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We present here investigations on a hole doped layered perovskite, La1.2Sr0.8CoO4. In these systems concentration of the holes in host matrix decides ratio of the Co2+/Co3+ and magnetic phase diagram. In addition, spin-state of the Co3+ ions has an important role in magnetic correlations. Magnetization measurements show the signature of some exotic phases like cluster glass or canonical spin-glass and ferromagnetic ordering (Tc ∼ 211 K). In addition, calculated temperature variation of order parameter by also including the spin-state variations of Co3+ manifests that the orbital magnetic moments are not quenched here and has an important role in overall magnetic moment at low temperatures.

A detailed magnetic study on the Fe substituted NdCrTiO5 is presented. Phase purity of polycrystalline NdCr0.5Fe0.5TiO5 prepared by conventional solid-state technique has been confirmed by synchrotron X-ray diffraction (SXRD). Temperature-dependent magnetization (MT) measurements indicate a transition to an antiferromagnetic (AFM) type of magnetic ordering at T N ∼ 10 K. AFM type of order is also supported by magnetocaloric data. Magnetic hysteresis as well as a type of magnetic relaxation behavior collected below T N (at 5 K) indicate the presence of weak ferromagnetism.

Environmentally benign $(1\text{\ensuremath{-}}x)\mathrm{Ba}({\mathrm{Ti}}_{0.8}{\mathrm{Zr}}_{0.2}){\mathrm{O}}_{3}\text{\ensuremath{-}}x({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}){\mathrm{TiO}}_{3}$ (BZT-BCT) ceramics are promising materials due to their remarkable high piezoresponse [Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)]. In this Letter, by focusing on local and average structure in combination with macroscopic electromechanical and dielectric measurements we demonstrate the structure property relationship in the tetragonal BZT-BCT ceramic. During high-temperature cubic to tetragonal phase transformation, polar nanoregions are manifested through the spontaneous volume ferroelectrostriction at temperatures below $\ensuremath{\sim}477$ K. Temperature-dependent local structural investigations across the Zr $K$ edge extended x-ray absorption fine-structure spectroscopy reveal an anomalous collaboration between the ${\mathrm{ZrO}}_{6}$ and ${\mathrm{TiO}}_{6}$ octahedra. These octahedra compromise their individuality during polarization development. The presence of domains of submicron size embedded inside the macroscopic ferroelectric regions below ${T}_{m}$, as well as their hierarchical arrangement, is observed by piezoresponse force microscopy. Effects of the existence of the structural/polar heterogeneities below ${T}_{m}$ are observed also when polarizabilities of the poled and unpoled samples are compared; the poled sample is found to be more susceptible to the electric field. In addition, by using electric field dependent x-ray diffraction studies we also show that this ceramic under field exhibits a reduction of tetragonal distortion, which is consistent with earlier reports.

${[{\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}]}_{0.62}[{\mathrm{CoO}}_{2}]$, a two dimensional misfit metallic compound, is famous for its rich phases accessed by temperature, i.e., high temperature spin-state transition, metal-insulator transition (MIT) at intermediate temperature ($\ensuremath{\sim}100\phantom{\rule{4pt}{0ex}}\mathrm{K}$), and low temperature spin density wave (SDW). It enters into a SDW phase below ${T}_{\text{MIT}}$ which becomes long range at 27 K. Information on the independent role of misfit layers (rocksalt/${\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}$ and triangular/${\mathrm{CoO}}_{2}$) in these phases is scarce. By combining a set of complementary macroscopic (DC magnetization and resistivity) and microscopic (neutron diffraction and x-ray absorption fine structure spectroscopy) measurements on pure (CCO) and Tb substituted in the rocksalt layer of CCO (CCO1), magnetic correlations in both subsystems of this misfit compound are unraveled. CCO is found to exhibit glassiness, as well as exchange bias (EB) effects, while CCO1 does not exhibit glassiness, albeit it shows weaker EB effect. By combining local structure investigations from extended x-ray absorption fine structure (EXAFS) spectroscopy and neutron diffraction results on CCO, we confirm that the SDW arises in the ${\mathrm{CoO}}_{2}$ layer. Our results show that the magnetocrystalline anisotropy associated with the rocksalt layer acts as a source of pinning, which is responsible for EB effect. Ferromagnetic clusters in the ${\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}$ layer affects the SDW in ${\mathrm{CoO}}_{2}$ and ultimately glassiness arises.

