K. Dey

Polar octahedral rotations, cation displacement and ferroelectricity in multiferroic SmCrO3

Our thorough synchrotron diffraction studies provide a clue on the origin of ferroelectricity in SmCrO3. Careful observation demonstrates that polar order develops in the paramagnetic state. Rietveld refinement of the diffraction data confirms that emergence of polar order is correlated with the structural transformation from centrosymmetric Pbnm to non-centrosymmetric Pna21 space group of the distorted orthorhombic structure. Rotations of polar CrO6 octahedra and Sm displacement are proposed to be correlated with the emergence of polar order, which is extended over a wide temperature range and increases gradually with decreasing temperature. This is consistent with the relaxor behavior as evident from the frequency-dependent dielectric response satisfying the Vogel-Fulcher law. A non-collinear to collinear spin transformation is suggested well below the spin reorientation transition. Appearance of ferroelectricity without any correlation to the antiferromagnetic order in SmCrO3 suggests a new class of ferroelectricity. All-electron full-potential first-principles calculation demonstrates significant Sm-Cr hybridization near the Fermi level, which substantiates the experimental findings.

Atypical multiferroicity of HoCrO3 in bulk and film geometry

We report ferroelectricity in antiferromagnetic HoCrO3 with a reasonably large value of spontaneous electric polarization (∼0.32 μC cm−2 at 10 K) from the measurement of pyroelectric current. The appearance of ferroelectricity is further confirmed in the film geometry. Intriguingly, the onset of polar order is observed at a significantly higher temperature (∼240 K) than the Neel temperature (TN = 142 K). The survival of remanent polarization is confirmed from the electric polarization hysteresis loops above and below TN for polycrystalline and film specimens. Structural analysis suggests that polar oxygen octahedral rotations and Ho displacements in the non-centrosymmetric Pna21 space group engineer ferroelectricity. The results indicate that occurrence of polar order in HoCrO3 resembles neither the typical case of proper ferroelectrics nor the improper ferroelectrics.

Cryogenic magnetocaloric effect in zircon-type RVO4 (R = Gd, Ho, Er, and Yb)

Low-temperature magnetic refrigeration has attracted the attention of the cryogenics research community in the areas of technology in space science and in the liquefaction of hydrogen or other fuel gases, although it has been less researched compared to room-temperature magnetic refrigeration. Here, we report that the RVO4 (R = Gd, Ho, Er, and Yb) series is promising for low-temperature cryogenic refrigeration. GdVO4 is the most promising material, exhibiting a maximum magnetocaloric effect (ΔSmaxM) of 41.1 J kg−1 K−1 at 3 K for a change in field (ΔH) of 50 kOe. The values of ΔSmaxM are also promising, with 7.94 J kg−1 K−1 at 15 K, 19.7 J kg−1 K−1 at 2 K, and 17.4 J kg−1 K−1 at 2 K with ΔH = 50 kOe for HoVO4, ErVO4, and YbVO4, respectively. The maximum value of adiabatic temperature change (ΔTmaxad) attributed to the large magnetocaloric effect is 18 K for GdVO4 with ΔH = 50 kOe. The results further indicate that demagnetization with 50 kOe field close to the liquid hydrogen temperature at 20.3 K leads to the remarkable decrease of temperature to 3.5 K. Satisfying its potential as a refrigerant material, the thermal hysteresis of magnetization is negligible, and the values of ΔSmaxM and ΔTmaxad are as high as 12.4 J kg−1 K−1 and 8.5 K for 20 kOe change in field, which can be achieved without a superconducting magnet.

