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1
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Dynamical Structure Factors of Quasi-One-Dimensional Magnets of the Type of CsNiF_3

100%
Lisý V., Tóthová J.
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issue 5
953-955
EN
By a direct derivation of the equations of motion for the spins in CsNiF_3 we show that the extensively used sine-Gordon equation fails to describe the dynamics of this 1D magnet. Instead of this soliton-bearing model we use the spin-wave theory and, without going to the continuum approximation, calculate the dynamic structure factors of the scattering of neutrons on CsNiF_3. Complete analytical solutions for the dynamic structure factors in the frequency domain are obtained both within the classical consideration and with quantum corrections.
2
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Hydrodynamic Memory in the Motion of Charged Brownian Particles across the Magnetic Field

100%
Tóthová J., Lisý V.
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issue 5
1051-1053
EN
An exact solution of the Langevin equation is given for a charged Brownian particle driven in an incompressible fluid by the magnetic field, taking into account the hydrodynamic aftereffect. The stochastic integro-differential Langevin equation is converted to a deterministic equation for the particle mean square displacement. We have found the mean square displacement and other time correlation functions describing the particle motion. For the motion along the field the known results from the theory of the hydrodynamic motion of a free Brownian particle are recovered. The correlation functions across the field contain at long times the familiar Einstein terms and additional algebraic tails. The longest-lived tail in the mean square displacement is proportional to t^{1/2}. At short times the motion is ballistic and independent of the magnetic field.
3
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Effect of Magnetic Field on the Fluctuations of Charged Oscillators in Viscoelastic Fluids

100%
Lisý V., Tóthová J.
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vol. 126
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issue 1
413-414
EN
In the present work the generalized Langevin equation is solved for the motion of a charged Brownian oscillator in a magnetic field, when the thermal random force is exponentially correlated in the time. This model is consistent with the assumption that the medium has weakly viscoelastic properties. The velocity autocorrelation function, time-dependent diffusion coefficient and mean square displacement of the particle have been calculated. Our solutions generalize the previous results from the literature and are obtained in a way applicable to other problems of the Brownian motion with memory.
4
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Effect of Stochastic Dynamics on the Nuclear Magnetic Resonance in a Field Gradient

100%
Tóthová J., Lisý V.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
1111-1113
EN
In the present contribution, the attenuation function S(t) for an ensemble of spins in a magnetic-field gradient is calculated through an accumulation of the phase shifts in the rotating frame resulting from the changes of the particle displacements. The found S(t) is applicable for any kind of the stochastic motion of spins, including their non-Markovian dynamics with memory. Depending on the considered system, both the classical expressions valid for normal diffusion at long times and new formulae for the short-time Brownian motion can be obtained. Our method is also applicable to the NMR pulse sequences based on the refocusing principle. This is demonstrated by describing the spin echo experiment developed by Hahn.
5
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Electro-Rheological Properties of Transformer Oil-Based Magnetic Fluids

61%
Paulovičová K., Tóthová J., Rajňák M., Timko M., Kopčanský P., Lisý V.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
1141-1143
EN
The aim of this work was to study rheological behavior of nanofluids affected by electric field and temperature. We used transformer oil-based magnetic fluids, the suspensions of permanently magnetized colloidal particles (Fe_3O_4) coated by a stabilizing surfactant and immersed in transformer oil. The rheological characterization of transformer oil-based magnetic fluid was performed using the rotational rheometer MCR 502 in the shear rate from 10 to 1000 s¯¹.The strength of electric field was changed in the interval 0-6 kV cm¯¹. The flow curves and viscosity functions detected at three different temperatures 25, 50, and 75°C disclose rheological characteristics of samples, first of all the viscosity growth under increasing strength of electric field.
6
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Rheological and Thermal Transport Characteristics of a Transformer Oil Based Ferrofluid

52%
Paulovičová K., Tóthová J., Rajňák M., Wu Z., Sundén B., Wadsö L., Tobiáš T., Kopčanský P., Timko M., Lisý V.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
564-566
EN
Ferrofluids based on insulating liquids are intensively studied as a potential substitute of liquid dielectric in high voltage technologies. In this work we focus on the experimental investigation of flow and thermal transport characteristics of a ferrofluid based on transformer oil (Mogul) and iron oxide nanoparticles. The magneto-rheological behavior of the ferrofluid was studied by a rotational rheometer in the shear rate range from 1 to 1000 s¯¹ and magnetic field up to 1 T. By means of a thermal constants analyzer and a transient plane source method we obtained the thermal conductivity, specific heat and thermal diffusivity values for the studied oil and the ferrofluid. It is shown that the Newtonian character of the ferrofluid changes to a non-Newtonian with application of the magnetic field. The notable magneto-viscous effect has been observed especially at low shear rates. We found that the doping of the transformer oil by 3 wt% of the nanoparticles results in a thermal conductivity enhancement by about 3.2%.
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