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.     

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.     

Neutron powder diffraction study of the crystal and magnetic structures of Fe2SiS4

The orthorhombic Fe₂SiS₄ (space group no. 62, Pnma) showing olivine-type structure has been studied. Measurements using neutron powder diffraction experiments were performed at 295, 140, 120, 40, 20 and 10 K in order to determine the crystal and magnetic structures of Fe₂SiS₄ at low temperatures. No crystallographic phase transition was observed between 295 and 10 K while two magnetic transitions were found. One transition around 127 K denotes a change from the paramagnetic (P) to an antiferromagnetic (AF) state while at 30 K the appearance of a ferrimagnetic (Fi) state is observed. Different magnetic models are presented. Relations are found between indirect magnetic interactions, on the one hand, and the Fe(4a), Fe(4c) and sulfur positions, on the other hand, and relations between the crystal structure and the magnetic models are given. The neutron powder diffraction results are compared with results, already reported, of Mössbauer and magnetization measurements.     

Neutron diffraction studies of the diluted spinel ferrite ZnxMg0.75−xCu0.25Fe2O4

Neutron diffraction studies have been performed in the temperature range 1050 K⩾T⩾10 K on the spinel system Zn_xMg_0.75−xCu_0.25Fe₂O₄ (x=0.0, 0.25, 0.5 and 0.75) prepared in the ceramic sintering method. Cation distributions in all the four compositions have been determined from the analysis of neutron data. A perturbed ferrimagnetic order has been observed in the compositions with x values lying in the low to intermediate range, where apparently, randomly canted spins and small fluctuating clusters are superimposed on the ferrimagnetic long range order. A complete breakdown of the ferrimagnetic stability occurs at x=0.75 as indicated by the absence of magnetic long range order. A huge diffuse scattering signal appears at the low-Q region broadening the (1 1 1) diffraction line in the low temperature neutron patterns of this composition indicating the formation of large magnetic clusters. A cluster spin glass state is suggestive for this composition.     

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.     

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.     

Field-dependent anomalies in the thermodynamic and transport properties of RuSr2GdCu2O8—Evidence for a spin-flop transition

Thermopower and magnetization studies of epitaxial thin films and sintered polycrystalline ceramics of RuSr₂GdCu₂O₈ show evidence for the occurrence of a spin-flop transition, consistent with earlier neutron diffraction data. Realignment of spins in a magnetic field enhances T_c by up to 4 K, presumably due to reduced magnetic pair-breaking. Specific heat and powder AC susceptibility measurements give strong evidence for bulk superconductivity. High temperature magnetization measurements indicate in-plane ferromagnetic correlations which remain to be reconciled with low-temperature neutron diffraction evidence for conventional G-type antiferromagnetism.     

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.     

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.     

Magnetic structure of the ternary germanide Ce3Ni2Ge7

The ternary germanide Ce₃Ni₂Ge₇ has been studied by means of neutron powder diffraction and Ce L_III X-ray absorption (XAS). This compound which orders antiferromagnetically below T_N=7.2(2) K, crystallizes in the orthorhombic (Cmmm space group) La₃Co₂Sn₇-type structure where Ce atoms occupying two inequivalent crystallographic sites: Ce1 at 2d site and Ce2 at 4i site. Below T_N, the antiferromagnetic structure of Ce₃Ni₂Ge₇ is collinear but only the Ce2 atoms carry a magnetic moment (1.98(2) μ_B at 1.4 K). The absence of ordered magnetic moment on Ce1 atoms can be correlated to the average valence v=3.03(1), determined by X-ray absorption spectroscopy, suggesting an intermediate valence state of cerium in the 2d site.     

Magnetic properties of RCr2Si2 compounds (R=Tb,Er)

We report on magnetic properties of RCr₂Si₂ compounds with R=Tb, Er. The polycrystalline samples were characterized by powder X-ray diffraction and found to be isostructurally crystallizing in the ThCr₂Si₂-type tetragonal structure. The samples were further investigated by specific heat, AC-susceptibility and magnetization methods in the temperature range 2–900 K and in magnetic fields up to 9 T. The magnetic measurements revealed the magnetic ordering of the rare-earth moments below about 2 K, whilst no high-temperature ordering of the Cr moments was observed. The evidence of for Cr magnetism is corroborated by results of first-principles calculations based on the density functional theory (DFT).     

