Sanjay Kumar Upadhyay

Electro-caloric effect in lead-free Sn doped BaTiO3 ceramics at room temperature and low applied fields

Structural, dielectric, ferroelectric (FE), 119Sn Mossbauer, and specific heat measurements of polycrystalline BaTi1–xSnxO3 (x = 0% to 15%) ceramics are reported. Phase purity and homogeneous phase formation with Sn doping is confirmed from x-ray diffraction and 119Sn Mossbauer measurements. With Sn doping, the microstructure is found to change significantly. Better ferroelectric properties at room temperature, i.e., increased remnant polarization (38% more) and very low field switchability (225% less) are observed for x = 5% sample as compared to other samples and the results are explained in terms of grain size effects. With Sn doping, merging of all the phase transitions into a single one is observed for x ≥ 10% and for x = 5%, the tetragonal to orthorhombic transition temperature is found close to room temperature. As a consequence better electro-caloric effects are observed for x = 5% sample and therefore is expected to satisfy the requirements for non-toxic, low energy (field) and room temperature b...

Evolution of magnetic and dielectric properties in Sr-substituted high-temperature multiferroic YBaCuFeO5

We report the evolution of structural, magnetic and dielectric properties due to partial substitution of Ba by Sr in the high temperature multiferroic YBaCuFeO5. This compound exhibits ferroelectric and antiferromagnetic transitions around 200 K and these two phenomena are presumed to be coupled with each other. Our studies on magnetic and dielectric properties of the YBa1-xSrxCuFeO5 (x = 0.0, 0.25 and 0.5) show that substitution of Sr shifts magnetic transition towards higher temperature whereas dielectric transition to lower temperature. These results points to the fact that magnetic and dielectric transitions get decoupled as a result of chemical pressure in form of Sr substitution. The nature of magnetodielectric coupling changes across the series with the presence of higher order coupling terms. Additionally in these compounds glassy dynamics of electric dipoles is observed at low temperatures.

Study of (1−x) BaTiO3–x Ni0.5Zn0.5Fe2O4 (x = 5, 10 and 15%) magneto-electric ceramic composites

Abstract Structural, magnetic, ferroelectric and magneto-electric studies of BaTiO3 (BTO)–x Ni0.5Zn0.5Fe2O4 (NZFO) ceramic composites are reported. The fraction of NZFO, i.e., ferrite phase is varied from 0% to 15%. The prepared ceramics are studied using X-ray diffraction, Raman spectroscopy, magnetization and ferroelectric measurements. Using X-ray diffraction and Raman spectroscopy measurements, the bi-phase formation of the composites is confirmed. From the high temperature magnetization measurements, anomaly in the magnetization is observed at ferroelectric transition temperature of BTO phase indicating the presence of magneto-electric coupling. Positive Up Negative Down (PUND) measurements are employed to capture the true ferroelectric polarization, i.e., true switched charge density (QS). The strength of ME coupling is studied by measuring the QS before and after magnetic poling and it is observed that for x = 10% composition the ME coupling is maximum for the studied compositions.

A rock-salt-type Li-based oxide, Li 3 Ni 2 RuO 6, exhibiting a chaotic ferrimagnetism with cluster spin-glass dynamics and thermally frozen charge carriers

The area of research to discover new Li containing materials and to understand their physical properties has been of constant interest due to applications potential for rechargeable batteries. Here, we present the results of magnetic investigations on a Li compound, Li3Ni2RuO6, which was believed to be a ferrimagnet below 80 K. While our neutron diffraction (ND) and isothermal magnetization (M) data support ferrimagnetism, more detailed magnetic studies establish that this ferrimagnetic phase exhibits some features similar to spin-glasses. In addition, we find another broad magnetic anomaly around 40–55 K in magnetic susceptibility (χ), attributable to cluster spin-glass phenomenon. Gradual dominance of cluster spin-glass dynamics with a decrease of temperature (T) and the apparent spread in freezing temperature suggest that the ferrimagnetism of this compound is a chaotic one. The absence of a unique freezing temperature for a crystalline material is interesting. In addition, pyroelectric current (Ipyro) data reveals a feature in the range 40–50 K, attributable to thermally stimulated depolarization current. We hope this finding motivates future work to explore whether there is any intriguing correlation of such a feature with cluster spin-glass dynamics. We attribute these magnetic and electric dipole anomalies to the crystallographic disorder, intrinsic to this compound.

Reduced leakage in epitaxial BiFeO3 films following oxygen radio frequency plasma treatment

Epitaxial BiFeO3 (BFO) films were deposited using pulsed laser deposition method. The prepared films were characterized using x-ray diffraction, x-ray reflectivity, ferroelectric loop tracer, and leakage current measurements before and after oxygen plasma treatment. The leakage current of the films, a crucial parameter in device applications, is observed to be reduced by two orders of magnitude with oxygen plasma treatment at room temperature. P-E hysteresis loops were observed in oxygen plasma treated BFO films. The observed results indicate the usefulness of oxygen radio frequency plasma treatment (RF 13.56 MHz), which is an effective and low temperature processing technique, in such lossy ferroelectric thin films.

Study of Aging & De-Aging Behaviour in Un-Doped Polycrystalline Ferroelectric BaTiO3

Aging and de-aging behavior of un-doped polycrystalline BaTiO3 (BTO) is studied with the P-E measurements. The double hysteresis (P-E) loop is observed for the aged sample and it is observed that the relationship between the aging time and the developed internal bias field is similar to as that of reported for acceptor doped BTO and other ferroelectric materials in literature. However, the relaxation rates are quite different as compared to that of acceptor doped BTO. This is explained in terms of defect concentration. The de-aging behaviour is studied with different methods viz., thermal relaxation, combined use of temperature and applied electric field. Heating the sample slightly above its TC and cooling in the presence of external applied dc- field is observed to completely remove the pinching and bring the internal bias field to zero.

