Chalcogenide bulk spinels with general formula CuCr 2 X 4 (X = S, Se, and Te) are ferromagnetic and p-type metallic materials with the thermoelectric figure of merit of 0.15. They can be used to dope or alloy with related semiconducting spinels. Therefore, it is expected that their nanosized crystallites display also unique properties and new potential applications. This paper presents the results of dc and ac magnetic measurements, including the higher harmonics of ac magnetic susceptibility as well as electrical conductivity and thermoelectric power of the CuCr2S4 nanospinels. These studies showed that decreasing the size of crystallites to nanometer scale leads to a dramatic change in their physical properties.

Chalcogenide spinels show a variety of physical properties and are very good candidates for electronic and high-frequency applications. We report the measurements of magnetic susceptibility, magnetic isotherm, electrical conductivity, thermoelectric power and calculations of the superexchange and double-exchange integrals made for singlecrystalline Cu[CrxHfy]Se4 spinels. The results showed a ferromagnetic order of magnetic moments below the Curie temperatures of 390K and, an increase in the splitting of the zero-field cooled and field cooled susceptibilities with increasing Hf-content below the room temperature suggesting a slight spin-frustration and a rapid transition from semiconducting to metallic state at room temperature. A quantitative evaluation of the exchange Hamiltonian showed that the total hopping integral rapidly decreased and the bandwidth of the 3d t2g band due to Cr3+ and Cr4+ ions strongly narrowed from 0.76eV for y = 0 to 0.28eV for y = 0.14. The narrowing of this band appears to be responsible for semiconducting properties of the Hf-doped CuCr2Se4 spinels below the room temperature.

The crystal structure, magnetic isotherm, magnetic susceptibility, electrical conductivity and specific heat measurements for single-crystalline ZnxDyyCrzSe4 (where x+y+z≈3) spinels are presented. A semiconducting behavior with the activation energy of 0.53eV, an antiferromagnetic order with a Néel temperature TN=22K and a strong ferromagnetic exchange evidenced by a positive Curie–Weiss temperature θ=79, 71 and 70K with increasing Dy-content in the sequence 0.05, 0.13 and 0.19 were established. Below TN the magnetic field dependence of magnetization, M(H), shows two peaks at critical fields Hc1 and Hc2. The values of Hc1 decrease slightly with temperature, especially for the larger Dy-content, while the values of Hc2 drop rapidly with temperature. The magnetic contribution to the specific heat displays a sharp peak at TN, which is strongly shifted to much lower temperatures in the applied magnetic fields. Similar behavior was found for the temperature dependence of the specific heat C(T) plotted as C(T)/T vs. T. The value of the magnetic and phonon contribution to the entropy at TN and at H=0 is only ∼4.8, ∼4.4 and ∼4.2Jmol−1K−1/Cr3+ for y=0.05, 0.13 and 0.19, respectively, much lower than the average magnetic contribution Sm=(z/2)Rln(2S+1)=12.33Jmol−1K−1/Cr3+ calculated for Cr3+ ion with S=3/2, as the dysprosium one is paramagnetic.

CuCr2Te4 can be obtain by mechanical alloying followed by heat treatment. The obtained phase crystallizes in the spinel-type structure of the space group Fd3m. The calculated crystallite size equals to 100 nm. Magnetic susceptibility measurements showed ferrimagnetic order below 21 K.

•The new single crystals ZnCr2Se4 doped by tin were successfully synthesized.•ZFC and FC splitting for all investigated crystals was observed.•Spin-glass-like behaviour for a large Sn content was discovered.

The series of chromium selenides Gd-doped ZnCr2Se4 (from 0.07 Gd to 0.21 Gd) was synthesized in polycrystalline form using ceramic method from stoichiometric amounts of elements. The phase and structure determination by X-ray diffraction showed the main cubic normal spinel structure and traces amount of phases ZnSe and the selenium. Magnetic measurements showed an antiferromagnetic order for all compositions with the Neel temperature T-N similar to 22 K, a change of slope at the first critical field lid of about 11.5 kOe for T = 2 K characteristic for a metamagnetic transition, a breakdown of the helical spin arrangement at the second critical field Ha of about 60 kOe for T = 2 K, and splitting of the zero-field-cooling and field-cooling susceptibilities below the freezing temperature T-f = 7.3 K suggesting magnetic frustration. Specific heat measurements exhibited first-order anomalies at T-N. These effects are interpreted in terms of the superexchange integrals for the first two coordination spheres including structural defects and non-stoichiometry. (C) 2015 Elsevier B.V. All rights reserved.

