N. V. Baranov

Extra T-linear specific heat contribution induced by the f–d-exchange in Gd–Ni binary compounds

Specific heat measurements performed for Gd–Ni binary compounds with 1:2, 1:1 and 3:1 stoichiometry have revealed an additional contribution to the coefficient γ of the T-linear term of the low-temperature specific heat in comparison with that obtained for isostructural paramagnetic counterparts R–Ni (R = Y, Lu, La). This extra contribution Δγ is found to grow linearly with increasing Gd concentration from nearly zero for GdNi5 up to Δγ≈21.5 mJ g-at.−1 K−2 for Gd3Ni. To explain the appearance of Δγ and its dependence on the strength of the f–d exchange interaction in Gd-rich binary compounds with Ni a new mechanism enhancing the effective mass of conduction electrons is suggested. The increased γ value in Gd-containing compounds is attributed to spin fluctuations induced by the f–d exchange interaction in the hybridized 3d–5d electron subsystem.

Onset of magnetism in Y1−xGdxCo2: effect on the heat capacity and electrical resistivity

Results of the electrical resistivity, magnetoresistance and specific heat measurements are presented for Y1−xGdxCo2 in the vicinity of the critical Gd concentration xc ≈ 0.15 for the onset of a long-range magnetic order. Pronounced maxima of the coefficient γ of the T-linear specific heat, Debye temperature and residual resistivity as a function of concentration are found to exist at x = 0.1, i.e. just below xc. These anomalies, together with the existence of a minimum on the temperature dependences of the electrical resistivity at 0 < x < 0.15, are ascribed to the substantial influence of localized spin density fluctuations induced by f–d exchange in the itinerant d-electron subsystem of Co. The maximal extra contribution to the residual resistivity Δρresmax in Y1−xRxCo2 pseudobinaries is found to depend on the critical concentration as well as on the type of R ions, while the value of Δγmax depends only on the critical concentration of R ions. The significant upturns on Cp/T versus T2 dependences observed in Y1−xGdxCo2 at 0 < x < 0.15 may be indicative of a non-Fermi-liquid behaviour of the d-electron subsystem caused by the partial substitution of Gd for Y.

High-field magnetization and magnetic structure of Tb3Co

The measurements of the magnetization in high steady and pulsed fields together with neutron diffraction measurements on a powder sample and on a single crystal have been performed to study the magnetic state of the Tb3Co compound. It has been shown that the modulated antiferromagnetic structure which exists in Tb3Co below TN = 82 K transforms to the incommensurate magnetic structure with a strong ferromagnetic component along the c-axis with further cooling below the critical temperature Tt≈72 K. The phase transition from the high-temperature to the low-temperature magnetic state at Tt is of first order. The incommensurability of the low-temperature magnetic structure of Tb3Co is attributed to the non-Kramers character of the Tb3+ ion in combination with competition between the indirect exchange interaction and the low-symmetry crystal electric field.

Spin fluctuations in Gd3Rh induced by f–d exchange: the influence on the T-linear specific heat

The results of specific heat, magnetization and magnetic susceptibility measurements for the antiferromagnetic (AF) compound Gd3Rh are presented. Below its N?el temperature TN = 112 K, this compound exhibits a field-induced phase transition from the AF to the ferromagnetic state in a relatively small magnetic field (~0.4 T). Specific heat measurements revealed a giant enhancement of the coefficient of the T-linear specific heat in Gd3Rh (? = 118 mJ mol?1 K?2) in comparison with that observed for the isostructural nonmagnetic partner Y3Rh (? = 11 mJ mol?1 K?2). An anomalous behaviour of the heat capacity as well as a set of peculiarities of magnetic properties in Gd3Rh are associated with the existence of spin fluctuations induced by f?d exchange interaction in the d-electron subsystem that arises from features of the crystal and electronic structure of the R3M-type compounds.

Irreversible field-induced magnetic phase transitions and properties of Ho3Co

The results of magnetic susceptibility, magnetization, electrical resistivity and specific heat measurements performed on Ho3Co single crystals show that this compound exhibits two different antiferromagnetic structures: AFII at 8 K<T< 22 K and AFI below Tt≈8 K. Below the Neel temperature TN = 22 K the application of a magnetic field along the main crystallographic directions induces magnetic phase transitions which are accompanied by giant magnetoresistance. At T<Tt the field-induced phase transitions along the c- and b-axes are found to be irreversible, and a small ferromagnetic component is observed along the a-axis. These peculiarities are associated with the non-Kramers character of the Ho ion and with the presence of a complex incommensurate magnetic structure of Ho3Co below TN. The temperature coefficient of the electrical resistivity for Ho3Co above TN over a wide temperature range is found to differ from that observed for other R3Co compounds. Such a behaviour is attributed to the presence of an additional contribution to the conduction electron scattering by spin fluctuations induced by f–d exchange in the itinerant d-electron subsystem. The value of this extra contribution and its temperature range is suggested to depend on the spin value of the R ion. The excess of the effective magnetic moment per R ion, which is observed in Ho3Co and in other R3M type compounds, is also attributed to spin fluctuations induced by f–d exchange.

