Sudipta Goswami

Multiferroic coupling in nanoscale BiFeO3

Using the results of x-ray and neutron diffraction experiments, we show that the ferroelectric polarization, in ∼22 nm particles of BiFeO3, exhibits a jump by ∼30% around the magnetic transition point TN (∼635 K) and a suppression by ∼7% under 5T magnetic field at room temperature (≪TN). These results confirm the presence of strong multiferroic coupling even in nanoscale BiFeO3 and thus could prove to be quite useful for applications based on nanosized devices of BiFeO3.

Particle size dependence of magnetization and noncentrosymmetry in nanoscale BiFeO3

The saturation magnetization (MS), antiferromagnetic transition point (TN), and the off-center displacements of Bi and Fe ions have been measured as a function of particle size in nanoscale BiFeO3. TN decreases down to ∼550 K for particles of size ∼5 nm from ∼653 K in bulk, while MS rises by more than an order of magnitude. Analysis of crystallographic structure from Rietveld refinement of x-ray diffraction patterns shows significant rise in off-center displacements of Bi (δBi) and Fe (δFe) ions within a unit cell with the decrease in particle size. The net unit-cell polarization PS too, is found to be larger in nanoscale regime.

Room temperature multiferroicity in orthorhombic LuFeO3

From the measurement of dielectric, ferroelectric, and magnetic properties, we observe simultaneous ferroelectric and magnetic transitions around ∼600 K in orthorhombic LuFeO3. We also observe suppression of the remanent polarization by ∼95% under a magnetic field of ∼15 kOe at room temperature. The extent of suppression of the polarization under magnetic field increases monotonically with the field. These results show that even the orthorhombic LuFeO3 is a room temperature multiferroic of type-II variety exhibiting quite a strong coupling between magnetization and polarization.

Large magnetoelectric coupling in nanoscale BiFeO3 from direct electrical measurements

Science Foundation Ireland (SFI Principal Investigator (PI) Project No. 11/PI/1201, SFI FORME Strategic Research Cluster Award No. 07/SRC/I1172, ISCA grant (SFI: 12/ISCA/2493)); Indo-Ireland joint program (DST/INT/IRE/P-15/11); Council of Scientific and Industrial Research, India (CSIR Research Associateship)

Determination of intrinsic ferroelectric polarization in lossy improper ferroelectric systems

We measured the intrinsic hysteretic polarization in lossy improper and nanoferroelectric systems where the nonhysteretic polarization and leakage are large and the relaxation takes place over a broader time scale. We used different measurement protocols such as standard single triangular voltage pulse, a pulse train of Positive Up Negative Down, and an even more complicated pulse train of fourteen voltage pulses and compared the results obtained. We show that a protocol which sends a train of fourteen pulses is more appropriate for extracting relaxed (i.e., time scale independent) and intrinsic remanent polarization for these samples. We also point out that it is possible to select and design an appropriate measurement protocol depending on the magnitude of polarization and leakage of the system.

Spontaneous exchange bias in a nanocomposite of BiFeO3-Bi2Fe4O9

X iv :1 30 4. 22 05 v1 [ co nd -m at .m tr lsc i] 8 A pr 2 01 3 Spontaneous exchange bias in a nanocomposite of BiFeO3-Bi2Fe4O9 Tuhin Maity, Sudipta Goswami, Dipten Bhattacharya, Gopes C. Das, and Saibal Roy a) Micropower-Nanomagnetics Group, Microsystems Center, Tyndall National Institute, University College Cork, Lee Maltings, Dyke Parade, Cork, Ireland Nanostructured Materials Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700032, India School of Materials Science, Jadavpur University, Kolkata 700032, India

A training effect on electrical properties in nanoscale BiFeO3

We report our observation of the training effect on dc electrical properties in a nanochain of BiFeO3 as a result of large scale migration of defects under the combined influence of electric field and Joule heating. We show that an optimum number of cycles of electric field within the range zero to ~1.0 MV cm(-1) across a temperature range 80-300 K helps in reaching the stable state via a glass-transition-like process in the defect structure. Further treatment does not give rise to any substantial modification. We conclude that such a training effect is ubiquitous in pristine nanowires or chains of oxides and needs to be addressed for applications in nanoelectronic devices.

Anomaly in structural noncentrosymmetry around T-N in bulk and nanoscale BiFeO3

Using high resolution powder neutron diffraction data, we show that there is a distinct anomaly in the structural noncentrosymmetry around the magnetic transition point TN for bulk and nanoscale BiFeO3. It appears that the structural noncentrosymmetry - which gives rise to the ferroelectric polarization - is suppressed anomalously by ~1% (of the average noncentrosymmetry at above the magnetic transition) in the bulk sample and by ~12% in the nanoscale sample as the magnetic transition is approached from higher temperature. This observation shows that the multiferroic coupling improves in the nanoscale sample which is expected to brighten the application prospects of nanoscale BiFeO3 in nanospintronics-based sensor devices.

Large magnetocapacitance in electronic ferroelectric manganite systems

We have observed a sizable positive magnetocapacitance (∼5%–90%) in perovskite Pr0.55Ca0.45MnO3 and bilayer Pr(Sr0.1Ca0.9)2Mn2O7 system under 5 T magnetic field across 20–100 K below the magnetic transition point TN. The magnetodielectric effect, on the other hand, exhibits a crossover: (a) from positive to negative for the perovskite system and (b) from negative to positive for the bilayer system over the same temperature range. The bilayer Pr(Sr0.1Ca0.9)2Mn2O7 system exhibits a sizable anisotropy as well. We have also noticed the influence of magnetic field on the dielectric relaxation characteristics of these systems. These systems belong to a class of improper ferroelectrics and are expected to exhibit charge/orbital order driven ferroelectric polarization below the transition point TCO. Large magnetocapacitance in these systems shows a typical multiferroic behavior even though the ferroelectric polarization is small in comparison to that of other ferroelectrics.