O. F. Demidenko

Effect of the orbital ordering on the transport and magnetic properties of MnSe and MnTe

This paper reports on the results of measurements of the conductivity in MnSe and MnTe polycrystalline samples under thermal cycling in the temperature range 80 < T < 300 K in magnetic fields of up to 5 kOe. Manganese selenide MnSe is found to exhibit a magnetoresistive behavior below the Néel temperature. The specific features revealed in the temperature dependences of the magnetic susceptibility and the electrical resistivity in some temperature ranges are accounted for in terms of magnetic ordering, which is mediated by the interaction of the pseudoorbital moments with spins.

Magnetic and electrical properties of cation-substituted sulfides MexMn1 − xS (Me = Co, Gd)

The temperature dependences of the specific magnetization σ and the electrical resistivity ρ of MexMn1 − xS single crystals (Me= Co, Gd; x= 0.05) have been studied in the temperature range 80 K < T < 1000 K. The samples under study have revealed the presence of a spontaneous magnetic moment below the Néel temperature (TN) and ferromagnetic clusters in Gd0.05Mn0.95S in the temperature range 146 K < T < 680 K. Substitution of gadolinium for manganese initiates a transition from p-type to n-type conduction. The change in the conduction type is accompanied by an increase in the electrical resistivity at 300 K by approximately one order of magnitude and, accordingly, by a decrease in the activation energy. The magnetic and electrical properties of the crystals under study have been interpreted in terms of the cluster model with temperature-dependent ferromagnetic exchange and an electron localized in this cluster.

Transport properties and ferromagnetism of CoxMn1 − xS sulfides

We have studied the resistivity and thermoelectromotive force (thermo emf) in a temperature range of T = 80–1000 K, the magnetic susceptibility and magnetization in a temperature range of T = 4.2–300 K at an external magnetic field of up to 70 kOe, and the structural characteristics of CoxMn1−xS sulfides (0 ≤ x ≤ 0.4). Anomalies in the transport properties of these compounds have been found in the temperature intervals ΔT1 = 200–270 K and ΔT2 = 530–670 K and at T3 ∼ TN. The temperature dependences of the magnetic susceptibility, magnetization, and resistivity, as well as the current-voltage characteristics, exhibit hysteresis. In the domain of magnetic ordering at temperatures below the Néel temperature (TN), the antiferromagnetic CoxMn1−xS sulfides possess a spontaneous magnetic moment that is explained using a model of the orbital ordering of electrons in the t2g bands. The influence of the cobalt-ion-induced charge ordering on the transport and magnetic properties of sulfides has been studied. The calculated values of the temperatures corresponding to the maxima of charge susceptibility, which are related to a competition between the on-site Coulomb interaction of holes in various subbands and their weak hybridization, agree well with the experimental data.

Crystal structure and electrical properties of GdxMn1 − xS and TixMn1 − xSe solid solutions

Regions of the existence of sulfide GdxMn1 − xS and selenide TixMn1 − xSe solid solutions have been identified. Their electrical and thermoelectric properties have been studied in the temperature range 80–900 K. It has been established that the substitution of Gd2+ and Ti2+ ions for Mn2+ cations initiates reversal of the type of charge carrier with respect to the starting compounds MnS and MnSe. The cation substitution in solid solutions brings about a change from the hole conduction (α > 0) characteristic of the manganese monosulfide and monoselenide to the electronic conduction (α < 0).

Electrical conductivity and thermopower in Cox Mn1 − x S sulfides

This paper reports on the results of investigations into the structural, electrical, and thermoelectrical properties of sulfides CoxMn1 − xS (0 ≤ x ≤ 0.4) in the temperature range 80–950 K. It is established that the thermopower coefficient α decreases significantly with an increase in the cobalt concentration in the lattice of the α-MnS compound. The CoxMn1 − xS compounds with cobalt concentrations in the range 0 ≤ x ≤ 0.3 are semiconductors with hole conduction (α > 0), whereas the compound with x = 0.4 exhibits metallic conduction (α < 0). It is found that the band gap Eg of the compounds under investigation varies in the range from 1.46 eV for α-MnS (x = 0) to 0.26 eV for CoxMn1 − xS (x = 0.4).

