Magnetic And Transport Properties Of Layer Perovskite Manganites

The perovskite manganites are attracted much interest in recent years because of the colossal magnetoresistance effect, Jahn-Teller distortion, charge ordering, orbital ordering, phase separation, anisotropy, metal-insulator transition, aging effect and memory effect. Furthermore, they are promising to be used in high density magnetic recording and magnetic sensor. It goes without saying that these attractive phenomena must be related to the micro-mechanisms of manganites. The understanding of these mechanisms is very important for the development of the condensed matter of physics. In the present dissertation, the magnetic and transport properties of the single and bilayer manganites are investigated, including some samples of single crystal. The main results are as follows:1. For La_0.6-xGd_xSr_1.4MnO₄(x=0, 0.1,0.2, 0.4, 0.6, LGSM214)polycrystal, its magnetism is composed of two parts: the spin-glass-like magnetism of Mn and the paramagnetism of Gd³⁺ . The Gd³⁺ ions' paramagnetism leads to the increase of the low temperature magnetism of LGSM as the Gd content increases. Above 100K, all the samples show thermo activation conduction behavior. Due the strong AFM coupling between Mn, no evident MR effect is observed until the magnetic field is up to 5T.2. The polycrystal of La_1.3Sr_1.7Mn₂O₇ shows good FM at low temperatures, however, when being doped a little amount of Dy (La_1.3-yDy_ySr_1.7Mn₂O₇y=0.05 and 0.1), the low temperature magnetisms of the three samples is different greatly and the intermediate temperature magnetisms are very similar. The behavior can be understood by considering the preferential occupancy of Dy.3. For Fe-doping polycrystals of La_2-2xSr_1+2xMn_1.88Fe_0.12O₇ (x=0.3, 04 and 0.55), the magnetism of the sample changes from weak FM to AFM with increasing of x and themetal-insulator transition disappears. The ZFC and FC M - T curves split at low temperatures. Mossbauer measurements indicate all the samples have large energy splitting ^leV, and the splitting increases with x. This result is consistent with the fact that resistivity increases with x at RT4. For preparation of single crystal of bilayer manganites, we grew x=0.4 La2-2xSri+2xMn207 single crystal as the first attempt. Magnetic and transport measurements indicate: this compound possesses a great anisotropy;the ground stateis ferromagnetic and rc=120K;the magnetic moments lie in ab-plane preferentially;the resistivity is very low indicating the high quality;there is a evident metal-insulator transition and a large MR effect around Tc.5. The measurements of Lai 3Sr! 7Mn! 88Fe0i2Mn2O7 single crystal indicate: after the doping of Fe, the crystal shows spin glass behaviors, and the easy axis rotates from c-axis to the ab-plane. The transport measurements indicate phase separation in the sample. The magnetoresistance effect is suppressed after the system enters spin glass state. The Fe-doping does not induce lattice distortion or new type interaction, however, the properties of the sample are different from any 327LaSr sample with the hole concentration in the range of 0.3 to 0.4. This suggests that the action of Mn on the properties of manganites is very special, and it can not be replaced by other ions.6. The antiferromagnetism and charge ordering in La2-2xSri+2xMn207 (x=0.55 and 0.6) single crystals are studied. AFM and charge ordering are the two main characteristics of these two samples.7. The low temperature magnetism and magnetoresistance in LaRo.2Sri+2xMn207 (R=Eu, Gd, Y) single crystals are investigated, there are strong time dependent effects and .rich MR phenomena.