Effects of transition metal doping on multiferroic ordering in ni3v2o8 and fevo4

We have studied the effects of doping both non-magnetic and magnetic ions on the phase transitions and multiferroic ordering in two multiferroic oxides; Ni3V2O8 and FeVO4. Magnetic, dielectric, specific heat, polarization and AC susceptibility measurements were used to track changes in phase transition temperatures. We found that the two higher temperature magnetic transitions in Ni3V2O8; TH = 9.1 K and TL = 6.3 K are suppressed to lower temperatures with all transition metal dopants. For Zn doping, the rates of the suppression of both TH and TL with dopant fraction are consistent with simple site dilution for two-dimensional spin systems, with the suppression of TH consistent with Ising spins and the suppression of TL consistent with Heisenberg spins. However, samples remain multiferroic at least up to 15% Zn doping. Conversely, spin-1/2 Cu doping strongly suppresses both transitions, for which the multiferroic magnetic structure is completely suppressed at only 10% Cu doping. However, below 10% Cu doping, the samples show enhanced ferroelectric polarization, and a sizable net magnetization also develops. With spin-3/2 Co doping, suppression is very small, with the multiferroic transition persisting even at 30% doping and the material showing Ni3V2O8 magnetic characteristics up to 80% doping. On the Co rich side of the composition, we find that the magnetic ordering temperatures for Co3V2O8 are suppressed rapidly with Ni doping. With higher spin dopants (e.g. spin-2 Fe and spin-5/2 Mn), suppression remains fairly low. We also present phase diagrams for (Ni1-xMx)3V2O8 (M = Zn, Cu, Co, Fe and Mn). These studies suggest that the spin structures in Ni3V2O8 responsible for the development of ferroelectric order are relatively robust against perturbations produced by both magnetic and non-magnetic dopants, with the most significant disruption of the magnetic structure developing for Cu doping. In the case of FeVO4, we find that the magnetoelectric coupling in FeVO4 is remarkably robust to dopants with minimal suppression in transition temperatures for 3 transition metal dopants, Zn, Cr and Fe. We observe clear reversible polarization even at 20% doping suggesting the multiferroic order persists even at a large doping fraction. It is possible that the low symmetry and the 3-dimensional spin structure of FeVO4 are responsible for this remarkable robustness against dopants. These results indicate multiferroic ordering in Ni3V2O8 and FeVO4 show high resilience to dopants which can be extremely important in tuning the properties of multiferroics in general. Cu doped Ni3V2O8 studies show it is possible to develop a sizable net magnetization simultaneously with strong polarization and good magnetoelectric coupling which is a much desirable property to have for device development