Relevancy of phase separation between electrical and thermal properties in La0.8Sr0.2MnO3 thin films

For the purpose of finding the relevancy of phase separation between electrical and thermal properties, La_0.8Sr_0.2MnO₃ thin films, deposited on quartz glass substrates, were studied for temperature dependent resistance and thermal emittance. Based on the phase separation concept, metal phase volume fraction f was calculated from the temperature dependence of resistance and emittance properties by a phenomenological model and two-energy-level Boltzmann distribution, respectively. The two sets of f coincide with each other very well. The results show f plays an important role in the electrical transportation and thermal properties, and the two properties are essentially correlated by f.     

Evidence of glassy ferromagnetic phase and kinetic arrest of electronic phase in Sm0.35Pr0.15Sr0.5MnO3 manganites

The effect of doping of rare earth Pr³⁺ ion as a replacement of Sm³⁺ in Sm_0.5Sr_0.5MnO₃ is investigated. Temperature dependent dc and ac magnetic susceptibility, resistivity, magnetoresistance measurements on chemically synthesized (Sm_0.5−xPr_x)Sr_0.5MnO₃ show various unusual features with doping level x=0.15. The frequency independent ferromagnetic to paramagnetic transition at higher temperature (∼191 K) followed by a frequency dependent reentrant magnetic transition at lower temperature (∼31 K) has been observed. The nature of this frequency dependent reentrant magnetic transition is described by a critical slowing down model of spin glasses. From non-linear ac susceptibility measurements it has been confirmed that the finite size ferromagnetic clusters are formed as a consequence of intrinsic phase separation, and undergo spin glass-like freezing below a certain temperature. There is an unusual observation of a 2nd harmonic peak in the non-linear ac susceptibility around this reentrant magnetic transition at low temperature (∼31 K). Arrott plots at 10 and 30 K confirm the existence of glassy ferromagnetism below this low temperature reentrant transition. Electronic- and magneto-transport measurements show a strong magnetic field—temperature history dependence and strong irreversibility with respect to the sweeping of magnetic field. These results are attributed to the effect of phase separation and kinetic arrest of the electronic phase in this phase separated manganite at low temperatures.     

Unusual magnetization suppression induced by higher magnetic field in (La0.83Bi0.17)0.67Ca0.33MnO3

Magnetization behavior of (La_0.83Bi_0.17)_0.67Ca_0.33MnO₃ has been investigated in the temperature range from 100 to 180 K. A metamagnetic transition was observed in the temperature region, where the magnetization was measured after a zero-field-cooling from room temperature to a selected temperature. Experimental results show that, after a higher magnetization route, the field-increasing branches of the magnetization curves shows an unusual training effect: below a magnetic field H₀, the applied magnetic field enhances the value of magnetization; however, above H₀ the magnetic field suppresses the value, and the behavior cannot be totally attributed to the enhancement effect of the applied magnetic field on ferromagnetic phase fraction. It is proposed that, in the two-phase coexistence region, the higher magnetic field promotes the phase separation and leads to both the fraction of ferromagnetic domain and the stabilization of antiferromagnetic domain increase.     

Oxygenation-induced two peaks in the self-doped La0.9MnO3+δ compounds

A series of self-doped La_0.9MnO_3+δ compounds have been fabricated by the solid-state reaction method with different post-annealing and oxygenation processes. The temperature dependence of resistivity measured at several magnetic fields indicated that all of these samples have an insulator–metal transition with a large negative colossal magnetoresistance (CMR) ratio. The resistivity, maximum CMR ratio and peak position are dominated by the oxygen content. As the oxygen content increases, the resistivity decreases, the peak position shifts to higher temperature and the maximum CMR ratio increases, respectively; meanwhile the peak number in the R–T curve is changed from 1 to 2, then from 2 to 1. The temperature dependencies of magnetization and specific heat show the evidence of magnetic phase transition in these samples. We think that two peaks contributed by the different oxygen-induced ferromagnetic phases resulted in the phase separation of the sample.     

