Structural and magnetic phase transitions of kagome-like compounds REBaCo4O7 (RE=Dy,Ho, Er,Tm, Yb,Lu)

In a temperature range 5–300 K the specific heat C(T) on a new mixed valence cobalt oxides REBaCo₄O₇ (RE=Dy, Ho, Er, Tm, Yb, Lu) was investigated. The first-order structural phase transitions from hexagonal P6₃mc to orthorhombic Cmc₂1 phase was indicated by a peak-like anomaly in C(T) curves at T_S～160, 178, 224, and 280 K for RE=Lu, Yb, Tm, and Er correspondingly. The magnetic phase transitions was indicated as the changes of slope on the C(T) curves were found at corresponding temperatures: T_N～50, 74, 98, and 98 K for RE=Lu, Yb, Tm, and Er, correspondingly.     

Superconductivity and thermal properties of sulphur doped FeTe with effect of oxygen post annealing

Here, we report the synthesis and characterization of sulphur-substituted iron telluride i.e. FeTe_1?xS_x; (x = 0–30 %) system and study the impact of low temperature oxygen (O₂) annealing as well. Rietveld analysis of room temperature X-ray diffraction (XRD) patterns shows that all the compounds are crystallized in a tetragonal structure (space group P4/nmm) and no secondary phases are observed. Lattice constants are decreased with increasing S concentration. The parent compound of the system i.e. FeTe does not exhibit superconductivity but shows an anomaly in the resistivity measurement at around 78 K, which corresponds to a structural phase transition. Heat capacity C_p(T) measurement also confirms the structural phase transition of FeTe compound. Superconductivity appears by S substitution; the onset of superconducting transition temperature is about 8 K for FeTe_0.75S_0.25 sample. Thermoelectric power measurements S(T) also shows the superconducting transition at around 7 K for FeTe_0.75S_0.25 sample. The upper critical fields H_c2(10%), H_c2(50%) and H_c2(90%) are estimated to be 400, 650 and 900 kOe respectively at 0 K by applying Ginzburg Landau (GL) equation. Interestingly, superconducting volume fraction is increased with low temperature (200 °C) O₂ annealing at normal pressure. Detailed investigations related to structural (XRD), transport [S(T), R(T)H], magnetization (AC and DC susceptibility) and thermal [C_p(T)] measurements for FeTe_1?xS:O₂ system are presented and discussed.     

Magnetic characterisation and hydrogen absorption characteristics of Pr3(Fe,Ti)29Hx

An interstitial Pr₃(Fe,Ti)₂₉ hydride was synthesised by gas-phase hydrogenation on Pr₃(Fe,Ti)₂₉ powder using H₂. The reaction kinetics between Pr₃(Fe,Ti)₂₉ and H₂ gases was studied in a constant-volume reactor. The sample starts to rapidly absorb hydrogen, interstitially, at about 533 K. Absorption passes through a maximum at about 598 K (1.6 H/f.u) and then interstitial hydrogen desorption takes place up to the temperature of 673 K. By cooling to room temperature, the sample absorbs more hydrogen, interstitially, reaching the value of 3.6 H/f.u. By remaining at room temperature, the sample absorbs even more hydrogen reaching the value of 5.2 H/f.u. The lattice expansion observed is 2.1% and the Curie temperature, T_C, increased from 392 to 518 K. The hydride exhibits saturation magnetisation, M_S, of 145.4 and 157.5 Am²/kg at room temperature (RT) and at 5 K, respectively, anisotropy field, H_A, of 2.1 T (RT) and 4.5 T (5 K) and average hyperfine field, H_eff, of 23.3 T (RT). The magnetic anisotropy of Pr₃(Fe,Ti)₂₉ hydride is the same as that of the parent compounds, easy-cone-like, changing only in the cone angle (from 34° to 26°).     

Giant magnetoresistance in the Ge-rich magnetocaloric compound,Gd5(Si0.1Ge0.9)4

We have measured the zero-field electrical resistivity in the temperature range 5–295 K and magnetoresistance in magnetic fields of up to 12 T of Gd₅(Si_0.1Ge_0.9)₄. The resistivity changes drastically at the magnetostructural first-order transition (T_C≅80 K on heating). This transition can be induced reversibly by the application of an external magnetic field above T_C, producing a concomitant giant magnetoresistance (GMR) effect, Δρ/ρ≅−50%. This study demonstrates that (in addition to giant magnetocaloric and magnetoelastic effects) GMR can be tuned between ∼20 and ∼290 K in Gd₅(Si_xGe_1−x)₄ with x⩽0.5 by simply adjusting the Si : Ge ratio.     

