Paramagnetic Insulator Research Articles

Charge-spin-coupled electrical transport properties in EuMoO3/SrTiO3superlattices

We investigate the structural and electrical properties of perovskite oxide superlattices composed of a ferromagnetic metal, EuMoO${}_{3}$, and a paramagnetic insulator, SrTiO${}_{3}$, grown on GdScO${}_{3}$ (110) substrates by pulsed laser deposition. The results of x-ray diffraction show that the superlattices are formed as designed, while considerable interface roughening is recognized by high-resolution scanning transmission electron microscopy. In the transport properties, the temperature dependence of resistivity changes from metallic to insulating behaviors on reducing the thickness of EuMoO${}_{3}$. An anomalous Hall effect emerges in metallic superlattices, and its amplitude clearly corresponds to conductivity. Negative magnetoresistance is observed in all superlattices and is more pronounced in the insulating superlattices. These results indicate the existence of spin-canted and low-conductivity dead layers near the heterointerfaces due to the suppressed exchange coupling between electrons and local Eu 4${f}^{7}$ spins.

Topological Phase Transitions Driven by Magnetic Phase Transitions inFexBi2Te3(0≤x≤0.1) Single Crystals

We propose a phase diagram for Fe(x)Bi2Te3 (0≤x≤0.1) single crystals, which belong to a class of magnetically bulk-doped topological insulators. The evolution of magnetic correlations from ferromagnetic to antiferromagnetic gives rise to topological phase transitions, where the paramagnetic topological insulator of Bi2Te3 turns into a band insulator with ferromagnetic-cluster glassy behavior around x∼0.025, and it further evolves to a topological insulator with valence-bond glassy behavior, which spans over the region from x∼0.03 up to x∼0.1. This phase diagram is verified by measuring magnetization, magnetotransport, and angle-resolved photoemission spectra with theoretical discussions.

Mott–Hubbard transition in V2O3 revisited

AbstractThe isostructural metal‐insulator transition in Cr‐doped V2O3 is the textbook example of a Mott–Hubbard transition between a paramagnetic metal (PM) and a paramagnetic insulator. We review recent theoretical calculations as well as experimental findings which shed new light on this famous transition. In particular, the old paradigm of a doping‐pressure equivalence does not hold, and there is a microscale phase separation for Cr‐doped V2O3.

Magnetization in electron- and Mn- doped SrTiO3

Mn-doped SrTiO3.0, when synthesized free of impurities, is a paramagnetic insulator with interesting dielectric properties. Since delocalized charge carriers are known to promote ferromagnetism in a large number of systems via diverse mechanisms, we have looked for the possibility of any intrinsic, spontaneous magnetization by simultaneous doping of Mn ions and electrons into SrTiO3 via oxygen vacancies, thereby forming SrTi1−xMnxO3−δ, to the extent of making the doped system metallic. We find an absence of any enhancement of the magnetization in the metallic sample when compared with a similarly prepared Mn doped, however, insulating sample. Our results, thus, are not in agreement with a recent observation of a weak ferromagnetism in metallic Mn doped SrTiO3 system.

Relevancy between Thermochromic and Magnetic Property of La<sub>1-x</sub>Sr<sub>x</sub>MnO<sub>3</sub> (x=0.2 and 0.33) Smart Radiation Thin Film Materials Prepared by Magnetron Sputtering

La1-xSrxMnO3 thin films with x=0.2 and 0.33 were prepared by magnetron sputtering method for its potential application on thermal control of spacecraft. The materials show a phase transition from ferromagnetic metal phase with a low infrared emittance to paramagnetic insulator phase with a high emittance. Because of the thermochromic property, they can automatically change their infrared emittance greatly in response to environment temperature and thermal load and keep the spacecraft electronic components working normally. A superconduction quantum interference device magnetometer was used to study magnetization over the temperature range 100–315 K. Temperature dependence of total hemispherical emittance was carried out in a liquid nitrogen cooled vacuum chamber by a steady state calorimetric method. Thermal emittance results indicate an obvious tuneability of both La0.8Sr0.2MnO3 and La0.67Sr0.33MnO3 thin films. Compared to La0.8Sr0.2MnO3 thin film, La0.67Sr0.33MnO3 thin film has a higher transition temperature and bigger emittance tuneability. Based on phase separation concept, the thermal emittance was fitted by two-energy-level Boltzmann distribution of metal phase volume fraction fH. The measured magnetization curves were fitted in terms of mean-field approximation theory. The volume fraction of ferromagnetic phase fM was not coinciding with the fH very well. This result showed the fH and the fM plays a different role on thermal emittance property.

