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Register a new userRole of oxygen vacancy in the spin-state change and magnetic ordering in SrCoO$_{3-delta}$
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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.
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We investigated theoretically electronic and magnetic properties of the perovskite material SrCoO$_{3-delta}$ with $deltaleq 0.15$ using a projector-augmented plane-wave method and a Greens function method. This material is known from various experiments to be ferromagnetic with a Curie temperature of 260$,$K to 305$,$K and a magnetic moment of 1.5${,mu_text{B}}$ to 3.0${,mu_text{B}}$. Applying the magnetic force theorem as it is formulated within Greens function method, we calculated for SrCoO$_{3-delta}$ the magnetic exchange parameters and estimated the Curie temperature. Including correlation effects by an effective $U$ parameter within the GGA$+U$ approach and verifying this by hybrid functional calculations, we obtained the Curie temperatures in dependence of the oxygen deficiency close to the experimental values.
It has been well established that both in bulk at ambient pressure and for films under modest strains, cubic SrCoO$_{3-delta}$ ($delta < 0.2$) is a ferromagnetic metal. Recent theoretical work, however, indicates that a magnetic phase transition to an antiferromagnetic structure could occur under large strain accompanied by a metal-insulator transition. We have observed a strain-induced ferromagnetic to antiferromagnetic phase transition in SrCoO$_{3-delta}$ films grown on DyScO$_3$ substrates, which provide a large tensile epitaxial strain, as compared to ferromagnetic films under lower tensile strain on SrTiO$_3$ substrates. Magnetometry results demonstrate the existence of antiferromagnetic spin correlations and neutron diffraction experiments provide a direct evidence for a G-type antiferromagnetic structure with Neel temperatures between $T_N sim 135,pm,10,K$ and $sim 325,pm,10,K$ depending on the oxygen content of the samples. Therefore, our data experimentally confirm the predicted strain-induced magnetic phase transition to an antiferromagnetic state for SrCoO$_{3-delta}$ thin films under large epitaxial strain.
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.
We report measurements and analysis of magnetization, resistivity and thermopower of polycrystalline samples of the perovskite-type Co/Rh oxide La$_{0.8}$Sr$_{0.2}$Co$_{1-x}$Rh$_x$O$_{3-delta}$. This system constitutes a solid solution for a full range of $x$,in which the crystal structure changes from rhombohedral to orthorhombic symmetry with increasing Rh content $x$. The magnetization data reveal that the magnetic ground state immediately changes upon Rh substitution from ferromagnetic to paramagnetic with increasing $x$ near 0.25, which is close to the structural phase boundary. We find that one substituted Rh ion diminishes the saturation moment by 9 $mu_B$, which implies that one Rh$^{3+}$ ion makes a few magnetic Co$^{3+}$ ions nonmagnetic (the low spin state), and causes disorder in the spin state and the highest occupied orbital. In this disordered composition ($0.05le x le 0.75$), we find that the thermopower is anomalously enhanced below 50 K. In particular, the thermopower of $x$=0.5 is larger by a factor of 10 than those of $x$=0 and 1, and the temperature coefficient reaches 4 $mu$V/K$^2$ which is as large as that of heavy-fermion materials such as CeRu$_2$Si$_2$.
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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