Electron spin resonance on GdxMo6Se8 (x = 1.0 and 1.2)

Electron spin resonance of Gd³⁺ in Gd_xMo₆Se₈ (x = 1.0 and 1.2) has been measured in X-band and in K-band between 1.4 and 290 K. The results indicate that the conduction electron - Gd³⁺ spin exchange coupling constant is positive and equal to +0.011 eV, in agreement with previous determinations based on superconductivity and static magnetic susceptibility measurements.     

Low-temperature study of magnetic ordering in gadolinium orthophosphate

The zero-field heat capacity shows an antiferromagnetic ordering of Gd³⁺ in gadolinium orthophosphate at 0.8 K. The application of the external magnetic field leads to the splitting of the Gd³⁺ ground-state multiplet. The antiferromagnetic ordering becomes gradually suppressed with increasing field, and the loss of the long-range magnetic ordering with a threshold field between 0.2 and 0.5 T is indicated by heat-capacity data. Estimated entropy of the anomaly due to magnetic ordering or the Schottky-type anomaly (above 0.5 T) is close to Rln8 as expected for Gd³⁺ ground-state multiplet. Magnetization measurements above 2 K corroborate this magnetic behaviour.     

Calorimetric evidence of antiferromagnetism in GdBa2Cu3O6+δ

Heat capacity measurements between 1.8 and 10K have been made on the superconductor GdBa₂Cu₃O_6+δ with δ ⋍ 0.7 and T_c ⋍ 94K. A well defined magnetic transition prevails at 2.22K. The entropy involved agrees well with the complete antiferromagnetic ordering of Gd³⁺ ions.     

Coexistence of superconductivity and long-range antiferromagnetic order in Gd1.2Mo6Se8

Neutron diffraction experiments confirm that the ternary rare earth compound Gd_1.2Mo₆Se₈ orders antiferromagnetically in the superconducting state at a Neél temperature T_N=0.75 K.     

Low-temperature heat capacity and ac magnetic susceptivility of the re-entrant ferromagnetic superconductor Ho1.2Mo6S8

Thermal hysteresis in the ac magnetic susceptibility and a spike-shaped anomaly in the heat capacity, superimposed on a feature due to ferromagnetic ordering, have been observed at the coupled superconducting to normal-ferromagnetic transition in Ho_1.2Mo₆S₈.     

Magnetic ordering in the superconducting state of rare earth molybdenum sulphides, (RE)1.2Mo6S8 (RE = Tb, DyandEr)

The resistance and ac susceptibility measurements on Tb_1.2Mo₆S₈, Dy_1.2Mo₆S₈ and Er_1.2Mo₆S₈ revealed a magnetic ordering in their superconducting state at 0.9, 0.4 and 0.15 K, respectively. Some of the important features of the ordering in each compound are discussed with reference to their upper critical field curves.     

Low temperature heat capacity of rare earth molybdenum sulfides

Pronounced anomalies in the heat capacity of Gd_1.2Mo₆S₈, and probably Dy_1.2Mo₆S₈ are associated with the previously reported antiferromagnetic transition which occurs in the superconducting state in each of these compounds.     

Spin resonance of conduction electrons and EPR of localized moments in a low-dimensional organic conductor [Pd(dddt)2]Ag1.5Br3.5

The EPR spectra of a quasi-two-dimensional organic metal [Pd(dddt)₂]Ag_1.5Br_3.5 contain signals due to the spin resonance of conduction electrons (CESR) and signals due to the localized magnetic moments of Ag²⁺. The system of Ag²⁺ ions exhibits a strong indirect antiferromagnetic exchange interaction characterized by the Weiss constant Θ=−280(25) K, which leads to magnetic ordering at a temperature of T ₀=70(10) K. The same temperature T ₀ corresponds to a strong anomaly in the CESR linewidth. The observed anomaly in the CESR linewidth, as well as some features of the temperature dependence of conductivity in the system studied, are explained by the interaction between conduction electrons and Ag²⁺ ions localized in the anion layers (π-d interaction) and by antiferromagnetic ordering of the spins of Ag²⁺ magnetic ions.     

