Temperature dependence of the dielectric permittivity of BaTiO3-CoFe2O4 ceramic composites

Two-phase ceramic composites of cobalt ferrite dispersed in a barium titanate matrix were prepared and their dielectric permittivity was characterized. A rounding of their dielectric constant near the ferroelectric transition temperature of the BaTiO3 phase was observed. A generalized brick-wall model, taking into account the BaTiO3 contribution, fitted with the Landau-Devonshire theory, and the CoFe2O4 permittivity, experimentally determined, was used in order to obtain their effective dielectric constant. The effective model was applied to the discussion and fitting of the temperature dependence of the dielectric permittivity of the composites. The Landau-Devonshire coefficients for the BaTiO3 phase were also obtained.This work has been financially supported by the Portuguese Foundation for Science and Technology (FCT), through the project POCI/CTM/60181/2004

Influence of Grain Size Dispersion on the Magnetic Properties of Nanogranular BaTiO3-CoFe2O4 Thin Films

Thin film nanogranular composites of cobalt ferrite (CoFe2O4) dispersed in a barium titanate (BaTiO3) matrix were deposited by laser ablation with different cobalt ferrite concentrations (x). Their structural and magnetic properties were characterized. The films were polycrystalline and composed by a mixture of tetragonal-BaTiO3 and CoFe2O4 with the cubic spinel structure. A slight (111) barium titanate phase orientation and (311) CoFe2O4 phase orientation were observed. The lattice parameter of the CoFe2O4 was always smaller than the bulk value indicating that the cobalt ferrite was under compressive stress. From atomic force microscopy a broad distribution of grain sizes was observed in the nanocomposites, with a significant amount of smaller grains (<40nm) from the CoFe2O4 phase. The magnetic measurements show an increase of the magnetic moment from the low concentration region where the magnetic grains are more isolated and their magnetic interaction is small, towards the bulk value for higher CoFe2O4 content in the films. A corresponding decrease of coercive field with increasing cobalt ferrite concentration was also observed, due to the higher inter-particle magnetic interaction (and reduced stress) of the agglomerated grains.This work has been financially supported by the Portuguese Foundation for Science and Technology (FCT) and FEDER, through the project POCI/CTM/60181/2004

Structural and magnetic properties of nanogranular BaTiO3-CoFe2O4 thin films deposited by laser ablation on Si/Pt substrates

Thin film nanocomposites of cobalt ferrite (CoFe2O4) dispersed in barium titanate (BaTiO3) matrix, have been deposited with different cobalt ferrite concentrations (from 20% to 70% CoFe2O4), as well as pure barium titanate and cobalt ferrite thin films (end members). The films were prepared by pulsed laser ablation on platinum covered Si(001) substrates. The films structure was studied by X-ray diffraction and their surface was examined by scanning electron microscopy (SEMI). The magnetic properties were measured in a SQUID magnetometer. The results show that the deposited films are polycrystalline with a slight (111) barium titanate phase orientation and (311) COFe2O4 phase orientation. The grain sizes measured from the X-ray diffraction peak widths, for both phases. arc in the range 40nm to 100nm. However. as the concentration of the cobalt ferrite increases. the grain size of the BaTiO3 phase decreases, from 100nm to 30nm, up to 40% CoFe2O4 concentration beyond which the BaTiO3 grain size has an approximately constant value near 30nm. On the other hand the cobalt ferrite grain size does not show a clear trend with increasing cobalt ferrite concentration. fluctuating in the range 20nm to 30nm. The magnetic measurements show an increase of the magnetic moment from the low concentration region where the, magnetic grains are more isolated and their magnetic interaction is small, towards the bulk value at higher CoFe2O4 concentrations. Also, a strong reduction of the magnetization with increasing temperature was observed, due to the corresponding decrease of the magnetocristalline anisotropy of the cobalt ferrite.This work has been financially supported by the Portuguese Foundation for Science and Technology (FCT), through the project POCI/CTM/60181/2004

Stress induced magnetic anisotropy on BaTiO3-CoFe2O4 nanogranular composite thin films

The influence of stress on the magnetic properties of BaTiO3-CoFe2O4 nanocomposites deposited by laser ablation on platinum covered Si(001) substrates, was characterized. The cobalt ferrite phase was under compressive strain in the growth direction that progressively relaxed as its concentration increased. A stress induced perpendicular magnetic anisotropy was observed, that decreased with increasing CoFe2O4 content, due to the relaxation of stress in the films.This work has been financially supported by the Portuguese Foundation for Science and Technology (FCT) and FEDER, through the projects POCI/CTM/60181/2004, FEDER/POCTI/0155/94, and through the Associated Laboratory-IN

