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Register a new userUnusual magneto-elastic forces stabilize the insulating ferromagnetic state of manganites: the La(0.8)Ca(0.2)$MnO(3) case
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The ferromagnetic and insulating state observed in La$_{1-x}$Ca$_{x}$MnO$_3$, 0.125$<$x$<$0.2, is characterized by structural and magnetic anomalies below T$_C$, similar to those observed in the x$_{Sr}$$approx$1/8. A neutron scattering study of the superlattice {bf Q$_0$}= (0,0,1+/4)$_{cub}$ peak, and of the magnetic excitations are reported in the x$_{Ca}$=0.2 sample. The occurrence of this superstructure is associated with the observation of a gap in the spin dynamics, at a {bf q$_0$} wave-vector ({bf q$_0$}={bf Q$_0$}-$tau$) with the same modulus $|${bf q$_0$}$|$ in all directions, which divides the dispersion into two regimes. For $|${bf q}$|$$<$$|${bf q$_0$}$|$ the dispersion is splitted into two or three curves. For $|${bf q}$|$$>$$|${bf q$_0$}$|$, magnetic excitations lock on acoustic and optic phonon energies, revealing a new kind of magneto-vibrational coupling. We suggest an analysis in terms of two distinct magnetic couplings, associated with two ferromagnetic media involved into a collective state.
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We have studied the temperature evolution of the inverse photoemission spectra of phase separated La$_{0.2}$Sr$_{0.8}$MnO$_{3}$. To identify the features in the room temperature experimental spectra, band structure calculations using Korringa-Kohn-Rostoker Greens function method were carried out. We find that the features generated by local moment disorder calculations give a better match with the experimental spectrum. In the insulating phase, we observed unusually an increased intensity at around the Fermi level. This puzzling behaviour is attributed to the shift in the chemical potential towards the conduction band. The present results clearly show the importance of unoccupied electronic states in better understanding of the phase separated systems.
We report on a high-pressure Raman study on two members of the La$_{1-x}$Ca$_x$MnO$_{3-delta}$ manganite family ($x=0.20$, $delta=0$ and $delta=0.08$). The results obtained for the $delta=0$ sample show a different behavior in the low and high pressure regimes ascribed to the onset of a new pressure-activated interaction previously invoked in other manganite compounds. The comparison of our results with literature data gives further support to the identification of the Jahn-Teller active stretching mode and shows that pressure-induced octahedral symmetrization is more effective in systems exhibiting a lower metallic character. On the contrary the new interaction sets in at pressure which decreases on increasing the metallic character of the system indicating a relevant role of the Mn-Mn hopping integral in its activation.
We report an experimental study of the time dependence of the resistivity and magnetization of charge-ordered La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ under different thermal and magnetic field conditions. A relaxation with a stretched exponential time dependence has been observed at temperatures below the charge ordering temperature. A model using a hierarchical distribution of relaxation times can explain the data.
We report an unusual giant linear magnetostrictive effect in the ferrimagnet Gd$_{2/3}$Ca$_{1/3}$MnO$_3$ ($T_{c} approx$80 K). Remarkably, the magnetostriction, negative at high temperature ($T approx T_{c}$), becomes positive below 15 K when the magnetization of the Gd sublattice overcomes the magnetization of the Mn sublattice. A rather simple model where the magnetic energy competes against the elastic energy gives a good account of the observed results and confirms that Gd plays a crucial role in this unusual observation. Unlike previous works in manganites where only striction associated with 3$d$ Mn orbitals is considered, our results show that the lanthanide 4$f$ orbitals related striction can be very important too and it cannot be disregarded.
This paper has been withdrawn by authors.
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