Search

The influence of Co and Ni ion doping on the crystal structure, phase transitions, and magnetic properties of the Fe7Se8 system has been investigated. The compounds studied are (Fe0.987Ni0.013)7Se8, (Fe0.955Ni0.045)7Se8, (Fe0.915Ni0.085)7Se8, (Fe0.89Ni0.11)7Se8, (Fe0.79Ni0.21)7Se8, (Fe0.975Co0.025)7Se8, (Fe0.951Co0.049)7Se8, and (Fe0.91Co0.09)7Se8 as well as the parent compound Fe7Se8. The crystals are grown using a modified Bridgman method and exist as a hexagonal NiAs-like structure with an ordered distribution of Fe vacancies that determine their magnetic properties. The substitution effect, inducing a systematic decrease in the unit-cell volume due to the ionic radii of doping ions being smaller than the radii of Fe2+ ions, is a source of chemical pressure, acting on the magnetic subsystem and determining the magnetic ordering temperature TC and the spin-reorientation temperature (TSRT). As the most important result of this paper, it was shown that hydrostatic and chemical pressures are strongly correlated in the investigated system. The character of magnetic phase transitions has been designated in the vicinity of TC and TSRT temperatures, and the magnetic entropy change ΔSm and refrigeration potential magnitudes have been determined for this temperature range. These magnetocaloric parameters were shown to depend strongly on chemical pressure. A phenomenological model developed based on the experimental measurements demonstrated that the magnetic properties of the system depend strongly on the crystal field acting on Fe2+ ions and the ratio of the numbers of Fe2+ and Fe3+ ions. [ABSTRACT FROM AUTHOR]

The magnetocaloric effect has been studied in high quality single crystals of Fe7Se8 (3c type) grown by using Bridgman's method. Magnetization and magnetocaloric effect measurements have been carried out in a magnetic field up to 5 T over the temperature range from 2 to 490 K. The spin reorientation transition from the easy c-axis to the easy c-plane, proceeding in an abrupt fashion, as a first-order phase transition, has been observed near the temperature TR ≈ 125 K. The magnetization curves in the vicinity of this transition were shown to have an S-shape with a clear hysteresis. The first order metamagnetic field induced transitions have been identified above and below TR. The conventional magnetocaloric effect related to the metamagnetic transitions has been found above TR, while below TR the inverse magnetocaloric effect was clearly seen. The existence of both kinds of magnetocaloric effect is important from the point of view of large rotating field entropy change in Fe7Se8 single crystals. The refrigeration capacity associated with a second order phase transition from the ferrimagnetic to the paramagnetic state at the Néel temperature TN ≈ 450 K was found to be weaker than that appearing near TR. The giant anisotropy of the magnetocaloric effect was related to the magnetic anisotropy of Fe7Se8 crystals. The one-ion model of the magnetocaloric effect has been developed and its predictions have been compared with experimental data. [ABSTRACT FROM AUTHOR]

Temperature dependence of magnetic susceptibility of the ErAl3(BO3)4 single crystal was measured in the temperature range of T = 2−15 K at the external magnetic field of H = 100 Oe applied along the a and c crystal-lographic axes. The influence of spin-spin correlations on the magnetic susceptibility has been identified. It was estimated that the value of Curie-Weiss temperature is θAF ≈ −0.45 K. [ABSTRACT FROM AUTHOR]

New data on the state of Tb3+ doping ions in the EuAl3(BO3)4 single crystals have been obtained by studying the electron paramagnetic resonance spectra within the X band and optical absorption spectra at 4 K in the visible range. Additionally, the specific heat of these crystals was measured and analyzed. It has been shown that the Tb3+ ions substitute the trivalent europium ones and lower crystal symmetry. In the crystal field, the ground multiplet of Tb3+ splits in such a way that two singlets located in close proximity are the lowest energy levels. It has been shown that the temperature dependence of the linewidth can be described by the exponential function, which is characteristic of the Aminov-Orbach processes. [ABSTRACT FROM AUTHOR]

A substantial magnetic contribution into specific heat capacity is found in monocrystalline ErAl3(BO3)4 at temperatures below 5 K. The results of measured ESR spectra at 4,2 K confirm manifestation of short-range magnetic order as well. The obtained data is discussed invoking the model accounting for spin-spin interaction between rare-earth ions in paired clusters. [ABSTRACT FROM AUTHOR]

A rotational magnetocaloric effect is predicted in the TbAl3(BO3)4 crystal based on the results of measuring field dependences of magnetization and the temperature dependence of heat capacity. The effect is modeled using the quasi-doublet approximation. Changes to the crystal entropy at constant temperature are determined, as well as at varying temperature under adiabatic conditions. The refrigerant capacity of the crystal is estimated. It is shown that terbium aluminum borate is a promising material for magnetic cooling. [ABSTRACT FROM AUTHOR]

The magnetic investigations as a function of the temperature and magnetic field for the rubidium neodymium double tungstate RbNd(WO4)2 single crystal have been performed. The magnetization was measured in the temperature range from 4.2 to 100 K and for the magnetic field up to 1.5 T. The crystal field and exchange parameters were found. [ABSTRACT FROM AUTHOR]

