Search

We have investigated the temperature dependence of the magnetic susceptibility $\chi(T)$ of rare-earth cobaltites RCoO$_3$ (R= La, Pr, Nd, Sm, Eu) in the temperature range $4.2-300$ K and also the influence of hydrostatic pressure up to 2 kbar on their susceptibility at fixed temperatures $T=78 $ and 300 K. The specific dependence $\chi(T)$ observed in LaCoO$_3$ and the anomalously large pressure effect (d ln $\chi$/d$P\sim -100$ Mbar$^{-1}$ for $T = 78$ K) are analyzed in the framework of a two-level model with energy levels difference $\Delta$. The ground state of the system is assumed to be nonmagnetic with the zero spin of Co$^{3+}$ ions, and magnetism at a finite temperature is determined by the excited magnetic spin state. The results of the analysis, supplemented by theoretical calculations of the electronic structure of LaCoO$_3$, indicate a significant increase in $\Delta$ with a decrease in the unit cell volume under the hydrostatic pressure. In the series of RCoO$_3$ (R= Pr, Nd, Sm, Eu) compounds, the volume of crystal cell decreases monotonically due to a decrease in the radius of R$^{3+}$ ions. This leads to an increase in the relative energy $\Delta$ of the excited state (the chemical pressure effect), which manifests itself in a decrease in the contribution of cobalt ions to the magnetic susceptibility at a fixed temperature, and also in a decrease in the hydrostatic pressure effect on the susceptibility of RCoO$_3$ compounds, which we have observed at $T=300$ K.

The effect of pressure on magnetic properties of LaCoO$_3$ is studied experimentally and theoretically. The pressure dependence of magnetic susceptibility $\chi$ of LaCoO$_3$ is obtained by precise measurements of $\chi$ as a function of the hydrostatic pressure $P$ up to 2 kbar in the temperature range from 78 K to 300 K. A pronounced magnitude of the pressure effect is found to be negative in sign and strongly temperature dependent. The obtained experimental data are analysed by using a two-level model and DFT+U calculations of the electronic structure of LaCoO$_3$. In particular, the fixed spin moment method was employed to obtain a volume dependence of the total energy difference $\Delta$ between the low spin and the intermediate spin states of LaCoO$_3$. Analysis of the obtained experimental $\chi(P)$ dependence within the two-level model, as well as our DFT+U calculations, have revealed the anomalous large decrease in the energy difference $\Delta$ with increasing of the unit cell volume. This effect, taking into account a thermal expansion, can be responsible for the temperatures dependence of $\Delta$, predicting its vanishing near room temperature., Comment: 7 pages, 9 figures

The structure and physical properties of superconducting compounds Y(La)-Ba(Sr)-Cu-O are studied, the compounds being prepared by the method of cryogenic dispersion of a charge consisting of premix oxides and carbonates. Electrical conductivity and critical current density of the superconductors are measured over a wide temperature range of 10~$mK$ to 300~$K$. Degradation of the superconductor critical parameters in time and structural characteristics, magnetic susceptibility, heat capacity and acoustic properties are studied, and current-voltage characteristics are determined., Comment: 9 pages, 11 figures

Magnetic susceptibility $\chi$ of the polycrystalline sample of samarium monosulfide was measured as a function of the hydrostatic pressure $P$ up to 2 kbar at liquid nitrogen and room temperatures using a pendulum-type magnetometer. A pronounced magnitude of the pressure effect is found to be positive in sign and strongly temperature dependent: the pressure derivatives of $\chi$, d\,ln$\chi$/d$P$, are $6.3\pm0.5$ and $14.2\pm1$ Mbar$^{-1}$ at 300 and 78 K, respectively. The obtained experimental results are discussed within phenomenological approaches., Comment: 5 pages, 4 figures

