Dielectric properties of the spin-1/2 dimer compounds Ba3Cr2O8 and Sr3Cr2O8

We have investigated the dielectric and magnetic properties of the spin-1/2 compounds Ba₃Cr₂O₈ and Sr₃Cr₂O₈. For Ba₃Cr₂O₈, the real part dielectric constant ε′(T) exhibit a frequency-independent peak near the Jahn-Teller transition temperature T_JT = 70 K. However, no anomaly in the ε′(T) curve is observed near T_JT = 275 K for the isostructural compound Sr₃Cr₂O₈. The difference in the ε′(T) behaviors may be attributed to the different onset temperatures of the phonon mode splitting in the samples. Dielectric relaxation analysis revealed that extrinsic contribution due to Maxwell–Wagner effect dominated at high temperatures for both compounds. The magnetization data reveal the occurrence of antiferromagnetic interactions between the Cr⁵⁺ ions and the χ(T) curves can be described by the Bleaney–Bowers equation for both compounds.     

Magnetic and magnetotransport properties of single-crystalline R_2PdGe_6(R = Pr,Gd and Tb)

Single crystals of R_2PdGe_6(R = Pr, Gd and Tb) compounds were grown by the Bi-flux method. Pr_2PdGe_6 is an antiferromagnetic compound with Néel temperature T_N= 15 K, in which a field-induced magnetic transition(spin flip) occurs when a magnetic field is applied along either a or b axis; a small magnetization and hysteresis loop were observed when a field is applied along c axis. Gd_2PdGe_6 is a collinear antiferromagnetic compound with T_N= 37 K along b axis. Tb_2PdGe_6 is an antiferromagnetic compound with T_N= 48 K and its hard magnetization direction is along b axis. The temperature dependences of the resistance of the entire three compounds present inflection points at the respective T_N. A large resistance(as well as magnetoresistance) change can be found at the spin flip transition of Pr_2PdGe_6, but the change is not obvious at the spin flop transition of Gd_2PdGe_6.     

Temperature and Magnetic Field-Induced Spin Reorientation in Rare-Earth Perovskite ErFe<Subscript>0.75</Subscript>Cr<Subscript>0.25</Subscript>O<Subscript>3</Subscript>

We report the synthesis of a single-phase rare-earth perovskite ErFe_0.75Cr_0.25O₃ polycrystalline and its magnetic properties. A transition occurs at temperature T _N = 120 K below which we observe a weak magnetic moment from the canted antiferromagnetism. Interestingly, ErFe_0.75Cr_0.25O₃ reveals the compensation-like behavior at T _comp?like = 27 K, where the net magnetic moments of transition-metal ions are antiparallel and equal to the induced net moment of Er³⁺ ions, and the paramagnetic contribution of Er³⁺ moment presenting a nonzero magnetization. The temperature-dependent magnetization measurement shows a spin reorientation transition from Γ₄ to Γ₁ at 6 K. Furthermore, it is also observed that there is a spin-flop transition at low temperature induced by external magnetic field in Γ₁ state (antiferromagnetic state). The interaction between (Fe/Cr)-3d and Er-4f electrons drives an extremely interesting spin reorientation transition which is highly sensitive to magnetic field and temperature.     

Evidence of Weak Ferromagnetism in Pb(Fe2/3W1/3)O3 Powders by Means of Non-resonant Microwave Absorption

We report on the effects of temperature and dc magnetic field on non-resonant microwave absorption measurements at X-band (8.8–9.8 GHz) in Pb(Fe_2/3W_1/3)O₃ powders, for the 294–425 K temperature range. Two techniques are used: the magnetically modulated microwave absorption spectroscopy (MAMMAS) and the low-field microwave absorption (LFMA). MAMMAS response showed distinctive features associated with para-antiferromagnetic transition at T _N =334.3 K, and below T _N a weak ferromagnetism is also observed, and it is attributed to canting of Fe⁺³ ion sublattices in the antiferromagnetic matrix. The low-field microwave absorption (LFMA) is used to give further knowledge on this material, where this technique also gives evidence of the magnetic transition, suggesting a weak ferromagnetism at room temperature.     

