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Vladimír Sechovský

Publications and source records attributed to Vladimír Sechovský.

32 records · Page 2Linked to original sources
arXiv2017

Electrical resistivity across the tricriticality in itinerant ferromagnet

We investigate the discontinuous ferromagnetic phase diagram near tricritical point in UCo 1-x Ru x Al compounds by electrical resistivity measurements. Separation of phases in UCo 0.995 Ru 0.005 Al at ambient pressure and in UCo 0.990 Ru 0.010 Al at pressure of 0.2 GPa and disappearance of ferromagnetism at 0.4 GPa is confirmed. The exponent of temperature dependence of electrical resistivity implies change from Fermi liquid behavior to non-Fermi liquid at 0.2 GPa and reaches minimum at 0.4 GPa. Our results are compared to results obtained on the pure UCoAl and explanation for different exponents is given.

cond-mat.mtrl-sci↗
arXiv2017

Quantum ferromagnet in the proximity of the tricritical point

Echoes of quantum phase transitions (QPTs) at finite temperatures are theoretically and experimentally challenging and unexplored topics. Particularly in metallic quantum ferromagnets the experimental investigations are hampered by an intricate preparation of sufficiently pure samples and the access to the proper coordinates in parameter space. The present study shows that it is possible to tune a specific system at easily accessible conditions to the vicinity of its quantum phase transition. The physics is demonstrated on Ru-doped UCoAl, driven by pressure or substitution to and across the tricritical point and follows the first-order transition line to the theoretically presumed QPT. These findings open the possibilities for further in-depth studies of classical and quantum critical phenomena at easily reachable conditions.

cond-mat.mtrl-sci↗
arXiv2016

Uranium ferromagnet with negligible magnetocrystalline anisotropy - $\mathrm{U_{4}Ru_{7}Ge_{6}}$

Strong magnetocrystalline anisotropy is a well-known property of uranium compounds. The almost isotropic ferromagnetism in $\mathrm{U_{4}Ru_{7}Ge_{6}}$ reported in this paper represents a striking exception. We present results of magnetization, AC susceptibility, thermal expansion, specific heat and electrical resistivity measurements performed on a $\mathrm{U_{4}Ru_{7}Ge_{6}}$ single crystal at various temperatures and magnetic fields and discuss them in conjunction with results of first-principles electronic-structure calculations. $\mathrm{U_{4}Ru_{7}Ge_{6}}$ behaves as an itinerant 5$f$-electron ferromagnet ($T_{\mathrm{C}}= 10.7 K$, $μ_{\mathrm{S}}= 0.85 μ_{\mathrm{B}}/f.u.$ at 1.9 K. The ground-state easy-magnetization direction is along the [111] axis of the cubic lattice. The anisotropy field $μ_{0}H_{\mathrm{a}}$ along the [001] direction is only of $\sim0.3 T$, which is at least 3 orders of magnitude smaller value than in other U ferromagnets. At $T_{r}=5.9 K$ the easy magnetization direction changes for [001] which holds at temperatures up to $T_{\mathrm{C}}$. The magnetoelastic interaction induces a rhombohedral (tetragonal) distortion of the paramagnetic cubic crystal lattice in case of the [111]([001]) easy-magnetization direction. The rhombohedral distortion is connected with two crystallographically inequivalent U sites. The ab initio calculated ground-state magnetic moment of $1.01 μ_{\mathrm{B}}/f.u.$ is oriented along [111]. The two crystallographically inequivalent U sites are a consequence of spin-orbit coupling of the U 5$f$-electrons. In the excited state which is only 0.9 meV above the ground state the moment points to the [001] direction in agreement with experiment.

