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Register a new userUnconventional slowing down of electronic recovery in photoexcited charge-ordered La$_{1/3}$Sr$_{2/3}$FeO$_3$
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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.
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With x-ray absorption spectroscopy we investigated the orbital reconstruction and the induced ferromagnetic moment of the interfacial Cu atoms in YBa$_2$Cu$_3$O$_{7}$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ (YBCO/LCMO) and La$_{2-x}$Sr$_{x}$CuO$_4$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ (LSCO/LCMO) multilayers. We demonstrate that these electronic and magnetic proximity effects are coupled and are common to these cuprate/manganite multilayers. Moreover, we show that they are closely linked to a specific interface termination with a direct Cu-O-Mn bond. We furthermore show that the intrinsic hole doping of the cuprate layers and the local strain due to the lattice mismatch between the cuprate and manganite layers are not of primary importance. These findings underline the central role of the covalent bonding at the cuprate/manganite interface in defining the spin-electronic properties.
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.
Polycrystalline La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) thin films were synthesized by pulsed laser ablation on single crystal (100) yttria-stabilized zirconia (YSZ) substrates to investigate the mechanism of magneto-transport in a granular manganite. Different degrees of granularity is achieved by using the deposition temperature (T$_{D}$) of 700 and 800 $^{0}$C. Although no significant change in magnetic order temperature (T$_C$) and saturation magnetization is seen for these two types of films, the temperature and magnetic field dependence of their resistivity ($rho$(T, H)) is strikingly dissimilar. While the $rho$(T,H) of the 800 $^{0}$C film is comparable to that of epitaxial samples, the lower growth temperature leads to a material which undergoes insulator-to-metal transition at a temperature (T$_{P}$ $approx$ 170 K) much lower than T$_C$. At T $ll$ T$_P$, the resistivity is characterized by a minimum followed by ln $emph{T}$ divergence at still lower temperatures. The high negative magnetoresistance ($approx$ 20$%$) and ln $emph{T}$ dependence below the minimum are explained on the basis of Kondo-type scattering from blocked Mn-spins in the intergranular material. Further, a striking feature of the T$_D$ = 700 $^{0}$C film is its two orders of magnitude larger anisotropic magnetoresistance (AMR) as compared to the AMR of epitaxial films. We attribute it to unquenching of the orbital angular momentum of 3d electrons of Mn ions in the intergranular region where crystal field is poorly defined.
The relationship between the magnetic interaction and photoinduced dynamics in antiferromagnetic perovskites is investigated in this study. In La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$ thin films, commensurate spin ordering is accompanied by charge disproportionation, whereas SrFeO${}_{3}$ thin films show incommensurate helical antiferromagnetic spin ordering due to increased ferromagnetic coupling compared to La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$. To understand the photoinduced spin dynamics in these materials, we investigate the spin ordering through time-resolved resonant soft X-ray scattering. In La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$, ultrafast quenching of the magnetic ordering within 130 fs through a nonthermal process is observed, triggered by charge transfer between the Fe atoms. We compare this to the photoinduced dynamics of the helical magnetic ordering of SrFeO${}_{3}$. We find that the change in the magnetic coupling through optically induced charge transfer can offer an even more efficient channel for spin-order manipulation.
We have measured the contribution of magnetic domain walls (DWs) to the electric resistance in epitaxial manganite films patterned by electron-beam lithography into a track containing a set of notches. We find a DW resistance-area (RA) product of ~2.5 10^(-13) Ohm/m^2 at low temperature and bias, which is several orders of magnitude larger than the values reported for 3d ferromagnets. However, the current-voltage characteristics are highly linear which indicates that the DWs are not phase separated but metallic. The DWRA is found to increase upon increasing the injected current, presumably reflecting some deformation of the wall by spin-transfer. When increasing temperature, the DWRA vanishes at ~225K which is likely related to the temperature dependence of the film anisotropy.
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