Manoj K. Singh

The Almeida–Thouless line in BiFeO3: is bismuth ferrite a mean field spin glass?

Low-temperature magnetic properties of epitaxial BiFeO(3) (BFO) thin films grown on (111) SrTiO(3) substrates have been studied. Zero-field-cooled and field-cooled magnetization curves show a large discrepancy beginning at a characteristic temperature T(f) that is dependent on the magnetic field strength. T(f)(H) varies according to the well known de Almeida-Thouless line [Formula: see text] suggesting an acentric long-range spin-glass behavior and mean field system.

Microwave dielectric and Raman scattering studies on bismuth zinc niobate thin films

Bismuth zinc niobate (Bi1.5Zn1.0Nb1.5O7) thin films were deposited on fused silica substrates by pulsed laser deposition. Its phase composition and microstructure were characterized by x-ray diffraction and atomic force microscopy. The as deposited films were amorphous and all of them were crystallized after a postdeposition annealing at 600 °C for 30 min in air. The crystallite size of the films was found to be varying between 31 and 60 nm with respect to the oxygen pressure in the deposition chamber. The films were found to be less oriented with the decreasing crystallite size. The films deposited under different oxygen pressures showed a microwave dielectric permittivity in between 98–124 and a loss tangent in between 0.005 and 0.007 over the X-band frequency range. From the Raman scattering data of the bulk and thin films, it was found that the Raman modes in thin films are relatively more intense and additional Raman modes were appearing in the films in the low frequency range, which reveal that the ...

Raman scattering measurements of phonon anharmonicity in CuAlO2 thin films

CuAlO2 thin films were grown on single crystalline sapphire substrates with c-axis orientation by rf sputtering method. The x-ray diffraction data indicate the formation of delafossite structure and tend to be oriented along (001). Temperature dependent Raman spectra of CuAlO2 thin films were measured from 80 to 1273 K, and we observed two optical modes at Eg (∼418 cm−1) and A1g(∼767 cm−1) showing anomalous frequency and linewidth shifts with temperature, which were interpreted as an experimental evidence of combined effect of lattice expansion and anharmonic phonon-phonon interaction in CuAlO2. At high temperature, polaronic state and change in effective mass due to lattice expansion also affect the frequency shift and the linewidth of the observed Raman modes.

Multiferroics properties in BiFeO3/CoFe2O4 heterostructures thin films deposited on (111) SrTiO3

The multiferroic properties of BiFeO3-CoFe2O4 (BFO-CFO) heterostructures thin film grown on (111) SrTiO3 (STO) substrates by employing pulsed laser deposition (PLD) method, have been studied.X ray diffraction analysis reveals the presence of BFO and CFO in two separate phases. The heterostructures BFO/CFO/STO(111) thin films show well – shaped hysteresis loop with saturated magnetization and coercive field, which are much larger than pure BFO/STO thin film. The frequency dependent dielectric anomalies in BFO-CFO multilayer are attributed to the interfacial effect across the presence of CFO as a separate phase embedded in BFO matrix.

Raman spectra and dielectric studies in Ti substituted Bi2 (Zn2/3Nb4/3)O7 pyrochlores

Bi2Zn2/3-x/3Nb4/3–2x/3TixO7 (m – BZNT) with x = 0 to 0.4 were synthesized in the conventional solid state route. The dielectric constants of these ceramics at microwave frequencies are found to be increasing from 70 to 114 when x increased from 0 to 0.4. Raman scattering studies were carried out to investigate the effect of Ti4+ substitution on the dielectric properties of Bi2 (Zn2/3Nb4/3)O7 (m-BZN) ceramics. The observed Raman peaks in the m-BZNT are slightly shifted towards lower frequency in comparison to that of m-BZN clearly indicated that the Ti4+ ions are indeed occupying substitution sites in the host m – BZN. The anomalous change in intensity and full with at half maxima (FWHM) of Raman modes at 749 and 847 cm−1 suggest change in oxygen octahedral closely related to dielectric properties of m-BZNT.

Phonon anomalies in multiferroic BiFeO3 epitaxial thin films with rhombohedral (R3c) symmetry

Raman spectrum of (111) oriented BiFeO3 (BFO) thin film was studied in temperature range between 27 °C - 500 °C. Observed three sharp A1 Raman modes at 136, 162, and 212 cm¿1 show anomalous changes in frequencies, line width and integrated intensity between span of temperature range 250¿ 400 °C i.e. around magnetic phase transition (TN). The sign and magnitude of such anomalous behavior appears to be an experimental evidence of perturbation in antiferromagnetic ordering coupled with octahedral tilting reveals strong spin-phonon coupling around TN. The soft mode behavior of A1 Raman modes at 136 and 162 cm¿1 was observed directly and reveals a decrease of the Curie temperature (TC) of the strained film, which was originally suggested by M. S. Kartavtseva and co workers [Thin Solid Films, 515 (2007) 6416].

Structural properties of multiferroic CuFeO2 thin films prepared by RF sputtering

CuFeO2 thin films were grown on single crystalline sapphire substrates with c-axis orientation by rf sputtering method. The reitveld refinement analysis of X - ray diffraction data indicates the formation of delafossite structure and tended to be oriented along (001). Raman-scattering study of these thin films reveals that the films were oriented along (001) with rhombohedral R3¿m symmetry. Temperature dependent Raman spectra of CuFeO2 thin films were measured from 80K to1273K and we observed two optical modes at Eg (296cm¿1) and Ag (638cm¿1) showing anomalous frequency shifts with temperature, which were interpreted as an experimental evidence of combined effect of lattice expansion and anharmonic phonon - phonon interaction in CuFeO2.

Anomalous magnetic ordering transition and induced Raman scattering in BiFeO3 thin films

Low temperature magnetic properties and Raman spectrum of highly (111)-oriented rhombohedral BiFeO 3 (BFO) thin films epitaxially grown on (111) SrTiO 3 substrates by pulsed laser deposition have been studied. Zero field cooled (ZFC) and field cooled (FC) magnetization curves display divergence at characteristic temperature T irr ( H ), which are magnetic field dependent, suggesting a spin-glass like behavior of the film. The change in magnetic ordering results in anomalous phonon behavior such as the extraordinary enhancement and appearance of new Raman modes due to Brillouin zone folding below ∼200 K.