Influence of parity-forbiddenness in the onset of mixed valence state in Sm1−x GdxS

The electronic structure of Sm and Gd in Sm_1−x Gd_xS has been experimentally studied by the x-ray shift method. Besides the well known strong increase of Sm valence at x≈0.15 (associated with the electronic transition to the mixed valence state), a decrease of Sm valence for x⩾0.9 has been observed for the first time. This phenomenon is explained as due to the 4f-5d hybridization on neighboring Sm atoms involved in the onset of the mixed valence state, and is interpreted as a manifestation of parity violation forbiddenness. Fiz. Tverd. Tela (St. Petersburg) 39, 1017–1019 (June 1997)     

Metals near a magnetic instability

Zero-temperature magnetic phase transitions exhibit an abundance of nearly critical magnetic fluctuations that allow to probe the traditional concepts of the metallic state. For the prototypical heavy-fermion compound, CeCu_6−x Au _x , a breakdown of the Fermi-liquid properties may be tuned by Au concentration, hydrostatic pressure, or magnetic field. The d-electron weak itinerant ferromagnet ZrZn₂, on the other hand, was recently found to display superconductivity in coexistence with ferromagnetism.     

Field and pressure response of Yb compounds close to a quantum critical point

YbCu_5−x Al _x provides the possibility to tune ground state properties by a change of the valence due to the Cu/Al substitution, by pressure as well as by the application of a magnetic field. Near to the critical concentration x _cr≈1.5 non-Fermi-liquid properties (NFL) are obvious, obeying hyperscaling. If magnetic order sets in for x>1.5, the application of moderate magnetic fields quenches order and again NFL features become evident. Hyperscaling in this case indicates strongly interacting spin fluctuations.     

Magnetic state of nickel-zinc ferrites at high zinc concentrations

Neutron diffraction and magnetic methods are used to investigate ferrites from the system Zn_xFe_1−x [Ni_1−x Fe_1+x ]O₄. In these investigations, no diffraction effects were observed that would indicate ordered positions for the perpendicular projections of spins at 4.2 K over the entire ferrimagnetic range of concentrations x. However, the high-field magnetic susceptibility and intense small-angle scattering of neutrons observed at helium temperatures in samples with x>0.45 are evidence of local angular structures with effective sizes of 1–10 nm. The temperatures at which these local angular structures are disrupted are determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1503–1504 (August 1998)     

Influence of structural deformations on the magnetic properties of Jahn-Teller complexes of divalent copper in CaxSr1−x F2 mixed crystals

Crystals of variable constituency Ca_xSr_1−x F₂:Cu (0⩽x⩽0.05, x≅0.5, 1.0⩾x⩾0.95) are investigated by EPR and x-ray structural analysis. The electron-Zeeman, intrinsic hyperfine, and ligand hyperfine interaction parameters are determined; they characterize the magnetic properties of the copper paramagnetic complexes formed in the crystals. Models of the molecular structures of these complexes are discussed, along with the influence of structural deformations and vibronic interactions on their magnetic properties. Fiz. Tverd. Tela (St. Petersburg) 40, 445–451 (March 1998)     

Interplay of negative pressure and hydrogen chemical effects in CeRhSn from first principles

Investigations within the local spin density functional theory (LSDF) of the intermetallic hydride system CeRhSnH_x were carried out for discrete model compositions in the range 0.33 ≤x_H ≤ 1.33 with the purpose of assessing the change of the cerium valence state in the neighborhood of the experimental hydride composition, CeRhSnH_0.8. In agreement with experiment, the analyses of the electronic and magnetic structures and of the chemical bonding properties point to trivalent cerium for 1 ≤x_H ≤ 1.33. In contrast, for lower hydrogen amounts the hydride system stays in an intermediate-valent state for cerium, like in CeRhSn. The influence of the insertion of hydrogen is addressed from both the volume expansion and chemical bonding effects. The latter are found to have the main influence on the change of Ce valence character. Spin polarized calculations point to a finite magnetic moment carried by the Ce 4f states; its magnitude increases with x_H in the range 1 ≤x_H ≤ 1.33.     

