Alexander V. Andreev

Magnetic phase transitions and magnetoelastic phenomena in UNiGa under pressure

Thermal expansion and magnetostriction of UNiGa single crystal have been measured at the range of pressures up to 2.2 GPa and magnetic fields up to 1.5 T. The results are reported with a special emphasis on anomalies accompanying the magnetic phase transitions and the effect of pressure on the critical parameters (transition temperature and magnetic field). Formation of a new antiferromagnetic phase induced by pressures above 1.8 GPa is revealed. Analysis of results enabled us to construct P-T magnetic phase diagrams for fields up to 1.5 T. The magnetoelastic effects are discussed in terms of marked variations in electronic structure accompanying transitions between different magnetic structures with various couplings of U magnetic moments along the c -axis and involvement of strong spin-orbit interaction.

Electronic properties ofα−UH3stabilized by Zr

Pure hydride of the α-UH3 type without any β-UH3 admixture was prepared by high-pressure hydrogenation of bcc U stabilized by Zr. Such material, characterized by a general formula (UH3)1-x Zr x , is stable in air at ambient and elevated temperatures. H release is observed between 400-450 °C similar to β-UH3. Its stability allowed to measure magnetic properties, specific heat, and electrical resistivity in a wide temperature range. Despite rather different crystal structure and inter-U spacing, the electronic properties are almost identical to β-UH3. Its ferromagnetic ground state with Curie temperature TC ≈ 180 K (weakly and non-monotonously dependent on Zr concentration) and U moments of 1.0 μB indicate why mixtures of α- and β-UH3 exhibited only one transition. Magnetic ordering leads to a large spontaneous magnetostriction ωs = 3.2*10-3, which can be explained by the increase of the spin moment between the paramagnetic (Disordered Local Moment) and the ferromagnetic state. The role of orbital moments in magnetism is indicated by fully relativistic electronic structure calculations.

Spin-lattice effects in selected antiferromagnetic materials (Review Article)

Spin-lattice effects play an important role in many magnetic materials. In this short review, we give some examples of such effects studied in low-dimensional, frustrated as well as uranium-based antiferromagnets. Utilizing ultrasound measurements at low temperatures and high magnetic fields provides valuable information on the spin-strain interactions. Specifically phase transformations and critical phenomena in magnetic systems with strong spin-lattice interactions are fruitful grounds for sound-velocity and sound-attenuation measurements.

Pressure induced anisotropic magnetovolume phenomena in Lu2Fe17 Intermetallics

Magnetisation and magnetic susceptibility of a Lu2Fe17 single crystal have been studied under hydrostatic pressure up to 1.2 GPa at temperatures down to 5 K using a SQUID magnetometer. The ferromagnetic phase of Lu2Fe17 is suppressed rapidly above a critical pressure P C = 0.4 GPa in the whole temperature range below the critical temperature T C . A magnetic phase diagram of Lu2Fe17 has been constructed using results of the magnetic susceptibility measurements under pressure. A pressure induced incommensurate antiferromagnetic phase exhibits metamagnetic transitions with the increasing critical magnetic field H C under pressure. Taking into account recent neutron diffraction data, the pressure induced anisotropic changes of the lattice parameters of the Lu2Fe17 are discussed.

Magnetic properties of a URhSi single crystal

Abstract The ferromagnetic behavior of URhSi (TiNiSi-type orthorhombic crystal structure) resembles its isostructural analogue URhGe, the well-known “ferromagnetic superconductor”. We report on first results of magnetization measurements of a URhSi single crystal with respect to a magnetic field and temperature. The magnetization data and specific-heat behavior point to a Curie temperature T C =10.5 K . At lower temperatures we observe a strong magnetic anisotropy with the dominant spontaneous magnetic moment (0.47μB) along the c-axis (similar to URhGe). Along the a- and b-axis, much smaller components are indicated. A strong anisotropy is observed also in paramagnetic range.

Magnetic Anisotropy of U2(Fe1-xCox)13.6Si3.4 (0≤x≤0.5)

Single crystals of the U2(Fe1-xCox)13.6Si3.4 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) solid solutions (hexagonal crystal structure of the Th2Ni17 type) were prepared for first time and their magnetization was measured at 4.2 K along the principal axes. The alloys exhibit a large basal-plane type magnetocrystalline anisotropy. Strong non-linearity of hard-direction magnetization curves and especially first-order magnetization process (FOMP) at low x values needs for description a large second anisotropy constant K2 which cannot be provided by the Fe sublattice and is, therefore, a strong evidence for magnetic ordering of U sublattice. K2 turns from negative to positive with increasing x at x = 0.2 which is reflected in disappearance of FOMP. The molecular spontaneous magnetic moments Ms decreases from 23 to 14 μB/f.u. whereas the Curie temperatures TC increases from 530 K to 595 K with increasing x from 0 to 0.5.

Collapse of 5 f -Electron Ferromagnetism in UPtAl Under High Pressures

UPtAl exhibits a ferromagnetic ordering of U magnetic moments at temperatures below T C =42.5 K. The magnetic-ordering transition is accompanied by an anomaly in the temperature dependence of electrical resistivity. This allows us to determine the value of Curie temperature from 𝜌 vs. T data that can be measured at very high pressures, at which reliable magnetization measurements are difficult. We report on resistivity measurements performed on an UPtAl single crystal under hydrostatic pressures p h 8 GPa. It was observed that the initial increase of T C with p becomes gradually reduced for p >2 GPa until the maximum T C value of , 48 K is reached between 4 and 6 GPa that is followed by a progressively developing downturn of the T C ( p ) curve. The latter result is attributed to the approaching collapse of the U 5 f -moment ferromagnetism. Low-temperature resistivity data point to a rapidly reduced magnetic anisotropy at highest pressures.

NMR and NQR studies of UCoAl with 5f-band metamagnetism

Abstract We have carried out NMR and NQR measurements of 27 Al and 59 Co in UCoAl, in which occurs a meta-magnetic transition in a small magnetic field of 0.8 T below 16 K. 27 Al NMR signal and two kinds of 59 Co NMR signals were observed under an external field from 2.4 to 5 T with a fixed frequency of 40 MHZ at 77 K. The temperature dependence of the nuclear spin-lattice relaxation rate in 59 Co NQR was also measured from 1.4 to 100 K.

Synthesis and structural/physical properties of U3Fe2Ge7: a single-crystal study.

A single crystal of U3Fe2Ge7 was synthesized by the tin-flux method, and its structural and electronic properties were studied. The compound crystallizes in the orthorhombic crystal structure of La3Co2Sn7 type with two Wyckoff sites for the U atoms. U3Fe2Ge7 displays a ferromagnetic order below TC = 62 K. Magnetization measurements in static (up to 14 T) and pulsed (up to 60 T) magnetic fields revealed a strong two-ion uniaxial magnetic anisotropy. The easy magnetization direction is along the c axis and the spontaneous magnetic moment is 3.3 μB per formula unit at 2 K. The moment per Fe atom is 0.2 μB, as follows from Mössbauer spectroscopy. The magnetic moments are oriented perpendicular to the shortest inter-uranium distances that occur within the zigzag chains in the ab plane, contrary to other U-based isostructural compounds. The magnetization along the a axis reveals a first-order magnetization process that allows for a quantitative description of the magnetic anisotropy in spite of its enormous energetic strength. The strong anisotropy is reflected in the specific heat and electrical resistivity that are affected by a gap in magnon spectrum.

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.