Panlin Zhang

Magnetic and transport properties of layered La1.2Sr1.8Mn2 - xTxO7 (T = Fe, Co, Cr)

The magnetic and transport properties of layered manganese oxides La1.2 Sr1.8 Mn2 - x Tx O7 (T = Fe, Co, Cr) are reported. The undoped oxide La1.2 Sr1.8 Mn2 O7 is ferromagnetic (FM) below 125 K where an insulating-metallic (I-M) transition occurs. Fe and Co doping have similar effects on the properties of La1.2 Sr1.8 Mn2 O7 , resulting in quick weakening of FM ordering, occurrence of a cluster-glass or spin-glass state for x > 0.07, immediate disappearance of the I-M transition and enhancement of low-temperature magnetoresistance. But a different effect is observed for Cr doping. In La1.2 Sr1.8 Mn2 - x Crx O7 , although the I-M transition also disappears quickly, FM ordering is maintained even for x = 0.5. These doping effects could be understood by considering the different exchange interactions between Mn and T (Fe, Co, Cr) and the corresponding change of the double-exchange interaction in La1.2 Sr1.8 Mn2 - x Tx O7 .

Effect of Fe doping on magnetic properties and magnetoresistance in La1.2Sr1.8Mn2O7

We report the effect of Fe doping on magnetic properties and magnetoresistance (MR) in the layered perovskite La1.2Sr1.8Mn2O7. Fe doping results in the weakening of ferromagnetic (FM) ordering, and the occurrence of a low-temperature spin-glass state in La1.2Sr1.8Mn2−xFexO7. Resistivity increases sharply with increasing Fe concentration. The metal–insulator transition disappears at a very low doping level (x=0.01), but the low-temperature MR effect is enhanced. These effects result from the replacement of Mn3+ by the doped Fe ions, which interrupts the path of double–exchange interaction and destroys the FM metallic state. Due to the anisotropic interaction in the layered perovskite, the magnetic properties and MR effect are more sensitive to Fe doping than those in ABO3-type perovskites.

Effect of structure and morphology on resistive loss at 10 GHz of the large‐area laser‐deposited YBa2Cu3O7 thin films

Three large‐area YBa2Cu3O7(YBCO) superconducting thin films have been laser‐deposited under almost identical conditions. However, the microwave surface resistance Rs of the films deviated greatly, they are 280 μΩ, 3.78 mΩ, and 97 mΩ, respectively. It was found that the structure and morphology of the films greatly influence the resistive losses at 10 GHz and 77 K of the YBCO thin films. Different loss mechanisms were discussed. For high Rs, the large angle grain boundaries were the dominate defect in the thin films and increased the Rs of the film markedly. For low Rs, it was mainly due to the misaligned ‘‘123’’ grains in the thin films and the intrinsic loss. For Rs up to the mΩ range, besides the misaligned 123 grains, domain boundaries and nonsuperconducting outgrowths of different sizes appeared and this caused the rise of Rs value. All these results were given experimentally and discussed theoretically.

Effect of Ga substitution on the structure and magnetic properties of Ho2Fe17−xGax (x=0–8) compounds

The Rietveld analysis of x-ray diffraction data shows that Ho2Fe17−xGax (x=0–8) solid solutions crystallize with the Th2Ni17 structure for x⩽2, the Th2Zn17 structure for x⩾4, and the two coexisting phases for x=3. The unit-cell volume increases linearly with a slope of 8.7 A3 per Ga atom. At lower gallium concentrations, Ga atoms occupy the 18f, 18h sites in the Th2Zn17 structure (or 12j, 12k sites in the Th2Ni17 structure) and avoid the 6c, 9d sites. For higher Ga content (x⩾6) Ga atoms still avoid the 9d site, but preferentially occupy 6c and 18f sites for x=6–8 while the occupancy factor of the 18h site decreases. Gallium substitution decreases the saturation magnetization linearly with a slope of 2.5μB. The compounds Ho2Fe17−xGax (x=7,8) possess uniaxial magnetocrystalline anisotropy at room temperature. Doping first increases, then decreases the Curie temperature until this finally rebounds for x>6. The “anomalous” enhancement of the Curie temperature for x=7, 8 may be correlated with the variation o...