Magnetic order, field-induced phase transitions and magnetoresistance in the intercalated compound Fe0.5TiS2

Measurements of the magnetic susceptibility, magnetization, electrical resistivity and neutron diffraction have been performed for the compound Fe(0.5)TiS(2) in which Fe atoms are intercalated between S-Ti-S tri-layers. It has been shown that this compound with a monoclinic crystal structure exhibits an antiferromagnetic (AF) ground state below the Néel temperature T(N) ≈ 140 K. Small deviations from the stoichiometry and some disordering effects caused by the additional low-temperature heat treatment do not affect substantially the AF state in Fe(0.5)TiS(2). According to neutron diffraction data the magnetic structure at 2 K is described by the propagation vector k = (1/4,0,1/4). The Fe magnetic moments with a value of (2.9 ± 0.1) μ(B) are directed at an angle of (78.5 ± 1.8)° to the layers. Application of the magnetic field at T < T(N) induces a metamagnetic phase transition to the ferromagnetic (F) state, which is accompanied by the large magnetoresistance effect (|Δρ/ρ| up to 27%). Below 100 K, the field-induced AF-F transition is found to be irreversible, as evidenced by magnetoresistance and neutron diffraction measurements. The magnetization reversal in the metastable F state is accompanied at low temperatures by substantial hysteresis (ΔH ~ 100 kOe) which is associated with the Ising character of Fe ions.

A cluster-glass magnetic state in R5Pd2 (R?=?Ho, Tb) compounds evidenced by AC-susceptibility and neutron scattering measurements

AC- and DC-susceptibility, high-field magnetization and neutron diffraction measurements have been performed in order to study the magnetic state of R5Pd2 (R = Ho, Tb) compounds. The results show that both compounds undergo cluster-glass freezing upon cooling below Tf. According to the neutron diffraction a long-range magnetic order is absent down to 2 K and magnetic clusters with short-range incommensurate antiferromagnetic correlations exist not only below Tf but also in a wide temperature range above the freezing temperature (at least up to 2Tf). A complex cluster-glass magnetic state existing in Ho5Pd2 and Tb5Pd2 down to low temperatures results in rather complicated magnetization behavior in DC and AC magnetic fields. Such an unusual magnetic state in compounds with a high rare-earth concentration may be associated with the layered type of their crystal structure and with substantial atomic disorder, which results in frustrations in the magnetic subsystem.

Magnetic state and properties of the Fe0.5TiSe2 intercalation compound

The Fe0.5TiSe2 compound with a monoclinic crystal structure has been prepared by intercalation of Fe atoms between Se-Ti-Se sandwiches in the layered structure of TiSe2. The crystal and magnetic structures, electrical resistivity, and magnetization of the Fe0.5TiSe2 compound have been investigated. According to the neutron diffraction data, the Fe0.5TiSe2 compound has a tilted antiferromagnetic structure at temperatures below the Néel temperature of 135 K, in which the magnetic moments of Fe atoms are antiferromagnetically ordered inside layers and located at an angle of approximately 74.4° with respect to the layer plane. The magnetic moment of Fe atoms is equal to (2.98 ± 0.05)μB. The antiferromagnetic ordering is accompanied by anisotropic spontaneous magnetostrictive distortions of the crystal lattice, which is associated with the spin-orbit interaction and the effect of the crystal field.

Magnetism of compounds with a layered crystal structure

This review presents the results of investigations of the crystal structure, magnetic ordering, magnetic anisotropy, and magnetic phase transformations in compounds of the RT2Z2 and RT6Z6 type (R is a rare-earth metal; T is a transition metal; and Z = Si, Ge, or Sn) and also in intercalated dichalcogenides of transition metals, such as MxTX2 and RxTX2. A specific feature of these compounds is a layered character of their crystal structures, in which the atoms that have a magnetic moment are located in separate crystallographic layers. Inside the layers of magnetic atoms and between the layers, there act different (in energy) exchange interactions of different type, which leads to a variety of magnetic structures and magnetic phase transitions in these compounds and makes them suitable objects for the investigation of physical phenomena inherent in quasi-two-dimensional magnetic systems.

Transport properties and polarization phenomena in intercalated AgxHfSe2 compounds

The electrical properties of intercalated AgxHfSe2 compounds (x = 0.1, 0.2) have been investigated for the first time. Investigations have been performed using various current electrodes, which make it possible to pass either the electron current or the ion current across the sample. Polarization effects, which indicate the self-consistent migration of charge carriers in the samples, have been found for the samples at room temperature. Based on the characteristic features of polarization decay, coefficients of conjugated chemical diffusion have been evaluated.