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  1. 57 Acta Physica Polonica A

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  1. 1 2018

  2. 15 2017

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  4. 16 2014

  5. 4 2012

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  1. 57 Kováč J.

  2. 14 Kopčanský P.

  3. 14 Timko M.

  4. 8 Diko P.

  5. 6 Antal V.

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1
Content available remote

Minor Loop Scaling Rules for Finemet Type Soft Magnetic Cores

100%
Varga L., Kováč J.
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EN
The validity of the Steinmetz law (P=K B^{n}) has been analyzed using the tanh model, by splitting the minor loops in three or two components, corresponding to different magnetization processes at different excitation levels. It is shown that the original Steinmetz law with the exponent n=1.6 is valid only at and above the coercive field, for intermediate and high induction values, where all the three magnetization processes are present. At small magnetization levels the exponent is about 3, much higher than the original proposed value.
2
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Rayleigh Region in Amorphous and Nanocrystaline FINEMET Alloy

100%
Novák L., Kováč J.
Acta Physica Polonica A
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2014
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vol. 126
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issue 1
126-127
EN
The magnetization processes in region of small excitation magnetic fields - Rayleigh region, were investigated on the amorphous and nanocrystalline FINEMET ribbons. The significant differences in behaviour of these materials were found. This could be explained by the difference in the value of magnetoelastic anisotropy as well as by the different thickness of the domain walls in these materials.
3
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Influence of Pulse Heating on Rayleigh Area of Amorphous FINEMET-Type Alloy

81%
Novák L., Kováč J., Hubač L.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
753-755
EN
The effect of pulse annealing on magnetization processes in FINEMET-type alloy in weak magnetic fields (the Rayleigh area) is studied in this paper. Rapidly quenched FINEMET ribbon in the original amorphous state was chosen for the study. The significant differences in behavior of studied materials are ascribed to the differences in internal mechanical stresses, which bring about the changes in total magnetic anisotropy and consequently also changes in domain walls thickness.
4
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AC Magnetic Susceptibility Study of Fe and Co Based Nanoparticles

81%
Zeleňáková A., Kováč J., Zeleňák V.
Acta Physica Polonica A
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2014
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vol. 126
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issue 1
222-223
EN
We have investigated magnetic properties of three different samples composed of magnetic Fe or Co core and protective Au or Pt metallic shell, namely Fe@Au, Co@Au and Fe@Pt samples, with sizes of about 10 nm, 8 nm and 5 nm respectively. We have measured and analysed the experimental data of real and imaginary component of ac susceptibility through theoretical laws for non-interacting and interacting particles and we have estimated the strength of their magnetic interactions. Our analyses showed the increase of inter-particle interactions with decreasing particles size.
5
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Study of the Magnetization Processes in Amorphous and Nanocrystalline FINEMET by the Numerical Decomposition of the Hysteresis Loops

81%
Kováč J., Novák L., Varga L.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
732-734
EN
The magnetization processes in amorphous and nanocrystalline FINEMET ribbons were studied by the numerical decomposition of the quasi-static hysteresis loop to the contributions of the domain wall movement, the domain rotations, and the domain wall annihilation and nucleation processes following the hyperbolic T(x) model of hysteresis. The hysteresis data measured during decrease of the excitation magnetic field were used for the separation of these processes. The significant differences in behavior of these two materials were found. In amorphous state the domain rotations component dominates whereas in nanocrystalline state the domain wall movement component prevails. These differences are reflected in the anisotropy field distributions as well.
6
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Effect of 1D Ordering on Magnetic Properties of Iron Nanoparticles coated by Silica Shell

81%
Zeleňáková A., Hrubovčák P., Zeleňák V., Kováč J.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
561-563
EN
Peculiar magnetic properties of 1D arranged composite consisting of Fe nanoparticles coated by insulating SiO2 layer were studied. Silica layer promotes the self-organization of 4 nm iron nanoparticles to the chain-like structures. Magnetization study shows that prepared nanoparticles exhibit very strong inter-particle magnetic interactions, which lead to long-range ordering of nanoparticles magnetic moments. Magnetic properties show superferromagnetic behaviour. The low value of room temperature coercivity, the existence of electrical insulating silica layer and small size of iron nanoparticles favour studied material for potential usage in microelectronic devices designed for high voltage signals modulated by high frequencies.
7
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Mechanochemical Preparation and Magnetic Properties of Fe₃O₄/ZnS Nanocomposite