Magnetic ordering in the perovskites Eu1−x CaxMnO3 (0⩻x⩻0.5)

The magnetic properties of Eu_1−x Ca_xMnO₃ have been investigated. As the calcium content increased up to x=0.2, the magnetization and the blocking temperature of the magnetic moments of clusters increased and the magnetic anisotropy decreased. As the calcium content increased further, the magnetization decreased, while the “freezing” temperature of the magnetic moments increased. Anomalies of the magnetic properties were observed in compositions with x=0.4 and 0.5 at T=40 K; these anomalies are attributed to a transition to the antiferromagnetic state in the charge-ordered phase. Fiz. Tverd. Tela (St. Petersburg) 39, 117–120 (January 1997)     

Imperfection of the Sm sublattice and valence instability in compounds based on SmB6

The magnetic susceptibility, electrical resistance, specific heat, and thermal expansion coefficient of SmB₆, Sm_0.8B₆, and Sm_1–x La_xB₆ (x=0.1 and 0.2) are measured in the temperature range T=4–300 K. The dispersion curves of the acoustic phonon branches in lanthanum-doped samples are studied. A combined analysis of the results confirms the existence of an activation gap in the electron density of states in both nonstoichiometric and lanthanum-doped compounds. The anomalies in the electronic component of the thermal expansion coefficient are associated to a considerable degree with the temperature variation of the valence and, like the magnetic susceptibility, reflect features of the f-electron excitation spectrum. It is found that lanthanum doping does not lead to significant changes in the anomalies in the phonon spectrum of SmB₆. It is established that the homogeneous intermediate-valent state of the samarium ion is fairly stable and is maintained when the perfection of the Sm sublattice is violated. Zh. éksp. Teor. Fiz. 115, 1024–1038 (March 1999)     

Advances in the study of intermediate-valence and heavy-fermion effects in rare-earth and uranium systems

Summary Three intermetallic systems, Yb−Al, U−Au and Ce−Co, are studied as most typical examples of intermediate-valence, heavy-fermion and coexistence of both intermediate-valence and heavy-fermion effects, respectively. YbAl₂ and YbAl₃, the two phases stable at low temperatures in the equilibrium phase diagram of Yb−Al, are both intermediate-valence systems with Yb valence values of 2.33 and 2.79 at room temperature. The modulation of these values by variations of external parameters like temperature, pressure and chemical substitutions with proper atoms was studied by structural, magnetic and spectroscopical techniques. Two phases are given also by U with Au at low temperatures with compositions UAu₂ and U₁₄Au₅₁: both systems exhibit heavy-fermion properties although with different ordering mechanisms. U₁₄Au₅₁ is an antiferromagnetic heavy-fermion system with a Nèel temperature around 23 K, UAu₂ shows spin-fluctuating behaviour with a saturation in theT→0 K magnetic susceptibility. Effects of substitutions with thorium were investigated only in U₁₄Au₅₁: competition between strong antiferromagnetic coupling and Kondo-like behaviours is observed. The Ce-rich side of the Ce−Co phase diagram shows a single Ce₂₄Co₁₁ phase stable at room temperature interest in the present work but in CeCo₂, which was demonstrated to be intermediate valent by resonant photoemission and XAS experiments and to exibit low magnetic moment and superconducting transition. The coexistence of intermediate-valence and heavy-fermion behaviours observed in Ce₂₄Co₁₁ and of intermediate valence and superconductivity in CeCo₂ is shown by the analyses of XPS, XAS and magnetic experiments with suitable phenomenological models. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Influence of unstable valence of cerium ions on the crystal field in ReNi compounds

