Magnetic phase diagram of diluted spinel Zn1−xCuxCr2Se4 system

By using mean field theory, we have evaluated the nearest-neighbour and the next-neighbour super-exchange J₁(x) and J₂(x), respectively, for Zn_1−xCu_xCr₂Se₄ in the range 0?x?1. The intraplanar and the interplanar interactions are deduced. High-temperature series expansions are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random phase approximation. The magnetic phase diagram is deduced. A spin glass phase is predicted for intermediate range of concentration. The results are comparable with those obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) have been deduced. The values are comparable to those of the 3D Heisenberg model, and are insensitive to the dilution x.     

Study of magnetic properties of Mn1−xCuxCr2S4 by: High-temperature series expansions

The high-temperature series expansions method applied in the systems Mn_1−xCu_xCr₂S₄ in the range 0?x?1. The exchange interactions and the magnetic exchange energies are calculated by using the probability law. The high-temperature series expansions have been applied in the spinel Mn_1−xCu_xCr₂S₄ systems, combined with the Padé approximants method, to determine the magnetic phase diagram, i.e. T_C versus dilution x. The critical exponent associated with the magnetic susceptibility (γ) is deduced. The obtained value of γ is insensitive to the dilution ratio x and may be compared with other theoretical results based on 3D Heisenberg model.     

Heat capacity measurements of itinerant electron magnetism in Y3Ni13−xCOxB2 system

The heat capacity of the Y₃Ni_13−xCo_xB₂ series has been measured from 300 mK to RT. The magnetic ordering phase transitions have been characterized as second-order type and the T_c's determined. The electronic contribution to the low-temperature heat capacity for x=0 yields an electronic constant γ=54 mJ mol K², which is higher than those of YNi₅ and YNi₄B, proving experimentally that its density of states at the Fermi surface is larger than in those other compounds. The substitution of Ni by Co increases γ linearly. Electronic band calculations could explain these features.     

Magnetic structure of TbNiAl4

Magnetisation and specific heat measurements, in the range 2 K to room temperature, demonstrate that three magnetic phases exist for the intermetallic compound TbNiAl₄. Powder neutron diffraction, also carried out over a wide temperature range, establishes that the intermediate magnetic phase is incommensurate, and confirms that the lowest temperature phase has a linear antiferromagnetic structure with a (0 1 0) propagation vector. The respective transition (Néel) temperatures, in zero applied magnetic field are 34.0 and 28.0 K.     

High temperature magnetic ordering in La2RuO5

Magnetic susceptibility, heat capacity and electrical resistivity measurements have been carried out on a new ruthenate, La₂RuO₅ (monoclinic, space group P2₁/c) which reveal that this compound is a magnetic semiconductor with a high magnetic ordering temperature of 170 K. The entropy associated with the magnetic transition is 8.3 J/mol K close to that expected for the low spin (S=1) state of Ru⁴⁺ ions. The low temperatures specific heat coefficient γ is found to be nearly zero consistent with the semiconducting nature of the compound. The magnetic ordering temperature of La₂RuO₅ is comparable to the highest known Curie temperature of another ruthenate, namely, metallic SrRuO₃, and in both these compounds the nominal charge state of Ru is 4+.     

Superconducting NbB2: An ab initio study of elastic constants

The five different elastic constants of the superconducting NbB₂ are calculated for the first time by ab initio density functional method with both correlation and exchange potentials. In the absence of experimental data, the results are compared with those of other related diborides. The fully relaxed and isotropic bulk moduli are also estimated and the implication of their comparison is made.     

Magnetic structure and phase diagram of the frustrated spinel Cd1−xZnxCr2Se4

In attempt to characterise the magnetic ordering in the whole composition range of the Cd_1−xZn_xCr₂Se₄ system, various magnetic measurements were performed on both crystalline and polycrystalline samples with 0?x?1. The magnetic properties of the system are typical of a ferromagnet below x=0.4 and of a complex antiferromagnet one above x=0.6. In this work the intermediate region was carefully studied. The variations of both M(T) and χ_ac at low fields suggest that transitions from ferromagnetic to Gabay–Toulouse ferromagnetic-spin-glass mixed phase at low temperature occur in the range 0.41?x?0.58. The high-temperature susceptibility measurements show that for the whole concentration range the system obeys Curie–Weiss laws. The results can be explained by the coexistence of competing interactions (ferromagnetic between nearest neighbours and antiferromagnetic between higher order neighbours) and disorder due to the random substitution between zinc and cadmium ions in the tetrahedral sites of the spinel lattice. An experimental magnetic phase diagram of the system is established.     

Magnetic properties of MnCr2O4 nanoparticle

The exchange interactions and the magnetic exchange energies are calculated by using the mean field theory and the probability law of Zn_1−xMn_xCr₂O₄ nanoparticles. The high-temperature series expansions have been applied in the spinels Zn_1−xMn_xCr₂O₄ systems, combined with the Padé approximants method, to determine the magnetic phase diagram, i.e. T_C versus dilution x. The critical exponent associated with the magnetic susceptibility (γ) is deduced. The obtained value of γ is insensitive to the dilution ratio x and may be compared with other theoretical results based on the 3D Heisenberg model.     