Evolution from antiferromagnetic to paramagnetic Kondo insulator with increasing hybridization; XPS studies

We present the Ce 3d x-ray photoemission (XPS) spectra for CeM2Al10 (M=Ru, Os, Fe) from which we determined the on-site hybridization between the f and conduction electron states, Δcf, and the 4f-level occupancy, nf. Those parameters have been obtained using the Gunnarsson-Schönhammer approach. We found Δcf stronger for the Kondo insulator CeFe2Al10 than for the remaining compounds with Ru and Os. We discuss the type of behaviour of CeM2Al10 on the base of the earlier theoretical phase diagram obtained within the Anderson-lattice model.

Percolation Model of the Temperature Dependence of Resistivity in Pr0.67A0.33MnO3 (A = Ba or Sr) Manganites

The analyses of resistivity experimental results of Pr0.67Ba0.33MnO3 (PBMO) and Pr0.67Sr0.33MnO3 (PSMO) manganites are presented. The electrical resistivity curves are fitted with the phenomenological percolation model, which is based on the phase segregation of ferromagnetic–metallic (FMM) clusters and paramagnetic–insulating (PMI) regions. The estimated results are in good agreement with experimental data. We found that the transition to the metallic state occurs if the volume fraction of the ferromagnetic phase reaches a percolation threshold, suggesting that the percolation of ferromagnetic (FM) domains is responsible for the observed metal–insulator (M–I) transition. According to the percolation model, we found that the energy gap of the quasi-particles in the phase separated FM and PM states is significantly smaller for PBMO than that for PSMO confirming that PSMO is more conductive than PBMO. We also found that the volume fraction of the ferromagnetic phase has the same physical meaning as the reduced magnetization.

Low-energy excitations in strongly correlated materials: A theoretical and experimental study of the dynamic structure factor in V2O3

This work contains an experimental and theoretical study of the dynamic structure factor at large momentum transfer $|Q|\ensuremath{\sim}4$ \AA{}${}^{\ensuremath{-}1}$ of the strongly correlated transition-metal oxide V${}_{2}$O${}_{3}$. We focus in particular on the transitions between $d$ states that give rise to the spectra below 6 eV. We show that the main peak in this energy range is mainly due to ${t}_{2g}\ensuremath{\rightarrow}{e}_{g}^{\ensuremath{\sigma}}$ transitions, and that it carries a signature of the phase transition between the paramagnetic insulator and the paramagnetic metal that can already be understood from the joint density of states calculated at the level of the static local density approximation. Instead, in order to obtain theoretical spectra that are overall similar to the measured ones, we have to go beyond the static approximation and include at least crystal local field effects. The latter turn out to be crucial in order to eliminate a spurious peak and hence allow a safe comparison between theory and experiment, including an analysis of the strong anisotropy of the spectra.

Dielectric and magnetic permittivities of three new ceramic tungstates MPr2W2O10 (M = Cd, Co, Mn)

Broadband dielectric spectroscopy measurements revealed an anomalously large relative permittivity value (ε r = 884) for MnPr2W2O10, a smaller value (ε r = 156) for CoPr2W2O10 and the smallest value (ε r = 22) for CdPr2W2O10 at low frequency (ν = 0.1 Hz) and above room temperature in the insulating and paramagnetic state. Below 273 K, the relative permittivity (ε r ∼ 24) did not depend significantly on frequency for all the tungstates under study. Electrical resistivity, thermoelectric power, electron paramagnetic resonance, magnetic susceptibility and magnetization provided experimental evidence that the studies tungstates were paramagnetic insulators with low n-type conduction. Only in the case of MnPr2W2O10 was a ferrimagnetic order below 45 K observed. These effects are discussed within the framework of Maxwell–Wagner polarization, chemical covalent bonds and porosity mechanism.

Rubidium superoxide: Ap-electron Mott insulator

Rubidium superoxide, RbO${}_{2}$, is a rare example of a solid with partially filled electronic $p$ states, which allows us to study the interplay of spin and orbital order and other effects of strong electronic correlations in a material that is quite different from the conventional $d$ or $f$ electron systems. Here we show, using a combination of density functional theory (DFT) and dynamical mean-field theory, that at room temperature RbO${}_{2}$ is indeed a paramagnetic Mott insulator. We construct the metal-insulator phase diagram as a function of temperature and Hubbard interaction parameters $U$ and $J$. Due to the strong particle-hole asymmetry of the RbO${}_{2}$ band structure, we find strong differences compared to a simple semielliptical density of states, which is often used to study the multiband Hubbard model. In agreement with our previous DFT study, we also find indications for complex spin and orbital order at low temperatures.

Electrical and magnetic properties of CdRE2W2O10 tungstates (RE=Y, Nd, Sm, Gd–Er)

Electrical resistivity, thermoelectric power, broadband dielectric spectroscopy, magnetic susceptibility, and magnetization provide experimental evidence that the CdRE2W2O10 tungstates (RE=Y, Nd, Sm, Gd–Er) are magnetically disordered insulators with low relative dielectric permittivity (εr∼15). These results are characteristic for paramagnetic insulators having the screened electrons on the filled shells. Some of them behave like superparamagnets with the blocking temperature of 30K, depending on strength of the spin-orbit coupling.

