A. V. Andreev

Thermal properties of metamagnetic transition in heavy-fermion systems

Abstract In order to investigate the metamagnetic anomaly (MA) observed in several heavy-fermion compounds, field dependence of specific heat C(B) and magnetocaloric effect have been measured in the prototypical 4f-electron compound CeRu2Si2, and compared to that of a 5f-electron metamagnet UCoAl. In the C/T versus B curves, an unexpected double-peak structure is found in CeRu2Si2, suggesting that MA is closely related to a sharp peak in the density of states of the hybridized quasiparticle bands. On the other hand, for UCoAl, only a small decrease in C/T is observed, although both compounds exhibit similar bulk magnetic properties.

U Ternaries with ZrNiAl Structure - Lattice Properties.

The bonding properties of UTX compounds having the hexagonal ZrNiAl-type structure display a pronounced anisotropy. The compressibility and thermal expansion reach values several times higher for the basal plane than along the c-axis. This situation also occurs for the Th-containing counterparts, whereas it is absent in isostructural materials based on rare earths. This phenomenon can be attributed to the participation of the delocalized 5f states in the bonding, leading to compression of the 5f charge towards the basal plane. This scenario has been investigated theoretically by ab-initio band structure calculations performed for URhAl.

Specific-heat anomaly of metamagnetism on PrFe4P12 and UCoAl

Abstract The field dependence of specific heat has been measured at low temperatures in order to study two different types of f-electron metamagnetic systems. The antiferromagnet PrFe4P12 with Neel temperature of 6.4 K shows a sharp peak structure in C/T at the spin flip metamagnetic transition. In the high field state, the magnetic contribution Cm/T shows an anomalous huge value ∼1.4 J/K 2 mol . UCoAl shows the metamagnetic transition from paramagnetic ground state at ∼1 T, where C/T at 0.5 K shows an apparent decrease reflecting a change of the density of states. In the low-field state, ferromagnetic spin-fluctuation dominates in an anomalous temperature dependence of C.

Specific-Heat Anomaly around Metamagnetic Transition in UCoAl

We have measured the specific heat and magnetocaloric effect of an UCoAl single crystal down to ∼0.1 K in fields applied along the c -axis to investigate the thermodynamic aspects of the metamagnetic behavior. Hysteresis behavior observed in both physical quantities below ∼3 K confirms the first order character of the metamagnetic transition. We have found that an apparent decrease of the electronic part of specific heat C e / T by 20% at 0.25 K with increasing field across the metamagnetic transition field of ∼1 T, evidencing the change in the density-of-states at the Fermi energy. C e / T gradually decreases with increasing temperature in zero field. This anomalous temperature dependence can be explained by taking into account a spin fluctuation contribution in the low-field state.

Magnetic properties of UFeGe

UFeGe crystallizes in the orthorhombic structure of the TiNiSi type with a monoclinic distortion. Magnetic susceptibility and electrical resistivity measurements point to paramagnetic behaviour with characteristic features of spin fluctuations. Electronic structure calculations reveal a strong hybridization of the 5f and 3d states with the Fermi energy located in a minimum of the density of states.

Band metamagnetism and related phenomena in Er(Co1-xSix)2

We report on magnetic, transport, and cohesive properties of Er(Co1−xSix)2 compounds in varying external conditions (T,H,p). The localized Er 4f magnetic moments in ErCo2 order ferromagnetically below TC=33 K. The Co 3d band states form no moment above TC, but below TC the majority and minority 3d sub-bands split due to strong 4f−3d exchange interaction. Consequently, Co moments of ∼1 μB appear coupled antiparallel to the Er moments. This is accompanied by a ∼50% resistivity drop and an abrupt lattice expansion. The sharp magnetization, resistivity, and volume anomalies in the critical temperature region indicate a first order magnetic phase transition. Detailed comparative studies reveal that the onset of Co magnetizm appears at T0∼(TC−1) K. Si substitution for Co enhances both, TC (up to 66 K for x=0.1) and ΔT=TC−T0, but the lattice parameter remains unchanged. In opposite, both the magnetovolume and resistivity anomaly at T0 become reduced. Pressure effect on TC(T0) is large and negative, ∂ ln TC/∂p∼−2...