Vibrational anisotropy of MnO6 octahedron and phase separation in La0.67−yPryCa0.33MnO3 manganites

The role of vibrational anisotropy of Mn³⁺O₆ octahedron in the phase separation behavior of La_0.67−yPr_yCa_0.33MnO₃ (x=0, 0.15, 0.25 and 0.30) has been investigated by means of magnetization M, internal friction Q⁻¹, Young's modulus E along with the X-ray powder diffraction measurements. For the samples with y=0 and 0.15, the Q⁻¹ exhibits three peaks in the ferromagnetic region, which are attributed to the intrinsic inhomogeneity of ferromagnetic phase, i.e. the electronic phase separation with the coexistence of insulating and conducting phases. However, both the samples with y=0.25 and 0.30 undergo a magnetic phase separation with the coexistence of the antiferromagnetic and ferromagnetic phases, and the Q⁻¹ peaks related to the electronic phase separation have not been observed. In addition, the Q⁻¹ exhibits a peak in the paramagnetic region for all samples, which may result from the formation of magnetic clusters. We observed that the evolution from electronic to magnetic phase separation is close related to the rapid increase in the ratio of two kinds of Jahn-Teller distortion modes Q₃ and Q₂, i.e. Q₃/Q₂. A schematic phase diagram is given in the text, and it is suggested that the enhancement of vibrational anisotropy of Mn³⁺O₆ octahedron plays a key role in the evolution from electronic to magnetic phase separation.     

Immiscibility in the NiFe2O4-NiCr2O4 spinel binary

The solid solution behavior of the Ni(Fe_1−nCr_n)₂O₄ spinel binary is investigated in the temperature range 400-1200 °C. Non-ideal solution behavior, as exhibited by non-linear changes in lattice parameter with changes in n, is observed in a series of single-phase solids air-cooled from 1200 °C. Air-annealing for 1 year at 600 °C resulted in partial phase separation in a spinel binary having n=0.5. Spinel crystals grown from NiO, Fe₂O₃ and Cr₂O₃ reactants, mixed to give NiCrFeO₄, by Ostwald ripening in a molten salt solvent, exhibited single-phase stability down to about 750 °C (the estimated consolute solution temperature, T_cs). A solvus exists below T_cs. The solvus becomes increasingly asymmetric at lower temperatures and extrapolates to n values of 0.2 and 0.7 at 300 °C. The extrapolated solvus is shown to be consistent with that predicted using a primitive regular solution model in which free energies of mixing are determined entirely from changes in configurational entropy at room temperature.     

Effect of Cr doping on the magnetic and electrical transport properties of charge-ordered Bi0.5Ca0.5MnO3

An experimental study on the magnetic and electrical transport properties of the manganites Bi_0.5Ca_0.5Mn_1−xCr_xO₃ (BCMCO) (0≤x≤0.12) is carried out. The results show that Cr doping can suppress the charge-ordering transition, favoring the ferromagnetic clusters. For x=0.12, the charge-ordering transition disappears but a very broad paramagnetic-ferromagnetic-like transition is detected at the Curie temperature T_C=72.6 K. It is caused by phase separation or coexistence of the charge-ordering and ferromagnetic phase. Moreover, the critical Cr content to destroy charge ordering phase in BCMCO does not match the general monotonous tendencies shown by Cr-doped Re_0.5Ca_0.5MnO₃ (Re-rare-earth). These differences are ascribed to the fact that the ground state in BCMCO differs markedly from the ferromagnetic metallic phase in Cr-doped Re_0.5Ca_0.5MnO₃ compounds.     

Metastable states in phase separated electron doped Sm0.15Ca0.85MnO3

In order to study the mechanism behind the phase separation scenario in the Sm_0.15Ca_0.85MnO₃ compound, magnetization and resistivity measurements have been carried out in pulsed magnetic fields up to 50 T at temperatures 4.2 K<T<200 K. It is found that external magnetic field causes a collapse of a C-type AFM (P2₁/m) phase resulting in field-induced insulator-metal transition, which is irreversible below T₁=75 K. In zero field the content of a G-type phase in the mixed C-G state can vary from 10 to 17% at T=10 K. A set of metastable states with different volume ratios of G-type to C-type phases is observed below T₁ depending on the history of the sample. The obtained results indicate that the phase separation plays a dominant role for the electric and the magnetic properties of this material.     

