Papers by Tuhin Subhra Maity
Journal of the American Ceramic Society
Single-phase multiferroic materials are of considerable interest for future memory and sensing ap... more Single-phase multiferroic materials are of considerable interest for future memory and sensing applications. Thin films of Aurivillius phase Bi7Ti3Fe3O21 and Bi6Ti2.8Fe1.52Mn0.68O18 (possessing six and five perovskite units per half-cell, respectively) have been prepared by chemical solution deposition on c-plane sapphire. Superconducting quantum interference device magnetometry reveal Bi7Ti3Fe3O21 to be antiferromagnetic (TN = 190 K) and weakly ferromagnetic below 35 K, however, Bi6Ti2.8Fe1.52Mn0.68O18 gives a distinct room-temperature in-plane ferromagnetic signature (Ms = 0.74 emu/g, μ0Hc =7 mT). Microstructural analysis, coupled with the use of a statistical analysis of the data, allows us to conclude that ferromagnetism does not originate from second phase inclusions, with a confidence level of 99.5%. Piezoresponse force microscopy (PFM) demonstrates room-temperature ferroelectricity in both films, whereas PFM observations on Bi6Ti2.8Fe1.52Mn0.68O18 show Aurivillius grains undergo ferroelectric domain polarization switching induced by an applied magnetic field. Here, we show for the first time that Bi6Ti2.8Fe1.52Mn0.68O18 thin films are both ferroelectric and ferromagnetic and, demonstrate magnetic field-induced switching of ferroelectric polarization in individual Aurivillius phase grains at room temperature.
Journal of Applied Physics
We have observed a large as well as path-dependent spontaneous exchange bias (HSEB) (∼30–60 mT) i... more We have observed a large as well as path-dependent spontaneous exchange bias (HSEB) (∼30–60 mT) in a nanocomposite of BiFeO3-Bi2Fe4O9 across 5–300 K when it is measured in an unmagnetized state following zero-field cooling and appropriate demagnetization. The path dependency yields a variation in the exchange bias depending on the sign of the starting field and the path followed in tracing the hysteresis loop. The asymmetry thus observed—ΔHSEB—is found to be decreasing nonmonotonically across 5–300 K with a peak around 200 K. The ΔHSEB together with large HSEB could have significant ramification in tuning the exchange bias driven effects and consequent applications.
Phys. Rev. Lett.
We observe an enormous spontaneous exchange bias (∼300–600 Oe)—measured in an unmagnetized state... more We observe an enormous spontaneous exchange bias (∼300–600 Oe)—measured in an unmagnetized state following zero-field cooling—in a nanocomposite of BiFeO3 (∼94%)-Bi2Fe4O9 (∼6%) over a temperature range 5–300 K. Depending on the path followed in tracing the hysteresis loop—positive (p) or negative (n)—as well as the maximum field applied, the exchange bias (HE) varies significantly with ∣-HEp∣>∣HEn∣. The temperature dependence of HE is nonmonotonic. It increases, initially, till ∼150 K and then decreases as the blocking temperature TB is approached. All these rich features appear to be originating from the spontaneous symmetry breaking and consequent onset of unidirectional anisotropy driven by “superinteraction bias coupling” between the ferromagnetic core of Bi2Fe4O9 (of average size ∼19 nm) and the canted antiferromagnetic structure of BiFeO3 (of average size ∼112 nm) via superspin glass moments at the shell.
In this paper, the research focus is how to entangle magnetic dipoles to control/engineer magneti... more In this paper, the research focus is how to entangle magnetic dipoles to control/engineer magnetic properties of different devices at a submicron/nano scale. Here, we report the generation of synthetic arrays of tunable magnetic dipoles in a nanomodulated continuous ferromagnetic film. In-plane magnetic field rotations in modulated Ni45Fe55 revealed various rotational symmetries of magnetic anisotropy due to dipolar interaction with a crossover from lower to higher fold as a function of modulation geometry. Additionally, the effect of aspect ratio on symmetry shows a novel phase shift of anisotropy, which could be critical to manipulate the overall magnetic properties of the patterned film. The tendency to form vortex is in fact found to be very small, which highlights that the strong coupling between metastable dipoles is more favorable than vortex formation to minimize energy in this nanomodulated structure. This has further been corroborated by the observation of step hysteresis, magnetic force microscopy images of tunable magnetic dipoles, and quantitative micromagnetic simulations. An analytical expression has been derived to estimate the overall anisotropy accurately for nanomodulated film having low magnetocrystaline anisotropy. Derived mathematical expressions based on magnetic dipolar interaction are found to be in good agreement with our results.