The crystal and magnetic structure of Ti-substituted LaCrO3

The crystal and magnetic structures of LaCrO₃ and La(Cr_0.90Ti_0.10)O₃ have been investigated between 5 and 350 K by means of neutron powder diffraction and DC magnetic measurements. Both compounds are characterized by an antiferromagnetic G_x-type ordering at low temperature. Structural features suggest the occurrence of Ti in the tetra-valent state. Despite the mixed valence induced by Ti-substitution leading to the occurrence of the Jahn-Teller species Cr²⁺, no evidence for long range ferromagnetism can be detected. On account of a miscibility gap, a higher degree of Ti-substitution at the Cr site cannot be achieved; as a consequence the solid state solubility of Ti in LaCrO₃ at 1573 K has been ascertained.     

Itinerant ferromagnetism to insulating spin glass in SrRu1−xCuxO3 (0 ≤ x ≤ 0.3)

The ferromagnetic metallic oxide, SrRuO₃ (T_C ∼ 165 K) undergoes structural, magnetic and metal-insulator transitions upon substitution of Cu at the Ru-site. For x = 0.2 in SrRu_1−xCu_xO₃, the structure becomes a tetragonal with the space group I4/mcm and there is a signature of both ferromagnetic (T_C = 65 K) and antiferromagnetic (T_N = 32 K) ordering due to possible magnetic phase separation. The antiferromagnetism arises due to short range ordering of Cu- and Ru-moments. Jahn-Teller distortion of (Ru,Cu)-O₆ octahedra indicates that the copper ions are in 2+ oxidation state with ⁶t2g³eg electronic configuration. For x ≥ 0.1, narrowing of Ru-4d bandwidth by the substitution of Cu ions results in semiconducting behavior. For x = 0.3, the ac and dc susceptibility measurements indicate a spin glass behavior. The origin of spin glass behavior has been attributed to competing ferromagnetic and antiferromagnetic interactions.     

Cu, Pu and Fe High T c Superconductors: All the Same Mechanism

The more than 20 years old Cu high-T _c superconductors exhibit as undoped parent materials antiferromagnetism. Upon doping the long-range antiferromagnetism disappears and only short-range antiferromagnetic clusters remain which show a spin pseudo-gap. There are no good ideas why long-range antiferromagnetism disappears upon the appearance of superconductivity because antiferromagnetism and superconductivity are compatible. A breakthrough has come about with the discovery of a plutonium (Pu) containing alloy PuCoGa₅ with a T _c of 18.5 K. In principle not very exciting, but in the field of actinides T _c??s are not more than 3 K because of the high mass and corresponding low phonon energies. The compound is a high-T _c material in the field of actinides. But also this Pu-containing compound is a short-range antiferromagnet with a spin pseudo-gap. A pattern starts to develop! As well in the Cu as in the Pu compounds, some magnetic ions Cu²⁺ and Pu³⁺ are replaced upon doping with nonmagnetic Cu³⁺ or spontaneously with nonmagnetic Pu²⁺ ions, thus a mixed valence configuration appears with nonmagnetic states (spin holes) in antiferromagnetic clusters. The newly discovered Fe pnictide superconductors, however, have only one valence, Fe²⁺ above and below T _N, the Néel temperature of 150 K, as well above and below T _c, as judged by the isomer shift of the Mössbauer effect. However, doping with fluorine, replacing oxygen, not only introduces electrons, but changes locally the crystal field acting on the iron ions. Divalent iron 3d⁶ has a high-spin configuration $\mathrm{t}_{2}^{4}\mathrm{e}^{2}$ in a magnetic ??₅ configuration and a nonmagnetic low-spin configuration $\mathrm{t}_{2}^{6}$ in a ??₁ state. So with the same valence we can have a magnetic and a nonmagnetic configuration, triggered by variation of the local crystal field induced by doping, causing again spin holes. We show that these spin holes in antiferromagnetic clusters have an attractive interaction and combine to make nonmagnetic bipolarons, which can condense and lead to superconductivity.     

