Metamagnetism in the Cr2V4-xMoxO13+0.5x solid solutions

Magnetic and Electrical Characteristics of Nd3+ and Mn2+-Co-doped Calcium Molybdato-Tungstates Single Crystals

Single crystals of Ca1-3x-yMny□xNd2x(MoO4)1-3x(WO4)3x molybdato-tungstates (□ 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 ∼ 8) and loss tangent (tan δ ∼ 0.01) both up to 400 K and up to 1 MHz, as well as the Fermi energy (∼0.04 eV) and the Fermi temperature (∼500 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.

Magnetic and Electrical Properties of CoRE2W2O10 Ceramic Materials

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.

Magnetic and Electrical Characteristics of Nd3+-Doped Lead Molybdato-Tungstate Single Crystals

Single crystals of Pb_1-3x▯_xNd_2x(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.

Effect of Gd3+ Substitution on Thermoelectric Power Factor of Paramagnetic Co2+-Doped Calcium Molybdato-Tungstates

A series of Co²⁺-doped and Gd³⁺-co-doped calcium molybdato-tungstates, i.e., Ca_1−3x−yCo_yxGd_2x(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.

Study of the Structure, Magnetic, Thermal and Electrical Characterisation of ZnCr2Se4: Ta Single Crystals Obtained by Chemical Vapour Transport

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 ZnCr_2-xTa_xSe₄ (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 ZnCr_2-xTa_xSe₄ 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 T_N~22 K was observed. However, a small amount of Ta significantly reduced the second critical field (H_c2) 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 H_c2 reduction can lead to strong competition among AFM and FM interactions and spin frustration, as the specific heat under magnetic fields H < H_c2 shows a strong field decrease in T_N.

Influence of Crystallite Size on the Magnetic Order in Semiconducting ZnCr2Se4 Nanoparticles

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 T_f = 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 T_f 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.

Dielectric and magnetic characteristics of Ca1-x Mn x MoO4 (0 ≤ x ≤ 0.15) nanomaterials

Scheelite-type Ca_1-x Mn _x MoO₄ (x = 0.0, 0.01, 0.05, 0.10 and 0.15) nanomaterials were successfully synthesized via a combustion route. Dielectric studies showed a weak n-type electrical conductivity characteristic for insulators and low relative permittivity (ε _r < 15) decreasing with increasing Mn²⁺ content. CaMoO₄ and Mn²⁺-doped nanomaterials are chemically compatible with Al and Ag electrodes and promising for low-temperature co-fired ceramic applications. Magnetic studies showed, at room-temperature diamagnetism for pure CaMoO₄, the balance between diamagnetism and paramagnetism for Ca_1-x Mn _x MoO₄ (x = 0.01) and paramagnetic behaviour when 0.05 ≤ x ≤ 0.15 as well as the short-range antiferromagnetic interactions growing in strength as Mn²⁺ content increases. The Landé factor fitting procedure showed a spin-only contribution to the magnetic moment. CaMoO₄ matrix unexpectedly revealed the residual paramagnetism at low temperatures derived probably from the molybdenum ions having unpaired 4d electrons as well as a paramagnetic-diamagnetic transition at 70 K.