High field Shubnikov-De-Haas effect and magnetoresistance in the organic metal (TMTSF)2 ClO4

The transverse magnetoresistance of (TMTSF)₂ ClO₄ has been investigated in magnetic fields up to 32 T at several temperatures down to 4.2 K and different angles between B and the crystalline axis. Shubnikov-de-Haas oscillations are observed directly on the magnetoresistance at fields higher than 10 T giving a fundamental field of 259± 10 T in a direction close to c¹ axis.     

Mössbauer study of the in-plane anisotropy in annealed amorphous Fe81B13.5Si3.5C2 ribbons

Mössbauer spectra of amorphous Fe₈₁B_13.5Si_3.5C₂ ribbons (METGLAS 2605 SC) annealed in transverse magnetic field at 594 and 654 K were recorded for various orientations of the ribbons. It is determined that the in-plane projection of the anisotropy is 17° off the transverse axis for the 594 K annealed specimen and 10° off the transverse axis for the 654 K specimen.     

Beats of quantum oscillations of the hall coefficient and resistivity in semiconductor alloys n-Bi0.93Sb0.07 at a small deviation of the magnetic field direction from the trigonal axis

Quantum oscillations of the resistivity ??₂₂ and Hall coefficient R _12.3 in the semiconductor alloy n-Bi_0.93Sb_0.07 have been studied at H ?? C ₃ and j ?? C ₁ in magnetic fields to 14 T and at temperatures of 1.5, 4.5, 10, and 20 K. At temperatures of 1.5 and 4.5 K, beats of quantum oscillations of ??₂₂ and R _12.3 due to a small deviation of the magnetic field H from the crystallographic C ₃ axis have been observed. To determine the oscillation period ?? _i , cyclotron mass m _ci , cyclotron frequency ?? _ci , and extreme section S _extri, experimentally measured quantum oscillation beats have been compared with the model beats of oscillations of three harmonic functions, two of which have close frequencies. The deviation of the parameters ?? _i , m _ci , and S _extri from the same parameters when the magnetic field H exactly coincides with the trigonal C ₃ axis has made it possible to estimate the magnetic field H deflection angle from the trigonal C ₃ axis, which is ??1°.     

Quantum oscillations of Hall resistance in bismuth bicrystals with twist low-angle internal boundaries

Quantum oscillations of the Hall resistance ρ_ij(B) of bismuth bicrystals are investigated in magnetic fields up to 35 T. It is found that the twist low-angle internal boundary possesses n-type conductivity and comprises a central part and two adjacent layers, which are characterized by the specific features of the Fermi surface of electrons.     

Asymmetry with respect to the magnetic field direction in the interaction between the quantum states of two coupled superconducting rings

The interaction between the quantum states of two circularly asymmetric superconducting aluminum rings forming figure eight, threaded by a magnetic flux and biased by an external sinusoidal ac current with zero dc component, has been investigated. Quantum oscillations in the dependence V _dc(B) of the rectified dc voltage on magnetic field for these structures have been measured at different external currents and temperatures close to critical. Fourier and wavelet analyses of the function V _dc(B) have revealed, along with the two fundamental ring frequencies, various combination frequencies; this fact is indicative of interaction in the structure. Deviation of the function V _dc(B) from oddness with respect to the magnetic field direction has been found for the first time.     

Co-Si-B and Fe-Co-B amorphous alloys: Induced anisotropy and various magnetic properties

The dependence on the metalloid content of some magnetic properties of Co_100−x(Si_0.6B_0.4)_x (22.5 ⩽ x ⩽ 30) and Co₇₅Si_25−xB_x (10 ⩽ x ⩽ 25) amorphous alloys has been studied.Ribbons were subjected to different kinds of heating treatments: field annealing, stress annealing and stress-field annealing (tensile stress and longitudinal magnetic field applied simultaneously). While the anisotropies induced by simple field annealings are of the order of magnitude of 0.1 kJm^-3, the anisotropy induced by stress-field annealing can reach values up to 0.5 kJm^-3. The preferred axis is longitudinal for most of the annealing conditions. The temperature and composition dependence of the magnetostriction have been studied too.Stress, field and stress-field induced anisotropies have also been measured in Co₆₆Fe₉B₂₅ samples (λ_s > 0). In this case the preferred axis is transverse to the ribbon axis.     

