| NiCo2O4(NCO)is an interesting mixed valence state transition metal oxide with spinel structure.Due to its unique physical and chemical properties,NCO can be widely used in photoelectric conversion and energetic materials,such as solar cells,electrocatalysis and lithium batteries.In order to make good use of NCO material,it is very important to understand its cation distribution,electrical transport and magnetic properties.Compared with bulk materials,the films have significant surface effect and size effect,and the films are favorable for high-density integration of devices.As thin films are always epitaxially grown on various substrates,the influence of substrates on properties of thin film is a very important research project.Epitaxial strain and the crystal orientation is important factors which can affect on the properties of the film.In this work,the effects of lattice strain and crystal orientation on the properties of the NCO film are systematically studied.The main work and results are as follow:1.It is important to use different substrates to tune the stress of epitaxial films by using the difference of lattice constants.We used two kinds of substrates,LaAlO3 and Si.The NiCo2O4 films with growth temperature range from room temperature to 600? were prepared on(111)LAO and(100)Si substrate and the effects of epitaxial stress on the magnetic,electrical transport and optical properties of the films were investigated.The two substrates bring compressive stress for the NCO film,which results in a large out of-plane lattice constants of the NCO film.The stress of the NCO film on(100)Si is larger and the distance between the magnetic ions of the film is small.Therefore,the double exchange action and the super exchange effect in the NCO film on Si substrate are larger,which leads to the greater conductivity and saturation magnetization.2.In order to study the effect of different crystal orientation on the physical properties of NiCo2O4 thin films,epitaxial NCO thin films were prepared on(111)LaAlO3 and(100)LaAlO3.The effect of crystal orientation on the electrical and magnetic properties of NCO thin films,especially the anomalous Hall effect is studied.The substrate produces the same stress on the film.The film has good epitaxial quality and smooth surface.Because of the weak local effect,negative magnetoresistance is found in the samples.The fitting results prove that the negative magnetoresistance of the NCO film is related to the scattering of conduction electrons by local magnetic moments.The distance between the magnetic atoms in(100)NCO plane is less than that of(111)NCO,which leads to the good conductivity and saturation magnetization of the(100)NCO film.At the same time,because of the instability of the tetrahedral structure,the magnetic ions present non-coplanar magnetic strucuture,and the low temperature presents the topological Hall effect.3.By changing oxygen pressure during the film growth process,we found that the structure and magnetoelectric properties of the films changed.Under the background vacuum(0 mTorr),the films failed to grow epitaxially,and there were various defects in the samples.When the oxygen pressure is higher(20?50 mTorr),the NCO films show better electrical and magnetic properties.At 50 mTorr,the conductivity of the film is as high as 186 S/cm,which is higher than that reported previously(178 S/cm).When the oxygen pressure is higher(200?300 mTorr),the ion collisions between the oxygen ions and the targets bombarded by the target can not form the crystalline state,which makes the cation defects,the conductivity and the magnetic properties weaken. |
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