Phase transition in a vortice lattice in a Bi2212:Pb single crystal

The magnetooptical method was used to investigate the penetration of a magnetic flux into a single crystal of a high-temperature superconductor (Bi_0.84Pb_0.16)_2.2Sr₂CaCu₂O₈ in crossed magnetic fields. It is shown that at low temperatures the penetration of the magnetic flux is anisotropic: the flux moves preferentially along the magnetic field applied in the plane of the sample, and the anisotropy grows as the temperature increases. At a temperature T_m = 54±2 K, there occurs a sharp change in the character of penetration of the magnetic field into the superconductor; the direction of the flux ceases be dependent on the direction and magnitude of the magnetic field applied in the plane of the sample. In this case, the transition temperature T_m is independent of the applied magnetic field. The effect is interpreted in terms of the concepts of a phase transition in the system of vortices, which is related to a sharp decrease in the correlations in the position of vortices in various CuO planes, i.e., with the transition from three-dimensional to two-dimensional behavior of the vortex structure.     

First-order phase transition in potassium dysprosium tungstate induced by the field renormalization and softening of the elastic moduli in the vicinity of a structural Jahn-Teller-type transition

The magnetostriction of KDy(WO₄)₂ single crystals is measured in an external magnetic field at temperatures below the temperature of a structural phase transition of the Jahn-Teller type. A steplike irreversible variation in the elastic strain is observed to occur with an increase in the magnetic field applied along the a or b axis of the monoclinic cell of the crystal. The residual change in the strain is retained after changing the sign of the magnetic field. The return to the initial state characterized by field-induced jumps in the strain is possible only after thermal cycling well above the structural phase transition temperature. The theory of this phenomenon is developed using a phenomenologically derived thermodynamic potential of the elastic sub-system that takes into account the crystal symmetry and the field renormalization of the elastic moduli. The jumplike transitions are interpreted as being due to the magnetic softening of the elastic moduli in the vicinity of the structural phase transition temperature.     

Resistivity superconducting transition in single-crystalline Cd0.95Ni0.05Sb system consisting of non-superconducting CdSb and NiSb phases

Resistivity superconducting transition has been for the first time found in single crystal of two-component 0.95(CdSb)–0.05(NiSb) system. End members of the system are not superconductors under normal conditions. Insulating behavior in temperature dependence of the electrical resistivity, which is due to hopping conductivity, precedes the transition. The resistivity superconducting transition is rather broad, since at cooling down the electrical resistivity starts to fall at 10.5 K, whereas zero resistivity is reached only at ~2.3 K. Longitudinal magnetic field gradually depresses superconductivity and shifts the superconducting transition to lower temperatures. Under magnetic field above 0.5 T, superconductivity is totally destroyed. Main features observed in the resistivity superconducting transition, including its unusually big width and insulating electrical behavior above the transition, can be related to inhomogeneity of the single crystal studied. According to XRD and SEM examinations, the single crystal consists of major CdSb phase and minor NiSb phase. The NiSb phase forms inhomogeneities in the CdSb matrix. Micro-sized needle-like NiSb crystals and nano-sized Ni_1-xSb_x clusters can be considered as typical inhomogeneities.     

Magnetostructural phase transitions in manganese arsenide single crystals

The effect of an external magnetic field with a strength up to 140 kOe on the phase transitions in manganese arsenide single crystals has been investigated. The existence of unstable magnetic and crystal structures at temperatures above the Curie temperature T _C = 308 K has been established. The displacements of manganese and arsenic atoms during the magnetostructural phase transition and the shift in the temperature of the first-order magnetostructural phase transition in a magnetic field have been determined. It has been shown that the magnetocaloric effect in a magnetic field of 140 kOe near the Curie temperature T _C is equal to ??T ?? 13 K. A model of the superparamagnetic state in MnAs above the temperature T _C has been proposed using the data on the magnetic properties and structural transformation in the region of the first-order magnetostructural phase transition. It has been demonstrated that, at temperatures close to T _C, apart from the contribution to the change in the entropy from the change in the magnetization there is a significant contribution from the transformation of the crystal lattice due to the magnetostructural phase transition.     

