Charge-ordered La0 Research Articles

Surface morphology and magnetic properties in Pr0.7Sr0.3MnO3 layer grown on charge-ordered La0.5Ca0.5MnO3 layer

We report magnetic behaviors in Pr0.7Sr0.3MnO3 (PSMO) layer grown on charge-ordered La0.5Ca0.5MnO3 (LCMO) layer. PSMO/LCMO bilayer film was prepared by pulsed laser deposition on SrTiO3 (001) substrate. Large particulates at the surface of PSMO layer impart inhomogeneous composition in film, resulting in the variation of film's crystalline structure. We find an exchange bias (EB) in the bilayer film with relative roughness and inhomogeneous surface, whereas smooth and homogeneous surface does not lead to an EB effect. It is believed that an antiferromagnetic structure is formed in the inhomogeneous PSMO layer, which causes the EB effect in this system. The present results are discussed and possible explanations are provided based on the random-field model of EB and the related research results.

Effect of the ionic radius of A-cations on the magnetic and magnetocaloric properties of charge-ordered manganite La0.5Ca0.5–x Sr x MnO3 (0 < x < 0.5)

The effect of substitution of Sr2+ ions for Ca2+ ions on the magnetic and magnetocaloric properties of a charge-ordered La0.5Ca0.5MnO3 manganite has been studied. The direct and inverse magnetocaloric effects corresponding to the paramagnetic–ferromagnetic and ferromagnetic–antiferromagnetic transitions have been detected. It has been shown that the substitution Ca2+ → Sr2+ leads to the suppression of the charge-ordered phase and to an increase in the Curie temperature TC. The evolution of the destruction of the charge-ordered phase is nonmonotonic, which is associated with the change in the type of antiferromagnetic ordering.

Positive exchange-bias and giant vertical hysteretic shift in La0.3Sr0.7FeO3/SrRuO3 bilayers

The exchange-bias effects in the mosaic epitaxial bilayers of the itinerant ferromagnet (FM) SrRuO3 and the antiferromagnetic (AFM) charge-ordered La0.3Sr0.7FeO3 were investigated. An uncharacteristic low-field positive exchange bias, a cooling-field driven reversal of positive to negative exchange-bias and a layer thickness optimised unusual vertical magnetization shift were all novel facets of exchange bias realized for the first time in magnetic oxides. The successive magnetic training induces a transition from positive to negative exchange bias regime with changes in domain configurations. These observations are well corroborated by the hysteretic loop asymmetries which display the modifications in the AFM spin correlations. These exotic features emphasize the key role of i) mosaic disorder induced subtle interplay of competing AFM-superexchange and FM double exchange at the exchange biased interface and, ii) training induced irrecoverable alterations in the AFM spin structure.

Spin reorientation and giant dielectric response in multiferroic La1.5Sr0.5NiO4+δ

A multiferroic sample of La1.5Sr0.5NiO4+δ was prepared by conventional solid-state reaction. X-ray diffraction study revealed La1.5Sr0.5NiO4+δ exhibiting a single phase with a tetragonal structure (F4K2Ni-perovskite-type structure, and space group of I4/mmm). An iodometric titration method was used to determine non-stoichiometric oxygen concentrations (δ) in the La1.5Sr0.5NiO4+δ ceramic. We found δ = −0.017, which corresponds to a doping level of nh = x + 2δ = 0.466 (with x being Sr2+ content). The field-cooled (FC) magnetization curve indicated that there was a strong increase in spin ordering (SO) at temperature TSO ∼ 100 K. Below TSR ∼ 50 K, spins undergo a spin reorientation (SR) turning away from the stripe direction on cooling. Its dielectric constant ε (ω , T) = ε′(ω,T) − iε ″(ω,T) was estimated by basing on capacitance and conductance curves measured at different temperatures and frequencies. Dielectric relaxations of charge-ordered La1.5Sr0.5NiO4+δ ceramic were investigated in a broad temperature range. The giant dielectric constant (104–106) with a low dielectric loss of 10−2–10−1 was also found.

Giant Dielectric Response with an Electric Field in Charge-Ordered La1−xCaxMnO3Compounds

The effect of electric fields on the dielectric properties of charge-ordered La1−xCaxMnO3 compounds is reported. A low-frequency giant dielectric tunability can be obtained under a bias field as low as of the order of ∼10 V/cm. The analyses suggest that the giant dielectric tunability under low bias fields could be derived from the extrinsic contributions such as intergrains and Schottky barriers. In addition, the tunability strongly depends on the stability of the charge-ordered state of the compounds. These findings make it possible to achieve a high tunability of nonferroelectrics in low electric fields, indicating that it is promising for potential applications, such as tunable devices.

Hyperfine Interactions in Half-Doped and 2/3-Doped Charge-Ordering Manganites

The present work is devoted to the theoretical interpretation of nuclear magnetic resonance (NMR) spectra on nonmagnetic lanthanum ion in charge-ordered La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3 compounds. It is shown that the anisotropic hyperfine interaction, which arises due to the polarization of lanthanum’s external p-shells, plays a crucial role in spectrum forming. Our model allows to explain experimental spectra. The work is partially supported by CRDF REC-005.

