S. L. Gnatchenko

Anomalous optical phonons in FeTe chalcogenides: Spin state, magnetic order, and lattice anharmonicity

Polarized Raman-scattering spectra of non-superconducting, single-crystalline FeTe are investigated as function of temperature. We have found a relation between the magnitude of ordered magnetic moments and the linewidth of A1g phonons at low temperatures. This relation is attributed to the intermediate spin state (S=1) and the orbital degeneracy of the Fe ions. Spin-phonon coupling constants have been estimated based on microscopic modeling using density-functional theory and analysis of the local spin density. Our observations show the importance of orbital degrees of freedom for the Fe-based superconductors with large ordered magnetic moments, while small magnetic moment of Fe ions in some iron pnictides reflects the low spin state of Fe ions in those systems.

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 the antiferromagnetic spin-1/2 chain system beta-TeVO4

The magnetic susceptibility and magnetization of high-quality single-crystal beta-TeVO4 are reported. We show that this compound, made of weakly coupled infinite chains of VO5 pyramids sharing corners, behaves as an S = 1/2 one-dimensional Heisenberg antiferromagnet. From magnetic experiments, we deduce the intrachain antiferromagnetic coupling constant, J/k(B) = 21.4 +/- 0.2 K. Below 5 K, a series of three phase transitions at 2.26, 3.28, and 4.65 K is observed.

X-ray study of Nd0.5Sr0.5MnO3 manganite structure above and below the ferromagnetic metal–antiferromagnetic insulator spontaneous phase transition

The crystal structure of the manganite Nd0.5Sr0.5MnO3 is studied at temperature T=300 and 77.3 K by means of an x-ray diffractometer. It is shown that the transition from the ferromagnetic metallic state to the antiferromagnetic insulating charge-ordered state is accompanied by a lowering of the symmetry of the structure from orthorhombic to monoclinic. The space-group symmetry of the orthorhombic and monoclinic phases is identified as Imma and P21/m, respectively. Twinning of the crystal and the formation of a twin domain structure with coherent boundaries in the (00l) crystallographic planes are found.The crystal structure of the manganite Nd0.5Sr0.5MnO3 is studied at temperature T=300 and 77.3 K by means of an x-ray diffractometer. It is shown that the transition from the ferromagnetic metallic state to the antiferromagnetic insulating charge-ordered state is accompanied by a lowering of the symmetry of the structure from orthorhombic to monoclinic. The space-group symmetry of the orthorhombic and monoclinic phases is identified as Imma and P21/m, respectively. Twinning of the crystal and the formation of a twin domain structure with coherent boundaries in the (00l) crystallographic planes are found.

Antiferromagnetic resonance in Mn2P2S6

The antiferromagnetic resonance (AFMR) spectra along the principal magnetic axes x, y, z of the compound Mn2P2S6 have been investigated in detail in a wide range of frequencies (8–142GHz) and magnetic fields (up to 75kOe) at T=4.2K. It is shown that this compound is a biaxial magnetic substance. At H=0, there are two gaps in the spin-wave spectrum: (101.46±0.1) and (115.52±0.1)GHz. The AFMR data were used to determine the effective magnetic-anisotropy fields: Ha1=0.619kOe and Ha2=0.803kOe. An anomalous interaction of the AFMR branches in Mn2P2S6 has been detected, along with the appearance of coupled spin–spin oscillations of the same symmetry. An additional absorption in the AFMR spectrum is a local mode associated with breakdown of translational order in a low-dimension magnet. It is shown for the first time that the appearance of additional absorption peaks depends on the sample-cooling rate. It is proven that the observed peaks are associated with the internal modes of the domain boundaries.