Synthesis and characterization of CaLaFe11O19

A new ferrimagnetic compound with the chemical formula CaLaFe₁₁O₁₉ has been synthesized by solid state reaction between the respective oxides and their structural, electrical and magnetic properties have been studied. One magnetic Fe³⁺ ion in CaLaFe₁₂O₁₉ is replaced by La³⁺ ion. The crystallographic results show the compound is hexagonal magnetoplumbite. The electrical conductivity has been measured from 300 to 800 K. The activation energy changes at Curie temperature (653 K). The compound is ferrimagnetic from 300 to 653 K and above T_c it acts as a paramagnetic. Variation of inverse molar susceptibility has been measured at various temperature in paramagnetic region and Curie molar constant (C_m) is calculated. AC susceptibility measurements are made at room temperature. The Seebeck coefficient (S) measurements show that the compound is n-type.     

Magnetic properties of GdCo12B6 compound under high pressures

The effects of hydrostatic pressure up to 10 kbar on Curie temperature T_C, compensation temperature T_COMP and spontaneous magnetization M_S of ferrimagnetic GdCo₁₂B₆ compound have been studied. Two antiferromagnetically coupled sublattices that are carrying magnetization of typically 0.42 μ_B/Co atom and 7 μ_B/Gd cancel out at compensation temperature at about 50 K and magnetic ordering temperature T_C=163±2 K. The volume dependence of intrinsic magnetic properties of the GdCo₁₂B₆ compound has been determined by studying it under hydrostatic pressure. The observed increase of M_S with pressure (dM_S/dp=+0.005 μ_B kbar^?1 at 5 K) is attributed predominantly to the pressure induced decrease of Co magnetic moments. The crucial role of Co in this behavior is confirmed by the change of sign of the pressure slope at temperatures above T_COMP and by the fact that the estimated decrease of m_Co is also quite comparable with pressure induced decrease of M_S in YCo₁₂B₆ (dM_S/dp=?0.007 μ_B kbar^?1). The decrease of m_Co is also responsible for the increase of T_COMP with pressure (dT_COMP/dp=+0.06 K kbar^?1). The decrease of T_C with pressure (dT_C/dp=?0.55 K kbar^?1) is comparable to the decrease observed on RCo₁₂B₆ compounds with non-magnetic R and can be attributed to the volume dependence of Co–Co exchange interactions. The remarkable role of the hybridization as a consequence of small distances between Co and B atoms could be a background of this rather unexpected volume stability of magnetic properties.     

Giant magnetic refrigerant capacity in Ho3Al2 compound

Magnetic properties and magnetocaloric effects (MCEs) of the intermetallic Ho₃Al₂ compound are investigated by magnetization and heat capacity measurements. Two successive magnetic transitions, a spin-reorientation (SR) transition at T_SR=31 K followed by a ferromagnetic (FM) to paramagnetic (PM) transition at T_C=40 K, are observed. Both magnetic transitions contribute to the MCE and result in a large magnetic entropy change (ΔS_M) in a wide temperature range. The maximum values of ?ΔS_M and adiabatic temperature change (ΔT_ad) reach 18.7 J/kg K and 4.8 K for the field changes of 0–5 T, respectively. In particular, a giant value of refrigerant capacity (RC) is estimated to be 704 J/kg for a field change of 5 T, which is much higher than those of many potential refrigerant materials with similar transition temperatures.     

Transverse voltage in superconductors at zero applied magnetic field

A systematic study of the transverse voltage at zero magnetic field in the superconducting state is reported. The effects of warming rate, temperature, applied magnetic field, and electrical current on the transversal resistance (R_XY) of polycrystalline superconducting sample are taken into account. At zero magnetic field two peaks are observed in R_XY(T) curves which are related to the double superconducting transition in the R_XX(T) component. In the superconducting (R_XX = zero) and normal states no transverse voltage has been detected at zero magnetic field as expected. The results are discussed within the framework of the motion of Abrikosov and Josephson vortices and anti-vortices. A new scaling relation between transverse and longitudinal components given by R_XY ～ dR_XX/dT has been confirmed.     

Pressure effect on magnetic properties of RCo12B6 (R=Y,Ce) compounds

The effect of pressure on magnetic properties of YCo₁₂B₆ and CeCo₁₂B₆ was studied in temperature range 5–300 K at pressures up to 9 kbar. The Curie temperature T_C and spontaneous magnetization M_S decrease with pressure for both compounds. The decrease can be attributed mostly to the volume dependence of both, the Co magnetic moment and the exchange interactions. The hybridization of the p–d states as a consequence of small distances between the Co and B atoms can be one reason of the relatively low pressure effects (ΔT_C/Δp=?0.39±0.02 K/kbar, d ln M_S/dp=?0.0013±0.0002 kbar^?1) in YCo₁₂B₆. Higher volume sensitivity of magnetic properties of CeCo₁₂B₆ in comparison with YCo₁₂B₆ can be attributed to the pressure induced changes of the Ce f- and Co d-states.     

