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Register a new userThe effect of Cu substitution on the A1g mode of La(0.7)Sr(0.3)MnO3 manganites
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We report on the first Raman data of Cu substituted La(1-y)Sr(y)Mn(1-x)Cu(x)O3 (0 < x < 0.10 and 0.17 < y < 0.3, accordingly in order to have the same Mn(4+)/[Mn(4+)+Mn(3+)] ratio), collected in the frequency range 100-900 cm-1 and at room temperature, with parallel and crossed polarizations of the incident and scattered light. Spectra were fitted with a Drude-Lorentz model, and peaks at 190-220 and 430 cm-1, together with two broad structures centered at near 500 and 670 cm-1, have been found. We also have observed that the A1g mode is substantially shifted with increasing Cu substitution. The A1g phonon shift is a linear function of the tolerance factor t and the rhombohedral angle, thus following the structural changes of the MnO6 octahedra in the system.
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The magnetic dead layers in films a few nanometers thick are investigated for La$_{0.7}$Sr$_{0.3}$MnO$_3$ on (001)-oriented SrTiO$_3$ (STO), LaAlO$_3$ (LAO) and (LaAlO$_3$)$_{0.3}$(Sr$_2$TaAlO$_6$)$_{0.7}$ (LSAT) substrates. An anomalous moment found to persist above the Curie temperature of the La$_{0.7}$Sr$_{0.3}$MnO$_3$ films is not attributed to the films, but to oxygen vacancies at or near the surface of the substrate. The contribution to the moment from the substrate is as high as 20 $mu$B/nm$^2$ in the case of STO or LSAT. The effect is increased by adding an STO cap layer. Taking this d-zero magnetism into account, extrapolated magnetic dead layer thicknesses of 0.8 nm, 1.5 nm and 3.0 nm are found for the manganite films grown on LSAT, STO and LAO substrates, respectively. An STO cap layer eliminates the LSMO dead layer.
The viscous Gilbert damping parameter governing magnetization dynamics is of primary importance for various spintronics applications. Although, the damping constant is believed to be anisotropic by theories. It is commonly treated as a scalar due to lack of experimental evidence. Here, we present an elaborate angle dependent broadband ferromagnetic resonance study of high quality epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ films. Extrinsic effects are suppressed and we show convincing evidence of anisotropic damping with twofold symmetry at room temperature. The observed anisotropic relaxation is attributed to the magnetization orientation dependence of the band structure. In addition, we demonstrated that such anisotropy can be tailored by manipulating the stain. This work provides new insights to understand the mechanism of magnetization relaxation.
Charge transfer induced interfacial ferromagnetism and its impact on the exchange bias effect in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ correlated oxide heterostructures were investigated by soft x-ray absorption and x-ray magnetic circular dichroism spectra in a temperature range from 10 to 300 K. Besides the antiferromagnetic Ni$_3^+$ cations which are naturally part of the NdNiO$_3$ layer, Ni$_2^+$ ions are formed at the interface due to a charge transfer mechanism involving the Mn element of the adjacent layer. They exhibit a ferromagnetic behavior due to the exchange coupling to the Mn$_4^+$ ions in the La$_{0.7}$Sr$_{0.3}$MnO$_3$ layer. This can be seen as detrimental to the strength of the unidirectional anisotropy since a significant part of the interface does not contribute to the pinning of the ferromagnetic layer. By analyzing the line shape changes of the x-ray absorption at the Ni L$_{2,3}$ edges, the metal-insulator transition of the NdNiO$_3$ layer is resolved in an element specific manner. This phase transition is initiated at about 120 K, way above the paramagnetic to antiferromagnetic transition of NdNiO$_3$ layer which measured to be 50 K. Exchange bias and enhanced coercive fields were observed after field cooling the sample through the Neel temperature of the NdNiO$_3$ layer. Different from La$_{0.7}$Sr$_{0.3}$MnO$_3$/LaNiO$_3$, the exchange bias observed in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ is due to the antiferromagnetism of NdNiO$_3$ and the frustration at the interface. These results suggest that reducing the interfacial orbital hybridization may be used as a tunable parameter for the strength of the exchange bias effect in all-oxide heterostructures which exhibit a charge transfer mechanism.
We report on first principles calculations of the electronic structure of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrTiO$_{3}$ junction with two possible types of interface terminations. We find that the La$_{0.7}$Sr$_{0.3}$O/TiO$_{2}$ interface preserves the interlayer ferromagnetic coupling between the interface MnO$_{2}$ layer and the bulk. The other interface, MnO$_{2}$/SrO, favours antiferromagnetic coupling with the bulk. By inserting two unit cells of undoped LaMnO$_{3}$ at the interface the ferromagnetism is recovered. This is understood in terms of the doping level and the mobility of carriers near the interface.
The electric, magnetic, and thermal properties of three perovskite cobaltites with the same 30% hole doping and ferromagnetic ground state were investigated down to very low temperatures. With decreasing size of large cations, the ferromagnetic Curie temperature and spontaneous moments of cobalt are gradually suppressed - $T_C=130$ K, 55 K and 25 K and $m = 0.68 mu_B$, 0.34 $mu_B$ and 0.23 $mu_B$ for Nd$_{0.7}$Sr$_{0.3}$CoO$_3$, Pr$_{0.7}$Ca$_{0.3}$CoO$_3$ and Nd$_{0.7}$Ca$_{0.3}$CoO$_3$, respectively. The moment reduction with respect to moment of the conventional ferromagnet La$_{0.7}$Sr$_{0.3}$CoO$_3$ ($T_C=230$ K, $m = 1.71 mu_B$) in so-called IS/LS state for Co$^{3+}$/Co$^{4+}$, was originally interpreted using phase-separation scenario. Based on the present results, mainly the analysis of Schottky peak originating in Zeeman splitting of the ground state Kramers doublet of Nd$^{3+}$, we find, however, that ferromagnetic phase in Nd$_{0.7}$Ca$_{0.3}$CoO$_3$ and likely also Pr$_{0.7}$Ca$_{0.3}$CoO$_3$ is uniformly distributed over all sample volume, despite the severe drop of moments. The ground state of these compounds is identified with the LS/LS-related phase derived theoretically by Sboychakov textit{et al.} [Phys. Rev. B textbf{80}, 024423 (2009)]. The ground state of Nd$_{0.7}$Sr$_{0.3}$CoO$_3$ with an intermediate cobalt moment is inhomogeneous due to competing of LS/LS and IS/LS phases. In the theoretical part of the study, the crystal field split levels for $4f^3$ (Nd$^{3+}$), $4f^2$ (Pr$^{3+}$) and $4f^1$ (Ce$^{3+}$ or Pr$^{4+}$) are calculated and their magnetic characteristics are presented.
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