Determination of valence state of Mn ions in Pr1−xAxMnO3−δ (A = Ca, Sr) by Mn-L3 X-ray absorption near-edge structure analysis

The valence states of the Mn ions in Pr1−xAxMnO3−δ (A=Ca, Sr) are investigated by Mn-L3 X-ray absorption near-edge structure (XANES) analysis. The spectral fine structures in the Mn-L3 XANES analysis show a significant difference between Pr0.5Ca0.5MnO3−δ and Pr0.5Sr0.5MnO3−δ, a paramagnetic insulator and a ferromagnetic metal, respectively, at room temperature, whereas the spectral structures of Pr0.7Ca0.3MnO3−δ and Pr0.7Sr0.3MnO3−δ, paramagnetic insulators, are almost identical. These results indicate that the valence states of the Mn ions in these materials are highly correlated with their magnetic and electrical properties. A significant difference was also found between the Mn-L3 XANES profiles of Pr1−xCaxMnO3−δ and the profiles formed by the linear combination of the Mn-L3 XANES spectra of PrMnO3 (Mn3+) and CaMnO3 (Mn4+). This difference indicates that the Mn ions in these materials do not have a mixed-valence state of 3+ and 4+, but have an intermediate valence state o 3+ and 4+.

Effects of substrate-induced-strain on the structural and transport properties of La0.7Ca0.3MnO3 thin films

We have studied the structural and electrical properties of epitaxial La0.7Ca0.3MnO3 (LCMO) thin films prepared by metal organic deposition under different types and degrees of substrate-induced strain. 40-nm-thick films have been epitaxially grown on single-crystalline (LaAlO3)0.3–(SrAlTaO6)0.7 (negligible tensile strain), SrTiO3 (tensile strain) and LaAlO3 (compressive strain) substrates. High-resolution X-ray diffraction and reciprocal space maps demonstrate a direct correlation between the crystalline quality and the substrate-induced strain. The electrical properties were found to be strongly dependent on the substrate used. The temperature dependence of resistivity curves was fitted using various approaches in different phases (below and above the ferromagnetic transition temperature TP). In the ferromagnetic metallic phase, ρ(T) follows a Tα power law. The obtained values of the coefficient α (3.5–4) indicate that the electrical transport in our films is a combination of spin wave scattering processes and electron–magnon or two-magnon scattering phenomena. In the paramagnetic insulator phase, the activation energy EA and the variable range hopping characteristics (characteristic temperature T0) were found to be strongly dependent on the strain-induced lattice mismatch between the LCMO and the substrate used.

Microstructural and magnetotransport properties of La1 − xCaxMnO3 (0.45 ≤ x ≤ 0.60) thin films

In the present work we focus on the coexistence of magnetic-electronic phases and hence phase separation in La1−xCaxMnO3 thin films in the composition range of 0.45≤x≤0.60. The oriented polycrystalline thin films were prepared by nebulized spray pyrolysis technique on single crystalline substrate LaAlO3 [LAO (001)]. The lattice parameters are observed to decrease with increasing in Ca content and the structural/microstructural disorder is also seen to increase concomitantly. As the Ca content is increased from x=0.45 to 0.55, the paramagnetic insulator (PMI) to ferromagnetic metal (FMM) transition temperatures (TC/TIM) are suppressed. This is accompanied by (i) appreciable suppression of the TC, (ii) sharp reduction in the magnetic moment, (iii) occurrence of a peak in the magnetization below TC, (iv) stronger bifurcation of the ZFC–FC magnetization, and (v) decline in the magnetization in the lower temperature regime. At x=0.60, the FMM disappears and a weak FM insulator like phase is observed. In the vicinity of x=0.55, the magnetic ground state resembles spin cluster glass. This is also confirmed by the occurrence of a hysteresis loop in the resistivity of the x=0.55 measured during the cooling and warming cycle. The transition broadening up to x=0.55 and the vanishing of the two transitions in x=0.60 film along with the simultaneous decrease in the magnetic moment and enhanced resistivity show that the observed variations are due to the increase in the antiferromagnetic-charge ordered (AFM-CO) phase fraction at higher Ca content. This is also confirmed by the decrease in magnitude of the magnetoresistance and its temperature dependence. Our results demonstrate that, there is strong phase separation in the overdoped region (0.50<x≤0.60) due to the presence of multiple magneto-electric phases.

Insulator-to-metal transition in vanadium sesquioxide: does the Mott criterion work in this case?

