The charge ordering in , which occurs on cooling the ferromagnetic metallic ground state, is readily destroyed on application of a magnetic field of 6 T. For , for which the ground state is charge ordered, on the other hand, magnetic fields have no effect on the charge ordering. In order to understand such a marked difference in charge-ordering behaviour of the manganates, we have investigated the structure as well as the electrical and magnetic properties of compositions (Ln = Nd, Sm, Gd and Dy) wherein varies over the range 1.17-1.13 Å. The lattice distortion index, D, and charge-ordering transition temperature, , for the manganates increase with the decreasing . The charge-ordered state is transformed to a metallic state on applying a magnetic field of 6 T in the case of , but this is not the case with the analogous Sm, Gd and Dy manganates with less than 1.17 Å. In order to explain this behaviour, we have examined the -dependence of the Mn-O-Mn bond angle, the average Mn-O distance and the apparent one-electron bandwidth, obtained from these structural parameters. It is suggested that the extraordinary sensitivity of the charge ordering to arises from factors other than those based on the Mn-O-Mn bond angle and average Mn-O distances alone. It is possible that the competition between the covalent mixing of the oxygen orbital with the A-site and B-site cation orbitals plays a crucial role. Strain effects due to size mismatch between A-site cations could also cause considerable changes in .

The crystal structure of the anion-deficient perovskite Ca2.5Sr0.5GaMn2O8 has been studied at 290 and 5 K by neutron diffraction (290 K; space group Pcm21, a = 5.4294(1), b = 11.3722(3), c = 5.2983(1)Å). The vacant oxide sites order to create a structure in which perovskite bilayers consisting of MnO6 octahedra are isolated from each other along [010] by a single layer of GaO4 tetrahedra. At 5 K the material is antiferromagnetic with an ordered magnetic moment of 3.09(1) μB per Mn cation. Magnetic susceptibility measurements suggest that short-range magnetic ordering within the bilayers occurs above 200 K, and muon spin relaxation data show that the transition to long-range magnetic order occurs between 150 and 125 K. The resistivity of Ca2.5Sr0.5GaMn2O8 decreases by an order of magnitude at 125 K, and ∼50% magnetoresistance is seen in a field of 80 kOe at 110 K.

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A spin glass state is observed in the double perovskite oxide Sr 2 FeCoO 6 prepared through sol-gel technique. Initial structural studies using x rays reveal that the compound crystallizes in tetragonal I4/m structure with lattice parameters, a ¼ 5.4609(2) Å and c ¼ 7.7113(7) Å . The temperature dependent powder x ray diffraction data reveal no structural phase transition in the temperature range 10-300 K. However, the unit cell volume shows an anomaly coinciding with the magnetic transition temperature thereby suggesting a close connection between lattice and magnetism. Neutron diffraction studies and subsequent bond valence sums analysis show that in Sr 2 FeCoO 6 , the B site is randomly occupied by Fe and Co in the mixed valence states of Fe 3 þ /Fe 4þ and Co 3þ /Co 4þ . The random occupancy and mixed valence sets the stage for inhomogeneous magnetic exchange interactions and in turn, for the spin glass state in this double perovskite, which is observed as an irreversibility in temperature dependent dc magnetization at T f $ 75 K. Dynamical scaling analysis of v 0 (T) yields a critical temperature T ct ¼ 75.14(8) K and an exponent z ¼ 6.2(2) typical for spin glasses. The signature of presence of mixed magnetic interactions is obtained from the thermal hysteresis in magnetization of Sr 2 FeCoO 6 . Combining the neutron and magnetization results of Sr 2 FeCoO 6 , we deduce that Fe is in low spin state while Co is in both low spin and intermediate spin states. V

The itinerant elecmn ferromagnetism in Lal-,Sr,CoO, (0.1 < I < 0.5) has been reinvestigated. AC "gnetic susceptibility studies on these systems have been carried out for Le first time. The nature of ihe susceptibility indicales the freezing of superparamagnetic clusters af low values of x (< 0.3) and long-range ferromapetic order at higher values of x (2 0.3). Electrical resistivity studies show that itinerant electron behaviour with a positive temperature coefficient of resislance at low temperatures is obtained for x > 0.2. Metallization and longrange ferromagnetic order thus Seems to be associated with different ori'pins Chemical systematics obtained from the changes in lanice paramems show !hat Ihc expansion of LaCoO, on the substitution of La by Sr stabilizes the paramagnetic s u s of CO ions which are in predominantly diamagnetic low-spin state in LaCoO,. The core S = 1 spins of hivalant CO seems to be coupled ferromagnetically by the itinerant electron in a manner similar to the Zener doubleexchange mechanism.