Effect of a strong magnetic field on the quadrupole and magnetic orders and crossover in the Jahn-Teller magnet DyVO<Subscript>4</Subscript>

The effect of a strong magnetic field H||[001] on magnetic properties of the Jahn-Teller compound DyVO₄ is studied. New phase transitions discovered and investigated experimentally and theoretically include the breaking of quadrupole order (enhancement of the crystal symmetry) and breaking of antiferromagnetic order as well as the effect of convergence of energy levels of the Dy³⁺ ion (crossover). The differential magnetic susceptibility of a DyVO₄ crystal is measured in fields up to 36 T in the temperature interval 1.4?15 K. The observed magnetic susceptibility anomalies and phase transitions are described using a unified theoretical approach.     

Hidden reentrant superconducting phase in a magnetic field in (TMTSF)<Subscript>2</Subscript>ClO<Subscript>4</Subscript>

We suggest explanation of the high upper critical magnetic field, perpendicular to conducting chains and parallel to conducting layers H _c2 ^b′ ≃ 6 T, experimentally observed in the organic superconductor (TMTSF)₂ClO₄. In particular, we show that H _c2 ^b′ can be higher than both the quasiclassical upper critical field and Clogston-Chandrasekhar paramagnetic limit in a singlet quasi-one-dimensional superconductor. We predict the coexistence of the hidden Reentrant and Larkin-Ovchinnikov-Fulde-Ferrell phases in a magnetic field. Our results are compared to the recent experimental data and shown to be in a good agreement with the experiments.     

Induced quadrupole effects near a crossover in a tetragonal TbLiF4 sheelite in a strong magnetic field up to 50 T

The anomalies of magnetic properties of TbLiF₄ caused by the interaction of the energy levels of a rare-earth ion in a strong magnetic field up to 50 T directed along the [100] and [110] axes are studied experimentally and theoretically. The jumps in magnetization M(H) and the maxima of the differential magnetic susceptibility dM(H)/dH are found in critical fields H _c = 28 and 31 T, where the lower component of the excited doublet approaches the ground-state singlet of a Tb³⁺ ion. Based on the crystal-field model with known interaction parameters, we calculated the Zeeman effect and the magnetization and magnetic susceptibility curves for the TbLiF₄ crystal, which adequately describe magnetic anomalies and critical parameters of a crossover. It is shown that the jumpwise change in the α- and γ-symmetry quadrupole interactions in TbLiF₄ caused by changes in the corresponding quadrupole moments during the crossing of energy levels leads, in accordance with experiments, to a decrease in the critical field H _c by approximately 4 T and an increase in the maximum of the differential susceptibility dM(H)/dH near the crossover more than twofold. This behavior can be considered as an analog of the induced quadrupole transition caused by a change of the ground state of the rare-earth ion during crossover.     

Magnetic and structural phase transitions in YMn<Subscript>2</Subscript>O<Subscript>5</Subscript> ferromagnetoelectric crystals induced by a strong magnetic field

Magnetic, magnetoelectric, and magnetoelastic properties of YMn₂O₅ ferromagnetoelectric single-crystals are investigated in strong pulsed magnetic fields of up to 250 kOe and in static magnetic fields of up to 12 kOe. It is found that, in YMn₂O₅ at T < T_N=42 K, a transverse weakly ferromagnetic moment of σ ₀=0.8 G cm³/g exists that is oriented along axis a and is attributed to the magnetoelectric interaction. When a magnetic field is directed along axis b, which is likely to be the axis of antiferromagnetism, a spin-flop transition is observed that is accompanied by jumps in magnetostriction and electric polarization. When a magnetic field is directed along axis a, the temperature of ferroelectric transition shifts from 20 to 25 K at H≈200 kOe. A theoretical analysis of the experimental results is given within phenomenological theory with regard to the fact that a YMn₂O₅ compound belongs to noncollinear antiferromagnetic crystals even in the exchange approximation.     

Effect of Gd-Mn exchange on phase transitions in GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript> induced by a strong magnetic field

Complex studies of the magnetic, magnetoelectric, and magnetoelastic properties of GdMn₂O₅ single crystals in strong pulsed magnetic fields are carried out in order to obtain additional indirect information on the character of the rare-earth and manganese spin ordering. It is shown that magnetic ordering of Gd³⁺ spins affects the manganese sublattice spin orientation and initiates new magnetic phase transitions. The observed magnetoelectric properties of the GdMn₂O₅ system are interpreted in terms of the theory of phase transitions.     

Electrical properties of the perovskite-like phase of CaCoCu<Subscript>2</Subscript>V<Subscript>4</Subscript>O<Subscript>12</Subscript> at pressures of up to 50 GPa

The results from experimental investigations on the effect of high pressures (10–50 GPa) on the electrical properties and magnetoresistance of the perovskite-like high-pressure state of CaCoCu₂V₄O₁₂ are presented. The pressure ranges of substantial changes in the behavior of electrical characteristics are determined.     