The electronic energy structures and magnetic properties of iron-based compounds with group VI elements (FeTe, BiFeO$_3$, SrFe$_{12}$O$_{19}$ and SrCoTiFe$_{10}$O$_{19}$) are studied using the density functional theory (DFT) methods. Manifestations of different types of chemical bonds in magnetism of these compounds are studied theoretically. Calculations of electronic structures of these systems were performed using the generalized gradient approximation (GGA) for description of the exchange and correlation effects within DFT. For SrFe$_{12}$O$_{19}$ and SrCoTiFe$_{10}$O$_{19}$ hexaferrites the GGA+$U$ method was also employed to deal with strongly correlated 3$d$-electrons. The calculations have revealed distinctive features of electronic structure of the investigated iron-based compounds with strongly correlated 3$d$-electrons, which can be responsible for their peculiar structural and magnetic properties., Comment: 7 pages, 8 figures

Magnetic susceptibility $\chi$ of Bi$_{2-x}$Mn$_{x}$Se$_3$ ($x = 0.01-0.2$) was measured in the temperature range $4.2-300$ K. For all the samples, a Curie-Weiss behaviour of $\chi(T)$ was revealed with effective magnetic moments of Mn ions corresponding to the spin value S=5/2, which couple antiferromagnetically with the paramagnetic Curie temperature $\Theta\sim -50$ K. In addition, for the samples of nominal composition $x$ = 0.1 and 0.2 the effect of a hydrostatic pressure $P$ up to 2 kbar on $\chi$ has been measured at fixed temperatures 78 and 300 K that allowed to estimate the pressure derivative of $\Theta$ to be d$\Theta$/d$P\sim-0.8$ K/kbar. Based on the observed behaviour of $\Theta$ with varied Mn concentration and pressure, a possible mechanism of interaction between localized Mn moments is discussed., Comment: 7 pages, 6 figures

Possible influence of spatially inhomogeneous distribution of strontium impurities on the temperature of antiferromagnetic (AFM) ordering in ceramic samples La2-xSrxCuO4 has been revealed., Comment: 6 pages, 3 figures 16-th International Meeting "Ordering in Minerals and Alloys", 12-17 September 2013, Rostov-on Don, Tuapse, Russia V 1, pp 124-127

The influence of uniform pressures $P$ up to 5 kbar on the superconducting transition temperature $T_c$ was studied for the FeSe$_{1-x}$Te$_x$ ($x=0$, 0.85, 0.88 and 0.9) system. For the first time, we observed a change in sign of the pressure effect on $T_c$ when going from FeSe to tellurium rich alloys. This has allowed to specify the pressure derivative d$T_c$/d$P$ for the system as a function of composition. The observed dependence was compared with results of the {\em ab initio} calculations of electronic structure and magnetism of FeSe, FeTe and FeSe$_{0.5}$Te$_{0.5}$, and also with our recent experimental data on pressure effects on magnetic susceptibilities of FeSe and FeTe compounds in the normal state. This comparison demonstrates a competing interplay between superconductivity and magnetism in tellurium rich FeSe$_{1-x}$Te$_x$ compounds, Comment: 7 pages, 6 figures

A detailed magnetization study for the novel FeSe superconductor is carried out to investigate the behavior of the intrinsic magnetic susceptibility $\chi$ in the normal state with temperature and under hydrostatic pressure. The temperature dependencies of $\chi$ and its anisotropy $\Delta \chi=\chi_{|}-\chi_{\bot}$ are measured for FeSe single crystals in the temperature range 4.2-300 K, and a substantial growth of susceptibility with temperature is revealed. The observed anisotropy $\Delta \chi$ is very large and comparable with the averaged susceptibility at low temperatures. For a polycrystalline sample of FeSe, a significant pressure effect on $\chi$ is determined to be essentially dependent on temperature. Ab initio calculations of the pressure dependent electronic structure and magnetic susceptibility indicate that FeSe is close to magnetic instability with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume and especially on the height $Z$ of chalcogen species from the Fe plane. The change of $Z$ under pressure determines a large positive pressure effect on $\chi$ which is observed at low temperatures. It is shown that the literature experimental data on the strong and nonmonotonic pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with calculated behavior of the density of electronic states at the Fermi level., Comment: 8 pages, 10 figures