Magnetism of UCoGe and U3Co4Ge7

A series of UCoGe and U₃Co₄Ge₇ polycrystalline samples has been prepared by arc melting and studied with respect to the phase composition and crystal structure, magnetization, a.c. susceptibility, electrical-resistivity and specific-heat behavior (down to 350 mK). U₃Co₄Ge₇ has been found to exhibit a spontaneous magnetization below T_C=21 K. Clear anomalies at T_C typical for a ferromagnetic transition have been observed in a.c. susceptibility, electrical-resistivity and specific-heat data. No additional anomaly, which would indicate the second magnetic phase transition below T_C reported in the literature, has been indicated. In all our UCoGe samples a transition to superconductivity has been revealed. On the other hand, no clear evidence of any transition to ferromagnetism with zero-field cooling down to 1.8 K has been obtained. The zero-field state is most probably governed by strong ferromagnetic spin fluctuations and seems to transform to a ferromagnetic state only when applying a magnetic field ≥10 mT. Simultaneously, an increase of the superconducting transition temperature is increasing with a magnetic field up to 10 mT and starts to decrease when increasing the field above this value. Measurements on bulk samples, which are by rule textured, indicate strong magnetocrystalline anisotropy in both investigated compounds.     

Giant magneto-caloric effect around room temperature at moderate low field variation in La0.7(Ca1−xSrx)0.3MnO3 perovskites

Among the perovskite manganites, a series of La_1?xCa_xMnO₃ has the largest magneto-caloric effect (MCE) (|ΔS_m|_max=3.2–6.7 J/kg K at ΔH=13.5 kOe), but the Curie temperatures, T_C, are quite low (165–270 K). The system of LaSrMnO₃ has quite high T_C but its MCE is not so large. The manganites La_0.7(Ca_1?xSr_x)_0.3MnO₃ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25) have been prepared by solid state reaction technique with an expectation of large MCE at room temperature region. The samples are of single phase with orthorhombic structure. The lattice parameters as well as the volume of unit cell are continuously increased with the increase of x due to large Sr²⁺ ions substituted for smaller Ca²⁺ ions. The field-cooled (FC) and zero-field-cooled (ZFC) thermomagnetic measurements at low field and low temperatures indicate that there is a spin-glass like (or cluster glass) state occurred. The Curie temperature T_C increases continuously from 258 K (for x=0) to 293 K (for x=0.25). A large MCE of 5 J/kg K has been observed around 293 K at the magnetic field change ΔH=13.5 kOe for the sample x=0.25. The studied samples can be considered as giant magneto-caloric materials, which is an excellent candidate for magnetic refrigeration at room temperature region.     

Improved critical current density in Zn doped YBCO single crystals

Magnetic measurements were made using pure YBCO and Zn doped YBa₂(Cu_1?xZn_x)₃O_7?σ. Single crystals with Zn concentration of 0.5%, 1.5%, 3.0% and 4.3%. The magnetic hysteresis loops for these samples were measured in the temperature range 0.1 ? T/T_c ? 0.96 under magnetic fields of 5 T using SQUID. It was found that the critical current density J_c increased for low Zn content samples up to 3% Zn concentration compared to pure YBCO sample and decreased for the higher Zn content samples. These values varied consistently when compared at magnetic fields of 1 T and 3 T. Moreover Zn doped samples showed significant values of J_c in the temperature range of 0.7–0.9T_c, close to critical temperature compared to pure YBCO sample. The irreversibility field H_irr was also enhanced in this temperature range showing consistent decrease with increase of Zn concentration. The peak field H_p above H_c1 and irreversibility field H_irr, both show power law dependence of the form H = m₁(1 ? T/T_c)^m2 in the temperature range of 0.75–0.96T_c. The values of parameter m₂ increased from 1.44 to 1.95 for the samples up to 3% Zn content and decreased to 1.37 for higher Zn contents. The ratio H_irr/H_p was found to be 3–4 for the lower Zn content samples and was 7–8 for the sample with high Zn content indicating more disorder for higher Zn content samples. The region between peak field H_p and irreversibility field H_irr was broadened with the increase of Zn concentration. The strong effect of Zn substitution in modifying behavior of these samples even at elevated temperatures is possibly due to the changes in the anisotropy of our samples with the increase of Zn concentration and also due to the locally induced changes in magnetic moments by Zn substitution.     

