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1
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Superconductivity of Niobium Thin Films in the BiOCl/Nb Heterostructures

100%
Lotnyk D., Komanický V., Bunda V., Feher A.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
1030-1032
EN
In the current paper, electrical transport properties of 25 nm thick Nb films sputtered on the photosensitive semiconductor BiOCl were investigated in the temperature range 7.5 K ≤ T ≤8.5 K. The influence of green (532 nm) and red (640 nm) laser excitations on resistive superconducting transitions of the niobium thin films on a silicon glass and BiOCl single crystal substrates were studied. The temperature dependences of the resistivity for Nb are in good agreement with the McMillan model which indicates the strong influence of the inverse proximity effect induced by the interface. The increased influence of the BiOCl/Nb interface under laser excitation corresponds to the raising of the ratio of the density of normal to superconductivity carriers in the Tıghtarrow0 limit and this observation is in agreement with the photoconductivity study of BiOCl single crystals.
2
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Magnetocaloric Effect in NiCl₂(bipy) at Low Temperatures

100%
Ráczová K., Čižmár E., Feher A.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
922-924
EN
Magnetothermal properties of the compound NiCl₂(bipy), where bipy = 4,4'-bipyridine, were studied. The single-ion anisotropy of Ni(II) ions was estimated and its influence on magnetocaloric effect in NiCl₂(bipy) was investigated. Above the metamagnetic transition a maximum in the isothermal change of the magnetic entropy is reached near 11 K at field change from 0T to 7T with peak value -ΔS = 6J K¯¹ kg¯¹. The temperature dependence of ΔS above 7K agrees with the assumed easy-axis type of single-ion anisotropy. The inverse magnetocaloric effect, typical for ordered antiferromagnets, was observed in the ordered state of NiCl₂(bipy) only for isothermal magnetization in small fields up to 2T.
3
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EPR Spectra of the Genuine-Organic and Metal-Organic TCNQ-Based Anion Radical Salts

100%
Botko M., Čižmár E., Kajňaková M., Feher A.
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vol. 126
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issue 1
252-253
EN
We have measured the electron paramagnetic resonance spectra of genuine-organic anion-radical salts [Me-2,6-di-Me-Py](TCNQ)_{2}, [Me-3,5-di-Me-Py](TCNQ)_{2}, [N-Me-OH-Me-Py](TCNQ)_{2}, [N-Et-OH-Me-Py](TCNQ)_{2}, and metal-organic anion-radical salt [Mn-phen_{3}](TCNQ)_{2}·H_{2}O in the temperature range from 2 K to 300 K in the X-band frequency range. A comparison study of the shape of spectra, g-factors and linewidths with respect to the composition and crystal structure of studied compounds is presented. While the genuine-organic anion-radical salts display a typical EPR response originating from TCNQ pairs, the spectra of metal-organic anion-radical salt are dominated by the contribution of transition metal.
4
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Magnetic Properties of Anion-Radical Salts Based on TCNQ Anion Radical

88%
Radváková A., Kajňaková M., Feher A., Ziolkovskiy D., Starodub V.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
499-502
EN
We report the experimental results of anion-radical salts based on TCNQ anion radical. DC magnetic susceptibility measurements in the temperature range from 2 K to 300 K and magnetization measurements were performed on two samples: (N-Me-2, 5-di-Me-Pz)(TCNQ)_2 and (N-Me-2, 6-di-Me-Pz)(TCNQ)_2 using a Quantum Design MPMS.
5
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Magnetic Specific Heat of Anion-Radical Salt [Ni(bipy)₃](TCNQ)₄·(CH₃)₂CO at Very Low Temperatures

88%
Šoltésová D., Čižmár E., Vasylets G., Starodub V., Feher A.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
925-927
EN
Thermodynamic studies of the anion-radical salt system [Ni(bipy)₃](TCNQ)₄·(CH₃)₂CO, where TCNQ is 7,7',8,8'-tetracyano-quinodimethane, are reported. The anion-radical salt systems based on TCNQ belong to a material class in which the arrangement of the anion-radical salt has considerable impact on the charge transfer and magnetic properties. The crystal structure of the studied compound consists of [Ni(bipy)₃]⁺² cations containing Ni⁺² ions and four types of crystallographically independent anion-radicals TCNQ^{·-} (A, B, C and D). These TCNQ^{·-} radicals form two different types of TCNQ^{·-} stacks (AABB and CCDD), where a strong exchange interaction is expected. The temperature dependence of the specific heat of a single crystal was studied in magnetic fields up to 5 T and in the temperature range from 0.4 K to 30 K. The temperature dependence of specific heat displays a broad Schottky-like maximum above 0.4 K. Using a single-ion approximation, the analysis of the temperature dependence of the specific heat below 10 K yields values for the anisotropy parameters, D/k_{B}=-1.95 K and E/k_{B}=0.3 K. These results suggest that the observed maximum in the specific heat originates from Ni⁺² ions while the exchange interaction between the transition metal ions and the TCNQ is negligible.
6
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Experimental Study of Magnetic Properties of Alumina Ceramics