Photoinduced effect in charge-ordering La0.5Ca0.5MnO3 film

Charge-ordered La0.5Ca0.5MnO3 film has been deposited on (100)-LaAlO3 substrate using rf magnetron sputtering method. X-ray diffraction analysis shows that the bulk has orthorhombic structure and the film has the better epitaxial character. The charge-ordering transition temperature is about 280 K by fitting the resistance-temperature curve using the variable-range hopping model. The decrease in the resistance of the film irradiated by the laser is caused by the delocalization effect of the correlated electron system. The photoinduced relaxation exhibits different processes when the laser is on and off, which can be attributed to thermal fluctuation.

Ultrasonic anomalies in La1∕3Sr2∕3MO3 (M=Fe, Mn)

The longitudinal ultrasonic velocity (Vl), magnetization, specific heat, and resistivity of polycrystalline La1∕3Sr2∕3FeO3 have been measured between 20 and 300 K. The material undergoes charge-ordering (CO) transition at about 200 K accompanied by an antiferromagnetic ordering. Upon cooling down from 300 K, a substantial softening in Vl above the charge-ordering transition temperature TCO and dramatic stiffening below TCO are observed. This feature is similar in character to that of charge-ordered La1∕3Sr2∕3MnO3, and gives clear evidence for electron-phonon coupling in La1∕3Sr2∕3FeO3. Below 181 K, another unexpected softening in Vl is observed, which is attributed to the breathing-type distortion of an Fe-O octahedron.

The role of the cooperative Jahn–Teller effect in the charge-ordered La1−xCaxMnO3 (0.5⩽x⩽0.87) manganites

Based on the magnetoresistance, magnetization, ultrasound, and crystallographic data, we studied the role of the cooperative Jahn–Teller (JT) effect in the charge-ordered (CO) state for La1−xCaxMnO3. We found that, with increasing the fraction of Q3 mode of JT distortion and decreasing that of Q2 mode in the CO state, the magnetic structure evolves from charge-exchange-type to C-type and the orbital ordering changes from 3dx2−r2/3dy2−r2-type to 3dz2−r2-type, with the strength of ferromagnetism and the phase separation tendency being suppressed. At the same time, the stability of the CO state and the cooperative JT lattice distortion increase. These effects imply that the cooperative JT effect with different vibration modes is the key ingredient in understanding the essential physics of the CO state.

Transport, magnetic, specific heat, internal friction, and shear modulus in the charge ordered La0.25Ca0.75MnO3 manganite

The resistivity, magnetization, specific heat, shear modulus, and internal friction in the charge-ordered (CO) La0.25Ca0.75MnO3 were studied. The electronic conduction in the CO state was found to obey Mott's variable-range hopping model with a change in the localization length ξ near TN. A large decrease of the magnetization was observed between the Néer temperature TN and the charge ordering transition temperature TCO, which was attributed to the consequence of the orbital ordering. Drastic stiffening in shear modulus and an internal friction peak appeared just below TCO, and were explained as due to the coupling between the lattice strain and the orbital degree of freedom. A small upturn of the magnetization and a small softening of the ultrasound near TCA≈42 K suggest the evolution of spin canting from the matrix of long-range antiferromagnetic state.

Internal friction and Jahn–Teller effect in the charge-ordered La1−xCaxMnO3 (0.5⩽x⩽0.87)

The Jahn–Teller effect in the charge-ordered (CO) state for La1−xCaxMnO3 (0.5⩽x⩽0.87) was studied by measuring the low-temperature powder x-ray diffraction, internal friction, and shear modulus. We find that the electron–lattice interaction with the static Jahn–Teller distortion is the strongest near x≈0.75 in the CO state. It was particularly observed that a crossover of the Jahn–Teller vibration mode from Q2 to Q3 near x=0.75 induces crossovers of the crystal structure from tetragonally compressed to tetragonally elongated orthorhombic, and of the magnetic structure from CE-type to C-type near x=0.75. The experimental results give strong evidence that the Jahn–Teller effect not only plays a key role in stabilizing the CO state, but also determines the magnetic and crystal structures in the CO state for La1−xCaxMnO3.

Columnar defect induced phase transformation in epitaxial La0.7Ca0.3MnO3 films

Epitaxial La0.7Ca0.3MnO3 thin films on the SrTiO3(100) surface have been irradiated with 250 MeV Ag17+ ions at different nominal fluence values in the range of 5×1010–4×1011 ions/cm2, resulting in columnar defects. At low fluences these defects cause changes in material properties that are small and scale linearly with dosage. Above a threshold fluence value ∼3×1011 ions/cm2 dramatic changes are observed, including an order of magnitude increase in the resistivity and 50 K drop in the Curie temperature. Transmission electron microscopy measurements show that the changes are associated with a phase transformation of the undamaged region between the columnar defects. The transformed phase has a diffraction pattern very similar to that seen in charge-ordered La0.5Ca0.5MnO3. We propose that above a critical level of ion damage, strains caused by the presence of the columnar defects induce a charge-ordering phase transition that causes the observed dramatic changes in physical properties. We speculate that a conceptually similar surface-induced charge ordering may be responsible for the “dead layer” observed in very thin strained films, and the dramatic changes in optical properties induced by polishing, and that an impurity-induced charge ordering causes the extreme sensitivity of properties to lattice substitution.