To examine the effect of hybridization of 4f electrons with conduction electrons on the crystal field potential using neutron spectroscopy, we studied the effects of the crystal electric field (CEF) in intermetallic compounds of the type ReNi, in which chemical substitution is followed by a transition of the cerium ions from an intermediate valence state to the Kondo state. Measurements were performed both on cerium ions in the compounds Ce_1−x La_xNi (x=0.5, 0.8), where they have a whole-number population of the 4f shell, and on the paramagnetic impurity ion Nd in the series of compounds Re_1−x Nd_xNi (Re=Ce, La, Y), in which the cerium ions are found either in an intermediate valence state or in the Kondo state. From the neutron inelastic magnetic scattering spectra on Nd ions, we have reconstructed the crystal field parameters in ReNi compounds and calculated the CEF level diagram of Ce ions in these compounds as functions of the interion distances Re-Ni. The results of our calculations are in good agreement with the experimentally determined splitting diagram of the ground-state multiplet of the Ce ions. We have determined that as the degree of hybridization with the conduction electrons grows the CEF potential varies considerably and the effective splitting of the 4f shell of the cerium ions increases. The estimated energy scale of the splitting of the ground-state multiplet of the Ce³⁺ ions in the ReNi CEF (Δ_CEF∼15 meV) turns out to be commensurate with the Kondo temperature (T _K ;140 K for CeNi. Analysis indicates that the CEF potential has a substantial effect on the formation of the valence-unstable ground state of the f shell in this compound. Zh. éksp. Teor. Fiz. 113, 1731–1747 (May 1998)     

Frozen magnetic states in quasi-two-dimensional Cr1/3−x NixTaS2 compounds with competing exchange interactions

A study of the dynamic and static magnetic susceptibility of quasi-two-dimensional, doubly intercalated compounds Cr_1/3−x Ni_xTaS₂ (0.08⩽x⩽0.23) is reported. It is shown that cooling from paramagnetic phase in this concentration range produces a spin-glass-type frozen magnetic state. In contrast to the classical 3D spin glasses, however, this state sets in within a broad temperature range rather than at a phase transition. A magnetic phase diagram of the studied compounds has been constructed. Fiz. Tverd. Tela (St. Petersburg) 39, 1801–1805 (October 1997)     

Ferrimagnetic phase transition in the system Li0.5Fe2.52−x GaxO4 in the vicinity of the multicritical point of the x-T phase diagram

In connection with the problem of identifying magnetic states in the vicinity of x ₀ (the multicritical point of the x-T diagrams of spin-glass systems) a study has been made of properties that can be exploited to determine the presence of a thermodynamic phase transition at the Curie point T _C and the distinctive features of the transition, specifically, the temperature dependence of the magnetic part of the specific heat C _m(T), the temperature dependence of the low-field magnetization σ _H(T), and (with a view toward examining critical behavior in a magnetic field) the magnetization isotherms σ _H(T). The investigated object is the system of dilute ferrimagnetic spinels Li_0.5Fe_2.5−x Ga_xO₄, in which every type of magnetic state has spatially inhomogeneous cluster structures. The results obtained for a sample with x=1.45 indicate that the classical criteria of a ferrimagnetic second-order phase transition at T _C=(97±2) K occur for x∼x ₀. The results of similar investigations for a sample with x=1.6, which exists in the cluster spin-glass state for T<T _f=22 K and in an uncorrelated cluster state of the superparamagnetic type for T>T _f, are also given for comparison with the preceding case. Zh. éksp. Teor. Fiz. 114, 2065–2077 (December 1998)     

Magnetic phases in Mn<Subscript>1</Subscript>_<Subscript>x</Subscript>Fe<Subscript>x</Subscript>WO<Subscript>4</Subscript> studied by neutron powder diffraction