Electron spin resonance study of Fe doping effect in La0.67Ca0.33MnO3

Electron spin resonance (ESR) study was carried out on La_0.67Ca_0.33Mn_1−xFe_xO₃ (x=0.0, 0.04) samples. The temperature dependence of the ESR spectra indicates the presence of phase separation above and below T_C in x=0.0 and 0.04 sample, respectively. The increase of the g-value in the high-temperature region indicates the existence of local spin correlations even in the paramagnetic state. The activation energy obtained from both the temperature dependence of the ESR intensity and linewidth exhibits a smaller value in the Fe-doped sample. Our study suggests that the ferromagnetic spin correlations would be significantly weakened by a slight doping of Fe ions on Mn sites.     

Thickness dependence of electroresistance in La0.67Ca0.33MnO3 epitaxial films

The electroresistance (ER) of La_0.67Ca_0.33MnO₃ (LCMO) epitaxial thin films with different thicknesses was studied. For the 110 nm thick LCMO film, its ER shows a maximum at T_p, where the resistance shows a peak, and decreases to zero at lower temperatures. While for the 30 nm thick LCMO film, its ER is remarkable in a wide temperature range. Another interesting observation in this work is that the electric current can tune the magnetoresistance of the ultrathin LCMO thin film. The results were discussed by considering the coexistence of ferromagnetic metallic phase with the charge ordered phase, and the variation of the phase separation with film thickness and electric current. This work also demonstrates that electric current can tune the magnetoresistance of the manganites, which is helpful for their applications.     

Amorphous-crystalline dual-layer structures resulting from metastable liquid phase separation in （Fe50Co25B15Si10）80Cu20 melt-spun ribbons

（Fe50Co25B15Si10）80Cu20 ribbons are prepared by using the single-roller melt-spinning method. A dual-layer structure consisting of a （Fe, Co）-rich amorphous phase and a Cu-rich crystalline phase forms due to metastable liquid phase separation before solidification. The magnetic hysteresis loops of the as-quenched and annealed samples are measured at room temperature. It is indicated that the coercivity of the ribbon is almost zero in the as-quenched state. The crystallization leads to the increase of coercivity and decrease of saturation magnetization.     

Phase and electrical behaviour in the Bi14W1 − xLaxO24 − 3x/2 system

The compositional and thermal dependencies of phase and electrical behaviour of compositions in the system Bi₁₄W_1 − xLa_xO_{24 − 3x/2} (0.00 < x < 1.00) have been studied by X-ray powder diffraction, differential thermal analysis and a.c. impedance spectroscopy. The system exhibits polymorphism and phase separation, which shows both compositional and thermal dependence. Compositions with x = 0.25 and x = 0.50 exhibit a single phase tetragonal structure at room temperature. In contrast, the x = 0.75 composition at room temperature shows a mixture of a cubic phase and a secondary β-Bi₂O₃ related tetragonal phase. A full solid solution is observed at high temperatures, corresponding to the occurrence of a δ-Bi₂O₃ type phase. The appearance of the various phases correlates well with the observed electrical behaviour. The x = 0.75 composition exhibits exceptionally high conductivity at high temperatures (σ₈₀₀ = 1.34 S cm^− 1), but also shows significant phase separation at lower temperatures.     

Hyperfine characterization of Bi1.9Te0.1SrNb1.9Hf0.1O9

The Aurivillius type oxide Bi_1.9Te_0.1SrNb_1.9Hf_0.1O₉ has been studied by Perturbed Angular Correlations spectroscopy using ¹⁸¹Ta probes. The spin precession curves were measured from room temperature up to 873 K. Two sites are occupied by probes and the temperature dependence of both indicates a continuous phase transition at about 625 K. One site is ordered while the other is disordered. This situation is analyzed in terms of simple models already applied to perovskites. The transition temperature of the solid solution Bi_2−xTe_xSrNb_2−xHf_xO₉ (with 0≤x≤0.5) shows a strong dependence on composition.     

