Half-doped La0 Research Articles

Observation of pressure-induced variation in magnetization and magnetic anisotropy for half-doped La0.5Ca0.5MnO3 nanoparticles

The effects of pressure on the magnetic properties of half-doped La0.5Ca0.5MnO3(LCMO) were investigated. Pressure was found to decrease the transition temperature (Tc) from the paramagnetic (PM) phase to the ferromagnetic (FM) phase, from 226 K to 223 K. Moreover, the pressure was also involved in decreasing the transition temperature (TN) for the FM-antiferromagnetic (AFM) phase from 180 K to 175 K. Below TN, the inhomogeneous AFM phase, where both the FM and AFM phases coexist, is observed in LCMO. Pressure induces changes in the zero-field-cooled magnetization (ΔMZFC) exhibiting ΔMZFC ≈ 0 in the PM region above 285 K.

Direct and inverse magnetocaloric effect in half-doped La0,5Ca0,5MnO3 and Pr0,5Sr0,5MnO3

Direct and inverse magnetocaloric effect in half-doped La0,5Ca0,5MnO3 and Pr0,5Sr0,5MnO3

Impact of hydrogenation on the structural, dielectric and magnetic properties of La0.5Ca0.5MnO3

Impact of hydrogenation on the structural, dielectric and magnetic properties of half-doped La0.5Ca0.5MnO3 manganate have been investigated. Polycrystalline sample of La0.5Ca0.5MnO3 was prepared through solid state reaction at ~ 1300 °C. Subsequently, the prepared sample was annealed at 600 °C for 6 h in a continuous flow of hydrogen gas in a tubular reduction furnace. Single-phase orthorhombic structure of the hydrogenated La0.5Ca0.5MnO3 in the space group Pnma was confirmed by Rietveld refinement of PXRD and FTIR analysis. SEM micrographs revealed well resolved grains of ~ 2 μm in a good crystalline sample. Hydrogenated La0.5Ca0.5MnO3 showed a sharp optical absorption peak in the UV range with a bandgap energy of 4.96 eV. Hydrogenation of La0.5Ca0.5MnO3 leads to significant reduction in the magnitude of dielectric constant and tangent loss at room temperature. Paramagnetic character of La0.5Ca0.5MnO3 at 300 K is retained upon the hydrogenation but the low temperature magnetic properties get modified dramatically. Temperature and field dependent magnetization measurements showed that hydrogenated La0.5Ca0.5MnO3 undergoes paramagnetic to antiferromagnetic transition at TN = ~ 110 K and in a disordered magnetic phase below 50 K. Systematic enhancements in the magnitude of saturation magnetization (Ms), coercivity (Hc), remanence (Mr) and squareness ratio at 50 K and 20 K indicate for the coexistence of antiferromagnetic and weak ferromagnetic order due to competitive magnetic exchange interactions between Mn3+ and Mn4+ ions. This study shows that hydrogenation plays key role in the modification of dielectric and magnetic properties of La0.5Ca0.5MnO3.

Investigation of magnetic response of charge ordering in half-doped La0.5Ca0.5MnO3 manganite

La0.5Ca0.5MnO3 (LCMO) polycrystalline sample was synthesized by the solid-state reaction method. The result of X-ray diffraction shows that the LCMO is single phase. We have studied the magnetic response of charge-ordered (CO) state in the LCMO manganite. The magnetic hysteresis loop at 100 K displays an interesting butterfly-type structure, which indicates the existence of CO anti-ferromagnetic (AFM) structure in LCMO. The temperature-dependent magnetization measurements show a paramagnetic (PM) to ferromagnetic (FM) transition at Curie temperature (TC ≈ 130 K) and a FM to AFM transition at Neel temperature (TN ≈ 50 K), measured under different fields of 100, 500 and 5000 Oe. Moreover, there is an interesting intercross in zero-field cooling (ZFC) and filed-cooling (FC) measurements, i.e., an intercross of the ZFC/FC curves in the CO LCMO manganite. These present magnetism measurements are discussed and possible explanations are provided by the existence of CO AFM state.

Resonant x-ray scattering in La1-xSr1+xMn04 (x≥0.5): Incommensurate-lattice modulation vs. Charge-stripe models

Using resonant x-ray scattering at the Mn K-edge, we have investigated the nature of the charge and lattice modulation in the La1-xSr1+xMnO4 (x=0.5 and 0.6) manganites. Resonant reflections (h±ε, h±ε, 0) and (h±2ε, h±2ε, 0) of the tetragonal I4/mmm structure with a modulation vector of 2e=l-x were found in the insulating phases of both manganites but the intensity of these reflections is much weaker for La0.4Sr1.6MnO4. Resonant x-ray scattering data for the two samples are well explained by the presence of two types of sinusoidal modulations of the oxygen displacements, transverse and longitudinal to the tetragonal [110] direction. The amplitude of the oxygen displacements for any of the modulations decreases with the hole doping, in agreement with the change from a commensurate (x=0.5) to an incommensurate (x=0.6) ordered phase. The different polarization and azimuthal behaviour of the two sets of resonant reflections rule out any kind of stripe model composed by Mn3+-like and Mn4+-like charge-ordering. The maximum charge disproportionation among the different Mn atoms in the unit cell is about 0.15 e− and 0.04 e− for the x=0.5 and x=0.6 samples, respectively. These results thus confirm the existence of a charge-density-wave ordering in both the commensurate-phase of the half-doped La0.5Sr1.5MnO4 and the incommensurate-phase of the over-doped La0.4Sr1.6MnO4 manganites.

