Antiferromagnetic phase transition in garnet-type AgCa2Mn2V3O12 and NaPb2Mn2V3O12

Ferrimagnetism has been extensively studied in garnets, whereas it is rare to find the antiferromagnet. Present work will demonstrate antiferromagnetism in the two Mn–V-garnets. Antiferromagnetic phase transition in AgCa₂Mn₂V₃O₁₂ and NaPb₂Mn₂V₃O₁₂ has been found, where the magnetic Mn²⁺ ions locate only on octahedral A site. The heat capacity shows sharp peak due to antiferromagnetic order with the Néel temperature T_N=23.8 K for AgCa₂Mn₂V₃O₁₂ and T_N=14.2 K for NaPb₂Mn₂V₃O₁₂. The magnetic entropy change over a temperature range 0–50 K is 13.9 J K^?1 mol-Mn²⁺-ions^?1 for AgCa₂Mn₂V₃O₁₂ and 13.6 J K^?1 mol-Mn²⁺-ions^?1 for NaPb₂Mn₂V₃O₁₂, which are in good agreement with calculated value of Mn²⁺ ion with spin S=5/2. The magnetic susceptibility shows the Curie–Weiss behavior over the range 29–350 K. The effective magnetic moment μ_eff and the Weiss constant θ are μ_eff=6.20 μ_B Mn²⁺-ion^?1 and θ=?34.1 K (antiferromagnetic sign) for AgCa₂Mn₂V₃O₁₂ and μ_eff=6.02 μ_B Mn²⁺-ion^?1 and θ=?20.8 K for NaPb₂Mn₂V₃O₁₂.     

Structural and magnetic properties of the double perovskite Sr2MnWO6

The nuclear and magnetic structure and the magnetic properties of the polycrystalline double perovskite Sr₂MnWO₆ have been studied. Rietveld analysis of neutron powder diffraction (NPD) data at T=295 K shows that the sample is tetragonal (space group P4₂/n, a=8.0119(4) Å, c=8.0141(8) Å). Some additional magnetic diffraction peaks were found in the NPD pattern at 10 K, which can be accounted for by antiferromagnetic ordering of spins at the Mn sites. The magnetic unit cell is doubled in all three unit axes directions (a=b=15.9984(8) Å, c=16.012(2) Å) and the manganese moments are coupled antiferromagnetically along the unit cell axes. The total magnetic moment of Mn²⁺ is found to be 2.27(7) μ_B. The antiferromagnetic behaviour was confirmed from magnetisation measurements. The transition from a paramagnetic to an antiferromagnetic state takes place at 13.0±0.1 K.     

Electrochemical and magnetic properties of Li4 / 3Mn2 / 3O2–LiCoO2 solid solution

The compounds Li_(4−x)/3Mn_2(1−x)/3Co_xO₂ (0 < x < 0.5) were prepared by the sol–gel technique. X-ray diffraction patterns of these compounds were identified as α-NaFeO₂ type layered structure, though some super-structure lines, related to the ordered array of Li and transition metal ions in the transition metal layer, were observed. The magnetic susceptibility exhibited an antiferromagnetic transition around 40 K for x < 0.2, however the specimens with x > 0.3 had no magnetic transition. The magnetic percolation may explain these magnetic variations. The electrochemical performances were evaluated for the compound of x = 0.5, and it was seen that the electrochemical properties were sensitive to the potential window. Additionally, it was also found that the discharge capacity strongly depended on the preparation temperature; it took a maximum value at the preparation temperature of 900 °C. The discharge capacity is sensitive not only to the cation mixing degree but also to the particle size.     

