DEPARTMENT OF MAGNETIC AND ELASTIC PROPERTIES OF SOLIDS

Magnetic properties of solids at low temperatures

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Staff of Research Group

Alexey V. Fedorchenko fedoreghataruslpostchenko@ileghataruslpostt.khareghataruslpostkov.ua Scopus ResearcherID GScholar ORCID	Head of Department, Head of Group, Dr.rer.nat., PhD.
Nadezhda V. Ushakova useghataruslposthakova@ileghataruslpostt.khareghataruslpostkov.ua	Engineer

Main directions of studies

• Investigation of magnetic properties of the layered double hydroxides intercalated with different anions. This activity supported by the consortium "EMP - European Microkelvin Platform" which is funded under the European Union’s Horizon 2020 programme Integrating and opening existing national and regional research infrastructures of European interest, grant agreement No. 824109.
• Investigation of magnetic properties of the lead-free multiferroic perovskites and layered double hydroxides. This activity supported by project TUMOCS - TUneable Multiferroics based on oxygen OCtahedral Structures. This project has received funding from European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 645660.
• Experimental studies of the magnetic properties of anisotropic transition metal compounds with metalloids, such as iron-based high-temperature superconductors, transition metal diborides, and itinerant antiferromagnets based on d-elements.
• Investigation of magnetic properties and effects related to phase transitions in solids.
• Experimental studies of magnetic properties of carbon nanotubes modified with 3d metals (iron and cobalt).
• Investigation of structural defects with the magnetic methods.

Main results

• Magnetic properties of new multiferroic BiFe_0.5Sc_0.5O₃ perovskite was studied in magnetic fields up to 50 kOe in the temperature range of 5–300 K. The compound shows a weak ferromagnetic component below the Néel temperature, T_N∼220 K. A wide cusp-like anomaly of the temperature dependent magnetic susceptibility was revealed below 200 K. The anomaly is dependent on the magnetic field applied and can be associated with the glassy magnetic behavior of the compound.
• Magnetic properties of polycrystalline multiferroic Bi_0.65La_0.35Fe_0.5Sc_0.5O₃ were studied over the temperature range of 5−300 K in magnetic fields up to H=50 kOe. Derivative of the initial magnetization curves demonstrates a temperature-dependent anomaly in fields of H=15−25 kOe. Also, an anomaly of the temperature dependent magnetization measured in ZFC regime in magnetic fields of 6−7 kOe has been found. Origin of both anomalies is associated with inhomogeneous magnetic state of the compound.
• Magnetic behavior of Co^II(n)Al^III layered double hydroxides (LDHs) (n = Co/Al = 2 and 3) intercalated with nitrate was studied as a function of temperature. Co(2)Al–NO₃ and Co(3)Al–NO₃ demonstrate anomalous behavior of their temperature dependence magnetic susceptibility in a higher temperature range: between 75 and 175 K, both the paramagnetic Curie temperature and the effective magnetic moment change in a non-monotonous way.
• Exchange bias phenomenon, evident of antiferromagnetic–ferromagnetic phase segregation state, has been observed in (Nd_1-xY_x)_2/3Ca_1/3MnO₃ (x = 0, 0.1) compounds at low temperatures. A contribution to the total magnetization of the compounds due to the ferromagnetic phase has been evaluated. It has been found that yttrium doping leads to the growth of the ferromagnetic phase fraction.
• The magnetization of FeSe_1-xTe_x (x = 0, 0.5, and 1.0) compounds has been studied in magnetic fields up to 50 kOe and at temperatures of 2 - 300 K. The superconducting transition was observed at T_c ~ 8 K and 13.6 - 14.2 K in FeSe_0.963 and FeSe_0.5Te_0.5, respectively. The intrinsic magnetic susceptibility of FeSe_0.963, FeSe_0.5Te_0.5, and FeTe was estimated to increase gradually with Te content. For FeTe a drastic drop in χ(T) with decreasing temperature was found at T_N ~ 70 K, which is presumably related to antiferromagnetic ordering.
• A detailed magnetization study for the novel FeSe superconductor is carried out to investigate the behavior of the intrinsic magnetic susceptibility χ in the normal state with temperature and under hydrostatic pressure. The temperature dependences of χ and its anisotropy are measured for FeSe single crystals in the temperature range 4.2-300 K, and a substantial growth of susceptibility with temperature is revealed.
• The magnetic susceptibility of the compound LaFeAsO_0.85F_0.1 was investigated in the temperature range 4.2 - 300 K, and the superconducting transition was detected at 26 K. A clear cusp at T_M ~ 135 K was observed in the low-field dependence of the magnetic susceptibility χ(T) with a sharp rise of χ with further cooling to TC ~ 26 K. The obtained magnetization data M(H) indicate the emergence of an FM phase at T≤T_M.
• The temperature dependencies of the magnetic susceptibility χ and its anisotropy were measured for the hexagonal single crystalline TB₂ compounds (T = Sc, Ti, Zr, Hf, V and Cr) in the temperature range 4.2-300 K. It was found that χ varies strongly and nonmonotonously with the filling of the hybridized d-band, being almost temperature independent and the largest for the diborides of group 4 metals.
• Magnetic properties of multi-walled carbon nanotubes modified with iron MWCNT+Fe are studied in detail in the temperature range 4.2-300 K. The ZFC and FC temperature dependences of the magnetic susceptibility reveal a number of magnetic phase transitions in this Fe-MWCNT system, including a Verwey transition in Fe₃O₄ particles which are presumably outside the MWCNT. It has been shown that both the residual magnetization M_R and the coercive field H_c decrease monotonically with increasing temperature.
• The magnetic properties of multi-walled carbon nanotubes (MWCNTs) modified with cobalt nanoparticles were studied in the temperatures and magnetic field range of (4.2-290) K and (0.3–50) kOe, respectively. Magnetization studies revealed two principal groups of Co nanoclusters in the synthesized MWCNT/Co sample. The larger particles have a size from a few to about 10 nm. They are ferromagnetic, at least up to 300 K, with the coercive field of about 600 Oe. The smaller Co particles have the atomic-scale size. They are responsible for a pronounced paramagnetic behavior observed at low temperatures. The magnetic characterization provided the remanent magnetization value (M_R) of about 0.07 emu/g (40% of the saturation moment), while the coercive field (H_c) value amounts to 600 Oe. Both parameters M_R and H_c slightly decrease with the rise of temperature. The substantial magnetization increase observed at low temperatures suggests the existence of nano Co clusters (in the atomic scale size).
• In the region of the equilibrium solid solutions of lithium in aluminum an anomalous low-temperature peak of diamagnetism is observed which is due to the presence of a boundary point on a line of band degeneracy directly below the Fermi level.

Experimental equipment

• SQUID-magnetometer (self-made) for measuring magnetic susceptibility and magnetization of solids in magnetic fields up to 50 kOe at temperatures of 4.2 ÷ 320 K with the possibility of studying the influence of uniaxial compression.
• A magnetometer for measuring the ac-susceptibility in weak magnetic fields at temperatures
  of 78 ÷ 350 K in the frequency range 1 ÷ 200 kHz is being developed.