cond-mat.str-el↗
arXiv2016

Peculiar Magnetism of UAu$_{2}$Si$_{2}$

Single-crystalline UAu$_2$Si$_2$ has been grown by a floating-zone melting method, and its magnetic, thermal and transport properties have been investigated through measurements of magnetization, specific heat and electrical resistivity to reveal its peculiar magnetism. It is shown that UAu$_2$Si$_2$ undergoes a second-order phase transition at \tm = 19 K, which had been believed to be ferromagnetic ordering in the literature, from a paramagnetic phase to an uncompensated antiferromagnetic phase with spontaneous magnetization along the tetragonal $c$-axis (the easy magnetization direction). The magnetic entropy analysis points to the itinerant character of 5f electrons in the magnetic ordered state of UAu$_2$Si$_2$ with large enhancement of the electronic specific heat coefficient of $γ$ $\sim$ 150 mJ/K$^2$mol at 2 K. It also reveals the relatively isotropic crystalline electric field effect of this compound, with contrast to the other relative isostructural compounds. The observed magnetization curves strongly suggest that there is a parasitic ferromagnetic component developing below $\sim$ 50 K in high coercivity with the easy axis along the tetragonal $c$-axis. The results are discussed in the context of evolution of magnetism within the entire family of isostructural U$T_2$Si$_2$ compounds.

cond-mat.str-el↗
arXiv2016

Pressure-induced quantum phase transition in the itinerant ferromagnet UCoGa

In this paper, we report the results of a high pressure study of the itinerant 5f-electron ferromagnet UCoGa. The work is focused on probing the expected ferromagnet-to-paramagnet quantum phase transition induced by high pressure and on the general features of the P-T(-H) phase diagram. Diamond anvil cells were employed to measure the magnetization and electrical resistivity under pressures up to ~ 10 GPa.At ambient pressure, UCoGa exhibits collinear ferromagnetic ordering of uranium magnetic moments μU ~ 0.74 μB (at 2 K) aligned along the c-axis of the hexagonal crystal structure below Curie temperature TC = 48K. With the application of pressure, gradual decrease of both, TC and the saturated magnetic moment, has been observed up to pressures ~ 6 GPa. This is followed by a sharp drop of magnetic moment and a sudden disappearance of the magnetic order at the pressure of 6.5 GPa, suggesting a first-order phase transition, as expected for a clean system. The low temperature power law dependence of the electrical resistivity shows distinct anomalies around the ~ 6 GPa, consistent with the pressure evolution of the magnetic moment and the ordering temperature. The tricritical point of the UCoGa phase diagram is located at approximately ~ 30 K and ~ 6 GPa.

cond-mat.mtrl-sci↗
arXiv2015

Magnetism, Superconductivity and Quantum Criticality in the Multi-Site Cerium Heavy Fermion Compound Ce3PtIn11

The properties of the novel heavy fermion superconductor Ce$_3$PtIn$_{11}$ are investigated by thermodynamic and transport measurements at ambient and under hydrostatic pressure. At ambient pressure the compound exhibits two successive magnetic transitions at $T_1$ $\approx$ 2.2 K and $T_{\rm N}$ $\approx$ 2 K into antiferromagnetically ordered states and enters into a heavy fermion superconducting phase below $T_{\rm c}$ $\approx$ 0.32 K. The coexistence of long-range magnetic order and superconductivity is discussed in the context of the existence of the two crystallographically inequivalent Ce-sites in the unit cell of Ce$_3$PtIn$_{11}$. The experimental data allow us to construct the pressure-temperature phase diagram.

cond-mat.str-el↗
arXiv2015

Evolution of ferromagnetic and non-Fermi liquid state with doping:the case of Ru doped UCoGe

We have investigated the impact of Ru substitution for Co on the behavior of the ferromagnetic superconductor UCoGe by performing x-ray diffraction, magnetization, specific heat and electrical resistivity measurements on polycrystalline samples of the $\mathrm{UCo}_{1-x}\mathrm{Ru}_{x}\mathrm{Ge}$ series ($0\geq x\leq0.9$). The initial Ru substitution up to $x\approx0.1$ leads to a simultaneous sharp increase of the Curie temperature and spontaneous magnetization up to maximum values of $T_{\mathrm{C}}=8.6 K$ and $M_{\mathrm{S}}=0.1 μ_{\mathrm{B}}$ per formula unit, respectively, whereas superconductivity vanishes already for $x\approx0.03$. Further increase of the Ru content beyond $x\approx0.1$ leads to a precipitous decrease of both, $T_{\mathrm{C}}$ and $M_{\mathrm{S}}$ towards a ferromagnetic quantum critical point (QCP) at $x_{\mathrm{cr}}=0.31$. Consequently the $T-x$ magnetic phase diagram consists of a well-developed ferromagnetic dome. We discuss the evolution of ferromagnetism with $x$ on the basis of band structure changes due to varying 5$f$-ligand hybridization. This scenario is supported by the results of electronic structure calculations and consideration of the simplified periodic Anderson model. The analysis of the temperature dependencies of the electrical resistivity and heat capacity at low temperatures of the samples in the vicinity of the QCP reveals a non-Fermi liquid behavior and assigns the ferromagnetic quantum phase transition to be most likely of a continuous Hertz-Millis type.

