NiCo_(2)O_(4)薄膜的制备及其磁电性能研究

Preparation and Magnetoelectric Properties of NiCo_(2)O_(4) Thin Films

采用脉冲激光沉积(PLD)技术,在氧化镁(MgO(001))、钛酸锶(SrTiO_(3)(001))和铝酸镁(MgAl_(2)O_(4)(001))3种衬底上,外延生长钴酸镍(NiCo_(2)O_(4))薄膜。采用原子力显微镜(AFM)和X射线衍射仪(XRD)对NiCo_(2)O_(4)薄膜进行形貌表征和结构分析,测试结果表明NiCo_(2)O_(4)薄膜具有平整的表面和优异的结晶度。采用综合物性测量系统(PPMS)测试NiCo_(2)O_(4)薄膜的电输运及磁性能,通过对比发现在MgO(001)和SrTiO_(3)(001)衬底上生长的NiCo_(2)O_(4)薄膜呈半导体特性,并且磁性较弱,而在MgAl_(2)O_(4)(001)衬底上生长的NiCo_(2)O_(4)薄膜具有金属性和室温亚铁磁性,并且表现出明显的垂直磁各向异性。综合分析表明,不同衬底上制备的NiCo_(2)O_(4)薄膜受外延应变、沉积温度(T_(g))及氧分压(p_(O2))的影响,会表现出差异较大的磁电特性。通过优化衬底的选择以及生长参数的调整,本文可以制备出具有垂直磁各向异性的室温亚铁磁金属NiCo_(2)O_(4)薄膜,可用于微型自旋电子学器件的制备。

The epitaxial NiCo_(2)O_(4)thin films were grown on MgO(001),SrTiO_(3)(001)and MgAl_(2)O_(4)(001)substrates at different oxygen pressures and temperatures by pulsed laser deposition system(PLD).The lattice constants of MgO,SrTiO_(3),MgAl_(2)O_(4) and NiCo_(2)O_(4)were 0.8432,0.781,0.8083 and 0.8114 nm,respectively.The lattice mismatch between the NiCo_(2)O_(4)films and the different substrates were 3.9%,3.7%and 0.38%,respectively.The surface morphology and the structural characterize of NiCo_(2)O_(4)thin films were charac⁃terized by atomic force microscope(AFM)and X-ray diffraction(XRD).AFM results suggested that the NiCo_(2)O_(4)films growth on MgO and SrTiO_(3) substrates presented some particles on the surface and larger roughness.When NiCo_(2)O_(4)films grew on MgAl_(2)O_(4) substrate,the films surfaces were smooth and the roughness was smaller.The reason for the greater roughness of NiCo_(2)O_(4)films growth on high oxy⁃gen pressures was attributed to the oxygen diffusion in the films during deposition process which would lead to an increase in the oxy⁃gen content in the NiCo_(2)O_(4)films,resulting in the formation of some particles on the film surface.XRD results showed that the highquality crystal structures existed in these NCO thin films.The 2θpeak positions of NiCo_(2)O_(4)films grown on MgO,SrTiO_(3) and MgAl_(2)O_(4) substrates were at 44.9°,43.9°and 44.4°,respectively.The shift in the peaks of NiCo_(2)O_(4)films grown on different substrates was due to a large lattice mismatch between the different substrates and NiCo_(2)O_(4)films.The magnetic and electrical properties of NiCo_(2)O_(4)films were measured by physical property measurement system(PPMS)combined with external resistivity and vibrating sample magnetome⁃ter(VSM)option.According to the resistance dependence on temperature test results,the resistance of NiCo_(2)O_(4)films growth on the MgO substrate increased slowly with the decrease of temperature.And with the increase of oxygen pressure,the resistance of NiCo_(2)O_(4)films presented increase tendency.The resistance evolution area of these NiCo_(2)O_(4)films was from 1×104 to 1×107Ω.Similar to NiCo_(2)O_(4)films grown on MgO substrate,the resistance of NiCo_(2)O_(4)films growth on SrTiO_(3) substrate increased slowly with the decrease tempera⁃ture from 50 to 300 K.When the test temperature was below 50 K,the resistance increased rapidly with the decrease of temperature.And the resistance increased gradually with the increase of deposition temperature.It was shown that the resistance evolution area was from 1×103 to 1×106Ω.It was obvious that NiCo_(2)O_(4)films grown on MgO and SrTiO_(3) substrates presented semi-conducting.The resis⁃tance dependence on temperature test results of NiCo_(2)O_(4)films growth on MgAl_(2)O_(4) substrates were also presented in this work.It was showed that the resistance of NiCo_(2)O_(4)film prepared under the conditions of 350℃and 20.0 Pa decreased with the decrease of tempera⁃ture in the entire temperature range.It was obvious that NiCo_(2)O_(4)film growth on this condition presented metal conductivity.While the resistance dependence on temperature curves of NiCo_(2)O_(4)thin films growth on other conditions presented semiconductor characteristic.The factors of lattice mismatch,deposition temperature,and oxygen pressure could affect the conductivity of NiCo_(2)O_(4)films on different substrates.According to magnetic hysteresis loop(M-H)and magnetic dependence of temperature curves(M-T)test results,NiCo_(2)O_(4)films growth on MgO and SrTiO_(3) substrates had weakly magnetic properties due to the influence of epitaxial strain.However,NiCo_(2)O_(4)films prepared on MgAl_(2)O_(4) substrate exhibited room temperature ferrimagnetism and perpendicular magnetic anisotropy by comparing M-H curves at 100 and 300 K under out-of-plane and in-plane magnetic field measurements.Meanwhile,the saturation magnetization of NiCo_(2)O_(4)film at 100 and 300 K were 4.2×10^(-10) and 2.0×10^(-10 )A·m^(2)·g^(-1) in out-of-plane direction magnetic fields,respectively.The co⁃ercive field was 0.06 and 0.03 T,respectively.The reason for the saturation magnetizations of these NiCo_(2)O_(4)films at 100 K stronger than at 300 K was due to the thermal effect.Corresponding to M-H curves,M-T test results showed that NiCo_(2)O_(4)films prepared on MgAl_(2)O_(4) substrates had room temperature ferrimagnetism.In conclusion,NiCo_(2)O_(4)films grown on MgO and SrTiO_(3) substrates had the semi-conducting and weak magnetic properties.While the NiCo_(2)O_(4)films grown on MgAl_(2)O_(4) substrates could present the metallic,room temperature ferrimagnetism,and perpendicular magnetic anisotropic properties.The films with metallic behavior had a large satu⁃ration magnetization.The metallic behavior in NiCo_(2)O_(4)films could be tuned by the parameters of growth temperature and oxygen pres⁃sure.Therefore,NiCo_(2)O_(4)films would present different magnetoelectric properties with the different parameters of strain,deposition temperature(Tg)and oxygen pressure(pO2).In this work,the room temperature ferrimagnetic and metallic NiCo_(2)O_(4)films could be pre⁃pared by optimized the growth parameters.And,this perpendicular magnetic anisotropic NiCo_(2)O_(4)films could be used in the micro-spin⁃tronics devices.This study demonstrated that changing growth conditions and tuning substrates was a useful way to control the ferrimag⁃netism and the anisotropy of the NiCo_(2)O_(4)films,which would provide an additional degree of freedom in designing spintronic devices based on spinel oxides including the NiCo_(2)O_(4).