Synthesis, structure, and magnetism of a new heavy-fermion antiferromagnet, CePdGa6

A new compound, CePdGa₆, and its isostructural analog, LaPdGa₆ have been synthesized by flux growth and characterized by single-crystal X-ray diffraction. The compounds adopt a tetragonal structure with P4/mmm space group, Z=1. The lattice parameters for CePdGa₆ are and and and for LaPdGa₆. Magnetic and thermal measurement have revealed that CePdGa₆ is a heavy-fermion with the specific heat coefficient and Ce f moments order antiferromagnetically along c-axis at . Reconfiguration of spin occurs at to induce a ferromagnetic component only in the a-b plane. This strong anisotropy in the magnetism might be related to its unique layered structure.     

Metamagnetic phase transition in a diruthenium compound with interpenetrating sublattices

The diruthenium compound [Ru₂(O₂CMe)₄]₃[Cr(CN)₆] may be the only known material that contains two weakly-coupled, magnetically-ordered sublattices occupying the same three-dimensional volume. Due to the strong easy-plane anisotropy on each Ru₂ complex, the moment of each sublattice is constrained to one of the eight cubic diagonals. At low fields, the two sublattices are antiferromagnetically aligned by weak dipolar and deformation energies. But above a metamagnetic critical field of about 1000 Oe, the sublattice moments become ferromagnetically aligned and the net magnetization increases dramatically. We have successfully modeled this metamagnetic transition by assuming that the individual sublattice spin configurations are only weakly distorted by the magnetic field. This model suggests that the ground state of each sublattice undergoes a phase transition at a pressure of about 7 kbar. The drop in the sublattice moment and the rise in the sublattice susceptibility above 7 kbar can be explained by a high- to low-spin transition (S = 3/2 to 1/2) on the mixed-valent diruthenium complexes.     

Physical properties and anisotropies of the RNiGe3 series (R = Y, Ce-Nd, Sm, Gd-Lu)

We have studied RNiGe₃ (R=Y, Ce-Nd, Sm, Gd-Lu) single crystals by measuring crystal structure and stoichiometry, magnetic susceptibility, magnetization, electrical resistivity, magnetoresistance, and specific heat. Clear anisotropies as well as antiferromagnetic ordering in the RNiGe₃ series (R=Ce-Nd, Sm, Gd-Tm) have been observed above 1.8 K from the magnetic susceptibility. A metamagnetic transition in this family (except for R=Sm) was detected at 2 K for applied magnetic fields below 70 kOe. The electrical resistivity of this series follows metallic behavior in the high temperature region. Below the antiferromagnetic ordering temperature a significant anisotropy is exhibited in the resistivity and magnetoresistance for different current directions. The anisotropic magnetic, transport, and thermal properties of RNiGe₃ compounds are discussed in terms of Ni site occupancy as well as a combination of the effect of formation of a magnetic superzone gap and the crystalline electric field.     

Effects of different annealing atmospheres on magnetic properties in La2CoMnO6 single crystals

We have successfully synthesized the high-quality single crystals of double perovskite La₂CoMnO₆ by flux method and investigated the effects of different annealing conditions on the magnetic properties. In an as-grown crystal, ferromagnetic order along the c axis arises below 204 K, and the spin-glass-type behavior inherent from ionic disorders is also observed. Upon annealing in different gas environments, the reconstructed magnetic states induce a metamagnetic transition (after Ar annealing), and increase Curie temperature (after O₂ annealing). After quenching, the magnetic transition temperature decreases significantly, comprising lots of defects and disorders. Our results provide adequate procedures to control magnetic properties of double-perovskite compounds.     

Metamagnetism and long range ordering in μ-1,3 bridging transition metal thiocyanato coordination polymers

Reaction of M(SCN)₂ (M = Mn, Fe, Ni) with pyridine (pyr) in aqueous solution at room temperature leads to the formation of the literature known pyridine-rich 1:4 compounds of composition [M(SCN)₂(pyridine)₄] (M = Mn (1-Mn), Fe (1-Fe), Ni (1-Ni)) reported recently. On heating, the 1:4 compounds decompose into their corresponding pyridine-deficient 1:2 compounds of composition [M(SCN)₂(pyridine)₂]_n (M = Mn (2-Mn), Fe (2-Fe), Ni (2-Ni)) which decompose on further heating. In the crystal structure of the pyridine-deficient 1:2 compounds the metal cations are coordinated by four N-atoms of two pyridine ligands and two N-bonded thiocyanato anions, each in mutually trans orientation, and by two S-atoms of two adjacent thiocyanato anions in a slightly distorted octahedral geometry. The thiocyanato anions bridge the metal cations into one-dimensional (1D) polymeric chains. IR spectroscopic investigations on the pyridine-deficient 1:2 compounds are in agreement with the presence of μ-1,3 bridging thiocyanato anions. Magnetic measurements of the pyridine-rich 1:4 compounds show only Curie-Weiss paramagnetism whereas for the pyridine-deficient 1:2 compounds an antiferromagnetic ordering for [Mn(NCS)₂(pyridine)₂]_n (2-Mn) and metamagnetic behavior for [Ni(NCS)₂(pyridine)₂]_n (2-Ni) is found. For [Cu(NCS)₂(pyridine)₂]_n (2-Cu) Curie-Weiss paramagnetic behavior is observed. [Fe(NCS)₂(pyridine)₂]_n (2-Fe) shows metamagnetic behavior, which was already investigated but remeasured for a more detailed characterization.     