Magnetic ordering temperature, initial splitting and effective g-factor of the ground quasi-doublet of a Tb3+ ion were determined by investigating the heat capacity and ESR in a TbAl3(BO3)4 single crystal. The parameters of the magnetic interaction were calculated. [ABSTRACT FROM AUTHOR]

We studied the temperature dependence of heat capacity CP(T) of a quasi-one-dimensional magnetic system β-TeVO4, in a zero magnetic field and a temperature region of 0.1K ≤ T ≤ 300 K. The CP(T) dependence shows phase transitions at temperatures of 4.65, 3.28, and 2.32 K. Is it established that at a temperature of TN = 4.65K there is a phase transition from a paramagnetic to an antiferromagnetic state. We provide a theoretical description of the heat capacity’s temperature dependence. [ABSTRACT FROM AUTHOR]

A substantial magnetic contribution into specific heat capacity is found in monocrystalline ErAl3(BO3)4 at temperatures below 5 K. The results of measured ESR spectra at 4,2 K confirm manifestation of short-range magnetic order as well. The obtained data is discussed invoking the model accounting for spin-spin interaction between rare-earth ions in paired clusters. [ABSTRACT FROM AUTHOR]

A rotational magnetocaloric effect is predicted in the TbAl3(BO3)4 crystal based on the results of measuring field dependences of magnetization and the temperature dependence of heat capacity. The effect is modeled using the quasi-doublet approximation. Changes to the crystal entropy at constant temperature are determined, as well as at varying temperature under adiabatic conditions. The refrigerant capacity of the crystal is estimated. It is shown that terbium aluminum borate is a promising material for magnetic cooling. [ABSTRACT FROM AUTHOR]

Magnetic ordering temperature, initial splitting and effective g-factor of the ground quasi-doublet of a Tb3+ ion were determined by investigating the heat capacity and ESR in a TbAl3(BO3)4 single crystal. The parameters of the magnetic interaction were calculated. [ABSTRACT FROM AUTHOR]

We studied the temperature dependence of heat capacity CP(T) of a quasi-one-dimensional magnetic system β-TeVO4, in a zero magnetic field and a temperature region of 0.1K ≤ T ≤ 300 K. The CP(T) dependence shows phase transitions at temperatures of 4.65, 3.28, and 2.32 K. Is it established that at a temperature of TN = 4.65K there is a phase transition from a paramagnetic to an antiferromagnetic state. We provide a theoretical description of the heat capacity’s temperature dependence. [ABSTRACT FROM AUTHOR]

A series of FeSe 0.5 Te 0.5 bulk samples have been prepared using the high gas pressure and high-temperature synthesis (HP-HTS) method to optimize the growth conditions for the first time and investigated for their superconducting properties using structural, microstructure, transport, and magnetic measurements to reach the final conclusions. Ex situ and in situ processes are used to prepare bulk samples under a range of growth pressures using Ta-tube and without Ta-tube. The parent compound synthesized by convenient synthesis method at ambient pressure (CSP) exhibits a superconducting transition temperature of 14.8 K. Our data demonstrate that the prepared FeSe 0.5 Te 0.5 sealed in a Ta-tube is of better quality than the samples without a Ta-tube, and the optimum growth conditions (500 MPa, 600 °C for 1 h) are favorable for the development of the tetragonal FeSe 0.5 Te 0.5 phase. The optimum bulk FeSe 0.5 Te 0.5 depicts a higher transition temperature of 17.3 K and a high critical current density of the order of >10 4 A/cm 2 at 0 T, which is improved over the entire magnetic field range and almost twice higher than the parent compound prepared using CSP. Our studies confirm that the high-pressure synthesis method is a highly efficient way to improve the superconducting transition, grain connectivity, sample density, and pinning properties of a superconductor.

The enhanced performance of superconducting FeSe 0.5 Te 0.5 materials with added micro-sized Pb and Sn particles is presented. A series of Pb- and Sn-added FeSe 0.5 Te 0.5 (FeSe 0.5 Te 0.5 + x Pb + y Sn; x = y = 0-0.1) bulks are fabricated by the solid-state reaction method and characterized through various measurements. A very small amount of Sn and Pb additions ( x = y ≤ 0.02) enhance the transition temperature ( T c onset ) of pure FeSe 0.5 Te 0.5 by ~1 K, sharpening the superconducting transition and improving the metallic nature in the normal state, whereas larger metal additions ( x = y ≥ 0.03) reduce T c onset by broadening the superconducting transition. Microstructural analysis and transport studies suggest that at x = y > 0.02, Pb and Sn additions enhance the impurity phases, reduce the coupling between grains, and suppress the superconducting percolation, leading to a broad transition. FeSe 0.5 Te 0.5 samples with 2 wt% of cometal additions show the best performance with their critical current density, J c , and the pinning force, F p , which might be attributable to providing effective flux pinning centres. Our study shows that the inclusion of a relatively small amount of Pb and Sn ( x = y ≤ 0.02) works effectively for the enhancement of superconducting properties with an improvement of intergrain connections as well as better phase uniformity.