P. Shahi

Effect of Pr- and Nd- doping on structural, dielectric, and magnetic properties of multiferroic Bi0.8La0.2Fe0.9Mn0.1O3

Bi0.8La0.15RE0.05Fe0.9Mn0.1O3 (where RE = Pr and Nd) have been prepared via conventional solid state route. The Rietveld refinement of X-ray diffraction patterns shows that both systems crystallize in orthorhombic Pnma space group. Raman modes observed for these two systems indicate that both systems are very close to orthorhombic Pnma structure. Appearance of prominent A1-3 and weak E-2 modes in Bi0.8La0.15Nd0.05Fe0.9Mn0.1O3 indicate the presence of chemically more active Bi-O covalent bonds (which favors stereochemical activity of Bi lone pair electrons) in comparison to Bi0.8La0.15Pr0.05Fe0.9Mn0.1O3. Moreover, Bi0.8La0.15Nd0.05Fe0.9Mn0.1O3 system shows higher dielectric constant, low dielectric loss, and higher magnetization value in comparison to Bi0.8La0.15Pr0.05Fe0.9Mn0.1O3 system. Ferroelectric transition temperature decreases to 460 °C for both systems in comparison to 710 °C of Bi0.8La0.2Fe0.9Mn0.1O3. The improved dielectric and magnetic response suggests Bi0.8La0.15Nd0.05Fe0.9Mn0.1O3 a better mu...

Low temperature magnetic and transport properties of LSMO-PZT nanocomposites†

Nanocomposites of La0.7Sr0.3MnO3–PbZr0.52Ti0.48O3 (LSMO–PZT) with varying PZT content have been synthesized by sol–gel method. XRD, HRTEM, SEM and EDX studies confirm the coexistence of both components (viz. LSMO and PZT) in the composites and reveal that the PZT nanoparticles occupy the surface and the grain boundaries (GBs). The M–H measurement of LSMO–PZT composites exhibits a weak ferromagnetic nature with low coercivities of 0.0123 T and 0.0142 T at 300 K and 80 K, respectively, and the highest magnetic moment (nB) achieved is 2.069 μB at 80 K for x = 0.01. The M–T studies confirm that the transition temperature (Tc) ≈ 360 K and the maximum entropy changes (ΔSm) are 0.40, 0.35 and 0.25 mJ Kg−1 K−1 at the magnetic field Hmax = 100 Oe for x = 0.01, 0.02 and 0.05, respectively. The average particle size of LSMO, PZT, and LSMO–PZT nanocomposites range between 24–30 nm, as confirmed from XRD, TEM, and magnetic measurements. Electrical resistivity studies show that the insulator–metal transition temperature (TIM) decreases rapidly with addition of a small amount of PZT and then remains almost constant with a higher concentration of PZT. The conduction in the insulator region is governed by tunneling between magnetic phases and in the metallic region the conduction shows non-Fermi liquid behavior. The resistivity increases with increasing PZT concentration. The energy barriers and scatterings are responsible for the increase in higher resistivity observed. The magneto-resistance (%MR) has been found to decrease with decreasing temperature. Variation in the MR at low temperature in LSMO-the PZT nanocomposite is due to the effect of the large spin polarization and pinning of domain walls at the GBs.

Raman effect and magnetic properties of doped TbMnO3

The room temperature Raman scattering, structural and temperature variation of magnetic properties of Y- and Ni-doped TbMnO3 are investigated. Both for Y and Ni doping, the Neel temperature (TN) decreases. Y doping reduces effectively both the JTb–Tb and JMn–Mn exchange interactions, whereas Ni doping on the Mn site decreases only the JMn–Mn exchange interaction. The reduction in TN in the Ni-doped sample cannot be explained with simple exchange integral. It might be the case that Ni2+ doping changes the polarization flop observed in TbMnO3 which in effect may change the exchange interaction. In the room temperature Raman spectra for TbMnO3 and Y-doped TbMnO3 eight modes are observed. When Ni is doped, modes due to MnO6 bending and in-plane O2 stretching broaden and few modes disappear due to the induced lattice disorder.

Magnetic and optical properties of Fe doped crednerite CuMnO2

A geometrically frustrated magnetic CuMnO2 system has been investigated because of its rich magnetic properties. Neutron diffraction, synchrotron X-ray, magnetic, X-ray photoemission spectroscopy (XPS) and UV-Visible spectroscopy measurements have been carried out on CuMnO2 and 5% Fe doped CuMnO2 samples. Fe doping reduces the distortion. Moreover, Fe doping induces the ferromagnetic coupling between ab planes. The value of magnetization is increased with Fe doping but coercivity is decreased. These might be due to the direct Mn–Mn exchange and Mn–O–Cu–O–Mn super–super exchange interactions. The UV-Vis data indicate the appearance of new energy bands in these compounds. The XPS study indicates that Fe is in the 3+ state.