Cobalt ferrite thin films deposited by electrophoresis on p-doped Si substrates

The structural and magnetic properties of cobalt ferrite (CoFe2O4) thin films deposited by electrophoresis on p-doped Si(001) substrates have been characterized. The films were polycrystalline and composed by cobalt ferrite with the cubic spinnel structure. The observed decrease of the coercive field with the sixth power of the grain size was indicative of a competition between the magnetocrystalline anisotropy and the exchange coupling energy, on these randomly oriented nanosized grained films.J. Barbosa and M.P. Proenca gratefully acknowledge a PhD grant from Fundacao para a Ciencia e Tecnologia (SFRH/BD/41913/2007 and SFRH/BD/43440/2008, respectively)

Structural and magnetic properties of nanogranular BaTiO3-CoFe2O4 thin films deposited by laser ablation on Si/Pt substrates

Thin film nanogranular composites of cobalt ferrite (CoFe2O 4) dispersed in a barium titanate (BaTiO3) matrix were deposited by laser ablation with different cobalt ferrite concentrations (x). The films were polycrystalline and composed by a mixture of tetragonal- BáTiO3 and CoFe2O4 with the cubic spinnel structure. A slight (111) barium titanate phase orientation and (311) CoFe2O4 phase orientation was observed. As the concentration of the cobalt ferrite increased, the grain size of the BaTiO 3 phase decreased, from 91nm to 30nm, up to 50% CoFe 2O4 concentration, beyond which the BaTiO3 grain size take values in the range 30-35nm. On the other hand the cobalt ferrite grain size did not show a clear trend with increasing cobalt ferrite concentration, fluctuating in the range 25nm to 30nm. The lattice parameter of the CoFe2O4 phase increased with increasing x. However, it was always smaller than the bulk value indicating that, in the films, the cobalt ferrite was under compressive stress that was progressively relaxed with increasing CoFe2O4 concentration. The magnetic measurements showed a decrease of coercive field with increasing x, which was attributed to the relaxation of the stress in the films and to the increase of particle agglomeration in bigger polycrystalline clusters with increasing cobalt ferrite concentration.This work has been financially supported by the Portuguese Foundation for Science and Technology (FCT), through the project POCI/CTM/60181/2004

Structural and magnetic characterization of LaSrMnO3 thin films deposited by laser ablation on MgO substrates

La2/3Sr1/3MnO3-delta thin films were deposited by laser ablation on MgO substrates under low oxygen pressure cool down. Their structural and magnetic properties are presented. The magnetic and electrical resistivity measurements indicate a reduction of the Curie and the metal-insulator transition temperatures due to the formation of magnetic inhomogeneneous films, where clusters of a metallic phase are mixed in a magnetically disordered insulating matrix. By a low-angle X-ray reflectivity study we show that the thin films are chemically inhomogeneous with an oxygen deficiency in bulk of the film when compared with the film/air interfacial region

High-field magnetoresistance of La0.67Sr0.33MnO3 thin films deposited on LiNbO3 substrates

Colossal magnetoresistive manganites have been widely studied due to their potential use in sensor and device applications. In this work, La0.67Sr0.33MnO3 thin films were deposited by pulsed laser ablation on LiNbO3 substrates and magnetoresistance measurements were performed using pulsed magnetic fields up to 25 T. The corresponding magnetoconductance of the films was fitted in order to obtain the grain boundary (GB) contribution to the transport properties. The observed temperature dependence of the fitting parameters was indicative of antiferromagnetism across GB spins and reflected the progressive reduction of magnetic ordering with increasing temperature.This work was supported in part by Project No. FEDER/POCTI 155/94 from Fundação para a Ciência e Tecnologia (FCT). I.T. Gomes, A. Pereira and J. Barbosa gratefully acknowledge Ph.D. grants (SFRH/BD/36348/2007, SFRH/BD/22373/2005 and SFRH/BD/41913/2007, respectively) from FCT