Nanometric Size Effect on Magnetic Ordering in Half-doped La0.5Ca0.5MnO3 Manganite: EPR Probing

Measurements of the X-band electron paramagnetic resonance and the model fittings of the linewidth and intensity versus temperature dependences were employed to probe the finite-size effect on the magnetic ordering in half-doped La0.5Ca0.5MnO3 manganite. It is shown that complex mixed magnetic ordering in bulk samples, comprised of the antiferromagnetic and the charge-ordered (AFM/CO) component, together with the ferromagnetic one, appear to be unstable regarding this effect; the AFM/CO phase is suppressed in nano-samples, which agrees with the data of magnetic measurements. In addition, we have managed to show that the FM state, appearing in nano-grains, is frustrated due to (i) an AFM-like core-shell interaction and (ii) a strong spin disorder at the surfaces and core-shell transition layers.

Electron paramagnetic resonance study of size and nonstoichiometry effects on magnetic ordering in half-doped La0.5Ca0.5MnO3 manganite

The X-band electron paramagnetic resonance in La0.5Ca0.5MnO3 was studied. Fitting the measured temperature dependence of the EPR linewidth and intensity to established models were employed to probe the effects of finite-size and nonstoichiometry. Regarding these effects, the complex mixed magnetic ordering inherent in bulk samples was shown to be unstable. The antiferromagnetic (AFM) and charge ordered component is suppressed in nanosamples. But the ferromagnetic (FM) state, appearing in nanograins, is frustrated due to (i) an AFM-like core-shell interaction and (ii) a strong spin disorder at the surfaces and core-shell transition layers. In addition, a weak nonstoichiometry which does not affect the crystal structure in bulk induces a strong decrease of the FM component in its magnetic ground state.

Interplay between the orbital and magnetic order in monolayerLa1−xSr1+xMnO4 manganites

A two-dimensional model which describeseg electrons in a monolayer of an undoped and half doped manganiteLa1−xSr1+xMnO4 is studied using correlated wavefunctions. The effective Hamiltoniantakes into account the kinetic energy, the crystal field splitting betweenx2−y2 and 3z2−r2 orbitals, and on-site Coulomb interactions foreg electrons. Theyinteract with S = 3/2 spins due to t2g electrons, which are treated as frozen core spins. Furthermore, the model includesantiferromagnetic superexchange interaction between core spins, and the coupling betweeneg electrons and Jahn–Teller modes. The model reproduces the antiferromagnetic order in the undopedLaSrMnO4 compound, withoccupied 3z2−r2 orbitalsand elongated MnO6 octahedra along the direction perpendicular to the Mn–O plane. In half dopedLa0.5Sr1.5MnO4 manganite one finds robust chequerboard-like charge order using realistic parameters.However, the experimentally observed CE phase is more difficult to stabilize, and wediscuss the necessary conditions to obtain it within the present model. Altogether, weconclude that the Jahn–Teller effect plays a crucial role in the entire regime ofdoping.

Evidence for the charge-ordered state and phase separation at room temperature in half-doped La0.5Ca0.5MnO3 films

The microstructure and the magnetotransport properties of La0.5Ca0.5MnO3 films, prepared by rf magnetron sputtering on a LaAlO3 substrate, have been investigated. The electron-diffraction and the high-resolution electron-microscopy studies reveal the coexistence at room temperature of double-period modulated (charge-ordered) and unmodulated (charge-disordered) phases. It is suggested that the high-temperature fragmentation of film results in the magnetic-phase-separated state at low temperatures.

Magnetic field induced properties of manganite perovskites with colossal magnetoresistance (invited)

We present a systematic study of the magnetotransport and magnetic properties of the half-doped La0.5Ca0.5MnO3+δ system. The solid is a metamagnet which undergoes a first-order antiferromagnet (AFM) to ferromagnet (FM) phase transition under a field or by changing temperature. Associated with the AFM–FM transition is an insulator to metal transition. A maximum 109-fold magnetoresistance ratio has been observed at 4.2 K between the least and the most conductive states. At low T (⩽50 K), we have also observed two additional metastable electronic states in the canted AFM state at certain fields. The resistivity of each state differs from one another by at least one order of magnitude. The existence of these multiple states may be related to the unique charge- and spin-ordered state of the half-doped manganite.