On the room temperature multiferroic BiFeO<Subscript>3</Subscript>: magnetic,dielectric and thermal properties

Magnetic dc susceptibility between 1.5 and 800 K, ac susceptibility and magnetization, thermodynamic properties, temperature dependence of radio and audio-wave dielectric constants and conductivity, contact-free dielectric constants at mm-wavelengths, as well as ferroelectric polarization are reported for single crystalline BiFeO₃. A well developed anomaly in the magnetic susceptibility signals the onset of antiferromagnetic order close to 635 K. Beside this anomaly no further indications of phase or glass transitions are indicated in the magnetic dc and ac susceptibilities down to the lowest temperatures. The heat capacity has been measured from 2 K up to room temperature and significant contributions from magnon excitations have been detected. From the low-temperature heat capacity an anisotropy gap of the magnon modes of the order of 6 meV has been determined. The dielectric constants measured in standard two-point configuration are dominated by Maxwell-Wagner like effects for temperatures T > 300 K and frequencies below 1 MHz. At lower temperatures the temperature dependence of the dielectric constant and loss reveals no anomalies outside the experimental errors, indicating neither phase transitions nor strong spin phonon coupling. The temperature dependence of the dielectric constant was measured contact free at microwave frequencies. At room temperature the dielectric constant has an intrinsic value of 53. The loss is substantial and strongly frequency dependent indicating the predominance of hopping conductivity. Finally, in small thin samples we were able to measure the ferroelectric polarization between 10 and 200 K. The saturation polarization is of the order of 40 μC/cm², comparable to reports in literature.     

Segmental dynamics of poly(methyl phenyl siloxane) confined to nanoporous glasses

The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DS), temperature modulated DSC (TMDSC) and neutron scattering (NS). Nanoporous glasses with pore sizes of 2.5–20 nm have been used. DS and TMDSC experiments show that for PMPS in 7.5 nm pores the molecular dynamics is faster than in the bulk which originates from an inherent length scale of the underlying molecular motions. For high temperatures the temperature dependence of the relaxation rates for confined PMPS crosses that of the bulk state. Besides finite states effects also the thermodynamic state of nano-confined PMPS is different from that of the bulk. At a pore size of 5 nm the temperature dependence of the relaxation times changes from a Vogel/Fulcher/Tammann like to an Arrhenius behavior where the activation energy depends on pore size. This is in agreement with the results obtained by NS. The increment of the specific heat capacity at the glass transition depends strongly on pore size and vanishes at a finite length scale between 3 and 5 nm which can be regarded as minimal length scale for glass transition to appear in PMPS.     

Specific heat of nanostructured superconducting tin in magnetic fields

Specific heat of tin nanoparticles, which are embedded in porous glass with average pore size ～7 nm, has been investigated in the low-temperature region in magnetic fields up to 2 T. The temperatures of the transition into the superconducting state in various magnetic fields have been determined for tin nanostructured in porous glass. The H _c -T _c phase diagram has been constructed. The upper critical field has been evaluated and the electronic specific heat coefficient and the Debye temperature have been refined. These results have been discussed within the structural model of tin nanoparticles in porous glass.     

Magnetic properties of porous glass-CuO nanocomposites

The magnetization of the porous glass nanocomposite with CuO nanoparticles embedded in the pores has been studied in the temperature range from 1.8 to 350 K for different pore fillings. It has been shown that the magnetic properties of these nanocomposites depend significantly on pore filling. It has been found that, in the low-temperature range for the nanocomposite with a pore filling of 55% and for pressed CuO, the ZFC and FC susceptibilities diverge, a feature which has been almost absent in the nanocomposite with 21% filling. It has been demonstrated that the kink in the temperature dependence of magnetization, which corresponds to the paramagnetic-multiferroic phase transition, does not shift in the sample with a larger pore filling as compared to that observed in the bulk sample.     

