A. V. Fedorchenko

Magnetic and superconducting properties of FeSe1−xTex (x∼0, 0.5, and 1.0)

The magnetization of FeSe1−xTex (x∼0, 0.5, and 1.0) compounds has been studied in magnetic fields up to 50kOe and at temperatures of 2–300K. The superconducting transition was observed at Tc∼8K and 13.6–14.2K in FeSe0.963 and FeSe0.5Te0.5, respectively. For most of the samples, nonlinearity of the magnetization curves in the normal state gives evidence of a common, substantial presence of ferromagnetic impurities in these compounds. By taking these impurity effects into account, the intrinsic magnetic susceptibility χ of FeSe0.963, FeSe0.5Te0.5, and FeTe was estimated to increase gradually with Te content. For FeTe a drastic drop in χ(T) with decreasing temperature was found at TN∼70K, which is presumably related to antiferromagnetic ordering. To shed light on the observed magnetic properties, ab initio calculations of the exchange enhanced magnetic susceptibility are performed for FeSe and FeTe in the local spin density approximation.

Magnetic properties of superconducting FeSe in the normal state

A detailed magnetization study for the novel FeSe superconductor is carried out to investigate the behavior of the intrinsic magnetic susceptibility χ in the normal state with temperature and under hydrostatic pressure. The temperature dependences of χ and its anisotropy Δχ = χ([parallel]) - χ([perpendicular]) are measured for FeSe single crystals in the temperature range 4.2-300 K, and a substantial growth of susceptibility with temperature is revealed. The observed anisotropy Δχ is very large and comparable to the averaged susceptibility at low temperatures. For a polycrystalline sample of FeSe, the significant pressure effect on χ is determined to be essentially dependent on temperature. Ab initio calculations of the pressure-dependent electronic structure and magnetic susceptibility indicate that FeSe is close to magnetic instability, with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume and especially on the height Z of chalcogen species from the Fe plane. The change of Z under pressure determines a large positive pressure effect on χ, which is observed at low temperatures. It is shown that the literature experimental data on the strong and nonmonotonic pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with the calculated behavior of the density of electronic states at the Fermi level.

Anisotropy of the magnetic properties and the electronic structure of transition-metal diborides

The temperature dependences of the magnetic susceptibility χ and its anisotropy Δχ=χ∥−χ⊥ have been measured for hexagonal single crystals of transition-metal diborides MB2 (M=Sc,Ti,V,Zr,Hf) in the temperature interval 4.2–300K. It is found that the anisotropy Δχ is weakly temperature-dependent, a nonmonotonic function of the filling of the hybridized p−d conduction band, and largest for group-IV transition metals. First-principles calculations of the electronic structure of diborides and the values of the paramagnetic contributions (spin and Van Vleck) to their susceptibility show that the behavior of the magnetic anisotropy is due to the competition between Van Vleck paramagnetism and orbital diamagnetism of the conduction electrons.

Pressure effects on the magnetic susceptibility of FeTex (x\simeq 1.0 )

The magnetic susceptibility χ of FeTe(x) compounds (x approximately 1.0) was studied under hydrostatic pressure up to 2 kbar at fixed temperatures of 55, 78 and 300 K. Measurements were taken both for polycrystalline and single crystalline samples. At ambient pressure, with decreasing temperature a drastic drop in χ(T) was confirmed at T approximately 70 K, which appears to be closely related to antiferromagnetic ordering. The obtained results have revealed a puzzling growth of susceptibility under pressure, and this effect is enhanced by lowering the temperature. To shed light on the pressure effects in the magnetic properties of FeTe, ab initio calculations of its volume dependent band structure and the exchange enhanced paramagnetic susceptibility were performed within the local spin density approximation.

Anomalous diamagnetism in aluminum-lithium alloys

In the region of the equilibrium solid solutions of lithium in aluminum an anomalous low-temperature peak of diamagnetism is observed which is due to the presence of a boundary point on a line of band degeneracy directly below the Fermi level. The position and structure of the peak in the average valence function are analogous to those studied previously in the aging systems Al–Mg and Al–Zn under suitable heat treatment. The value of the lithium impurity scattering parameter for the electron states in the vicinity of the point of degeneracy mentioned is estimated, and the linear relation of that parameter with impurity-related electrical resistance in aluminum alloys is established.

Specific features of the magnetic properties of RB4 (R = Ce, Sm and Yb) tetraborides. Effects of pressure

The temperature dependence and the effect of pressure P up to 2 kbar on the magnetic susceptibility χ of the tetraborides SmB4 and YbB4 was studied. For the compound CeB4, the electronic structure and magnetic susceptibility were calculated from first principles as a function of the atomic volume. The results show that in the studied tetraborides, rare-earth ions (Ce4+, Sm3+ and Yb2.8+) exhibit different valence states, which determines the specific features of their magnetic properties. In particular, the obtained pressure derivatives of susceptibility dlnχ/dP for cerium, samarium and ytterbium tetraborides are −2, −0.6 and +2.7 (in units of Mbar−1), respectively, which are characteristic for the exchange-enhanced itinerant paramagnetism, Van Vleck ionic paramagnetism with a stable f-shell, and the magnetism of rare-earth ions in the intermediate valence state.

