Doped Mn-based Oxide Magnetoresistance

Perovskite manganites Re1 A MnO3 (ReLa, Pr, etc., A=Ca, Sr, Ba, etc.) have attracted intensive interest due to their colossal magnetoresista.nce (CMR) effect and potential applications in information field. In this thesis, we focused our studies on the improvement of magnetoresistance (MR) properties of manganites at room temperature (RI) and the characterization of CMR manganite films prepared by means of a simple technology, metal-organic decomposition (MOD).We first fabricated bulk materials of two-element-doped La0 67(Ca0 64 IBa04)MnO by solid state reaction, which presented CMR effect at RI. The Curie temperature (Ta) of the sample was 312 K, near which sharp metal-semiconductor transition and CMR peak occurred. Under 6 kOe applied filed, the MR ratio of the sample reached 24.7% at the CMR peak temperature 306 K and still reached 20% at 300 K (RI), much larger than those ofLa067Ca033MnO and La067Ba033MnO bulk samples, which were 2% and 2.4%, respectively.The effect of preparation technique on the CMR effect of La0 67Pb0 33MnO bulk materials was investigated. High sintering temperature and long sintering time led to great loss of Pb in the samples and heavy decline of T~, but the samples still remained the typical characteristics of CMR effect. In the samples sintered at lower temperature with deceased sintering time, structural defects such as porosity were observed, which were responsible for the smooth metal-semiconductor transition far below the ferromagnetic transition in these samples and the monotonic increase of MR rate with decreasing temperature.By means of MOD technique, thin films of CMR La1 A, MnO (A=Ca, Sr, Ba) were successftilly synthesized on amorphous quartz substrates, which was not reported in other references. The films had polycrystalline structure with [202] preferred orientation. In these films, La0 67Sr0 33MnO2 samples exhibited a good linear MR effect with applied field at RI. The MR ratio reached 5% under 10 kOe field at RI. However, La067Ba033MnO~ and La0 67Ca0 33MnO~ samples showed very weak MR effect at RT. Due to the oxygen defect, all the films had lower T~ than their corresponding bulk samples. Porous structure was also displayed in the films, which resulted in smooth metal-semiconductor transition far below ferromagnetic transition and monotonic increase of MR rate with decreasing temperature. At low temperature, all films presented large low-field MR effect and perpendicular anisotropy ofMR. The low-field MR effect was correlated with the spin dependent transport of carriers near grain boundaries. The perpendicular anisotropy of MR was caused by the demagnetizing effect. With increasing temperature, low-field MR declined for the offset effect of thermal hopping of carriers on the conduction. Above T0, both lowfield MR effect and perpendicular anisotropy of MR would disappear.For the La0 67Sr0 33MnO films deposited on the other different substrates, low-field MR effect was also shown at low temperature. Except the films deposited on A12O3 ceramic substrate, all films presented perpendicular anisotropy of MR at lowtemPerature. At RT a1l the films deposited on different substrates had linear ameffect with field. About 2.4%~5% are ratio was obtained in these films at RT underl0 kOe field. Furthermore, anna1ing conditions only slightly affected the areproperties of La,,,Sr0,,MnO. n1ms. Especially, annealing technology could hardlychange the good linearity of MR effect and ffe ratio at RT.For La,-.SrxMnO. nlms with different doping level x (x=0.l, 0.33, 0.6, 0.8),samples with doping level x s 0.6 presented ferromagnetic transition and MR'effect.Sdriilar to La0,,Sr0,,MnO. films, La0,Sr,.,MnO. nlIns also had meta1-semiconductortransition. However, metal-sendconduCtor tfansition was not observed in thetemperature dependence of resistivity of the films with doping level x 2 0.6 andL%,Sr0,MnO. nlms did not show am effect. At RT only the sample with dopinglevel x=0.33 exhibited linear MR effect.The doping effects of other elements...