The quadruple perovskite CaMn$_7$O$_{12}$ is a topical multiferroic, in which the hierarchy of electronic correlations driving structural distortions, modulated magnetism, and orbital order is not well known and may vary with temperature. x-ray resonant elastic scattering (XRES) provides a momentum-resolved tool to study these phenomena, even in very small single crystals, with valuable information encoded in its polarization- and energy-dependence. We present an application of this technique to CaMn$_7$O$_{12}$. By polarization analysis, it is possible to distinguish superstructure reflections associated with magnetic order and orbital order. Given the high momentum resolution, we resolve a previously unknown splitting of an orbital order superstructure peak, associated with a distinct \textit{locked-in} phase at low temperatures. A second set of orbital order superstructure peaks can then be interpreted as a second-harmonic orbital signal. Surprisingly, the intensities of the first- and second-harmonic orbital signal show disparate temperature and polarization dependence. This orbital re-ordering may be driven by an exchange mechanism, that becomes dominant over the Jahn-Teller instability at low temperature.

Layered misfit cobaltate ${[{\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}]}_{0.62}[{\mathrm{CoO}}_{2}]$, which emerged as an important thermoelectric material [A. C. Masset et al. Phys. Rev. B 62, 166 (2000)], has been explored extensively in the last decade for the exact mechanism behind its high Seebeck coefficient. Its complex crystal and electronic structures have inhibited consensus among such investigations. This situation has arisen mainly due to difficulties in accurate identification of the chemical state, spin state, and site symmetries in its two subsystems (rocksalt $[{\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}]$ and triangular $[{\mathrm{CoO}}_{2}]$). By employing resonant photoemission spectroscopy and x-ray absorption spectroscopy along with charge transfer multiplet simulations (at the Co ions), we have successfully identified the site symmetries, valencies, and spin states of the Co in both layers. Our site-symmetry observations explain the experimental value of the high Seebeck coefficient and also confirm that the carriers hop within the rocksalt layer, which is in contrast to earlier reports where hopping within triangular ${\mathrm{CoO}}_{2}$ layer has been held responsible for the large Seebeck coefficient.

Hole-doped La2CoO4 compounds have been widely studied due to their multifaceted behavior such as charge ordering, spin ordering and spin blockade. Hole doping in La2CoO4 causes appearance of the Co3+ along with the Co2+, which makes it an interesting model system for study of the spin-state transitions. In hole-doped samples, the spin-states of the Co3+ ions at large have been reported as the low or intermediate state. We report here a study on the Sr doping in La2CoO4. The Sr doping has been found to stabilize the crystal structure. By combining structural, electronic structure and electrical transport results, it is observed that the high-spin state is present in all the La2−xSrxCoO4 (x = 0.5, 0.8, 0.9 and 1.0) samples. The high-spin state has been found to be concomitant with the delocalization of the e g electrons. In addition, in these materials, a colossal thermopower has been observed for first time in the cryogenic temperatures.

The origin of multiferroicity in ${\mathrm{NdCrTiO}}_{5}$ has been examined. A first-order phase transition, due to breaking of the inversion symmetry, near the low-temperature antiferromagnetic ordering (${T}_{N}\ensuremath{\sim}21$ K) where spontaneous electric polarization also appears, is confirmed. From synchrotron x-ray powder diffraction measurements, the structure at low temperatures is found to be noncentrosymmetric (space group $Pba2$), justifying the polar behavior of this compound below ${T}_{N}$. Temperature-dependent Raman measurements reveal that the lattice is strongly coupled with the spins, pointing to the spin lattice correlations in a noncentrosymmetric space group being responsible for invoking the ferroelectricity below 21 K. Through time-of-flight neutron powder diffraction experiments, it is confirmed that the magnetic sublattices, both Nd and Cr, simultaneously order at ${T}_{N}$, and at temperatures below 15 K the Cr moments are found to be saturated while the Nd moments continue to grow until 6 K.

Insulator to metal (IMT) transition (T$_t$ $\sim$ 341 K) in the VO2 accompanies transition from an infrared (IR) transparent to IR opaque phase. Tailoring of the IMT and associated IR switching behavior can offer potential thermochromic applications. Here we report on effects of the W and the Tb doping on the IMT and associated structural, electronic structure and optical properties of the VO2 thin film. Our results show that the W doping significantly lowers IMT temperature ($\sim$ 292 K to $\sim$ 247 K for 1.3\% W to 3.7\% W) by stabilizing the metallic rutile, $\it{R}$, phase while Tb doping does not alter the IMT temperature much and retains the insulating monoclinic, $\it{M1}$, phase at room temperature. It is observed that the W doping albeit significantly reduces the IR switching temperature but is detrimental to the solar modulation ability, contrary to the Tb doping effects where higher IR switching temperature and solar modulation ability is observed. The IMT behavior, electrical conductivity and IR switching behavior in the W and the Tb doped thin films are found to be directly associated with the spectral changes in the V 3$\it{d_{\|}}$ states., 11 pages and 11 figures