BaT2As2 single crystals (T = Fe,Co, Ni) and superconductivity upon Co-doping

The crystal structure and physical properties of BaFe₂As₂, BaCo₂As₂, and BaNi₂As₂ single crystals are surveyed. BaFe₂As₂ gives a magnetic and structural transition at T_N = 132(1) K, BaCo₂As₂ is a paramagnetic metal, while BaNi₂As₂ has a structural phase transition at T₀ = 131 K, followed by superconductivity below T_c = 0.69 K. The bulk superconductivity in Co-doped BaFe₂As₂ below T_c = 22 K is demonstrated by resistivity, magnetic susceptibility, and specific heat data. In contrast to the cuprates, the Fe-based system appears to tolerate considerable disorder in the transition metal layers. First principles calculations for BaFe_1.84Co_0.16As₂ indicate the inter-band scattering due to Co is weak.     

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.     

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.     

Accumulating evidence for the two-step magnetic ordering in the borocarbides DyNi2B2C and DyCo2B2C

Accumulating evidence for a two-step magnetic ordering in the borocarbide DyNi₂B₂C is summarized, including earlier overlooked evidence for the initial magnetic transition and a recent magnetization study of polycrystalline samples. The two-step ordering involves initial two-dimensional ferromagnetic ordering in the DyC (basal) planes at T_N (=16.3 K), gradual build-up of the three-dimensional (3D) alternate stacking of ferromagnetic planes, and a final 3D ordering in the AF–I-related structure at a lower temperature T_o (=10.4 K), depicting a first-order transition. Supporting evidence for the two-step magnetic ordering in DyNi₂B₂C comes from point-contact spectroscopy measurements in the normal state for DyNi₂B₂C–Ag contact, and from similar behaviour of PrNi₂B₂C and (Pr_0.91Dy_0.09)Ni₂B₂C. In the isostructural borocarbide DyCo₂B₂C the two magnetic transitions (at 7.8 and 2.6 K) deduced from the specific-heat measurements are also attributed to a two-step magnetic ordering.     

New superconducting RbFe2As2: A first-principles investigation

RbFe₂As₂ has recently been reported to be a bulk superconductor with T_c = 2.6 K in the undoped state, in contrast to undoped BaFe₂As₂ with a magnetic ground state. We present here the results of the first-principles calculations of the structural, elastic and electronic properties for this newest superconductor and discuss its behaviour in relation to other related systems.     

Magnetic anisotropy and magnetostriction of Lu2Fe17 single crystal

The magnetic properties of Lu₂Fe₁₇ single crystal have been studied by means of magnetization, susceptibility and magnetostriction measurements. The unusual magnetic behavior with two magnetic phase transitions has been observed in magnetic fields up to 50 Oe. The magnetostriction of the Lu₂Fe₁₇ compound has the maximum at temperature T≈285 K at which the paraprocess makes the main contribution to the magnetization.     

Superconductivity in Nb2InC

In this work the Nb₂InC phase is investigated by X-ray diffraction, heat capacity, magnetic and resistivity measurements. Polycrystalline samples with Nb₂InC nominal compositions were prepared by solid state reaction. X-ray powder patterns suggest that all peaks can be indexed with the hexagonal phase of Cr₂AlC prototype. The electrical resistance as a function of temperature for Nb₂InC shows superconducting behavior below 7.5 K. The M(H) data show typical type-II superconductivity with H_C1 ～ 90 Oe at 1.8 K. The specific heat data are consistent with bulk superconductivity. The Sommerfeld constant is estimated as γ ～ 12.6 mJ mol^?1 K^?1.     

Specific heat of TbMn2(H,D)2

Specific heat (SH) measurements on TbMn₂(H,D)₂ powders have been performed in the temperature range from 2 to 350 K, in zero magnetic field and in 9 T. Due to the low heat conductivity of the samples, the measurements were carried out on a mixed Cu- and sample-powder pellet. For TbMn₂, the anti-ferromagnetic phase transition was manifest by a single SH peak at T_N=47 K, whereas a double SH peak at 281 and 288 K and an upturn below 5 K were observed for the hydride sample. Upon applying the magnetic field of 9 T, the SH upturn was suppressed, whereas no visible influence was found on the specific heat in the whole temperature range above 10 K as well as on the double peak.     