Insulator-metal transition in V3O5

The results of a study of the insulator-metal phase transition in V₃O₅ occuring at T = 450 K are presented. Conductivity, thermopower, magnetic, structural and optical data are reported. On a basis of the data obtained we conclude that the high-temperature phase is a poor metal with strong electron correlations and localized magnetic moments. The metal-non-metal transition is presumably driven by the spatial ordering of V³⁺ and V⁴⁺ ions.     

Heat capacity and magnetic phase transitions of rare-earth orthoferrite HoFeO3

Heat capacity of two rare-earth orthoferrites HoFeO₃ and LuFeO₃ were measured between 1.8 and 200 K. A distinctly large and two small heat capacity anomalies were detected for HoFeO₃ under zero magnetic field around 3.3, 53 and 58 K, respectively. The low-temperature anomaly with a peak at 3.3 K is due to the ordering of Ho³⁺ ions and the estimated magnetic entropy for this transition was favorably compared with the expected (R ln 2). Application of magnetic field significantly affects the positions and the magnitudes of the anomaly at 3.3 K. Energies of low-lying levels of the lowest J-term of Ho³⁺ ion were roughly estimated through analysis of the Schottky heat capacity.     

Magnetoelectric coupling driven dielectric anomaly in non-polar system SeCuO3

The non-polar material SeCuO₃, which contains Cu²⁺ with S=1/2$S=1/2$ spin and Se⁴⁺, has a highly distorted perovskite structure due to the small radii of Se⁴⁺ cations. The dielectric constant displays a critical decrease at 25 K, at which temperature the ferromagnetic ordering of the Cu²⁺ spin appears, suggesting a strong coupling between the magnetic and dielectric properties. Studies on SeCuO₃ show that the magnetic and electrical subsystems reciprocally correlate via the hybridization of sp and pd. We conclude that the spin–pair correlation along the b-axis plays a significant role in the decrease of the dielectric constant around the magnetic transition temperature via the magnetoelectric coupling, and successfully explain the dielectric anomaly. The obtained dielectric constant and the magnetocapacitance for SeCuO₃ are quantitative agreement with experimental results.     

Low temperature magnetization measurements on heavy rare earth molybdenum sulfides, (RE)1.2Mo6S8

Magnetization measurements on the heavy rare earth molybdenum sulfides, (RE)_1.2Mo₆S₈ (RE = Gd, Tb, Dy and Ho) were performed to further clarify the natures of their magnetic orders found in our recent resistivity and ac susceptibility measurements below 1 K. The variations of their magnetic and superconducting transition temperatures along the series of the rare earth ions in the Periodic Table are interpreted by means of available theories.     

Electron spin resonance studies at the phase transition in YbInCu4

In the intermetallic compound YbInCu₄ the Gd³⁺-ESR and the static susceptibility were measured in the temperature range 1.5K–300 K, i.e. both below and above the valence phase transition which occurs in this material at T_ph≈50 K. The Gd³⁺ resonance is mainly determined by exchange interaction of Gd³⁺ ions with fluctuating Yb³⁺ ions via conduction electrons (RKKY coupling) both below and above this transition. Arguments are presented that at low temperature YbInCu₄ is a dense Kondo system (mixed valent state).     

Coexistence of superconductivity and long-range antiferromagnetic order in the compound ErxMo6Se8

Low temperature specific heat and magnetic susceptibility measurements indicate that superconductivity and long-range antiferromagnetic order coexist in the compound Er_xMo₆Se₈ (x = 1.0 and 1.2).     

Synthesis and properties of InxMo6Se8-yIy indium insertion compounds

We present a study of the properties of the series In_xMo₆Se_8-yI_y (y=0.5, 1.0, 2.0) having the hexagonal-rhombohedral Mo₆Se₈ structure type. The materials were prepared by a low temperature (525 °C) insertion technique. For y=0.5, only the new superconducting phase In_.5Mo₆Se_7.5I_.5 was found. For y=1.0 and y=2.0, the In_xMo₆Se_8-yI_y phases represent the first Chevrel phases which are solid solutions with respect to indium insertion. In_xMo₆Se₇I (0 < × < 1) presents a phase diagram which indicates phenomena associated with an In⁺¹-In⁺³ valence change as x increases. In_xMo₆Se₆I₂ (0 < × < 0.66) is a well-behaved solid solution of In³⁺ in Mo₆Se₆I₂. For both y=1.0 and y=2.0, an increase in In³⁺ concentration increases the rhombohedral angle into the 94°– 95° range and the hexagonal unit cell volumes into the range of 940 Å ³ at maximum indium concentrations. At higher temperatures, the indium abstracts iodide from the cluster units which then condense to form known phases with larger clusters. Even though Mo₆X₈ phases are stabilized by partial halogen substitution, attempts to synthesize niobium Chevrel-type phases stabilized by halogen-chalcogen substitution were unsucessful.     