cond-mat.str-el↗
arXiv2015

Single crystal study of layered U$_{n}$RhIn$_{3n+2}$ materials: case of the novel U$_{2}$RhIn$_{8}$ compound

We report on the single crystal properties of the novel U$_{2}$RhIn$_{8}$ compound studied in the context of parent URhIn$_{5}$ and UIn$_{3}$ systems. The compounds were prepared by In self-flux method. U$_{2}$RhIn$_{8}$ adopts the Ho$_{2}$CoGa$_{8}$-type structure with lattice parameters a $= 4.6056(6)$ Å and c $= 11.9911(15)$ Å. The behavior of U$_{2}$RhIn$_{8}$ strongly resembles that of the related URhIn$_{5}$ and UIn$_{3}$ with respect to magnetization, specific heat and resistivity except for magnetocrystalline anisotropy developing with lowering dimensionality in the series UIn$_{3}$ vs. U$_{2}$RhIn$_{8}$ and URhIn$_{5}$. U$_{2}$RhIn$_{8}$ orders antiferromagnetically below T$_{\textrm{N}}$ $= 117$ K and exhibits a slightly enhanced Sommerfeld coefficient $γ= 47$ mJ$\cdot$mol$^{-1}\cdot$K$^{-2}$. Magnetic field leaves the value of Néel temperature for both URhIn$_{5}$ and U$_{2}$RhIn$_{8}$ unaffected up to 9 T. On the other hand, T$_{\textrm{N}}$ is increasing with applying hydrostatic pressure up to 3.2 GPa. The weak temperature dependence of $χ(T)$ in all studied compounds might be attributed to the mainly itinerant nature of 5f electrons. The character of uranium 5f electron states of U$_{2}$RhIn$_{8}$ was studied by first principles calculations based on the density functional theory. The overall phase diagram of U$_{2}$RhIn$_{8}$ is discussed in the context of magnetism in the related URhX$_{5}$ and UX$_{3}$ (X = In, Ga) compounds.

cond-mat.str-el↗
arXiv2015

Anisotropic magnetic properties of URhIn$_{5}$ compound

We report on synthesis and anisotropic physical properties of URhIn$_{5}$. High quality single crystals were grown in In-flux. The compound undergoes a second order phase transition into an antiferromagnetic state at T$_{\textrm{N}}$ = 98 K. The transition is field independent up to 9 T. An increase of the resistivity $ρ$ with j along the [100], [110] and [001] tetragonal axis indicates a spin-density-wave induced order with the gap opening first along the [001] direction. The magnetic susceptibility $χ$ = M/H exhibits a strong anisotropy. Above T $=$ 200 K, $χ$(T) follows Curie-Weiss law with the effective moment of $μ_{\textrm{eff}}$ = 3.71 $μ_{\textrm{B}}/$U and the Weiss temperatures of $θ_{\textrm{P}}^{[100]} = -900$ K and $θ_{\textrm{P}}^{[001]} = -500$ K for H $\parallel$ [100] and H $\parallel$ [001] respectively. The characteristic Kondo-like temperature for URhIn$_{5}$ yields T$_{\textrm{K}} =$ 125 K.