NDMAP: the best material to study the Haldane's prediction

As predicted by Haldane, spin, S=1 one-dimensional (1D) Heisenberg antiferromagnet (HAF) has an energy gap between the singlet ground state and first excited triplet. On application of magnetic field, the triplet state Zeeman splits and the energy of one of the triplet state becomes zero at a critical field, H_c. Above H_c the system recovers magnetism. Then, we expect that a quasi-1D HAF will show a magnetic long-range ordering (LRO) at low temperatures due to the inter-chain coupling. This field-induced LRO has not been observed before due to complication of the crystal structure in the materials studied so far and/or technical difficulty.From a heat capacity measurement on a single crystal of an S=1 quasi-Q1D HAF, Ni(C₅H₁₄N₂)₂N₃(PF₆), we found an anomaly at a temperature in finite fields indicating a field-induced phase transition. A magnetic LRO is confirmed by a neutron diffraction measurement on the same sample. The temperature versus magnetic field phase diagram of this compound is constructed and discussed.     

Ultrasonic study of dysprosium in a magnetic field

The ultrasonic attenuation of longitudinal waves propagating along the c axis of single crystal dyprosium is reported, as a function of the applied basal plane field in the paramagnetic region, and as a function of the temperature, at constant applied basal plane field, in the spin-spiral region. In the paramagnetic region, anomalous attenuation behavior is explained on the basis of competing spin-polarization and spin-fluctuation effects. Two anomalous maxima in the temperature dependence of the attenuation were observed: one near T_N is attributed to spin fluctuations associated with short range ferromagnetic ordering; another one at 130 K is attributed to a magnetic phase transition from a fanstructure phase, intermediate between the spin-spiral and ferromagnetically ordered phase     

Temperature and magnetic field induced structural transformation in Si-doped CeFe2: An in-field X-ray diffraction study

Using the X-ray powder diffraction technique at various temperatures and applied magnetic fields, we have studied the magnetostructural properties of Ce(Fe_0.95Si_0.05)₂. The X-ray diffraction data establish quantitative relationships between bulk magnetization and the evolution of structurally distinct phases with magnetic field and temperature, and confirm the distinct features of a first-order phase transition such as supercooling and superheating, metastability, and phase co-existence of different structural polymorphs. We observe the lattice volume mismatch across the structural phase transition, which appears to be the cause for the step behavior of the magnetization isotherms at low temperatures. The present study shows that the lattice distortion has to be treated explicitly, like spin, along with the effects of lattice–spin coupling to account for the magnetization behavior of this system. This structure template can resolve the issue of kinetics in this material as observed in different time scale measurements and with different experimental protocols.     

Jahn-Teller phase transitions in TbcGd1−cVO4

The temperatures at which the cooperative Jahn-Teller phase transitions occur in Tb_cGd_1?cVO₄ have been determined from Raman and optical birefringence measurements. Over a narrow range of c there are two transition temperatures; between these temperatures the crystal is distorted, but a higher and lower temperatures the crystal has the same undistorted structure. This unusual behaviour was predicted earlier, and is accurately described by molecular field theory.     

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.     

Ultrasonic attenuation study of erbium in a magnetic field

The ultrasonic attenuation coefficient of longitudinal waves propagating along the c-axis of a single crystal of erbium was measured over a temperature range which covers the transition temperature from the paramagnetic to the sinusiodal phase T₆ = 83.8 K and the transition temperature from the sinusoidal to elliptical phase T_⊥ = 53.9 K. These measurements were performed as a function of temperature in the presence of constant magnetic fields and at constant temperatures as a function of applied magnetic fields. A magnetic field parallel to the c-axis shifted T₆ to lower temperatures and reduced the peak attenuation. A broad new maximum emerged on the high temperature side of the field-shifted T₆. Application of H perpendicular to the c-axis produced little change in either T₆ or T_⊥. A model introduced by Tachiki and Mackawa is used to discuss the results.     