Nano-estruturas compósitas multiferroicas

Tese de doutoramento - Programa Doutoral em Física (MAP-FIS)Os materiais multiferroicos magnetoelétricos, caracterizam-se por apresentem um acoplamento entre os seus graus de liberdade elétricos e magnéticos, e têm despertado, em anos recentes, um grande interesse científico e tecnológico. A origem deste acoplamento pode, por um lado, ser intrínseca ao próprio material, ou pode resultar de um produto de propriedades de dois materiais distintos. Combinando um cerâmico piezoelétrico com um material magnetostritivo, as interações elásticas entre as fases, resultam num acoplamento mecânico que, por sua vez, dá origem a um comportamento magnetoelétrico do compósito. Nas estruturas compósitas, existem três geometrias principais de mistura entre fases. A geometria 2- 2 que consiste numa multicamada, a geometria 1-3 em pilares da fase magnetostritiva dispersos numa matriz piezoelétrica, e a geometria 0-3 em grãos da fase magnetostritiva dispersos na numa matriz piezoelétrica. Apesar da elevada área de interface entre fases, a elevada condutividade associada aos materiais magnetostritivos e a consequente elevada corrente de fuga, dificulta a aplicação de campos elétricos nas geometrias do tipo 1-3. Por outro lado, nos filmes com estruturas do tipo 2-2, o substrato limita as deformações mecânicas das fases. Torna-se assim importante desenvolver novas estruturas com geometrias mais favoráveis que evitem ou limitem estes problemas, de modo a melhorar a resposta dos compósitos multiferroicos. Em filmes nano-estruturados, as suas propriedades magnetoelétricas, dependem fortemente da geometria de mistura de fases, assim como das suas propriedades morfológicas e estruturais. As propriedades de um filme magnetoelétrico compósito, são portanto em grande medida determinadas, pelas técnicas e parâmetros de deposição utilizados. O presente trabalho de doutoramento, foi dedicado ao estudo de filmes compostos por um cerâmico piezoelétrico, titanato de bário (BaTiO3), e por um material magnetostritivo, ferrite de cobalto (CoFe2O4) ou níquel (NiFe2O4), depositados sobre substratos de silício platinado. Foram utilizados dois métodos de deposição, dando origem a duas geometrias de mistura distintas. Por um lado, foram produzidos por ablação laser, filmes nanogranulares, onde grãos de ferrite de cobalto ou de níquel, estão dispersos numa matriz de titanato de bário, naquilo que constitui uma geometria do tipo 0-3. As propriedades estruturais e magnéticas destes filmes, foram estudadas em função da concentração de ferrite. Para tal, foram utilizadas técnicas como difração de raios-X (XRD), espetroscopia Raman, microscopia de força atómica (AFM) e magnetometria SQUID. Os resultados obtidos, demonstram que os filmes são policristalinos e compostos por grãos de titanato de bário com tamanhos que variam entre os 45nm e os 20nm, e por grãos de ferrite com tamanhos de grão da ordem de 30nm. No caso do sistema BaTiO3/CoFe2O4 observou-se que devido às diferenças de coeficientes de expansão térmicos do BaTiO3 e do CoFe2O4, a ferrite está sobre tensões mecânicas de tração no plano, que relaxam à medida que a sua concentração nas amostras aumenta. À tensão de tração no plano, é associado um desvio para o azul na posição dos modos Raman atribuídos ao CoFe2O4, naquilo que constitui o designado efeito piezoespetroscópico. É discutida ainda, a presença de uma anisotropia magnética perpendicular ao plano, em função das tensões de tração no plano, a que a fase de ferrite está sujeita. O mais baixo coeficiente de expansão térmica da ferrite de níquel em relação ao titanato de bário na fase cúbica, leva a que no sistema composto por BaTiO3/NiFe2O4, sejam observadas tensões mecânicas compressivas na fase de ferrite de níquel, tensões que resultam num desvio para o vermelho na posição dos modos Raman correspondentes. Por outro lado, foram também produzidas nano-estruturas compostas por grãos de ferrite de cobalto dispersos no substrato por deposição eletroforética, sendo posteriormente cobertos posteriormente por um filme de titanato de bário depositado por ablação laser. A estrutura resultante, pode ser considerada uma estrutura com um tipo de conectividade mista, consistindo numa estrutura do tipo 2-2, onde a camada superior é uma camada piezoelétrica e a camada inferior uma camada compósita, onde os grãos magnetostritivos estão dispersos numa matriz piezoelétrica como uma estrutura do tipo 1-3. Nesta geometria, é espectável que a área de interface seja maximizada relativamente às típicas estruturas em bicamada evitando as correntes de fuga características das estruturas do tipo 1-3. Foram caracterizadas as propriedades estruturais, dielétricas e magnéticas destes filmes, sendo dado especial enfase à forma como estas propriedades variam com a concentração relativa de ferrite. Para tal, foram utilizadas técnicas de caracterização como difração de raios-X, microscopia eletrónica