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Magnetocaloric Effect in Materials with the First Order Transitions - Direct Measurements

100%
Kamarád J., Kaštil J., Arnold Z., Javorský P., Sechovský V.
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issue 5
1000-1001
EN
The results of direct measurements of the magneto-caloric characteristics Δ T_{ad} and the indirect determination of Δ S_{m} of pure Gd around the Curie temperature and of the FeMnP_{0.45}As_{0.55} and Ni_{45}Co_5Mn_{31}Ga_{19} intermetallics in the vicinity of the first order magnetic and structural phase transitions are presented and discussed. A pronounced temperature and field dependence of the first order transitions in the studied materials manifests restrictions applying these materials in magnetic refrigeration techniques. An effective use of the Ericsson and the Brayton refrigeration cycles with FeMnP_{0.45}As_{0.55} in the role of a refrigerant is discussed.
2
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Spin Disordered Resistivity of the Heusler Ni₂MnGa-based Alloys

100%
Kamarád J., Kaštil J., Albertini F., Fabbrici S., Arnold Z.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
1072-1074
EN
Electrical resistivity of the selected Heusler off-stoichiometric (NiCo)₂Mn(GaIn) alloys was studied in a wide range of temperature and magnetic field. A step-like change of resistivity (Δρ ≈24 μΩcm) was detected in the off-stoichiometric Ni_{1.85}Mn_{1.21}Ga_{0.94} alloy at temperature of martensitic structural transition. This Δρ is much more significant than one in the stoichiometric Ni₂MnGa alloy. In the case of the off-stoichiometric (NiCo)₂Mn(GaIn) alloys, an enormous change of resistivity, Δρ ≈ 200 μΩcm, accompanies the structural transition. Simultaneously, the maximum of the spin disordered resistivity ρ_{sd}(T) of austenite phase of the alloys is slightly dependent on composition of the alloy and vary from ≈30 μΩcm up to ≈45 μΩcm, in good agreement with theoretical calculations. Due to high sensitivity of the structural transition temperature of the alloys to magnetic field, the very pronounced magnetoresistance effects have been observed in the studied alloys.
3
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Transport Properties of CeCo_{12}B_6 in Vicinity of Phase Transition

86%
Skorokhod Y., Arnold Z., Míšek M., Kamarád J., Isnard O., Mayot H.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
259-262
EN
Study of resistivity and specific heat on CeCo_{12}B_6 compound was performed in a temperature range 2.5-300 K. The specific heat exhibits pronounced λ-type anomaly in the vicinity of the Curie temperature T_C. Using the Debye-Einstein model the following parameters were obtained: the Debye temperature θ_D = 185 K, coefficient of the electron specific heat γ_{el} = 115 mJ/mol K. The parabolic temperature dependence of resistivity ρ(T) = ρ_0 + A_ρT^2 was observed below T_C in agreement with ferromagnetic state. Comparison of dρ/dT(T) with C_M(T) allows to confirm their similar character, accordingly to theory of critical behavior.
4
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Magnetic and Transport Properties of CeCo_{12}B_6 and CeFe_2Co_{10}B_6 Compounds under Pressure

86%
Míšek M., Arnold Z., Skorokhod Y., Kamarád J., Isnard O., Mayot H.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
263-266
EN
Study of pressure effects on magnetic and transport properties of CeCo_{12}B_6 and CeFe_2Co_{10}B_6 was performed in temperature range 5-300 K at pressures up to 9 kbar. The significant decrease in the Curie temperature T_C as a consequence of Fe substitution from (135±1) K to (85±1) K is not accompanied by the changes of saturation magnetization. The decrease in T_C with pressure is the same for both samples -ΔT_C/Δp ≈ -0.5 K/kbar. The relative decrease in saturation magnetization with pressure is about 0.3%/kbar for both samples. Pressure induced resistivity changes were below 1%. The relatively low pressure effect on both the T_C and magnetization can be attributed to the hybridization as a consequence of small distances between the Co and B atoms.
5
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Effect of Pressure on Magnetic Properties of Hexacyanochromates

86%
Mitróvá Z., Maťaš S., Mihalik M., Zentková M., Arnold Z., Kamarád J.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
469-472
EN
We present the study of pressure effect on magnetic properties of TM^{2+}_3[Cr^{III}(CN)_6]_2·nH_2O ferrimagnets and ferromagnets (TM = Cr and Co) under pressures up to 0.9 GPa. Applied pressure strengthens super-exchange interaction in Cr^{2+}-prussian blue analogues with dominant antiferromagnetic interaction J_{AF} leading to increase in the Curie temperature T_C (ΔT_c/Δp = 29.0 K/GPa) and reduces T_C of Co^{2+}-prussian blue analogues with dominant ferromagnetic interaction J_F (ΔT_c/Δp = -1.8 K/GPa). The rise of J_{AF} interaction is attributed to the enhanced value of the single electron overlapping integral S. On the other hand, the applied pressure slightly affects bonding angles between magnetic ions mediated by the cyano-bridge and reduces the strength of magnetic coupling. Changes of the magnetization curve with pressure can be attributed to changes of magnetic anisotropy. The reduction of magnetization with pressure observed on Cr^{2+}-prussian blue analogues can be explained by pressure induced transition from Cr^{2+} high spin state to Cr^{2+} low spin state. All pressure induced changes are reversible.
6
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Effect of Pressure on Magnetic Properties of TM_3[Cr(CN)_6]_2 · nH_2O Nanoparticles

73%
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.
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