Dielectric properties and phase transitions of (Pb0.87La0.02Ba0.1)(Zr0.6Sn0.4−xTix)O3 ceramics with compositions near AFE/RFE phase boundary

The electrical properties and phase transition behavior of (Pb_0.87La_0.02Ba_0.1)(Zr_0.6Sn_0.4−xTi_x)O₃ solid solutions (PLBZST, 0.04≤x0.2) were investigated by the X-ray diffraction, permittivity, pyroelectric current, and P-E electric hysterisis loops. As the composition x increased from 0.04 to 0.2, the antiferroelectric ceramics (x≤0.07, AFE) with tetragonal phase changed to the ferroelectric relaxors (RFE, 0.09≤x). AFE ceramics showed a peculiar diffuse phase transition and dielectric relaxation at the low temperature (down to −100 °C) due to a frustration between AFE and FE state. With an increase in composition x, electrically field-induced AFE-FE switching field (E_AFE-FE) and AFE-paraelectric (PE) phase transition temperature (T_c) are depressed in the temperature (T)-Ti composition (x) phase diagram, a FE-AFE-PE triple phase point (T_tr) with the lowest transition temperature occurred at x=0.09. The pyroelectric currents under an application of various external electric field (E) were measured to identify a T-E phase diagram of the PLBZST compound.     

Crystallographic study of the phase transition in (Me4P)2ZnBr4

The compound (Me₄P)₂ZnBr₄, a member of the β-K₂SO₄ structure class, undergoes a phase transition at 84°C from the room temperature space group P12₁/c1 to the parent Pmcn structure. The room temperature structure corresponds to a ferrodistortive transition of B_1g symmetry at the zone center. At room temperature, the compound has lattice constants a=9.501(1), b=16.055(2), c=13.127(2) Å and β=90.43(1)°. For the high temperature phase, the orthorhombic cell has dimensions a=9.466(2), b=16.351(3) and c=13.284(2) Å. The structures consist of two crystallographically independent Me₄P⁺ cations and the ZnBr₄²⁻ anions. In the room temperature phase, all three ionic species show substantial displacement from the mirror plane perpendicular to the a-axis that exists in the high temperature phase, as well as rotations out of that plane. The thermal parameters of the cations are indicative of substantial librational motion. Measurements of lattice parameters have been made at 2-5°C intervals over the temperature range 40-140°C. The changes in the lattice constants appear continuous at T_c (within experimental limits) indicating that the phase transition is likely second-order. The a lattice constant shows an anomalous shortening as T_c is approached. Thermal expansion coefficients are calculated from this data. An application of Landau theory is used to derive the temperature dependencies of spontaneous shear strain and corresponding elastic stiffness constants associated with the primary order parameter.     

Phase transition of [(C6H5)3PCH3]+(TCNQ)-2 and electrical transport properties

The electrical conductivity at 10GHz, the dielectric constant, and the thermoelectric power (TEP) of [(C₆H₅)₃PCH₃]⁺(TCNQ)^-₂, from 230 up to 400 K, have been measured. This organic quasi-one-dimensional solid undergoes a first order phase transition at 314 K. At the transition the conductivity increases by a factor of 2.2 and the activation energy drops to 0.26 from 0.31 eV. At 314 K TEP decreases abruptly from -75 to -60μVK^-1 and remains almost constant for T > 314 K. The dielectric permeability ?₀ is constant and equal to 5 in the low temperature phase, increases abruptly by 7% at the transition, and then depends strongly on temperature in the high temperature phase. Results of the high temperature phase are interpreted in terms of a strongly correlated salt.     

