|
|
Pressure-induced changes of magnetic behavior of RCo2-type compounds
Valenta, Jaroslav ; Prchal, Jiří (advisor) ; Kamarád, Jiří (referee)
The diploma thesis is focused on the study of pressure-induced changes in a behavior of RECo2-type compounds. HoCo2 and its substitution of Si instead of Co in Ho(Co1-xSix)2, x = 0,025 were chosen as a representative compounds from this family of materials. Measurements of electric resistivity, heat capacity, magnetization and AC magnetic susceptibility under ambient pressure showed presence of three magnetic transitions for both samples: the change of the easy magnetization direction at TR ~ 16 K, the Curie temperature TC ~ 80 K and the flipping of magnetization of Co magnetic moments into the parallel direction with Ho magnetic moments at Tf ~ 125 K for HoCo2. Under hydrostatic pressures up to 3 GPa, the TR shifts to higher temperatures whereas TC and Tf, both decrease. This probably occurs due to the weakening of the Co magnetism and the strength of exchange interaction between Ho-Co sublattices. The experiment of muon spin rotation demonstrates a pressure influence to the decay of Co magnetic clusters, which occurs at lower temperatures with increasing pressure. It is evident that the hydrostatic pressure directly influences the exchange interaction between Co-Co.
|
|
|
Influence of the external pressure on the magnetic behavior of the material
Valenta, Jaroslav ; Prchal, Jiří (advisor) ; Prokleška, Jan (referee)
This thesis deals with magnetic properties of the HoCo2 compound, which is composed of rare earth and transition metal elements. The study is focused on properties at ambient pressure and changes of the magnetic behavior of the material under hydrostatic pressure. At the ambient pressure, there was performed analysis of the sample quality with a finding of presence of three magnetic transitions: flipping temperature magnetization of Co clusters Tf = (125 ± 1) K, Curie temperature TC = (79,5 ± 0,1) K and spin-reorientation temperature TR = (16,4 ± 0,5) K. When applying the hydrostatic pressure up to 3 GPa, there was observed a shift of the temperatures of the magnetic transitions and in the case Tf and TR their consequent vanishing. From the shifts of the TC and Tf towards lower temperatures and from the vanishing of the Tf, it was concluded that Co magnetism is being suppressed by the influence of the hydrostatic pressure.
|
|
|
Pressure-induced changes of magnetic behavior of RCo2-type compounds
Valenta, Jaroslav
The diploma thesis is focused on the study of pressure-induced changes in a behavior of RECo2-type compounds. HoCo2 and its substitution of Si instead of Co in Ho(Co1-xSix)2, x = 0,025 were chosen as a representative compounds from this family of materials. Measurements of electric resistivity, heat capacity, magnetization and AC magnetic susceptibility under ambient pressure showed presence of three magnetic transitions for both samples: the change of the easy magnetization direction at TR ~ 16 K, the Curie temperature TC ~ 80 K and the flipping of magnetization of Co magnetic moments into the parallel direction with Ho magnetic moments at Tf ~ 125 K for HoCo2. Under hydrostatic pressures up to 3 GPa, the TR shifts to higher temperatures whereas TC and Tf, both decrease. This probably occurs due to the weakening of the Co magnetism and the strength of exchange interaction between Ho-Co sublattices. The experiment of muon spin rotation demonstrates a pressure influence to the decay of Co magnetic clusters, which occurs at lower temperatures with increasing pressure. It is evident that the hydrostatic pressure directly influences the exchange interaction between Co-Co.