We report the effect of SiO2 coating on the structural, magnetic and dielectric properties of che... more We report the effect of SiO2 coating on the structural, magnetic and dielectric properties of chemically synthesized high purity BiFeO3 particles. The as synthesized BiFeO3 particles demonstrate properties comparable with those reported for bulk BiFeO3. On the other hand, the structural measurement on BiFeO3@SiO2 shows that, the SiO2 coating has anisotropically compressed the lattice of BiFeO3 particles and stimulates the variation in the electron density. This affects magnetic and dielectric behavior of material. Frequency dependent dielectric constant study at low temperature (20 K to 325 K) revealed slight reduction (8 – 10 %) in the dielectric constant of BiFeO3@SiO2 particles compared to uncoated BiFeO3 particles. The study reveals five anomalies at 234, 206, 146, 84 and 25 K located close proximity to the linear magnetodielectric coupling and spin reorientation transitions. The loss tangent (≈ 10-3) and ac conductivity (≈ 10-8 Ω-1cm-1) of BiFeO3@SiO2 particles are orders of magnitude lower than those observed for the BiFeO3 particles. The magnetic measurement shows the existence of room temperature ferromagnetism in BiFeO3@SiO2 particles with average value of magnetic moment per Fe atom ≈ 0.030 μB and appreciable coercivity as high as 120 Oe. The canted spin structure in the surface shell of BiFeO3@SiO2 particles show an enhanced magnetic property and shifted hysteresis loop. The magnetic measurement in close proximity to dielectric transitions revealed enhanced magnetization, suggest presence of anisotropies. It has been observed that, SiO2 coating alters the properties of BiFeO3 particles. Our dielectric and magnetic measurements show enhanced coupling among the electric and magnetic ordered parameters in BiFeO3@SiO2 core – shell particles compared to uncoated BiFeO3 particles. The magnetic and dielectric properties of SiO2 coated BFO are similar to nanoparticles of BiFeO3 where interface plays significant role.
nsti.org
We demonstrate a simple and cost-effective route, based around block copolymer patterning, to cre... more We demonstrate a simple and cost-effective route, based around block copolymer patterning, to create dense arrays of highly ordered iron oxide nanodots at substrate surface. The methodology creates hexagonally arranged features via a process of selective block copolymer inclusion and allows dimensional and structural control of both features and patterns at large scale. Spectroscopic, microscopic and magnetic measurements show the nanodots have uniform size and shape and their placement mimics the original self-assembled block copolymer pattern. The magnetic properties are quantified and are commensurate with well-isolated, size monodisperse nanodots exhibiting superparamagnetism, have potential biomedical applications. It is suggested that these nanodots have good thermal stability and strong adherence to the substrate surface, making them useful for technological applications.
Highly dense hexagonal ordered arrays of superparamagnetic iron oxides nanodots are fabricated by... more Highly dense hexagonal ordered arrays of superparamagnetic iron oxides nanodots are fabricated by a simple and cost-effective route. Spectroscopic, microscopic and magnetic measurements show that the nanodots have uniform size, shape and their placement mimics the original self-assembled block copolymer pattern. The nanodots show good thermal stability and strong adherence to the substrate surface, making them useful for practical device applications.
Journal of Applied Physics, Jan 1, 2011
We report the temperature and frequency dependent dielectric measurements and ac magnetic suscept... more We report the temperature and frequency dependent dielectric measurements and ac magnetic susceptibility of chemically synthesized DyFeO3 nanoparticles (size ∼50–60 nm). The measurement of the dielectric properties was carried out in a broad temperature (20–325 K) and frequency (1–106 Hz) range. The non-Debye type dipolar relaxation phenomenon was observed in the DyFeO3 nanoparticles, as confirmed by the Cole–Cole plots. The higher values of ɛ′ at the lower frequencies are explained on the basis of the Maxwell–Wagner model. The Cole–Cole analysis enabled us to separate the contribution of relaxation times, resistance and capacitance in grain and grain boundaries in DyFeO3 nanocrystals. We found that with increasing temperature, the contribution of grain boundary resistance increases in comparison to the grain resistance. We also performed spin relaxation studies in a broad temperature and frequency range. Both the in-phase (χ′) and out-of-phase (χ″) components of the ac magnetic susceptibilities of the DyFeO3 nanoparticles show a clear evidence at spin reorientation transition around 70 K and TN (Dy) around 4 K as reported by us in our earlier study using the dc magnetic behaviour.
