Strain-induced magneto-transport properties of manganite thin films
Open Access
- Author:
- Hu, Yufeng
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 21, 2003
- Committee Members:
- Qi Li, Committee Chair/Co-Chair
Vincent Henry Crespi, Committee Member
Nitin Samarth, Committee Member
Tom Mallouk, Committee Member - Keywords:
- manganite
Strain
low-field
magnetoresistance
anisotropy
magneto-transport - Abstract:
- This dissertation consists of three parts. The first part focuses on the studies of the low-field magnetoresistance (LFMR) properties of strained manganite thin films, namely, La0.67A0.33MnO3 thin films (A=Ca, Sr, Ba) and Pr1-xSrxMnO3 thin films (x=0.2, 0.25, 0.33 and 0.4). In the second part, we discuss the studies of the high field magnetoresistance anisotropy properties of the strained thin films. The observed large anisotropic magnetoresisance cannot be explained by the existing theories. In the last part, we presented some preliminary data on the manganite nanostructures. Chapter 1 briefly overviews the studies of the colossal magnetoresistance in the manganite materials, and the low-field magnetoresistance in manganite thin films. It also gives an overview of the theories involved in this dissertation, which covered the conventional theories and recently proposed theory on the domain wall resistance in ferromagnetic materials, and the theories on the magnetoresistance anisotropy. Chapter 2 describes the experimental methods to prepare the high quality manganite thin films, i.e., the pulse-laser deposition technique, and the characterization of the structural, electrical and magnetic properties of the films using X-ray diffraction (XRD), atomic force microscopy (AFM) and other measurements. Chapter 3 focuses on the low-field magnetoresistance properties of the strained La0.67A0.33MnO3 and Pr1-xSrxMnO3 thin films. First, strain-induced large low-field magnetoresistance (LFMR) has been systematically studied in La0.67Ca0.33MnO3 (LCMO), La0.67Sr0.33MnO3 (LSMO) and La0.67Ba0.33MnO3 (LBMO) thin (50-200 Å) films grown on different substrates, such as LaAlO3(001) (LAO), NdGaO3(110) (NGO) and SrTiO3 (001) (STO). Due to the lattice mismatch between the films and the substrates ranging from -2.6% to +1%, compressive and tensile strains were imposed to the films grown on LAO and STO substrates, respectively. Large LFMR was observed in LCMO and LSMO thin films on LaAlO3 substrates, and positive magnetoresistance was observed in most of the films grown on SrTiO3 substrates. The large LFMR is strongly dependent on the film thickness, temperature and the composition of the manganites. Second, strain-induced large LFMR has been systematically studied in Pr1-xSrxMnO3 (x=0.2, 0.25, 0.33, 0.4) ultrathin films (45-300 Å) grown on LAO and STO substrates. The films are under compressive (grown on LAO substrates) or tensile (grown on STO substrates) strain imposed by the lattice mismatch with the substrates. Large LFMR was observed in all compressive-strained Pr1-xSrxMnO3 films when the magnetic field is applied perpendicular to the film plane, but the LFMR ratio is larger for the films with lower carrier concentration (x). Large LFMR was identified as domain wall resistance in the compressive strained LCMO, LSMO and Pr1-xSrxMnO3 thin films. The effect will be discussed based on strain-induced magnetic anisotropy and unconventional domain walls. Chapter 4 is devoted to the high field anisotropic magneto-transport properties of strained manganite thin films. A systematic study of the out-of-plane (magneto-crystalline) anisotropic magnetoresistance (AMR) has been conducted for the differently strained Pr1-xSrxMnO3 (x=0.2, 0.25, 0.33, 0.4) ultrathin films (40-300 Å). The compressive- and tensile- strained ultrathin Pr1-xSrxMnO3 films show anomalously large out-of-plane AMR, but with opposite signs. In contrast, the almost strain free Pr1-xSrxMnO3/NGO films with the same thickness show much lower out-of-plane AMR. As the thickness increases, the AMR of all the films becomes small. In addition, the field dependence of the out-of-plane AMR shows a crossover at the metal-insulator transition temperatures for all three types of the films. The in-plane AMR of the films are smaller regardless of the film thickness and the substrate. Out-of-plane AMR on strained La0.67A0.33MnO3 thin films was also presented, and the large out-of-plane AMR was observed in tensile strained films in this series. The anomalous AMR results in the strained Pr1-xSrxMnO3 thin film are explained in terms of the orbit ordering and spin-orbit coupling. It is originated from the energy band splitting of the manganese eg level and the differential occupation probability of the dx2-y2 and the d3z2-r2 orbital due to the static Jahn-Teller distortion. However, the large AMR, indicating an enhanced spin-orbital coupling due to strain, have not been discussed in the current theories. In chapter 5 we presented some preliminary data on the manganite nanostructures. We found that, while the manganite nanostructure maintains all the LFMR and DWR properties as in the un-patterned manganite thin films, it shows the nonlinear I-V characteristics below and well above Tc. And the I-V curves were fitted very well with the Simmons tunneling model, which indicates that tunneling is a dominant factor in the transport property. At last, we conclude the results of this dissertation and outline the future works in chapter 6.