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1
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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.
2
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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.
3
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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.
4
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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.
5
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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.
6
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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.
7
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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.
8
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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.
9
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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.
10
Content available remote

Ti-Induced Ferromagnetism and the Specific Heat of CaTi_xRu_{1-x}O_3 (x=0, 0.005, 0.03)

76%
Zorkovská A., Baran A., Kajňaková M., Feher A., Šebek J., Šantavá E., Bradarić I.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
355-358
EN
The influence of Ti substitution on the specific heat of the CaTi_xRu_{1-x}O_3 system at low concentrations x=0, 0.005, and 0.03 was studied in the temperature range of 2-300 K at magnetic fields up to 9 T. Small peak was revealed in the C/T vs. T^2 dependence at around 3 K, which are field sensitive (the electronic specific heat coefficient γlinearly decreases with the increase in magnetic field), and might be connected to some kind of magnetic ordering. The coefficient γis suppressed also by Ti substitution.
11
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[Ni(en)(H_2O)_4][SO_4]·2H_2O - an S = 1 Molecular Magnet with Easy-Plane Anisotropy

64%
Tarasenko R., Orendáčová A., Tibenská K., Potočňák I., Kajňaková M., Vlček A., Orendáč M., Feher A.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
481-484
EN
Structural analysis of [Ni(en)(H_2O)_4][SO_4]·2H_2O was performed and it suggests that the crystal field should play a dominant role in the magnetic properties of the system. This conjecture coincides well with the specific heat and susceptibility behaviour. The analysis confirmed that the compound can be treated as a spin 1 single molecule magnet with nonmagnetic ground state introduced by easy-plane single-ion anisotropy D/k_B≈11 K and neglecting in-plane anisotropy E/D <0.1.
12
Content available remote

Magnetic Properties of an S=2 Ladder Spin Model Applied to a New Quasi-One-Dimensional Magnet [Mn(phen)_{3}](TCNQ)_{2}·H_{2}O

64%
Botko M., Cheranovskii V., Vasilets G., Čižmar E., Kajňaková M., Anders A., Starodub V., Feher A.
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vol. 126
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issue 1
20-21
EN
The effective spin S=2 Heisenberg ladder model with free-spin admixtures was proposed for the study of the low-temperature magnetic properties of the complex compound [Mn(phen)_{3}](TCNQ)_{2}·H_{2}O. The temperature dependence of magnetic susceptibility was found to be close to experimental data.
13
Content available remote

CuBr_2(C_{10}H_8N_2) - S = 1/2 Two-Dimensional Rectangular Heisenberg Antiferromagnet

64%
Čižmár E., Kajňaková M., Orendáčová A., Orendáč M., Feher A., Anders A., Park J., Meisel M.
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issue 5
973-974
EN
Magnetic susceptibility and X-band electron spin resonance study of a two-dimensional Heisenberg antiferromagnetic system CuBr_2(bipy), where bipy = C_{10}H_8N_2 is 4,4'-bipyridyl, has been performed in the temperature range from 300 K down to 2 K. A rhombic anisotropy of the g-factor was obtained from X-band EPR measurements with g_{x} = 2.037, g_{y} = 2.100 and g_{z} = 2.219. The temperature dependence of the magnetic susceptibility with a round maximum observed at 28 K suggests antiferromagnetic type of short-range order. No signature of the magnetic long-range order in studied compound was observed. The comparison of the magnetic susceptibility data with a two-dimensional rectangular Heisenberg model yields the values of intralayer exchange couplings J/k_{B} = - 47 K and J'/k_{B} = - 9.4 K.
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