|
|
|
Ultrapure metallic materials
Prachařová, Marcela ; Kalbáčová Vejpravová, Jana (advisor)
Title: Ultrapure metallic materials Author: Bc. Marcela Janatová Department: Department of Condensed Matter Physics Supervisor: RNDr. Jana Poltierová Vejpravová, Ph.D. Supervisor's e-mail address: jana@mag.mff.cuni.cz Abstract This thesis deals with crystal structure and magnetic properties of RCr2Si2 (R = Pr, Tb, Er) and RCr2Si2C (R = La, Ce, Pr, Nd) compounds. All prepared samples were characterized by powder X-ray diffraction and investigated by specific-heat, magne- tization and AC-susceptibility measurements in the temperature range 2-900 K and in magnetic fields up to 9 T. The results suggest non-trivial magnetic ordering of the Cr sublattice in all studied compounds, accompanied with ferromagnetic ordering of the Pr and Nd magnetic moments below the Curie temperature of 30 and 21 K in PrCr2Si2C and NdCr2Si2C, respectively, and possible metamagnetism in CeCr2Si2C. For RCr2Si2C compounds, the experimental evidence of Cr magnetism is corrobora- ted by results of extended first-principles calculations based on the density functional theory. Keywords: RCr2Si2 compounds; RCr2Si2C compounds; Magnetic ordering; Heat ca- pacity; Magnetization 1
|
|
|
Magnetism in the RCo2-type compounds
Šebesta, Jakub ; Prchal, Jiří (advisor) ; Prokleška, Jan (referee)
Magnetic properties of RCo2 compounds have been studied since the sec- ond half of the last century. However, there was recently observed new mag- netic state, so-called parimagnetism. The aim of this thesis was to find the real behavior of TmCo2, which exhibits many discrepancies in the published results, by means of preparation of high-quality single-crystals, its characteri- zation around the ordering temperature and to study parimagnetic behavior. Obtained results showed differences between the samples, which should be caused by the instability of Co and Tm magnetism. It might be the reason of discrepancies in the literature. The results however show the same struc- ture of measured curves with two present phase transition. We suggest two possible interpretations: a) magnetic reorientation of Tm magnetic moments below the ordering temperature of the Tm sub-lattice or b) consequent or- dering of the rare-earth sub-lattice followed by ordering of the Co sub-lattice at lower temperature, which seems to be more realistic. We observed pari- magnetic anomalies at 38 K, 80 K, 168 K only in the case of single-crystals containing small amount of impurity phase, which probably affects the for- mation of magnetic clusters. 1
|
|
|
Ultrapure metallic materials
Prachařová, Marcela ; Kalbáčová Vejpravová, Jana (advisor)
Title: Ultrapure metallic materials Author: Bc. Marcela Janatová Department: Department of Condensed Matter Physics Supervisor: RNDr. Jana Poltierová Vejpravová, Ph.D. Supervisor's e-mail address: jana@mag.mff.cuni.cz Abstract This thesis deals with crystal structure and magnetic properties of RCr2Si2 (R = Pr, Tb, Er) and RCr2Si2C (R = La, Ce, Pr, Nd) compounds. All prepared samples were characterized by powder X-ray diffraction and investigated by specific-heat, magne- tization and AC-susceptibility measurements in the temperature range 2-900 K and in magnetic fields up to 9 T. The results suggest non-trivial magnetic ordering of the Cr sublattice in all studied compounds, accompanied with ferromagnetic ordering of the Pr and Nd magnetic moments below the Curie temperature of 30 and 21 K in PrCr2Si2C and NdCr2Si2C, respectively, and possible metamagnetism in CeCr2Si2C. For RCr2Si2C compounds, the experimental evidence of Cr magnetism is corrobora- ted by results of extended first-principles calculations based on the density functional theory. Keywords: RCr2Si2 compounds; RCr2Si2C compounds; Magnetic ordering; Heat ca- pacity; Magnetization 1
|
|
|
Pressure-induced changes of magnetic behavior of RCo2-type compounds
Valenta, Jaroslav
The diploma thesis is focused on the study of pressure-induced changes in a behavior of RECo2-type compounds. HoCo2 and its substitution of Si instead of Co in Ho(Co1-xSix)2, x = 0,025 were chosen as a representative compounds from this family of materials. Measurements of electric resistivity, heat capacity, magnetization and AC magnetic susceptibility under ambient pressure showed presence of three magnetic transitions for both samples: the change of the easy magnetization direction at TR ~ 16 K, the Curie temperature TC ~ 80 K and the flipping of magnetization of Co magnetic moments into the parallel direction with Ho magnetic moments at Tf ~ 125 K for HoCo2. Under hydrostatic pressures up to 3 GPa, the TR shifts to higher temperatures whereas TC and Tf, both decrease. This probably occurs due to the weakening of the Co magnetism and the strength of exchange interaction between Ho-Co sublattices. The experiment of muon spin rotation demonstrates a pressure influence to the decay of Co magnetic clusters, which occurs at lower temperatures with increasing pressure. It is evident that the hydrostatic pressure directly influences the exchange interaction between Co-Co.
