N. V. Selezneva

Magnetic order, field-induced phase transitions and magnetoresistance in the intercalated compound Fe0.5TiS2

Measurements of the magnetic susceptibility, magnetization, electrical resistivity and neutron diffraction have been performed for the compound Fe(0.5)TiS(2) in which Fe atoms are intercalated between S-Ti-S tri-layers. It has been shown that this compound with a monoclinic crystal structure exhibits an antiferromagnetic (AF) ground state below the Néel temperature T(N) ≈ 140 K. Small deviations from the stoichiometry and some disordering effects caused by the additional low-temperature heat treatment do not affect substantially the AF state in Fe(0.5)TiS(2). According to neutron diffraction data the magnetic structure at 2 K is described by the propagation vector k = (1/4,0,1/4). The Fe magnetic moments with a value of (2.9 ± 0.1) μ(B) are directed at an angle of (78.5 ± 1.8)° to the layers. Application of the magnetic field at T < T(N) induces a metamagnetic phase transition to the ferromagnetic (F) state, which is accompanied by the large magnetoresistance effect (|Δρ/ρ| up to 27%). Below 100 K, the field-induced AF-F transition is found to be irreversible, as evidenced by magnetoresistance and neutron diffraction measurements. The magnetization reversal in the metastable F state is accompanied at low temperatures by substantial hysteresis (ΔH ~ 100 kOe) which is associated with the Ising character of Fe ions.

Transport properties and polarization phenomena in intercalated AgxHfSe2 compounds

The electrical properties of intercalated AgxHfSe2 compounds (x = 0.1, 0.2) have been investigated for the first time. Investigations have been performed using various current electrodes, which make it possible to pass either the electron current or the ion current across the sample. Polarization effects, which indicate the self-consistent migration of charge carriers in the samples, have been found for the samples at room temperature. Based on the characteristic features of polarization decay, coefficients of conjugated chemical diffusion have been evaluated.

Influence of copper intercalation on the resistive state of compounds in the Cu-HfSe2 system

Polycrystalline samples of intercalated compounds CuxHfSe2 have been synthesized for the first time and their electrical resistivity has been measured at both direct current and alternating current (with a frequency ranging from 200 Hz to 150 kHz) in the temperature range 80–300 K. It has been shown that the intercalation of copper atoms between three-layer Se-Hf-Se blocks leads to an increase in the electrical resistivity of the samples, as well as to a more pronounced activated character of the temperature dependence of the electrical resistivity. A time dependence of the electrical resistivity of the CuxHfSe2 samples at room temperature, which is associated with the presence of copper ions in the sample, has been determined.

Specific features of the charge and mass transfer in a silver-intercalated hafnium diselenide

The specific features of the charge transfer in intercalated samples of AgxHfSe2 have been studied for the first time by alternating current (ac) impedance spectroscopy. It has been found that relaxation processes in an ac field are accelerated with increasing silver content in the samples. The complex conductivity (Y) shows a frequency dispersion described by power law Y ∼ ωs, which is characteristic of the hopping conductivity mechanism. The AgxHfSe2 compounds demonstrate shorter relaxation times as compared to those observed in hafnium diselenide intercalated with copper atoms, and this fact indicates that the charge carrier mobility in the silver-intercalated compounds is higher. The possibility of silver ion transfer in AgxHfSe2 is confirmed by the measurements performed by the method of electrochemical cell emf.

Suppression and inducement of the charge-density-wave state in Cr x TiSe2

The x-ray diffraction, electrical resistivity and thermal expansion measurements have been employed to study how the intercalation of Cr atoms into TiSe2 matrix affects the crystal structure, formation of the charge density wave (CDW) and electrical properties. The intercalation of a small amount of Cr atoms (up to x ~ 0.03) is observed to suppress the CDW formation. The electrical resistivity of Cr x TiSe2 compounds with the Cr concentrations 0.03  ⩽  x  ⩽  0.20 shows a metallic-type behavior; while in the concentration range 0.25  ⩽  x  ⩽  0.5, the resistivity shows an anomalous behavior indicating the reappearance of the transition to a CDW-like state; further growth of the Cr content up to x  =  0.6 again leads to the metallic-type resistivity. For the compound Cr0.25TiSe2, the phase transition below 160 K together with abnormal change in the electrical resistivity is found to be accompanied by anomalies in the lattice parameters and thermal expansion behavior; this transition is classified as first-order type. It has been found that despite the intercalation of Cr atoms some Ti-Se bonds in the Se-Ti-Se tri-layers of Cr x TiSe2 with x  ⩽  0.5 have nearly the same lengths as in the host lattice TiSe2, which apparently allows the transition to be returned to the CDW-like state.