Critical current density and vortex dynamics in uranium irradiated Co-doped BaFe2As2

We report the effect of defects introduced by heavy-ion irradiation with 2.6 GeV uranium ions at several matching fields in single crystalline Ba(Fe_0.925Co_0.075)₂As₂. The suppression rate of T_c at lower matching fields is larger than that at higher matching fields. The critical current density calculated from magnetic hysteresis loop is enhanced up to 4.1 × 10⁶ A/cm² at 2 K. Clear dips in magnetic hysteresis loops near zero field are observed at high matching fields. Field dependence of normalized relaxation rate is suppressed, and the relationship between the dip and the relaxation rate is discussed.     

Superconducting properties of the pyrochlore oxide Cd2Re2O7

We report the superconducting properties of the pyrochlore oxide Cd₂Re₂O₇. The bulk superconducting transition temperature T_c is about 1.0 K, and the upper critical field H_c2 determined by the measurement of specific heat under magnetic fields is 0.29 T. The superconducting coherence length is estimated to be 34 nm. Specific heat data measured on single crystals suggest that the superconducting gap of Cd₂Re₂O₇ is nodeless.     

Magnetism in PrIr2Si2: A single crystal study

We present the results of magnetization, susceptibility and specific-heat measurements of the high-temperature (HT) and low-temperature (LT) phases of PrIr₂Si₂ performed on single-crystalline samples. The HT and LT phases adopt the tetragonal CaBe₂Ge₂-type and ThCr₂Si₂-type structure, respectively. We have found no magnetic phase transition for the HT phase at temperatures down to 2 K. On the other hand, the LT phase apparently orders antiferromagnetically (AF) at 45.5 K and undergoes a transition to another AF phase at T_t=23.7 K. Complexity of the magnetic phase diagram is amplified by two metamagnetic transitions induced by magnetic field applied along the c-axis at temperatures below T_t. The results will be discussed with respect to other polymorphic compounds PrNi₂As₂ and UCo₂Ge₂.     

Crystal structure,thermal expansion and magnetic properties of Nd2Cu0.8Ge3 compound

A new ternary intermetallic compound, Nd₂Cu_0.8Ge₃, was synthesized and its crystal structure was determined by Rietveld refinement of X-ray powder diffraction data. The Nd₂Cu_0.8Ge₃ compound crystallizes in space group I4₁/amd (No. 141), with a tetragonal a-ThSi₂ structure type, and a=0.41783(2) nm, c=1.43689(9) nm, Z=2 and D_calc=7.466 g/cm³. Using the high temperature powder X-ray diffraction (HTXRD) technique, the lattice thermal expansion behavior of the compound was investigated in the temperature range of 298–648 K, and the result shows that its unit-cell parameters increased anisotropically when temperature increased. The magnetic susceptibility measured in the temperature range of 5–300 K indicated antiferromagnetic order of Nd₂Cu_0.8Ge₃ at low temperatures, and the magnetic susceptibility can be well described over the range of 50–300 K using Curie–Weiss law. The calculated effective magnetic moment (μ_eff) is 3.53 μ_B and dominated by the contribution of the Nd³⁺ ions.     

Stable barium compounds in YBa2Cu3Oy superconductors

The stability of various amounts of Ba₃Cu₃In₄O₁₂ (334) or BaTbO₃ (BTO) in a sintered YBa₂Cu₃O_y (YBCO) matrix was examined. Samples with added 334 or BTO exhibited critical temperatures (T_c) above 90 K for up to 20 vol.% addition and improved critical current densities (J_c) under a magnetic field. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis indicated that 334 and BTO did not react with the YBCO matrix under the sintering conditions used. The normalized J_c under a magnetic field of 1 T reached a maximum at 14 vol.% of 334 addition and 20 vol.% BTO addition. YBCO thin films with added BTO showed a gradual decrease in the T_c with increasing BTO content. YBCO films with added 334 showed a constant T_c of 87 K up to a 334 content of 4 vol.%.     

