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Register a new userStructural and physical properties of $SrMn_{1-x}Ru_xO_3$ perovskites
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We combine the results of magnetic and transport measurements with neutron diffraction data to construct the structural and magnetic phase diagram of the entire family of SrMn$_{1-x}$Ru$_{x}$O$_3$ ($0 leqslant x leqslant 1$) perovskites. We have found antiferromagnetic ordering of the C type for lightly Ru-substituted materials ($0.06 leqslant x leqslant 0.5$) in a similar manner to $R_{y}$Sr$_{1-y}$MnO$_3$ ($R$=La, Pr), due to the generation of Mn$^{3+}$ in both families of manganite perovskites by either $B$-site substitution of Ru$^{5+}$ for Mn$^{4+}$ or $A$-site substitution of $R^{3+}$ for Sr$^{2+}$. This similarity is driven by the same ratio of $d^4$ / $d^3$ ions in both classes of materials for equivalent substitution level. In both cases, a tetragonal lattice distortion is observed, which for some compositions ($0.06 leqslant x leqslant 0.2$) is coupled to a C-type AF transition and results in a first order magnetic and resistive transition. Heavily substituted SrMn$_{1-x}$Ru$_{x}$O$_3$ materials are ferromagnetic due to dominating exchange interactions between the Ru$^{4+}$ ions. Intermediate substitution ($0.6 leqslant x leqslant 0.7$) leads to a spin-glass behavior instead of a quantum critical point reported previously in single crystals, due to enhanced disorder.
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Orthorhombic Y$_{1-x}$Ca$_x$MnO$_3$ ($0 leq x leq 0.5$) was prepared under high pressure and the variations with $x$ of its structural, magnetic, electrical properties and the polarized Raman spectra were investigated. The lattice parameters change systematically with $x$. Although there are strong indications for increasing disorder above $x = 0.20$, the average structure remains orthorhombic in the whole substitutional range. Ca doping increases conductivity, but temperature dependence of resistivity $rho$(T) remains semiconducting for all $x$. The average magnetic exchange interaction changes from antiferromagnetic for $x < 0.08$ to ferromagnetic for $x > 0.08$. The evolution with $x$ of the Raman spectra provides evidence for increasingly disordered oxygen sublattice at $x geq 0.10$, presumably due to quasistatic and/or dynamical Jahn-Teller distortions.
GdCo$_5$ may be considered as two sublattices - one of Gd and one of Co - whose magnetizations are in antiparallel alignment, forming a ferrimagnet. Substitution of nickel in the cobalt sublattice of GdCo$_5$ has been investigated to gain insight into how the magnetic properties of this prototype rare-earth/transition-metal magnet are affected by changes in the transition metal sublattice. Polycrystalline samples of GdCo$_{5-x}$Ni$_x$ for 0 $ leq x leq $ 5 were synthesized by arc melting. Structural characterization was carried out by powder x-ray diffraction and optical and scanning electron microscope imaging of metallographic slides, the latter revealing a low concentration of Gd$_2$(Co, Ni)$_7$ lamellae for $x leq 2.5$. Compensation - i.e. the cancellation of the opposing Gd and transition metal moments is observed for $1 leq x leq 3$ at a temperature which increases with Ni content; for larger $x$, no compensation is observed below 360 K. A peak in the coercivity is seen at $x approx 1$ at 10K coinciding with a minimum in the saturation magnetization. Density-functional theory calculations within the disordered local moment picture reproduce the dependence of the magnetization on Ni content and temperature. The calculations also show a peak in the magnetocrystalline anisotropy at similar Ni concentrations to the experimentally observed coercivity maximum.
Understanding the structural underpinnings of magnetism is of great fundamental and practical interest. Se_{1-x}Te_{x}CuO_{3} alloys are model systems for the study of this question, as composition-induced structural changes control their magnetic interactions. Our work reveals that this structural tuning is associated with the position of the supposedly dummy atoms Se and Te relative to the super-exchange (SE) Cu--O--Cu paths, and not with the SE angles as previously thought. We use density functional theory, tight-binding, and exact diagonalization methods to unveil the cause of this surprising effect and hint at new ways of engineering magnetic interactions in solids.
We report on synthesis, structural characterization, resistivity, magnetic and thermal expansion measurements on the as yet unexplored $delta$-phase of FeSe$_{1-x}$, here synthesized under ambient- (AP) and high-pressure (HP) conditions. We show that in contrast to $beta$-FeSe$_{1-x}$, monophasic superconducting $delta$-FeSe$_{1-x}$ can be obtained in off-stoichiometric samples with excess Fe atoms preferentially residing in the van der Waals gap between the FeSe layers. The AP $delta$-FeSe$_{1-x}$ sample studied here ($T_c$ $simeq$ 8.5,K) possesses an unprecedented residual resistivity ratio RRR $simeq$ 16. Thermal expansion data reveal a small feature around $sim$90,K, which resembles the anomaly observed at the structural and magnetic transitions for other Fe-based superconductors, suggesting that some kind of magnetic state is formed also in FeSe. %indicative of a fluctuating magnetic ordering. For HP samples (RRR $simeq$ 3), the disorder within the FeSe layers is enhanced through the introduction of vacancies, the saturated magnetic moment of Fe is reduced and only spurious superconductivity is observed.
A lack of spatial inversion symmetry gives rise to a variety of unconventional physics, from noncollinear order and Skyrmion lattice phases in magnetic materials to topologically-protected surface states in certain band insulators, to mixed-parity pairing states in superconductors. The search for exotic physics in such materials is largely limited by a lack of candidate materials, and often by difficulty in obtaining crystals. Here, we report the single crystal growth and physical properties of the noncentrosymmetric tungsten aluminide cage compounds Al$_4$W and Al$_5$W, alongside related molybdenum aluminides in which spin-orbit coupling should be significantly weaker. All compounds are nonmagnetic metals. Their high conductivities suggest the opportunity to find superconductivity at lower temperatures, while the limits we can place on their transition temperatures suggest that any superconductivity may be expected to exhibit significant parity mixing.
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