AuB2 in comparison to superconducting MgB2-an ab initio study

The noble metal diboride AuB₂, a potential candidate for superconductor, is studied by an ab initio method in comparison to the superconducting MgB₂. The results, described in terms of equilibrium lattice constants, bulk modulus, pressure derivative of bulk modulus and their in- and out-of-plane linear values, volume coefficient of T_c, density of states, band structure, show some similarity as well as dissimilarity between the behaviour of the two compounds. The implications for the behaviour are discussed.     

High elastic modulus in b-axis-oriented single crystal V2O5

The elastic moduli of V₂O₅ are of great importance for the assessment of the buildup of strain during thin-film growth as well as for the analysis of defect generation and propagation. The usually rather small crystal dimensions make a precise experimental determination of the elastic constants highly challenging and only very little is known about the temperature dependent strain evolution. Here large V₂O₅ single crystals grown with different parameter sets are investigated by ultrasonic pulse experiments and by sampled continuous wave ultrasound spectroscopy. The elastic modulus C₂₂ is determined to be 220 GPa at room temperature. Temperature dependent investigations of the elastic behavior show that ultrasonic experiments are suitable for highly sensitive detection of oxygen-deficient phases in rapidly grown samples. In addition they indicate the presence of temperature dependent elastic instabilities in inhomogeneous samples.     

Specific heat and magnetic ordering of NdNi2B2C

In this work, the specific heat of NdNi₂B₂C was computed with the three sets of crystal-field parameters proposed by previous authors. All curves of the heat capacity plotted with the calculated results exhibit sharp peaks around the magnetic transition temperature TN as experimentally observed. To understand the mechanism of its magnetic ordering, we also calculated the magnetization of the material in low temperature region with the ground crystal-field (CF) level, the two lowest CF levels, and the full CF levels of J=9/2 multiplet respectively for comparison. Using the two eigenstates of the ground CF level, we derived a formula for 〈Jx〉 with mean-field approach for theoretical analysis. Both our numeric and theoretical results suggest that the two lowest CF levels play dominant roles in the magnetic process of the material below TN. It is also very interesting to notice that the ground CF level itself results in a larger TN, but the inclusion of the first excited CF doublet in calculations instead hinders the magnetic ordering, leading to a weakly reduced transition temperature.     

Field dependent susceptibility of the 2D ferromagnet (CH3NH3)2CuCl4 in the paramagnetic phase

The temperature dependence of the ac susceptibility (χ) at constant applied magnetic field was investigated in the paramagnetic region of the quasi-2D ferromagnet (CH₃NH₃)₂CuCl₄. Above the Curie temperature (T_C=8.85 K) a maximum in the χ(T,H) curves was observed at T_m(H). The temperature at the maximum increases with increasing applied field. This anomaly is related to short range fluctuations close the order transition. The behavior of T_m(H) is governed by the gap exponent of the scaling function (Δ=γ+β). We found Δ=2.2±0.1 in very good agreement with the previously known values of γ and β.     

Possible anisotropy gap in La1.35Sr1.65Mn2O7 and La1.5Sr0.5NiO4 detected through specific heat and magnetization measurements

Magnetization and specific heat measurements, as a function of temperature, were performed on single crystals of La_1.35Sr_1.65Mn₂O₇ and La_1.5Sr_0.5NiO₄, under different applied magnetic fields (H). The specific heat in La_1.35Sr_1.65Mn₂O₇ was decreased for H=9 T parallel to the crystal c axis, compared with H=0, possibly due to a suppression of spin-wave excitations (magnons) in that ferromagnetic bilayer structure. On the other hand, the applied magnetic field had no effect in the specific heat of the antiferromagnetic La_1.5Sr_0.5NiO₄. For H=9 T and below the temperature of 4 K the specific heat data, for each crystal, was well fitted by an exponential decay law. This allowed the calculation of energy gaps around 1 meV for both compounds, in close agreement with Δ=2μ_BH for an expected energy gap in the magnon spectrum. Detailed magnetization measurements showed monotonic variations below 4 K and a steep increase close to 2 K. Both magnetization and specific heat measurements suggest the existence of an anisotropy gap in the energy spectrum of La_1.35Sr_1.65Mn₂O₇ and La_1.5Sr_0.5NiO₄.     

A mechanical spectrum study of bilayer cuprate La1.82Sr0.18CaCu2O6+δ

The mechanical spectrum of bilayer cuprate La_1.82Sr_0.18CaCu₂O_6+δ (La2126) was measured using the vibrating reed method from 30 K to room temperature at kilohertz frequency. A clear modulus softening was observed at 223 K accompanied by a sharp internal friction peak, which evidences a phase transition. Another broad internal friction peak around 250 K was observed, accompanied by a large modulus change. This contrasts with the broad internal friction peak of Ca doped Y Ba₂Cu₃O_7−δ (Y123) for which no clear modulus change was observed. The main contribution to the broad internal friction peak of La2126 is expected to be from the glass transition of the interstitial oxygen atoms.     