cond-mat.str-el↗
arXiv2013

Influence of symmetry on Sm magnetism studied on SmIr2Si2 polymorphs

Polycrystalline samples of SmIr2Si2 formed at room temperature both the low temperature phase (LTP) and the metastable high temperature phase (HTP), respectively, depending on the heat treatment. The samples were studied by X-ray powder diffraction, DTA, specific-heat and magnetization measurements with respect to temperature and magnetic field. The first order LTP to HTP polymorphic phase transition has been determined showing the huge temperature hysteresis of 264°C caused by the high energy barrier due to the change of stacking of Sm, Ir and Si basal plane sheets within the transition. Both polymorphs show indications of antiferromagnetic order at low temperatures. The considerably different magnetic phase transitions determined for the LTP and HTP confirm the strong influence of crystal structure symmetry on magnetism in the two polymorphs. The magnetism in SmIr2Si2 exhibits typical features caused by the specific behavior of Sm3+ ion characterized by energy nearness of the ground state and first excited state and crystal field influence. Interpretation of experimental results is corroborated by results of ab initio electronic structure calculations.

cond-mat.str-el↗
arXiv2013

Complex Magnetic Phase Diagram of a Geometrically Frustrated Sm Lattice: SmPd2Al3 case

Magnetism in SmPd2Al3 was investigated on a single crystal by magnetometry and neutron diffraction. SmPd2Al3 represents a distinctive example of the Sm magnetism exhibiting complex magnetic behavior at low temperatures with four consecutive magnetic phase transitions at 3.4, 3.9, 4.3 and 12.5 K. The rich magnetic phase diagram of this compound reflects the specific features of the Sm3+ ion, namely the energy nearness of the ground-state multiplet J = 5/2 and the first excited multiplet J = 7/2 in conjunction with strong crystal field influence. Consequently, a significantly reduced Sm magnetic moment in comparison with the theoretical Sm3+ free-ion value is observed. Despite the strong neutron absorption by natural samarium and the small Sm magnetic moment (~ 0.2 μB) we have successfully determined the magnetic k-vector (1/3, 1/3, 0) of the phase existing in the temperature interval 12.5 - 4.3 K. This observation classifies the SmPd2Al3 compound as a magnetically frustrated system. The complex magnetic behavior of this material is further illustrated by kinetic effects of the magnetization inducing rather complicated magnetic structure with various metastable states.

cond-mat.str-el↗
arXiv2012

Superconductivity in the YIr2Si2 and LaIr2Si2 Polymorphs

We report on existence of superconductivity in YIr2Si2 and LaIr2Si2 compounds in relation to crystal structure. The two compounds crystallize in two structural polymorphs, both tetragonal. The high temperature polymorph (HTP) adopts the CaBe2Ge2-structure type (space group P4/nmm) while the low temperature polymorph (LTP) is of the ThCr2Si2 type (I4/mmm). By studying polycrystals prepared by arc melting we have observed that the rapidly cooled samples retain the HTP even at room temperature (RT) and below. Annealing such samples at 900C followed by slow cooling to RT provides the LTP. Both, the HTP and LTP were subsequently studied with respect to magnetism and superconductivity by electrical resistivity, magnetization, AC susceptibility and specific heat measurements. The HTP and LTP of both compounds respectively, behave as Pauli paramagnets. Superconductivity has been found exclusively in the HTP of both compounds below Tsc (= 2.52 K in YIr2Si2 and 1.24 K in LaIr2Si2). The relations of magnetism and superconductivity with the electronic and crystal structure are discussed with comparing experimental data with the results of first principles electronic structure calculations.

cond-mat.supr-con↗
arXiv2010

Low magnetic field phase diagram of UCoGe

We propose an alternate scenario for the relation between ferromagnetism (FM) and superconductivity (SC) in UCoGe at ambient pressure. On the basis of neutron polarimetry measurement performed in strictly zero field on a single-crystal at ambient pressure, we claim that the SC phase does exist in spite of lack of long-range FM order. Moreover, the SC regime dominates to the paramagnetic (PM) state in small external magnetic fields. Above the critical SC temperature, TSC = 0.64 K, the zero-field state is characterized by a regime with strong FM spin fluctuations, suggesting the proximity of the FM quantum critical point. We present an ambient-pressure phase diagram of UCoGe based on our resistivity, magnetization and neutron polarimetry results.

cond-mat.supr-con↗