Exchange anisotropy in polycrystalline FeNi∕FeMn films with hysteresis loop asymmetry

The process of magnetization reversal of a FeNi(50A)∕FeMn(50A) polycrystalline film prepared in a magnetic field has been investigated at temperatures ranging from 25to300K. The external field was oriented in the film plane along the easy or difficult axis of the ferromagnetic layer. In the process of magnetization reversal of the film along the easy axis, strong asymmetry of the hysteresis loop is observed together with an exchange shift. As temperature decreases, the asymmetry becomes more pronounced and the shift increases. The field dependences of the magnetization of the film are symmetric and are not shifted when the external field is applied along the difficult axis. The magnetization reversal process is examined on the basis of a model that takes account of the appearance of high-order exchange anisotropy in polycrystalline films. It is shown that the observed strong asymmetry of the hysteresis loop is associated with the formation of a canted phase and the existence of a metastable state. As the ...

Photoinduced light absorption and dichroism of Ca3Mn2Ge3O12 garnet as a probe of electronic processes and intrinsic electric fields

Measurements of photoinduced light absorption in Ca3Mn2Ge3O12 revealed some unusual features: a saturation with the pumping intensity and a broad straggling of relaxation times with a predominance of very long times. These experimental facts cannot be understood in terms of photoinduced absorption centers associated with impurities or lattice defects, but are naturally explained within the notion of random electric fields of active charges. Active charges are produced by light pumping via the dissociation of coupled pairs of charges (consisting of a Mn-hole coupled with a compensating negative impurity or a negatively charged vacancy) which exist in the ground state. Such active charges create electric fields in a larger volume than coupled pairs, thus enhancing the probability for forbidden optical transitions. On the other hand, the random fields of active charges promote hopping of holes and hence the relaxation of photoinduced effects. A broad distribution of random-field magnitudes gives rise to a ve...

Formation of noncollinear spin configurations during magnetization reversal in multilayered Fe/Si films

Abstract Magnetization reversal processes in Fe/Si multilayered films have been studied by means of the longitudinal magneto-optical Kerr effect at room temperature. The multilayers have been grown by DC sputtering on the single-crystal GaAs substrate. The Kerr effect curves had areas with an invariable or slightly varying rotation and had different remanent rotation value in the zero magnetic field for different directions of the magnetic field in the film plane. The observed features are related to the formation of stable and metastable noncollinear spin configurations. The experimental results are explained in the framework of the theory, which takes into account the competition between the bilinear exchange term on the one hand and the biquadratic exchange or cubic magnetic anisotropy terms on the other hand.

Magnetooptical studies of the H–T phase diagram of an Fe/Si multilayered film

The magnetization reversal of a multilayered Fe/Si film having cubic magnetocrystalline anisotropy is investigated in the temperature interval 25–300 K by magnetooptical methods. It is found that the growth of the biquadratic exchange interaction as the temperature is lowered causes a spontaneous second-order phase transition from a collinear antiferromagnetic state to a noncollinear state. The presence of cubic anisotropy in the film gives rise to spontaneous and magnetic-field-induced first-order phase transitions between noncollinear states. Magnetooptical studies permit constructing the H–T magnetic phase diagram of the multilayered Fe/Si film for an orientation of the external field along the hard magnetization axis [110]. A calculation of the H–T phase diagram in the framework of a model taking into account the bilinear exchange and cubic anisotropy, with constants I1 and K that are assumed to be independent of temperature, and also the biquadratic exchange with a linearly temperature-dependent cons...

Domain structure of the antiferromagnetic insulating state in Nd0.5Sr0.5MnO3

Optical reflectivity studies of the ferromagnetic metal (FMM) to antiferromagnetic insulator (AFI) phase transition are performed on Nd0.5Sr0.5MnO3 manganite in a wide temperature and magnetic field range. The formation of a domain structure in the AFI state during the FMM−AFI phase transition is observed. It is shown that the two types of domains observed are energetically equivalent states. On the basis of the experimental results and symmetry analysis we conclude that these domains are crystal twins. The twin domain structure of the AFI state in the Nd0.5Sr0.5MnO3 is visible in reflected unpolarized light due to a different tilting of the surface in the domains. The two-phase domain structure FMM+AFI formed in the vicinity of the phase transition is also studied. It is found that a thermodynamically equilibrium two-phase stripe domain structure does not develop. The absence of the magnetic intermediate state is due to the large energy of the interphase wall, which results in the stripe structure period...