A low temperature internal friction study of Y0.85Ca0.15Ba2Cu3O7−δ with different oxygen content

The ceramic sample of Y_0.85Ca_0.15Ba₂Cu₃O_7−δ was prepared by standard solid-state reaction method, and samples with different oxygen concentration were obtained by quenching from high temperature. The internal friction was measured using the vibrating reed method from liquid-nitrogen temperature to room temperature at kilohertz frequency. An internal friction peak was observed around 250 K in Y_0.85Ca_0.15Ba₂Cu₃O_7−δ quenched from 1023 K. The peak is related to the one observed around 220 K (labeled as P3 peak) in undoped YBa₂Cu₃O_7−δ (Y123). This result shows the dependence of P3 peak on carriers density and P3 peak has a strong correlation to the abnormal behavior of Y123 in the underdoped range. The variation of two low temperature thermal activated relaxation peaks (P1 and P2) on oxygen content were also investigated. And consistent explanations were given based on all recent researches.     

On the phase diagrams of the ferromagnetic superconductors UGe2 and ZrZn2

A general phenomenological theory is presented for the phase behavior of ferromagnetic superconductors with spin-triplet electron Cooper pairing. The theory accounts in detail for the temperature-pressure phase diagram of ZrZn₂, while the main features of the diagram for UGe₂ are also described. Quantitative criteria are deduced for the U-type (type I) and Zr-type (type II) unconventional ferromagnetic superconductors with spin-triplet Cooper electron pairing. Some basic properties of quantum phase transitions are also elucidated.     

Effect of carbon doping on thermodynamic behavior of MgB2

The thermodynamic behavior of carbon doped MgB₂ has been studied using a rigid ion model (RIM). The model potential consists of the long-range Coulomb, the short-range repulsive and the van der Waals interactions. This model has successfully explained the cohesive and thermodynamic properties of Mg(B_1−xC_x)₂ (x=0.0, 0.02, 0.05, 0.075, 0.1, 0.2). The properties studied are the cohesive energy, molecular force constant, Restrahlen frequency, compressibility, Debye temperature and Gruneisen parameter. Our results on Restrahlen frequency and Debye temperature are in reasonably good agreement with the available experimental data. In addition, we have computed the specific heat C_p for Mg(B_1−xC_x)₂ (x=0.2) as a function of temperature T in the range 16 K?T?1000 K. We have also shown the variation of specific heat C_p with doping concentration at room temperature (300 K). The calculated specific heat C_p for Mg(B_1−xC_x)₂ (x=0.2) in the temperature range 16 K?T?22 K for which experimental results are available, agrees pretty well with the experimental data.     

Pressure-induced structural phase transition in ReO3

We have investigated the pressure-induced structural phase transition in ReO₃ by neutron diffraction on a single crystal. We collected neutron diffraction intensities from the ambient and high pressure phases at P=7 kbar and refined the crystal structures. We have determined the stability of the high pressure phase as a function temperature down to T=2 K and have constructed the (P-T) phase diagram. The critical pressure is P_c=5.2 kbar at T=300 K and decreases almost linearly with decreasing temperature to become P_c=2.5 kbar at T=50 K. The phase transition is driven by the softening of the M₃ phonon mode. The high pressure phase is formed by the rigid rotation of almost undistorted ReO₆ octahedra and the Re-O-Re angle deviates from 180°. We do not see any evidence for the existence of the tetragonal (P4/mbm) intermediate pressure phase reported earlier.     

Determination of Curie temperature by Arrott plot technique in Gd5(SixGe1−x)4 for x>0.575

Determination of Curie temperature by plotting magnetic moment vs. temperature curves requires a small applied field, which influences the measurement and temporarily disturbs the temperature of the sample especially for highly magnetocaloric materials. The Arrott plot technique was therefore used in order to determine the Curie temperature for a magnetocaloric Gd₅Si_2.7Ge_1.3 (x=0.675) single crystal sample. This technique was compared with other methods such as the inflection point technique and the line projection method. The results show how applied magnetic field influences the determination of Curie temperature. Using the Arrott plot the second-order transition Curie temperature for Gd₅Si_2.7Ge_1.3 was determined to be 304 K.     

Heat capacity and magnetic properties of pyrochlore Hg2Os2O7

Hg₂Os₂O₇, which has the cubic pyrochlore structure, remains metallic down to the liquid helium temperature unlike its isostructural counterpart Cd₂Os₂O₇, which shows metal-insulator transition at 226 K. Magnetization and heat capacity data for Hg₂Os₂O₇ are presented. The magnetic anomaly at T_N=88 K shares many characteristics in common with the metal-insulator transition in Cd₂Os₂O₇, though Hg₂Os₂O₇ remains metallic below T_N. The heat capacity C_p shows no or very little change in the magnetic entropy around T_N, supporting the view that there is no long-range ordering of localized spins. The measured value of electronic heat-capacity coefficient γ=21 mJ K⁻²mol⁻¹ is comparable to the value obtained from band-structure calculation on Cd₂Os₂O₇, suggesting that mass-enhancement is small in Hg₂Os₂O₇. There is a pronounced peak in C_p/T³ at 13.1 K, which corresponds to a peak in the phonon density of states at 40 cm⁻¹.