Ashok Kumar

In the last decade, a tremendous amount of research works have been carried out in the area of multiferroic nanostructure materials in order to meet the requirement of science and technology for the future microelectronic industries. The potential functionalities of any kind nanostructure materials lies in the vicinity of large surface to volume ratio compare to the bulk counterpart. The new technology for microelectronics require small dimension, robust mechanical properties, higher degree of electrical and electronic properties and finally high output power with low dissipation energy. The area of ferroelectric and multiferroic nanostructures are vast, this chapter deals with the different variety of multiferroic nanostructure with different dimensionality (D) 0D 1D, 2D and 3D and their state of art characterization process. Possible and feasible theoretical modeling and related experimental facts of already established multiferroics nanostructures will be described. It will also cover the advantages and the limitations of these nanostructures in design, development, and particularly for device applications.

We report nearly single phase Pb(Zr0.14Ti0.56Ni0.30)O3-delta(PZTNi30) ferroelectric having large remanent polarization (15-30 {\mu}C/cm2), 0.3-0.4 V open circuit voltage (VOC), reduced band gap (direct 3.4 eV, and indirect 2.9 eV), large ON and OFF photo current ratio, and the fast decay time. Reasonably good photo current density (1-5 {\mu}A/cm2) was obtained without gate bias voltage which significantly increased with large bias field. Ferroelectric polarization dictates the polarity of VOC and direction of short circuit current (ISC), a step forward towards the realization of noncentrosymmetric ferroelectric material sensitive to visible light.

In‐plane temperature dependent dielectric behavior of BiFeO₃ (BFO) as‐grown thin films show diffuse but prominent phase transitions near 450 (±10) K and 550 K with dielectric loss temperature dependences that suggest skin layer effects. The 450 K anomalies are near the “transition” first reported by Polomska et al. [Phys. Status Solidi 23, 567 (1974)]. The 550 K anomalies coincide with the surface phase transition recently reported [Marti et al., Phys. Rev. Lett. 106, 236101 (2011)]. In addition, anomalies are found at low temperatures: After several experimental cycles, the dielectric loss shows a clear relaxor‐like phase transition near what was previously suggested to be a spin reorientation transition (SRT) temperature (∼201 K) for frequencies 1 kHz < f < 1 MHz which follow a nonlinear Vogel–Fulcher (V–F) relation; an additional sharp anomaly is observed near ∼180 K at frequencies below 1 kHz. As emphasized recently by Cowley et al. [Adv. Phys. 60, 229 (2011)], skin effects are expected for all relaxor ferroelectrics. Using the interdigital electrodes, experimental data and a theoretical model for in‐plane longitudinal and transverse direct magnetoelectric (ME) coefficient are presented.

One-dimensional ferroelectric nanostructures, carbon nanotubes (CNT) and CNT-inorganic oxides have recently been studied due to their potential applications for microelectronics. Here, we report coating of a registered array of aligned multi-wall carbon nanotubes (MWCNT) grown on silicon substrates by functional ferroelectric Pb(Zr,Ti)O3 (PZT) which produces structures suitable for commercial prototype memories. Microstructural analysis reveals the crystalline nature of PZT with small nanocrystals aligned in different directions. First-order Raman modes of MWCNT and PZT/MWCNT/n-Si show the high structural quality of CNT before and after PZT deposition at elevated temperature. PZT exists mostly in the monoclinic Cc/Cm phase, which is the origin of the high piezoelectric response in the system. Low-loss square piezoelectric hysteresis obtained for the 3D bottom-up structure confirms the switchability of the device. Current-voltage mapping of the device by conducting atomic force microscopy (c-AFM) indicates very low transient current. Fabrication and functional properties of these hybrid ferroelectric-carbon nanotubes is the first step towards miniaturization for future nanotechnology sensors, actuators, transducers and memory devices.

In-plane temperature dependent dielectric behavior of BiFeO3 (BFO) as-grown thin films show diffuse but prominent phase transitions near 450 (+/-10) K and 550 K with dielectric loss temperature dependences that suggest skin layer effects. The 450 K anomalies are near the "transition" first reported by Polomska et al. [Phys. Stat. Sol. 23, 567 (1974)]. The 550 K anomalies coincide with the surface phase transition recently reported [Xavi et al. PRL 106, 236101 (2011)]. In addition, anomalies are found at low temperatures: After several experimental cycles the dielectric loss shows a clear relaxor-like phase transition near what was previously suggested to be a spin reorientation transition (SRT) temperature (~ 201 K) for frequencies 1 kHz < f < 1MHz which follow a nonlinear Vogel-Fulcher (V-F) relation; an additional sharp anomaly is observed near ~180 K at frequencies below 1 kHz. As emphasized recently by Cowley et al. [Adv. Phys. 60, 229 (2011)], skin effects are expected for all relaxor ferroelectrics. Using the interdigital electrodes, experimental data and a theoretical model for in-plane longitudinal and transverse direct magnetoelectric (ME) coefficient are presented.

Millimeter-diameter planar devices of glass/ZnO:Al/BiFeO₃/La_0.67Sr_0.33CoO₃ (LSCO) heterostructures were fabricated by pulsed laser deposition (PLD) techniques. Diode-like behavior with high short-circuit current (SSC ∼ 4 mA/cm²) and open-circuit voltage (OCV ∼ 0.22 V) was obtained under the illumination of about 1% of maximum solar energy. Impedance spectroscopy revealed that electrode/dielectric interface and grain-boundary conduction are mainly responsible for the photo-current. Electrode/dielectric interface, grain boundary impedance, and low-frequency ac conductivity change by almost three orders of magnitude under weak light. Relaxation time of the photo-carriers changes from 80 ms to 96 μs suggesting that with optimal collecting instruments, one should expect currents several orders higher.

Here, we report the tuning of room-temperature magnon frequencies from 473 GHz to 402 GHz (14%) and magnetic moment from 4 to 18 emu/cm³ at 100 Oe under the application of external electric fields (E) across interdigital electrodes in BiFeO₃ (BFO) thin films. A decrease in magnon frequencies and increase in phonon frequencies were observed with Magnon and phonon Raman intensities are asymmetric with polarity, decreasing with positive E (+E) and increasing with negative E (−E) where polarity is with respect to in-plane polarization P. The magnetoelectric coupling (α) is proved to be linear and a rather isotropic α = 8.5 × 10⁻¹² sm⁻¹.

Epitaxial biferroic relaxors of formula Pb[(Zr_0.53Ti_0.47)_0.4(Fe_0.67W_0.33)_0.6]O₃ (PZTFW) were grown by pulsed laser deposition (PLD) techniques. Raman spectra revealed a global Fm3m symmetry with two high-frequency A_1g modes that imply the existence of polar nano-regions (PNRs). Near room temperature, frequency-dependent dielectric patterns follow Vogel-Fulcher (VF) relation that implies relaxor ferroelectric phase, however, a frequency-dependent, high-temperature, Maxwell-Wagner space charge-based phase transition was also noticed. A large and irreversible magnetization with a sharp cusp in zero-field-cooled (ZFC) magnetization with low ZFC magnetic field advocates Heisenberg spin-glass behavior. Observation of slim ferroelectric hysteresis below the freezing temperature (T_f), PNRs, and spin glass confirms biferroic-relaxor nature.