Gd), ultra-high vacuum components, and the electrical energy it takes to run the manufacturing process are all very costly. To produce quality films requires a time and energy consuming chamber preparation (gettering) to produce films with a relative cooling power (RCP) of an order of magnitude smaller (~70 J/kg) than can be obtained with Gd-alloy microwires (~800 J/kg). The increased surface area for an array of wires as compared to a laminate structure allows for more efficient heat transfer. For all of these reasons, we turned the focus onto Gd-alloy microwires. In the latter part of this thesis, we have discussed the Gd-alloy microwires as a function of magnetocaloric parameters of magnetic entropy change, adiabatic temperature change, and refrigerant capacity (RC). We have demonstrated two effective methods for improving the RC of the microwires through creating novel biphase nanocrystalline/amorphous structures via thermal annealing and directly from adjusted melt-extraction. Through studying the effects of chemical doping, as well as studying arrays of microwires with a range of Curie temperature (TC) values, we have designed a new magnetic bed structure that has potential applications as a cooling device for micro-electro-mechanical systems and energy-conversion devices.

Orientadores: Sergio Gama, Nilson Antunes de Oliveira / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-08T01:08:08Z (GMT). No. of bitstreams: 1 Campos_Arianade_D.pdf: 9214751 bytes, checksum: 227e7e0b1cc697ea73f810066346ff6d (MD5) Previous issue date: 2006 / Resumo: Neste trabalho descreveremos a obtenção dos compostos da família MnA s1-xAx (A= Te,P, Sb) e Mn1-xFe xAs para várias concentrações. Dividimos este trabalho em duas etapas, a primeira via obtenção em forno de alta pressão e a segunda via obtenção em forno tubular em tubos de quartzos. A primeira etapa, ainda se dividiu em obtenção indireta e direta dos materiais. Na obtenção indireta dos materiais, focamos nosso trabalho nos compostos de MnAs e MnSb para a produção da série MnAs1-xSbx. Na obtenção direta, partimos dos elementos para sintetizar os materiais, utilizando o mesmo método adotado na obtenção indireta. Na segunda etapa do trabalho, obtemos os compostos diretamente em tubos de quartzo. As amostras produzidas foram caracterizadas por difração de raios-X, microscopia óptica, microscopia eletrônica de varredura utilizando a técnica de WDS e, finalmente, análises magnéticas para a obtenção do efeito magnetocalórico de cada material, e assim a avaliação destes materiais como possíveis candidatos a materiais refrigerantes. Após o cálculo do efeito magnetocalórico, utilizamos um modelo fenomenológico que considera a dependência da temperatura crítica da fase magnética na mudança de volume, o modelo utilizado parte das descrições propostas por Bean e Rodbell que correlaciona fortes interações magnetoelásticos com a transição de fase de primeira ordem / Abstract: In this work we describe the obtaining processes of the MnAs1-x Ax (A= Te, P, Sb) and Mn1-xFexAs series for several concentrations. We divided this work in two stages: in the first one the samples were obtained using a high pressure furnace and in the second one using a resistive furnaces with the samples sealed in quartz tubes. The first stage, can be split in direct and indirect obtaining of the materials. In the indirect obtaining of the materials, our work was focused on the MnAs and MnSb compounds for the production of the series MnAs1-xSbx. In the direct obtaining, we synthesized the materials directly from the elements, using the same method adopted in the indirect obtaining. In the second stage of the work, we obtained the samples directly from the elements in quartz tubes. The produced samples were characterized by ray-X diffraction, optical microscopy, electron microscopy using the WDS technique and finally magnetic analysis for the calculation of the magnetocaloric effect of each material and, in this way evaluate these materials as possible candidates to refrigerant materials. After the calculation of the magnetocaloric effect, we used a phenomenological model that considers the dependence of the critical temperature of the magnetic phase in the volume change, the model used part of the descriptions proposed by Bean and Rodbell [1] that correlates strong magnetoelastic interaction with the first order phase transition / Doutorado / Física da Matéria Condensada / Doutor em Ciências

The structural, electronic, magnetic, magnetocaloric, and transport properties of doped Ni-Mn-(In, Sn) based Heusler alloys were studied using neutron diffraction, x-ray diffraction (XRD), differential scanning calorimetry (DSC), high field magnetization, specific heat, x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and hydrostatic pressure measurements. The adiabatic temperature change (∆Tad) by a direct method and through thermomagnetic measurements in magnetic fields up to 14 T has been performed for these alloys. Also the mixed effect of pressure and magnetic field on the transition temperature of these alloys are discussed. In order to develop new magnetocaloric and multifunctional materials, the synthesis and characterization of Heusler alloys in reduced dimensions, i.e., ribbons and thin films has been performed. In addition, the structural, magnetic, and magnetocaloric properties of Ni-based binary alloys were investigated, including saturation magnetization and Curie temperature (TC) for the possible applications in self controlled magnetic hyperthermia applications.

