Effect Of Doping On Intrinsic Properties Of The Colossal Magnetoresistance Materials

Peroveskite Manganites are the subject of intense study for the fields of condensed physics and materials physics in recent years due to the discovery of colossal Magnetoresistance (CMR) effect near Curie Temperature Tc and 100% spin polarization, as well as the complex interaction between electrons,lattices and spins in these compounds. Double Exchange (DE) is an important theory in interpretation of the electrical transport and magnetic properties of manganites. However, the effect of doping on DE and the possibility of DE hetero-ionic coupling between Mn and other ions has not yet been conclusive, and remains an open question. In this thesis, we base on the polycrystalline manganese oxides of La_2/3Ca_1/3MnO₃ and La_2/3Sr_1/3MnO₃, introduce the foreign elements to A- and B-sites by a special method. The electrical transport and magnetic properties of the doped manganites have been investigated, which provides experimental and theoretic basis for the enhancement of CMR at room temperature and whether or not DE coupling between hetero-ionic. The main investigations are shown as followed:1. An introduction of spintronics has been presented. More attentions have been paid on the structure, electrical transport behavior, and magnetic properties of perovskite manganites, which are considered as the most promising magneto electronics materials. On the basis of the effect to DE and CMR by substituting in perovskite manganites, the basic thinking of subject selection and investigation significance is put forward.2. A series of samples have been prepared by decreasing the value of and by fixing and increasingσ², in order to study the effect to DE and CMR by substituting at A-sites. It is found that the angle of Mn-O-Mn bond deviates 180o and the DE is weaken with the decreasing and increasingσ², which results in the decrease of T_c and an enhancement of CMR.3. Cr³⁺ has the same electronic configuration as Mn⁴⁺, so whether or not DE coupling between them remains an open question. A special formula of La_2/3+yCa_1/3-yMn_1-yCr_yO₃ with [Mn³⁺]/([Mn⁴⁺]+[Cr³⁺]) ratio fixed at optimal proportion 2:1 is designed and a comparison of the magnetic and electrical transport properties with La_2/3Ca_1/3Mn_1-yCr_yO₃. It is found that the substituting Cr for Mn in manganites shows inefficiency in lowering Tc and Tp. At lower doping level, there are two peaks in the resistivity versus temperature curve for La_2/3Ca_1/3Mn_1-yCr_yO₃ and the additional peak disappears for La_2/3+yCa_1/3-yMn_1-yCr_yO₃. Compared with La_2/3Ca_1/3Mn_1-yCr_yO₃, the magnetization and conductivity are enhanced in La_2/3+yCa_1/3-yMn_1-yCr_yO₃ in the low temperature range. we suggest that there exists the poor DE between Mn³⁺ and Cr³⁺ coupling at high temperature so that Cr³⁺ cannot play the role of Mn⁴⁺ in the Mn³⁺-O-Cr³⁺ interaction. With the decrease in temperature, Cr³⁺ partially plays the role of Mn⁴⁺ in the Mn³⁺-O-Cr³⁺ interaction, Mn³⁺ and Cr³⁺ ions turn to be favor of ferromagnetically ordering, probability of the electron transfer between Mn³⁺ and Cr³⁺ ions increases, which would promote the conductivity. The DE interaction between Mn³⁺ and Cr³⁺ is similar to the DE interaction between Mn³⁺ and Mn⁴⁺ but the former is weaker than that for the latter.4. Similarly, at y≤0.1, the resistivity versus temperature curve for La_2/3Sr_1/3Mn_1-yCr_yO₃ displays double peaks, however, there has an intrinsic characteristic peak in the curve of La_2/3+ySr_1/3-yMn_1-yCr_yO₃. Compared with La_2/3Sr_1/3Mn_1-yCr_yO₃, the conductivity is enhanced in La_2/3+yCa_1/3-yMn_1-yCr_yO₃ in the low temperature range. Furthermore, there is a visible enhancement of CMR around room temperature, which corresponds with the enhancement of the DE interaction between Mn³⁺ and Cr³⁺ with a decreasing in temperature.5. A series of double-doping samples of La_(2+4y)/3Ca_(1-4y)/3Mn_1-yCu_yO₃ with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been prepared and compared to La_2/3Ca_1/3Mn_1-yCu_yO₃ by the measurement of the structural, magnetic and electrical transport properties. It is found that Cu doping acts the role of fluxing action in La_2/3Ca_1/3Mn_1-yCu_yO₃ but cannot act it in La_(2+4y)/3Ca_(1-4y)/3Mn_1-yCu_yO₃. The Tp of La_2/3Ca_1/3Mn_1-yCu_yO₃ maintains around the Tp of La_2/3Ca_1/3MnO₃, but the Tp of La_(2+4y)/3Ca_(1-4y)/3Mn_1-yCu_yO₃ shifts towards lower temperature with the increasing of the doping content, meanwhile, the magnetoresistance of La_(2+4y)/3Ca _(1-4y)/3Mn_1-yCu_yO₃ is enhanced at a wide temperature window.6. In the samples La_2/3Sr_1/3Mn_1-yCu_yO₃ and La(2+4y)/3Sr(1-4y)/3Mn_1-yCu_yO₃, Cu doping cannot act the role of fluxing action in those manganites. The Tp of all samples shift towards lower temperature region with the increasing of doping content and the resistivity increase. Compared to La_2/3Sr_1/3Mn_1-yCu_yO₃, the CMR effect is enhanced in La(2+4y)/3Sr(1-4y)/3Mn_1-yCu_yO₃ especially at the room temperature region.7. The Tc and Tp of La_(2+4y)/3(Ca,Sr)_(1-4y)/3Mn_1-yCu_yO₃ are lower than that of La_2/3(Ca,Sr)_1/3Mn_1-yCu_yO₃, which implies that the ratio of Mn³⁺/Mn⁴⁺ maintained at 2:1 except for Cu2+ cannot make samples a good ferromagnetism. The influence of tolerance factor, andσ2 by altering the ratio of A-sites elements on magnetic properties and DE may be the main factor for the double-doping manganites.As a conclusion, the special doping method on A- and B-sites in manganites is a distinct technique to clarify the influence on DE and experimentally test the possibility of DE hetero-ionic coupling between Mn and other ions in a certain extent. Such a process presents a new way to study DE in manganites, which would be beneficial to search out the room temperature magnetoresistance materials also.