We present a detailed study on the magnetic structure and magnetoelectric properties of several double perovskites $R_2\mathrm{CoMn}{\mathrm{O}}_6$ ($R=\mathrm{Ho}$, Tm, Yb, and Lu). All of these samples show an almost perfect $(\sim 94\%)$ ordering of $\mathrm{C}{\mathrm{o}}^{2+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$ cations in the unit cell. Our research reveals that the magnetic ground state strongly depends on the $R$ size. For samples with larger $R$ (Ho and Tm), the ground state is formed by a ferromagnetic order ($F$ type) of $\mathrm{C}{\mathrm{o}}^{2+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$ moments, while $R$ either remains mainly disordered (Ho) or is coupled antiferromagnetically (Tm) to the Co/Mn sublattice. For samples with smaller $R$ (Yb or Lu), competitive interactions lead to the formation of an $E$-type arrangement for the $\mathrm{C}{\mathrm{o}}^{2+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$ moments with a large amount of extended defects such as stacking faults. The $\mathrm{Y}{\mathrm{b}}^{3+}$ is partly ordered at very low temperature. The latter samples undergo a metamagnetic transition from the $E$ into the $F$ type, which is coupled to a negative magnetodielectric effect. Actually, the real part of dielectric permittivity shows an anomaly at the magnetic transition for the samples exhibiting an $E$-type order. This anomaly is absent in samples with $F$-type order, and, accordingly, it vanishes coupled to the metamagnetic transition for $R=\mathrm{Yb}$ or Lu samples. At room temperature, the huge values of the dielectric constant reveal the presence of Maxwell-Wagner depletion layers. Pyroelectric measurements reveal a high polarization at low temperature, but the onset of pyroelectric current is neither correlated to the kind of magnetic ordering nor to the magnetic transition. Our study identified the pyroelectric current as thermally stimulated depolarization current and electric-field polarization curves show a linear behavior at low temperature. Therefore, no clear ferroelectric transition occurs in these compounds.

• Received 21 April 2017

DOI:https://doi.org/10.1103/PhysRevB.96.024409