Tuning the Growth Pattern in 2D Confinement Regime of Sm2O3 and the Emerging Room Temperature Unusual Superparamagnetism

Programming the reaction chemistry for superseding the formation of Sm2O3 in a competitive process of formation and dissolution, the crystal growth patterns are varied and two different nanostructures of Sm2O3 in 2D confinement regime are designed. Among these, the regular and self-assembled square platelets nanostructures exhibit paramagnetic behavior analogous to the bulk Sm2O3. But, the other one, 2D flower like shaped nanostructure, formed by irregular crystal growth, shows superparamagnetism at room temperature which is unusual for bulk paramagnet. It has been noted that the variation in the crystal growth pattern is due to the difference in the binding ability of two organic ligands, oleylamine and oleic acid, used for the synthesis and the magnetic behavior of the nanostructures is related to the defects incorporated during the crystal growth. Herein, we inspect the formation chemistry and plausible origin of contrasting magnetism of these nanostructures of Sm2O3.

Magnetoelectric coupling and exchange bias effects in multiferroic NdCrO3.

We report ferroelectricity around  ∼88 K that appears well below T N (∼25 K), unlike other members of RCrO3 series. A synchrotron diffraction study suggests that the occurrence of ferroelectricity in NdCrO3 is coupled to the structural transformation from centrosymmetric Pnma to a non-centrosymmetric Pna21 space group. A strong magnetoelectric coupling is observed in the electric polarization [P(T)]. This coupling is significantly influenced by the magnetic field cooling effect, suggesting an exchange bias effect in P(T). This exchange bias effect is also revealed by the systematic shift of the magnetic hysteresis loops below T(N). The rare occurrence of an exchange bias effect in both the magnetic and electric polarizations associated with a strong magnetoelectric coupling is of fundamental interest, as well as being attractive for technological applications close to liquid nitrogen temperature.

Magnetic memory in nanocrystalline α-Fe2O3 embedded in reduced graphene oxide

Single phase iron oxide (α-Fe2O3) of nearly regular ellipsoidal shape, embedded in reduced graphene oxide (RGO) has been prepared by the chemical route. In the chemical synthesis, a modified Hummerss method is used to prepare graphene oxide (GO). The reduction of GO to RGO is carried out using hydrazine hydrate. Interestingly, X-ray diffraction (XRD), high resolution Transmission Electron Microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy confirm α-Fe2O3 without evidence of trace amounts of Fe3O4. The results point out that the oxide phase of Fe i.e., α-Fe2O3 does not undergo any reduction process. Structural, morphological and spectroscopic characterizations have been performed by XRD, FESEM, TEM and XPS studies. Investigation of dc magnetization studies at low temperatures and low magnetic fields infer, super spin glass (SSG) behavior as manifested through relaxation dynamics and zero field cooled (ZFC) magnetic memory effects.

Magnetic memory effect in self-assembled nickel ferrite nanoparticles having mesoscopic void spaces

Self-assembled magnetic nanoparticles, which are fascinating for their potential applications in high-density data storage, nanoscale electronics, sensors, and medicines, attracted significant attention over the years. Controlled self assembly of magnetic nanoparticles is an art of chemical synthesis, because self assembly does not correspond to the thermodynamically minimum energy state. In order to construct the self-assembled nanostructure an input of external energy is required to direct the self assembly. In this article we report a novel approach of fabricating nanocrystalline and mesoporous nickel ferrite of 5–9 nm size particles by using supramolecular assembly of lauric acid (anionic surfactant) as a structure directing agent under hydrothermal conditions. Thermal, field and time variation of magnetization as well as careful studies of ageing effect clearly demonstrates strong interparticle interaction of the nanoparticle assembly. Interestingly, the memory effect is observed at low temperature as a consequence of interparticle interaction of the nickel ferrite nanoparticle assembly.

Multiferroicity around Verwey transition in Fe3O4 thin films

We report observation of finite remanent polarization (∼10-80 nC/cm2) and reasonably strong magnetoelectric coupling in thin film of Fe3O4 within a temperature range from ∼40 K to Verwey transition temperature TCO ∼120 K. Both the intrinsic capacitance and remanent ferroelectric polarization decrease by ∼20-60% under 0-50 kOe field within this temperature range. The measurements using conventional techniques do not yield convincing results as the ferroelectric polarization is small and loss is large. However, use of more sophisticated protocol - which eliminates the contribution from loss and nonhysteretic polarization effectively - provides reliable information about remanent polarization and its change under magnetic field around the Verwey transition.