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Register a new userEffect of antiferromagnetic spin correlations on lattice distortion and charge ordering in Pr$_{0.5}$Ca$_{1.5}$MnO$_{4}$
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We use neutron scattering to study the lattice and magnetic structure of the layered half-doped manganite Pr$_{0.5}$Ca$_{1.5}$MnO$_4$. On cooling from high temperature, the system first becomes charge- and orbital- ordered (CO/OO) near $T_{CO}=300$ K and then develops checkerboard-like antiferromagnetic (AF) order below $T_{N}=130$ K. At temperatures above $T_{N}$ but below $T_{CO}$ ($T_N<T<T_{CO}$), the appearance of short-range AF spin correlations suppresses the CO/OO induced orthorhombic strain, contrasting with other half-doped manganites, where AF order has no observable effect on the lattice distortion. These results suggest that a strong spin-lattice coupling and the competition between AF exchange and CO/OO ordering ultimately determines the low-temperature properties of the system.
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The magnetic correlations in the charge- and orbital-ordered manganite La(0.5)Sr(1.5)MnO(4) have been studied by elastic and inelastic neutron scattering techniques. Out of the well-defined CE-type magnetic structure with the corresponding magnons a competition between CE-type and ferromagnetic fluctuations develops. Whereas ferromagnetic correlations are fully suppressed by the static CE-type order at low temperature, elastic and inelastic CE-type correlations disappear with the melting of the charge-orbital order at high temperature. In its charge-orbital disordered phase, La(0.5)Sr(1.5)MnO(4) exhibits a dispersion of ferromagnetic correlations which remarkably resembles the magnon dispersion in ferromagnetically ordered metallic perovskite manganites.
We report low temperature specific heat measurements of Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ ($0.3leq x leq 0.5$) and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ with and without applied magnetic field. An excess specific heat, $C^{prime}(T)$, of non-magnetic origin associated with charge ordering is found for all the samples. A magnetic field sufficient to induce the transition from the charge-ordered state to the ferromagnetic metallic state does not completely remove the $C^{prime}$ contribution. This suggests that the charge ordering is not completely destroyed by a melting magnetic field. In addition, the specific heat of the Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ compounds exhibit a large contribution linear in temperature ($gamma T$) originating from magnetic and charge disorder.
We studied the charge-orbital ordering in the superlattice of charge-ordered insulating Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ and ferromagnetic metallic La$_{0.5}$Sr$_{0.5}$MnO$_3$ by resonant soft x-ray diffraction. A temperature-dependent incommensurability is found in the orbital order. In addition, a large hysteresis is observed that is caused by phase competition between insulating charge ordered and metallic ferromagnetic states. No magnetic phase transitions are observed in contrast to bulk, confirming the unique character of the superlattice. The deviation from the commensurate orbital order can be directly related to the decrease of ordered-layer thickness that leads to a decoupling of the orbital-ordered planes along the c axis.
We present resonant soft X-ray scattering (RSXS) results from small band width manganites (Pr,Ca)MnO$_3$, which show that the CE-type spin ordering (SO) at the phase boundary is stabilized only below the canted antiferromagnetic transition temperature and enhanced by ferromagnetism in the macroscopically insulating state (FM-I). Our results reveal the fragility of the CE-type ordering that underpins the colossal magnetoresistance (CMR) effect in this system, as well as an unexpected cooperative interplay between FM-I and CE-type SO which is in contrast to the competitive interplay between the ferromagnetic metallic (FM-M) state and CE-type ordering.
Interest for layered Ruddlesden-Popper strongly correlated manganites of Pr$_{0.5}$Ca$_{1.5}$MnO$_4$ as well as to their thin film polymorphs is motivated by the high temperature of charge orbital ordering above room temperature. We report on the tailoring of the c-axis orientation in epitaxial RP-PCMO films grown on SrTiO$_3$ (STO) substrates with different orientations as well as the use of CaMnO$_3$ (CMO) buffer layers. Films on STO(110) reveal in-plane alignment of the c-axis lying along to the [100] direction. On STO(100), two possible directions of the in-plane c-axis lead to a mosaic like, quasi two-dimensional nanostructure, consisting of RP, rock-salt and perovskite building blocks. With the use of a CMO buffer layer, RP-PCMO epitaxial films with c-axis out-of-plane were realized. Different physical vapor deposition techniques, i.e. ion beam sputtering (IBS), pulsed laser deposition (PLD) as well as metalorganic aerosol deposition (MAD) are applied in order to distinguish between the effect of growth conditions and intrinsic epitaxial properties. For all deposition techniques, despite their very different growth conditions, the surface morphology, crystal structure and orientation of the thin films reveal a high level of similarity as verified by X-ray diffraction, scanning and high resolution transmission electron microscopy. We found that for different epitaxial relations the stress in the films can be relaxed by means of a modified interface chemistry. The charge ordering in the films estimated by resistivity measurements occurs at a temperature close to that expected in bulk material.
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