Structure,superconductivity and magnetism of new rare earth-rhodium silicides RE2Rh3Si5 of U2Co3Si5-type

New ternary silicides RE₂Rh₃Si₅ (RE = Y, La, Nd, Sm, Gd, Tb, Dy, Ho, Er) have been prepared. They crystalize in the U₂Co₃Si₅-type structure. Only the silicides containing the diamagnetic rare earths Y and La show a superconducting transition at T_Cr = 4.4 ± 0.2K and T_Cr = 2.7 ± 0.1K respectively, those with magnetic rare earths order antiferromagnetically at low temperature.     

Synthesis, structure and exchange bias in Cr2O3/CrO2/Cr2O5 particles

We report on the synthesis, structure and magnetic properties of a novel exchange bias system with Cr₂O₃/CrO₂/Cr₂O₅ interfaces. Chromium oxide particles with mixed chromium valences were prepared by sintering CrO₃ in air. X-ray diffraction patterns show that CrO₃ lost its oxygen gradually with increasing temperature and time through Cr₃O₈, Cr₂O₅, CrO₂, and finally Cr₂O₃ at temperatures above 760 K. X-ray photoelectron spectra indicate a low CrO₂ content and a binding energy of 579.3 eV for Cr 2p_3/2 photoelectrons in Cr₂O₅. Chromium dioxide was found to stably coexist with Cr₂O₃ and Cr₂O₅ in the particles. Magnetic measurements show hysteresis loop shifts in the sample, indicating an exchange bias induced by antiferromagnetic Cr₂O₃/Cr₂O₅ in ferromagnetic CrO₂. An exchange bias of 9 mT at 5 K and a coercivity of 26.3 mT were observed in the chromium oxide particles containing CrO₂.     

EuFe2As2: Magnetic Structure and Local Charge Distribution Anisotropies as Seen by Resonant X-ray Scattering

Non-resonant and element specific magnetic X-ray scattering has been used to determine the orientation of Eu and Fe magnetic moments in EuFe₂As₂ iron pnictide. Experiments have been carried out on single crystal samples at the ESRF. Resonant measurements on magnetic reflections at the Eu L₃ absorption edge indicate that the orientation of the Eu moments in the antiferromagnetic phase (T<T _N=19 K) lie parallel to the crystallographic a-axis. In addition, non-resonant magnetic X-ray measurements indicate that the Fe moments are aligned along the same direction in the spin density-wave ordered phase (T<T _S=190 K). The temperature dependence of the integrated intensities suggests that the Fe magnetic sublattice is barely affected by the onset of Eu ordering at T _N. The observation of non-zero resonant intensity on nuclear-forbidden reflections with wavevector corresponding to the Fe magnetic propagation vector at both the Eu L₃ and As K absorption edges may be interpreted as the result of the polarization of the Eu 5d and As 4p electronic bands via hybridization with the Fe 3d states.     

Structural and magnetic properties of LaFe0.5Cr0.5O3 studied by neutron diffraction, electron diffraction and magnetometry

The structural and magnetic properties of the perovskite type compound LaFe_0.5Cr_0.5O₃ have been studied by temperature dependent neutron powder diffraction and magnetization measurements. Rietveld refinement of the neutron diffraction data shows that the compound crystallizes in an orthorhombic perovskite structure with a random positioning of the Fe and Cr cations at the B sublattice. The magnetic structure at 10 K is a collinear antiferromagnetic one with the magnetic moment per site being equal to 2.79(4) μ_B. Magnetisation measurements confirm the overall antiferromagnetic behaviour. Moreover, it indicates a weak uncompensated magnetic moment close to the transition temperature T_N ≈ 265 K. This moment can be described by a magnetic cluster state, which remains up to 550 K. Electron diffraction patterns along with high-resolution transmission electron microscopy images reveal that the crystallites are composed by domains of different orientation, which share the same cubic perovskite sub-cell reflections.     

