| We have investigated various materials with regard to the effects of pressure, field, and substitution on their multiferroic properties. In MnWO4, the Mn+2 ions can be substituted by magnetic Fe+2, or non-magnetic Zn+2 ions, since MnWO4, FeWO4, and ZnWO4 are isomorphic. The multiferroic properties of Mnl-x FexWO4 (x = 0.02, 0.035, 0.05 and 0.1) and Mn1-xZnxWO4 (x = 0.02, 0.05, 0.1, 0.2 and 0.4) single crystals were investigated. We discovered the suppression (Fe substitution), or enhancement (Zn substitution) of the ferroelectric (FE) polarization. External magnetic field (H), induces re-entrant ferroelectricity, or restores FE/spiral magnetic phase in Mn 1-xFexWO4. The temperature-concentration (x)- magnetic field (H) phase diagrams for both solid solutions were completely resolved from polarization, dielectric constant, specific heat, and magnetic measurements. The anisotropic Heisenberg model was solved under mean field approximation to qualitatively understand the effects of substitution and field. Neutron diffraction studies confirm our conclusions. We further investigated the effect of hydrostatic pressure on FE polarization in different multiferroic compounds, for example, in Ni3V 2O8 MnWO4 and YMn2O5. The pressure-temperature phase diagram was constructed for these compounds. Pressure completely suppresses the FE polarization of Ni3V 2O8 and MnWO4. However, in YMn2O5, external pressure did reverse the sign of the polarization at low temperature. We have conducted high resolution thermal expansion measurements revealing significant lattice anomalies, and correlated with the observed effects of pressure.;We found that HoFe3(BO 3)4 is unique among rare-earth iron borate family, RFe3(BO3)4 since it shows spontaneous polarization, P(T) below TN, and decreases with increasing magnetic field. Whereas NdFe 3(BO3)4 shows large polarization upon changing magnetic field, but it doesn't show spontaneous polarization. The Magneto-electric properties of single crystals of Ho 1-xNdx Fe3(BO3)4 (x = 0.0, 0.25, 0.5, 0.75) was studied thoroughly. The magnetic field (H) - temperature phase diagram has completely been resolved for H ∥ C axis and H ∥ A axis from the thermodynamic, magnetic, and polarization data. In contrast to HoFe3(BO3)4 the solid solution of Ho1-x NdxFe3(BO3) 4 exhibits both spontaneous polarization and the polarization due to magnetoelectric effect. We further investigated the magnetic, magnetostriction, and magnetoelectric properties of d-electron free rare-earth aluminum borate TmAl3(BO3)4 between room temperature and 2 K. The polarization along a and c directions reaches up to 300 muC/m2 at 70 kOe with the field applied along the a axis. The magnetoelectric polarization is proportional to the magnetostriction. This compound show that the rare-earth moment itself can generate a large magnetoelectric effect which makes it easier to understand the origin of the magnetoelectricity. |
