ABSTRACT
CuCr2Se4 nanoparticles were obtained by the high-energy ball milling of CuCr2Se4 single crystals, which had a size of approximately 32 nm after 5 h of milling. Structural, magnetic, and electrical studies have shown that a reduction in CuCr2Se4 single crystals to the nanosize leads to (1) a weakening of ferromagnetic interactions, both long and short range, (2) a lack of saturation of magnetization at 5 K and 70 kOe, (3) a change in the nature of electrical conductivity from metallic to semiconductor, and (4) a reduction in the thermoelectric power factor S2σ by an order of magnitude of 400 K. The above results were considered in terms of the parameters of the band model, derived from the high-temperature expansion of magnetic susceptibility and from the diffusive component of thermoelectric power. Theoretical calculations showed a significant weakening of both the superexchange and double exchange mechanisms, a reduction in the [Cr3+,Cr4+] band width from 0.76 to 0.19 eV, and comparable values of the Fermi energy and the activation energy (0.46 eV) in the intrinsic region of electrical conductivity. The main advantage of high-energy ball milling is the ability to modify the physicochemical properties of already existing compounds for desired applications.
ABSTRACT
Single crystals of Ca1-3x-yMnyâ¡xNd2x(MoO4)1-3x(WO4)3x molybdato-tungstates (â¡ denotes vacant sites) with a scheelite-type structure have been successfully grown by the Czochralski technique in an inert atmosphere. This paper presents the results of structural, optical, magnetic, and electrical properties, as well as the broadband dielectric spectroscopy measurements of single crystals with different Nd3+ ion concentrations, i.e., when x = 0.0050 or x = 0.0098, and with constant content of Mn2+ ions, i.e., when y = 0.0050. Our magnetic studies have shown that substitution of diamagnetic Ca2+ ions in the CaMoO4 matrix with paramagnetic Nd3+ ones with a content not exceeding 0.02 and having a screened 4f-shell revealed a significant effect of orbital diamagnetism and Van Vleck's paramagnetism. Both single crystals have revealed residual electrical conductivity without an intrinsic region and a change of sign of the Seebeck coefficient at ca. 230 K. Dielectric spectroscopy measurements have shown constant values of relative permittivity (εr â¼ 8) and loss tangent (tan δ â¼ 0.01) both up to 400 K and up to 1 MHz, as well as the Fermi energy (â¼0.04 eV) and the Fermi temperature (â¼500 K) determined for both crystals from the diffusion component of thermopower. These results suggest the presence of shallow acceptor and donor levels in the studied crystals.
ABSTRACT
Microcrystalline samples of CoRE2W2O10 tungstates (RE = Y, Dy, Ho, Er) were prepared by a high-temperature solid-state reaction and then sintered into a ceramic form for unique properties and potential applications. For this purpose, structural, microscopic, ultraviolet-visible (UV-vis), magnetic, electrical, and thermoelectric measurements were performed. These studies showed a monoclinic structure, paramagnetism, short-range antiferromagnetic interactions in all samples, long-range ferrimagnetic interactions only in CoY2W2O10, poor n-type conductivity of 6.7 × 10-7 S/m at room temperature, strong thermal activation (Ea1 = 0.7 eV) in the intrinsic region, a strong increase in the power factor (S2σ) above 300 K, a Fermi energy (EF) of 0.16 eV, and a Fermi temperature (TF) of 1800 K. The above studies suggest that anion vacancy levels, which act as doubly charged donors, and to a lesser extent, the mixed valence band of cobalt ions (Co2+, Co3+), which are located below the bottom of the conduction band and below the Fermi level, are responsible for electron transport.
ABSTRACT
This study aimed to obtain and investigate ZnCr2Se4 single crystals doped with rhenium. The single crystals were obtained by applying chemical vapour transport. An X-ray study confirmed the cubic (Fd3¯m) structure of the tested crystals. Thermal, magnetic, electrical, and specific heat measurements accurately determined the physicochemical characteristics, which revealed that the obtained single crystals are p-type semiconductors with antiferromagnetic order below the Néel temperature TN = 21.7 K. The Debye temperature had a value of 295 K. The substitution of Re-paramagnetic ions, possessing a screened 5d-shell, in place of Zn-diamagnetic ions, caused an increase in the activation energy, Fermi energy, and Fermi temperature compared to the pure ZnCr2Se4. The boost of the dc magnetic field induced a shift of TN towards lower temperatures and a spin fluctuation peak visible at Hdc = 40 and 50 kOe. The obtained single crystals are thermally stable up to 1100 °C.
ABSTRACT
Single crystals of Pb1-3xâ¯xNd2x(MoO4)1-3x(WO4)3x (PNMWO) with scheelite-type structure, where ⯠denotes cationic vacancies, have been successfully grown by the Czochralski method in air and under 1 MPa. This paper presents the results of structural, optical, magnetic and electrical, as well as the broadband dielectric spectroscopy measurements of PNMWO single crystals. Research has shown that replacing diamagnetic Pb2+ ions with paramagnetic Nd3+ ones, with a content not exceeding 0.01 and possessing a screened 4f-shell, revealed a significant effect of orbital diamagnetism and Van Vleck's paramagnetism, n-type electrical conductivity with an activation energy of 0.7 eV in the intrinsic area, a strong increase of the power factor above room temperature for a crystal with x = 0.005, constant dielectric value (~30) and loss tangent (~0.01) up to room temperature. The Fermi energy (~0.04 eV) and the Fermi temperature (~500 K) determined from the diffusion component of thermopower showed shallow donor levels.
