J.B. Shi

Co doping effect on the crystal structure, magnetoresistance and magnetic properties of an (La0.7Ba0.3)(Mn1−xCox)O3 system with x=0−1.0

The effects of chemical size and valence on the crystal structure, magnetoresistance and magnetic properties were investigated in a perovskite-like (La 0.7 Ba 0.3 )(Mn 1 -xCox)O3 system with x = 0 - 1.0. The samples with x ≤ 0.3 and x ≥ 0.9 are of a cubic phase, but an orthorhombic phase is observed in the samples with 0.4 ≤ x ≤ 0.8. The lattice parameter, a, based on the cubic phase, at first decreases with increasing x up to 0.7, then slightly varies as x ≥ 0.8. The metal-insulator transition shows up in the compositions for x ≤ 0.2, and subsequently disappears as x ≥ 0.3. All samples exhibit ferromagnetism at low temperatures, but the long-range magnetic ordering in pure manganate (x = 0) becomes a cluster glass-type (short-range) ferromagnetic order due to the introduction of Co. The saturation magnetization decreases with increasing Co content for x ≤ 0.7, then increases as x ≥ 0.7. Our recent study using an X-ray absorption spectroscope has revealed that Co 2+ /Co 3+ and Mn 3+ /Mn 4+ , but no Co 4+ or Mn 2+ ions were observed in this entire series. The magnetotransport and magnetic properties are closely related to the average valences of Mn and Co ions.

Evolution of Magnetotransport Properties and Spin-Glass Behavior of the La0.7-xNdxPb0.3MnO3 System

The effect of neodymium substitution on the lanthanum site in La0.7-xNdxPb0.3MnO3 perovskite polycrystalline has been systematically studied. The substitution of neodymium for lanthanum causes structure change from rhombohedral to orthorhombic. Ferromagnetism is suppressed as neodymium content increases. The magnetic order changes from a ferromagnetic long-range order with x=0.0 to a spin-glass nature with x=0.7. Besides, the ferromagnetic-paramagnetic transition temperature values are lower for the neodymium phases than for the lanthanum ones. Therefore, the introduction of neodymium degrades symmetry of the structure, diminishes the spin-coupling exchange interaction, and results in changes of magnetic properties. This fact is in good agreement with increasing the B value in Blochs T3/2 law and decreasing the spin-wave stiffness parameter D in quadratic dispersion relation.

Magnetic behavior in the La0.7Pb0.3Mn1−xCoxO3 perovskite compounds

Compositions with the rhombohedral structure have been successfully synthesized in the La 0.7 Pb 0.3 Mn 1-x Co x O 3 system. DC magnetization and AC susceptibility measurements were carried out for the entire series. The Curie temperature T c decreases with increasing x from 306 K (x = 0.0) down to 60 K (x = 0.9). The antiferromagnetic state was found near the Co-rich side (x = 1.0). Co doping tends to destroy the double exchange in Mn 4+ -O-Mn 3+ and broadens the coexistence region of the cluster-glass and ferromagnetic states.

Structure characterization and magnetic properties of Ln0.7Pb0.3MnO3 (Ln=La, Nd, and Pr) manganites

Abstract Ln 0.7Pb0.3MnO3 (Ln=La, Nd, Pr) oxides have been synthesized by using the conventional ceramic method. The magnetic measurements show that Tc values alter considerably by substituting different rare earth ions. The zero field cooled (ZFC) magnetization curve of La manganite shows almost the same magnitude with the field cooled (FC); however, the curve of Nd manganite drops to zero at low temperatures. The magnetization is hardly saturated by the external field for Pr and Nd manganites due to the spin-glass nature. These facts are in good agreement with the lower ionic radii of the Nd ions and the corresponding large distortion of their perovskite structures.

X-ray absorption spectroscopy study of the La0.7Ba0.3Mn1−xCoxO3 system

Abstract We report the transition-metal (Mn and Co) K-edge X-ray absorption spectroscopy (XAS) study of a series of La0.7Ba0.3Mn1−xCoxO3 ( 0⩽x⩽1 ) samples. Systematic chemical shifts to higher energy with the Co content ( x ) are observed in both Mn and Co K-edge spectra. The chemical shift to higher energy in Mn K-edge spectra is caused by the decreased Mn3+/Mn4+ ratio due to Co substitution of Mn. The Co K-edge spectra show that the Co valence is between 2+ and 3+, which also increases with x . The effect of electronic structures on the physical properties are briefly discussed.

Structural and magnetic properties in La0.7Pb0.3Mn1-XCoxO3 systems

Abstract In this study we present the results of an extensive structural investigation of the La 0.7 Pb 0.3 Mn 1− X Co X O 3 (0≦ x ≦1) system. Powder X-ray diffraction shows a single-phase rhombohedral distorted perovskite structure for all samples. There is a gradual decrease in the lattice parameter as the Co content increases. In addition, we have systematically investigated the magnetic properties by using a SQUID magnetometer. The magnetoresistances are also measured and a giant negative magnetoresistance of over 50% can be obtained.

Magnetization processes in polycrystalline (La0.7-xNdx)Pb0.3MnO3

Abstract Magnetization processes for the Nd substitution on the La site in polycrystalline (La 0.7− x Nd x )Pb 0.3 MnO 3 have been systematically studied. The low-field magnetic behavior observed in ZFC and FC curves shows a long-range spin order for La-rich compounds and a spin-glass state for Nd-rich ones. Magnetic hysteresis loops with high field show square curves for all samples. The hysteresis behavior can be modeled by the formula of M = M s (1– a / H n ) where 0≦ n ≦1. The term a / H n expresses the deviation of magnetization from saturation. The larger factor n and smaller factor a for La-rich samples resulting in sharper square curves which should be associated with the long-range spin order of magnetic moment.

Magnetic and transport properties of La0.7−xPrxPb0.3MnO3

Abstract The substitution of Pr on the La site in La1−xPrxPb0.3MnO3 has been studied in order to probe into the physical mechanisms of colossal magnetoresistance (CMR) behavior. The magnetic order changes from a long-range order with x=0.0 to a short-range nature with x=0.7. The ferro-paramagnetic and metal-insulator transition temperature values decrease as Pr content increases. The highest MR ratio value is 16.9% for x=0.0 against 41.6% for x=0.7 in an applied field of 1 T . It is suggested that the mean ionic radius of the La-site ions may plays an important role on the CMR effect.

Crystal structures and magnetic properties of the double perovskite (Ca2−2xSr2x)FeMoO6

Abstract Chemical size effect on structural and magnetic properties in (Ca2−2xSr2x)FeMoO6 has been examined. The compounds crystallize in the monoclinic system for 0.0⩽x 318 K (x=0.0) to 393 K (x=1.0) . At 82 K , the saturation magnetization of all samples remains MS⩽3.5μB/formula unit compared to a theoretical spin-only moment 4μB/formula unit for a perfectly ordered compound. This discrepancy is probably due to a slight disorder and valence-fluctuation between the Fe and Mo atoms.

Synthesis characterization and magnetic properties of the La0.7−xSmxPb0.3MnO3 manganites

Abstract The magnetic and transport properties of mixed-valence manganites La0.7−xSmxPb0.3MnO3 (x=0,0.1 and 0.3) are investigated. The x=0 phase is indexed in a rhombohedral structure ( R 3 c ) while the rest exhibit orthorhombic symmetry (Pnma). The increase of x causes a decrease in the spin–coupling interaction between the moments of Mn. Hence, the decrease of ferromagnetic transition temperature from 321 to 152 K and the saturation magnetization from 80.36 to 78.33 emu/g can be affected by the substitution of La by Sm.