HoMnO3 films were grown on pure and Nb-doped SrTiO3 (001) substrates by pulsed laser deposition. ... more HoMnO3 films were grown on pure and Nb-doped SrTiO3 (001) substrates by pulsed laser deposition. The films grew epitaxially with the c-axis along the substrate normal. Varying the deposition temperature between 650 and 850 °C did not significantly affect the structural and magnetic properties of the films, whereas growth in oxygen partial pressures below 0.01 mbar lead to a degradation of the structural properties. Some of the films had a ferromagnetic-like magnetic phase transition at about 45 K, probably related to Mn3O4 precipitates; this magnetic response was isotropic. The Ho sublattice was found to be paramagnetic down to 5 K, but showing a pronounced anisotropy with the c-axis being the hard axis. The films showed a distinct dielectric anomaly at 16 K that depended on voltage and slightly on frequency in the range between 1 kHz and 1 MHz. The magnetoelectric effect was large with an in-plane field of 8 T suppressing the dielectric anomaly completely.
Solid State …, Jan 1, 2010
Cupric oxide nanoparticles of ∼8–10 nm width and 40–45 nm length self assembled as large particle... more Cupric oxide nanoparticles of ∼8–10 nm width and 40–45 nm length self assembled as large particles ∼1–1.5 μm have been investigated, in the 10–325 K temperature range, using magnetic and dielectric measurements. In magnetic measurements a single broad peak at ∼230 K in a zero field cooled sample has been observed. Coercivity, in magnetization measurements at 10 K, suggests that the nanoparticles are core-shell type particles with an antiferromagnetic core and a ferromagnetic shell. Dielectric measurements, at various frequencies from 3.7 Hz to 949 kHz, exhibit a sharp peak at 284 K followed by weak anomalies around 213 and 230 K.
The Journal of Physical …
We report the temperature-dependent Raman and dielectric spectroscopy of chemically synthesized B... more We report the temperature-dependent Raman and dielectric spectroscopy of chemically synthesized BiFeO3 nanoparticles (average size 50−60 nm). The Raman spectra (90−700 K) show two sets of transitions in the lowest Raman E mode, associated with Bi−O bond motion situated in close proximity to the spin reorientation transitions reported for BiFeO3, thereby indicating the existence of possible coupling between magnons and phonons for particle size below the helical order parameter (62 nm). These transitions are slightly shifted in temperature in comparison to the bulk single crystals. We also observe a step-like behavior in Raman peak position around the Neel temperature, suggesting that the phonons are influenced by the magnetic ordering in nanosized BiFeO3. The heat-flow measurements show two sharp endothermic peaks at 1094 and 1223 K representing rhombohedral to orthorhombic or monoclinic transition followed by transition into the cubic phase above 1200 K. The low temperature (20−325 K), frequency-dependent (1−106 Hz) dielectric constant and loss tangent measurements show that the loss tangent (10−3) and ac conductivity values (10−8 Ohm−1-cm−1) are orders of magnitude lower than the reported values for BiFeO3 ceramics, indicating high levels of ionic purity of our samples. The real part of the permittivity shows a slight reduction in its value (30) in comparison to the bulk single crystals. Similar to the Stokes Raman shift, its temperature-dependent dielectric constant also shows four weak anomalies at 85, 168, 205, and 230 K situated in close proximity to the spin reorientation transitions, indicating magnetoelectric coupling.
Journal of Applied …, Jan 1, 2010
The rare earth orthochromites are extremely interesting due to the richness of their optical, die... more The rare earth orthochromites are extremely interesting due to the richness of their optical, dielectric, and magnetic properties as well as due to their multiferroic properties which make them suitable materials to study in the nanoregime. However, the wet-chemical synthesis of these materials in nanosize is nontrivial. Here, we report for the first time, the detailed Raman spectra as well as magnetic and dielectric properties of chemically synthesized GdCrO3 nanoparticles of size ranging from 40 to 60 nm. The magnetic properties are dictated by competing Cr3+–Cr3+, Gd3+–Cr3+, and Gd3+–Gd3+ superexchange interactions in different temperature regions, resulting into an antiferromagnetic ordering at 167 K due to the Cr3+–Cr3+ followed by weak ferromagnetic ordering due to the onset of Cr3+–Gd3+ interactions. At lower temperature, it shows weak antiferromagnetic ordering due to Gd3+–Gd3+ interaction. Below 95 K, GdCrO3 nanoparticles showed the presence of negative magnetization due to Gd3+ and Cr3+ interactions resulting into weak ferromagnetic coupling. The Raman spectroscopy shows the characteristic Raman shifts indicating that below 450 cm−1, Gd3+ ions play a dominant role in determining the phonon frequencies of GdCrO3, and above 450 cm−1, the Cr+3 ions dominate. We also present for the first time the low temperature dielectric constant and loss tangent data for GdCrO3 in a broad temperature and frequency range. The dielectric constant shows a decrease in comparison to the bulk values due to the size dependent effects. It also shows a peak centered at around 320 K above which it shows a sharp decrease. The dielectric loss value in GdCrO3 nanoparticles is quite small and shows an interesting frequency dependent anomaly at lower temperature which might be due to the coupling between magnetic and dielectric order parameters.
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Papers by Tuhin Subhra Maity