88%
Tarasenko R., Orendáčová A., Orendáč M., Kajňaková M., Feher A.
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issue 5
1067-1068
EN
Magnetic properties of alumina ceramics were studied within a program focused at design of microcalorimeters and experimental setup for specific heat and magnetocaloric measurements. The investigations were concentrated on the determination of amount of magnetic impurities that can significantly influence results of physical measurements at low temperatures. Our experimental studies of heat capacity, susceptibility and magnetization clearly indicate that alumina ceramics contains magnetic impurities, primarily ions with spin 5/2. The average concentration of magnetic ions was estimated, n ≈ 2000 ppm. More details of analysis of experimental data are discussed in the paper.
7
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Experimental Study of Magneto-Structural Correlations in Low-Dimensional Quantum Magnets Cu(en)Cl₂ and Cu(tn)Cl₂

88%
Tarasenko R., Lederová L., Orendáčová A., Orendáč M., Feher A.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
420-422
EN
The present work is focused on the investigation of magneto-structural correlations in Cu(en)Cl₂ and Cu(tn)Cl₂. A comparative study of powder susceptibility and magnetization of both compounds revealed that the replacement of tn by en ligand did not affect single-ion properties. On the other hand, the structure modification led to significant reduction of magnetic interactions as well as lowering the crystal symmetry. The impact of the high pressure on magnetic properties of the compounds is discussed.
8
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Experimental Study of Magnetocaloric Effect in Dipolar Magnet KEr(MoO_4)_2

88%
Hanko J., Orendáč M., Kajňaková M., Orendáčová A., Feher A.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
465-468
EN
We report on magnetothermodynamic properties of single crystal KEr(MoO_4)_2 which were investigated from nominally 0.4 K to 20 K in magnetic fields up to 4 T. Using the available specific heat data the diagram of temperature and magnetic field dependence of the total entropy was constructed. Predictions for temperature dependences of the relative temperature variations during the adiabatic demagnetization and the entropy variation during the isothermal demagnetization were calculated from the specific heat data. The obtained results suggest that unlike common refrigerants, in KEr(MoO_4)_2 the quantities describing magnetic cooling remain nearly constant in the temperature range 2-10 K.
9
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Magnetocaloric Study of S=1/2 Low-Dimensional Heisenberg Antiferromagnet [Cu(phen)(H_2O)_2]SO_4

88%
Sedláková L., Orendáč M., Kajňaková M., Orendáčová A., Feher A.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
473-476
EN
We present the experimental study of magnetocaloric effect on powder sample [Cu(phen)(H_2O)_2]SO_4 (phen = phenantroline = C_{12}H_8N_2) in the temperature range from nominally 0.2 K to 4 K and in magnetic fields up to 2 T. The values of total entropy were extracted from the experimental data obtained by adiabatic demagnetization. The comparison was performed with the temperature dependence of the entropy for S=1/2 paramagnet calculated at selected magnetic fields. The deviations observed below 0.6 T were attributed to internal field mediated by exchange coupling. In addition, the temperature response observed in [Cu(phen)(H_2O)_2]SO_4 revealed very short relaxation time below 1 K. The comparison of the behavior observed at the end of the demagnetization in [Cu(phen)(H_2O)_2]SO_4 and other systems suggests that the magnetocaloric effect can serve as an alternative tool for studying slow magnetic relaxation.
10
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Physical Properties of "HTSC/Photosemiconductor" Micro- and Nanoplates Based Junctions