ABSTRACT
Monocrystalline chalcogenide spinels ZnCr2Se4 are antiferromagnetic and semiconductor materials. They can be used to dope or alloy with related semiconducting spinels. Therefore, their Pb-doped display is expected to have unique properties and new potential applications. This paper presents the results of dc and ac magnetic measurements, including the critical fields visible on the magnetisation isotherms, electrical conductivity, and specific heat of the ZnCr2S4:Pb single crystals. These studies showed that substituting the diamagnetic Pb ion with a large ion radius for the Zn one leads to strong short-range ferromagnetic interactions in the entire temperature range and spin fluctuations in the paramagnetic region at Hdc = 50 kOe.
ABSTRACT
A series of Co2+-doped and Gd3+-co-doped calcium molybdato-tungstates, i.e., Ca1-3x-yCoyxGd2x(MoO4)1-3x(WO4)3x (CCGMWO), where 0 < x ≤ 0.2, y = 0.02 and represents vacancy, were successfully synthesized by high-temperature solid-state reaction method. XRD studies and diffuse reflectance UV-vis spectral analysis confirmed the formation of single, tetragonal scheelite-type phases with space group I41/a and a direct optical band gap above 3.5 eV. Magnetic and electrical measurements showed insulating behavior with n-type residual electrical conductivity, an almost perfect paramagnetic state with weak short-range ferromagnetic interactions, as well as an increase of spin contribution to the magnetic moment and an increase in the power factor with increasing gadolinium ions in the sample. Broadband dielectric spectroscopy measurements and dielectric analysis in the frequency representation showed a relatively high value of dielectric permittivity at low frequencies, characteristic of a space charge polarization and small values of both permittivity and loss tangent at higher frequencies.
ABSTRACT
The new series of single-crystalline chromium selenides, Ta-doped ZnCr2Se4, was synthesised by a chemical vapour transport method to determine the impact of a dopant on the structural and thermodynamic properties of the parent compound. We present comprehensive investigations of structural, electrical transport, magnetic, and specific heat properties. It was expected that a partial replacement of Cr ions by a more significant Ta one would lead to a change in direct magnetic interactions between Cr magnetic moments and result in a change in the magnetic ground state and electric transport properties of the ZnCr2-xTaxSe4 (x = 0.05, 0.06, 0.07, 0.08, 0.1, 0.12) system. We found that all the elements of the cubic system had a cubic spinel structure; however, the doping gain linearly increased the ZnCr2-xTaxSe4 unit cell volume. Doping with tantalum did not significantly change the semiconductor and magnetic properties of ZnCr2Se4. For all studied samples (0 ≤ x ≤ 0.12), an antiferromagnetic order (AFM) below TN~22 K was observed. However, a small amount of Ta significantly reduced the second critical field (Hc2) from 65 kOe for x = 0.0 (ZnCr2Se4 matrix) up to 42.2 kOe for x = 0.12, above which the spin helical system changed to ferromagnetic (FM). The Hc2 reduction can lead to strong competition among AFM and FM interactions and spin frustration, as the specific heat under magnetic fields H < Hc2 shows a strong field decrease in TN.
ABSTRACT
This paper reports on the electrical and broadband dielectric spectroscopy studies of Zn2-xMgxInV3O11 materials (where x = 0.0, 0.4, 1.0, 1.6, 2.0) synthesized using a solid-state reaction method. These studies showed n-type semiconducting properties with activation energies of 0.147-0.52 eV in the temperature range of 250-400 K, symmetric and linear I-V characteristics, both at 300 and 400 K, with a stronger carrier emission for the matrix and much less for the remaining samples, as well as the dipole relaxation, which was the slowest for the sample with x = 0.0 (matrix) and was faster for Mg-doped samples with x > 0.0. The faster the dipole relaxation, the greater the accumulation of electric charge. These effects were analyzed within a framework of the DC conductivity and the Cole-Cole fit function, including the solid-state density and porosity of the sample. The resistivity vs. temperature dependence was well fitted using the parallel resistor model. Our ab initio calculations also show that the bandgap increased with the Mg content.
ABSTRACT
Structural, electrical, magnetic, and specific heat measurements were carried out on ZnCr2Se4 single crystal and on nanocrystals obtained from the milling of this single crystal after 1, 3, and 5 h, whose crystallite sizes were 25.2, 2.5, and 2 nm, respectively. For this purpose, the high-energy ball-milling method was used. The above studies showed that all samples have a spinel structure, and are p-type semiconductors with less milling time and n-type with a higher one. In turn, the decrease in crystallite size caused a change in the magnetic order, from antiferromagnetic for bulk material and nanocrystals after 1 and 3 h of milling to spin-glass with the freezing temperature Tf = 20 K for the sample after 5 h of milling. The spin-glass behavior for this sample was derived from a broad peak of dc magnetic susceptibility, a splitting of the zero-field-cooling and field-cooling susceptibilities, and from the shift of Tf towards the higher frequency of the ac susceptibility curves. A spectacular result for this sample is also the lack of a peak on the specific heat curve, suggesting a disappearance of the structural transition that is observed for the bulk single crystal.
