Tuning of the Multiferroic Properties of Selected Materials by Magnetic Field, Chemical Doping, and Pressure

This works will explore how magnetic field, chemical doping, and hydrostatic pressure affect type-II multiferroic materials. Above a critical field, a ferroelectric polarization is induced by an externally applied magnetic field and its reversal is observed in coexisting multiferroic phases of Mn1-xCoxWO4. The sign reversal of the polarization is explained on the basis of strong inter-domain coupling between the two coexisting multiferroic phases and the preserved chirality of the spin helix across their domain boundary. Also observed at low cobalt doping, was the continuous rotation of the polarization in the magnetic field, thereby charging the ferroelectric domain walls. Next, the evolution of different magnetic and multiferroic phases was explored up to 30% nickel doping in MnWO4 via magnetic, ferroelectric, heat capacity and neutron diffraction measurements. Nickel doping quickly suppressed the paraelectric phase observed at lower nickel dopings and suppressed the multiferroicity at higher doping in MnWO4. The anisotropy and anti-correlation effect between Mn and Ni spins forced the suppression of polarization which was observed for higher nickel concentration. Lastly the effect of pressure up to 18 kbars was explored for GdMn2O5 and RCrO3 (R = Ho, Dy, Gd). A new ferroelectric phase was induced above a critical pressure and also at a higher temperature, than the already reported multiferroic phase for GdMn2O5. The new ferroelectric phase is explained on the basis of pressure induced decoupling of Gd-moments and Mn-spins in the commensurate phase. The decoupled Mn spins order at higher temperature which gives rise to the new ferroelectric phase. On the other hand, the pressure effect on Neel temperature of the multiferroic RCrO3 (R = Ho, Dy, Gd) shows that Neel temperature of RCrO3 increases with pressure.