Interactions magnetiques intra- et interclusters dans les pentafluorures RuF5 et OsF5

The nature of the magnetic interactions in RuF₅ and OsF₅ has been investigated using neutron diffraction and magnetic measurements under high applied fields. RuF₅ and OsF₅ are magnetically ordered at respectively T_N = 5 and 6 K. The magnetic structures have been determined. A model based on isolated tetranuclear clusters M₄F₂₀ (M = Ru or Os) has been applied to explain the magnetic behavior above the Néel temperature. The intracluster exchange interactions are antiferromagnetic. The fitting of the magnetization data leads to an exchange constant of $\text{J}\text{k}\text{= ?7.7}\text{K}$ for RuF₅, in good agreement with that calculated from the thermal variation of the magnetic susceptibility.     

Multiple Superconducting Gaps and Anisotropic Spin Fluctuations in Hole-Doped and Electron-Doped Iron-Pnictides: NMR Studies

We present extensive ⁷⁵As-NMR data on the electron-doped pnictides PrFeAsO_0.89F_0.11 (T _c=45 K), LaFeAsO_0.92F_0.08 (T _c=23 K), and the hole-doped Ba_0.72K_0.28Fe₂As₂ (T _c=31.5 K) single crystal. We find that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin lattice relaxation rate 1/T ₁ decreases below T _c with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below T _c and shows a step-wise T variation as well. These results indicate spin-singlet superconductivity with multiple gaps.     

Effect of chromium substitution on structure and magnetic properties of Bi2Fe4O9

Orthorhombic Bi₂Fe_4 − xCr_xO₉ (x = 0.0, 0.25, and 0.75) nanoplatelets were synthesized by a simple hydrothermal method. The structure, morphology, and magnetic properties of the obtained powders have been characterized. Calculation of the lattice parameters of Bi₂Fe_4 − xCr_xO₉, as well as bond lengths and angles, was carried out by X-ray diffraction Rietveld refinement. The volumes of the metal-oxygen tetrahedra and octahedra were calculated to be sequentially increasing as the Cr doping level increases. The samples undergo an antiferromagnetic transition at 250 ± 5 K. The magnetic moments of the samples increase with higher Cr doping level. The 3d electron spin state for Fe³⁺ in the as-prepared samples is different, which is possibly due to the distortion of Fe-O tetrahedra and octahedra in the crystal structure after chromium substitution.     

Lead nitroprusside: A new precursor for the synthesis of the multiferroic Pb2Fe2O5, an anion-deficient perovskite

In order to investigate the formation of multiferroic oxide Pb₂Fe₂O₅, the thermal decomposition of Pb[Fe(CN)₅NO] has been studied. The complex precursor and the thermal decomposition products were characterized by IR and Raman spectroscopy, thermal analysis, powder X-ray diffraction (PXRD), scanning electron microscopy and magnetic measurements. The crystal structure of Pb[Fe(CN)₅NO] was refined by Rietveld analysis. It crystallizes in the orthorhombic system, space group Pnma. The thermal decomposition in air produces highly pure Pb₂Fe₂O₅ as final product. This oxide is an anion deficient perovskite with an incommensurate superstructure. The magnetic measurements confirm that Pb₂Fe₂O₅ shows a weak ferromagnetic signal probably due to disorder in the perfect antiferromagnetic structure or spin canting. The estimated ordering temperature from the fit of a phenomenological model was 520 K. The SEM images reveal that the thermal decomposition of Pb[Fe(CN)₅NO] produces Pb₂Fe₂O₅ with small particle size.     

Fluorination of Bi<Subscript>1.8</Subscript>Fe<Subscript>1.2</Subscript>SbO<Subscript>7</Subscript> pyrochlore solid solutions

A technique has been developed for fluorinating the pyrochlore oxide Bi_1.8Fe_0.2FeSbO₇, and a compound with the composition Bi_1.8Fe_1.2SbO_7–x/2F_x has been obtained. The synthesized oxyfluoride also has the pyrochlore structure (sp. gr. Fd3m), with a lattice parameter a = 10.4443(1) Å (R _wp = 5.2). It has been shown that the charge balance upon fluorine substitution for oxygen is maintained not through partial reduction of Fe³⁺ to Fe²⁺ but through the incorporation of fluorine into oxygen vacancies. The magnetic behavior of the fluorinated pyrochlore phase is determined by the persisting frustration of the octahedral sublattice, which is responsible for the development of a spin glass state below T _f = 12 K. The fluorination-induced changes in the anion sublattice led to an increase in the antiferromagnetic exchange interaction between neighboring Fe³⁺ ions and changes in the dynamic properties of the spin glass phase.     

