Suppression of charge-ordering and appearance of magnetoresistance in a spin-cluster glass manganite La0.3Ca0.7Mn0.8Cr0.2O3

The magnetic properties of electron-doped manganite La0.3Ca0.7MnO3 and La0.3Ca0.7Mn0.8Cr0.2O3 polycrystalline samples prepared by sol-gel technique have been investigated between 5 and 300 K in magnetic fields ranging from 0 to 5 T. The transition at 260 K, attributed to charge ordering in La0.3Ca0.7MnO3, is completely suppressed in the Cr-substituted sample while the onset of a magnetic remanence followed by the appearance of a magnetic irreversibility at lower temperatures is observed in both samples. These features indicate that ferromagnetic clusters coexist with either an antiferromagnetic phase for La0.3Ca0.7MnO3 or a spin-cluster glass phase for La0.3Ca0.7Mn0.8Cr0.2O3 at the lowest temperatures. The exponential temperature dependence of the resistivity for the Cr-substituted sample is consistent with the small polaron hopping model for 120 K < T < 300 K, while the data are better described by Mott's hopping mechanism for T < 120 K. Whereas the parent compound La0.3Ca0.7MnO3 is known to show no magnetoresistance, a large negative magnetoresistance is observed in the La0.3Ca0.7Mn0.8Cr0.2O3 sample below 120 K. The appearance of the CMR is attributed to spin dependent hopping between spin clusters and/or between ferromagnetic domains

Effects of Gd 3+ doping on structural and dielectric properties of PZT (Zr:Ti=52:48) piezoceramics

The purpose of this research is to study the effect of doping Gd into Pb(Zr 0.52 Ti 0.48 )O 3 ceramics prepared by solid state reaction. X-ray diffraction patterns show that all PGZT samples are of tetragonal structure and the highest doping should be no more than 2 mole % Gd at which the unreacted oxides start to appear. The electron spin resonance (ESR) spectra of PGZT's indicate that Gd 3+ can enter both A site of the perovskite structure instead of only A site as widely believed. The ESR peaks resonance shift towards low fields as the concentration is higher, which is due to the change in crystal field experienced by Gd 3+ ions. At x =0.001, 0.005 and 0.01 dopings, two sets of powder ESR signals arising from Gd 3+ (4f 7, spin 7/2) ions at A site. The first set shows some fine structure having strong absorption peaks centered at 76.26 mT (g= 8.550). The second is a seven-peak spectrum centered at 206.01 mT (g= 3.165), which belongs to the Gd 3+ ions at B sites. Furthermore, the overlapped ESR strong absorption peaks from 309.17 mT to 314.49 mT (g= 2.2818-2.1087) belong to Gd 3+ of unreacted Gd 2O 3. The local environments of Gd 3+ ions were verified from the calculated ESR spectra using appropriate spin Hamiltonian parameter, i.e. gyromagnetic tensor g, zero-field splitting D and hyperfine tensor A. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2005