Magnetic measurements have been carried out in different LaNi{1-x}MnxO{3+delta} samples with 0.1 ... more Magnetic measurements have been carried out in different LaNi{1-x}MnxO{3+delta} samples with 0.1 leq x leq 0.9. All these samples show two magnetic anomalies, one at relatively high temperature characteristic of a ferromagnetic ordering and the other at low temperature, typical of magnetic relaxation phenomena. Neutron diffraction patterns indicate that long-range ferromagnetic ordering is only achieved for x geq 0.5. Neutron
We report here the structure and magnetic properties of LaNi3/4Mn1/4O3. X-ray diffraction and tra... more We report here the structure and magnetic properties of LaNi3/4Mn1/4O3. X-ray diffraction and transmission electron microscopy experiments demonstrate that this sample is a single perovskite phase and not a biphasic system as was thought in the past. The thermal evolution of the ac magnetic susceptibility shows two large magnetic anomalies at 45 and 262 K. Isothermal magnetization measurements indicate the presence of a very low ferromagnetic component below 262 K. The neutron diffraction experiments, instead, do not detect a magnetic contribution for this sample in the whole temperature range measured. The complex magnetic behaviour can be explained as a glassy ferromagnetic state, a phase with properties typical of both systems: ferromagnets and spin-glasses.
ABSTRACT The LaMn1-xGaxO3 series (x=0.05, 0.1, 0.15, 0.3, 0.4, 0.5, 0.6, 0.8, 0.9) was studied by... more ABSTRACT The LaMn1-xGaxO3 series (x=0.05, 0.1, 0.15, 0.3, 0.4, 0.5, 0.6, 0.8, 0.9) was studied by means of ac magnetic susceptibility, magnetization, and neutron diffraction as a function of temperature. The crystal and magnetic structures have been refined from the neutron diffraction data. The replacement of Mn by Ga induces a continuous decrease of the tetragonal distortion of the MO6 (M=Mn,Ga) octahedron, being practically regular for x>~0.6. Simultaneously, the Ga substitution induces a ferromagnetic component explained in terms of a canting of the A-type antiferromagnetic structure. The ferromagnetic component increases with increasing the Ga content up to x=0.5, where a collinear ferromagnetism is observed. For x>~0.6, the system evolves from collinear ferromagnetism into superparamagnetism. The occurrence of superparamagnetism for heavily doped samples (x=0.8 or 0.9) suggests that Mn atoms tend to form clusters.
We have performed a complete resonant x-ray scattering (RXS) study at the Mn K- edge of LaMnO3. (... more We have performed a complete resonant x-ray scattering (RXS) study at the Mn K- edge of LaMnO3. (h 0 0), (0 k 0) and (0 0 1) reflections with h, k and 1=odd, were measured between 10 and 300 K. We observed strong resonant peaks at the three types of reflections. They exhibit dipolar character with a change of the
ABSTRACT The LaMn1-xGaxO3 series has been studied by means of x-ray absorption spectroscopy at bo... more ABSTRACT The LaMn1-xGaxO3 series has been studied by means of x-ray absorption spectroscopy at both Mn and Ga K edges. A homovalent substitution between Mn3+ and Ga3+ can be deduced from the x-ray absorption near-edge structure (XANES) spectra. Moreover, the evolution of the prepeaks in the Mn K-edge XANES spectra seems to indicate a strong mixing of the Mn3+ 3d states among near neighbors. The local geometric structure around Ga3+ barely changes along the series showing that GaO6 is a quite rigid octahedron. On the other hand, the local geometric structure around Mn3+ (in the MnO6) changes along the series evolving from the well-known cooperative tetragonal distortion (Jahn-Teller type) in LaMnO3 to a regular MnO6 octahedron for x˜0.6. The latter exhibits large Debye-Waller factors indicating a spread of Mn-O distances (non-Jahn-Teller type). This confirms the high instability of the Mn3+O6 octahedral local symmetry, the possible distortions being not limited to the tetragonal ones. We also discuss the role of the single-ion Jahn-Teller effect as a mechanism of Mn3+ stabilization in solids.
We reconsider the interpretation of x-ray resonant scattering experiments performed on the so-cal... more We reconsider the interpretation of x-ray resonant scattering experiments performed on the so-called charge-ordered manganites with Mn 3+/Mn 4+= 1. The comparison between these experiments and our x-ray absorption data shows that no real Mn 3+/Mn 4+ charge ...
