|
|
Kooperativní jevy v cerových sloučeninách na hranici magnetismu
Moudřík, Jan ; Prokleška, Jan (advisor) ; Javorský, Pavel (referee)
This work reports on physical properties of a novel CeCo0.715Si2.285 compound. The compound crystallizes in the I-4m2 space group structure with extremely elongated unit cell (a = 4.13˚A , c = 32.84˚A) containing BaAl4 structural patterns. In zero magnetic field it orders antiferromagnetically at TN = 10.0K. Under application of magnetic field along the c-axis it manifests numerous magnetic transitions in small fields (B < 0.5T), resembling the so-called 'devil's staircase' behaviour (e.g. CeSb, CeCoGe3). The magnetization is almost constant from 1T up to 45T for H||c but considerably reduced (0.3µB/Ce) with respect to the free Ce3+ ion. For fields applied along the a-axis typical behaviour for a hard axis in a material with uniaxial anisotropy is observed. The performed single crystal neutron diffraction experiment did not allow complete determination of magnetic structure. 1
|
|
|
Structure discontinuity in the RTX hexagonal materials
Rusňák, Martin ; Prchal, Jiří (advisor) ; Javorský, Pavel (referee)
As it was discovered earlier the crystal lattice of ternary hexagonal RTX compounds exhibits a criticality for specific values of the ratio of the lattice parameters c/a around the interval of 0.565 - 0.575. To explore this effect in more detail, we have prepared polycrystalline samples of Ce1-xYxPdAl and SmPdAl and monocrystalline SmPdAl, SmNiAl. In case of Ce1-xYxPdAl series the forbidden range of c/a values was found to evolve with composition and temperature. Structural transformation is temperature independent in these compounds. SmPdAl compound is single-phased in the whole temperature range even though the c/a ratio crosses the gap of forbidden values common for other compounds. However a criticality in evolution of c/a is present at temperature 500 K. Anomaly visible on magnetic and transport properties, which is connected with change of structure, was found at approximately the same temperature. Explanation based on enhanced thermal movement of the atoms is proposed. Besides, also magnetic measurements at low temperatures were performed. The magnetic ordering temperature Tc = 15.3 K and dominant antiferromagnetic order was found. Monocrystalline SmNiAl exhibits very similar behaviour to the SmPdAl.
|
|
|
Infrared Spectroscopy of Multiferroics
Goian, Veronica ; Kamba, Stanislav (advisor) ; Javorský, Pavel (referee) ; Dubroka, Adam (referee)
Infrared Spectroscopy of Multiferroics Author: Veronica Goian Institute: Department of Dielectrics, Institute of Physics of the Academy of Sciences, Na Slovance 2, 182 21 Prague 8 Abstract: We have investigated numerous multiferroic and magnetoelectric materials mainly using infrared (IR) spectroscopy. Nevertheless, the studies were frequently combined with radio-frequency, microwave, THz, Raman and structural measurements provided by our colleagues, as well as by magnetic and elastic investigations, where we participated. Our main aim was the complex study of quantum-paraelectric antiferromagnet EuTiO3 in the form of crystals, ceramics and thin films. Near 300 K we have discovered an antiferrodistorive phase transition from cubic mPm3 to tetragonal I4/mcm structure in bulk EuTiO3 and explained its low-frequency dielectric properties by anomalous polar phonon behavior. Large and anisotropic magnetodielectric effect, which we found in EuTiO3, was successfully explained and experimentally confirmed by observation of tuning of phonon frequency with magnetic field. Our IR studies of tensile strained EuTiO3 thin films revealed a displacive ferroelectric phase transition near 250 K. Our American colleagues revealed the ferromagnetic order below 4.2 K in the same strained EuTiO3 thin film. In such way we have...
