J. Šebek

Specific features of the formation of the ground state in PrB6

The temperature dependences of the thermal conductivity and magnetization of a PrB6 single crystal in magnetic fields of 0–14 T have been measured. An analysis of the data amassed has revealed that, apart from the well-known magnetic phase transitions at T ∼ 7 and ∼4 K, there appears a spontaneous magnetization with a relatively small magnetic moment in PrB6 at temperatures below 20 K, which is initiated by the splitting of the ground state, apparently, due to dynamic structure distortions.

Single-crystal study of U(Co1−xTx)Al compounds for T=Fe and Ni

Abstract UCoAl has a paramagnetic ground state, although the c -axis susceptibility shows a maximum at T max ≈20 K . The low- T state is easily destabilized in a c -axis magnetic field or by doping by suitable elements. We report on first magnetization and specific-heat study of U(Co 0.95 T 0.05 )Al single crystals for T =Fe and Ni in fields up to 10 T . In both cases we confirmed the strong uniaxial anisotropy typical also for UCoAl. The Ni substitution yields a considerable increase of B c whereas UCo 0.95 Fe 0.05 Al is ferromagnetic below 30 K . A detailed analysis of specific-heat data for UCoAl and UCo 0.95 Ni 0.05 Al in 0 and 10 T allows extracting entropy contributions due to spin fluctuations.

Investigation of in-gap states in SmB6 ∗ )

We have investigated the low temperature properties of the narrow-gap semiconductor SmB6 by means of electrical resistivity and specific heat measurements. Results imply that the residual resistivity below about 3 K is non-activated and the corresponding state, which is formed within the in-gap states, has a metallic-like nature. Heat capacity measurements confirmed the metallic-like properties of the in-gap states and revealed, moreover, an enhancement of the specific heat below 2 K which is more expressive for the sample with a lower amount of impurities. The observed behaviour can be attributed to the formation of a coherent state within the in-gap states of this compound.

Low temperature AC susceptibility of UCoGe crystals

The recent discovery of the magnetic ordering and the superconductivity in the UCoGe motivate the more detailed investigation of its properties. In this paper we study three mono-crystalline samples (prepared by different techniques) by means of the AC susceptibility. For this measurement the Physical Property Measurement System (PPMS) from Quantum Design was used, using the simple self–made extension allowing transparent (for the user) measurement of AC susceptibility with the 3He option (major aspect will be discussed). Our result show strong dependence of the single crystal physical properties (possible suppression of superconductivity and magnetism) on the method used for sample preparation.

Magnetic Phase Transitions in U3Al2Si3

Magnetic phase transitions to magnetic ordered state have been investigated on a U3Al2Si3 textured sample by measurements of electrical resistivity, magnetization and specific heat in the temperature range 2 – 70 K for magnetic field up to 14 T. The magnetic measurements confirmed a magnetic ordering at Tc ≈ 40 K but on contrary to previous reports we have observed a remarkable magnetic anisotropy of the easy plane type with magnetization Me0=1.6μB/f.u. extrapolated to zero field for magnetic easy axis and Mho= 0.2μB/f.u. for hard axis. The magnetic phase transition is accompanied by broad anomalies in resistivity and heat capacity data. The applied magnetic field smears out the maximum in resistivity at T = 26 K and the maximum in specific heat at T = 36 K as well as shifts them to higher temperatures. Magnetic contribution to specific heat Cmag has maximum at the ordering temperature Tc = 39 K and surprisingly it is non-zero up to about 100 K well above Tc. The magnetic entropy saturates to value 35 J/mol K that is close to 3 R ln 4. Around the ordering temperature Tc the entropy is about 3 R ln 2. There is a surplus of the specific heat above Tc and an anomaly in magnetization at 60 K that is induced by the magnetic field. It could be an evidence of some kind of order that has not been observed yet.

Anisotropy of the hyperfine enhanced nuclear ferromagnet PrNi5

The magnetic ordering in PrNi5 has been investigated in external fields up to 72 mT and at temperatures from below the nuclear ordering temperature up to about 20 mK. The anisotropy of magnetization was observed for two monocrystalline samples with different crystallographic axes parallel to the external field. The results show that the nuclear magnetic moments of Pr order in the basal plane of the hexagonal crystal structure.

Influence of Ru on the Magnetic Properties of Y2T17 (T = Co, Fe)

Influence of Ru substitution on magnetic properties of Y2Fe17 and Y2Co17 (hexagonal structure of the Th2Ni17-type) has been studied on single-crystalline samples. Whereas in the Y2Fe17-xRux system ferromagnetism is transformed rapidly into antiferromagnetism with a field-induced metamagnetic transition, in Y2Co17-xRux only a monotonous decrease of spontaneous magnetic moment, Curie temperature and first anisotropy constant is observed.

Magnetic anisotropy and hidden martensitic transition in V3Si

Magnetization, electrical resistivity and heat capacity have been measured on a single crystal V3Si in the range of (2-25) K and in magnetic field up to 14 T. A different behavior of magnetization for two orientations of the crystal has been found. In one orientation the magnetization displays a clear ferromagnetic character and below Tc coexistence of ferro-magnetism and superconductivity with a peak-effect in the vicinity of upper critical field Hc2. The specific heat measurements show sharp lambda anomaly corresponding to a transition to superconductive state and an additional anomaly around 15 K when applied field suppresses the superconductivity below this temperature.

Magnetic Anisotropy of PrNi5 Single Crystal at Millikelvins

Magnetic ordering in the hyperfine enhanced nuclear ferromagnet PrNi5 was investigated in external magnetic fields up to 72 mT at temperatures from below the ordering temperature up to about 20 mK. An anisotropy of the magnetization and ac susceptibility was observed for two single crystalline samples having a or c crystallographic axis parallel to the external field. The results show that the nuclear magnetic moments of Pr are ordered in the basal plane of the hexagonal crystal structure of PrNi5.

Calculation of Magnetic Properties and Specific Heat for the Nuclear Enhanced Ferromagnet PrNi5

A general model hamiltonian for hyperfine enhanced magnetic materials is presented. The use of the model is not limited only to the nuclear regime, but the model treats also situations when the strength of the exchange interaction is close to and above the threshold value. The model is solved numerically for PrNi5 and the results are compared with recent ultra-low temperature experiments. In particular, the behavior of magnetization, susceptibility, and specific heat of PrNi5 is studied for two external field geometries and anisotropic properties are pointed out. The effect of the quadrupole interaction is examined in detail. The question of the correlations among interacting ions is also addressed.