Magnetic properties of (La1−xPrx)6Fe11Al3 compounds

The synthesis, structure and magnetic properties of (La1-xPrx)(6)Fe11Al3 (x = 0.0-1.0) compounds are investigated in this work. Upon substitution of Pr for La, the unit-cell volume decreases linearly with the Pr content, but the long-range magnetic-ordering temperature rises. A first-order magnetic process is observed at 5 K, which becomes more evident with increasing Pr concentration while the Pr concentration has little effect on the value of the critical field of the transition. With increasing temperature, the magnetic state of the compounds changes from antiferromagnetic to spin cluster, and finally to the paramagnetic state

Magnetoresistance in layered perovskites La1.2-xNdxSr1.8Mn2O7

Layered perovskites La1.2-xNdxSr1.8Mn2O7 (x = 0 similar to 0.6, 1.0 and 1.2) have been investigated with respect to their magnetotransport properties. Substitution of Nd for La results in gradual changes of resistivity and magnetoresistance (MR) effect from being similar to La1.2Sr1.8Mn2O7 to resembling Nd1.2Sr1.8Mn2O7.For the low-doped samples (x less than or equal to 0.4), the insulator-metal (IM) transition temperature T-P and the temperature of MR maximum T-MR decrease, but the MR maximum MRmax increases, with x increasing. The samples with x greater than or equal to 0.5 which are insulating also show large MR effect, and the largest MR ratio is observed in x = 0.5 and 0.6 for which a field-induced I-M transition is observed, with Delta rho/rho(0) = 98% below 20 K at a field of 6 T. Ln the layered perovskites, the lanthanlide ions play an important role and the MR effect can be chemically controlled by tuning the La/Nd ratio

Magnetic and transport properties of vanadate PrVO3

The magnetic and electric properties of the vanadate PrVO3 with the GdFeO3 -type orthorhombic structure are reported. The temperature-dependent magnetization displays an obvious irreversibility under zero-field cooling (ZFC) and field cooling (FC), and two magnetic phase-transition temperatures 20 K and 90 K, which may correspond to the spin reorientation and the magnetic ordering, respectively. A canted antiferromagnetic configuration is derived from the low spontaneous magnetization at 5 K. In the sintered bulk, a high coercivity 2.4 T is due to the pinning of the domain wall. The resistance measurement shows that the compound is an insulator at low temperatures.

Fe-Fe bond length and exchange interaction in Y2Fe17-xGax (x = 0-8) compounds

The Ga-occupancy factors and the Fe-Fe bond lengths resulting from the Rietveld refinement of X-ray diffraction data show a critical point at a Ga content between x = 5 and 6. The Fe-Fe exchange-interaction constant J(FeFe) derived from the Curie temperature increases linearly with increasing Ga concentration when x less than or equal to 4, remains almost unchanged for x = 4-6, then increases sharply when x > 6. The variation of the Fe-Fe exchange-interaction constant may be qualitatively explained by the variation of the Ga preferential occupancy aad the Fe-Fe bond lengths

Magnetic and electric properties of layered perovskites Nd2−2xSr1+2xMn2O7 (x=0.3–0.5)

Magnetic and electric properties of layered perovskites Nd2 - 2xSr1 + 2xMn2O7 (x = 0.3, 0.4 and 0.5) are sensitive to the doping content x. The sample with x = 0.5 is antiferromagnetic (AFM) and insulating. On decreasing x, the AFM ordering is suppressed and a canted AFM or weak ferromagnetic (FM) ordering appears, and the resistivity decreases. The sample with x = 0.4 still shows insulating behavior, but a metal-insulator transition is observed for x = 0.3. By suggesting the presence of a competition between AFM super-exchange interaction and FM double-exchange interaction, the doping dependence of magnetic and electric properties can be understood

Field-induced transition in the layered manganite LaEu0.2Sr1.8Mn2O7

We report on the effect of substitution of Eu on the structure, magnetic and transport properties of the layered manganese oxides La1.2-xEuxSr1.8Mn2O7 (x = 0-0.4). With Eu substituting for some of the La, the MnO6 octahedron shows an anomalous lattice distortion, especially in the case of x = 0.2. Eu substitution suppresses the ferromagnetic (FM) metallic state, resulting in a FM cluster-glass state for xleq0.2 and no magnetic ordering for x>0.2. LaEu0.2Sr1.8Mn2O7 (x = 0.2) undergoes a metamagnetic transition from the proposed layered FM cluster-glass state to the long-range FM state under a 4 T field, as well as a magnetic-field-induced insulating-metallic transition. The relationships among the lattice effect, exchange interactions and magnetotransport properties are discussed.