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Register a new userSite-substitution in GdMnO3 : effects on structural, electronic and magnetic properties
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We report on detailed structural, electronic and magnetic studies of GdMn$_{1-x}$Cr$_x$O$_3$ for Cr doping levels 0 $le$ $x$ $le$ 1. In the solid solutions, the Jahn-Teller distortion associated with Mn$^{3+}$ ions gives rise to major changes in the ${bc}$-plane sub-lattice and also the effective orbital ordering in the ${ab}$-plane, which persist up to the compositions $x$ $sim$ 0.35. These distinct features in the lattice and orbital degrees of freedom are also correlated with $bc$-plane anisotropy of the local Gd environment. A gradual evolution of electronic states with doping is also clearly seen in O $K$-edge x-ray absorption spectra. Evidence of magnetization reversal in field-cooled-cooling mode for $x$ $ge$ 0.35 coinciding the Jahn-Teller crossover, suggests a close correlation between magnetic interaction and structural distortion. These observations indicate a strong entanglement between lattice, spin, electronic and orbital degrees of freedom. The nonmonotonic variation of remnant magnetization can be explained by doping induced modification of magnetic interactions. Density functional theory calculations are consistent with a layer-by-layer type doping with ferromagnetic (antiferomagnetic) coupling between Mn (Cr) ions for intermediate compound ($x$ = 0.5), which is distinct from that observed for the end members GMnO$_3$ and GdCrO$_3$.
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Cubic spinel CoCr2O4 has attained recent attention due to its multiferroic properties. However, the Co site substitution effect on the structural and magnetic properties has rarely been studied in thin film form. In this work, the structural and magnetic properties of Co1-xNixCr2O4 (x = 0, 0.5) epitaxial thin films deposited on MgAl2O4 (100) and MgO (100) substrates to manipulate the nature of strain in the films using pulsed laser deposition (PLD) technique are presented. The epitaxial nature of the films was confirmed through X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements. Raman measurements revealed a disappearance of characteristic A1g and F2g modes of the CoCr2O4 with increase in the Ni content. Atomic force microscopy (AFM) studies show a modification of the surface morphology upon Ni substitution. Magnetic measurements disclose that the ferrimagnetic Curie temperature (Tc) of the CoCr2O4 in thin film grown on MgAl2O4 (100) and MgO (100) substrates were found to be 100.6 +/- 0.5 K and 93.8 +/- 0.2 K, respectively. With Ni substitution the transition temperatures significantly get enhanced from that of CoCr2O4. X-ray photoelectron spectroscopy (XPS) suggests Cr3+ oxidation states in the films, while Co ions are present in a mixed Co2+/Co3+ oxidation state. The substitution of Ni at Co site significantly modifies the line shape of the core level as well as the valence band. Ni ions are also found to be in a mixed 2+/3+ oxidation state. O 1s core level display asymmetry related to possible defects like oxygen vacancies in the films.
We have investigated the Vanadium- (V) substituted Ni-Zn-Co ferrites where the samples were prepared using solid-state reaction technique. The impact of V5+ substitution on the structural, magnetic, dielectric and electrical properties of Ni-Zn-Co ferrites has been studied. XRD analysis confirmed the formation of a single-phase cubic spinel structure. The lattice constants have been calculated both theoretically and experimentally along with other structural parameters such as bulk density, X-ray density and porosity. The FESEM images are taken to study the surface morphology. FTIR measurement is also performed which confirms spinel structure formation. The saturation magnetization (Ms), coercive field (Hc) and Bohr magneton (B) were calculated from the obtained M-H loops. The temperature dependent permeability is studied to obtain the Curie temperature. Frequency and composition dependence of permeability was also analyzed. Dielectric behavior and ac resistivity are also subjected to investigate the frequency dependency. An inverse relationship was observed between the composition dependence of dielectric constant and ac resistivity. The obtained results such as the electrical resistivity, dielectric constants and magnetic properties suggest the appropriateness of the studied ferrites in microwave device applications.
We have performed an experimental study of the crystal structure, lattice-dynamics, and optical properties of PbCrO4 (the mineral crocoite) at ambient and high pressures. In particular, the crystal structure, Raman-active phonons, and electronic band-gap have been accurately determined. X-ray-diffraction, Raman, and optical-absorption experiments have allowed us also to completely characterize two pressure-induced structural phase transitions. The first transition is isostructural, maintaining the monoclinic symmetry of the crystal, and having important consequences in the physical properties; among other a band-gap collapse is induced. The second one involves an increase of the symmetry of the crystal, a volume collapse, and probably the metallization of PbCrO4. The results are discussed in comparison with related compounds and the effects of pressure in the electronic structure explained. Finally, the room-temperature equation of state of the low-pressure phases is also obtained.
To date, germanene has only been synthesized on metallic substrates. A metallic substrate is usually detrimental for the two-dimensional Dirac nature of germanene because the important electronic states near the Fermi level of germanene can hybridize with the electronic states of the metallic substrate. Here we report the successful synthesis of germanene on molybdenum disulfide (MoS$_2$), a band gap material. Pre-existing defects in the MoS$_2$ surface act as preferential nucleation sites for the germanene islands. The lattice constant of the germanene layer (3.8 $pm$ 0.2 AA) is about 20% larger than the lattice constant of the MoS$_2$ substrate (3.16 AA). Scanning tunneling spectroscopy measurements and density functional theory calculations reveal that there are, besides the linearly dispersing bands at the $K$ points, two parabolic bands that cross the Fermi level at the $Gamma$ point.
The electronic and magnetic properties of transition metal dichalcogenides are known to be extremely sensitive to their structure. In this paper we study the effect of structure on the electronic and magnetic properties of mono- and bilayer $VSe_2$ films grown using molecular beam epitaxy. $VSe_2$ has recently attracted much attention due to reports of emergent ferromagnetism in the 2D limit. To understand this important compound, high quality 1T and distorted 1T films were grown at temperatures of 200 $^text{o}$C and 450 $^text{o}$C respectively and studied using 4K Scanning Tunneling Microscopy/Spectroscopy. The measured density of states and the charge density wave (CDW) patterns were compared to band structure and phonon dispersion calculations. Films in the 1T phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the 1T-film shows a CDW pattern with 4a $times$ 4a periodicity which is the 2D version of the bulk CDW observed in this compound. Our phonon dispersion calculations confirm the presence of a soft phonon at the correct wavevector that leads to this CDW. In contrast, the first layer of distorted 1T phase films shows a strong stripe feature with varying periodicities, while the second layer displays no observable CDW pattern. Finally, we find that the monolayer 1T $VSe_2$ film is weakly ferromagnetic, with ~ $3.5 {mu}_B$ per unit similar to previous reports.
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