Structural, Magnetic, and Magnetoelectric Correlation in bulk, nano, and Doping Modified Trirutile Fe2TeO6

Abstract

In this thesis, the correlation among lattice, spin, and charge degrees of freedom are investigated in bulk, nano, and doping modified inverse trirutile Fe2TeO6 (FTO). The bulk and doped materials are synthesized via solid-state reaction route. The nano polycrystalline FTO has been synthesized by the sol-gel process. Structural, magnetic, and magnetoelectric (ME) properties are studied for bulk material. The detailed investigations reveal of the rare existence of d5 off-centering, weak ferroelectric polarization and demonstrate its correlation with observed magnetism and ME coupling in the apparent centrosymmetric (P42/mnm) G type (TN~210 K) antiferromagnet FTO compound. The origin of ferroelectricity (FE) is associated with both lattice and asymmetric electron density distribution around the ion cores as concluded by ellipsoid analysis (EA) and charge density distribution analysis following the maximum entropy method (MEM). Apart from the surprising FE of d5 origin, the existence of d(5/2)-d(5/2) dimeric interaction to manifest broad maximum in the magnetic susceptibility above TN and change in spin dynamics at 150 K is evidentially reported. Simultaneous manifestations of all the correlated phenomena are probed via neutron diffraction, magnetization, heat capacity, and muon spectroscopic measurements. The emergence of magnetic order and the magnetoelectricity solely depends on the same ion (Fe3+) though unusual ME multiferroicity is found well above TN. ME coupling is observed in the spin-phonon coupling, magnetic field-dependent polarization, ME voltage, and magnetostrain measurements. Intrabilayer exchange coupling via the double oxygen bridged Fe-O1-Fe pathway is proposed to play a dominating role to exhibit the negative nonlinear ME behavior at 300 K. Interbilayer exchange via Fe-O2-Fe pathways dominantly determines the hysteretic nonlinear ME coupling below TN. The observation of renormalization of different Raman modes below 210 K suggests the existence of spin-phonon coupling in the material. The coupling strength is quantified in the range 0.1-1.2 cm-1 following the mean field approximation and two-spin cluster approximation. The spin-phonon coupling is realized to be mediated by asymmetric stretching of Fe-O2 in Fe-O2-Fe exchange pathways. The observed nonlinear ME coupling signifies magnetoelasticity as manifested in the temperature and magnetic field-dependent strain measurement. Hence, the rare existence of FE, magnetic order, and ME coupling induced by the same d5 ion is demonstrated in bulk FTO. Sol-gel synthesized nanocrystalline FTO sample is characterized for structural, magnetic, morphological, and ME properties. Higher distortion with diffuse p-d hybridization (Fe-O bond) in Fe3+ polyhedra is observed in nano FTO compared to the bulk material. Nano-spherical morphology with particle size 10-40 nm is observed in the synthesized samples. Signature of reduction in surface coordination is observed in XPS and Raman spectroscopy study. TN is observed to be shifted to 167 K from 210 K, as seen in bulk. The shift towards low temperature is explained by the finite size effect. The broad anomaly in magnetic susceptibility above TN is well ascribed by 5/2-5/2 dimeric interaction fitting. A weak ferromagnetic induction is observed due to reduced surface coordination in the nano sample. The room temperature ferroelectric and ME properties of the nano FTO change significantly due to the finite size effect, reduced surface coordination and diffuse nature of Fe-O covalent bonding. Polarization (Pr) value (0.098 μC/cm2) increases in nano-FTO. A nonmonotonous increase in the remanent polarization is noticed when an external magnetic field is applied to the sample. This is a clear indication of prevailing substantial ME coupling in the nano sample at room temperature. The quantification of magnetoelectricity is done by directly measuring the ME voltage (V) in the presence of a varying dc magnetic field (H), and the ME coefficients are obtained using a quadratic relation in H. The values so obtained for the first order (α/d), second-order (β/d), and third-order (γ/d) ME coefficients are ∼0.22 mV cm−1 Oe−1, ∼−1.70 × 10−2 mV cm−1Oe−2, and ∼0.72 × 10−6 mV cm−1Oe−3, respectively. Nb is doped (FTON5: 5% and FTON10: 10%) at the Te position to study the doping-induced modification based on a theoretically proposed strategy to increase TN. No change is observed up to 10% Nb doping. Instead, a weak ferromagnetic induction is observed on increasing Nb concentration. The remanent polarization decreases with increase Nb doping concentration. The nonlinear magnetic field dependency is fitted with the equation containing three terms, and the different coupling coefficients are obtained as; α/d~0.32 mVcm-1Oe-1, β/d~5.13×10-3mVcm-1Oe-2, γ/d~5.13×10-5 mVcm-1Oe-3. Similarly, for FTON5, the nonlinear fitting corresponds to value of coefficients as, α/d~0.15 mVcm-1Oe-1, β/d~1.65×10-3mVcm-1Oe-2, γ/d~6.40×10-5 mVcm-1Oe-3. In case of FTON10 linear voltage response is observed with, α/d~0.37 mVcm-1Oe-1, β/d~ 2.88×10-3mVcm-1Oe-2. Hence, new room temperature ME material with the unconventional ME coupling is reported in bulk and nanocrystalline FTO. The explored ME properties can be exploited to have potential technological applications.