Charge transport mechanism in intercalated CuxHfSe2 compounds

Alternating current resistivity measurements have been performed for the first time on intercalated CuxHfSe2 (0 ≤ x ≤ 0.18) samples using the impedance spectroscopy technique together with direct current measurements. The results obtained indicate the hopping mechanism of charge transport in CuxHfSe2 compounds. It has been found that an increase in the copper content in samples enhances relaxation processes. The ac conductivity exhibits frequency dispersion described by the universal dynamic response.

Crystal and magnetic structures of Cr1∕3NbSe2 from neutron diffraction

Neutron diffraction measurements of the Cr intercalated niobium diselenide Cr1∕3NbSe2 together with magnetization measurements have revealed that this compound exhibits ferromagnetic ordering below TC = 96 K unlike a chiral helimagnetic order observed in the sulfide compound Cr1∕3NbS2. As derived from neutron diffraction data, the Cr magnetic moments μCr = 2.83 ± 0.03 μB in Cr1∕3NbSe2 are aligned within basal plane. The discrepancy in the magnetic states of Cr1∕3NbS2 and Cr1∕3NbSe2 is ascribed to the difference in the preferential site occupation of Cr ions in crystal lattices. In Cr1∕3NbSe2, the Cr ions are predominantly distributed over 2b Wyckoff site, which determines a centrosymmetric character of the crystal structure unlike Cr1∕3NbS2, where the Cr ions are mainly located in 2c position and the crystal structure is non-centrosymmetric.

Layer-preferential substitutions and magnetic properties of pyrrhotite-type Fe7-yMyX8 chalcogenides (X = S, Se; M = Ti, Co).

A comparative study of four series of pyrrhotite-type chalcogenide compounds Fe(7-y)M(y)X(8) (X = S, Se) with substitution of Ti or Co for iron has been performed by means of x-ray and neutron powder diffraction, and by magnetization measurements. In Fe(7-y)M(y)X(8) compounds having a ferrimagnetic order at y = 0, the substitution of either Ti or Co for iron is observed to result in a monotonous decrease of the magnetic ordering temperature, while the resultant magnetization shows a non-monotonous behavior with a minimum around y = 1.0-1.5 in all the Fe(7-y)M(y)X(8) families except Fe(7-y)Co(y)Se(8). Suppression of a magnetically ordered state with substitutions in Fe(7-y)M(y)X(8) is ascribed to nearly zero values of Ti and Co magnetic moments, while the non-monotonous changes of the resultant magnetization are explained by the compensation of the sublattice magnetizations due to the non-random substitutions in alternating metallic layers. The difference in the cation partitioning observed in Fe(7-y)Ti(y)X(8) and Fe(7-y)Co(y)X(8) is attributed to the difference in the spatial extension of Ti and Co 3d orbitals. High coercive field values (20-24 kOe) observed at low temperatures in the Ti-containing compounds Fe(7-y)Ti(y)X(8) with y ⩾ 3 are suggested to result from the enhancement of Fe orbital moment due to the Ti for Fe substitution.

Order-disorder structural phase transition in the system of intercalated Ni atoms in the Ni0.5TiSe2 compound

Changes in the crystal structure of the Ni0.5TiSe2 intercalated compound have been thoroughly investigated using powder X-ray diffractometry in the temperature range from 93 to 700 K. It has been shown that upon heating to the temperature of about 400 K, this compound undergoes a phase transition from the monoclinic structure described by space group I12/m1 to the trigonal structure