but also have the potential ability to couple electric and magnetic polarizations which provides an additional degree of freedom in device design and applications. Consequently, ferro-electromagnetism is the subject of intensive investigations because these materials potentially offer a whole range of applications, including the emerging fields of spintronics, data-storage media, and multiple-state memories [2-6]. Ferromagnetic and ferroelectric ordering parameters are widely used to store binary information in magnetoresistive random access memory (MRAMs) devices [7] and ferroelectric random access memory (FeRAMs) devices [8], respectively. Unfortunately, ferroelectric ferromagnets (or ferrimagnets) are very scarce and the search for a material with both large and finite polarization and magnetization at room temperature is still in progress. To reach this goal, the first step is to obtain materials with magneto-electric coupling. Among all known multiferroics, the only compound that satisfies these criteria is bismuth ferrite (BFO). First synthesized in the late 1950s [9], BFO was shown to be a G-type antiferromagnet with a Néel temperature of 630 K by Kiselev et al. [10]. Later, Sosnowska et al. showed that the magnetic order of bulk BFO is not strictly collinear and that a cycloidal modulation with a period of 62 nm is present [11]. The ferroelectric mechanism in BFO is controlled by the stereochemical activity of the Bi 3+ 6s 2 lone pair, responsible of a charge transfer process from 6s 2 to formally empty 6p orbitals [12, 13] while the weak ferromagnetic property can be associated to the residual moment from the canted Fe 3+ spin structure [14]. The coupling effect between both magnetic and electric behaviours occurs through lattice distortion of BFO [15] and Khomskii has been emphasized the different ways to combine magnetism and ferroelectricity in mutiferroics materials [16]. The low thermal stability of BiFeO 3 provides an obstacle to conventional ceramics processing of this material, as the processing window is very narrow. The formation of
Pure and calcium-modified (Ca x Bi 1-x FeO 3 , x = 0.0, 0.1, 0.2, 0.30) thin films were fabricated on Pt(111)/Ti/SiO 2 /Si substrates by the soft chemical method using LaNiO 3 as the bottom electrode. Highly (200)oriented BFO film was coherently grown on LNO at 500°C. Ca-doped BiFeO 3 films have a dense microstructure and rounded grains. The conventional problem of the leakage current for the highest doped film was reduced from 10-5 to 10-10 with remarkable improvement in the film/electrode interface, chemical homogeneity, crystallinity, and morphology of the BFO film. Enhanced ferroelectricity was observed at room temperature due to the bottom electrode. Fatigue-free films were grown on LaNiO 3 bottom electrodes with no degradation after 1×10 10 switching cycles at an applied voltage of 5 V with a frequency of 1 MHz. After several tests the capacitors retained 77% of its polarization upon a retention time of 10 4 s. Room temperature magnetic coercive field measurements indicate that the magnetic behaviour is influenced by the nature of the bottom electrode.
Bi 3 NbO 7 (BNO) thin films were deposited on Pt/TiO 2 /SiO 2 /Si (100) and LaNiO 3 bottom electrode substrates at room temperature from the polymeric precursor method. X-ray powder diffraction was used to investigate the formation characteristics and stability range of the tetragonal modification of a fluorite-type solid solution. The results showed that this tetragonal, commensurately modulated phase forms through the intermediate formation of the incommensurately modulated cubic fluorite phase followed by the incommensurate-commensurate transformation. LaNiO 3 (LNO) bottom electrode strongly promotes the formation of high intensity (111) texture of BNO films. The dielectric constants of the films increased from 192 to 357 at 1 MHz with the bottom electrode while the leakage current behavior at room temperature of the films decreased from 10-7 to 10-8 A/cm 2 at a voltage of 5 V. The reduction of dc leakage current is explained on the basis of relative phase stability and improved microstructure of the material. The capacitance density of 75 fC/lm 2 , dielectric loss of 0.04 % at 1 MHz, and breakdown strength of about 0.30 MV/cm is compatible with embedded decoupling capacitors applications.
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