Magnetic properties of RPdIn (R=Gd–Er) compounds

AC susceptibility and DC magnetization measurements were performed for the RPdIn (R=Gd–Er) compounds both in the paramagnetic and in the ordered state. In opposite to GdPdIn, which is a ferromagnet (T_c=102 K), the other samples show a complex ferrimagnetic behavior with the additional transition at T_t<T_c. In the high-temperature phase (for T_t<T<T_c), a ferromagnetic interaction dominates, while in the low-temperature phase (for T⩽T_t) antiferromagnetic interactions with the magnetocrystalline anisotropy, especially strong for TbPdIn, come into play. The ordering temperatures are T_c=70, 34, 25 and 12.3 K for Tb-, Dy-, Ho- and ErPdIn respectively, while transition temperatures are T_t=6, 14 and 6 K for Tb-, Dy- and HoPdIn respectively. TbPdIn reveals an additional transition at 27 K connected with the intermediate ferrimagnetic phase. The critical fields for the magnetization process of the low-temperature phase are high (52 and 150 kOe for TbPdIn and 32 kOe for DyPdIn at T=4.2 K) yet these values decrease remarkably with increasing temperature. Results of the study are compared with magnetic and neutron diffraction data hitherto available. We state that irreversibility of the zero-field cooled–field cooled magnetization is not connected with the spin-glass phase claimed elsewhere.     

Antiferromagnetic ordering in the double perovskites La2−xSrxCoRuO6

The electric transport and magnetic susceptibility of double perovskites La_2?xSr_xCoRuO₆ have been studied over a temperature range up to 800–1000 K. The crystal and magnetic structure has been determined by neutron diffraction on two samples of the series, x=0.6 and 1.4, which represent the electron- and hole-doped systems with respect to “ideal” single-valent insulator x=1. The study shows that spins in both the Co and Ru f. c. c. like sublattices exhibit a long-range ordering of the antiferromagnetic type II (T_N=60 K for x=0.6 and T_N=60–80 K for x=1.4).     

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.     

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.     

Structural and magnetic ordering in La0.6Ca0.4MnO3

The structural and magnetic ordering in La_0.6Ca_0.4MnO₃ has been studied by neutron powder diffraction as a function of temperature between 15 and 300 K. The para-ferromagnetic transition at T∼250 K is accompanied by significant structural distortions in the form of octahedral Mn–O₆ rotations. At 15 K, the total refined ferromagnetic moment on the Mn site was obtained as 3.1 μ_B, in reasonable agreement with the total expected average moment of mixed Mn³⁺/Mn⁴⁺ matrix.     

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.     

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.     

Transport,thermal, and magnetic properties in heavy-fermion superconductor Ce2CoIn8

We have grown high purity single crystals of Ce₂CoIn₈ using the self-flux technique, and have investigated its transport, thermal, and magnetic properties, including the anisotropic features. Single crystals of Ce₂CoIn₈ were grown in the lower temperature region to avoid the formation of un-wanted phases such as CeCoIn₅ and CeIn₃ impurities. The results of the structural and physical measurements imply that the present single crystals have high purity. The electrical resistivity and specific heat measurements demonstrate that Ce₂CoIn₈ has a superconducting ground state with a distinct non-Fermi liquid character. This indicates that the superconductivity in Ce₂CoIn₈ arises out of the verge of the underlying quantum critical instability mediated by the antiferromagnetic correlations. Additionally, we investigate the crystalline electric field (CEF) energy scheme based on the temperature dependence of the specific heat and the anisotropic features in the susceptibility. We propose one of the CEF level schemes calculated on the basis of the CEF model that the first and second CEF states are located at Δ₁ = 82 K and Δ₂ = 102 K above the ground state doublet, respectively.