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1
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Magnetic Study of the Fe Coated by Au Nanoparticles

100%
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.
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Superparamagnetism in Cobalt Nanoparticles Coated by Protective Gold Layer

100%
Hrubovčák P., Zeleňáková A., Zeleňák V., Kováč J.
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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.
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Ordered Nanoporous Silica Modified with Nanoparticles of Lanthanide Oxides

100%
Zeleňáková A., Kapusta O., Zeleňák V.
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vol. 126
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issue 1
218-219
EN
Lanthanide oxide nanoparticles were encapsulated inside of pores of highly ordered periodic silica of SBA15 type with hexagonal symmetry. The magnetic properties of such nanoperticles were investigated. The structural characterization using the SAXS, XANES, XRD, and N_{2} adsorption measurements showed the presence of lanthanide oxides of Ln_{2}O_{3} type (Ln=La, Pr, Nd, Gd, Eu), with the size of about 5 nm, incorporated in nanoporous channel system. Their magnetic properties, studied by SQUID apparatus, showed the weak antiferromagnetic ordering at 2 K in the nanocomposites Gd_{2}O_{3}@SBA15, Pr_{2}O_{3}@SBA15 and Nd_{2}O_{3}@SBA15. This behaviour of the nanoparticles is caused by blocking process of magnetic moments, which at 300 K exhibit the superparamagnetism, evidenced from ZFC/FC magnetization.
4
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Magnetic and Structural Studies into the Effect of Solvent Exchange Process in Metal-Organic Framework MOF-76(Gd)

100%
Almáši M., Zeleňák V., Zeleňáková A.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
991-993
EN
Effect of solvent exchange/absence on magnetic and structural properties of microporous metal-organic framework MOF-76(Gd) has been performed. Three compounds have been prepared and characterized: as synthesized MOF-76(Gd)-DMF, which containing DMF (N,N'-dimethylformamide) and H₂O molecules in the cavity system, activated complex without solvents, MOF-76(Gd) and water exchanged sample MOF-76(Gd)-H₂O. The structural study showed that compounds undergo structural changes after activation/exchange processes. Changes were mainly observed in the magnetic properties and distances between Gd...Gd ions: 4.718 Å for MOF-76(Gd)-DMF, 4.326 Å for MOF-76(Gd)-H₂O and 4.296 Å for MOF-76(Gd). The magnetic properties of the samples were studied by magnetic susceptibility χ_{M}(T) and magnetization M(H). The low positive value of the Weiss constant, Θ = 1.54 K in compound MOF-76(Gd)-DMF shows on a very weak ferromagnetic interaction. On the contrary, the negative values of Θ in complexes MOF-76(Gd) (-5.58 K) and MOF-76(Gd)-H₂O (-6.23 K) confirm the existence of antiferromagnetic exchange interaction between Gd(III) ions.
5
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Exchange Bias in Iron-Oxide Particles Nanocasted in Periodic Porous Silica

100%
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.
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AC Magnetic Susceptibility Study of Fe and Co Based Nanoparticles

100%
Zeleňáková A., Kováč J., Zeleňák V.
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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.
7
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Effect of 1D Ordering on Magnetic Properties of Iron Nanoparticles coated by Silica Shell

100%
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.
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Scaling Analysis of the Magnetocaloric Effect in Co/Au Nanoparticles

100%
Hrubovčák P., Zeleňáková A., Zeleňák V., Franco V.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
795-797
EN
The system of superparamagnetic Co/Au bimetallic nanoparticles of average diameter 7 nm was investigated with respect to its magnetocaloric properties. DC magnetic measurements revealed the presence of field dependent zero field cooled M(T) maximum (6-8 K) and significant zero field cooled/field cooled irreversibility at low temperatures in the system. Documented thermal hysteresis disallow standard magnetic entropy change calculation from isothermal M(H) data, thus we attempted to employ zero field cooled M(T) data for this purpose. Magnetic entropy change was calculated employing the Maxwell relation. In maximal field variation of 1 T relative high magnetic entropy change for nanoparticles Δ S_M ≈ 0.7 J/(kg K) at T =9 K was observed. The data collapsed onto single universal curve after proper axis rescaling.
9
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Structure and Magnetic Properties of Three-Dimensional Gadolinium-Based Hybrid Framework

88%
Almáši M., Zeleňáková A., Císařová I., Bednarčík J., Zeleňák V.
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vol. 126
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issue 1
308-309
EN
In the present work we have focused on the preparation and magnetic study of coordination polymer formed by Gd(III) cations as nodes and formate (HCOO¯; FOR) anions as charge compensating linkers. The prepared complex with formula {[Gd(μ_{3}-FOR)_{3}]}_{n} was characterized by single-crystal X-ray diffraction, and high-energy powder X-ray diffraction. The structural study showed that complex is formed by 3D polymeric network with the shortest Gd-Gd, distances of 3.998 Å. The magnetic properties of the complex were studied by magnetic susceptibility χ_{M}(T) and magnetization M(H) measurements. The results show on the weak antiferromagnetic coupling at low temperatures represented by the Weiss constant θ=-0.468 K. The value of effective magnetic moment μ_{eff}=7.57μ_{B}, which was estimated from the experimental data is close to the theoretical value for systems with S=7/2. Correlation between crystal structure of complexes and magnetic properties is presented.
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Fe₂O₃ and Gd₂O₃ Nanoparticles Embedded in Mesoporous Silica: Magnetic Properties Comparison

