Magnetic order and magnetoelectric properties of R2CoMnO6 perovskites ( R=Ho , Tm, Yb, and Lu)

Abstract

We present a detailed study on the magnetic structure and magnetoelectric properties of several double perovskites R 2 CoMnO 6 (R= Ho, Tm, Yb and Lu). All of these samples show an almost perfect (~94%) ordering of Co 2+ and Mn 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 Co 2+ and Mn 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 Co 2+ and Mn 4+ moments with a large amount of extended defects such as stacking faults. The Yb 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=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.3

Tunable magnetization steps in mixed valent ferromagnet Eu2CoMnO6

“…A sharp and positive peak of d χ /d T was observed at ~ 114 K, which represents an additional domain wall de-pinning process associated with the predominant long-range ferromagnetic order 37 , 38 . The χ values for the two different orientations exhibit strong magnetic anisotropy, which suggests that the spins are nearly aligned along the c axis, consistent with the neutron diffraction results that the ferromagnetic moments are aligned closed to the c -axis 20 , 21 .…”

“…In the case that misplaced magnetic ions are frustrated, the portion of anti-sites may increase further. Thus, the observed magnetic moments are compatible with the amount of antisite defects estimated as 6–8% from the neutron diffraction experiments on the isostructrual compounds 20 , 21 . To verify the oxygen content of the as-grown crystal, we used a thermogravimetric and differential thermal analysis under 5% H 2 /Ar atmosphere.…”

“…These series of compounds could be refined within two possible space groups, orthorhombic Pbnm (or Pnma ) and monoclinic P 2 1 / n 23 . However, the recent neutron diffraction studies on polycrystalline R 2 CoMnO 6 (R = Y, Ho, and Tm) clearly demonstrate that the magnetic structure is accompanied by alternating Co 2+ and Mn 4+ spins 20 , 21 . The refinement result of the same magnetic moments for both Co and Mn ions suggests that valences of the ions are Co 2+ and Mn 4+ corresponding to high spin states ( S = 3/2).…”

Strong magnetoelectric coupling in mixed ferrimagnetic-multiferroic phases of a double perovskite

“…The formation of anti-sites in addition to the dominant FM order 27,29,30 of Co 2+ and Mn 4+ moments is known as the mechanism for the observed magnetic exchange bias in polycrystalline Y 2 CoMnO 6 19 . In Tm 2 CoMnO 6 and Er 2 CoMnO 6 (ECMO), the neutron diffraction studies confirm that the order of Co 2+ and Mn 4+ moments is FM and the order of Er 3+ /Tm 3+ moments at lower temperature activates the additional ferrimagnetic (FIM) order between Er 3+ /Tm 3+ and ferromagnetic Co 2+ /Mn 4+ sublattices 31–33 . The FIM order exhibits an inversion of the magnetic hysteresis loop in polycrystalline ECMO 34 .…”

Magnetic inversion symmetry breaking and spin reorientation in Tb2MnNiO6 : A polar strong ferromagnet

“…For example, in compounds with nonsymmorphic space groups, if the magnetic atoms sit at special Wyckoff positions (e.g., with pointgroup symmetry −1, as the monoclinic R 2 MnMO 6 [M: Ni,Co] series), some propagation vectors at the zone boundary of the paraelectric phase favor polar symmetries and the occurrence of type II multiferroicity. An example is the collinear E phase in monoclinic P2 1 /n double perovskites (DPs) presenting upup-down-down (↑↑↓↓) spin chains along a particular crystallographic direction (e.g., R 2 MnCoO 6 , R = Yb, Lu [3]). The presence of a magnetic propagation vector k = (0, 0, 1/2) implies that the glide plane is necessarily broken.…”