The temperature dependences of resistance, R(T), of two single-crystals La_{2}CuO_{4+\delta} samples have been studied with the aim to detect a possible change in the R(T) behavior induced by paramagnetic-antiferromagnetic (PM-AFM) transition. One of the samples with \delta \lesssim 0.01, was fairly homogeneous in oxygen distribution (not phase-separated) with Neel temperature T_{N}\approx 266 K. Conductivity of this sample has been determined by Mott's variable-range hopping below T_N. The other, far less resistive, sample with \delta \approx 0.05, was inhomogeneous (phase-separated) showing both PM-AFM (T_N\approx 205 K) and superconducting (T_c\approx 25 K) transitions. It is found that for the homogeneous sample the resistivity decreases above T_N far faster with temperature than below it (for both directions of measuring current, parallel and perpendicular to basal CuO_2 planes). A similar behavior of conductivity near PM-AFM transition is also found for the phase-separated and less resistive sample. In this case a clear kink in R(T) curve near T_N\approx 205 K can be seen. Furthermore, a transition to metallic (dR/dT>0) behavior occurs far enough above T_N. The observed behavior of the samples studied is related to increased delocalization of charge carriers above T_N. This is in accordance with decrease in the AFM correlation length and corresponding enhancement of the hole mobility above T_N known for low-doped lanthanum cuprates., Comment: 11 pages, 6 figures, submitted to Physica B

It is found that a strong inhomogeneous distribution of oxygen in single crystal La_{2}CuO_{4+\delta} can lead to the formation of isolated superconducting inclusions, the average hole concentration being no more than 0.0024 per Cu atom. The behavior of the conductivity in the magnetic field below T=20 K is consistent with possible existence of a insulating low-temperature magnetic phase (spin density waves), which is typical of cuprates with excess oxygen., Comment: 4 pages with 3 figs, a translation from Russian into English of paper submitted to Izvestiya Ross. Akad. Nauk, Ser. Fizicheskaya

Magnetic susceptibility of the isostructural Ce(Ni{1-x}Cu{x})5 alloys (0< x <0.9) was studied as a function of the hydrostatic pressure up to 2 kbar at fixed temperatures 77.3 and 300 K, using a pendulum-type magnetometer. A pronounced magnitude of the pressure effect is found to be negative in sign and to depend strongly and non-monotonously on the Cu content, showing a sharp maximum in vicinity of x = 0.4. The experimental results are discussed in terms of the Ce valence change under pressure. It has been concluded that the fractional occupation of the f-states, which corresponds to the half-integer valence of Ce ion (3.5), is favorable for the valence instability in alloys studied. For the reference CeNi5 compound the main contributions to magnetic susceptibility and their volume dependence are calculated ab initio within the local spin density approximation (LSDA), and appeared to be in close agreement with experimental data., Comment: 5 pages, 4 figures

The ultrasonic properties of La$_{1-x}$Ca$_{x}$MnO$_{3}$ ($x\approx 0.25$) with the Curie temperature $T_C$ about 200 K are studied. Temperature dependences of longitudinal and transverse sound velocities were measured in zero magnetic field and for different constant magnetic fields as well. The ultrasonic study is supported by magnetic, resistive, magnetoresistive, structural and other measurements of the sample that facilitate interpretation of the results obtained. The magnetic field influence on sound properties found in this study presents some new features of the interplay between the elastic and magnetic properties of these compounds. It is shown that the paramagnetic-ferromagnetic transition in the sample studied is first order, but can become second order under the influence of applied magnetic field., Comment: submitted to Phys. Rev. B

The DC transport properties of and the radio-frequency (RF) wave absorption (at 2.525 MHz) in a sample of La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ prepared by floating-zone method are measured. The Curie temperature, $T_{c}$, of the sample is about 374 K. Giant temperature and magnetic-field variations in RF absorption are found in the vicinity of $T_{c}$. Relative change of the RF absorption in magnetic field (magnetoabsorption) is about 67% in field 2.1 kOe and about 55% in field 1 kOe. This giant magnetoabsorption effect can be used to develop RF devices controlled by temperature and low magnetic field. A weak temperature dependence of magnetoabsorption for the sample studied in the range from room temperature to about 350 K makes it especially attractive for practical use. The RF study supplemented with transport, magnetoresistive and magnetic measurements enables us to discuss the optimal properties of manganite samples for observation of giant magnetoabsorption in low field., Comment: Submitted to J. Magn. Magn. Mater., 14 pages including 7 figures