CuCr1.65Se4 nanoparticles crystallize in the monoclinic Cr2Se3-type structure of the space group I2/m. The average crystallite size basing on the line broadening is less than 10 nm. With decrease of the size of grains a change from ferromagnetic to ferrimagnetic order, a lack of the magnetization saturation and a strong spin orbit coupling visible in the large value of the Lande factor gχ = 2.72 are observed. The change in magnetic order is caused by the change of the crystalline symmetry from the cubic phase to monoclinic one.

CuCr1.65Se4 nanopaxticles crystallize in the monoclinic Cr2Se3-type structure of the space group I2/m. The average crystallite size basing on the line broadening is less than 10 nm. With decrease of the size of grains a change from ferromagnetic to ferrimagnetic order, a lack of the magnetization saturation and a strong spin orbit coupling visible in the large value of the Lande factor g(x) = 2.72 are observed. The change in magnetic order is caused by the change of the crystalline symmetry from the cubic phase to monoclinic one.

The ZnCr2Se4:Dy-single crystals were prepared by chemical vapor transport in closed silica tubes using ZnSe and DySe with CrCl3 as the transport agent. Three crystals with different Dy-content were studied by XRD and magnetic measurements in order to determine the influence of dysprosium on their structural and magnetic properties. The results are inconclusive with respect to the location of Dy-ions admixed into ZnCr2Se4. Composition obtained by Scanning Electron Microprobe points towards tetrahedral sites of the spinel structure, which is not confirmed by bulk XRD. Magnetic measurements of the obtained single crystals corroborated the influence of dysprosium on magnetic properties, which boosted the antiferromagnetic component of the magnetism.

CuCr1.65Se4 nanoparticles crystallize in the monoclinic Cr2Se3-type structure of the space group I2/m. The average crystallite size basing on the line broadening is less than 10 nm. With decrease of the size of grains a change from ferromagnetic to ferrimagnetic order, a lack of the magnetization saturation and a strong spin orbit coupling visible in the large value of the Lande factor gχ = 2.72 are observed. The change in magnetic order is caused by the change of the crystalline symmetry from the cubic phase to monoclinic one.

Abstract. Phase and structure analysis of CuCr2-xGdxSe4 (x=0.1÷0.3) synthesized using a solid state reaction method were carried out using X-Ray powder diffraction. All obtained compounds were found to be multi phase with the spinel phase being dominant up to x=0.1, indicating low solubility limit of gadolinium. Two compounds CuCr1.95Gd0.05Se4 and CuCr1.9Gd0.1Se4,. were chosen for a detailed structure refinement.

Polycrystalline compounds in the Zn1-xNdxCr2Se4 system were prepared by solid state reaction using selenides (ZnSe, Cr2Se3) and pure elements (Nd, Se) as starting materials. The structural properties were determined by X-ray diffraction and the chemical composition confirmed by SEM-EDX. The observed symmetry is cubic, space group Fd3m, while the lattice parameter varies from 10.4955(7)Å to 10.4976(7)Å, and is larger than for the pure matrix. The solubility limit for the current synthesis route lies below x = 0.1. The magnetic moments, effective and saturation, increase with increasing amount of Nd ions. The Neel temperature TN and ΘCW drop, respectively, to 17.4K and 81K for x = 0.1, independently indicating that neodymium is incorporated into the spinel lattice and promotes antiferromagnetic coupling between the Cr3+ ions.

The polycrystalline compounds in the ZnCr2-xNdxSe4 system were prepared by the ceramic method in closed silica tubes using ZnSe, Nd2Se3, and Cr2Se3 as starting materials. The chemical composition and structural data has been determined by X-ray diffraction. The observed symmetry is cubic, space group Fd (3) over barm. For ZnCr1.95Nd0.05Se4 compounds, a pure spinel phase forms for stoichiometric and smaller amounts of ZnSe. For both compounds of ZnCr1.90Nd0.10Se4, near spinel phase, a foreign ZnSe phase exists. It may indicate that 10% doping is close to the solubility limit of Nd in ZnCr2Se4.