The magnetic structures of Mn_1-xFe_xWO₄ with x = 0.0, 0.16, 0.21, 0.225, 0.232, 0.24, 0.27, 0.29, and 1.0 were refined from neutron powder diffraction data. The magnetic phase diagram could be completed in the coexistence range of different magnetic structures up to x = 0.29. For the magnetic state at 1.5 K a commensurate antiferromagnetic structure with a propagation vector = (±1/4, 1/2, 1/2) was found for x ⩽ 0.22 while the magnetic spins order with = (1/2, 0, 0) for x ≥ 0.22. In the latter phase, additionally, weak magnetic reflections indexed to an incommensurate ordering with = (- 0.214, 1/2, 0.457) occur in the diffraction pattern up to x = 0.29 indicating the occurence of a reentrant phase. For 0.12 ⩽ x ⩽ 0.29 the low temperature phases are separated from a magnetic high temperature phase showing only magnetic reflections indexed to a spin arrangement with = (1/2, 0, 0). The magnetic phase diagram is discussed qualitatively considering random superexchange between the statistically distributed Mn²⁺- and Fe²⁺-ions in the coexistence range 0.12 ⩽ x ⩽ 0.29 of different magnetic structures related to those of pure MnWO₄ and FeWO₄. Received 9 October 2002 Published online 14 March 2003     

Spin-polarized first-principles study of ferromagnetism in zinc-blende In<Subscript>1−x</Subscript>Mn<Subscript>x</Subscript>Sb

First-principles calculations based on the density functional theory are performed to study the structural properties, spin-polarized electronic band structures, density of states and magnetic properties of the zinc blende In_{1− x} Mn _x Sb (x = 0.125, 0.25, 0.50, 0.75, 1.0). The calculated lattice constants of In_{1− x} Mn _x Sb obey the Vegard’s law with a marginal upward bowing. With the increase of Mn concentration in In_{1− x} Mn _x Sb, a transition from the semi-metallic to the half-metallic behavior happens such that the majority-spin valence states crosses the Fermi level and the minority-spin states have a gap at the Fermi level. A large exchange splitting (∼ 4 eV) is observed between Mn 3d states of the majority-spins and the minority-spins. The total magnetic moment of In_{1− x} Mn _x Sb half-metallic ferromagnets per Mn atom basis is 4μ _B. The total magnetic moment per Mn atom indicate that Mn atoms act as acceptors in InSb and contribute to holes in the lattice of InSb. Due to p-d hybridization, the free space charge of Mn reduces that results a loss in its magnetic moment. The loss in the magnetic moment of the Mn atoms is converted into a small local magnetic moments on the In and Sb sites.     

Low-temperature properties of the Ce(Pd1−xNix) system

The influence of substituting Pd by Ni is described in Ce(Pd_1-xNi_x)₃ alloys with x taken up to about 0.25. Thermal and magnetization measurements point out a transition from a non-magnetic state (CePd₃) to a ferromagnetic state for x > 0.05, with a Curie temperature K. The Ce-L_2,3 absorption edges and magnetic circular dichroism (XMCD) study reveals the coexistence of strong 4f hybridization and ferromagnetic order. The Ce-L_2,3 XMCD signal measured in CePd₃ demonstrates that in the Ce-based dense Kondo materials only the 4f¹ channel gives a magnetic response.Received: 2 September 2003, Published online: 15 March 2004PACS: 75.20.Hr Local moment in compounds and alloys; Kondo effect, valence fluctuations, heavy fermions - 75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena - 71.10.Hf Non-Fermi-liquid ground states, electron phase diagrams and phase transitions in model systems     

Magnetic and structural transitions in La1−x Sr x MnO3: T-x phase diagram

The electrical conductivity, magnetic susceptibility, magnetization, and submillimeter (v=5∓20 cm⁻¹) permittivity and dynamic conductivity of La_1−x Sr _x MnO₃ (0≤x≤ 0.45) single crystals are investigated. The anomalies in the temperature dependences of these quantities are identified with diverse magnetic and structural phase transformations, including antiferromagnetic and ferromagnetic ordering, structural transitions between strongly distorted (Jahn-Teller) and weakly distorted (pseudocubic) orthorhombic phases, structural transitions to a rhombohedral phase and unusual transitions to a polaron-ordering state. As a result, the complete T-x phase diagram of the system La_1−2x Sr _x MnO₃ is constructed in a wide interval of temperatures T=4.2∓1050 K and concentrations x=0−0.45. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 331–336 (25 August 1998)     