Magnetization of La- and Ce-doped CaMnO<Subscript>3</Subscript> manganite single crystals in a strong magnetic field

Studies of the magnetization curves of electron-doped single-crystal manganites Ca_{1 ? x} Ln _x MnO₃ (Ln = La³⁺, Ce⁴⁺; x ≤ 0.12) in strong pulsed magnetic fields of up to 350 kOe have revealed a metamagnetic transition in Ca_0.9Ce_0.1MnO₃ in the temperature range 77–190 K. The critical transition fields increase to ～350 kOe with the temperature decreasing to 100 K. The spin polarization is ～50% of the theoretical value. These results are interpreted as due to “melting” of the orbital/charge ordering below the temperature T _OO/CO = 185 K = T _N (of the C type AFM phase); this entails a decrease in the volume of the ordered phase with localized carriers and an increase in the volume of the ferromagnetic phase with delocalized carriers. The temperature and field dependences of the magnetization are used to compare two manganite systems in the region of the two-phase magnetic state.     

Exotic molecular states in a strong magnetic field: H<Stack><Subscript>3</Subscript><Superscript>(2+)</Superscript></Stack> ion

The existence of the molecular ion H ₃ ⁽²⁺⁾ in a magnetic field in a triangular and a linear configuration is discussed. A variational method (with an optimization of the form of the vector potential) is used. It is shown that, in the range of magnetic fields 10⁸<B<10¹¹ G, the system (pppe), with the protons forming an equilateral triangle perpendicular to the magnetic line, has a well-pronounced minimum in the total energy. Also, for B?10¹⁰ G, if the protons are situated along a magnetic line (linear configuration), a well-pronounced minimum in the total energy appears. Both configurations are unstable under the decays H(atom) + p + p and H ₂ ⁽⁺⁾ + p. A possible connection between the H ₃ ⁽²⁺⁾ molecular ion and a recently discovered absorption feature in a neutron-star atmosphere is discussed.     

Temperature dependence of coherent phonons in TbVO<Subscript>4</Subscript> crystal probed by ultrafast optical spectroscopy

We report on the monolithic integration of frequency converter and amplitude modulator in a single lithium niobate (LiNbO₃) chip by the use of focussed ultrashort laser pulses. The waveguiding structures are obtained by femtosecond-laser induced internal modification and the electrodes are ablated out of a gold-layer sputtered onto the sample surface.     

Perpendicular magnetic anisotropy in single-crystal Co<Subscript>50</Subscript>Pt<Subscript>50</Subscript>/MgO(100) films

The crystal structure and hysteretic magnetic properties of equiatomic single-crystal CoPt films applied on MgO substrates by magnetron sputtering, as well as modification of these properties by thermal annealing, are studied. Heat-treated films of thickness in the range 2<d≤16 nm exhibit perpendicular magnetic anisotropy. A correlation between the crystalline anisotropy constant of the CoPt films and the order parameter of the LI₀ superstructure in these alloys is found. The effect of a single-crystalline MgO substrate on the structure and magnetic properties of equiatomic CoPt films is revealed.     

Jahn-Teller transitions in Yb<Emphasis Type="Italic">X</Emphasis>O<Subscript>4</Subscript> (<Emphasis Type="Italic">X</Emphasis>=V,P) stimulated by a strong magnetic field

The effect of an external magnetic field directed along various symmetry axes of a crystal on Jahn-Teller-type structural phase transitions (quadrupole ordering) is studied in YbPO₄ and YbVO₄ crystals with zircon structure. In the absence of a magnetic field, the crystals are in a precritical state and do not exhibit a spontaneous quadrupole ordering. It is shown that, in a field H ∥ [110], the strain susceptibility χ_γ increases with the field and, at a sufficiently high field strength, an orthorhombic lattice deformation along the [100] axis arises in the crystals under study; i.e., a stimulated Jahn-Teller phase transition of γ symmetry occurs. Using interaction constants determined from independent experiments, we calculated phase diagrams and anomalies in the magnetic and magnetoelastic properties of the YbPO₄ and YbVO₄ crystals near the stimulated phase transitions, investigated the effect of various pairwise interactions on them, and analyzed possible experimental observations of the predicted effects.     

Nonlinear magnetic resonance in the (CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>)<Subscript>2</Subscript>CuBr<Subscript>4</Subscript> crystal

The nonlinear microwave absorption in the (CH₃NH₃)₂CuBr₄ antiferromagnetic crystal is investigated experimentally. The temperature and angular dependences of the parameters of nonlinear resonance and the dependences of these parameters on the microwave pump power are analyzed. It is found that the nonlinear properties deteriorate with decreasing temperature and the linear and nonlinear contributions are competitive in character.     