|
|
|
Physical phenomena in ytterbium- and cerium-based compounds
Fikáček, Jan ; Sechovský, Vladimír (advisor) ; Arnold, Zdeněk (referee) ; Andreev, Alexander (referee)
This work contains a study of CeRuSn, which undergoes two structural transitions at 290 K and 256 K both connected with large temperature hysteresis. During the transitions the lattice shrinks along the c axis. At low temperatures the compound orders antiferromagnetically below 2.8(1) K. A strong magnetocrystalline anisotropy is caused by very shortened Ce-Ru separations pointing approximately along the c axis. Due to a strong hybridization, two thirds of Cerium atoms are in a non integer valence state. For the first time synthesized single-crystals of YbPt2Si2 a Yb2Pt3Si5 show no magnetic ordering. A maximum, which is visible in the temperature dependences of magnetic susceptibility originate in thermal population of the magnetic Yb3+ state. Powered by TCPDF (www.tcpdf.org)
|
|
|
Pressure-induced changes of magnetic behavior of RCo2-type compounds
Valenta, Jaroslav ; Prchal, Jiří (advisor) ; Kamarád, Jiří (referee)
The diploma thesis is focused on the study of pressure-induced changes in a behavior of RECo2-type compounds. HoCo2 and its substitution of Si instead of Co in Ho(Co1-xSix)2, x = 0,025 were chosen as a representative compounds from this family of materials. Measurements of electric resistivity, heat capacity, magnetization and AC magnetic susceptibility under ambient pressure showed presence of three magnetic transitions for both samples: the change of the easy magnetization direction at TR ~ 16 K, the Curie temperature TC ~ 80 K and the flipping of magnetization of Co magnetic moments into the parallel direction with Ho magnetic moments at Tf ~ 125 K for HoCo2. Under hydrostatic pressures up to 3 GPa, the TR shifts to higher temperatures whereas TC and Tf, both decrease. This probably occurs due to the weakening of the Co magnetism and the strength of exchange interaction between Ho-Co sublattices. The experiment of muon spin rotation demonstrates a pressure influence to the decay of Co magnetic clusters, which occurs at lower temperatures with increasing pressure. It is evident that the hydrostatic pressure directly influences the exchange interaction between Co-Co.
|
|
|
Influence of the external pressure on the magnetic behavior of the material
Valenta, Jaroslav ; Prchal, Jiří (advisor) ; Prokleška, Jan (referee)
This thesis deals with magnetic properties of the HoCo2 compound, which is composed of rare earth and transition metal elements. The study is focused on properties at ambient pressure and changes of the magnetic behavior of the material under hydrostatic pressure. At the ambient pressure, there was performed analysis of the sample quality with a finding of presence of three magnetic transitions: flipping temperature magnetization of Co clusters Tf = (125 ± 1) K, Curie temperature TC = (79,5 ± 0,1) K and spin-reorientation temperature TR = (16,4 ± 0,5) K. When applying the hydrostatic pressure up to 3 GPa, there was observed a shift of the temperatures of the magnetic transitions and in the case Tf and TR their consequent vanishing. From the shifts of the TC and Tf towards lower temperatures and from the vanishing of the Tf, it was concluded that Co magnetism is being suppressed by the influence of the hydrostatic pressure.
|
guest ::