A new ternary carbide Dy2Mn2−xC5 (x=0.6): Preparation,crystal structure,and physical properties

The crystal structure, magnetic and electrical transport properties of the new ternary compound Dy₂Mn_2−xC₅ (x=0.6) have been investigated. According to X-ray powder diffraction the carbide crystallizes in its own structure type, space group I4/mmm, a=3.6421(2), c=15.7713(9) Å, R_B=0.062, R_p=0.134. The crystal structure contains isolated carbon atoms and C₂ dimers in square-bipyramidal holes and distorted bicapped square anti-prisms, respectively. Manganese atomic positions in the structure were found to be not fully occupied. Physical properties were studied in the temperature range down to 0.4 K. The electrical resistivity of Dy₂Mn_2−xC₅ (x=0.6) reveals its basically metallic nature with a positive temperature coefficient above about 30 K. A resistive anomaly at around 20 K indicates the appearance of an antiferromagnetic superzone boundary gap at low temperatures. A phase transition towards long range antiferromagnetic magnetic ordering below 19 K is further revealed by heat capacity and ac susceptibility data. Magnetization data refer to a non-trivial nature of the magnetic ground state which may be caused by the intrinsic structural disorder associated with random vacancies at the Mn site.     

Pressure-induced magnetic transition in MnRhAs

The magnetic properties of a Fe₂P-type intermetallic compound MnRhAs have been investigated under high pressure up to 8.0 GPa by AC susceptibility measurement. Initially, both the antiferromagnetic (AF(I)) to the canted state magnetic transition temperature T_t and the canted state to another antiferromagnetic one (AF(II)) transition temperature T_C increase with compression. At 4.0 GPa, however, T_t decreases abruptly, while the increasing rate of T_C becomes larger above this pressure. A pressure-induced magnetic phase transition was seen at around this pressure when T_t and T_C are plotted in the pressure–temperature phase diagram. The transition from the antiferromagnetic to the ferromagnetic state observed below 160 K with increasing pressure is not frequently observed.     

Effect of surface orientation and thickness on the magnetization of anisotropic FCC ferromagnetic films

The dependence on temperature of the layer magnetization of a Heisenberg ferromagnetic ultrathin film in presence of magnetocrystalline single-ion anisotropy was theoretically investigated in the framework of a Green's function approach using the random phase approximation (RPA). The effect of surface orientation and of film thickness N on the Curie temperature T_C was carefully investigated in the case of face centered cubic (FCC) films: the steepest increase of T_C(N) was found in the case of the FCC(1 1 1) orientation and the smoothest in the FCC(1 1 0) one. Our results for T_C(N) were successfully fitted by a finite-size scaling relation [T_C(∞)−T_C(N)]/T_C(N)=(N/N₀)^−λ, giving a shift exponent λ≃1.5, irrespectively of the surface orientation. Finally, the temperature evolution of the magnetization profile was analyzed, as well as its limiting shape at T_C.     

Magnetic properties and magnetocaloric effects in Gd1−xHoxNiIn intermetallic compounds

Magnetic properties and magnetocaloric effects (MCEs) of the intermetallic Gd_1?xHo_xNiIn (x=0?1) compounds have been evaluated by magnetization and heat capacity measurements. The Curie temperature T_C can be tuned from near 100 K to 20 K by substituting Ho for Gd atoms. In addition, all the compounds with Ho atoms undergo two successive magnetic transitions with the decrease of temperature: a paramagnetic (PM) to ferromagnetic (FM) transition around T_C and a spin-reorientation (SR) transition around 7?9 K. It is found that both transitions contribute to the magnetic entropy change (ΔS_M). For a field change of 5 T, the maximum values of ?ΔS_M for Gd_0.4Ho_0.6NiIn are 6 J/kg K at T_t=9 K and 10 J/kg K at T_C=52 K, respectively. These two ?ΔS_M peaks overlap partly and result in a wide working temperature range of MCE, and thus leading to the largest RC value of 443 J/kg in the Gd_1?xHo_xNiIn system.     