The resistive measurements were made to study the magnetic field-induced antiferromagnetic (AF) - weak ferromagnetic (WF) transition in La$_2$CuO$_4$ single-crystal. The magnetic field (DC or pulsed) was applied normally to the CuO$_2$ layers. The transition manifested itself in a drastic decrease of the resistance in critical fields of ~5-7 T. The study is the first to display the effect of the AF -WF transition on the conductivity of the La$_2$CuO$_4$ single-crystal in the parallel - to - CuO$_2$ layers direction. The results provide support for the 3-dimensional nature of the hopping conduction of this layered oxide., Comment: 8 pages, 7 figures, RevTex

The magnetic and transport properties of La_{0.5}Sr_{0.5}CoO_{3-\delta} film grown on a LaAlO_3 substrate by pulsed-laser deposition are studied. The properties are found to be influenced by the magnetic anisotropy and inhomogeneity. Magnetoresistance anisotropy is determined by the shape anisotropy of the magnetization and the strain-induced magnetic anisotropy due to the film-substrate lattice interaction. Indications of the temperature-driven spin reorientation transition from an out-of plane orderded state at low temperatures to an in-plane ordered state at high temperatures as a result of competition between the mentioned sources of magnetic anisotropy are found., Comment: 5 pages, 8 figures, submitted to Fiz. Nizk. Temp, an extended version of short communication in cond-mat/0207346

The magnetic and transport properties of La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta}$ film grown on a LaAlO$_3$ substrate by pulsed-laser deposition are studied. The properties are found to be influenced by a combined influence of the magnetic anisotropy and inhomogeneity. Magnetoresistance anisotropy is determined by the shape anisotropy and the strain-induced magnetic anisotropy due to the film-substrate lattice interaction. Indications of the temperature-driven spin reorientation transition from an out-of plane orderded state at low temperatures to an in-plane ordered state at high temperatures as a result of competition between the mentioned anisotropy sources are found., Comment: 3 pages, 4 figures, reported at Moscow Int. Symp. on Magnetism (MISM 2002), June 2002

The unusual non-linear effects in hopping conduction of single-crystal La_{2}CuO_{4 + \delta} with excess oxygen has been observed. The resistance is measured as a function of applied voltage U (10^{-3} V - 25 V) in the temperature range 5 K < T < 300 K. At relatively high voltage (U > 0.1 V) the conduction of sample investigated corresponds well to Mott's variable-range hopping (VRH). An unusual conduction behavior is found, however, in low voltage range (approximately below 0.1 V), where the influence of electric field and (or) electron heating effect on VRH ought to be neglected. Here we have observed strong increase in resistance at increasing U at T < 20 K, whereas at T > 20 K the resistance decreases with increasing U. The magnetoresistance of the sample below 20 K has been positive at low voltage and negative at high voltage. The observed non-Ohmic behavior is attributable to inhomogeneity of the sample, and namely, to the enrichment of sample surface with oxygen during the course of the heat treatment of the sample in helium and air atmosphere before measurements. At low enough temperature (below 20 K) the surface layer with increased oxygen concentration is presumed to consist of disconnected superconducting regions (with T_{c} about 20 K) in poor-conducting matrix. The results obtained demonstrate that transport properties of cuprate oxides may be determined in essential degree by structural or stoichimetric inhomogeneities. This should be taken into account at evaluation of "quality" of high-temperature superconductors on the basis of transport properties measurements., Comment: 12 pages, REVTex, 11 Postscript figures, To be published in Fizika Nizkikh Temperatur (published by AIP as Low Temperature Physics)