81%
Bujňáková Z., Zorkovská A., Kováč J.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
1120-1122
EN
Powder nanocomposite of Fe₃O₄/ZnS was prepared by mechanochemical synthesis in a planetary ball mill. In this reaction natural magnetite mineral Fe₃O₄ was used, together with zinc acetate (CH₃COO)₂Zn·2H₂O and sodium sulfide Na₂S·9H₂O, as precursors for the zinc sulfide ZnS. X-ray diffraction revealed that the sample is composed of small nanocrystalline particles, containing Fe₃O₄ and ZnS. The non-milled magnetite showed distinctive Verwey transition at around 120 K, this becomes suppressed after milling, as a consequence of structural disorder and presence of defects. Moreover, the reduction of saturation magnetization from 91 A m²/kg to 69.2 A m²/kg was observed, as a consequence of the milling process. The magnetization of the Fe₃O₄/ZnS nanocomposite was the lowest (34.5 A m²/kg), due to the milling and to the decreased weight fraction of the ferrimagnetic component. Nevertheless, the Fe₃O₄/ZnS sample demonstrates ferrimagnetic behavior as well, and its structure is less perturbed by milling, the Verwey transition, although less impressive, but is preserved.
8
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Magnetic Transition in La_{1/3}Nd_{1/3}Ca_{1/3}MnO_{3} Perovskite Induced by Electric Field at a Given Magnetic Field

81%
Baszyński J., Kowalczyk A., Kováč J.
Acta Physica Polonica A
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2000
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vol. 97
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issue 5
879-882
EN
We report the discovery of new effect in La_{1/3}Nd_{1/3}Ca_{1/3}MnO_{3} - the jump of magnetization induced by external electric field E (2.5-4)V/cm at a given magnetic field H (18-25) kOe at T=4.2 K. This effect is large when compared with a metamagnetic transition induced by the magnetic field in many substances. The origin of the magnetization jump induced by external electric field has not yet been explained. We assume that this transition is induced by the hopping of E-excited e_{g} electrons and that this is a source of a jumpwise increase in magnetization in the non-collinear magnetic phase within the perovskite. This effect can be attributed to a strong coupling between carriers induced by the electric field (the conduction electrons e_{g}), and the local magnetic moments (t_{2g}-localized spins) through a mechanism of the Zener double exchange.
9
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Clean Bulk YBaCuO Superconductors Doped by Paramagnetic Ions of Sm and Yb

81%
Jirsa M., Volochová D., Kováč J., Diko P.
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EN
The recently developed clean cuprate technology was employed to study the effect of tiny amounts of paramagnetic Yb and Sm ions on electromagnetic properties of bulk YBaCuO. Small samples of about 1.5×1.5×0.5 mm³ were measured by vibrating sample magnetometer in the field range ±9 T and temperatures 30 K to 10 K in the non-superconducting tetragonal state and 300 K to 70 K in the orthorhombic, superconducting state. In the latter case, we observed the reversible paramagnetic moment above T_{C}, similar as in the tetragonal state. Below T_{C} an irreversible magnetic moment was observed from which critical current density, J_c, was evaluated using the extended Bean model. The effect of doping by Yb and Sm ions on J_c was studied and a correlation between J_c and the total paramagnetic moment was found. Both J_c(B) and the normalized pinning force density as a function of the reduced magnetic field were analyzed in terms of the classical model and discussed with respect to the material anisotropy, giving us at least a rough idea on the type of the effective pinning type and mechanism.
10
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On the Mechanism of H-Induced Curie-Point Relaxation in Fe-Based Glassy Alloys

81%
Balla S., Kováč J., Novák L., Lovas A.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
55-59
EN
In Fe based glasses not solely the stress-sensitive magnetic properties, but also the amorphous Curie temperature (T_C^{am}) can be altered by hydrogen absorption or due to the low temperature "heat treatments" in liquid N_2 (-196°C). The sign and magnitude of this T_C^{am} shift is strongly compositional dependent, but it is also influenced by the thermal history of glasses (heat treatments). In this paper new experimental results are presented, which were obtained on FeCrB alloys, supporting the previous observations. An attempt of interpretation of the results is also made, which is based on the existence of the quenched-in phase reminiscences, being inherited from the liquid structure.
11
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Exchange Bias in Iron-Oxide Particles Nanocasted in Periodic Porous Silica