Nature of unusual spontaneous and field-induced phase transitions in multiferroics RMn2O5

Complex magnetic, magnetoelectric and magnetoelastic studies of spontaneous and field-induced phase transitions in TmMn₂O₅ were carried out. In the vicinity of spontaneous phase transition temperatures (35 and 25 K) the magnetoelectric and magnetoelastic dependences demonstrated the jumps of polarization and magnetostriction induced by the field ∼150 kOe. These anomalies can be attributed to the influence of magnetic field on the conditions of incommensurate-commensurate phase transition at 35 K and the reverse one at 25 K. In b-axis dependences the magnetic field-induced spin-reorientation phase transition was also observed below 20 K. Finally the magnetoelectric anomaly associated with metamagnetic transition is observed below the temperature of rare-earth subsystem ordering at relatively small critical fields of 5 kOe. This variety of spontaneous and induced phase transitions in RMn₂O₅ stems from the interplay of three magnetic subsystems: Mn³⁺, Mn⁴⁺, R³⁺. The comparison with YMn₂O₅ highlights the role of rare earth in low-temperature region (metamagnetic and spin-reorientation phase transitions), while the phase transition at higher temperatures between incommensurate and commensurate phases should be ascribed to the different temperature dependences of Mn³⁺ and Mn⁴⁺ ions. The strong correlation of magnetoelastic and magnetoelectric properties observed in the whole class of RMn₂O₅ highlights their multiferroic nature.     

Upper critical fields of superconducting superconducting rare-earth molybdenum selenides

Upper critical fields, H_c2(T), for superconducting LaMo₆Se₈, La_0.8Eu_0.2Mo₆Se₈, La_0.8Ce_0.2Mo₆Se₈ and PrMo₆Se₈ were measured. For the best material, LaMo₆Se₈, T_c = 11.3 K, (dH_c²/dT)_T=Tc=70 kG/K and and H_c² (4.2 K) = 370 kG. These selenide compounds show promise as very high field superconductors.     

The specific heat of C-phase Gd2O3

The specific heat of C-phase (almost cubic) Gd₂O₃ has been measured between 1.4 and 18 K. It shows a broad peak at 2.0 K. While the peak can be fit by a Schottky specific heat curve, the weight of all the experimental evidence to date indicates that the specific heat is due to magnetic ordering of the Gd⁺⁺⁺ ions. The implications of the results on the technological applications of C-phase Gd₂O₃ are discussed.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Growth, optical parameters, and spectroscopic properties of crystals of disordered scheelite-like molybdates NaLaxGd1 ? x(MoO4)2 (x = 0–1) activated by Tm3+ ions

Polarized optical absorption spectra in the range of 450–2000 nm have been recorded for the Czochralski-grown concentration series of single crystals of mixed scheelite-like molybdates NaLa _x Gd_{1 − x} (MoO₄)₂(x = 0–1) activated by Tm³⁺ ions. The intensity parameters Ω _t (t = 2, 4, 6) of the crystals grown were determined by the Judd-Ofelt method, and the values found were used to estimate the probabilities and emission branching ratios for a number of radiative transitions of Tm³⁺ ions. The main optical constants of an Na_0.909La_0.490Gd_0.498Tm_0.035Mo_2.020O_8.048 crystal were found and the temperature dependences of the refractive index for the ordinary and extraordinary waves were investigated in the temperature range of 293–373 K. The spectral dependences of the emission cross section for the expected ³ F ₄ → ³ H ₆ laser transition of Tm³⁺ ions in an Na_0.908La_0.301Gd_0.676Tm_0.048Mo_2.019O_8.043 crystal were measured in the spectral range of 1500–2000 nm. The gain cross section of the active medium on the ³ F ₄ → ³ H ₆ laser transition of Tm³⁺ ions in this crystal was calculated using the spectral dependences of the absorption and emission cross sections.