S. Neeleshwar

Critical exponents of the La0.7Sr0.3MnO3, La0.7Ca0.3MnO3, and Pr0.7Ca0.3MnO3 systems showing correlation between transport and magnetic properties

From the low-temperature (down to 10K) dc-magnetization data of the La0.7Sr0.3MnO3 (LSM), La0.7Ca0.3MnO3 (LCM), and Pr0.7Ca0.3MnO3 (PCM) systems, we estimated the critical exponents β, γ, and hence δ from the analysis of the modified Arrot plots. The exponent β estimated for the LCM system is less than that predicted by Heisenberg model and resides within the zone predicted by Ising model while for the LSM sample, β is higher than that predicted from the Heisenberg model which is considered to be due to the presence of dipole-dipole interaction arising from the large spin moment in the LSM system. The magnetization data of the PCM system cannot be fitted to the modified Arrot plots, which suggest highly inhomogeneous ground state even under 5T magnetic field. Both LSM and LCM have almost equal values of γ. Seebeck coefficient data indicate a crossover from higher-temperature n-type to lower-temperature p-type conductivity behavior in both LSM and LCM systems, while the semiconducting PCM system shows p-ty...

The intrinsic thermal conductivity of SnSe.

Brief Communication Arising, suggested the formation of SnO2 as a plausible cause for the higher thermal conductivity in polycrystalline SnSe, it must be noted that SnSe is not highly sensitive to air at ambient temperature5. It should also be noted that Zhao et al.1 suggested the presence of “plenty of vacancies and interstitials” in their samples. Some defects are entropically present in single crystals, but we doubt that the vacancies and interstitials can account for the observed 11%–12% deficiency in density at room temperature. It is well known that porosity significantly reduces the thermal transport, which emphasizes the importance of reporting packing density values in future publications to validate the intrinsic transport properties. In other words, true thermal conductivity cannot be obtained by simple normalization, because thermal diffusivity and density are interdependent quantities. Here, we are not expressing concerns about the measurement techniques or the self-consistency of measurements by Zhao et al.1 but about the intrinsic nature of their SnSe single crystals. Thermoelectricians have long aimed to optimize ZT by reducing κ, and ref. 1 reported exceptionally low thermal conductivity. Thus, the sole aim of this Comment is to correct the scientific record by stating that the ultralow κ value reported in ref. 1 is not intrinsic to fully dense single-crystalline SnSe. A single crystal, by definition, must have an experimentally measured density that is close to 100% of the theoretical density. Thus, the SnSe samples of ref. 1 cannot be classified as single crystalline and the thermal conductivity and figure of merit values for SnSe presented in ref. 1 are not intrinsic to single-crystalline SnSe. brief CommUniCATions Arising

Enhanced magnetoresistance and Griffiths phase induced by Mo substitution in La0.7Ca0.15Sr0.15Mn1−xMoxO3 (0 ⩽ x ⩽ 0.05)

Structural, magnetic and electrical properties of the La0.7Ca0.15Sr0.15Mn1−xMoxO3 (0 x 0.05) compounds have been investigated. Powder x-ray analysis reveals that the sample with x = 0 crystallizes in the rhombohedral (R3c) structure, whereas in the Mo doped samples the structure can be indexed by an orthorhombic (Pbnm) structure. The important observations of the magnetic and transport properties are: (i) the Mo substitution induces a distinct suppression of the metal‐insulator (TMI) and ferromagnetic (FM)—paramagnetic transition (TC) and the temperature of TMI was found to be higher than TC in the Mo-doped samples, (ii) the substitution of Mo enhances the magnetoresistance at room temperature, (iii) a large deviation from the Curie‐Weiss law well above TC in the Mo substituted samples indicates the existence of a Griffiths phase and (iv) long-range FM order persists in all samples with a linear decrease of saturation moment as x increases. These results are discussed in terms of the Mn-site disorder and opening of strong FM coupling between Mn 2+ ‐O‐Mn 3+ , due to the Mn 2+ ions induced by Mo 6+ at the expense of Mn 4+ ions in the La0.7Ca0.15Sr0.15Mn1−xMoxO3 system. (Some figures in this article are in colour only in the electronic version)

Chemical disorder-induced magnetism in FeSi2 nanoparticles

Iron disilicide in a bulk form is practically nonmagnetic. In contrast, nanoparticles of FeSi2 exhibit superparamagnetism with blocking temperatures ranging from 8K(15nm)to34K(55nm). Their relatively low saturation magnetization suggests that the magnetic behavior is associated with only a small fraction of Fe ions, which have a sufficient number of other Fe as nearest neighbors. The chemical disorder is presumably induced in the formation of nanoparticles. A spin glass-type anomaly below 10K observed in specific heat data gives a further evidence for the compositional heterogeneity.

