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Register a new userRevisiting the Magnetic Structure and Charge Ordering in La$_{1/3}$Sr$_{2/3}$FeO$_3$ by Neutron Powder Diffraction and Mossbauer Spectroscopy
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F. Li
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The magnetic ordering of La$_{1/3}$Sr$_{2/3}$FeO$_3$ perovskite has been studied by neutron powder diffraction and $^{57}$Fe Mossbauer spectroscopy down to 2 K. From symmetry analysis, a chiral helical model and a collinear model are proposed to describe the magnetic structure. Both are commensurate, with propagation vector k = (0,0,1) in R-3c space group. In the former model, the magnetic moments of Fe adopt the magnetic space group P3$_{2}$21 and have helical and antiferromagnetic ordering propagating along the c axis. The model allows only one Fe site, with a magnetic moment of 3.46(2) $mu_{rm{B}}$ at 2 K. In the latter model, the magnetic moments of iron ions adopt the magnetic space group C2/c or C2/c and are aligned collinearly. The model allows the presence of two inequivalent Fe sites with magnetic moments of amplitude 3.26(3) $mu_{rm{B}}$ and 3.67(2) $mu_{rm{B}}$, respectively. The neutron diffraction pattern is equally well fitted by either model. The Mossbauer spectroscopy study suggests a single charge state Fe$^{3.66+}$ above the magnetic transition and a charge disproportionation into Fe$^{(3.66-zeta)+}$ and Fe$^{(3.66+2zeta)+}$ below the magnetic transition. The compatibility of the magnetic structure models with the Mossbauer spectroscopy results is discussed.
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The charge ordered La$_{1/3}$Sr$_{2/3}$FeO$_{3-delta}$ (LSFO) in bulk and nanocrystalline forms are investigated using ac and dc magnetization, M{o}ssbauer, and polarised neutron studies. A complex scenario of short range charge and magnetic ordering is realized from the polarised neutron studies in nanocrystalline specimen. This short range ordering does not involve any change in spin state and modification in the charge disproportion between Fe$^{3+}$ and Fe$^{5+}$ compared to bulk counterpart as evident in the M{o}ssbauer results. The refinement of magnetic diffraction peaks provides magnetic moments of Fe$^{3+}$ and Fe$^{5+}$ are about 3.15$mu_B$ and 1.57$mu_B$ for bulk, and 2.7$mu_B$ and 0.53$mu_B$ for nanocrystalline specimen, respectively. The destabilization of charge ordering leads to magnetic phase separation, giving rise to the robust exchange bias (EB) effect. Strikingly, EB field at 5 K attains a value as high as 4.4 kOe for average size $sim$ 70 nm, which is zero for the bulk counterpart. A strong frequency dependence of ac susceptibility reveals cluster-glass like transition around $sim$ 65 K, below which EB appears. Overall results propose that finite size effect directs the complex glassy magnetic behavior driven by unconventional short range charge and magnetic ordering, and magnetic phase separation appears in nanocrystalline LSFO.
Ordered electronic phases are intimately related to emerging phenomena such as high Tc superconductivity and colossal magnetoresistance. The coupling of electronic charge with other degrees of freedom such as lattice and spin are of central interest in correlated systems. Their correlations have been intensively studied from femtosecond to picosecond time scales, while the dynamics of ordered electronic phases beyond nanoseconds are usually assumed to follow a trivia thermally driven recovery. Here, we report an unusual slowing down of the recovery of an electronic phase across a first-order phase transition, far beyond thermal relaxation time. Following optical excitation, the recovery time of both transient optical reflectivity and x-ray diffraction intensity from a charge-ordered superstructure in a La$_{1/3}$Sr$_{2/3}$FeO$_3$ thin film increases by orders of magnitude longer than the independently measured lattice cooling time when the sample temperature approaches the phase transition temperature. The combined experimental and theoretical investigations show that the slowing down of electronic recovery corresponds to the pseudo-critical dynamics that originates from magnetic interactions close to a weakly first-order phase transition. This extraordinary long electronic recovery time exemplifies an interplay of ordered electronic phases with magnetism beyond thermal processes in correlated systems.
A detailed electronic phase diagram of perovskite-type oxides Sr$_{1-x}$La$_x$FeO$_3$ $(0leq x leq 0.5)$ was established by synchrotron X-ray diffraction, magnetization, and transport measurements for polycrystalline samples synthesized by a high-pressure technique. Among three kinds of helimagnetic phases in SrFeO$_3$ at zero field, two of them showing multiple-${it q}$ helimagnetic spin textures tend to rapidly disappear in cubic symmetry upon the La substitution with $x$ less than 0.1, which accompanies the loss of metallic nature. On the other hand, the third helimagnetic phase apparently remains robustly in Sr$_{1-x}$La$_x$FeO$_3$ with $x$ higher than 0.1, followed by merging to the spin/charge ordered phase with $xsim 1/3$. We propose an important role of itinerant ligand holes on the emergence of multiple-${it q}$ states and a possible link between the third (putative single-${it q}$) helimagnetic phase in SrFeO$_3$ and the spin/charge ordered phase in Sr$_{2/3}$La$_{1/3}$FeO$_3$.
Thin films of perovskite oxides offer the possibility of combining emerging concepts of strongly correlated electron phenomena and spin current in magnetic devices. However, spin transport and magnetization dynamics in these complex oxide materials are not well understood. Here, we experimentally quantify spin transport parameters and magnetization damping in epitaxial perovskite ferromagnet/paramagnet bilayers of La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$ (LSMO/SRO) by broadband ferromagnetic resonance spectroscopy. From the SRO thickness dependence of Gilbert damping, we estimate a short spin diffusion length of $lesssim$1 nm in SRO and an interfacial spin-mixing conductance comparable to other ferromagnet/paramagnetic-metal bilayers. Moreover, we find that anisotropic non-Gilbert damping due to two-magnon scattering also increases with the addition of SRO. Our results demonstrate LSMO/SRO as a spin-source/spin-sink system that may be a foundation for examining spin-current transport in various perovskite heterostructures.
The magnetic ordering of the hexagonal multiferroic compound YbMnO$_3$ has been studied between 100 K and 1.5 K by combining neutron powder diffraction, $^{170}$Yb Mossbauer spectroscopy and magnetization measurements. The Yb moments of the two crystallographic sites order at two different temperatures, the $4b$ site together with the Mn moments (at $T_N simeq$85 K) and the $2a$ site well below (at 3.5 K). The temperature dependences of the Mn and Yb moments are explained within a molecular field model, showing that the $4b$ and $2a$ sites order via Yb-Mn and Yb-Yb interactions respectively. A simple picture taking into account the local Mn environment of the Rare earth R ($4b$) ion is proposed to couple R and Mn orders in hexagonal RMnO$_3$ manganites. The nature and symmetry of the R-Mn interactions yielding the R order are discussed.
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