de varrimento (SEM), espectroscopia de raios-X por dispersão em energia (EDX), espetroscopia Raman, espetroscopia dielétrica, de impedâncias e magnetometria SQUID. Foi observada a natureza policristalina das amostras, sendo compostas por CoFe2O4 numa estrutura de espinela cúbica, e por BaTiO3 numa estrutura onde coexistem as suas fases ortorrômbica e tetragonal. Observou-se que, tal como nas amostras nanogranulares, a fase de ferrite, está sujeita a tensões de tração no plano. Por outro lado, a fase de BaTiO3 está sujeita a tensões de compressão no plano. Às tensões mecânicas sobre as duas fases, foram também associados desvios na posição dos modos Raman característicos, tendo sido possível a determinação dos coeficientes piezoespetroscópicos correspondentes. Na caracterização destas amostras, dedicou-se particular interesse à relação entre as suas propriedades estruturais, estabilização das fases do titanato de bário (ortorrômbica, tetragonal e cúbica) e propriedades dielétricas. Identificou-se a transição estrutural entre as fases tetragonal e cúbica do BaTiO3 a ~380K, desviada portanto para baixas temperaturas, relativamente ao valor de estrutura maciça. Este desvio foi associado ao tamanho de grão, através de um modelo baseado na teoria de Landau-Ginzburg, que considera os grãos de titanato de bário como possuindo uma estrutura núcleo-capa, tendo a camada exterior uma estrutura cúbica e o núcleo uma estrutura tetragonal. Foi também observado um comportamento relaxor, associado à coexistência das fases tetragonal e ortorrômbica na região da temperatura ambiente. Através de ajustes com a equação de Cole-Cole aos resultados de espetroscopia dielétrica, e de ajustes com um circuito equivalente R-CPE aos resultados de espetroscopia impedâncias, identificaram-se quais os principais mecanismos físicos que determinam as propriedades dielétricas destas estruturas. Verificou-se também, que as propriedades magnéticas destas amostras, são em grande medida determinadas pelo processo de produção dos grãos de ferrite de cobalto. Por último, as medidas de espetroscopia Raman em função de um campo magnético, permitiram, nos dois tipos de amostras, encontrar evidências do acoplamento mecânico entre as duas fases. Em particular, o modo E(TO)+A1(TO) do BaTiO3, altera a sua posição com o aumento do campo magnético, devido ao acoplamento mecânico com a fase magnetostritiva de CoFe2O4. É discutido o efeito da concentração de ferrite e da geometria das fases, no acoplamento observado.In recent years, multiferroic magnetoeclectric materials, presenting a coupling between their electric and magnetic degrees of freedom, have been attracting much scientific and technological interest. From the point of view of materials design, this coupling can arise from an intrinsic origin (single-phase), or result from a product of properties of distinct materials (composites). By combining a piezoelectric ceramic and a magnetostrictive material, the elastic interactions between them provide the coupling mechanism inducing a magnetoelectric behavior. On composite structures, the phases can be organized in three main different geometries. The 2-2 geometry consists on a multilayer, in the 1-3 geometry magnetostrictive pillars are dispersed on a piezoelectric matrix and on the 0-3 geometry magnetostrictive grains are dispersed on a piezoelectric matrix. Although with a large interface area between phases, on the 1-3 geometry, the low electrical resistivity associated to magnetostrictive materials leads to leakage currents that limit the application of electric fields. On the other hand, on films with 2-2 structure, the substrate clamping effect, limits the mechanical deformations of phases. This opens the need to search for other potentially more favorable geometries. On composite films, their magnetoelectric response depends critically of their phase mixture geometry, as well as on their structural and morphological properties. For that, suitable deposition techniques and processing parameters need to be chosen and characterized. This thesis, is dedicated to the study of thin films composed by a piezoelectric ceramic, barium titanate (BaTiO3), and a magnetostrictive material, cobalt ferrite (CoFe2O4) or nickel ferrite (NiFe2O4), with different composite geometries. To do that, two different deposition methods have been used, and two different geometries have been obtained. In one hand, nanogranular films, where cobalt or nickel ferrite grains are dispersed on a barium titanate matrix, have been deposited by laser ablation. Their structural and magnetic properties have been studied as a function of ferrite content in the sample. For that, X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM) and SQUID magnetometry were used. The films present a polycrystalline structure, composed by a mixture of barium titanate and ferrite with a cubic spinel structure. In the case of BaTiO3/CoFe2O4 system, the results