Magnetic-field-induced phase transition in Tb0.95Bi0.05MnO3+δ multiferroic

The effect of a magnetic field on the dielectric properties of Tb_0.95Bi_0.05MnO_3+δ single crystals has been analyzed. It has been shown that the state of the crystal for temperatures of 5–440 K is inhomogeneous and restricted domains of polar and spin correlations are exhibited in it. A phase transition in which the inhomogeneous state of the crystal changes substantially has been observed at a temperature of T ? 180 and 225 K (in the absence and presence of a magnetic field, respectively). The high-temperature phase contains large dielectric domains with a high dielectric constant (ε ～ 10⁵) and thin conducting layers at the boundaries of these domains. The magnetic field significantly affects the state of both low-and high-temperature phases of the crystal, shifting the temperature of the phase transition between them, and induces an additional phase transition at T ? 441 K.     

The magnetic phases of NdCu2

The magnetic phase diagram of a NdCu₂ single crystal is investigated by means of specific-heat measurements and neutron diffraction as a function of temperature and external magnetic field applied along the crystallographic b-direction. In the low temperature region we observe three commensurate phases AF1, F1, and F2. Their magnetic structures all consist of ferromagnetic (bc)-planes with different stacking sequences along the a-direction comprising 5, 3 and 8 chemical unit cells, respectively. Above 2.8 T the system is in a ferromagnetically aligned state (F3). Furthermore, there is an incommensurate phase AF3 between the low temperature commensurate phases and the paramagnetic state and, in a very narrow temperature region of only 0.2 K, an intermediate phase AF2 between AF1 and AF3. The Nd-moments are oriented along the b-direction in all phases.     

Unusual magnetization suppression induced by higher magnetic field in (La0.83Bi0.17)0.67Ca0.33MnO3

Magnetization behavior of (La_0.83Bi_0.17)_0.67Ca_0.33MnO₃ has been investigated in the temperature range from 100 to 180 K. A metamagnetic transition was observed in the temperature region, where the magnetization was measured after a zero-field-cooling from room temperature to a selected temperature. Experimental results show that, after a higher magnetization route, the field-increasing branches of the magnetization curves shows an unusual training effect: below a magnetic field H₀, the applied magnetic field enhances the value of magnetization; however, above H₀ the magnetic field suppresses the value, and the behavior cannot be totally attributed to the enhancement effect of the applied magnetic field on ferromagnetic phase fraction. It is proposed that, in the two-phase coexistence region, the higher magnetic field promotes the phase separation and leads to both the fraction of ferromagnetic domain and the stabilization of antiferromagnetic domain increase.     

Ferromagnetic resonance observation of a phase transition in magnetic field-aligned Fe2O3 nanoparticles

Fe₂O₃ hematite (alpha) nanoparticles suspended in the liquid phase of the liquid crystal 4,4-azoxyanlsole (PAA) are cooled below the freezing temperature (397 K) in a 4000 G dc magnetic field. The in field solidification locks the direction of maximum magnetization of the particles parallel to the direction of the applied dc magnetic field removing the effects of dynamical fluctuations of the nanoparticles on the magnetic properties allowing a study of the intrinsic magnetic properties of the nanoparticles as well as the anisotropic behavior of the ferromagnetic resonance (FMR) signal. Freezing in PAA allows temperature-dependent measurements to be made at much higher temperature than previous measurements. The field position, line width and intensity of the FMR signal as a function of temperature as well as the magnetization show anomalies in the vicinity of 200 K indicative of a magnetic transition, likely the previously observed Morin transition shifted to lower temperature due to the small particle size. Weak ferromagnetism is observed below T_c in contrast to the bulk material where it is antiferromagnetic below T_c. The Raman spectrum above and below 200 K shows no evidence of a change in lattice symmetry associated with the magnetic transition.     