Unusual behavior of thermal conductivity of a crystalline-NaCl-opal nanocomposite

An opal-based nanocomposite has been prepared with NaCl incorporated in its pores. The nanocomposite was produced by impregnating the opal with a NaCl solution at room temperature. Thermal conductivity of the nanocomposite has been measured in the temperature range 4.2–300 K. The effective heat conductivity of the nanocomposite was found to be equal to that of pure opal. The observed phenomenon can be explained by assuming that NaCl resides in opal pores in the form of noncontacting needles, thus precluding heat propagation through it. Fiz. Tverd. Tela (St. Petersburg) 40, 379–380 (February 1998)     

Resistance and magnetic susceptibility of superconducting lead embedded in nanopores of glass

This paper reports on a study of the resistance and differential magnetic susceptibility χ _ac of lead embedded in nanosized glass pores with a diameter of ∼7 mm, which was performed at temperatures of 6–300 K and magnetic fields of up to 6 T. The field dependence of the resistance R(H) and the temperature dependences of the real, χ″(T), and imaginary, χ″(T), parts of magnetic susceptibility reveal indications of superconducting phase transitions associated with the volume and surface superconductivity of Pb nanopar ticles. The measurements of the field dependence of resistance have been used to set up the H _c -T _c phase diagram and to carry out a comparison with the study of the heat capacity performed on the same samples.     

DSC and elastic moduli studies on tellurite-vanadate glasses containing antimony oxide

xSb₂O₃-40TeO₂-(60 − x) V₂O₅ glasses with 0 ≤ x ≤ 10 (in mol%) have been prepared by rapid- melt quenching method. DSC curves of these ternary glasses have been investigated. The glass transition properties that have been measured and reported in this paper, include the glass transition temperature (T _g ), glass transition width (ΔT _g ), heat capacity change at glass transition (ΔC _P ) and fragility (F). Thermal stability, Poisson’s ratio, fragility and glass forming tendency of these glasses have been estimated, to determine relationship between chemical composition and the thermal stability or to interpret the structure of glass. In addition, Makishima and Makenzie’s theory was applied for determination of Young’s modulus, bulk modulus and shear modulus, indicating a strong relation between elastic properties and structure of glass. Generally, results of this work show that glass with x = 0 has the highest shear, bulk and Young’s moduli which make it as suitable candidate for the manufacture of strong glass fibers in technological applications; but it should be mentioned that glass with x = 8 has higher handling temperature and super resistance against thermal attack.     

Peculiar features of heat capacity for Cu and Ni nanoclusters

The heat capacity of copper and nickel clusters (from 2 to 6 nm in diameter) was investigated in the temperature range 200–800 K using molecular dynamics method and a modified tight-binding potential. The simulation results demonstrate a very good agreement with the available experimental data at T = 200 K and a fairy good agreement at higher temperatures. A number of regular trends are revealed in computer experiments which agree with the corresponding theoretical predictions. A conclusion is made that in the case of single free clusters the heat capacity may exceed the capacity of the corresponding bulk material. It is found that at 200 K, the copper nanocluster (D = 6 nm) heat capacity is higher by 10% and for nickel cluster by 13%. The difference diminishes with increasing the nanoparticles size proportionally to the relative number of surface atoms. A conclusion is made that very high values of the nanostructure heat capacity observed in laboratory experiments should not be attributed to free clusters, i.e., the effect in question is caused by other reasons.     

High-temperature heat capacity of Gd2CuO4

The heat capacity of Gd₂CuO₄ has been studied by differential scanning calorimetry in the temperature range 362–958 K. It has been found that the temperature dependence of the molar heat capacity has an extremum near 590 K; the extremum is related to a phase transition from the high-temperature tetragonal phase to the low-temperature orthorhombic phase.     

Critical behavior of the heat capacity of the manganite La<Subscript>0.87</Subscript>K<Subscript>0.13</Subscript>MnO<Subscript>3</Subscript>

The heat capacity of the manganite La_0.87K_0.13MnO₃ has been measured in the temperature range 80–350 K. The nature of the ferromagnetic phase transition and the critical properties of heat capacity near the Curie temperature have been studied. The regularities of variations in the universal critical parameters near the phase transition point have been established. The calculated critical exponent and amplitudes of the heat capacity with allowance for corrections on the scaling (α = −0.13 and A ⁺/A ⁻ = 1.178) correspond to the critical behavior of the 3D Heizenberg model.     