Atomic structure and transport and magnetic properties of the Sm1−x SrxMnO3 system

This is the first study of the temperature dependences of the atomic structure by neutron diffraction, as well as of the resistivity, differential magnetic susceptibility, and magnetoresistance of the ceramic system ¹⁵⁴Sm_1−x Sr_xMnO₃ (x∼0.16–0.4). Samples (x⩾0.3) having an initially orthorhombic structure transfer upon cooling from the insulating to the metallic state and exhibit giant magnetoresistance, which at liquid-helium temperature reaches as high as 90% in magnetic fields up to 30 kOe. At lower doping levels (x⩽0.25), the compound has monoclinic structure. The resistivity of such compounds in zero magnetic field displays insulating behavior upon lowering the temperature to 77 K. Fiz. Tverd. Tela (St. Petersburg) 40, 1271–1276 (July 1998)     

Photosensitivity of surface-barrier and point structures on Cd<Subscript>1 − x</Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te single crystals

Two new types of photosensitive structures are proposed and fabricated for the first time on solid-solution single crystals of diamondlike Cd_{1 − x} Mn _x Te (x = 0−0.7) magnetic semiconductors. The photoelectric properties of surface-barrier (In/Cd_{1 − x} Mn _x Te) and welded (weld/Cd_{1 − x} Mn _x Te) structures are studied at T=300 K. The photosensitivity spectra of these structures are analyzed and compared; as a result, the character of the interband optical transitions and the energy gaps of the Cd_{1 − x} Mn _x Te crystals are determined. These structures can be applied in magnetic photoelectronic devices.     

Field-induced valence transition in EuPtP<Subscript>1−x</Subscript>As<Subscript>x</Subscript>

The ternary compound EuPtP exhibits two valence transitions at T ₁ = 235 K and T ₂ = 190 K. In order to examine a field-induced valence transition of Eu, we synthesized EuPtP_1−x As _x compounds with 0.05 ≤ x ≤ 0.5 and studied the magnetic and valence behavior. The substitution of As for P increases the lattice volume linearly and decreases both valence transition temperatures, T ₁ and T ₂, in contrast to the behavior under external pressures. The magnetization process in a pulsed magnetic field revealed that EuPtP_0.5As_0.5 exhibits an onset of metamagnetic transition above 50 T with a large hysteresis, which evidences a first-order field-induced valence transition. The analysis of the magnetization curves of x = 0.5 at various temperatures has demonstrated that the field-induced transition is essentially the same as the transition induced by temperature at T ₁.     

Universal behavior of CePd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Rh<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> ferromagnet at the quantum critical point

The heavy-fermion metal CePd_1−x Rh _x can be tuned from ferromagnetism at x = 0 to the nonmagnetic state at some critical concentration x _c . The non-Fermi liquid behavior (NFL) at x ≃ x _c is recognized by the power-law dependence of the specific heat C(T) given by the electronic contribution susceptibility X(T) and volume expansion coefficient α(T) at low temperatures: C/T ∝ X(T) ∝ α(T)/T∝ 1/ √T. We also demonstrate that the behavior of the normalized effective mass M _N ^* observed in CePd_1−x Rh _x at x ≃ 0.8 agrees with that of M _N ^* observed in paramagnetic CeRu₂Si₂ and conclude that these alloys exhibit the universal NFL thermodynamic behavior at their quantum critical points. We show that the NFL behavior of CePd_1−x Rh _x can be accounted for within the frameworks of the quasiparticle picture and fermion condensation quantum phase transition, while this alloy exhibits a universal thermodynamic NFL behavior that is independent of the characteristic features of the given alloy such as its lattice structure, magnetic ground state, dimension, etc. The text was submitted by the authors in English.