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Register a new userMagnetic ordering in non-centrosymmetric CePdAl$_{3}$ and CePtAl$_{3}$
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Owing to the hybridization of ceriums localised 4$f$ electron and conduction band composed of $d$-electrons, cerium based intermetallics exhibit various kinds of magnetic interactions. In crystals, these can result in exotic types of magnetic ordering. In this study, we report a detailed single-crystal neutron diffraction study on CePdAl$_3$ and CePtAl$_3$. We have synthesized a large crystal of CePdAl$_3$, which crystallizes in a non-centrosymmetric, orthorhombic structure with space group $Cmc2_1$, a new, distorted variant of the tetragonal BaNiSn$_3$ structure observed in other Ce$T$Al$_3$ compounds, such as CePtAl$_3$. Low-temperature diffraction measurements showed that CePdAl$_3$ orders in a collinear antiferromagnetic structure below T$_N$=5.3 (1) K, with magnetic moments pointing along the $a$-axis direction and an ordered magnetic moment $mu$=1.64(3) $mu_B$/Ce$^{3+}$. Tetragonal CePtAl$_3$ shows a modulated, cycloidal type of ordering with $vec{k}=(frac{2}{3},0,0)$, and a transition temperature T$_N$=3.2 K. Symmetry analysis allows two types of ordering, which show modulation of both amplitude and direction of magnetic moments. These results allow to conclude that in Ce$T$Al$_3$ system the orthorhombic distortion ($T$=Pd, Ag) releases some underlying magnetic frustration that results in modulated types of magnetic ordering in tetragonal compounds ($T$=Cu,Au,Pt).
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Ruthenium-based perovskite systems are attractive because their Structural, electronic and magnetic properties can be systematically engineered. SrRuO$_3$/SrTiO$_3$ superlattice, with its period consisting of one unit cell each, is very sensitive to strain change. Our first-principles simulations reveal that in the high tensile strain region, it transits from a ferromagnetic (FM) metal to an antiferromagnetic (AFM) insulator with clear tilted octahedra, while in the low strain region, it is a ferromagnetic metal without octahedra tilting. Detailed analyses of three spin-down Ru-t$_{2g}$ orbitals just below the Fermi level reveal that the splitting of these orbitals underlies these dramatic phase transitions, with the rotational force constant of RuO$_6$ octahedron high up to 16 meV/Deg$^2$, 4 times larger than that of TiO$_6$. Differently from nearly all the previous studies, these transitions can be probed optically through the diagonal and off-diagonal dielectric tensor elements. For one percent change in strain, our experimental spin moment change is -0.14$pm$0.06 $mu_B$, quantitatively consistent with our theoretical value of -0.1 $mu_B$.
$rm CePt_3Si$ is a novel heavy fermion superconductor, crystallising in the $rm CePt_3B$ structure as a tetragonally distorted low symmetry variant of the $rm AuCu_3$ structure type. $rm CePt_3Si$ exhibits antiferromagnetic order at $T_N approx 2.2$ K and enters into a heavy fermion superconducting state at $T_c approx 0.75$ K. Large values of $H_{c2} approx -8.5$ T/K and $H_{c2}(0) approx 5$ T refer to heavy quasiparticles forming Cooper pairs. Hitherto, $rm CePt_3Si$ is the first heavy fermion superconductor without a center of symmetry.
We present the first-principles investigation of the structural, electronic, and magnetic properties of SrCoO$_{3-delta}$ ($delta=0, 0.25, 0.5$) to understand the multivalent nature of Co ions in SrCoO$_{3-delta}$ along the line of topotactic transition between perovskite SrCoO$_{3}$ and brownmillerite SrCoO$_{2.5}$. From the on-site Coulomb interaction $U$-dependent ground state of stoichiometric SrCoO$_{3}$, we show the proximity of its metallic ferromagnetic ground state to other antiferromagnetic states. The structural and magnetic properties of SrCoO$_{3-delta}$ depending on their oxygen-content provide an interesting insight into the relationship between the Co-Co distances and the magnetic couplings so that the spin-state transition of Co spins can understood by the change of $pd$-hybridization depending on the Co-Co distances. The emph{strong} suppression of the $dpsigma$-hybridization between Co $d$ and O $p$ orbitals in brownmillerite SrCoO$_{2.5}$ brings on the high-spin state of Co$^{3+}$ $d^{6}$ and is responsible for the antiferromagnetically ordered insulating ground state. The increase of effective Co-Co distances driven by the presence of oxygen vacancies in SrCoO$_{3-delta}$ is consistent with the reduction of the effective $pd$-hybridization between Co $d$ and O $p$ orbitals. We conclude that the configuration of neighboring Co spins is shown to be crucial to their local electronic structure near the metal-to-insulator transition along the line of the topotactic transition in SrCoO$_{3-delta}$. Incidentally, we also find that the textit{I2mb} symmetry of SrCoO$_{2.5}$ is energetically stable and exhibits ferroelectricity via the ordering of CoO$_{4}$ tetrahedra, where this polar lattice can be stabilized by the presence of a large activation barrier.
Magnetization measurements of LaCoO$_{3}$ have been carried out up to 133 T generated with a destructive pulse magnet at a wide temperature range from 2 to 120 K. A novel magnetic transition was found at $B>100$ T and $T>T^{*}=32pm 5$ K which is characterized by its transition field increasing with increasing temperature. At $T<T^{*}$, the previously reported transition at $Bsim65$ T was observed. Based on the obtained $B$-$T$ phase diagram and the Clausius-Clapeyron relation, the entropy of the high-field phase at 80 K is found to be smaller for about $1.5$ J K$^{-1}$ mol$^{-1}$ than that of the low-field phase. We suggest that the observed two high-field phases may originate in different spatial orders of the spin states and possibly other degrees of freedom such as orbitals. An inherent strong correlation of spin states among cobalt sites should have triggered the emergence of the ordered phases in LaCoO$_{3}$ at high magnetic fields.
$mathsf{Mn_{3}Sn}$ has recently attracted considerable attention as a magnetic Weyl semimetal exhibiting concomitant transport anomalies at room temperature. The topology of the electronic bands, their relation to the magnetic ground state and their nonzero Berry curvature lie at the heart of the problem. The examination of the full magnetic Hamiltonian reveals otherwise hidden aspects of these unusual physical properties. Here, we report the full spin wave spectra of $mathsf{Mn_{3}Sn}$ measured over a wide momentum - energy range by the inelastic neutron scattering technique. Using a linear spin wave theory, we determine a suitable magnetic Hamiltonian which not only explains the experimental results but also stabilizes the low-temperature helical phase, consistent with our DFT calculations. The effect of this helical ordering on topological band structures is further examined using a tight-binding method, which confirms the elimination of Weyl points in the helical phase. Our work provides a rare example of the intimate coupling between the electronic and spin degrees of freedom for a magnetic Weyl semimetal system.
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