<p> New metal-rich Gd5T4 magnetocaloric phases (T - p-element) were designed, synthesized and characterized. These phases exhibit a close relationship between the valence electron count, size effect, crystal structure and physical properties. The targeted cleavage of the interslab T-T dimers was achieved in the Gd5Si4-xPx, Gd5Si4-xSbx and GdsSi4_xBix systems. While in the Gd5Si4-xPx system only a change in the valence electron count was employed for the desired structural transformations, in the Gd5Si4-xSbx and Gd5Si4-xBix systems both the valence electron count and size effect were used to break the interslab dimers. Incorporation of large Bi atoms into the Gd5Si4 phase resulted in the complete cleavage of the interslab T-T bonds and lead to novel slab stacking sequences accompanied by stacking faults. </p> <p> The Gd5Si4_xPx and Gd5Si4-xSbx phases undergo ferromagnetic transitions within a wide temperature range. Values of the corresponding magnetic entropy changes indicate the presence of a conventional magnetocaloric effect. This is likely due to the temperature stability of the structures with the broken dimers which hinders first-order coupled magnetostructural transitions. </p> <p> Metal-rich ferromagnetic phases of the RE5NixT3-x composition with RE = Gd, Dy, Lu, T = Sb, Bi were derived from the parent RE5T3 binaries. Incorporation of nickel stabilizes the orthorhombic Yb5Sb3-type structures at 800 °C. The Gd5Ni0.96Sb2.04, Gd5Ni0.71Bi2.29 and Dy5Ni0.66Bi2.34 phases show relatively low values of magnetocaloric effect. </p> / Thesis / Doctor of Philosophy (PhD)

O efeito magnetocalórico, i.e., o aquecimento e/ou resfriamento de um material magnético sob variação do campo magnético aplicado é a base da refrigeração magnética.O efeito magnetocalórico é caracterizado pela variação da entropia em um processo isotérmico (O efeito magnetocalórico, i.e., o aquecimento e/ou resfriamento de um material magnético sob variação do campo magnético aplicado é a base da refrigeração magnética. O efeito magnetocalórico é caracterizado pela variação da entropia em um processo isotérmico (&#916;Siso) e pela variação da temperatura em um processo adiabático &#916;Tad.Apesar dos inúmeros trabalhos experimentais e teóricos publicados nessa área, muitos aspectos desse efeito ainda não são bem compreendidos.Nesse trabalho discutimos os efeitos da anisotropia sobre as propriedades magnetocalóricas de um sistema de momentos magnéticos localizados. Para essa finalidade, utilizamos um modelo de spins interagentes com um termo de anisotropia uniaxial do tipo DS2 z , onde D é um parâmetro. Nesse modelo, em que o eixo z é a direção de fácil magnetização, a magnitude do parâmetro de anisotropia e a direção do campo magnético aplicado têm um papel fundamental no comportamento das grandezas magnetocalóricas &#916;Siso e &#916;Tad. Realizamos um estudo sistemático para um sistema com J = 1 aplicando o campo magnético em diferentes direções. Os resultados mostram que, quando o campo magnético é aplicado ao longo da direção z, as grandezas magnetocalóricas apresentam o comportamento normal (valores positivos de &#916;Tad e valores negativos de &#916;Siso para &#916;B > 0). Quando o campo magnético é aplicado em uma direção diferente do eixo z, as grandezas magnetocalóricas podem apresentar o comportamento inverso (valores negativos de &#916;Tad e valores positivos de &#916;Siso para &#916;B > 0) ou o comportamento anômalo (troca de sinal nas curvas de &#916;Tad e &#916;Siso). Resultados equivalentes também foram obtidos para um sistema com J = 7=2. / The magnetocaloric effect, i.e., heating and/or cooling of a magnetic material subjected to magnetic field variation is the basis of magnetic refrigeration. The magnetocaloric effect is caracterized by the entropy change in an isothermic process (&#916;Siso) and by the temperature change in an adiabatic process (&#916;Tad). Despite the large number of experimental and theoretical works published in this area, there are many aspects of the magnetoccaloric effect which are not yet completely understood.In this work we discuss the effects of anisotropy on the magnetocaloric properties of a system of localized magnetic moments. In order to do that, we used a model of interacting spins with a uniaxial anisotropy term DS2 z , where D is a parameter. In this model, where the z axis is the easy magnetization direction, the magnitude of the anisotropy parameter and the direction of the applied magnetic field have an important role in the behavior of the magnetocaloric quantities &#916;Siso and &#916;Tad. We perform a systematic study for a system with J = 1 by applying the magnetic field in different directions. The results show that, when the magnetic field is applied in the z direction, the magnetocaloric quantities have the normal behavior (positive values of &#916;Tad and negative values of &#916;Siso with &#916;B > 0). When the magnetic field is applied in a direction different from the z axis, the magnetocaloric quantities can show the inverse behavior (negative values of &#916;Tad and positive values of &#916;Siso with &#916;B > 0) or the anomalous behavior (change of sign in the curves of &#916;Tad and &#916;Siso). Similar results have also been obtained for a system with J = 7=2.