Anisotropy of magnetic properties of Fe1+yTe

The magnetic properties of Fe(1+y)Te single crystals (y ≃ 0.1 ÷ 0.18) were studied at temperatures 4.2 ÷ 300 K. At an ambient pressure, with decreasing temperature a drastic drop in χ(T) was confirmed at T ≃ 60 ÷ 65 K, which appears to be closely related to the antiferromagnetic (AFM) ordering. It is found that the magnitudes of the anisotropy of magnetic susceptibility Δχ in the AFM phase are close in the studied samples, whereas the sign of the anisotropy apparently depends on the small variations of the excess iron y in Fe(1+y)Te samples. The performed DFT calculations of the electronic structure and magnetic properties for the stoichiometric FeTe compound indicate the presence of frustrated AFM ground states. There are very close energies and magnetic moments for the double stripe configurations, with the AFM axes oriented either on the basal plane or along the [0 0 1] direction. Presumably, both these configurations can be realized in Fe(1+y)Te single crystals, depending on the variations of the excess iron. This can provide different signs of magnetic anisotropy in the AFM phase, presently observed in the Fe(1+y)Te samples. For these types of AFM configuration, the calculations for the FeTe values of Δχ are consistent with our experimental data.

Magnetic properties and electronic structure of LaFeAsO0.85F0.1

The magnetic properties of the compound LaFeAsO0.85F0.1 were investigated by measurements of the dc magnetization for different values of the magnetic field H=0.02, 1.0 and 2.0T in the temperature range 4.2–300K. Superconducting behavior was found below 26K, whereas a distinct peculiarity in the low-field dependence of the magnetic susceptibility χ(T) was clearly observed at TM≃135K, which resembles a weak ferromagnetic (FM) response with saturation magnetic moment of about 10−4μB per formula unit at 50K. The transition at TM is presumably not governed by magnetic impurities but rather correlated with the antiferromagnetic (AFM) transition in undoped LaFeAsO at about the same temperature. We suggest that the observed magnetic properties of the LaFeAsO0.85F0.1 sample are due to an interplay of FM and AFM transitions, and are presumably related with an intrinsic feature of a small portion of the undoped LaFeAsO phase inherent in our sample. In order to shed light on the problem of magnetic instability of th...

Magnetic properties of Mn-doped Bi2Se3 compound: temperature dependence and pressure effects.

Magnetic susceptibility χ of Bi2-x Mn x Se3 (x  =  0.01-0.2) was measured in the temperature range 4.2-300 K. For all the samples, a Curie-Weiss behaviour of χ(T) was revealed with effective magnetic moments of Mn ions corresponding to the spin value S  =  5/2, which couple antiferromagnetically with the paramagnetic Curie temperature Θ ~ -50 K. In addition, for the samples of nominal composition x  =  0.1 and 0.2 the effect of a hydrostatic pressure P up to 2 kbar on χ has been measured at fixed temperatures 78 and 300 K that allowed to estimate the pressure derivative of Θ to be dΘ/dP ~ -0.8 K kbar(-1). Based on the observed behaviour of Θ with varied Mn concentration and pressure, a possible mechanism of interaction between localized Mn moments is discussed.

Low-temperature relaxation of magnetization in manganite Pr0.4Bi0.3Ca0.3MnO3

Low-temperature relaxation of magnetization in Pr0.4Bi0.3Ca0.3MnO3 ceramics was measured after cooling in a magnetic field and aging at temperatures from 5 K to 38 K near the transition temperature to the magnetically ordered state. It was found that the relaxation process has a thermoactivation character at the first stage and is described in terms of the Arrhenius equation. Its activation energy Q ≈ 1.1 meV is comparable with the antiferromagnetic interaction energy in this compound. The temperature–time dependence of the relaxation rate at the second stage is described by a power law. The relaxation slows down as the temperature approaches the critical value, which may be associated with the formation of a cluster structure in the studied compound under these conditions.Low-temperature relaxation of magnetization in Pr0.4Bi0.3Ca0.3MnO3 ceramics was measured after cooling in a magnetic field and aging at temperatures from 5 K to 38 K near the transition temperature to the magnetically ordered state. It was found that the relaxation process has a thermoactivation character at the first stage and is described in terms of the Arrhenius equation. Its activation energy Q ≈ 1.1 meV is comparable with the antiferromagnetic interaction energy in this compound. The temperature–time dependence of the relaxation rate at the second stage is described by a power law. The relaxation slows down as the temperature approaches the critical value, which may be associated with the formation of a cluster structure in the studied compound under these conditions.