Magnetic measurements have been carried out in different LaNi{1-x}MnxO{3+delta} samples with 0.1 ... more Magnetic measurements have been carried out in different LaNi{1-x}MnxO{3+delta} samples with 0.1 leq x leq 0.9. All these samples show two magnetic anomalies, one at relatively high temperature characteristic of a ferromagnetic ordering and the other at low temperature, typical of magnetic relaxation phenomena. Neutron diffraction patterns indicate that long-range ferromagnetic ordering is only achieved for x geq 0.5. Neutron
We report here the structure and magnetic properties of LaNi3/4Mn1/4O3. X-ray diffraction and tra... more We report here the structure and magnetic properties of LaNi3/4Mn1/4O3. X-ray diffraction and transmission electron microscopy experiments demonstrate that this sample is a single perovskite phase and not a biphasic system as was thought in the past. The thermal evolution of the ac magnetic susceptibility shows two large magnetic anomalies at 45 and 262 K. Isothermal magnetization measurements indicate the presence of a very low ferromagnetic component below 262 K. The neutron diffraction experiments, instead, do not detect a magnetic contribution for this sample in the whole temperature range measured. The complex magnetic behaviour can be explained as a glassy ferromagnetic state, a phase with properties typical of both systems: ferromagnets and spin-glasses.
ABSTRACT The LaMn1-xGaxO3 series (x=0.05, 0.1, 0.15, 0.3, 0.4, 0.5, 0.6, 0.8, 0.9) was studied by... more ABSTRACT The LaMn1-xGaxO3 series (x=0.05, 0.1, 0.15, 0.3, 0.4, 0.5, 0.6, 0.8, 0.9) was studied by means of ac magnetic susceptibility, magnetization, and neutron diffraction as a function of temperature. The crystal and magnetic structures have been refined from the neutron diffraction data. The replacement of Mn by Ga induces a continuous decrease of the tetragonal distortion of the MO6 (M=Mn,Ga) octahedron, being practically regular for x>~0.6. Simultaneously, the Ga substitution induces a ferromagnetic component explained in terms of a canting of the A-type antiferromagnetic structure. The ferromagnetic component increases with increasing the Ga content up to x=0.5, where a collinear ferromagnetism is observed. For x>~0.6, the system evolves from collinear ferromagnetism into superparamagnetism. The occurrence of superparamagnetism for heavily doped samples (x=0.8 or 0.9) suggests that Mn atoms tend to form clusters.
We have performed a complete resonant x-ray scattering (RXS) study at the Mn K- edge of LaMnO3. (... more We have performed a complete resonant x-ray scattering (RXS) study at the Mn K- edge of LaMnO3. (h 0 0), (0 k 0) and (0 0 1) reflections with h, k and 1=odd, were measured between 10 and 300 K. We observed strong resonant peaks at the three types of reflections. They exhibit dipolar character with a change of the
ABSTRACT The LaMn1-xGaxO3 series has been studied by means of x-ray absorption spectroscopy at bo... more ABSTRACT The LaMn1-xGaxO3 series has been studied by means of x-ray absorption spectroscopy at both Mn and Ga K edges. A homovalent substitution between Mn3+ and Ga3+ can be deduced from the x-ray absorption near-edge structure (XANES) spectra. Moreover, the evolution of the prepeaks in the Mn K-edge XANES spectra seems to indicate a strong mixing of the Mn3+ 3d states among near neighbors. The local geometric structure around Ga3+ barely changes along the series showing that GaO6 is a quite rigid octahedron. On the other hand, the local geometric structure around Mn3+ (in the MnO6) changes along the series evolving from the well-known cooperative tetragonal distortion (Jahn-Teller type) in LaMnO3 to a regular MnO6 octahedron for x˜0.6. The latter exhibits large Debye-Waller factors indicating a spread of Mn-O distances (non-Jahn-Teller type). This confirms the high instability of the Mn3+O6 octahedral local symmetry, the possible distortions being not limited to the tetragonal ones. We also discuss the role of the single-ion Jahn-Teller effect as a mechanism of Mn3+ stabilization in solids.
We reconsider the interpretation of x-ray resonant scattering experiments performed on the so-cal... more We reconsider the interpretation of x-ray resonant scattering experiments performed on the so-called charge-ordered manganites with Mn 3+/Mn 4+= 1. The comparison between these experiments and our x-ray absorption data shows that no real Mn 3+/Mn 4+ charge ...
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