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Register a new userLocal Structure and Its Effect on The Ferromagnetic Properties of La$_{0.5}$Sr$_{0.5}$CoO$_3$ thin films}
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We have used high-resolution Extended X-ray Absorption Fine-Structure and diffraction techniques to measure the local structure of strained La$_{0.5}$Sr$_{0.5}$CoO$_3$ films under compression and tension. The lattice mismatch strain in these compounds affects both the bond lengths and the bond angles, though the larger effect on the bandwidth is due to the bond length changes. The popular double exchange model for ferromagnetism in these compounds provides a correct qualitative description of the changes in Curie temperature $T_C$, but quantitatively underestimates the changes. A microscopic model for ferromagnetism that provides a much stronger dependence on the structural distortions is needed.
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We report detailed dc magnetization, linear and non-linear ac susceptibility measurements on the hole doped disordered cobaltite La$_{0.5}$Ba$_{0.5}$CoO$_3$. Our results show that the magnetically ordered state of the system consists of coexisting non-ferromagnetic phases along with percolating ferromagnetic-clusters. The percolating ferromagnetic-clusters possibly start a magnetic ordering at the Curie temperature of 201.5(5)~K. The non-ferromagnetic phases mainly consist of antiferromagnetic-clusters with size smaller than the ferromagnetic-clusters. Below Curie temperature the system exhibits an irreversibility in the field cooled and zero field cooled magnetization and frequency dependence in the peak of ac susceptibility. These dynamical features indicate towards the possible coexistence of spin-glass phase along with ferromagnetic-clusters similar to La$_{1-x}$Sr$_{x}$CoO$_3$ (x$geq$0.18), but the absence of field divergence in third harmonic of ac susceptibility and zero field cooled memory clearly rule out any such possibility. We argue that the spin-glass phase in La$_{1-x}$Sr$_{x}$CoO$_3$ (x$geq$0.18) is associated with the presence of incommensurate antiferromagnetic ordering in non-ferromagnetic phases which is absent in La$_{0.5}$Ba$_{0.5}$CoO$_3$. Our analysis show that the observed dynamical features in La$_{0.5}$Ba$_{0.5}$CoO$_3$ are possibly due to progressive thermal blocking of ferromagnetic-clusters which is further confirmed by the Wohlfarths model of superparamagnetism. The frequency dependence of the peak of ac susceptibility obeys the Vogel-Fulcher law with $tau_0approx10^{-9}$s. This together with the existence of an AT line in H-T space indicates the presence of significant inter-cluster interaction among these ferromagnetic-clusters.
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 report detailed dc magnetization, linear and non-linear ac susceptibility measurements on the hole doped disordered cobaltite La$_{0.5}$Ba$_{0.5}$CoO$_3$. Our results show that the magnetically ordered state of the system consists of coexisting non-ferromagnetic phases along with percolating ferromagnetic-clusters. The percolating ferromagnetic-clusters possibly undergo a 3D Hisenberg like magnetic ordering at the Curie temperature of 202(3) K. In between 202 and 220 K, the linear and non-linear ac susceptibility measurements show the presence of magnetic correlations even when the spontaneous magnetization is zero which indicates the presence of preformed short range magnetic-clusters. The characteristics of these short range magnetic-clusters that exist above Curie temperature are quite distinct than that of Griffiths phase e.g the inverse dc susceptibility exhibits an field independent upward deviation, and the second harmonic of ac susceptibility is non-negative. Below Curie temperature the system exhibit spin-glass like features such as irreversibility in the field cooled and zero field cooled magnetization and frequency dependence in the peak of ac susceptibility. The presence of a spin or cluster -glass like state is ruled out by the absence of field divergence in third harmonic of ac susceptibility and zero field cooled memory. This indicates that the observed spin-glass like features are possibility due to progressive thermal blocking of ferromagnetic-clusters which is further confirmed by the Wohlfarths model of superparamagnetism. The frequency dependence of the peak of ac susceptibility obeys the Vogel-Fulcher law with $tau_0approx 10^{-9}$ s. This together with the existence of an AT line in H-T space indicates the existence of significant inter-cluster interaction among these ferromagnetic-clusters.
Thin films of Pr0.5Ca0.5MnO3 manganites exhibiting charge/orbital-ordered properties with colossal magnetoresistance have been synthesized by the pulsed laser deposition technique on both (100)-SrTiO3 and (100)-LaAlO3 substrates. The effects of current-induced metallic-behavior of the films are investigated as a function of the temperature and the magnetic field. Calculations based on a heat transfer model across the substrate, and our resistivity measurements reveal effects of Joule heating on charge transport over certain ranges of temperatures and magnetic fields. Our results also indicate that a nonlinear conduction, which cannot be explained by homogeneous Joule heating of the film, is observed when the material is less resistive (10-2 W.cm). The origin of this behavior is explained with a model based on local thermal instabilities associated with phase-separation mechanism and a change in the long range charge-ordered state.
We report the study of magnetic and orbital order in Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ epitaxial thin films grown on (LaAlO$_3$)$_{0.3}$-(SrAl$_{0.5}$Ta$_{0.5}$O$_3$)$_{0.7}$ (LSAT) (011)$_c$. In a new experimental approach, the polarization and energy dependence of resonant soft x-ray scattering are used to reveal significant modifications of the magnetic order in the film as compared to the bulk, namely (i) a different magnetic ordering wave vector, (ii) a different magnetic easy axis and (iii) an additional magnetic reordering transition at low temperatures. These observations indicate a strong impact of the epitaxial strain on the spin order, which is mediated by the orbital degrees of freedom and which provides a promising route to tune the magnetic properties of manganite films. Our results further demonstrate that resonant soft x-ray scattering is a very suitable technique to study the magnetism in thin films, to which neutron scattering cannot easily be applied due to the small sample volume.
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