Strong-coupling constant in coordinate space

The coordinate-space behavior of (vector) strong-coupling constant in the background field α_B(r) is compared with that in standard perturbation theory α_v(r). The numerically calculated two-loop coupling constant α_B(r) is shown to exceed α_v(r) by 1–5% at very small distances, r?0.02 fm, and to be in agreement with lattice measurements of the static potential. At large distances, α_B(r) approaches the freezing value at r?0.5 fm. An analytic form of α_B(r) is proposed that approximates α_B(r) with a precision ?2% in the region r?0.5 fm.     

Metamagnetic states in single crystal HoNi<Subscript>2</Subscript>B<Subscript>2</Subscript>C at <Emphasis Type="Italic">T</Emphasis> ≈ 1.9 K: Torque magnetometry study

Metamagnetic transitions in single-crystal rare-earth nickel borocarbide HoNi₂B₂C have been studied at T ≈ 1.9 K with a Quantum Design torque magnetometer. With increasing field, transitions to antiferromagnetic, ferrimagnetic, non-collinear, and saturated paramagnetic states take place. The critical fields of the transitions depend crucially on the angle θ between applied field and the easy axis [110]. Measurements of torque along the c axis have been made while changing the angular direction of the magnetic field (parallel to basal tetragonal ab planes) and with changing field at fixed angle over a wide angular range. Two new phase boundaries in the region of the non-collinear phase have been observed, and the direction of the magnetization in this phase has been precisely determined. At low field the antiferromagnetic phase is observed to be multidomain. In the angular range around the hard axis (?6° ≤ ? ≤ 6°, where ? is the angle between the field and hard axis [100]) the magnetic behavior is found to be “frustrated” with a mixture of phases with different directions of the magnetization.     

Magneto-acoustic attenuation in AuSb2

Quantum oscillations in the ultrasonic attenuation in AuSb₂ were studied as a function of temperature, magnetic field and crystal orientation. The effective masses of the carriers associated with the F₅ and F₆ oscillations were measured in a (110) plane. For the F₅ oscillations, the Dingle temperature and apparent magnetic breakdown field appear to depend strongly upon orientation. For the F₆ oscillations, however, there were no signs of magnetic breakdown up to the highest magnetic fields available (70 kOe) and the Dingle temperature was roughly independent of orientation. From the acoustic velocities, the elastic constants were determined at 77 K: C₁₁ = (14·7 ± 0·9) × 10¹¹ dyne/cm², C₁₂ = (6·0 ± 0·9) × 10¹¹ dyne/cm², and C₄₄ = (2·59 ± 0·06) × 10¹¹ dyne/cm². These elastic constants give an adiabatic compressibility K_s = (1·13 ± 0·12) × 10^?12 cm²/dyne and a Debye temperature ?_D = (203 ± 15) K.     

Mössbauer study of aluminum-substituted hematites

A Mössbauer spectroscopic study is reported on a series of aluminum-substituted hematites, α(Fe_1-cAl_c)₂O₃, with c up to 0.32. These samples were prepared by heating aluminum-substituted goethites, αFeOOH, at 500°C. X-ray line broadening gives particle dimensions of ?200 Å to >1000 Å. Heating the samples to 900° improves crystallinity but reduces the maximum obtainable c to ?0.15. At 77 K for c?0.04 the magnetic structure is antiferromagnetic with the spins aligning close to the (111) axis. For c?0.08 and for all compositions at 298 K the spins are perpendicular to (111) and for this weakly ferromagnetic phase the supertransferred hyperfine field is (5±1) kOe per magnetic neighbor. Samples with 0.04?c<0.08 at 77 K have both magnetic phases present in varying proportions. The magnetic field at 298 K varies with aluminum content according to the 1/3-power law for the reduced sublattice magnetization. The asymmetric shape of the spectra for c?0.17 has been accounted for by a model based on the molecular field approximation with nearest neighbor superexchange interaction strength J_nn?15 K.     