Synthesis, structure, and magnetic properties of anion-substituted manganese chalcogenides

The anion-substituted solid solutions of the MnSe1 t xTex system have been synthesized. The crystal structure and magnetic properties of the synthesized solid solutions have been investigated. It has been shown that, in the concentration range 0 ≤ x ≤ 0.4, the solid solutions have a face-centered cubic structure. It has been revealed that an increase in the concentration of the substituting element in the MnSe1 − xTex system leads to an increase in the coefficient of thermal expansion of the sample. The investigation of the magnetic properties has been carried out at temperatures in the range 80 K < T < 1000 K in a magnetic field up to 8.6 kOe. It has been experimentally found that the type of antiferromagnetic order (the second type of ordering) remains unchanged over the entire concentration range up to x = 0.4 and that the paramagnetic Curie temperature and the Néel temperature decrease within the limits of 20%. Theoretical calculations have been performed using the Monte Carlo method, and the model of nanoclusters with an uncompensated antiferromagnetic moment has been proposed.

Peculiarities of Transport, Resonance and Optical Properties of the Anion-Substituted Manganese Chalcogenides

The transport, resonance and optical properties of anion-substituted manganese chalcogenides MnSe1-xTex in the 77-300 K temperature range in magnetic fields up to 1T are studied. The magnetoresistance effect with the maximum value in the vicinity of the Neel temperature for the composition x = 0.1 is revealed. EPR data indirectly indicate the type of the current carriers – lattice polarons. The changes in electron structure occurring due to the anionic substitution are studied using the optical methods.

Magnetic properties of CNT arrays synthesized by the injection CVD method at various catalyst concentrations

The arrays of multi-wall carbon nanotubes (CNTs) filled with ferromagnetic nanoparticles (MFCNTs) have been obtained by the high temperature pyrolysis of fluid hydrocarbon (o-xylene) in a mixture with the volatile source of catalyst (ferrocene) using Ar as the gas-carrier. The influence of the catalyst concentration cx (0.5%, 5%, and 10%) in the feeding solution on the composition, crystalline structure, morphology and, accordingly, magnetic properties of MFCNT arrays in a wide temperature range was investigated. The X-ray diffraction, SEM and TEM methods revealed that CNT arrays are filled by Fe3C and Fe phases and that the higher is the catalyst concentration in the feeding solution, the higher is Fe3C and Fe content in CNT arrays. Temperature dependence of the specific magnetization σ(T) shows that σ increases with the increasing of ferrocene concentration in a whole temperature range under investigation (78 ≤T < 600 K). It is shown that σ(T) follows the Bloch law in the temperature range 80-300 K with Bloch constant B=1.65.10-5 K-3/2 and the Stoner law at 300-480 K for samples with cx=10% and, correspondingly, 80-450 K with Bloch constant B=6.1.10-5 K-3/2 and 450-480 K for samples with cx=5%. The lower value of Bloch constant, which characterizes the exchange interaction, in the case of cx=10% might be attributed both to dimensional effects and the decrease of the effective magnetic momentum of Fe phases atoms. The hysteresis loops demonstrate that coercivity Hc(cx=5%) decreases, but Hc(cx=10%) is constant or even slightly increases with increasing the temperature. This phenomenon is explained by the increase both the saturation magnetization and shape anisotropy.

Magnetic properties of multiferroics Bi 1 – x Sm x FeO 3 synthesized under high pressure

The magnetic properties of the multiferroics obtained upon isovalent substitution of samarium cations for bismuth cations in BiFeO3. The samples have been synthesized by solid-phase reactions under conditions of a cold pressing at high (4 GPa) pressure. The correlation between the structure and the magnetic properties of the multiferroics has been found based on analyzing the experimental data.

The influence of electron irradiation on the magnetic properties of carbon nanotubes filled with Fe-phases composite

The arrays of multi-wall carbon nanotubes (CNTs) filled with Fe-containing nanocomposite have been studied from the point of view of their magnetic properties stability to electron irradiation. CNTs with different content of magnetic component were synthesized by CVD method using ferrocene/xylene mixture of varied proportion. The structure, composition and magnetic properties of CNTs irradiated with a dose of 1015 cm-2 were investigated by TEM, X-ray analysis and ponderomotive method, correspondingly. It was revealed that the morphology of the magnetic particles inside CNTs were changed what, along with radiation defects, lead the deterioration the magnetic properties of CNTs.