|
|
|
Magnetic properties of R2TIn8 and related tetragonal compounds
Čermák, Petr ; Javorský, Pavel (advisor) ; Isnard, Olivier (referee) ; Honda, Fuminori (referee)
Title: Magnetic properties of R2TIn8 and related tetragonal compounds Author: Petr Čermák Department / Institute: Department of Condensed Matter Physics Supervisor of the doctoral thesis: doc. Mgr. Pavel Javorský, Dr., Department of Condensed Matter Physics Abstract: Intermetallic compounds R2TIn8 (R = rare earth, T = transition metal), commonly called "218" because of stoichiometry, are structurally related to a class of well- known Ce-based heavy-fermions like CeCoIn5 or CeRhIn5. They are located between fully 3D cubic compound (e.g. CeIn3) and quasi-2D "115" superconductors, which makes them ideal candidates to study structural dimensionality effects on various properties. Recent developments in this field showed that it is possible to grow compounds with T = Pd or Pt with "218" stoichiometry. Therefore further study of "218" compounds is desired since much less is known about them compared to "115" compounds. We have focused mainly on the determination of magnetic structures and crystal field effects along the series of Rh based "218" compounds for various rare-earth elements. The single crystals of compounds with R = Nd, Tb, Dy, Ho, Er, Tm, La, Lu, Y were successfully grown. Results of bulk measurements (specific heat, susceptibility) together with magnetic structures determined from several neutron...
|
|
|
Magnetické vlastnosti Nd2RhIn8 a příbuzných tetragonálních sloučenin na bázi Nd
Pajskr, Karel ; Javorský, Pavel (advisor) ; Svoboda, Pavel (referee)
In this work we report on physical properties of Nd2RhIn8 and Nd2IrIn8 rare-earth intermetallic compounds from the RmTnX{2n+3m} compound family. The compounds crystallize in the P4/mmm space group. Both compounds are anti-ferromagnetic with Néel temperatures TN of 10.8 K, 12.5 K respectively and their magnetization curves show characterisic step-like transitions at low temperatures. The measured M(T) and/or M(H) magnetization curves were used to determine magnetic phase diagrams for field along the c-axis. The specific heat of Nd2IrIn8 was analyzed at temperatures T<TN with respect to the dimensionality of the magnetic excitations. In the T>TN region the Schottky specific heat and susceptibility were compared to calculations based on the crystal field scheme in Nd2IrIn8 obtained by ab-initio methods. Powered by TCPDF (www.tcpdf.org)
|
|
|
Magnetocaloric properties of rare-earth compounds
Kaštil, Jiří ; Javorský, Pavel (advisor) ; Kolomiets, Alexander (referee) ; Solzi, Massimo (referee)
This work presents study of magnetocaloric properties of compounds exhibiting unusual magnetic characteristics. Several systems were studied: TbNiAl, (Tb,Y)NiAl, TbNi(Al,In), TbFeAl, GdFeAl, Ni2MnGa based compounds and amorphous metallic alloy Gd-Co-Al-Y. Influence of magneto-crystalline anisotropy on magnetocaloric effect was studied on singlecrystalline sample of TbNiAl compound. The maximum of entropy change was measured with orientation of magnetic field along c axis and compared to polycrystalline sample a 100% increase was observed. Substitutions of Y and In in TbNiAl led to a change of magnetic ordering. Both substitution led to increase of RCP values of studied material. TbFeAl and GdFeAl compounds, characterized by partial disorder of Fe and Al atoms, showed magnetocaloric effect in wide temperature region which led to interesting values of RCP~350 J kg-1 . Very broad region of magnetocaloric effect was also observed on amorphous metallic material of Gd-Co-Al-Y. The effect of Er substitution in Ni2MnGa based compound on its magneto-structural transition, connected with inverse magnetocaloric effect, was studied. The direct measurement method of adiabatic temperature change is described and instrument for such measurement, developed in collaboration with FZU AVČR, v.v.i., is presented.
|
|
|
Extension of instrumental capabilities in cryomagnetic laboratory
Proschek, Petr ; Prokleška, Jan (advisor) ; Javorský, Pavel (referee)
Measurement of elastic properties (thermal expansion and magnetostriction) under (multi)extreme conditions is a difficult task. In the vicinity of the room temperature or above it an abundance of methods is available, with decreasing temperature and adding magnetic field and/or hydrostatic pressure their number is limited. Dilatometric cells (either planparaller or tilted plate design) provide superior sensitivity in low temperatures and applied magnetic fields, however, cannot be used in hydrostatic cell. Common choice for the measurement of thermal expansion under hydrostatic pressure are methods based on strain-gauges, with mediocre sensitivity and more importantly a difficult or even impossible usage at very low temperatures (T ≲ 3 K). Measurement of magnetic properties (especially magnetization) under (multi)extreme conditions is also a difficult task. In the vicinity of temperature 2 K and above it an abundance of methods is available, with decreasing temperature and adding magnetic field and/or hydrostatic pressure their number is limited. VSM system provides great sensitivity, but can not be used under 2 K and hydrostatic pressure. MPMS aparature provides pressure up to 9 GPa (diamond pressure cell), but still we can not apply lower temperatures than 2 K. Our aim is to develop a simple yet sensitive...