Metamagnetic transition in Na 0.85 CoO2 single crystals

We report the magnetization, specific heat, and transport measurements of a high quality Na(0.85)CoO2 single crystal in applied magnetic fields up to 14 T. At high temperatures, the system is in a paramagnetic phase. It undergoes a magnetic phase transition below approximately 20 K. For the field H||c, the measurement data of magnetization, specific heat, and magnetoresistance reveal a metamagnetic transition from an antiferromagnetic state to a quasiferromagnetic state at about 8 T at low temperatures. However, no transition is observed in the magnetization measurements up to 14 T for H perpendicular c. The low temperature magnetic phase diagram of Na(0.85)CoO2 is determined.     

Lattice effects in singlet ground state systems

We have investigated elastic, magnetic, thermal and structural properties of systems which exhibit induced magnetic behavior (TbP, PrSn₃, Pr₃Se₄, Pr₃Te₄). Particular emphasis is given to phonon effects due to the crystalline electric field, phase transitions, and magnetic fields. TbP has a first order magnetic phase transition and both TbP, PrSn₃ show a temperature dependence of the elastic constants, which can be explained with the known crystal field level scheme. The compounds Pr₃S₄, La₃S₄, Pr₃Se₄ exhibit structural instabilities at much higher temperatures than the magnetic or superconducting transition temperatures. Pr₃Te₄ does not exhibit a structural transition, but a strong elastic softening in a magnetic field. This effect is due to strong field dependent quadrupole transitions. All these systems are analyzed with respect to typical induced moment parameters.     

Influence of magnetic field on the electronic specific heat of the organic metal (BEDT-TTF)2KHg(SCN)4

Specific heat measurements of a single crystal of the organic metal (BEDT-TTF)₂KHg(SCN)₄ have been carried out at low temperatures and under a magnetic field of up to 14 T. A jump in the specific heat of about 0.1 J/mol·K, which corresponds to the antiferromagnetic phase transition, has been observed. The magnetic field is found to decrease the transition temperature at any field orientation. The strongest effect was found to take place in the field direction along the highly conducting ac plane. Zh. éksp. Teor. Fiz. 113, 1058–1063 (March 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

NMR study of the quasi-one-dimensional compound BaCo2V2O8

We report the results of a ⁵¹V nuclear magnetic resonance (NMR) study on the single crystal BaCo₂V ₂O₈ at temperatures between 3 and 300 K. The NMR features for fields both parallel and perpendicular to the c-axis have been identified. We found no substantial changes in the NMR shift and line width at low temperatures as a constant field of 7.06 T was applied along the c-axis. The NMR observations thus indicate the absence of magnetic long-range ordering under this field, consistent with the specific heat result which showed the disappearance of the magnetic transition beyond an external field of 4 T in BaCo₂V ₂O₈.     

Dynamic phase transitions and dynamic phase diagrams of the spin-2 Blume-Capel model under an oscillating magnetic field within the effective-field theory

The dynamic phase transitions are studied in the kinetic spin-2 Blume-Capel model under a time-dependent oscillating magnetic field using the effective-field theory with correlations. The effective-field dynamic equation for the average magnetization is derived by employing the Glauber transition rates and the phases in the system are obtained by solving this dynamic equation. The nature (first- or second-order) of the dynamic phase transition is characterized by investigating the thermal behavior of the dynamic magnetization and the dynamic phase transition temperatures are obtained. The dynamic phase diagrams are constructed in the reduced temperature and magnetic field amplitude plane and are of seven fundamental types. Phase diagrams contain the paramagnetic (P), ferromagnetic-2 (F₂) and three coexistence or mixed phase regions, namely the F₂+P, F₁+P and F₂+F₁+P, which strongly depend on the crystal-field interaction (D) parameter. The system also exhibits the dynamic tricritical behavior.