88%
Bunda V., Bunda S., Vashchuk F., Feher A., Kováč J.
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issue 5
1040-1041
EN
In this paper we report the formation process and electrophysical properties of 'high temperature superconductor (YBa_2Cu_3O_{7 - x})/photosemiconductor (BiOCl:Ti)' micro- and nanoplates based junction. The energy band model of ≪HTSC-semiconductor≫ junction is proposed.
11
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Thermodynamic and Magnetotransport Properties of High Quality Na_{0.77}CoO_{2} Single Crystals

88%
Baran A., Zorkovská A., Kajňaková M., Lin C., Feher A.
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vol. 126
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issue 1
360-361
EN
Heat capacity and electrical resistivity of high-quality Na_{0.77}CoO_{2} single crystals was systematically studied as a function of temperature and magnetic field. Anomalies at 20 K have been observed both in the heat capacity and the electrical resistivity. The broad bump in the heat capacity indicates a smeared magnetic phase transition. Magnetic fields up to 9 T, oriented perpendicularly to the ab plane, reduce the temperature of this anomaly in accordance with the assumption of A-type antiferromagnetic ordering. The low temperature upturn observed in resistivity below 20 K for slow cooling is also suppressed by the magnetic field. This anomaly is probably the consequence of the interplay of several different mechanisms, including the Kondo effect, electron-electron interactions, and electron-phonon scattering.
12
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Effect of Layered Structure on the Lattice Heat Capacity of the Rare Earth Molybdates

88%
Feher A., Stefányi P., Orendáčová A., Anders E. E., Zvyagin A. I.
Acta Physica Polonica A
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1991
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vol. 79
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issue 6
861-867
EN
Temperature dependence of the heat capacity of layered rare earth molybdates was measured in the temperature range from 0.4 K to 23 K. It is shown that the temperature dependence of the lattice heat capacity can be described by the "T", where n ≠ 3. It is shown that differently from KDy(MoO_{4})_{2} and CsDyEu(MoO_{4})_{2} for CsDy(MoO_{4})_{2} and CsGd(MoO_{4})_{2} we obtained the temperature term with n < 3, which can be connected with the 2D behaviour, manifested also in the so-called membrane effect.
13
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Experimental Studies of S=1 Heisenberg Planar Antiferromagnetic Chain Ni(C_{11}H_{10}N_{2}O)_{2}Ni(CN)_{4}

88%
Čizmár E., Černák J., Orendáč M., Herrmannsdörfer T., Feher A.
Acta Physica Polonica A
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2000
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vol. 97
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issue 3
579-582
EN
In this paper we discuss the results of experimental studies of specific heat and magnetic susceptibility of chain compound Ni(C_{11}H_{10}N_{2}O)_{2}Ni(CN)_{4}. We have observed a Schottky type anomaly at T=2.2 K in a temperature dependence of specific heat and the compound was identified as a S=1 Heisenberg antiferromagnetic chain with the subcritical exchange coupling D/k_{B}=5 K, D/|J|=5.2 and the strong in-plane anisotropy E/k_{B}=2.8 K. It has been found that the in-plane anisotropy affects the region of validity of a diluted exciton model [1] and a strong coupling model [2] for S=1 Heisenberg chains with the strong planar anisotropy. The analysis suggests that the in-plane anisotropy should be considered in any attempt to find a compound potentially located in the boundary of Haldane and "large-D" phases.
14
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Spin-Peierls Transition in (N-Me-Tetra-Me-Pz)(TCNQ)_{2}

88%
Kajňaková M., Botko M., Starodub V., Čižmár E., Feher A.
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vol. 126
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issue 1
254-255
EN
The new organic anion-radical salt (N-Me-Tetra-Me-Pz)(TCNQ)_{2}, (Pz = pyrazine) was studied as a low dimensional magnetic system. Heat capacity and EPR studies were performed in the temperature range from 2 to 300 K. The magnetic susceptibility was measured in the temperature range from 2 to 300 K and in magnetic fields of 100 mT and 1 T. The magnetic properties of this new system can be described as a dimerized Heisenberg spin S=1/2 chain possessing a spin-Peierls transition at 42 K.
15
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Slow Magnetic Relaxation in the Highly Anisotropic Layered Crystal CsNd(MoO₄)₂

88%
Tkáč V., Orendáčová A., Tarasenko R., Orendáč M., Feher A.
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EN
The present work is devoted to the magnetic relaxation study of the single crystal CsNd(MoO₄)₂ a layered rare-earth dimolybdate. AC susceptibility measurements performed in magnetic fields up to B=5 T applied along the easy and hard axis with frequency f ≈1 kHz revealed rather complex field-induced slow magnetic relaxation. Two relaxation regimes in different magnetic field intervals connected with magnetic interactions and possible single-ion relaxation mechanism were observed.
16
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Magneto-Structural Correlations in Cu(en)_2SO_4 - a new Magnet with Two-Dimensional Triangular Arrangement of Cu(II) ions