Dynamics of the magnetic flux component normal to the layers of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8</Subscript> in a tilted field

High-frequency losses in the strongly anisotropic layered superconductor Bi₂Sr₂CaCu₂O₈ are measured at 600 MHz under a magnetic field rocking about the ab plane. Anomalies in losses and hysteretic phenomena are found while performing periodic rocking, i.e., cycling the magnetic field component normal to the sample surface. Based on these observations, conclusions are drawn about the nature of magnetic-flux penetration into the superconductor. It is found that, in the range between 60 K and T _c , the dynamics of magnetic-flux vortex lines normal to the ab plane in the presence of a constant magnetic field applied parallel to this plane is governed by the critical penetration field H _c ^⊥* and the surface barrier in the presence of thermally activated vortex motion (giant flux creep). The dependences of H _c1 ^⊥* and the characteristic field of the surface barrier on the magnitude of the parallel magnetic field are measured.     

New magnetic states in copper metaborate CuB<Subscript>2</Subscript>O<Subscript>4</Subscript>

The static and resonance properties of copper metaborate CuB₂O₄ were experimentally studied in a magnetic field applied in the crystal tetragonal plane. The field-induced second-order phase transition to a weakly ferromagnetic state was observed in the temperature range 10–20 K. The low-field state is characterized by the absence of spontaneous moment, and it represents, presumably, a long-period helicoid. At temperatures below 2 K, two sequential first-order phase transitions were observed. They were accompanied by jumps in resonance absorption with a hysteresis upon changing field-scan direction. These transitions can be caused by the transformation of the incommensurate spin structure into the helicoidal states with periods commensurate with the lattice translation period.     

Surface critical magnetic field H<Subscript>c3</Subscript>(T) of a bulk superconductor MgB<Subscript>2</Subscript> using two-band Ginzburg-Landau theory

Two-band Ginzburg-Landau (TB G–L) equations for a bulk MgB₂ were solved analytically to determine the temperature dependence of surface critical magnetic fieldH _c3 (T). It is shown thatH _c3 (T) has the same temperature dependence with H_c2 (T), similar to the case of a single-band superconductor,H _c3 (T) = 166H _c2 (T). We use an elimination procedure for the decoupling of G–L equations of two-band superconductivity, which eases the calculations. It is expected that the temperature dependence forH _c3 (T) gives positive curvature nearT _c .     

Magnetic structure and phase diagram of Sr<Subscript>5</Subscript>Rh<Subscript>4</Subscript>O<Subscript>12</Subscript> frustrated Ising magnet

The magnetic structure of Sr₅Rh₄O₁₂ is based on Ising chains of rhodium ions with a variable valence, Rh³⁺-Rh⁴⁺. The ordering in the chains is assumed to be ferromagnetic. It has been shown that the magnetic structure and phase diagram of Sr₅Rh₄O₁₂ are well described in a model taking into account weak antiferromagnetic interactions between the nearest and next-nearest neighbors on the triangular lattice of ferromagnetic Ising chains. The ground state at low temperatures is the two-sublattice stripe phase; this phase in the magnetic field is transformed to the ferrimagnetic phase and, then, to the ferromagnetic phase. Small plateaus can be observed in the region of the transition from the ferrimagnetic phase to the ferromagnetic one.     

Effect of a magnetic field on the permittivity of 80%La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>/20%GeO<Subscript>2</Subscript> composite

The dielectric properties of a magnetoresistive conducting two-phase 80%La_0.7Sr_0.3MnO₃/20%GeO₂ (wt %) composite have been studied near the percolation threshold in magnetic fields from 0 to 15 kOe at frequencies of the measurement field from 5 kHz to 1 MHz. The samples have inductive impedances; i.e., their permittivities can be considered negative due to a high conductivity in this frequency range. The permittivity increases in magnitude in magnetic field, and the values of the magnetodielectric coefficient reach 23% at room temperature. The reasons for the effect of magnetic field on the dielectric permittivity of samples are discussed.     

Optical absorption spectra and magnetic phase transitions in TbFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Optical absorption spectra of the trigonal crystal of TbFe₃(BO₃)₄ in the vicinity of the ⁷F₆ → ⁵D₄ transition in a Tb³⁺ ion were studied as a function of temperature (2–70 K) and magnetic field strength (0–60 kOe) at 2 K. The splitting of the excited states of Tb³⁺ due to both the magnetic ordering of iron and an external magnetic field was determined. Abrupt splitting of the absorption lines of Tb³⁺ at temperature T_N of the magnetic ordering of the subsystem of iron was revealed, suggesting that the nature of such splitting is not entirely magnetic.