High Pressure Studies on Multiferroic Oxide Systems

In this work high pressure behaviors of both Type – I and Type – II multiferroic oxides are investigated using Raman spectroscopy, electric transport measurements and synchrotron radiation X- ray diffraction measurements. Possible technological applications of multiferroic materials due to their strong magnetoelectric coupling have attracted scientific community to predict and design new multiferroic materials at room temperature. Therefore, research activities in this field are continuously growing since the last decade after the discovery of Type – II multiferroic. In this background pressure can act as an efficient tool for fundamental understanding of related interactions and to stabilize multiferroics at room temperature. In this research work high pressure is generated in static method using gasketed diamond anvil cell for Raman scattering and X-ray diffraction measurements. Sample along with pressure marker are loaded inside the 100 μm hole of a metal gasket. A mixture of 4:1 methanol and ethanol is used to maintain hydrostatic condition inside the hole. High pressure X – ray diffraction measurements are carried out in Elettra synchrotron light source, Trieste, Italy in XRD1 and XPRESS beam line by employing a monochromatic X-ray beam. Electrical transport and magnetic measurements are carried out using Toroid anvil cell and a 300 ton hydraulic press. High pressure investigations up to 42 GPa are conducted on CuO which is a Type – II multiferroic in a short temperature range of 213K < T < 230K with both Tc and TN coinciding at 230 K. Observation of anomalous changes both in dielectric constant and dielectric loss in the pressure range 3.7–4.4 GPa and reversal of piezoelectric current with reversal of poling field direction indicate to a change in ferroelectric order in CuO at high pressures. A sudden jump in Raman integrated intensity of Ag mode at 3.4 GPa and observation of Curie-Weiss type behaviour in dielectric constant below 3.7 GPa lends credibility to above ferroelectric transition. A slope change in the linear behaviour of the Ag mode and a minimum in the FWHM of the same mode indicate indirectly to a change in magnetic ordering in CuO. High pressure behaviour of nanocrystaline YCrO3 reveals another interesting result. X-ray diffraction measurements show absence of any structural phase transition at high pressures, however anomalies are observed in the unit cell structural parameters at about 4.3 GPa. In the absence of any structural transition, anomalous behaviour of relevant Raman modes with minimum in the Raman band width at about same pressure strongly indicate towards a spin-phonon interaction. AC magnetic measurements in the toroid anvil apparatus show an anomalous enhancement of magnetic moment above 4 GPa indicating a collective response of nanoparticles. High pressure studies on 0.65 BiFeO3-0.35 PbTiO3 exhibit multiple structural phase transition in 0–34 GPa range where the system transformed from ambient tetragonal P4mm to a mixed phase of tetragonal and rhombohedral (P4mm + R3c) at 0.7 GPa, followed by transition to rhombohedral R3c at 0.8GPa, cubic Pm3m at 3.4 GPa and again to rhombohedral R3c structure at 6.1 GPa, respectively. The transformation to the cubic structure coincides with the ferroelectric-paraelectric transition. Above 6.6 GPa re-entrant ferroelectric behaviour is observed while the system transform from cubic to R3c phase. The disappearance and re-appearance of ferroelectric phase is supported by pressure induced spontaneous strain profile. We have also attempted to investigate BiGdO3 at room temperature as well as at high pressure. The sample is prepared using the solid-state reaction route. Ambient XRD analysis of BiGdO3 shows a cubic Fm3m phase having lattice parameter of 5.4658(4) A. Temperature dependent dielectric constant, piezoelectric current and electric filed dependent polarization measurements at room temperature suggest antiferroelectric behavior of this sample below 715K. Low temperature magnetic measurements indicate superparamagnetic behavior at 2 K. High pressures XRD measurements show cubic to orthorhombic structural transition at about 10 GPa accompanied by appearance of new Bragg reflection line. The volume expansion during the structural transition is reflected by the large drop in 640 cm-1 Raman mode frequency.