Low-doped Manganites Research Articles

Magnetic ground state of low-doped manganites probed by spin dynamics in a magnetic field

We study the spin dynamics of the manganites ${\mathrm{La}}_{0.9}{\mathrm{Ca}}_{0.1}\mathrm{Mn}{\mathrm{O}}_{3}$ and ${\mathrm{La}}_{0.91}{\mathrm{Sr}}_{0.09}\mathrm{Mn}{\mathrm{O}}_{3}$ in magnetic field. Previous studies in zero field have shown that the magnetic ground state is inhomogeneous with ferromagnetic $(F)$ droplets embedded in a canted antiferromagnetic (CAF) matrix. The spin dynamics consists of two spin wave branches in a high and a low-energy range, respectively. However, the assignment of these two branches either to a mean CAF medium or to $F$ droplets embedded in an AF matrix, was uncertain. The present study shows that these two branches do not follow the mean-field calculation for a homogeneous CAF state and do not correspond either to a phase separation between AF and $F$ states. The progressive spin-flop transitions observed for $H\ensuremath{\Vert}\mathbf{b}$ and $H\ensuremath{\Vert}\mathbf{a}+\mathbf{b}$, give rise to strong changes in the dispersion of the spin waves. The low-energy branch, only visible around $F$ zone centers at zero field, appears at any $q$-value in magnetic field. Moreover, as soon as the AF spin component deviates from its easy axis, leading to a decrease of the magnetic anisotropy, the high-energy branch is no more defined near the zone boundary. Its intensity is transferred to a higher flat level, likely related to defects in the CAF matrix. These new results strongly support a picture where these magnetic excitations arise from a modulated canted antiferromagnetic ground state consisting of two coupled $F$ and AF components.

Oxygen and disorder effect in the magnetic properties of manganite films

We have made a systematic study of the magnetic properties of low doped manganite films submitted to different oxygenation treatments. We have found that oxygenation dynamics depends critically of the strain field in the sample. The T C and the Mr increase as the oxygen content is increased. A decrease of the coercive field of the LSMO-STO films was observed, indicating that annealing treatments increase the oxygen content reducing oxygen vacancies.

Transport properties and magnetic domain structure in low-doped LaCaMnO manganite single crystals

The low temperature resistivity up-turn in lightly Ca doped LaCaMnO single crystals has been attributed to the domination of the tunneling mechanism in low temperature transport. The intrinsic tunnel barriers have been associated with spontaneous ferromagnetic domains walls. The magneto-optical technique has been employed to observe the relevant domain structure. A regular network of spontaneous ferromagnetic domains undergoes significant changes resulting in apparent rotation of the domain walls in the temperature range coinciding with the resistivity up-turn.

Magnetic ground state of low-doped manganites probed by spin dynamics under magnetic field

Abstract We present a neutron scattering study of spin dynamics under magnetic field in La 0.9 Ca 0.1 MnO 3 . In zero field, the spin wave spectrum consists of two branches, a high and a low-energy one. In applied field, the high-energy branch splits into two branches due to twinned domains. The gap of the new intermediate-energy branch strongly decreases above a spin-flop transition that occurs for H // b and H >2 T. Furthermore, this branch, that we could attribute to the twinned domain H // b , shows a q -discontinuity under field. The low-energy branch, measurable only around ferromagnetic zone centers at H =0, appears at all q -values under field.

Oxygen and disorder effect in the magnetic properties of manganite films

We have made a systematic study of the magnetic properties of low doped manganite films submitted to different oxygenation treatments. We have found that oxygenation dynamics depends critically of the strain field in the sample. The T C and the Mr increase as the oxygen content is increased. A decrease of the coercive field of the LSMO-STO films was observed, indicating that annealing treatments increase the oxygen content reducing oxygen vacancies .

Approach to the insulator–metal transition in low doped manganites by neutron scattering

In the phase diagram of La 1− x Sr x MnO 3 the compound with x=0.125, exhibits below T C=181 K, a ferromagnetic and metallic phase (FM). At T O′O″=159 K, a magneto-structural transition occurs towards a ferromagnetic and insulating (FI) phase with amazing properties. New superstructure Bragg peaks appear. At q =(000.5), the spin wave energies split into two levels and, along [0 0 1], they lock successively on the values of phonon modes. This suggests a complex ground state for this (FI) phase with mixed magnetic and phononic excitations. These results are discussed in the frame of existing theories.

Metastable conductivity in low-doped manganites

Experimental manifestations of current enforced metastable conductivity in La0.82Ca0.18MnO3 single crystals are discussed. Metastable conductivity manifests itself by complex current dependence, a pronounced bias dependent structure, resistance hysteresis, enhanced noise with non-Gaussian components, and long relaxation times. Spin polarized tunneling through intrinsic tunnel junctions associated with a network of twin boundaries has been tentatively associated with the phenomena observed.