81%
Zeleňáková A., Kováč J., Zeleňák V.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
357-359
EN
Iron-oxide nanoparticles were nanocasted in the periodic mesoporous silica matrix, consisting of two-dimensional hexagonally ordered channel system with the mean diameter of the channels about 7 nm. The magnetic measurements of dc magnetization confirm behavior typical of a superparamagnetic system, such as the irreversibility of the zero-field-cooled and field-cooled curves, presence of a maximum in zero-field-cooled curve related with blocking temperature T_C and revealing of coercivity H_C below T_C. The existence of negative exchange bias effect below T_C was confirmed in our system represented by value of exchange bias field H_{EB}=-970 Oe measured at the temperature 2 K.
12
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Magnetic Study of the Fe Coated by Au Nanoparticles

81%
Zeleňáková A., Kováč J., Kavečanský V., Zeleňák V.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
533-536
EN
Nanosized iron-gold magnetic nanoparticles with an average particle size 10 nm were prepared by a reverse micelle method. The magnetic properties measurements of DC and AC magnetization confirm behaviour typical of a superparamagnetic system, such as the irreversibility of the zero-field-cooled and field-cooled curves, the frequency dependence of a blocking temperature T_B, and revealing of coercivity H_C below blocking temperature. The quantitative analysis of AC susceptibility due to value of parameter C_1 = ΔT_B/(T_B Δ log f)=0.0242 confirming the existence of inter-particle interaction in our system.
13
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Superparamagnetism in Cobalt Nanoparticles Coated by Protective Gold Layer

81%
Hrubovčák P., Zeleňáková A., Zeleňák V., Kováč J.
Acta Physica Polonica A
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2014
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vol. 126
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issue 1
216-217
EN
In this work we present the study on structural and magnetic properties of fine cobalt nanoparticles coated by protective gold layer synthetized by microemulsion method. Structural measurements (powder XRD, HRTEM) confirmed spherical shape of the particles, their nanocrystalline character and presence of individual Co and Au phases. Investigation of magnetic properties of the particles revealed superparamagnetic behavior at higher temperatures and magnetic hysteresis at low temperatures. Average magnetic moment of individual particle m_{p}~86.3μ_{B} was established. The critical temperature, below which the magnetic moments of the particles are blocked, was T_{B}=6 K. Investigation of magnetic relaxation processes via magnetic susceptibility, detected one maximum attributed to single relaxation process, present in the particles. Obtained data, interpreted in terms of Neél-Arrhenius and Vogel-Fulcher theoretical models, confirmed the presence of strong magnetic interparticle interactions.
14
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Magnetic Properties of Fe@Pt Nanoparticles with Core/Shell Morphology

71%
Zeleňáková A., Zeleňák V., Bednarčík J., Lauda M., Kováč J.
Acta Physica Polonica A
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2010
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vol. 118
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issue 5
1002-1004
EN
We have investigated the magnetic properties of iron nanoparticles coated by platinum shell prepared by chemical procedure. Magnetic measurements using SQUID magnetometer confirm the behaviour typical for a superparamagnetic system. From the M(H) loop at 300 K the magnetic moment of each particle m_{P} ≈ 250 μ_{B} was calculated. The magnetic relaxation study by the in-phase χ' and out-of-phase χ" ac susceptibilities in frequency range 0.1-1000 Hz suggests the existence of two magnetic layers with different magnetic spin's dynamics and strong dipolar interactions between nanoparticles. The observed magnetic behaviour is caused by the blocking process of interacting superparamagnetic particles with magnetic iron core, diamagnetic platinum shell and spin-glass like behaviour magnetic interlayer between them.
15
Content available remote

Magnetic Properties of Mechanochemically Synthesized Mixed Oxides

71%
Kostova N., Zorkovská A., Kováč J., Velinov N., Baláž P.
Acta Physica Polonica A
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2014
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vol. 126
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issue 1
411-412
EN
The mixed oxides Fe_{2}O_{3}-ZnO have been obtained in nanocrystalline state by reactive milling in a high-energy planetary mill, from a stoichiometric mixture of hematite and ZnO. The magnetic properties of samples were evaluated by magnetization measurements and Mössbauer spectrometry. A post milling annealing promotes the solid state reaction and improves the zinc ferrite formation, paramagnetic ZnFe_{2}O_{4} phase is formed. Further mechanical activation leads to structural transformation into Wüstite-type (FeZn)O mixed oxide, with ferromagnetic hysteresis and increased magnetization.
16
Content available remote