have shown that, due to differences in the BaTiO3 and CoFe2O4 thermal expansion coefficients, the CoFe2O4 grains are under a tensile stress on plane, that relaxes as the ferrite concentration increases in the samples. The tensile stress was observed to induce blue-shifts on the CoFe2O4 Raman modes positions, in the so called piezospectroscopy effect. the presence of an out-of-plane magnetic anisotropy, induced by the stress state of the ferrite, is also discussed. The smaller NiFe2O4 thermal expansion coefficient, relative to cubic-barium titanate, leads to a compressive stress state of the ferrite phase on the BaTiO3/NiFe2O4 system. This stress state, in turn, gives rise to a red-shift on the nickel ferrite Raman modes. On the other hand, composite films have been obtained by electrophoretic-deposited CoFe2O4 grains, covered by a laser-ablated BaTiO3 layer. The final structure can be considered as a structure with mixed connective types: in one hand it consists of a 2-2 type structure with an upper piezoelectric layer and a bottom composite layer, on the other hand, on the composite bottom layer the magnetostrictive grains are dispersed on a piezoelectric matrix forming a 1-3 structure. In this mixed geometry it is expected that the interface area between the phases is increased relatively to typical bilayer films, while avoiding the 1-3 geometry leakage currents. The structural, dielectric and magnetic properties of these thin film systems have been characterized, as a function of changes in the CoFe2O4 content. For that, characterization techniques, such as X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), dielectric and impedance spectroscopy and SQUID magnetometry were used. The samples are polycrystalline and composed by CoFe2O4 in a cubic spinel structure and BaTiO3 with a structure with coexisting tetragonal and orthorhombic phases. As in laser-ablated nanogranular samples, the ferrite phase is under a tensile stress on the sample plane. On the other hand, the BaTiO3 phase is under a compressive stress. The mechanical stress on the two phases has been associated to Raman modes shifts and the corresponding piezospectroscopy coefficients have been determined. The relation between the structure and BaTiO3 phase stabilization (orthorhombic, tetragonal, cubic) and their dielectric properties on these samples is particularly interesting. The barium titanate tetragonalcubic phase transition has been identified at 380K, shifted to lower temperature relatively to bulk. This shift has been associated to the grain size effect, considering a model based on Landau-Ginzburg theory, where BaTiO3 grains are composed by a core-shell structure, with a cubic structure on shell and a tetragonal structure on core. Additionally a relaxor behavior has been observed and associated to the coexistence of barium titanate orthorhombic and tetragonal phases in the room temperature region. In addition with fits with Cole-Cole equation to dielectric spectroscopy results, and with R-CPE equivalent circuit to impedance spectroscopy results, the main physical mechanisms that determine the dielectric properties of these nanocomposites have been identified by fits to dielectric and impedance spectroscopy measurements. Also, it has been found that the magnetic properties are strongly dependent on the cobalt ferrite grains production process. In both kinds of films, evidence of stress-mediated magnetoelectric coupling between the phases was found by magnetic field dependent Raman spectroscopy. In particular, the BaTiO3 E(TO)+A1(TO) mode frequencies changed their position with increasing magnetic field, due to induced strains from the magnetostrictive CoFe2O4 phase. The effect of strains on the different phases is discussed.Fundação para a Ciência e Tecnologia (FCT) (SFRH/BD/41913/2007)

Barium titanate thin films deposited by electrophoresis on p-doped si(001) substrates

Barium titanate (BaTiO3) thin films have been prepared by electrophoretic deposition on p-doped and platinum covered silicon (Si) substrates. Their structure, nanostructure and dielectric properties were characterized. The as-deposited films were polycrystalline and composed by barium titanate nanograins with an average grain size ~9 nm. Annealing at high temperatures promoted grain growth, so that the samples annealed at 600 ºC presented average grain sizes ~24 nm. From Raman spectroscopy measurements it was found that the tetragonal (ferroelectric) BaTiO3 phase was stabilized on the films. Also, at higher annealing temperatures, cation disorder was reduced on the films. From measurements of the temperature dependence of the dielectric permittivity the corresponding paraelectric-ferroelectric phase transition was determined. The observed transition temperature (~100ºC) was found to be below the BaTiO3 bulk or thick film values, due to the small nanosized grains composing the films.Fundação para a Ciência e Tecnologia (FCT)FEDE