76%
Kajňaková M., Orendáčová A., Orendáč M., Feher A., Mal'arová M., Trávníček Z.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
507-510
EN
Cu(en)_2SO_4 (en = 1, 2-diaminoethane) was synthesized and its crystal structure determined. From a crystallographic point of view, the compound might represent an example for the realization of a S =1/2 Heisenberg magnet on a two-dimensional spatially anisotropic triangular lattice. Magnetic susceptibility and magnetization studies were performed to analyze the magneto-structural correlations and to characterize the magnetic subsystem of the present compound. The results indicate the presence of antiferromagnetic interactions between magnetic ions with zJ/k_B≈-14.6 K and short-range order near 7 K.
17
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Frustrated Zig-Zag Spin Chains Formed by Hydrogen Bonds in [Cu(H₂O)(OH)(tmen)]₂[Pd(CN)₄]·2H₂O

76%
Čižmár E., Orendáčová A., Orendáč M., Kuchár J., Feher A., Park J., Meisel M.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
940-942
EN
The magnetic properties of the novel dimeric compound [Cu(H₂O)(OH)(tmen)]₂[Pd(CN)₄]·2H₂O (tmen=N,N,N',N'-tetramethylethylenediamine) with modulated crystal structure were studied in the temperature range from 95 mK to 300 K. Magnetic measurements revealed a presence of weak antiferromagnetic exchange coupling in the compound. The temperature dependence of specific heat is characterized by the presence of a Schottky-like maximum at 0.47 K and a λ-anomaly at 0.28 K, indicating the formation of long-range order in the system. The comparison of the experimental data with theoretical predictions revealed the presence of antiferromagnetic intradimer exchange coupling J/k_{B}= -1.2 K and interdimer coupling of a similar strength mediated via hydrogen bonds between dimeric units forming a frustrated magnetic zig-zag chain structure.
18
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Magnetocaloric Effect in CsDy(MoO₄)₂

76%
Tarasenko R., Tkáč V., Orendáčová A., Orendáč M., Sechovský V., Feher A.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
414-416
EN
CsDy(MoO₄)₂ belongs to the family of binary alkaline rare-earth molybdates, with detectable low-temperature structural phase transitions, caused by the cooperative Jahn-Teller effect. Magnetocaloric studies of a single crystal of CsDy(MoO₄)₂ have been performed in the temperature range from 2 K to 18 K in magnetic fields up to 7 T applied along the crystallographic a axis. The maximum value of the isothermal entropy change is about 8 J/(kgK) and was achieved already in the fields above 2 T. The observed results suggest that the studied system can be used as a magnetic refrigerant at helium temperatures.
19
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Ferroelastic Phase Transition in a Layered Bismuth Oxychloride Single Crystals

76%
Bunda V., Bunda S., Vashchuk F., Feher A., Kajňaková M., Kováč J., Syrkin E.
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issue 5
1069-1070
EN
We report the results of low-temperature specific-heat, magnetization, and X-ray diffraction measurements on a bismuth oxyhalide (BiOCl) single crystal. We conclude that BiOCl in temperature region 150-230 K shows "antiferroelastic-paraelectric" phase transitions at critical temperatures T_{c1} = 167.5 K and T_{c2} = 214.6 K, respectively.
20
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Application of Superconductor/Photosemiconductor Contact Structures in Electronics

76%
Bunda V., Bunda S., Komanicky V., Botko M., Čižmár E., Feher A.
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vol. 126
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issue 1
362-363
EN
In recent years, two-dimensional (2D) nanostructured materials, such as nanoplates and nanosheets, have attracted much attention because of their unique electronic, magnetic, optical, and catalytic properties, which mainly arise from their large surface areas, nearly perfect crystallinity, structural anisotropy, and quantum confinement effects in the thickness. The 2D nanostructured materials can be used as building blocks for advanced materials and devices with designed functions in areas as diverse, as lasers, transistors, catalysis, solar cells, light emission diodes, chemical and biological sensors. We report physical properties of YBCO/BiOI contact structures and electrophysical properties of BiOI single crystal.
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