XRD and XPS characterization of mixed valence Mn3O4 hausmannite thin films prepared by chemical spray pyrolysis technique

Mesoporous Mn2O3 and Mn3O4 with Crystalline Walls

Many-body $GW$ calculations have emerged as a standard for the prediction of band gaps, band structures, and optical properties for main-group semiconductors and insulators, but it is not well established how predictive the $GW$ method is in general for transition metal (TM) compounds. Surveying the series of 3$d$ oxides within a typical $GW$ approach using the random-phase approximation reveals mixed results, including cases where the calculated band gap is either too small or too large, depending on the oxidation states of the TM (e.g., FeO/Fe${}_{2}$O${}_{3}$, Cu${}_{2}$O/CuO). The problem appears to originate mostly from a too high average $d$-orbital energy, whereas the splitting between occupied and unoccupied $d$ symmetries seems to be reasonably accurate. It is shown that augmenting the $GW$ self-energy by an attractive (negative) and occupation-independent on-site potential for the TM $d$ orbitals with a single parameter per TM cation can reconcile the band gaps for different oxide stoichiometries and TM oxidation states. In Cu${}_{2}$O, which is considered here in more detail, standard $GW$ based on wave functions from initial density or hybrid functional calculations yields an unphysical prediction with an incorrect ordering of the conduction bands, even when the magnitude of the band gap is in apparent agreement with experiment. The correct band ordering is restored either by applying the $d$-state potential or by iterating the wave functions to self-consistency, which both have the effect of lowering the Cu-$d$ orbital energy. While it remains to be determined which improvements over standard $GW$ implementations are needed to achieve an accurate ab initio description for a wide range of transition metal compounds, the application of the empirical on-site potential serves to mitigate the problems specifically related to $d$ states in $GW$ calculations.

https://link.aps.org/accepted/10.1103/PhysRevB.87.085112

Band-structure calculations for the 3 d transition metal oxides in G W

Very uniform and well shaped Mn₃O₄ nano-octahedra are synthesized using a simple hydrothermal method under the help of polyethylene glycol (PEG200) as a reductant and shape-directing agent. The nano-octahedra formation mechanism is monitored. The shape and crystal orientation of the nanoparticles is reconstructed by scanning electron microscopy and electron tomography, which reveals that the nano-octahedra only selectively expose {101} facets at the external surfaces. The magnetic testing demonstrates that the Mn₃O₄ nano-octahedra exhibit anomalous magnetic properties: the Mn₃O₄ nano-octahedra around 150 nm show a similar Curie temperature and blocking temperature to Mn₃O₄ nanoparticles with 10 nm size because of the vertical axis of [001] plane and the exposed {101} facets. With these Mn₃O₄ nano-octahedra as a catalyst, the photodecomposition of rhodamine B is evaluated and it is found that the photodecomposition activity of Mn₃O₄ nano-octahedra is much superior to that of commercial Mn₃O₄ powders. The anomalous magnetic properties and high superior photodecomposition activity of well shaped Mn₃O₄ nano-octahedra should be related to the special shape of the nanoparticles and the abundantly exposed {101} facets at the external surfaces. Therefore, the shape preference can largely broaden the application of the Mn₃O₄ nano-octahedra.

http://ematweb.cmi.ua.ac.be/emat/pdf/1690.pdf

Well shaped Mn₃O₄ nano-octahedra with anomalous magnetic behavior and enhanced photodecomposition properties.

We report the magnetic and microstructural properties of antiferromagnetic MnO nanoparticles with shells of ferrimagnetic ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$, which is opposite the usual arrangement of antiferromagnetically coated ferromagnetic nanoparticles. In addition, the antiferromagnetic MnO cores order at much higher temperature $({T}_{N}=118\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ than the ferrimagnetic ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ shells $({T}_{C}=43\phantom{\rule{0.3em}{0ex}}\mathrm{K})$---another reversal of the usual situation. The single crystal MnO cores, with rocksalt structure, are crystallographically aligned with the tetragonal spinel structure of the ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ shells. Particles field cooled in $50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ have large coercive force and exchange bias below ${T}_{C}$, e.g., 5800 and $2950\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$, respectively, at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The spontaneous magnetization at ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$ is $\ensuremath{\sim}20%$ of its value at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and remains finite for more than $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ above ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$. Hysteresis with exchange bias is present in this anomalous region. The MnO cores with their uncompensated spins are responsible for the behavior above ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$. The MnO cores have a blocking temperature of $95\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the hysteresis and exchange bias above ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$ results from the switching of the MnO spin lattices by their uncompensated spins. Analysis of the thermoremanent magnetization and field cooling and/or zero field cooling in $50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, and the dependence of exchange bias on the temperature at which the cooling field was applied support this model.

/pdf/antiferromagnetic-mno-nanoparticles-with-ferrimagnetic-mn-3-4c57gjds4f.pdf

Antiferromagnetic MnO nanoparticles with ferrimagnetic Mn 3 O 4 shells: Doubly inverted core-shell system

A neutron diffraction study has been made on the magnetic structure of single crystalline Mn3O4 from liquid helium temperature to the Neel point, 41K, and the nuclear structure at room temperature has been refined.

https://iopscience.iop.org/article/10.1088/0022-3719/7/2/019/pdf

O V Nielsen

Papers

The magnetic structure of Mn3O4 Hausmannite between 4.7K and Neel point, 41K