Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userCommensurate structural modulation in the charge- and orbitally-ordered phase of the quadruple perovskite (NaMn$_3$)Mn$_4$O$_{12}$
196
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
By means of synchrotron x-ray and electron diffraction, we studied the structural changes at the charge order transition $T_{CO}$=176 K in the mixed-valence quadruple perovskite (NaMn$_3$)Mn$_4$O$_{12}$. Below $T_{CO}$ we find satellite peaks indicating a commensurate structural modulation with the same propagation vector q =(1/2,0,-1/2) of the CE magnetic order that appears at low temperature, similarly to the case of simple perovskites like La$_{0.5}$Ca$_{0.5}$MnO$_3$. In the present case, the modulated structure together with the observation of a large entropy change at $T_{CO}$ gives evidence of a rare case of full Mn$^{3+}$/Mn$^{4+}$ charge and orbital order consistent with the Goodenough-Kanamori model.
rate research
Read More
By means of neutron powder diffraction, we investigated the effect of the polar Bi$^{3+}$ ion on the magnetic ordering of the Mn$^{3+}$ ions in BiMn$_3$Mn$_4$O$_{12}$, the counterpart with textit{quadruple} perovskite structure of the textit{simple} perovskite BiMnO$_3$. The data are consistent with a textit{noncentrosymmetric} spacegroup $Im$ which contrasts the textit{centrosymmetric} one $I2/m$ previously reported for the isovalent and isomorphic compound LaMn$_3$Mn$_4$O$_{12}$, which gives evidence of a Bi$^{3+}$-induced polarization of the lattice. At low temperature, the two Mn$^{3+}$ sublattices of the $A$ and $B$ sites order antiferromagnetically (AFM) in an independent manner at 25 and 55 K, similarly to the case of LaMn$_3$Mn$_4$O$_{12}$. However, both magnetic structures of BiMn$_3$Mn$_4$O$_{12}$ radically differ from those of LaMn$_3$Mn$_4$O$_{12}$. In BiMn$_3$Mn$_4$O$_{12}$ the moments $textbf{M}_{A}$ of the $A$ sites form an anti-body AFM structure, whilst the moments textbf{M}$_{B}$ of the $B$ sites result from a large and textit{uniform} modulation $pm textbf{M}_{B,b}$ along the b-axis of the moments textbf{M}$_{B,ac}$ in the $ac$-plane. The modulation is strikingly correlated with the displacements of the Mn$^{3+}$ ions induced by the Bi$^{3+}$ ions. Our analysis unveils a strong magnetoelastic coupling between the internal strain created by the Bi$^{3+}$ ions and the moment of the Mn$^{3+}$ ions in the $B$ sites. This is ascribed to the high symmetry of the oxygen sites and to the absence of oxygen defects, two characteristics of quadruple perovskites not found in simple ones, which prevent the release of the Bi$^{3+}$-induced strain through distortions or disorder. This demonstrates the possibility of a large magnetoelectric coupling in proper ferroelectrics and suggests a novel concept of internal strain engineering for multiferroics design.
Ultrasound velocity measurements of the orbitally-frustrated GeCo$_2$O$_4$ reveal unusual elastic instabilities due to the phonon-spin coupling within the antiferromagnetic phase. Shear moduli exhibit anomalies arising from the coupling to short-range ferromagnetic excitations. Diplike anomalies in the magnetic-field dependence of elastic moduli reveal magnetic-field-induced orbital order-order transitions. These results strongly suggest the presence of geometrical orbital frustration which causes novel orbital phenomena within the antiferromagnetic phase.
CaCu$_3$Fe$_4$O$_{12}$ exhibits a temperature-induced transition from a ferrimagnetic-insulating phase, in which Fe appears charge disproportionated, as Fe$^{3+}$ and Fe$^{5+}$, to a paramagnetic-metallic phase at temperatures above 210 K, with Fe$^{4+}$ present. To describe it, we propose a microscopic effective model with two interpenetrating sublattices of Fe$^{(4-delta)+}$ and Fe$^{(4+delta)+}$, respectively, being $delta$ the Fe-charge disproportionation. We include all $3d$-Fe orbitals: $t_{2g}$ localized orbitals, with spin 3/2 and magnetically coupled, plus two degenerate itinerant $e_g$ orbitals with local and nearest-neighbor (NN) electron correlations, and hopping between NN $e_g$ orbitals of the same symmetry. Allub and Alascio previously proposed a model to describe the phase transition in LaCu$_3$Fe$_4$O$_{12}$ from a paramagnetic-metal to an antiferromagnetic-insulator, induced by temperature or pressure, involving charge transfer between Fe and Cu ions, in contrast to Fe-charge disproportionation. With the model proposed for CaCu$_3$Fe$_4$O$_{12}$, modified to account for this difference between the two compounds, the density of states of the itinerant Fe orbitals was obtained, using Greens functions methods. The phase diagram for CaCu$_3$Fe$_4$O$_{12}$ was calculated, including phases exhibiting Fe-charge disproportionation, where the two eg orbitals in each site are symmetrically occupied, as well as novel phases exhibiting local orbital selectivity/asymmetric occupation of $e_g$ orbitals. Both kinds of phases may exhibit paramagnetism and ferromagnetism. We determined the model parameters which best describe the phase transition observed in CaCu$_3$Fe$_4$O$_{12}$, and found other phases at different parameter ranges, which might be relevant for other compounds of the ACu$_3$Fe$_4$O$_{12}$ family, which present both types of transitions.
We report a neutron powder diffraction study of $R$Mn$_7$O$_{12}$ quadruple perovskite manganites with $R$ = La, Ce, Nd, Sm, and Eu. We show that in all measured compounds concomitant magnetic ordering of the $A$ and $B$ manganese sublattices occurs on cooling below the N$mathrm{acute{e}}$el temperature. The respective magnetic structures are collinear, with one uncompensated Mn$^{3+}$ moment per formula unit as observed in bulk magnetisation measurements. We show that both LaMn$_7$O$_{12}$ and NdMn$_7$O$_{12}$ undergo a second magnetic phase transition at low temperature, which introduces a canting of the $B$ site sublattice moments that is commensurate in LaMn$_7$O$_{12}$ and incommensurate in NdMn$_7$O$_{12}$. This spin canting is consistent with a magnetic instability originating in the $B$ site orbital order. Furthermore, NdMn$_7$O$_{12}$ displays a third magnetic phase transition at which long range ordering of the Nd sublattice modifies the periodicity of the incommensurate spin canting. Our results demonstrate a rich interplay between transition metal magnetism, orbital order, and the crystal lattice, which may be fine tuned by cation substitution and rare earth magnetism.
Mn$_3$O$_4$ is a spin frustrated magnet that adopts a tetragonally distorted spinel structure at ambient conditions and a CaMn$_2$O$_4$-type postspinel structure at high pressure. We conducted both optical measurements and emph{ab} emph{initio} calculations, and systematically studied the electronic band structures of both the spinel and postspinel Mn$_3$O$_4$ phases. For both phases, theoretical electronic structures are consistent with the optical absorption spectra, and display characteristic band-splitting of the conduction band. The band gap obtained from the absorption spectra is 1.91(6) eV for the spinel phase, and 0.94(2) eV for the postspinel phase. Both phases are charge-transfer type insulators. The Mn 3emph{d} $t_2$$_g$ and O 2emph{p} form antibonding orbitals situated at the conduction band with higher energy.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