|
|
|
Study of properties of iron-containing nanoparticles stressing their application potential
Kubíčková, Lenka ; Kohout, Jaroslav (advisor) ; Javorský, Pavel (referee) ; Mašláň, Miroslav (referee)
Magnetic nanoparticles offer a plethora of application possibilities in various fields of human endeavors. The fundamental understanding of their physical properties, related to the constituent magnetic phase, surface termination, and possible coating, synthesis method, size, shape, or even clustering, is crucial for their effective use and optimization for the intended applications. This thesis aims to contextualize original results, concerning especially the structure and magnetic properties, obtained during fundamental research on nanoparticles of selected iron-containing systems and employ these findings in testing the nanoparticles in chosen applications. Iron presents an ideal constituting element due to its low cost, high abundance in the Earth's crust, exploitability, and low toxicity. The selected systems involved in this thesis comprise iron-containing oxides (ferrites, and various polymorphs of iron(III) oxide, including ε-Fe2O3 and doped counterparts), and sulfides (greigite, chalcopyrite), all of which exhibit specific properties such as magnetic or structural transitions. Among the studied applications, the largest attention is devoted to the use of magnetic nanoparticles as contrast agents in magnetic resonance imaging, and the analysis of their efficacy in contrast enhancement -...
|
|
|
Vibron states in cerium crystalline compounds
Doležal, Petr ; Javorský, Pavel (advisor) ; Detlefs, Blanka (referee) ; Rafaja, David (referee)
This thesis is focused on study of the vibron state in Ce intermetallics. The presence of a vibron state is a consequence of an enhanced magneto-elastic interaction, between phonons and 4f electrons of the Ce ion. The magneto-elastic interaction is usually weak and can be neglected, but here in CeAl2, CePd2Al2, CeCuAl3 and CeAuAl3 is considered to be strong enough, which leads to a bound state called the vibron state. A well determined crystal structure of these compounds is a necessary prerequisite to discuss this unique behaviour. Therefore our investigation is performed on the following levels: A crystal structure study of (Ce,La)Pd2Al2-xGax and CePt2Al2, by low temperature and high temperature X-ray powder diffraction; Investigation of bulk and transport properties of a CePt2Al2 single crystal using the specific heat, magnetisation and electrical resistivity measurements; Symmetry analysis of phonon modes and angular momentum operators in the model Hamiltonian, based on group theory; And finally the study of phonon dispersion curves in CePd2Al2 and LaPd2Al2 single crystals using inelastic X-ray scattering. All these results and preformed analyses lead to the following conclusions: The CePd2Al2 and LaPd2Al2 are incongruently melting phases. We found and described the way which has allowed us to...
|
|
|
Magnetic properties of Ce compounds studied by specific heat
Čermák, Petr ; Javorský, Pavel (advisor)
Materials containing the 4f (rare earth) or 5f (actinides) exhibit a large variety of interesting physical properties. The Ce-based compounds have a special place among the rare-earth compounds. The Ce atom contains only a single f-electron that is responsible for the magnetic behavior. The 4f states in compounds with the heavy rare earths have a well localized character, whereas many Ce-based compounds are on the borderline between the localized and itinerant behavior. These compounds show large variety of the magnetic ground states what is a result of the competition between the long-range order of the RKKY type and the screening of the localized moments by conduction electrons. We observe nonmagnetic states with a mixed valence (between Ce3+ and Ce4+), metallic systems with a long-range order of the Ce moments (ferromagnetic, antiferromagnetic or more complex structures). To analyze the electronic properties, the heat capacity data, and namely their low-temperature part, play an indispensable role. This diploma work comprise the sample preparation of selected cerium compounds, their phase characteristics and the heat capacity measurements at low temperatures (0.4 - 300 K). The main part is focused on the data analysis and comparison with theoretical models.
|
guest ::
RSS feed.