Magnetic Properties of Thorium Ferricyanide

71%
Göbl R., Zentková M., Kavečanský V., Kováč J., Zentko A.
Acta Physica Polonica A
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2000
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vol. 97
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issue 5
835-838
EN
The magnetic properties of Th_{3}[Fe(CN)_{6}]·10H_{2}O were investigated. It was shown that this compound is antiferromagnetically ordered in the low temperature region. The observed antiferromagnetic ordering is stable only in the low field.
17
Content available remote

Synthesis and Characterization of Magnetoferritin

71%
Mitróová Z., Melníková L., Kováč J., Timko M., Kopčanský P.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1318-1320
EN
The paper presents detailed experimental study of synthesis and characterization a bioinorganic magnetic molecule - magnetoferritin. Magnetoferritin with loading of iron ions per protein molecule in the range from 300 to 3000 was prepared. Size distribution analysis (transmission electron microscopy, dynamic light scattering) shows spherical nanoparticles with particle size distribution from 2 to 12 nm, and hydrodynamic diameter from 12 to 25 nm. The thermomagnetic curves measured after cooling the sample in zero field (zero-field cooling) and under the presence of the measurement field (field cooling) show superparamagnetic behavior with the blocking temperature T_{b} from 22 to 60 K and the magnetization loops measured below T_{b} (at 2 K) show the hysteresis with coercive field from 20 to 30 kA/m depending on the concentration of the magnetic nanoparticles.
18
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Study of Composites Consisting of High-T_{c} Superconductor and Fine Magnetic Particles

71%
Timko M., Kopčanský P., Kováč J., Koneracká M., Plecháček V.
Acta Physica Polonica A
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2000
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vol. 97
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issue 3
389-392
EN
The composite systems as a mixture of fine magnetic particles of Fe_{3}O_{4} and superconducting powder such as Bi_{1.8}Pb_{0.2}Sr_{2}Ca_{2}Cu_{3}O_{10+x} were prepared. The influence of internal magnetic field (generated by magnetic particles) on the superconducting properties of prepared composites were studied as a function of concentration of magnetic particles and their magnetic state. The observed data both of the shielding and the Meissner effect were compared with corresponding linear combination of pure signals of magnetite and superconductor, respectively. The large differences were observed for demagnetized samples. It means that result must be discussed in the frame of the distribution of internal magnetic field created by the magnetic particles.
19
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Influence of Spark Plasma Sintering on Microstructure and Properties of Bulk LaCaSrMnO Magnetocaloric Materials

71%
Zmorayová K., Antal V., Kováč J., Diko P., Noudem J.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
804-806
EN
Polycrystalline La_{0.67}Ca_{0.33-x}Sr_xMnO₃ (x=0.33, 0.03, 0) (LCSM) perovskite samples treated by spark plasma sintering technique were investigated. This study proved influence of the sintering temperature on microstructure of these perovskite ceramics. The thermogravimetric measurements revealed the possibility of increase of oxygen content in studied LCSM samples by additional annealing under oxygen atmosphere and low temperature. Finally, the effect of this annealing on magnetic properties was demonstrated.
20
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Crystalline Structure, Stoichiometry and Magnetic Properties of the Morasko Meteorite

71%
Idzikowski B., Kováč J., Diko P., Stankowski W., Muszyński A.
Acta Physica Polonica A
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2010
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vol. 118
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issue 5
1071-1073
EN
The composition and structure of iron-nickel alloys smithereens extracted from the Morasko (Poland) Meteorite (fell ∿ 5 ka BP) were investigated by optical metallographic techniques, scanning electron microscopy and electron microprobe analysis, thermal analyses, magnetic measurements and X-ray diffraction. Microstructural analysis by scanning electron microscopy and optical microscopy has shown that the sample is composed from large grains of Fe-Ni alloy in which secondary phase crystals with well developed crystal habits and the size about 10 micrometers are distributed. Thermal analyses confirmed that the transformation from alpha to gamma Fe-Ni solid solution appearing as a function of temperature corresponds to about 5 wt% Ni in the Fe-Ni alloy. Possible scenario of the extraterrestrial sample solidification is related to the microstructural and magnetic behavior.
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