Effects of A-site disorder in the properties of A₂CoMnO₆(A = La, Tb)

Abstract

We have studied the structural and physical properties of the La2-xTbxCoMnO6 series. The crystal and magnetic structures of these compounds were determined by x-ray and neutron diffraction techniques. All samples belong to the family of double perovskites with space group P21/n, but the Co/Mn ordering is not perfect, and antisite defects are formed. The concentration of these defects increases for intermediate compositions, indicating that La/Tb disorder influences the Co/Mn arrangement. A ferromagnetic ground state is established due to the strong Mn(4+)-O-Co(2+) superexchange interaction. For the intermediate compositions and at low temperature, the Co/Mn ordering is accompanied by the ordering of Tb(3+) moments in the ab-plane, indicating a mutual polarization between both sublattices. Macroscopic magnetic properties reveal that Curie temperature decreases as Tb content increases in correlation with the increase of the structural distortion. All samples show semiconducting behaviour, and overall the electrical resistivity increases with decreasing La-content. The dielectric constant (ε') has a value of around 12 at low temperatures for all samples, revealing the lack of permanent dipoles. The temperature dependence of ε' on warming exhibits a strong increase that depends heavily on the frequency of the electric field. This effect is ascribed to non-intrinsic effects such as contacts or internal barrier-layers.

X-ray absorption and emission spectroscopy study of Mn and Co valence and spin states inTbMn1−xCoxO3Cuartero1, Lafuerza2, Rovezzi3 et al. 2016Phys. Rev. B332160View full textAdd to dashboardCite

“…[10][11] , as previously reported [22,23]. In order to confirm these statements, we explicitly calculate XAS spectra using CTM4XAS code.…”

“…In particular, the onset of the FM ground state for x=0.5 is at T C =100 K. [11] A preliminary model from powder neutron diffraction patterns, suggests that Mn and Co moments are oriented within ac plane and both ions are meant to have the same magnetic moment, estimated as 2.23(8)μ B for x=0.5, which is below the 3 μ B expected for S=3/2 high spin Mn 4+ and Co 2+ ions but it seems directly related to the number of antisite defects [11]. However, magnetic long range ordering completely disappears for lower and higher Co concentrations (x<0.…”

“…Thus, further attempts to enhance magnetic long range ordering leading to atomic displacements and spin-driven ferroelectricity, pointed towards the dilution of Mn sublattice with magnetic ions like Cr 3+ [9] and Co 2+ /Co 3+ [10,11]. The first substitution promoted higher values for magnetic field coercivity and remanence for low Cr interactions, but G-type AFM long range order correlations appear for x≥0.5 similar to those of TbCrO 3 .…”

Magnetoelectric and structural properties ofY2CoMnO6: The role of antisite defects

“…[10], Y 2 CoMnO 6 , with Y 3+ (r = 1.075Å) [18] in the A position, develops an E-type ordering so compounds with smaller atoms in this position should show similar magnetic structure. This is the case for Yb-and Lu-based compounds (r = 1.042 and 1.032Å, respectively) [18], but not for other R 2 CoMnO 6 compounds (R = Ho, Tm, or Er), where the cationic radius of the A atom ranges between [18] 1.072 and 1.052Å and the conventional FM ordering is found [19].…”

“…However, there is no clear experimental evidence that Y 2 CoMnO 6 adopts an E-type magnetic ordering, and it contradicts other works that claim a different ordering [12][13][14]. A study in the YMn 1-x Co x O 3 series [12] indicated that YMn 0.5 Co 0.5 O 3 exhibits a partial Mn/Co ordering (around 75%) and a collinear FM ordering similar to the one found in La 2 CoMnO 6 or Tb 2 CoMnO 6 [15,16]. Accordingly, a recent magnetic study of Y 2 CoMnO 6 also reports a collinear FM ordering [13], though the refinements of the neutron patterns suggests a significant number of antisite defects (ASDs), i.e., Mn and Co exchange positions.…”