Conductivity oscillations in current-induced metastable states in low-doped manganite single crystals

Deterministic oscillations of current-induced metastable resistivity in changing voltage have been detected in La$_{0.82}$Ca$_{0.18}$MnO$_3$ single crystals. At low temperatures, below the Curie point, application of specific bias procedures switches the crystal into metastable resistivity state characterized by appearance of pronounced reproducible and random structures in the voltage dependence of the differential conductivity. In certain bias range equally spaced broad conductivity peaks have been observed. The oscillating conductivity has been tentatively ascribed to resonances in a quantum well within the double tunnel barrier of intrinsic weak-links associated with twin-like defect boundaries.

Microscopic State of Low Doped Manganites La1-xSrxMnO3 Probed by ESR

The electron spin resonance (ESR) measurements were performed in single crystals of La 1-x Sr x MnO 3 (0.05 ≤ x ≤ 0.125) in order to study the interplay of crystal field, Dzyaloshinsky-Moriya interaction, Jahn-Teller (JT) effect, and orbital order. The angular dependence of the ESR linewidth of an untwinned La 0.95 Sr 0.05 MnO 3 single crystal has been analyzed in the high-temperature approximation, which takes into account the microscopic geometry of the four nonequivalent Mn positions in the orthorhombic unit cell. A strong mixing of the |3z 2 - r 2 ) and |x 2 - y 2 ) states for the real orbital configuration was found. Magnetic inhomogeneities observed in the ESR spectra across the composition range 0.075 ≤ x ≤ 0.125 can be attributed to the presence of ferromagnetic clusters (magnetic spin polarons) in the paramagnetic state. New polaronic models are proposed.

Current-induced metastable resistive states with memory in low-doped manganites

The influence of dc current flow on the resistivity and phase transitions in low-doped ${\mathrm{La}}_{0.82}{\mathrm{Ca}}_{0.18}{\mathrm{MnO}}_{3}$ single crystals has been investigated. At low temperatures, where the resistivity strongly increases with decreasing temperature, dc current depresses resistivity in a way consistent with the domination of tunneling-conduction mechanisms. Current flow exceeding some threshold currents results in resistivity switching and metastability. Bipolar current sweep exceeding threshold currents in both positive and negative direction creates low-resistivity states in the sample. The low-resistivity state converts into a very-low- and stable-resistivity state under a stronger bipolar current sweep. Current-induced low-resistivity states are characterized by long-term memory persisting even after storing the sample for a few days at room temperature. The memory can be erased by ac current flow at high temperatures. The results are interpreted in terms of a spin-polarized tunnel conduction mechanism, which modifies phase-separation conditions along the percolation path.

Anomalous magnetic field dependence of T AF in La 0.95 Sr 0.05 MnO 3

The magneto-elastic properties of single-crystalline La0.95Sr0.05MnO3 have been studied ultrasonically. Our investigations focussed on the temperature interval where magnetic ordering starts to evolve and results in a spin canted antiferromagnetic ground state. In detail the experiments revealed that the magnetic order parameter in low-doped manganite is only weakly coupled to lattice strains. Furthermore, the anomalous temperature dependence of the order parameter as found resembles highly that in stoichiometric LaMnO3. However, the main and most surprising finding is that external magnetic fields favor the spin canted phase in La0.95Sr0.05MnO3. It is unclear at present how the exchange interaction can be tuned by magnetic fields in the way observed and we are not aware of existing theoretical concepts which might give a plausible explanation for the unexpected field dependent behavior of the critical temperature. We believe, however, that this behavior primarily results from the fact that the exchange interaction depends sensitively on the orbital configuration of the manganese d electrons.

Anomalous anisotropic ac susceptibility response ofLa1−xSrxMnO3(x≈1/8)crystals: Relevance to phase separation

The dynamic response of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ crystals $(x=0.1$ and 0.125) has been studied by ac susceptibility. A strongly frequency- and field-dependent susceptibility anomaly was observed along a and b axes in the intermediate temperature range between the onset of magnetic order and the orthorhombic--``pseudocubic,'' ${\mathrm{O}}^{\ensuremath{'}}\ensuremath{-}{\mathrm{O}}^{\ensuremath{''}},$ structural transition. No anomaly was present if the ac field was applied along the c axis. This unusual response, reminiscent of cluster-glass behavior, is attributed to a strongly anisotropic, magnetically nonuniform state. This observation would constitute a clear indication of phase separation in low-doped manganites.

Magnetic inhomogeneity and colossal magnetoresistance in manganese oxides

The magnetic and magnetotransport properties of the manganites, R 1− x A x MnO z (R = La, Pr, Nd; A = Ca, Sr, Ba), are strongly dependent on the doping level x and the oxygen stoichiometry z. For x ≥ ∼ 0.2, stoichiometric R 1− x A x MnO 3 was found to have a small temperature independent coercivity of 5–30 Oe and the saturation magnetization (∼ 90 emu/g) of fully aligned Mn ions. Oxygen deficient (or x < 0.2) R 1− x A x MnO z has a saturation magnetization smaller than that of fully aligned Mn spins and shows an enhancement and temperature dependence of the coercivity, and spin freezing behavior. The latter is characterized by a peak in zero field cooled (ZFC) magnetization. Our data suggest that oxygen deficient or low doped manganites are magnetically inhomogeneous. We discuss possible origins of the magnetic inhomogeneity and its influence on the colossal magnetoresistance.