Magnetization, ac and dc magnetic susceptibility measured in the zero-field-cooled mode were used to study the high spin-low spin transitions in polycrystalline Cu0.2Co0.76Cr1.83Se4 semiconductor. The real part component of fundamental susceptibility chi'(1)(T) and its second (chi(2)) and third (chi(3)) harmonics revealed two spectacular peaks at 128 K and at 147 K, confirming the appearance of the spin-crossover phenomenon.

Magnetization and magnetic susceptibility measured in the zero-field-cooled mode were used to study the spin crossover transition in polycrystalline CuxCoyCrzSe4 compounds. With increasing Co content a transition from ferromagnetic order via ferrimagnetic one to antiferromagnetic-like behaviour was observed. This transition is accompanied with a lowering symmetry from cubic to monoclinic and for the latter the spin crossover phenomenon occurs. These results are considered in a framework of the ligand-field split and the spin-orbit coupling.

A high spin (HS) – low spin (LS) transition has been discovered in the Co[Cr0.5Ga1.5]S4 spinel, where the Co ions occupy tetrahedral sites and the Cr and Ga ions octahedral ones. The latter are diluted by the non-magnetic Ga-ions. Application of magnetic fields revealed ferrimagnetic order with a Curie temperature TC = 126 K, a strong suppression of the magnetic susceptibility () and a slight shift of TC to lower temperatures. A cusp at 15.6 K on the ac (T) curve suggests a spin frustration of the re-entrant type. Characteristic for the HS-LS transition is an inflection point at 150 K on the 1/(T) curve, a lack of the Curie-Weiss region above TC and a small value of the magnetization, 4.76 emu/g at 4.4 K and at a magnetic field of 57.5 kOe.

Single phase materials with general formula CdxMeyCr2Se4 (where Me = Mn, Sn) were obtained using solid state synthesis method. Both compounds are ferromagnets with TC =115K (Sn) and 135K (Mn). The values of Curie-Weiss parameter ΘC-W = 135K (Sn) and 145K (Mn) are higher than respective values of TC indicating a presence of competing antiferromagnetic component. The values of lattice parameters are consistent with Sn and Mn replacing Cd. Slight cadmium deficiency has been also observed.

We have observed that doping CuCr2Se4 with tin introduces and then increases the tetrahedral deformation of the spinel structure, which can be connected with the Jahn-Teller (JT) distortion due to Cr2+ ions in antiprismatic environment. The presence of Cr2+ is corroborated by the increase of the saturation magnetization above the 6μB value expected for the 2 Cr3+ ions. In the high limit of the high Sn doping the deformation is decreased, which we attribute to elimination of the JT condition by the increased local distortions. With the tin doping the magnetic structure evolves from the ferromagnet, through spin-glass like to the antiferromagnetic due to increased AF coupling in the system. This is corroborated by the decreasing transition temperature and the Curie-Weiss parameter.

Single phase materials with general formula CdxMeyCr2Se4 (where Me = Mn, Sn) were obtained using solid state synthesis method. Both compounds are ferromagnets with TC =115K (Sn) and 135K (Mn). The values of Curie-Weiss parameter ΘC-W = 135K (Sn) and 145K (Mn) are higher than respective values of TC indicating a presence of competing antiferromagnetic component. The values of lattice parameters are consistent with Sn and Mn replacing Cd. Slight cadmium deficiency has been also observed.

The polycrystalline Co0.83Fe1.8Se4 compound was obtained using ceramic method. The chemical composition was verified using energy-dispersive X-ray fluorescence spectrometry (EDXRF) and EDS method. X-ray analysis was used to make phase and structure analysis. The Rietveld method was applied for structure refinement. Co0.83Fe1.8Se4 compound crystallized in monoclinic system, space group C . The magnetic susceptibility and magnetization were measured, they revealed ferrimagnetic properties of a sample.