Effect of Y-doping on the transport and magnetic properties of La0.5Sr0.5CoO3 and La0.7Sr0.3CoO3

The temperature variation of magnetization, resistivity, and thermoelectric power of undoped and Y-doped La0.7Sr0.3CoO3 and La0.5Sr0.5CoO3 samples have been investigated. Y-doping decreases the magnetization possibly due to the spin-state transition of Co ions. The low temperature conduction in (La1−yYy)0.7Sr0.3CoO3 is consistent with the variable range hopping. With Y-doping, value of the Seebeck coefficient increases as Y-doping decreases bandwidth and increases distortion. Seebeck coefficient value also reflects that the orbital stability increases with Sr concentration.

Effect of dilution of both A- and B- sites on the multiferroic properties of spinal Mott insulators

The structural, magnetic, electrical and transport properties of FeV2O4, by doping Li and Cr ions respectively in A and B sites, have been studied. Dilution of A-site by Li doping increases the ferri-magnetic ordering temperature and decreases the ferroelectric transition temperature. This also decreases the V-V distances which in effect increases the A-V coupling. This increased A-V coupling dominates over the decrease in A-V coupling due to doping of non-magnetic Li. On the other hand, Cr doping increases the ferri-magnetic ordering temperature but does not alter the ferroelectric transition temperature which is due to the fact that the polarization origin to the presence of almost non-substituted regions.

Effect of Zn doping on the magneto-caloric effect and critical constants of Mott insulator MnV2O4

X-ray absorption near edge spectra (XANES) and magnetization of Zn doped MnV2O4 have been measured and from the magnetic measurement the critical exponents and magnetocaloric effect have been estimated. The XANES study indicates that Zn doping does not change the valence states in Mn and V. It has been shown that the obtained values of critical exponents β, γ and δ do not belong to universal class and the values are in between the 3D Heisenberg model and the mean field interaction model. The magnetization data follow the scaling equation and collapse into two branches indicating that the calculated critical exponents and critical temperature are unambiguous and intrinsic to the system. All the samples show large magneto-caloric effect. The second peak in magneto-caloric curve of Mn0.95Zn0.05V2O4 is due to the strong coupling between orbital and spin degrees of freedom. But 10% Zn doping reduces the residual spins on the V-V pairs resulting the decrease of coupling between orbital and spin degrees of freedom.

Pressure induced effects on the chemical and magnetic structure of spinel MnV2O4

The influence of external pressure (P  ⩽  5 GPa) on both the structural and magnetic ordering in MnV2O4 has been investigated using neutron diffraction technique. The volume and the V-V distance decrease with pressure while the c/a ratio increases, suggesting a lowering of the distortion with pressure. Under ambient conditions this compound exhibits a structural transition (T S) from tetragonal to cubic at ~53 K and a magnetic transition (T N ) at ~56 K. It is found that with an increase in pressure to 5 GPa, T N increases (from 56 K to 80 K), dT N /dP  >  0, while T S decreases (from 53 K to 37 K). The non collinear magnetic structure in the tetragonal phase at 5 GPa and 10 K remains the same as at ambient pressure. However, the Mn and V sublattice, now exhibits distinct transition temperatures, [Formula: see text] ~ 80 K, and [Formula: see text] ~ 60 K. The transition to the cubic phase at T S is accompanied by a collinear alignment of the Mn and V spins and a reduction in the Mn moment. The region in which the structure remains in the cubic phase with collinear magnetic structure increases with pressure from ~3 K at ambient pressure to ~43 K at 5 GPa pressure.

Effect of Chemical Pressure at the Boundary of Mott Insulator to Itinerant Electron Limit Transition in Spinel Vanadates

The effect of chemical pressure on the structural, transport, magnetic and electronic properties of ZnV2O4 has been investigated by doping Mn and Co onto the Zn sites of ZnV2O4. With Mn doping the V-V distance increases and with Co doping it decreases. The resistivity and thermoelectric power data indicate that, as the V-V distance decreases, the system moves towards quantum phase transition. The transport data also indicate that the conduction is due to small polaron hopping. The chemical pressure shows a non-monotonous behaviour of charge gap and activation energy. On the other hand, when Ti is doped on the V-site of ZnV2O4, the metal metal distance decreases and, at the same time, T-N also increases.

Magnetic and structural properties of Zn doped MnV2O4

The magnetization, Neutron diffraction and X-ray diffraction of Zn doped MnV2O4 as a function of temperature have been measured. It has been observed, with increase of Zn the non-linear orientation of Mn spins with the V spins will decrease which effectively decrease the structural transition temperature more rapidly than Curie Temperature.