We have investigated temperature, frequency, and magnetic field dependent dielectric properties of polycrystalline Bi 0.9Ca 0.1FeO 2.95. Two dielectric anomalies, near 65 K and 260 K, were observed with the anomaly near 65 K exhibiting distinct frequency dependence as the peak temperature shifts with increasing frequency. The low-temperature dielectric relaxation data that can be fitted to a Vogel-Fulcher expression yielding a characteristic relaxation time of ∼10 -8 s are four orders larger than that of pure BiFeO 3 which may be the resultant of the chemical pressure induced by Ca doping. We also observed a switchable magneto-dielectric response in Bi 0.9Ca 0.1FeO 2.95 at room temperature. © 2012 American Institute of Physics.

Mamidanna Sri Ramachandra Rao

Department Of Physics

CHARACTERISTIC RELAXATION TIME

CHEMICAL PRESSURES

Dielectric anomaly

FOUR-ORDER

Frequency dependence

Low temperatures

Magneto-dielectric effects

Peak temperatures

Polycrystalline

Room temperature

SWITCHABLE

VOGEL-FULCHER

Bismuth

Bismuth compounds

Dielectric properties

Dielectric relaxation

Iron oxides

Magnetic fields

Magnetos

calcium

Fulltext

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IIT Madras

Dielectric relaxation and magneto-dielectric effect in polycrystalline Bi0.9Ca0.1FeO2.95

Applied Physics Letters

Temperature-dependent thermal conductivity of polycrystalline BiFeO3, Bi0.9Ba0.05Ca0.05FeO2.95 and Bi0.9Ca0.1FeO2.95 materials was measured using a direct heat pulse technique. Thermal conductivity of the BiFeO3-based materials is analyzed using a phonon model to probe the thermal transport mechanisms in these ferrites. It is found that the calculated thermal conductivity of the BiFeO3-based compounds is in good agreement with experimental data. The suppression of the low-temperature phonon peak in the thermal conductivity of the doped BiFeO3 materials is mainly attributed to the phonon-point-defect scattering. In addition, the contribution of optical phonon-magnon resonance scattering to optical phonon thermal transport reveals the presence of phonon-magnon coupling in these BiFeO3 materials. Finally, magneto-thermal conductivity measurements show the magnon thermal transport in pure and doped BiFeO3 systems. © 2015 IOP Publishing Ltd.

Journal of Physics D: Applied Physics

Phonon thermal transport and phonon-magnon coupling in polycrystalline BiFeO3 systems

Wedemonstrate that oxygen vacancies (VO) produced by aliovalent (Ca2+) doping in BiFeO3 (BCFO) and associated structural changes due toVO ordering result in systematic alteration of the bandgap (Eg) over a wide range from 1.5 eV to 2.3 eV. By contrast, the change in the bandgap of aCa2+ and Ti4+ co-doped BiFeO3 (BCFTO) system, wherein theVO formation is suppressed, is negligible. These contrastive results strongly confirm the role of oxygen vacancies in altering the bandgap of BCFO. Irrespective of doping, microstrain, which is found to be large (0.3 to 1.2%) below a critical size (dc.Q60 nm) also produces a small, yet linear change in the bandgap (Eg from 2.0 to 2.3 eV). The cubic phase stabilizes gradually in BCFO for xQ&gt;Q0-1 through an orthorhombic phase (for 0.05Q&lt;QxQ&lt;Q0-1), whereas it directly transforms for xQ&gt;Q0-1 in BCFTO. This change in BCFO at 300 Ksuggests a highpressure-like (or high-temperature-like) effect of the oxygen vacancies and dopants on the structure. Systematic variations in the relative intensities and peak positions of Fe d-d transitions in BCFO reveal the local changes in Fe-O-Fe coordination. These results along with XANES andHRTEMstudies substantiate the observed structural changes. © 2015 IOP Publishing Ltd.

Materials Research Express

Wide-range tunable bandgap in Bi1-xCaxFe1-yTiyO3-δ nanoparticles via oxygen vacancy induced structural modulations at room temperature

The influence of oxygen vacancies on the dielectric relaxation behavior of pure and Eu-substituted BiFeO3 nanoparticles synthesized by a sol-gel technique has been studied using impedance spectroscopy in the temperature range of 90 °C to 180 °C. The electric relaxation time and activation energy of the oxygen vacancies can be calculated from the Arrhenius equation, and found to be 1.26 eV and 1.76 eV for pure and Eu-substituted BiFeO 3, respectively. Substitution induces structural disorder and changes in the Fe-O-Fe bond angle, leading to alteration of the magnetic properties, observed from magnetic studies and evaluated using Rietveld refinement of the XRD patterns. X-ray photoelectron spectroscopy (XPS) confirms the shifting of the binding energy of the Bi 4f orbital, establishing Eu substitution at the Bi site. Calculation of the area under the Fe2+/Fe3+ (2p) and O (1s) XPS spectra gives approximate values of the oxygen vacancies. © 2014 the Partner Organisations.