Evidence of an intermediate phase in ternary Ge<Subscript>7</Subscript>Se<Subscript>93-x</Subscript>Sb<Subscript>x</Subscript> glasses

The compositional dependence of thermal properties, such as glass transition temperature (T_g), non-reversing enthalpy change (ΔH_NR) and the specific heat capacity change (ΔC_p) of melt quenched Ge₇Se_93-xSb_x (21 ≤ x ≤ 31) glasses, has been studied using alternating differential scanning calorimetry (ADSC) which is analogous to modulated differential scanning calorimetry (MDSC). The glass transition temperature, T_g, which is a measure of global connectivity of the glass, has been found to increase with the addition of Sb. In addition, a change in slope has been observed in the composition dependence of T_g at an average coordination 〈r〉 = 2.40. The experimentally observed compositional variation of glass transition temperature, has been compared with the theoretical predictions from the stochastic agglomeration theory (SAT) and has been found to be consistent. Further, a narrow thermally reversing window is seen in the compositional variation of the relaxation enthalpy (ΔH_NR), which is centered around 〈r〉 = 2.40. The change in specific heat capacity (ΔC_p) at T_g is also found to exhibit a distinct minima at 〈r〉 = 2.40, suggesting that the structural rearrangements for the liquid in the glass transition region are minimized around 〈r〉 = 2.4.     

Magnetic properties of the Nd<Subscript>0.5</Subscript>Gd<Subscript>0.5</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

The magnetic properties of the Nd_0.5Gd_0.5Fe₃(BO₃)₄ single crystal have been studied in principal crystallographic directions in magnetic fields to 90 kG in the temperature range 2–300 K; in addition, the heat capacity has been measured in the range 2–300 K. It has been found that, below the Néel temperature T _N = 32 K down to 2 K, the single crystal exhibits an easy-plane antiferromagnetic structure. A hysteresis has been detected during magnetization of the crystal in the easy plane in fields of 1.0–3.5 kG, and a singularity has been found in the temperature dependence of the magnetic susceptibility in the easy plane at a temperature of 11 K in fields B < 1 kG. It has been shown that the singularity is due to appearance of the hysteresis. The origin of the magnetic properties of the crystal near the hysteresis has been discussed.     

Low-energy lattice excitations in the decagonal Al-Ni-Fe and icosahedral Al-Cu-Fe quasicrystals and the (Al,Si)-Cu-Fe cubic phase

The specific heat of decagonal Al_71.3Ni_24.0Fe_4.7 and icosahedral Al₆₂Cu_25.5Fe_12.5 quasicrystals and the Al_55.0Si_7.0Cu_25.5Fe_12.5 cubic phase approximating the structure of the icosahedral alloy has been studied in the temperature range 4.2–40.0 K. All the three compounds exhibit low coefficients of the electronic heat capacity and pronounced deviations of the low-temperature lattice heat capacity from a cubic temperature law in the range 5–10 K. The results obtained by the thermodynamic method and inelastic neutron scattering have been compared and analyzed. It has been established that, at energies ɛ < 14 meV, the spectral density of thermal vibrations in the icosahedral quasicrystal is substantially higher than those in the cubic approximant and in decagonal quasicrystal.     

Low-temperature specific heat of crystalline and amorphous Eu2(MoO4)3

The specific heat C _total of crystalline and amorphous Eu₂(MoO₄)₃ is measured in the temperature interval 4.5–30 K. The amorphous state is obtained by applying pressure ∼7 GPa at room temperature. It is found that the specific heat of the crystal at T⩽7.5 K is described by a cubic function of temperature, while the specific heat of the amorphous sample has a strongly non-Debye character in the entire experimental temperature interval. The curve of C _total for amorphous europium molybdate is analyzed in a model of soft atomic potentials, and it is shown that it agrees well with universal low-temperature anomalies of the specific heat of classical glasses obtained by quenching from the liquid. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 8, 623–627 (25 October 1998)     

Temperature dependences of the order parameter for sodium nitrite embedded into porous glasses and opals

The temperature dependences of the order parameter η(T) for sodium nitrite NaNO₂ embedded in porous glasses with average pore diameters of 320 and 20 nm, as well as in artificial opals, have been investigated. It has been demonstrated that the dependence η(T) for sodium nitrite in the porous glass almost coincides with that for the bulk material, whereas this dependence for NaNO₂ in opals differs substantially from that observed in the bulk material and from those previously determined for sodium nitrite in porous glasses with average pore diameters of 3 and 7 nm. It has been revealed that the dependence of the order parameter for sodium nitrite in opals exhibits a temperature hysteresis (approximately equal to 8 K). The temperature dependence η(T) has been described using a simple model, which takes into account the nanopore diameter distribution existing in artificial opals.     

