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Magnetic Exchange Interactions and Estimation of T_N in CsNiF_3 from First Principles

100%
Legut D., Rusz J.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
503-506
EN
The CsNiF_3 has been for long time studied as a prototype of quasi-1D planar ferromagnetic system. At very low temperature the studied system is insulator and therefore the magnetic exchange interactions should decay very rapidly. We treated the magnetic exchange coupling within the Heisenberg model for the nearest neighbor interaction between the anti-ferromagnetically coupled Ni-chains. The influence of up to the second-nearest neighbors on the ferromagnetic exchange coupling along the Ni-chains was determined. The exchange interactions were calculated for the experimental volume by the density functional theory, within the all-electron approach using the local density approximation for the exchange and correlation. The Néel temperature was calculated by means of the mean-field theory and by the random-phase approximation method.
2
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f-Electron Behavior in Rare Earth Based Systems: Localization or Itinerancy?

81%
Biasini M., Rusz J., Ferro G., Czopnik A.
Acta Physica Polonica A
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2005
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vol. 107
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issue 4
554-561
EN
Measurement of the two-dimensional angular correlation of the electron-positron annihilation radiation complemented with ab initio calculations can provide decisive information about the character of the f-electrons in rare earth compounds. We provide examples of f-electron localized and f-electron itinerant systems, respectively. (i) In the case of the antiferromagnetic heavy fermion and superconductor CeIn_3 the multisheet Fermi surface, reconstructed from our measurements in the paramagnetic phase, agrees closely with the predictions of band structure calculations regarding the Ce 4f electrons as fully localized. (ii) On the other hand, our studies of the antiferromagnet actinide based UGa_3 in the paramagnetic phase, compared with calculations which include the effects due to the non-uniform positron density and the electron-positron correlations, produce a substantial evidence that an unconstrained 5f-electron itinerant description applies.
3
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Electronic Structure of PrCo_2Ge_2

71%
Diviš M., Vejpravová J., Rusz J., Prokleška J., Sechovský V.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
295-298
EN
The intermetallic compound PrCo_2Ge_2 exhibits very interesting physical properties: antiferromagnetism below T_N = 28 K, an order to order transition at T_R = 8 K, a rich magnetic phase diagram, and huge magnetocrystalline anisotropy as observed by recent experiments on a single crystal in a magnetic field up to 14 T. We performed first principles calculations based on the density functional theory. The localized 4f^2 configuration of Pr^{3+} was treated using the open core approximation. The electronic densities of states were obtained and the position of the Fermi level was found. The hybridization between the Co-3d, Pr-5d and Ge-4p states is discussed. The calculated magnetic moment of Co is found to be quite small (m_{Co} < 0.1 μ_B). The first principles calculations of the crystal field interaction were performed as well: the obtained crystal field parameters were used as the starting parameters for further refinement in comparison with experimental data of the magnetic susceptibility and specific heat. The crystal field splitting of the 4f states was determined.
4
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Magnetic Circular Dichroism in Electron Microscopy

71%
Rusz J., Novák P., Rubino S., Hébert C., Schattschneider P.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
599-644
EN
Although the analogies between the electron energy loss spectra and X-ray absorption spectra were recognized long ago, the possibility to observe the magnetic circular dichroism in the electron microscope was demonstrated only recently. This technique combines the atom specificity with the high spatial resolution of the transmission electron microscope. Being a very young technique, there are still many open questions concerning the optimization of the signal to noise ratio. In this paper we study the dichroic signal dependence on the sample thickness, acceleration voltage and incoming beam tilt angle of bcc-Fe and fcc-Ni.
5
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Point-Contact Spectroscopy of Crystalline Electric Field of Heterocontacts PrB_6 and NdB_6 with Pt

61%
Vasil'ová M., Pristáš G., Reiffers M., Flachbart K., Rusz J., Shitsevalova N.
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EN
Point-contact spectra of single crystals PrB_6 and NdB_6 are presented. We observed maxima connected with crystalline electric field excitations in agreement with previous results for applied voltage V>8 mV. Moreover, we observed maxima at 6 mV (PrB_6) and 3 mV (NdB_6), which are probably connected with phonon modes. Moreover, we observed phonon peak at 10.6 mV in PrB_6. Further measurements in magnetic fields and crystalline electric field calculations are necessary.
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