V. A. Desnenko

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.

Exchange bias in phase-segregated Nd2/3Ca1/3MnO3 as a function of temperature and cooling magnetic fields

Exchange bias (EB) phenomena have been observed in Nd2/3Ca1/3MnO3 colossal magnetoresistance perovskite below the Curie temperature TC ∼ 70 K and attributed to an antiferromagnetic–ferromagnetic (FM) spontaneous phase segregated state of this compound. Field cooled magnetic hysteresis loops exhibit shifts toward negative direction of the magnetic field axis. The values of exchange field HEB and coercivity HC are found to be strongly dependent of temperature and strength of the cooling magnetic field Hcool. These effects are attributed to evolution of the FM phase content and a size of FM clusters. A contribution to the total magnetization of the system due to the FM phase has been evaluated. The exchange bias effect decreases with increasing temperature up to TC and vanishes above this temperature with disappearance of FM phase. Relaxation of a non-equilibrium magnetic state of the compound manifests itself through a training effect also observed while studying EB in Nd2/3Ca1/3MnO3.

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.

Charge ordering, internal structural parameters, and magnetic susceptibility of Nd2∕3Ca1∕3MnO3: driving forces of a phase transition

The temperature dependences of the long diagonals dMn–O of the MnO6 octahedron and the magnetic susceptibility χ of Nd2∕3Ca1∕3MnO3 in the temperature interval 100–290K are investigated. The functions dMn–O(T) and χ(T) are found to have anomalies in the charge-ordering range (Tco≈212K). The sharp decrease of the diagonal dMn–O2s agrees with phase-transition notions, according to which the spatial modulation of the charge density is due to the modulation of the Mn–Mn bond lengths. The most likely driving forces of the transition are the Peierls lattice instability and Jahn–Teller stability of the MnO6 octahedron at the Mn3+ ions. The hysteresis of the temperature dependence χ(T) in the paramagnetic region shows indirectly that structural phase separation occurs together with the transition. The parameters of the function χ(T) indicate that ferromagnetic clusters consisting of one, two, or three ion pairs Mn3+–Mn4+ form in the system in the temperature intervals 274K⩽T⩽290K, 224K⩽T⩽252K, and 130K⩽T⩽198K, res...

Nonmonotonic temperature dependence of the spontaneous magnetization of the antiferromagnetic crystal LiCoPO4

The temperature dependence of the magnitude of the weak ferromagnetic moment is measured in the many-sublattice antiferromagnetic magnetoelectric crystal LiCoPO4. The dependence is found to be nonmonotonic—as the temperature is decreased below the Neel temperature TN, the ferromagnetic moment initially increases and then decreases markedly. It reaches its highest value, close to 0.12 G, at a temperature of around 10.5 K, which is roughly TN/2. The possible mechanisms for the appearance of weak ferromagnetism in LiCoPO4 and the causes of the nonmonotonic temperature dependence of the spontaneous magnetic moment are discussed. Among the possible mechanisms, there is a preference for one in which the weak ferromagnetism is due to a nonuniform antiferromagnetic structure which is proposed for this crystal.

Exchange interaction and magnetoresistance in La2/3Ca1/3MnO3: experiment and models

The magnetization M(T) and electrical resistivity ρ(T) of a La2/3Ca1/3MnO3 film have been studied in the temperature range 5 K⩽T⩽320 K in the magnetic field intervals 10 Oe⩽H⩽400 Oe and 0⩽H⩽50 kOe, respectively. It is found that the M(T)/M(0) value is larger than that predicted by the conventional molecular field model below the Curie point T=267 K, and that the ln ρ(T) dependence is close to linear in the temperature range 80 K<T<200 K (accordingly, ∂ ln ρ/∂T is constant in this region). A model of the electrical conductivity and magnetoresistivity of the system describing qualitatively the experimental results is proposed (the Δmτ model). The model includes a thermally activated (with characteristic energy Δ) mechanism of conductivity, dependence of the concentration and the effective mass (m) of the itinerant charge carriers on the magnetization, as well as scattering (with characteristic time τ) of the charge carriers by static breakings of the translational symmetry, thermal fluctuations of the magne...

Antisymmetric exchange in La-substituted BiFe0.5Sc0.5O3 system: symmetry adapted distortion modes approach

Abstract Neutron powder diffraction measurements on the 35 % La-substituted Bi1−xLaxFe0.5Sc0.5O3 composition revealed that the samples obtained under high-pressure (6 GPa) and high-temperature (1500 K) conditions crystalize into a distorted perovskite structure with the orthorhombic Pnma symmetry and the unit cell parameters: ao = 5.6745(2) Å, bo = 7.9834(3) Å and co = 5.6310(2) Å. A long-range magnetic ordering takes place below 220 K and implies a G-type magnetic structure with the moments 4.10(4)μB per Fe aligned predominately along the orthorhombic c-axis. The space group representation theory using the orthorhombic symmetry yields four bi-linear coupling schemes for the magnetic order parameters imposed by antisymmetric exchange interactions. The couplings are analysed based on symmetry adapted distortion modes defined in respect of the undistorted cubic perovskite structure. The approach allows a quantitative estimation of the coupling strength. It is shown that the experimentally found spin configuration combines the magnetic order parameters coupled by the atomic displacement modes with the largest amplitudes. The results indicate that the antisymmetric exchange is the dominant anisotropic term which fully controls the direction of the Fe3+ spins in the distorted perovskite lattice.

Magnetic structure of an incommensurate phase of La-doped BiFe0.5Sc0.5O3: Role of antisymmetric exchange interactions

A 20% substitution of Bi with La in the perovskite Bi1-xLaxFe0.5Sc0.5O3 system obtained under high-pressure and high-temperature conditions has been found to induce an incommensurately modulated structural phase. The room temperature X-ray and neutron powder diffraction patterns of this phase were successfully refined using the Imma(0,0,g)s00 superspace group (g=0.534(3)) with the modulation applied to Bi/La- and oxygen displacements. The modulated structure is closely related to the prototype antiferroelectric structure of PbZrO3 which can be considered as the lock-in variant of the latter with g =0.5. Below T_N = 220 K, the neutron diffraction data provide evidence for a long-range G-type antiferromagnetic ordering commensurate with the average Imma structure. Based on a general symmetry consideration, we show that the direction of the spins is controlled by the antisymmetric exchange imposed by the two primary structural distortions, namely oxygen octahedral tilting and incommensurate atomic displacements. The tilting is responsible for the onset of a weak ferromagnetism, observed in magnetization measurements, whereas the incommensurate displacive mode is dictated by the symmetry to couple a spin-density wave. The obtained results demonstrate that antisymmetric exchange is the dominant anisotropic interaction in Fe3+ based distorted perovskites with a nearly quenched orbital degree of freedom.

Pressure effect on electronic structure and magnetic properties of RNi5

Abstract Magnetic susceptibility of RNi 5 single crystals (R  Gd, Tb, Dy) was studied under uniform and uniaxial pressures at temperatures above T C . The uniform pressure derivatives of the paramagnetic Curie temperature ϑ and the crystalline electric field parameter B 2 0 appear to be small. On the other hand, ϑ and B 2 0 parameters display a large sensitivity to the uniaxial pressure due to related changes of c / a ratio. Ab initio electronic structure calculation was performed for GdNi 5 to evaluate the volume derivative of T C .

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.