Evidence for an extended critical region near the metamagnetic transition of UCoAl

Electrical resistivity and ac magnetic susceptibility studies of the 5f-metamagnet UCoAl reveal the presence of strong spin fluctuation effects for temperatures below 30 K and magnetic fields below the critical metamagnetic field μ0Hc≈1 T, where a non-Fermi liquid term ∼T3/2 is prominent in the electrical resistivity. A step-wise increase of the residual resistivity and an increasingly prominent Fermi liquid T2 term of the resistivity is observed as the field is increased though the metamagnetic transition.

Unusual 5f magnetism in the U2Fe3Ge ternary Laves phase: a single crystal study

Magnetic properties of the intermetallic compound U(2)Fe(3)Ge were studied on a single crystal. The compound crystallizes in the hexagonal Mg(2)Cu(3)Si structure, an ordered variant of the MgZn(2) Laves structure (C14). U(2)Fe(3)Ge displays ferromagnetic order below the Curie temperature T(C) = 55 K and presents an exception to the Hill rule, as the nearest inter-uranium distances do not exceed 3.2 Å. Magnetic moments lie in the basal plane of the hexagonal lattice, with the spontaneous magnetic moment M(s) = 1.0 μ(B)/f.u. at T = 2 K. No anisotropy within the basal plane is detected. In contrast to typical U-based intermetallics, U(2)Fe(3)Ge exhibits very low magnetic anisotropy, whose field does not exceed 10 T. The dominance of U in the magnetism of U(2)Fe(3)Ge is suggested by the (57)Fe Mössbauer spectroscopy study, which indicates very low or even zero Fe moments. Electronic structure calculations are in agreement with the observed easy-plane anisotropy but fail to explain the lack of an Fe contribution to the magnetism of U(2)Fe(3)Ge.

Hydrogen-induced changes in TbNiAl

Neutron diffraction was used to determine the deuterium sites in TbNiAlD1.28 and TbNiAlD0.8. Both samples were found to crystallize in the orthorhombic Amm2 space group. Our analysis reveals that deuterium in TbNiAlD1.28 occupies positions at x=0 and x=1/2 only, while TbNiAlD0.8 shows the formation of two different phases. For TbNiAlD1.28, additional magnetic reflections are observed below 16 K indicating antiferromagnetic order. On the other hand, at 10 K, only one of the TbNiAlD0.8 phases was found to be magnetic. In addition, our study of the localized vibrations of hydrogen in TbNiAlH1.39 and in its non-magnetic analog YNiAlH1.35 using inelastic neutron scattering supports the existence of two different hydrogen sites in the saturated hydrides.

Magnetism and related phenomena in RE(Co1-xSix)2 compounds

Abstract We report on crystal structure, magnetic, transport and dilatometric studies of the pseudobinary compounds RE(Co 1− x Si x ) 2 with RE = Nd, Ho and Er for x ≤ 0.15. The lattice volume of the Ho and Er-based compounds is almost composition invariable, whereas in Nd(Co 1− x Si x ) 2 for x between 0 and 0.15 it increases by ∼ 7%. Already small Si substitutions for Co ( x ≤ 0.075) induce a dramatic increase of T c both in the Ho and Er compounds without apparent loss of the first order character of the magnetic phase transition, whereas opposite changes of T c are observed in Nd analogues. These results, together with variations of resistivity and magnetovolume anomalies at T c and of the Co magnetic moment observed in all three systems is discussed in terms of expected changes of electronic structure and their influence on the hierarchy of exchange interactions and formation of the Co moment in this class of materials.