Reduced leakage current and improved ferroelectricity in magneto-electric composite ceramics prepared with microwave assisted radiant hybrid sintering

Structural, electrical and magnetic properties of magneto-electric composite ceramics viz., 0.9 BaTi0.95Sn0.05O3 (BTSO)- 0.1 Ni0.8Zn0.2Fe2O4 (NZFO) prepared with microwave assisted radiant hybrid sintering (MARH) are reported. Phase purity and isovalent substitution of Ti4+ by Sn4+ of the samples is confirmed from x-ray diffraction and 119Sn Mossbauer measurements respectively. Significant suppression of leakage current and improvement of ferroelectricity is observed for the composites prepared with MARH. The observed results are explained in terms of uniform dispersion of ferrite (NZFO) phase in the ferroelectric (BTSO) matrix as evidenced from back-scattered scanning electron micrographs.

Pyrocurrent anomalies and intrinsic magnetodielectric behavior near room temperature in Li 2 Ni 2 Mo 3 O 12 , a compound with distorted honeycomb and spin-chains

Keeping current interests to identify materials with intrinsic magnetodielectric behaviour near room temperature and with novel pyroelectric current anomalies, we report temperature and magnetic-field dependent behavior of complex dielectric permittivity and pyroelectric current for an oxide, Li2Ni2Mo3O12, containing magnetic ions with (distorted) honey-comb and chain arrangement and ordering magnetically below 8 K. The dielectric data reveal the existence of relaxor ferroelectricity behaviour in the range 160–240 K and there are corresponding Raman mode anomalies as well in this temperature range. Pyrocurrent behavior is also consistent with this interpretation, with the pyrocurrent peak-temperature interestingly correlating with the poling temperature. 7Li NMR offer an evidence for crystallographic disorder intrinsic to this compound and we therefore conclude that such a disorder is apparently responsible for the randomness of local electric field leading to relaxor ferroelectric property. Another observation of emphasis is that there is a notable decrease in the dielectric constant with the application of magnetic field to the tune of about −2.4% at 300 K, with the magnitude varying marginally with temperature. Small loss factor values validate the intrinsic behaviour of the magnetodielectric effect at room temperature.

Magnetic behavior of metallic kagome lattices, Tb3Ru4Al12 and Er3Ru4Al12

We report the magnetic behavior of two intermetallic-based kagome lattices, Tb3Ru4Al12 and Er3Ru4Al12, crystallizing in the Gd3Ru4Al2-type hexagonal crystal structure, by measurements in the range 1.8-300 K with bulk experimental techniques (ac and dc magnetization, heat capacity, and magnetoresistance). The main finding is that the Tb compound, known to order antiferromagnetically below (T N =) 22 K, shows glassy characteristics at lower temperatures ([Formula: see text]15 K), thus characterizing this compound as a re-entrant spin-glass. The data reveal that the glassy phase is quite complex and is of a cluster type. Since glassy behavior was not seen for the Gd analog in the past literature, this finding on the Tb compound emphasizes that this kagome family could provide an opportunity to explore the role of higher-order interactions (such as quadrupole) in bringing out magnetic frustration. Additional findings reported here for this compound are: (i) The plots of temperature dependence of magnetic susceptibility and electrical resistivity data in the range 12-20 K, just below T N , are found to be hysteretic leading to a magnetic phase in this intermediate temperature range, mimicking disorder-broadened first-order magnetic phase transitions; (ii) features attributable to an interesting magnetic phase co-existence phenomenon in the isothermal magnetoresistance in zero field, after travelling across metamagnetic transition fields, are observed. With respect to the Er compound, we do not find any evidence for long-range magnetic ordering down to 2 K, but this compound appears to be on the verge of magnetic order at 2 K.

Effect of Ni0.5Zn0.5Fe2O4 (NZFO) layer thickness on the magneto-electric properties of BaTiO3 (BTO)-NZFO composite bilayer thin films

Structural, magnetic, ferroelectric and magneto-electric (ME) studies of bilayer thin films of BaTiO3 (BTO)-Ni0.5Zn0.5Fe2O4(NZFO) prepared using pulsed laser deposition on platinized silicon substrate are reported. Thickness of BTO layer(≈400 nm) is kept the same and the thickness of the NZFO layer is varied from 25–150 nm to study the effects. The prepared films are studied using grazing incidence x-ray diffraction (GIXRD), Raman spectroscopy, atomic force microscopy (AFM), magnetization and ferroelectric loop tracer measurements. From GIXRD, all the films are found to be polycrystalline in nature. Raman data confirms the ferroelectric (tetragonal) and cubic phases of BTO and NZFO, respectively in the bilayers. It is observed from the AFM data that the growth of NZFO layer is correlated with that of bottom BTO layer for thickness 75 nm resulting in stress on the BTO phase evidenced from shifting of Raman mode corresponding to BTO. Room temperature magnetic and ferroelectric properties are shown by recording the room temperature M-H and P-E loops. Further, PUND (positive up and negative down) measurements are carried out to capture the true ferroelectric polarization of the films. ME properties of the films was probed by recording P-E measurements and PUND in the presence of external magnetic field and it is observed that the ME coupling is maximum for the film with NZFO layer thickness of 75 nm. The observed results are explained in terms of stress mediated ME coupling.