The thermopower of transition-metal double-perovskites: A sensitive probe of their electronic structures around the Fermi level

We report on a comparative study of S(T) for a series of transition-metal double-perovskites A₂BB′O₆ (A – Ca, Sr, Ba, and B, B′ = transition metal ions), some of them known to have half-metallic ground states. For Sr₂BB′O₆ with BB′ = CrMo, CrW, CrRe, FeMo, and FeRe (ferrimagnetic with high Curie temperatures), S(T) is metallic, for B′ = Mo and W it is n-type and for B′ =  Re, p-type. For A₂FeMoO₆ (A = Ca, Sr, Ba), the crystallographic differences (monoclinic, tetragonal and cubic space-groups, respectively) are accompanied by prominent differences in their (metallic) S(T). For the insulating Sr₂MnReO₆ and Ba₂MnReO₆, the onset of ferromagnetic order below T_c ∼ 120 K is marked by a steep drop of S(T) accompanied by only a slight change in the slope of ln ρ versus 1/T^1/2. Significant conclusions were drawn from the experimental results without the need for elaborate models.     

Inelastic neutron scattering on iron-based superconductor BaFe2(As,P)2

Inelastic neutron scattering has been performed on powder sample of an iron-based superconductor BaFe₂(As_0.65P_0.35)₂ with superconducting transition temperature (T_c) = 30 K, whose superconducting (SC) order parameter is expected to have line node. In the normal state, constant-E scan of dynamical structure factor, S(Q, E), exhibits a peak structure centered at momentum transfer Q ～ 1.20 Å^?1, corresponding to antiferromagnetic wave vector. Below T_c, the redistribution of the magnetic spectral weight takes place, resulting in the formation of a peak at E ～ 12 meV and a gap below 6 meV. The enhanced magnetic peak structure is ascribed to the spin resonance mode, evidencing sign change in the SC order parameter similar to other iron-based high-T_c superconductors. It suggests that fully-gapped s_± symmetry dominates in this superconductor, which gives rise to high-T_c (=30 K) despite the nodal symmetry.     

Effects of Cd2+ substitution on elastic properties and step-like anomalies in Tl0.9Bi0.1Sr1.8Yb0.2Ca1−xCdxCu1.99Fe0.01o7−δ superconductors

Cd-doped Tl_0.9Bi_0.1Sr_1.8Yb_0.2Ca_1−xCd_xCu_1.99Fe_0.01O_7−δ (x=0–0.4) bulk superconductor samples were prepared by solid-state reaction method, to examine the effect of Cd on ultrasonic velocity and elastic behavior of the samples. The samples were characterized by X-ray diffraction, DC electrical resistivity and temperature dependent ultrasonic velocity measurements. DC electrical resistivity measurement showed all the samples exhibit metallic normal-state behavior with the highest T_{c zero} observed at around 76.4 K (x=0.3). Ultrasonic velocity measurements at 80 K showed a non-linear increase in both absolute longitudinal and shear velocities as well as elastic moduli with Cd substitution with the largest increase observed for the x=0.3 sample. Temperature dependant longitudinal modulus showed elastic anomaly characterized by a step-like slope change at around 230 K for x=0 & x=0.3 and at around 250 K for x=0.4 with the x=0.3 sample showing the sharpest slope change. A comparison between experimental data and calculated lattice anharmonicity curve based on the model proposed by Lakkad, showed large deviation of the experimental longitudinal modulus curves for (x=0.3) from the calculated anharmonicity curves indicating that the elastic behavior was strongly influenced by the existence of the step-like longitudinal anomaly. On the other hand, our analysis using the Landau free energy model found that the anomalous step-like elastic behavior fitted well with the equation derived from the model for regions below and above the elastic anomaly temperature, T_A. The fitting indicated that the anomaly is related to a phase transition that is suggested to involve ordering of oxygen which introduces strain in the system.     

Ferromagnetic order and spin-glass behavior in multi-metallic compound Ni1.125Co0.375[Fe(CN)6] · 6.8H2O

Multi-metallic Prussian blue compound Ni_1.125Co_0.375[Fe(CN)₆] · 6.8H₂O has been synthesized. The Mössbauer spectroscopy at room temperature and IR spectra study revealed that the metal ions are bonded through cyanide ligand and the presence of low spin Fe^III(S = 1/2) and high spin Fe^III(S = 5/2) ions, as showed in these structure: Fe^III(S = 1/2)-CN-(Co^II/Ni^II)(96%) and Fe^III(S = 5/2)-NC-(Co^II/Ni^II) (4%). The Curie constant of C = 3.00 cm³ K mol⁻¹ and Weiss paramagnetic Curie temperature of θ = 16.43 K were observed in fitting according to Curie–Weiss law. These results indicate that there existed a ferromagnetic exchange interaction in the complexes. The observed value of coercive field (H_c) and remanent magnetization (M_r) at 4 K for the compound are 497 Oe and 1.03 Nβ. The presence of spin-glass behaviours in the compound is ascribed mainly to domain mobility or domain growth under different cooling conditions.