237Np Mössbauer study of a novel intermetallic NpRh2Si2

NpRh₂Si₂ (ThCr₂Si₂ structure) has been studied by ²³⁷Np Mössbauer spectroscopy between 4.2 and 100 K. The isomer shift value suggest a Np⁴⁺ electronic configuration. A single site combined magnetic plus quadrupole pattern is observed up to the magnetic ordering temperature of T_c=73(1) K. A Np magnetic moment of 1.4μ_B is deduced from the hyperfine field measured at 4.2 K. The magnetic moments are estimated to make an angle of either 90° or 34° with the tetragonal axis.     

Behaviour of ferrocholesterics under external magnetic fields

The influence of an external magnetic field on the orientational behaviour of a ferrocholesteric with a positive magnetic anisotropy is investigated. Both the phenomena arising when the field was switched on or switched off are considered. It is found that the field needed for a ferrocholesteric–ferronematic transition B_FC↑ is higher when compared to that obtained for the pure cholesteric (B_C↑). A similar result was obtained when estimating the critical field for the homeotropic ferronematic–ferrocholesteric (focal conic) transition, occurring when the magnetic field was decreased or switched off. We found that B_FC↓>B_C↓. These results are explained when considering that the magnetic moments of the magnetic powder are not oriented parallel to the liquid crystal molecular directors, therefore hindering their orientation under a magnetic field.     

The Mössbauer effect and magnetic phase transitions

The Mössbauer effect provides a direct method for identifying the spin axis in magnetic crystals and observing magnetic phase transitions. The order of the transition may be inferred from the Mössbauer spectrum. Phase changes can occur as a function of temperature (e.g. when the anisotropy fieldB _A changes sign) or as a function of applied magnetic field. In an antiferromagnet a field ?(2B _E B _A)^1/2 along the spin axis whereB _E is the exchange field causes the spin-flop transition which is normally first order (sharp) whereas the transition to the paramagnetic phase which occurs at higher fields?2B _E is second order (continuous). In quasi-one-dimensional crystals Mössbauer spectra show that the spin-flop transition is first order locally but occurs over a range of fields throughout the crystal, so that the first order character is masked in a conventional magnetization measurement. In fields applied at a finite angle>B _A/2B _E to the spin axis the transition becomes second order, i.e. a continuous rotation of the spins occurs. In canted antiferromagnets (or weak ferromagnets) the spin-flop transition is also continuous; in addition a “screw” re-orientation may be induced by fields applied perpendicular to the spin axis and arises from antisymmetric exchange. For crystals with lowT _N the hyperfine field changes when a magnetic field is applied and has a minimum at a phase transition; this may be used to map out the magnetic phase diagram.     

Magnetic properties of amorphous sputtered CoxSi1−x films

Magnetic properties have been measured by transverse magneto-optic Kerr effect (MOKE) in amorphous sputtered Co_xSi_1?x films which are ferromagnetic and amorphous at room temperature with 0.55<x<0.80. These as-sputtered films show an in-plane easy axis with the ratio of coercitive force in the hard axis H_c (HA) to easy axis H_c (EA) and the value of the blocking field H_B almost constant with 0.60<x<0.80. Transverse magnetic susceptibilities with bias magnetic field parallel (χ_t0) and perpendicular $\text{(χ}{}_{\mathrm{<mtext>t\pi </mtext><mtext>2</mtext>}}\text{)}$ to the easy axis has been measured. These measurements show a deviation of Hoffmann's micromagnetic law for the susceptibilities.     

Spin reorientation effects in GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> induced by applied field and temperature

Magnetic properties of GdFe₃(BO₃)₄ single crystals were investigated by ⁵⁷Fe-Mössbauer spectroscopy and static magnetic measurements. In the ground state, the GdFe₃(BO₃)₄ crystal is an easy-axis compensated antiferromagnet, but the easy axis of iron moments does not coincide with the crystal C₃ axis, deviating from it by about 20°. The spontaneous and field-induced spin reorientation effects were observed and studied in detail. The specific directions of iron magnetic moments were determined for different temperatures and applied fields. Large values of the angle between the Fe³⁺ magnetic moments and the C₃ axis in the easy-axis phase and between Fe³⁺ moments and the a₂ axis in the easy-plane phase reveal the tilted antiferromagnetic structure.     

