Study On The Magnetic Anisotropy Of Ferromagnetic Spinel-epitaxial NiFe₂O₄ Thin Films

With the continuous development of science and technology,the demands for electronic devices have become more and more diversified,functions such as miniaturization,low power consumption,energy saving and environmental protection have been the important indices for a series of multifunctional of devices,which leads to the trainsition of the study of bulk materials to low dimensional magnetic materials.Moreover,the magnetic anisotropy of magnetic materials is very important for the application of the relative devices.In order to study the magnetic anisotropy of thin films induced by strain,we fabricated highly epitaxial NiFe₂O₄?NFO?thin films by pulsed laser deposition.The magnetic anisotropy modulated by strain for various oriented NFO thin films has been investigated The main experimental results are as follows:1.We fabricated NFO thin film on?001?orientated STO single crystal substrate by pulsed laser deposition,and the optimal experimental parameters are as follows:substrate temperature is 700?,oxygen pressure is 15 Pa,target substrate distance is 60 mm,laser repetition is 5 Hz,laser energy is 270 mJ,and anneal temperature is 800?.The XRD patterns show the NFO thin films are epitaxial grown along the?001?crystallographic plane and are well crystallized with the small full width at half maximum.The AFM images and the XRR results indicate the surface of the thin film is flat with uniform grain size.The magnetic testing results reveal the thin film exhibits ferromagnetic property at room temperature.2.Using the optimal experimental parameters above,we fabricated NFO thin film on?001?,?110?and?111?oriented STO single crystal substrates by pulsed laser deposition,respectively.The XRD patterns show all the thin films are epitaxial grown along the corresponding substrates and are well crystallized.The AFM images and the XRR results indicate the surface of the thin film is flat with uniform grain size and the film is about 50 nm thickness.According to the results of XRD and TEM,the strain of three different orientations are calculated,which is consistent with RSMs.Through the measurement of in-plane and out-of-plane hysteresis loops of three epitaxial thin films,it is found that the magnetization state of the thin films is strongly dependent on the strain state.The strain induced by various orientations of STO changes from compressive to tensile for the three variously oriented thin films,which further causes switching of the magnetic easy axis from in-plane to out-of-plane.Furthermore,the different strain states affect the strength of the magnetic anisotropy and M_s values.3.Through the lattice mismatch between the thin film and the single crystal substrate,the magnetic anisotropy of NFO thin film can be modulated by different strain state.Considering the controllability of the strain,so we fabricated NFO thin film on STO,MgAl₂O₄?MAO?and MgO with?001?-orientated single crystal substrate by pulsed laser deposition.The XRD patterns show all the thin films are epitaxial grown along the corresponding substrates and are well crystallized.The AFM images and the XRR results indicate the surface of the thin film is flat with uniform grain size and the film is about 30 nm thickness.The XRD and RSMs spectra were used to analyze the strain state of the films grown on three different substrates.The hysteresis loops exhibit strong magnetic anisotropy in the in-plane and the out-of-plane at room temperature.Through the analysis,the strain induced by different single crystal substrates changes from compressive to tensile for the three kinds of thin films,which further causes switching of the magnetic easy axis from in-plane to out-of-plane.Our result have implications for better understanding of magnetic anisotropy and open a new route for the modulation of the magnetic easy axis.