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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.
2
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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.
3
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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.
4
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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.
5
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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.
6
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Investigation of Total Losses of Non-Oriented Electrical Steels

76%
Füzer J., Birčáková Z., Zeleňáková A., Hrubovčák P., Kollár P., Predmerský M., Huňady J.
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issue 5
1018-1019
EN
The aim of this work was to investigate the influence of microstructural features on the soft magnetic properties. The materials chosen for this work were two types of the Epstein strips of commercially produced non-oriented electrical steels by U.S. Steel Košice. The AC magnetic properties in 0.2-100 Hz frequency range were obtained by measurements of AC hysteresis loops by a fluxmeter based non-standard single sheet AC hysteresisgraph. The influence of the grain size and magnetic domains structure on the partial contributions (hysteresis, eddy current and anomalous) to the total losses is also discussed. The results of the frequency dependence of the total losses, measured by a single sheet loop tracer along the rolling direction and perpendicular to the rolling direction, show different contribution of partial components.
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