Antiferromagnetic interactions and Mössbauer study of LnMCuFeOδ Phases (Ln = Y,La; M = Ca,Sr, Ba)

A three-dimensional antiferromagnetic ordering 3D-AF atT _N ≈12K has been observed in these nonsuperconducting phases. In the compounds where a partial trivalent-divalent cationic substitution (Y by Ca or La by Sr) has been performed, this antiferromagnetic ordering is not observed above 4 K. Mössbauer spectroscopy studies indicate a unique iron environment and magnetic ordering; the spectra are consistent with the classic high spin Fe³⁺ although in an unusual fivefold coordination. Both the herein described transition and the formerly reported one at ～460 K are affected upon doping.     

Influence of the B-site ordering on the magnetic properties of the new La3Co2MO9 double perovskites with M = Nb or Ta

Double perovskites La₃Co₂NbO₉ and La₃Co₂TaO₉ have been prepared by both solid state and sol-gel synthesis. The crystal structures have been studied from X-ray and neutron powder diffraction data. Rietveld refinements show that the crystal structure is monoclinic (P2₁/n), with different degrees of ordering of B′ and B″ cations, with octahedra tilted according to the Glazer notation a⁻b⁻c⁺. Occupancy refinements show that the solid state materials are more B-site ordered than the sol-gel ones. Magnetization measurements show that these perovskites show two magnetic contributions, one with spontaneous magnetization and other with linear behaviour with the magnetic field associated to antiferromagnetic correlations. In the samples synthesized by solid state the spontaneous magnetization is more important than those synthesized by the sol-gel and present T_C of 62 K for Nb and 72 K for Ta. On the other hand, materials prepared by sol-gel have T_C 20 K for Nb and 40 K for Ta, respectively and major presence of the antiferromagnetic contribution. The competition between these magnetic behaviours is interpreted, by a microscopic point of view, as to be due to the different degrees of Co²⁺ ions disorder on the B site of the double perovskite structure. This disorder affects the ratio between the antiferromagnetic Co²⁺-O-Co²⁺ and the ferromagnetic Co²⁺-O-M⁵⁺-O-Co²⁺ couplings proposed for the system.     

Proximity effects on the spin density waves in X/Cr(001) multilayers (X = Sn, V, and Mn)

We present ab initio density functional calculations of the electronic structure and magnetic properties of X₂/Cr₃₆(001) and X₁/Cr₃₇(001) multilayers, with X = Sn, V and Mn, to investigate the impact of the proximity effects of the X layers on the spin density waves of the Cr slab. We find different magnetic profiles corresponding to the spin density wave and to the layered antiferromagnetic configurations. The nature of the different magnetic solutions is discussed in terms of the different interfacial environments in the proximity of Sn, V or Mn. The magnetic behavior at the interface is discussed in connection with the electronic structure through the density of electronic states projected at the interfacial X and Cr sites. We compare the results with those previously obtained for Fe₃/X₁/Cr₃₇/X₁(001) multilayers to analyze the role played by the ferromagnetic iron slab.     

On the longitudinal static and dynamic susceptibility of spin-1/2 Kondo systems

The impurity spin polarization, static susceptibility, and longitudinal impurity spin relaxation rate are calculated for thes-d model as function of temperature and magnetic field for ferromagnetic and antiferromagnetic exchange coupling. The thermodynamic functions and the dynamical susceptibility are obtained from the impurity relaxation spectrum, which is approximated by taking into account the infrared-like singularities. For antiferromagnetic coupling the zero-field susceptibility obeys a Curie-Weiss law1/χ～4.6(T+θ) for high and intermediate temperatures and it approaches the finite value1/χ～3.8θ for zero temperature. The zero-field relaxation rate is much larger than the Korringa value; it decreases with temperature and approaches the nonzero value1/T ₁～1.2θ for zero temperature. The relaxation rate decreases with increasing field. The results for the spin polarization agree well with the experimental data for the Cu:Fe alloy.