It is shown that the Mott criterion expressed by the simple relation a B(n c)1/3 ≈ 0.25 turns out to be quite successful in describing metal–insulator phase transitions not only in heavily doped semiconductors, but also in transition metal oxides such as VO2 and V2O3. It is found in this article that, in the case of a high-temperature transition ‘paramagnetic insulator – paramagnetic metal’ in vanadium sesquioxide, a B(n c)1/3 = 0.254. Difficulties connected with the analogous description of a low-temperature transition (‘paramagnetic metal – antiferromagnetic insulator’) in V2O3 are discussed.

Electric-field-induced shift of the Mott metal-insulator transition in thin films

The ground-state properties of a paramagnetic Mott insulator at half-filling are investigated in the presence of an external electric field using the inhomogeneous Gutzwiller approximation for a single-band Hubbard model in a slab geometry. We find that the metal-insulator transition is shifted toward higher Hubbard repulsions by applying an electric field perpendicular to the slab. The main reason is the accumulation of charges near the surface. The spatial distribution of site-dependent quasiparticle weight shows that it is maximal in a few layers beneath the surface, while the central sites where the field is screened have a very low quasiparticle weight. Our results show that above a critical-field value, states near the surface will be metallic, while the bulk quasiparticle weight is extremely suppressed but never vanishing, even for large Hubbard repulsions above the bulk zero-field critical value. Below the critical-field value, our results hint toward an insulating state in which the electric field is totally screened and the slab is again at half-filling.

Photoemission microscopy study of the two metal-insulator transitions in Cr-doped V2O3

We present a spectromicroscopy study of the two distinct metal-insulator transitions in (V1−xCrx)2O3, x = 0.011. The coexistence of metallic and insulating domains was observed with scanning photoelectron microscopy for both the paramagnetic insulator-paramagnetic metal and paramagnetic metal-antiferromagnetic insulator transitions, evidencing a clear correlation between their nucleation regions. Although these two transitions are very different in nature and underlying mechanism, in both cases the morphology of their phase separation is influenced by structural inhomogeneities. These results demonstrate the general relevance of strain caused by local lattice distortions in guiding the intrinsic tendency towards phase separation in Mott materials.

Role of structurally and magnetically modified nanoclusters in colossal magnetoresistance

It is generally accepted that electronic and magnetic phase separation is the origin of many of exotic properties of strongly correlated electron materials, such as colossal magnetoresistance (CMR), an unusually large variation in the electrical resistivity under applied magnetic field. In the simplest picture, the two competing phases are those associated with the material state on either side of the phase transition. Those phases would be paramagnetic insulator and ferromagnetic metal for the CMR effect in doped manganites. It has been speculated that a critical component of the CMR phenomenon is nanoclusters with quite different properties than either of the terminal phases during the transition. However, the role of these nanoclusters in the CMR effect remains elusive because the physical properties of the nanoclusters are hard to measure when embedded in bulk materials. Here we show the unexpected behavior of the nanoclusters in the CMR compound La(1-x)Ca(x)MnO(3) (0.4 ≤ x < 0.5) by directly correlating transmission electron microscopy observations with bulk measurements. The structurally modified nanoclusters at the CMR temperature were found to be ferromagnetic and exhibit much higher electrical conductivity than previously proposed. Only at temperatures much below the CMR transition, the nanoclusters are antiferromagnetic and insulating. These findings substantially alter the current understanding of these nanoclusters on the material's functionality and would shed light on the microscopic study on the competing spin-lattice-charge orders in strongly correlated systems.

Variational study of the magnetically induced metal-insulator transition in FeSi1−xGex

Recently, a minimal microscopic model for the magnetically induced metal-insulator transition in isostructural and isoelectronic alloys of FeSi${}_{1\ensuremath{-}x}$Ge${}_{x}$ has been introduced and solved within the mean-field approximation. Reasonable qualitative agreement with the experimental data has been found except for the type of transition between the paramagnetic insulator and the ferromagnetic metal. In this paper we argue that when correlation effects are taken into account, the model predicts a first-order metal-insulator transition in agreement with experiments.

Experimental search for the electron Electric Dipole Moment using solid state techniques

We report results of an experimental search for the permanent Electric Dipole Moment (EDM) of the electron using a solid state system. The experiment uses a paramagnetic insulator (gadolinium gallium garnet) with a large magnetic response at low temperatures. The presence of the electron EDM leads to a finite magnetization when the garnet sample is subjected to a strong electric field. The resulting magnetization can be measured using a superconducting quantum interference device (SQUID) as a magnetometer. With considerable efforts made towards controlling various sources of systematic effects, the experiment is currently free of spurious signals larger than the SQUID noise. We report the value of electron EDM of (−5.57 ± 7.98 ± 0.12) × 10−25e-cm with 120 hours of data.