Raman spectroscopy: Probing dynamics of water molecules confined in nanoporous silica glasses

In order to explore the influence of nanoscopic confinement on the vibrational properties of H-bonded liquids, we performed a detailed Raman scattering study, as a function of temperature, on water confined in 75 ? and 200 ? pores of a Gelsil glass. A detailed evaluation of the observed changes in the O-H stretching profile has been achieved by decomposing the O-H band into individual components, corresponding to those found for bulk water and associated to different levels of water connectivity. As main result, a similar effect produced by enlarging pore diameter and lowering T has been put into evidence. Again, the “structure-breaker” role of the GelSil glass on physisorbed water is confirmed and shown to be enhanced by the diminishing of the pore size.     

Molecular-dynamics simulation of the heat capacity for nickel and copper clusters: Shape and size effects

We have investigated the heat capacity of ideal Cu and Ni fcc clusters with diameters from 2 to 6 nm in the temperature range 200–800 K by the molecular-dynamics method using a modified tight-binding potential. Our analysis has shown consistency with the experimental results at temperatures of 200–300 K. The data obtained are also indicative of several regularities that are in agreement with the analytical calculations. We have concluded from the results of our computer simulations that the heat capacity in the case of isolated free clusters can exceed that of a bulk material, with this difference decreasing as the nanoparticle grows proportionally to the reduction in the fraction of surface atoms. The excess of the heat capacity for ideal copper and nickel nanoclusters with D = 6 nm at T = 200 K has been found to be 10% and 13%, respectively. Consequently, the large heat capacities of copper and nickel nanostructures observed in some real experiments cannot be related to the characteristics of free clusters. We hypothesize that these properties of a nanomaterial depend on the degree of agglomeration of its constituent particles, i.e., the surfaces and interphase boundaries of interconnected nanoclusters can have a strong effect. To test this hypothesis, we took nickel and copper clusters of various sizes (4000–7200 atoms) produced through the simulation of condensation from the gas phase. At high temperatures, we failed to adequately assess the role of the interphase boundaries in calculating the heat capacity of nanoparticles. The reason was the mass diffusion of Ni or Cu atoms to impart an energetically more favorable shape and structure to the synthesized clusters. At low temperatures, the heat capacity of such clusters exceeded that of clusters with an ideal shape and structure by a value from 3.2% to 10.6%. We have concluded that the Ni and Cu clusters produced in real experiments cannot be applied in devices using the thermal energy of such clusters without a preliminary optimization stage, because their external shape and interior structure are nonideal.     

Acoustic studies of melting and crystallization of indium-gallium alloy in porous glass

Temperature dependences of the velocity of the longitudinal ultrasonic waves in a nanocomposite on the basis of porous glass filled with gallium-indium alloy have been measured. Acoustic anomalies due to crystallization and melting of the alloy in nanopores have been revealed for the complete and partial cooling-heating cycles. A two-step temperature hysteresis loop between the curves of the velocity change upon cooling and heating has been found, the existence of which is related to the formation in pores of two types of mixtures, with α- and β-Ga. Stabilization of β-Ga in nanopores has been observed. It was shown that the conditions of confined geometry lead to a shift to low temperature of the melting regions for both mixtures in comparison with the regions of the coexistence of the liquid and solid phases in bulk alloy.     

Temperature dependence of the heat capacity and lattice constant of lanthanum and samarium hexaborides

The temperature dependence of isobaric heat capacity and [411] interplanar spacing in lanthanum and samarium hexaborides have been determined experimentally within the 5–300 K region. The variation of the lattice parameters and thermal expansion coefficients α(T) with temperature has been calculated. Fiz. Tverd. Tela (St. Petersburg) 40, 2051–2053 (November 1998)     

Thermal physical properties of ferroelectric ceramics PKR-7M near the diffuse phase transition

The thermal physical properties (heat capacity, thermal expansion coefficient, and deformation) of a relaxor piezoelectric ceramics based on the lead zirconate titanate PKR-7M have been studied in the temperature range 200–800 K. Diffuse anomalies have been revealed in the temperature dependences of the heat capacity and thermal expansion over wide temperature ranges 270–650 and 450–600 K, respectively. It has been shown that the anomalous behavior of the heat capacity is due to the manifestation of two-level states (Schottky anomalies). The results of the study have been discussed together with the data of structural studies.