Electrical And Magnetic Properties Of La_0.7-xHo_xSr_0.3MnO₃ Thin Films Prepared By Pulsed Laser Deposition

Perovskite manganite La_0.7Sr_0.3MnO₃ has broad potential application prospects,for its distinct electrical and magnetic properties,especially the unique colossal magnetoresistance?abbreviated as CMR?.The substitution of La³⁺by rare-earth may be an effective way to enhance the CMR effect and promote the practical application,while the lattice mismatch and orientations of substrates would also affect the high preferred orientation characteristics and lattice distortion of the films,which may further influence the performance.In the present study,we try to prepare La_0.7Sr_0.3MnO₃(La_0.7-xHo_xSr_0.3MnO₃,x=0,0.1,0.2,0.3,abbreviated as LHSMO)thin films by pulsed laser deposition,aiming to regulate the structural and electromagnetic properties of the films so that the CMR effect could be improved for the applications in magnetic field sensors,magnetic recording,and spintronic devices.LHSMO thin films were prepared on STO?001?substrate by pulsed laser deposition,and the effect of deposition parameters on the structure and morphology of the films were investigated.At higher deposition temperature,lager laser energy density and higher oxygen partial pressure,the crystallinity increased,the morphology became smoother and all the films grew preferentially along the c-axis direction.The optimum parameters were found to be laser energy density 2.5 J/cm²,deposition temperature 650°C,oxygen partial pressure 20 Pa.Based on the optimum deposition parameters,single-phased LHSMO epitaxial thin films with high crystallinity,smooth surface morphology and different compositions were grown on STO?001?substrate,and the influence of Ho-doping on the structural and electromagnetic properties was studied.Ho-doping would induce the lattice distortion and magnetic disorder into the system,which resulted in the decrease of both Curie temperature and saturation magnetization.However,the coercive field and the resistivity peak increased.The fitting results showed that the resistivity???data varied with temperature?T?as=++..+in low-temperature region,while it obeyed to the adiabatic small polaron hopping model in high-temperature region,and the conductivity activation energy increased with the increasing Ho contents.Additionally,the peak value of magnetoresistance increased from 16.9%to 68.8%,indicating that Ho-doping was effective to enhance the CMR effect.LHSMO thin films were further prepared on STO,LAO and MgO substrates with different lattice mismatches,and the effect of the substrates on the structural and electromagnetic properties were explored.The films showed 0.18%and 0.46%in-plane tensile strain when deposited on STO and MgO substrates,while those grown on LAO substrates were under an in-plane compressive strain of-0.64%.And the strength and types of the strain were critical to the properties of the films.From STO to MgO substrates,the strain of the films increased,and the Curie temperature of the films reduced from 178 K to 117 K,while the coercive field and the resistivity peak increased simultaneously.The films on LAO substrates possessed the maximum saturation magnetization and magnetoresistance?71.1%?for the biggest strain strength,but the Curie temperature was not the lowest due to its compressive strain.LHSMO thin films were deposited on substrates with different orientations,and the effect of orientations on the structural and electromagnetic properties were researched.From?001?to?111?orientations,the in-plane tensile strains increased from0.18%to 0.36%,the Curie temperature decreased from 178 K to 133 K,and the saturation magnetization reduced from 281 emu/cm³ to 213 emu/cm³,while the coercive field and resistivity peak increased gradually.Moreover,for the films with?001?,?110?and?111?orientations,the peak value of magnetoresistance were 68.8%?74.2%?90.4%,respectively.This illustrated that the high preferred orientation characteristics and lattice distortion of the films could be adjusted by choosing substrates with appropriate orientations and lattice mismatch in a degree,which could further improve the CMR effect.