Structural phase transitions and sodium ordering in Na0.5CoO2: a combined electron diffraction and Raman spectroscopy study

The nonstoichiometric Na_xCoO₂ system exhibits extraordinary physical properties that correlate with temperature and Na concentration in its layered lattice without evident long-range structure modification when conventional crystallographic techniques are applied. For instance, Na_0.7CoO₂, a thermodynamically stable phase, shows large thermoelectric power; water-intercalated Na_0.33CoO₂·1.3H₂O is a newly discovered superconductor with T_c∼4 K, and Na_0.5CoO₂ exhibits an unexpected charge ordering transition at around T_co∼55 K. Recent studies suggest that the transport and magnetic properties in the Na_xCoO₂ system strongly depend on the charge carrier density and local structural properties. Here we report a combined variable temperature transmission electron microscopy and Raman scattering investigation on structural transformations in Na_0.5CoO₂ single crystals. A series of structural phase transitions in the temperature range from 80 to 1000 K are directly identified and the observed superstructures and modulated phases can be interpreted by Na-ordering. The Raman scattering measurements reveal phase separation and a systematic evolution of active modes along with phase transitions. Our work demonstrates that the high mobility and ordering of sodium cations among the CoO₂ layers are a key factor for the presence of complex structural properties in Na_xCoO₂ materials, and also demonstrate that the combination of electron diffraction and Raman spectroscopy measurements is an efficient way for studying the cation ordering and phase transitions in related systems.     

Effect of the simultaneous substitution of two transition metals on the structural, magnetic and electrical properties of La0.6Sr0.4Mn1−2xCrxFexO3

The structural, magnetic and electrical properties of Cr and Fe simultaneously substituted in the perovskite La_0.6Sr_0.4Mn_1−2xCr_xFe_xO₃ have been studied. The presence of Cr and Fe had no significant effect on the structural properties. Curie temperature and saturation magnetization decrease with increase in Cr and Fe contents. For x=0.20 and 0.25, a steep drop of zero field-cooled (ZFC) magnetization at low temperature signifies the formation of cluster- or spin-glass state. A weak hysteresis at low fields seems to be an indication of phase separation. All the resulting magnetization curves can be explained by a superposition of both ferromagnetic and antiferromagnetic components. All the samples are semiconducting throughout the temperature range studied. Resistivity can be described by the adiabatic small polaron hopping and the variable range hopping model. It was found that the transport mechanism is dominated by the VRH model with an increase of Mott localization energy, which explains the increase of resistivity.     

The effect of hydrostatic pressure on the ferroelectric phase transition in [NH2(CH3)2]3[Sb2Cl9]

The effect of hydrostatic pressure on the ferroelectric phase transition temperature in [NH₂(CH₃)₂]₃[Sb₂Cl₉] (DMACA) has been studied by electric permittivity measurements at pressures up to 400 MPa. The pressure-temperature phase diagram is given. The phase transition temperature (T_c) increases with increasing pressure up to 150 MPa, passes through a maximum and then decreases with a further increase of pressure. The unexpected nonlinear decrease in T_c with pressure increasing above 150 MPa suggests that the mechanism of ferroelectric phase transition in DMACA is different from hitherto assumed.     

Birefringence,X-ray and neutron diffraction measurements on the structural phase transitions of (CH3NH3)2 MnCl4 and (CH3NH3)2FeCl4

We used optical birefringence, X-ray and neutron diffraction methods with single crystals to study the structural phase transitions of the perowskite-type layer structures of (CH₃NH₃)₂MeCl₄ with Me=Mn, Fe. The Mn-compound shows the following structural transitions at 394 K — a continuous order-disorder phase transition from tetragonal symmetry $\text{I4}\text{mmm}$ to orthorhombic space group Abma (Cmca in reference 10); at 257 K — a discontinuous transition to a second tetragonal modification; at 95 K — a discontinuous transition to a monoclinic phase. For the Fe-compound the corresponding transition temperatures are 328 K and 231 K, respectively. A low temperature monoclinic phase could not be observed. The lattice parameters of the different modifications were determined as a function of temperature. The temperature dependent course of the order parameter has been investigated for the order—disorder transition. For both compounds, all the methods used gave the same value for the critical exponent of β = 0.315.