Manifestation of quantum statistics in vibrational dynamics of poly(ethylene) crystals

The temperature dependences of the transverse expansion ?_⊥(T) and the longitudinal contraction ?_‖(T) (with respect to the axes of chain molecules) in large-sized poly(ethylene) (PE) crystal grains (100×60×60 nm) are measured using x-ray diffraction in the temperature range 5–380 K. The temperature dependence of the elongation of the molecular skeleton ?_C(T) is obtained by Raman spectroscopy. It is found that the dependences ?_⊥(T), ?_‖(T), and ?_C(T) exhibit a similar specific nonlinear behavior. Analysis of these dependences indicates that the nonlinearity is associated with the quantum statistics of transverse vibrations. The energies and amplitudes of zero-point (at T=0) transverse (torsional and bending) vibrations and the relevant zero-point components ?_‖(0) and ?_C(0) are estimated. It is revealed that the zero-point components make a considerable contribution to the dynamics of the PE crystal up to the melting temperature (～400 K).     

On the possibility of direct measurement of the magnetic field-induced phase transition in hematite by means of magnetooptical method

It is shown that a rotation ? and a deformation κ of the optical indicatrix appear during the transverse magnetic field-induced phase transition in hematite. Analytic expressions for ? and κ are deduced from the magnetization-dependent electromagnetic energy in the crystal. It is shown that during the phase transition, induced by increasing the temperature, the electromagnetic energy in the crystal. It is shown that during the phase transition, induced by increasing the temperature, the antiferromagnetic vector L = M₁ - M₂ is rotating from the three-fold C₃ axis toward the basal plane, which implies that the main axis of the optical indicatrix is not aligned in a general case with the magnetic field or the crystallographic axis although the magnetic moment (M₁ + M₂) is always parallel to the field. The linear magnetic birefringence is very sensitive to the magnetic phase in hematite, as described in previous experimental work, but the present analysis shows that a direct determination of the transverse field-induced phase transition can be obtained in hematite, by means of a magnetooptical method, only when large and non-uniform rotation (up to ninety degrees) and variation of the shape of the indicatrix are taken into account.     

Raman scattering spectra of B2-polariton and B2 (LO) modes in paraelectric KDP

The first observation of B₂-polariton and B₂ (LO) modes in the paraelectric phase of KDP has been made by the z(xy)z + Δy and z(xy)z? Raman scattering experiments, respectively. The B₂-polariton (θ = 0.20–0.48°) spectra has revealed both the Stokes and anti-Stokes peaks. The LO-mode which is nearly critically damped has been observed. From these results the lowest B₂ mode spectrum obtained by the y(xy)x Raman scattering has been reconfirmed to be oscillatory in nature, that is, a transverse optical phonon excitation.     

Magnetic birefringence of sound and magnetoacoustic oscillations in hematite

An experimental study of the magnetic birefringence of sound in the α-Fe₂O₃ easy-plane antiferromagnet is reported. The amplitude of transverse sound transmitted through the crystal along the C₃ trigonal axis was found to oscillate depending on the magnitude of the magnetic field H applied in the basal plane (H ⊥ C₃). The experiments provide qualitative support for the theory of this phenomenon developed earlier by Turov. Possible reasons for the substantial quantitative disagreement between theory and experiment observed in the field dependence of the sonic-amplitude oscillation period are discussed. Unannealed samples of the hematite revealed a 60° periodic dependence of the position of the intensity oscillation extrema on magnetic field orientation in the basal plane, which is associated with basal anisotropy of higher than second order. The observation of pronounced (～6×10³ Oe) effective magnetic anisotropy fields in the basal plane can be assigned to the existence of large residual strains in such samples.     

Quantum limit behavior of the magnetoresistance in Hg1-x-yCdxMnyTe alloys subjected to a high pulsed magnetic field

The transverse magnetoresistance and the Hall effect of a quaternary alloy semimagnetic semiconductor Hg_1-x-yCd_xMn_yTe were measured in high pulsed magnetic fields up to 30T. The investigation of the asymptotic power law (‘_p(B) ∝ B^α) in a quantum limit regime revealed that the short range type scattering is enhanced with increasing Mn composition, suggesting the scattering associated with the localized magnetic moment of Mn.