88%
Kapusta O., Zeleňáková A., Hrubovčák P., Girman V., Zeleňák V.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
860-862
EN
Nanocomposite materials containing Fe₂O₃ and Gd₂O₃ nanoparticles with the same concentration were prepared by nanocasting method. At this procedure silica matrix serves as nanoreactor for growth of nanoparticles. Temperature and external dc field dependences of the magnetization both samples were compared. Composite containing Fe₂O₃ nanoparticles shows superparamagnetic behaviour with blocking temperature around 45 K. Otherwise, paramagnetic properties were observed for the sample with Gd₂O₃ (above 10 K). Additionally, due to free pores the silica matrix could serve as medium to increase the number of bonded water molecules. These properties together with appropriate magnetic characteristics make studied materials suitable for magnetic resonance imaging applications.
11
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Magnetic Properties of Fe@Pt Nanoparticles with Core/Shell Morphology

88%
Zeleňáková A., Zeleňák V., Bednarčík J., Lauda M., Kováč J.
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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.
12
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Magnetocaloric Effect in Gd₂O₃@SiO₂ Nanocomposite

88%
Zeleňáková A., Hrubovčák P., Berkutova A., Kapusta O., Zeleňák V.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
810-812
EN
Nanocomposite consisting of Gd₂O₃ nanoparticles embedded in periodical porous silica matrix was investigated with respect to its magnetocaloric properties. Series of field (up to 5 T) dependence of magnetization data were recorded in temperature range 2-52 K. The data were subsequently processed employing Maxwell relation in order to calculate magnetic entropy change (Δ S_M) of the system. Examined nanocomposite exhibited reasonably high value of Δ S_M ≈ 29 J/(kg K) at maximal field change 5 T at the temperature of 2 K which suggests that this material could be feasible for cryomagnetic refrigeration applications.
13
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Effect of Pressure on Magnetic Properties of TM_3[Cr(CN)_6]_2 · nH_2O Nanoparticles

64%
Zentko A., Zentková M., Kavečanský V., Mihalik M., Mitróová Z., Arnold Z., Kamarád J., Cieslar M., Zeleňák V.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
489-493
EN
Effect of pressure on magnetic properties of magnetic nanoparticles, based on Prussian blue analogues, were studied in pressures up to 1.2 GPa. The Mn_3[Cr(CN)_6]_2·nH_2O and Ni_3[Cr(CN)_6]_2·nH_2O nanoparticles were prepared by reverse micelle technique. Transmission electron microscopy images show nanoparticles with average diameter of about 3.5 nm embedded in an organic matrix. The characteristic X-ray peaks of nanoparticles are more diffused and broader. Systems of nanoparticles behave as systems of interacting magnetic particles. The Curie temperature T_C is reduced from T_C = 56 K for Ni-Prussian blue analogues to T_C = 21 K for Ni-nanoparticles system and from T_C = 65 K for Mn-Prussian blue analogues to T_C = 38 K for Mn-nanoparticles system. One can explain this reduction of the Curie temperature and of the saturated magnetizationμ_s by dispersion of nanoparticles in an organic matrix i.e. by a dilution effect. Applied pressure leads to a remarkable increase in T_C for system of Mn-nanoparticles (ΔT_C/Δp = +13 K/GPa) and to only slight decrease in T_C for system of Ni-nanoparticles (ΔT_C/Δp = -3 K/GPa). The pressure effect follows behavior of the mother Prussian blue analogues under pressure. The increase in saturated magnetization, attributed to compression of the organic matrix, is very small.
14
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Neutron and EPR Study of Cu(tn)Cl_{2} - a~Two-Dimensional Spatially Anisotropic Triangular-Lattice Antiferromagnet

52%
Tarasenko R., Orendáčová A., Čižmár E., Maťaš S., Orendáč M., Zeleňák V., Pavlík V., Siemensmeyer K., Zvyagin S., Wosnitza J., Feher A.
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vol. 126
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issue 1
232-233
EN
We have studied the temperature dependence of the lattice parameters and the influence of spin anisotropy on the electron paramagnetic spectra of Cu(tn)Cl_2, an S=1/2 quasi-two-dimensional spatially-anisotropic triangular-lattice Heisenberg antiferromagnet. The variation of the resonance fields with temperature reflects the presence of an easy-plane exchange anisotropy with J_{z}/J_{x,y}<1 and g-factor anisotropy, g_{z}/g_{x,y}>1.
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