Large magnetocapacitance in electronic ferroelectric manganite systems

We have observed a sizable positive magnetocapacitance ($\sim$$5-90\%$) in perovskite Pr$_{0.55}$Ca$_{0.45}$MnO$_3$ and bilayer Pr(Sr$_{0.1}$Ca$_{0.9}$)$_2$Mn$_2$O$_7$ system under 5T magnetic field across 20-100 K below the magnetic transition point T$_N$. The magnetodielectric effect, on the other hand, exhibits a crossover: (a) from positive to negative for the perovskite system and (b) from negative to positive for the bilayer system over the same temperature range. The bilayer Pr(Sr$_{0.1}$Ca$_{0.9}$)$_2$Mn$_2$O$_7$ system exhibits a sizable anisotropy as well. We have also noticed the influence of magnetic field on the dielectric relaxation characteristics of these systems. These systems belong to a class of improper ferroelectrics and are expected to exhibit charge/orbital order driven ferroelectric polarization below the transition point T$_{CO}$. Large magnetocapacitance in these systems shows typical multiferroic behavior even though the ferroelectric polarization is small in comparison to that of other ferroelectrics.Comment: 6 pages with 5 embedded figures; accepted for publication in J. Appl. Phy

Determination of intrinsic ferroelectric polarization in lossy improper ferroelectric systems

We measured the intrinsic hysteretic polarization in lossy improper and nanoferroelectric systems where the nonhysteretic polarization and leakage are large and the relaxation takes place over a broader time scale. We used different measurement protocols such as standard single triangular voltage pulse, a pulse train of Positive Up Negative Down, and an even more complicated pulse train of fourteen voltage pulses and compared the results obtained. We show that a protocol which sends a train of fourteen pulses is more appropriate for extracting relaxed (i.e., time scale independent) and intrinsic remanent polarization for these samples. We also point out that it is possible to select and design an appropriate measurement protocol depending on the magnitude of polarization and leakage of the system. Published by AIP Publishing

Determination of intrinsic ferroelectric polarization in lossy improper ferroelectric systems

We measured the intrinsic hysteretic polarization in lossy improper and nanoferroelectric systems where the nonhysteretic polarization and leakage are large and the relaxation takes place over a broader time scale. We used different measurement protocols such as standard single triangular voltage pulse, a pulse train of Positive Up Negative Down, and an even more complicated pulse train of fourteen voltage pulses and compared the results obtained. We show that a protocol which sends a train of fourteen pulses is more appropriate for extracting relaxed (i.e., time scale independent) and intrinsic remanent polarization for these samples. We also point out that it is possible to select and design an appropriate measurement protocol depending on the magnitude of polarization and leakage of the system. Published by AIP Publishing

Field-Induced Spin-Structural Transition and Giant Magnetostriction in Ising Chain α‑CoV₂O₆

We have investigated the temperature and magnetic field dependence of magnetization, specific heat (<i>C</i>_p), and relative sample length change (Δ<i>L</i>/<i>L</i>₀) for understanding the field-induced spin-structural change in quasi-one-dimensional spin chain α-CoV₂O₆ which undergoes antiferromagnetic (AFM) transition below <i>T</i>_N = 15 K. Analysis of <i>C</i>_p(<i>T</i>) shows that an effective <i>S</i> = 1/2 Ising state is realized below 20 K, though the magnetic fluctuations persist well above <i>T</i>_N. <i>C</i>_p and the coefficient of linear thermal expansion (α) exhibit strong <i>H</i> dependence in the AFM state. We also observe a huge positive magnetostriction [Δ<i>L</i>(<i>H</i>)/<i>L</i>₀] below 20 K which does not show any tendency of saturation up to 9 T. With increasing field, a sharp and symmetric peak emerges below <i>T</i>_N in both <i>C</i>_p(<i>T</i>) and α­(<i>T</i>) due to field-induced first-order ferrimagnetic/ferromagnetic-paramagnetic transitions. The large value of magnetostriction below <i>T</i>_N suggests strong spin–lattice coupling in α-CoV₂O₆