O efeito magnetocalórico, i.e., o aquecimento e/ou resfriamento de um material magnético sob variação do campo magnético aplicado é a base da refrigeração magnética.O efeito magnetocalórico é caracterizado pela variação da entropia em um processo isotérmico (O efeito magnetocalórico, i.e., o aquecimento e/ou resfriamento de um material magnético sob variação do campo magnético aplicado é a base da refrigeração magnética. O efeito magnetocalórico é caracterizado pela variação da entropia em um processo isotérmico (&#916;Siso) e pela variação da temperatura em um processo adiabático &#916;Tad.Apesar dos inúmeros trabalhos experimentais e teóricos publicados nessa área, muitos aspectos desse efeito ainda não são bem compreendidos.Nesse trabalho discutimos os efeitos da anisotropia sobre as propriedades magnetocalóricas de um sistema de momentos magnéticos localizados. Para essa finalidade, utilizamos um modelo de spins interagentes com um termo de anisotropia uniaxial do tipo DS2 z , onde D é um parâmetro. Nesse modelo, em que o eixo z é a direção de fácil magnetização, a magnitude do parâmetro de anisotropia e a direção do campo magnético aplicado têm um papel fundamental no comportamento das grandezas magnetocalóricas &#916;Siso e &#916;Tad. Realizamos um estudo sistemático para um sistema com J = 1 aplicando o campo magnético em diferentes direções. Os resultados mostram que, quando o campo magnético é aplicado ao longo da direção z, as grandezas magnetocalóricas apresentam o comportamento normal (valores positivos de &#916;Tad e valores negativos de &#916;Siso para &#916;B > 0). Quando o campo magnético é aplicado em uma direção diferente do eixo z, as grandezas magnetocalóricas podem apresentar o comportamento inverso (valores negativos de &#916;Tad e valores positivos de &#916;Siso para &#916;B > 0) ou o comportamento anômalo (troca de sinal nas curvas de &#916;Tad e &#916;Siso). Resultados equivalentes também foram obtidos para um sistema com J = 7=2. / The magnetocaloric effect, i.e., heating and/or cooling of a magnetic material subjected to magnetic field variation is the basis of magnetic refrigeration. The magnetocaloric effect is caracterized by the entropy change in an isothermic process (&#916;Siso) and by the temperature change in an adiabatic process (&#916;Tad). Despite the large number of experimental and theoretical works published in this area, there are many aspects of the magnetoccaloric effect which are not yet completely understood.In this work we discuss the effects of anisotropy on the magnetocaloric properties of a system of localized magnetic moments. In order to do that, we used a model of interacting spins with a uniaxial anisotropy term DS2 z , where D is a parameter. In this model, where the z axis is the easy magnetization direction, the magnitude of the anisotropy parameter and the direction of the applied magnetic field have an important role in the behavior of the magnetocaloric quantities &#916;Siso and &#916;Tad. We perform a systematic study for a system with J = 1 by applying the magnetic field in different directions. The results show that, when the magnetic field is applied in the z direction, the magnetocaloric quantities have the normal behavior (positive values of &#916;Tad and negative values of &#916;Siso with &#916;B > 0). When the magnetic field is applied in a direction different from the z axis, the magnetocaloric quantities can show the inverse behavior (negative values of &#916;Tad and positive values of &#916;Siso with &#916;B > 0) or the anomalous behavior (change of sign in the curves of &#916;Tad and &#916;Siso). Similar results have also been obtained for a system with J = 7=2.

No description available.