${\mathrm{VO}}_{2}$ exhibits several insulating phases (monoclinic $M1,M2$, and triclinic $T)$, and the study of these phases is important for understanding the true nature of the insulator-to-metal transition (IMT) in ${\mathrm{VO}}_{2}$. These insulating phases have small but discernible crystallographic differences in the vanadium chains forming the dimers. Peculiarities of the electron localizations in the dimerized chains for many of the probes such as NMR make it difficult to characterize the true character of these phases [T. J. Huffman et al., Phys. Rev. B 95, 075125 (2017); J. Pouget et al., Phys. Rev. B 10, 1801 (1974)]. Here we present structural, electrical, ultrafast-reflectivity, and electronic structure studies of the $T$ phase Cr-doped ${\mathrm{VO}}_{2}$ and the $M1$ phase pure ${\mathrm{VO}}_{2}$ thin films, both grown by pulsed laser deposition under identical conditions. An intermediate $M2$ structure is observed in the Cr-doped ${\mathrm{VO}}_{2}$, while the pure ${\mathrm{VO}}_{2}$ directly goes from the insulating monoclinic $M1$ structure to a metallic rutile $R$ structure, manifested by temperature-dependent Raman spectroscopy. Temperature-dependent electronic structure studies utilizing x-ray near-edge absorption spectroscopy reveal that all these insulating phases (monoclinic $M1$ and $M2$ and triclinic $T)$ have similar electronic structures which place these insulating phases into the list of Mott-Hubbard insulators. This is a first combined experimental report on the electronic structure of all the three insulating phases, monoclinic $M1,M2$, and triclinic $T$.

Hole-doped antiferromagnetic La2CoO4 has been known to possess intriguing electronic and magnetic phases due to the complex temperature and magnetic field dependent behavior of Co3+ ions. Here, we report on magnetic and dielectric observations in the half hole-doped La1.5Sr0.5CoO4. A Griffiths-like phase has been observed in the high temperature region below TG (∼244 K). At low temperature (∼55 K), a metamagnetic phase has been observed where antiferromagnetic correlations appear. On further cooling, a static weak antiferromagnetic ordering appears at ∼30 K, and a charge-spin glass state has been realized at ∼14 K.Hole-doped antiferromagnetic La2CoO4 has been known to possess intriguing electronic and magnetic phases due to the complex temperature and magnetic field dependent behavior of Co3+ ions. Here, we report on magnetic and dielectric observations in the half hole-doped La1.5Sr0.5CoO4. A Griffiths-like phase has been observed in the high temperature region below TG (∼244 K). At low temperature (∼55 K), a metamagnetic phase has been observed where antiferromagnetic correlations appear. On further cooling, a static weak antiferromagnetic ordering appears at ∼30 K, and a charge-spin glass state has been realized at ∼14 K.

The tailoring and understanding of the metal-insulator transitions (MITs) in vanadium sesquioxide, V2O3, is of major interest for both applications and fundamental physics. V2O3 has been characterized by MIT and concurrent structural transition at ∼155 K; however, the nature of the MIT has remained more elusive. We investigated the MIT and the electronic structure (in metallic phase) of the pulsed laser deposition grown strained vanadium sesquioxide thin films on Si. The strained thin films synthesized here show the suppression (by ∼23 K) of the MIT to lower temperatures, whilst the structural transition temperature decreases only by ∼10 K. Our results systematically confirm that albeit the structural changes are crucial in V2O3, electronic transition seems to be of Mott-Hubbard type. Stabilization of the metallic phase in the strained V2O3 thin film has been manifested from resistivity data and observations of the increased crystal field and quasiparticle features.

Band gap engineering in ZnO thin films have been subject of intensive studies. The thin films of 2 wt % Fe and 2 wt % Ga doped ZnO and undoped ZnO were deposited on glass substrate by pulse laser deposition technique. Structural, optical and electronic structure properties of these thin films were investigated by X- Ray diffraction (XRD), UV-Vis spectroscopy and X-ray absorption spectroscopy (XAS), respectively. XRD studies show that all the thin films are highly oriented along the c-axis and maintain the wurtzite structure. Out of plane lattice parameter in Ga doped is smaller while in Fe doped is larger, compared to undoped ZnO. The band gaps of doped films have been found to increase due to doping of the Ga and Fe ions. XAS studies across O K edges of doped thin films show that the conduction band edge structure probed via oxygen 1s to 2p transitions have modified significantly in Ga doped sample.