Complex magnetism in a new alloy,Eu2PdSi3, with two crystallographically inequivalent sites

Lattice constants, electrical resistivity, heat capacity, AC and DC magnetic susceptibility and ¹⁵¹Eu Mössbauer effect studies on a new intermetallic compound, Eu₂PdSi₃, found to crystallize in an AlB₂-derived hexagonal crystal structure, are reported. The results establish that the Eu ions are in the divalent state down to low temperatures, undergoing two magnetic transitions, one at 40 K and the other at 10 K, pertaining to two different Eu sites. Interestingly, the minority Eu ions at the 2(b) sites order at a relatively high temperature (40 K) and this magnetic interaction is inferred to be ferromagnetic and quasi one-dimensional along the c-axis. The magnetic structure below 5 K following magnetic ordering (at 10 K) of the majority ions at 6(h) sites appears to be quite complex. In short, the crystallographic and (hence) the overall magnetic behaviors of this compound present an interesting situation.     

Magnetic orderings in La- and Y-substituted PrCo2Si2 compounds

The results of heat capacity (C), electrical resistivity (ρ) and magnetoresistance (Δρ/ρ) measurements on the compounds Pr_1−xLa_xCo₂Si₂ (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) and Pr_1−yY_yCo₂Si₂ (y=0.2, 1.0) are reported. The Pr sub-lattice dilution studies on the PrCo₂Si₂ compound indicate that in spite of the anomalously high antiferromagnetic ordering temperature (31 K), the RKKY interaction is responsible for magnetic ordering in this compound. The paramagnetic to antiferromagnetic transition temperature in Pr_0.8Y_0.2Co₂Si₂ is lower than that expected due to the positive chemical pressure effect. In contrast to the three magnetic transitions in PrCo₂Si₂ at 31, 17 and 9 K, there are only two magnetic transitions in Pr_0.8La_0.2Co₂Si₂ and Pr_0.8Y_0.2Co₂Si₂ above 3 K. The Δρ/ρ of PrCo₂Si₂ at 3 K is large (>20%) and positive for fields around 40 kOe. The metamagnetic transition observed in the Δρ/ρ data of Pr_1−xLa_xCo₂Si₂ and Pr_0.8Y_0.2Co₂Si₂ compounds has a systematic variation with composition.     

High coercivity Nd2(Fe,Co)14C-type ribbons prepared by melt spinning

Melt-spun Nd₁₃Dy₂Fe_77−xCo_xC₆B₂ (x=0, 5, 10, 15, 20) ribbons with a high coercivity more than 2 T have been obtained. It was found that the ribbons quenched at the optimum wheel speed 15 m/s (as-spun ribbons) mainly consist of ferromagnetic 2 : 14 : 1 phase and paramagnetic NdC₂ phase, and the ribbons spun at 25 m/s and subsequently annealed at 973 K for 15 min (as-annealed ribbons) are primarily composed of the magnetic 2 : 14 : 1 and 2 : 17 phases. The magnetization process of as-spun ribbons controlled by a pinning of the domain wall is different from that of as-annealed ribbons determined by a nucleation of the reverse domain. This significant difference originates possibly from the existence of paramagnetic NdC₂ phase acting as a pinning center in as-spun ribbons. In the as-annealed ribbons, the substitution of Co for Fe leads to increase of remanence (μ₀M_r), maximum energy product ((BH)_max) from 0.67 T, 9.7 MGOe for x=0 to 0.84 T, 14.4 MGOe for x=10, respectively. A coercivity of 2.74 T is obtained for as-quenched Nd₁₃Dy₂Fe_77−xCo_xC₆B₂ (x=0) ribbons.     

Investigation of magnetic properties of interacting Fe2O3 nanoparticles

The magnetic properties of Fe₂O₃ nanoparticles (average diameter ∅≅3 nm) in alumina (68% Fe₂O₃ in weight) have been investigated by magnetization measurements. The results indicate a superparamagnetic behavior of interacting particles, which block with decreasing temperature (the zero-field-cooled susceptibility shows a maximum at T≅145 K) with a distribution of relaxation times. A change of magnetic regime is observed below ∼60 K, due to the increasing interparticle interactions and local surface anisotropy.