Dalton Transactions

Role of oxygen vacancy and Fe-O-Fe bond angle in compositional, magnetic, and dielectric relaxation on Eu-substituted BiFeO3 nanoparticles

We have investigated diffuse reflectance spectra of the polycrystalline BiFeO 3, Bi 0.9Ba 0.05Ca 0.05FeO 2.95, and Bi 0.9Ca 0.1FeO 2.95 samples to demonstrate crystal-field spectroscopy, which enabled us to explore local distortion in the non-centrosymmetric FeO 6 octahedron, and also to study chemical pressure effect on the band gap and d-d transition band energies. The energy values of band gap and d-d transition bands of the doped BiFeO 3 samples were found to red-shift with the reduced unit cell volume. Absorption spectroscopic studies of the BiFeO 3 samples reveal that nature of the band gap is a direct. Raman spectroscopy study also reveals that the softening/hardening of the certain Raman modes in BiFeO 3 upon substitution of Ca. The chemical pressure induced changes in the band gap and d-d band energies, and the Raman modes of the Ca doped BiFeO 3, which also found to enhance its magnetic and magnetoelectric properties. © 2012 American Institute of Physics.

Journal of Applied Physics

Chemical pressure effect on optical properties in multiferroic bulk BiFeO 3

Structural and magnetic properties of polycrystalline BiFeO3, Bi0.9Ca0.1FeO2.95, Bi0.9Ba 0.05Ca0.05 FeO2.95, and Bi0.9Ba 0.1FeO2.95 ceramic samples were studied to establish the effects of doping in BiFeO3 on the magnetic property. X-ray diffraction data of the undoped and doped BiFeO3 samples were refined to a rhombohedral structure with space group R3c. X-ray photoelectron spectroscopy study showed the formation of a single-phase in both the undoped and doped BiFeO3 ceramics with Fe in the 3 valence state. Ca doped and Ba-Ca co-doped BiFeO3 ceramic samples show weak ferromagnetic ordering at room temperature. This observation makes Ca doped and Ba-Ca co-doped BiFeO3 samples an interesting material system for magnetoelectric coupling studies. © 2012 American Institute of Physics.

Weak ferromagnetic ordering in Ca doped polycrystalline BiFeO3

Our investigations using low-temperature Raman spectroscopy, dielectric, and magnetodielectric studies of as-prepared and vacuum-annealed BiFeO 3 ceramics revealed two dielectric anomalies near 25 and 281 K. Systematic studies were carried out to probe the anomaly near 25 K which exhibits clear frequency dependence and falls close to a previously observed magnetic transition. The low-temperature dielectric relaxation behavior yielded characteristic energy scales of ∼30 and ∼34 meV for the as-prepared and vacuum-annealed BiFeO3 samples, respectively. Raman spectra showed hardening of the E(TO6) and E(TO9) modes near 25 K, and the Raman shift in phonon frequencies is found to be broader in the vacuum-annealed BiFeO 3 than in the as-prepared BiFeO3. We observed a linear magnetodielectric coupling in both samples at 25 K. These experimental results suggest that coupling among spin, lattice, and dielectric properties persist to low temperatures in BiFeO3. © 2011 American Institute of Physics.

Dielectric relaxation near 25 K in multiferroic BiFeO3 ceramics

We report on the structural, thermal, and magnetic properties of polycrystalline BiFeO3 synthesized by sol-gel route. Powder x-ray diffraction data of the BiFeO3 sample was refined with rhombohedral structure with space group R3c. Magnetization and coercive field measurements showed weak ferromagnetic nature below 10 K. We also investigated the low temperature magnetocaloric effect in polycrystalline BiFeO3 ceramics. The anomalies observed in the magnetic entropy change of the BiFeO3 ceramics were close to that of low temperature phase transitions. The combined relative errors in the magnetic entropy change are found to vary from 4% to 15% with increase in temperature. © 2009 American Institute of Physics.

Low temperature magnetocaloric effect in polycrystalline BiFeO3 ceramics

Physical Review B

Effect of chemical substitution on the Néel temperature of multiferroicBi1−xCaxFeO3

Journal of Physics: Condensed Matter

Elastic and electrical anomalies at low-temperature phase transitions in BiFeO3

Dielectric relaxation and magneto-dielectric effect in polycrystalline Bi 0.9Ca 0.1FeO 2.95

Journal	Applied Physics Letters
ISSN	00036951
Open Access	No