Effect of the synthesis conditions on the magnetic and electrical properties of the BaFeO3−x oxide: A metamagnetic behavior

The BaFeO_2.95 oxide has been obtained from thermal decomposition of the [BaFe(C₃H₂O₄)₂(H₂O)₄] metallo-organic precursor at 800 °C under atmospheric oxygen pressure as small and homogeneous particles. From electronic paramagnetic resonance data, a metallic behavior in the 230-130 K temperature range has been observed. Magnetic measurements confirm the existence of a ferro-antiferromagnetic transition at 178 K. The magnetic properties of the BaFeO_2.95 oxide are strongly dependent on both temperature and magnetic field with a metamagnetic behavior. The synthesis conditions play an important role on the morphology and the electrical and magnetic properties. The syntherization of the sample produces a dramatic change in the transport properties and the existence of conductivity disappears.     

Synthesis, crystal structure and magnetic properties of an Os-containing pillared perovskite La5Os3MnO16

A new Os-containing, pillared perovskite, La₅Os₃MnO₁₆, has been synthesized by solid state reaction in sealed quartz tubes. This extends the crystal chemistry of these materials which had been known only for Mo and Re, previously. The crystal structure has been characterized by X-ray and neutron powder diffraction and is described in space group C-1 with parameters a=7.9648(9) Å; b=8.062(1) Å; c=10.156(2) Å, α=90.25(1)°, β=95.5(1)°; γ=89.95(2)°, for La₅Os₃MnO₁₆. The compound is isostructural with the corresponding La₅Re₃MnO₁₆ phase. A very short Os-Os distance of 2.50(1) Å was found in the dimeric pillaring unit, Os₂O₁₀, suggestive of a triple bond as demanded by electron counting. Nearly spin only values for the effective moment for Os⁵⁺ () and Mn²⁺ () were derived from magnetic susceptibility data. Evidence for magnetic transitions was seen near ∼180 and 80 K. Neutron diffraction data indicate that T_c is 170(5) K. The magnetic structure of La₅Os₃MnO₁₆ at 7 K was solved revealing that Os⁵⁺ and Mn²⁺ form ferrimagnetically coupled layers with antiferromagnetic interlayer ordering. The ordered moments are for Mn²⁺ and for Os⁵⁺, which are reduced from the respective spin only values of 5.0 and . The observation of net ferrimagnetic (antiparallel) intraplanar coupling between Os⁵⁺(t_2g³) and Mn²⁺(t_2g³e_g²) is interesting as it appears to contradict the Goodenough-Kanamori rules for 180° superexchange.     

Ferromagnetism Induced in UCoAl by Uniaxial Pressure

UCoAl exhibits attributes of an itinerant 5f-electron metamagnet. It is paramagnetic down to lowest temperatures but the c -axis susceptibility shows a maximum around T max , 20 K. When a field larger than B c , 0.6 T is applied along the c -axis of hexagonal structure a ferromagnetic ordering of U moments is induced at low temperatures. The critical parameters T max and B c are sensitive to alloying and hydrostatic pressure. In the latter case values of both, T max and B c , are increasing with increasing pressure, i.e. metamagnetism is suppressed towards a conventional paramagnetism. We report on results of an experiment with a c -axis uniaxial pressure, which has an opposite influence on T max and B c leading to ferromagnetism in zero field. A scenario of the physics of UCoAl is presented accounting for effects of hydrostatic- and uniaxial-pressure on the lattice and consequences in the 5f-electron delocalization and anisotropy of hybridization-mediated exchange interactions.     

Inducing magnetic order by Ru-substitution in PrFeSi

We have investigated the magnetic and transport properties across the transition from nonmagnetic to magnetically ordered state of Pr in PrFe_1−xRu_xSi polycrystalline samples. PrFeSi does not order magnetically, whereas by replacing Fe with Ru, magnetic ordering was induced for x=0.15. Increase in Ru content shifts the transition towards higher temperature. Metamagnetic transition observed in magnetization and magnetoresistance measurement shifted towards higher field with increase in Ru content. For x>0.15, a hump is observed in resistivity data possibly due to formation of a superzone gap.