Transport and magnetic properties of metallic La1−xPbxNiO3−δ (0.0⩽x⩽0.1)

Transport properties (resistivity and thermoelectric power) of Pb doped LaNiO3 viz. La1−xPbxNiO3−δ (0.0⩽x⩽0.1) show metallic behavior over a wide range of temperature (10–550K). Pb doping (up to 10%) at the La site does not destroy the metallic behavior of LaNiO3. The paramagnetic susceptibility χ decreases with Pb doping. Above 50K, χ is almost temperature independent and exhibits Pauli like features with a small additional Curie law contribution. The resistivity ρ increases with Pb doping though the thermoelectric power does not change proportionately, indicating that Pb doping does not introduce much disorder in the system. A linear T dependence of ρ observed above 150K suggests the importance of electron–phonon (el–ph) interactions but at temperature below 150K, ρ follows a T1.5 dependence. The estimated el–ph interaction constant λ increases (0.80–2.53) with Pb doping (x=0.0–0.1). No saturation of resistivity has been observed even up to 550K with x⩽0.1. The phonon frequency νph∼3444cm−1 of the undop...

Magnetic and superconducting properties of single crystals of Sr2HoRu1−xCuxO6 grown from high temperature solutions

Single crystals of Sr2HoRu1−xCuxO6 (with x=0–0.2), measuring 2–3mm across have been grown from PbO–PbF2 based solutions in the temperature range of 1250–1150°C. The crystals exhibit octahedral morphology and belong to the monoclinic space group P21∕n. While Sr2HoRuO6 is found to be antiferromagnetic with weak ferromagnetism below 30K, the solid solutions containing Cu exhibit a diamagnetic transition at 31K which increases in magnitude and temperature with increasing Cu. Through a correlation of magnetic and calorimetric properties, these crystals are concluded to be spin-glass superconductors.

Interface states and Barrier Heights on Metal/4H-SiC Interfaces

Available data on Schottky barrier heights on silicon and carbon rich faces of 4H-SiC have been carefully analyzed to investigate the mechanism of barrier formation on these surfaces. As in case of 3C and 6H-SiC, the barrier heights depend strongly upon method of surface preparation with a considerable scatter in the barrier height for a given metal-semiconductor system. However, for each metal the barrier height depends on the metal work function and strong pinning of the Fermi level has not been observed. The slopes of the linear relation between the barrier heights and metal work functions varies over a wide range from 0.2 to about 0.75 indicating that the density of interface states depends strongly on the method of surface preparation. By a careful examination of the data on barrier heights we could identify a set of nearly ideal interfaces in which the barrier heights vary linearly with metal work function approaching almost to the Schottky limit. The density of interface states for these interfaces is estimated to lie between 4.671012 to 2.631012 states/ cm2 eV on the silicon rich surface and about three times higher on the carbon rich faces. We also observed that on these ideal interfaces the density of interface states was almost independent of metal indicating that the metal induced gap states (MIGS) play no role in determining the barrier heights in metal-4H-SiC Schottky barriers.

Size effect on magnetic ordering in Ce3Al11

Abstract To study the size dependence of magnetic ordering, magnetic measurements have been made between 1.8 and 300 K on Ce3Al11 particles having an average particle size of 1400 A . The nanoparticles were single phase as confirmed by X-ray diffraction. At low temperatures a ferromagnetic transition occurs at T C =6.2 K , which is the same as that for the bulk material. On the other hand, the antiferromagnetic transition at T N =3.2 K for the bulk material is not visible down to 1.8 K . Meanwhile, the slightly smaller Curie constant of nanoparticles as compared to that of the bulk indicates a certain degree of demagnetization of Ce ions when the particle size is sufficiently reduced.

Low-temperature calorimetric evaluation of aluminum-lithium alloys

Low temperature calorimetric measurements have been made on a series of binary aluminum alloys containing 1–10 at. % lithium. The heat capacity data are analyzed in terms of an electronic and a lattice contribution. Accordingly, the low level of lithium addition yields appreciable changes in the electronic density-of-states at Fermi level and the characteristic Debye temperature of lattice. These results reflect certain degrees of electron localization between aluminum and lithium atoms, providing a fundamental basis for solid solution strengthening in the lightweight materials with great potential in aerospace applications.