Structural and Magnetic Properties of Novel Oxides

Transition-metal and rare earth based oxides belong to an attractive and challenging field of research in condensed matter physics. This class of oxide possesses a wide range of intriguing properties and reveals novel phenomena from insulator to superconductor and ferroelectricity to ferromagnetism. The competition and coexistence of different types of ground states gives rise to complex electronic and magnetic phases. This thesis entitled “Structural and Magnetic Properties of Novel Oxides” mainly concentrates on investigation of structural, optical, dielectric, electric and magnetic properties of some novel oxides such as BaTiO3, NiFe2O4, SrRuO3 and LaCrO3. Lattice distortion, defects and chemical composition are the key parameters to modify basic interactions to induce new behavior in oxides. Reduction of size to fundamental length scale i.e. coherence and cooperative length also significantly influences long range order in ferroelectric and ferromagnetic oxides.Research was conducted under the supervision of Prof. S K De of the Materials Science division under SPS [School of Physical Sciences]Research was carried out under CSIR fellowship and also CSIR & DST travel gran

Unveiling ferrimagnetic ground state, anomalous behavior of the exchange-bias field around spin reorientation, and magnetoelectric coupling in YbCr1−x_{1−x}Fex_{x}O3_{3}(0.1 ⩽x⩽\leqslant x \leqslant 0.6 )

We present a comprehensive experimental study of the magnetic structure, magnetic and dielectric properties of the rare-earth orthochromite-orthoferrite solid-solution series YbCr$_{1−x}$Fe$_{x}$O$_{3}$(0.1 $\leqslant x \leqslant$ 0.6 ). Room-temperature synchrotron x-ray diffraction analysis reveals the absence of any superlattice reflections, which excludes the formation of a B-site-ordered double-perovskite-like phase and establishes the complete solid solubility of Fe at the Cr site within the framework of orthorhombic Pbnm structure. We demonstrate that canted antiferromagnetic ground state of YbCrO$_3$ is converted to a ferrimagnetic with Fe doping, in addition to an increase in the magnetic ordering temperature. An unusual, second magnetic transition (first-order in nature) appears for $x\leqslant$0.3 samples below the ferrimagnetic transition temperature (e.g., at 70 K for x=0.4), which is identified as the spin reorientation of transition metal ions from the neutron powder diffraction measurements, and primarily, driven by the $f−d$ exchange interaction. A clear evidence of the anomalous behavior of coercivity and exchange bias field is found around the spin reorientation temperature, which is characterized by a significant change in the magnetocrystalline anisotropy due to spin reorientation of transition metal ions. Temperature-dependent dielectric data exhibit the magnetoelectric coupling as well as a ferroelectric relaxor-like state at the onset of ferrimagnetic ordering. Here, we reveal the anomalous behavior of the exchange bias field and significant magnetoelectric coupling around the spin reorientation and ferrimagnetic transitions, respectively, in YbCr$_{1−x}$Fe$_x$O3$_.

Exchange bias effect in BiFeO3-NiO nanocomposite

Ferromagnetic BiFeO3 nanocrystals of average size 11 nm were used to form nanocomposites (x)BiFeO3/(100 - x) NiO, x = 0, 20, 40, 50, 60, 80, and 100 by simple solvothermal process. The ferromagnetic BiFeO3 nanocrystals embedded in antiferromagnetic NiO nanostructures were confirmed from X-ray diffraction and transmission electron microscope studies. The modification of cycloidal spin structure of bulk BiFeO3 owing to reduction in particle size compared to its spin spiral wavelength (62 nm) results in ferromagnetic ordering in pure BiFeO3 nanocrystals. High Neel temperature (TN) of NiO leads to significant exchange bias effect across the BiFeO3/NiO interface at room temperature. A maximum exchange bias field of 123.5 Oe at 300K for x = 50 after field cooling at 7 kOe has been observed. The exchange bias coupling causes an enhancement of coercivity up to 235 Oe at 300 K. The observed exchange bias effect originates from the exchange coupling between the surface uncompensated spins of BiFeO3 nanocrystals and NiO nanostructures. (C) 2014 AIP Publishing LL