Magnetoelectric and structural properties ofY2CoMnO6: The role of antisite defects

Abstract

We have carried out an investigation on the magnetoelectric properties of the presumed multiferroic Y 2 CoMnO 6 with different degrees of Co/Mn atomic ordering. The magnetic ground state was studied by neutron diffraction, showing a collinear ferromagnetic (FM) ordering of Co and Mn moments with a small antiferromagnetic canting. No superstructure peaks from an E-type magnetic structure were detected in our measurements. Magnetic measurements reveal FM transitions with pinned magnetic domains. The degree of Co/Mn ordering affects the Curie temperature only a little, but has strong effects on the magnetic hysteresis loops, and the FM moment signal at high field increases with increasing such order. The loops display steps at critical fields whose number and extent depends on each specimen. The most ordered sample exhibits the greatest steps ascribed to the alignment of magnetic domains separated by antiphase boundaries. All samples are insulators exhibiting low dielectric loss and dielectric constants at low temperature. On warming, they show a step increase in the real dielectric permittivity accompanied by peaks in the dielectric loss typical of thermally activated hopping processes. 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 for these compounds that increases with increasing the Co/Mn ordering. There is no correlation between the magnetic transition and the onset of pyroelectric current. No significant changes are observed in the pyroelectric effect measured under an external magnetic field, so magnetoelectric coupling is negligible. This paper identifies the pyroelectric current as thermally stimulated depolarization current ascribed to the reorientation of defect dipoles with activation energy of about 0.05 eV. Therefore, no ferroelectric transition occurs in these compounds, discarding the existence of intrinsic magnetoelectric multiferroicity.

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 .…”

“…2 b, the spread of AC χ , which occurs below T C , is manifestly ascribed to the additional domain wall motions. The peak height is reduced without any shift upon increasing f , which would be attributed to a magnetic disaccommodation process of pinned domain walls 20 , 39 . The absence of the peak shift in AC χ does not support that the magnetic irreversibility in DC χ (Fig.…”

“…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).…”

“…The emergent properties arise from the intricate magnetic interactions and antiphase boundaries/antisite disorders between the mixed-valence magnetic ions 18 , 19 . In the case that a magnetic rare-earth ion is included, the additional ordering of the rare-earth magnetic moment at a lower temperature generates a significant modification of the magnetic properties 20 – 22 .…”

Thermally stimulated depolarization current properties of Co4Nb2O9 ceramics

“…TSDC technique had also been widely used to characterize defect properties in many materials, but its use in studying defects in multiferroics was seldom reported. The defects could influence physical properties of multiferroics greatly . They can induce ferromagnetism, generate an internal electric field to tune the polarization, pin ferroelectric domain, and even induce polarization memory effect .…”