Preparation Of High Entropy Spinel Oxide Epitaxial Films And Study On Magnetic And Electrical Properties

The development of many new magnetic devices is inseparable from ferromagnetic insulators,such as magnetic tunnel junctions and magnetic spin valves.Having a high Curie temperature,a high-symmetry crystal structure and good epitaxial growth quality is the key to depositing ferromagnetic insulators on single crystal substrates and performing device-based processing.However,the common ferromagnetic insulator materials often have problems like low Curie temperature or poor epitaxial growth.So it is difficult to integrate with other single crystal oxide films or substrates,and the corresponding heterostructures usually have rough surfaces and unstable performance.Therefore,it is necessary to design and prepare a new kind of ferromagnetic insulator material with a high Curie temperature and excellent epitaxial growth quality.A part of spinel transition metal oxides are ferromagnetic insulators with favorable overall properties,such as NiFe₂O₄、MnFe₂O₄、Fe₃O₄ and so on,but their resistivity is relatively low.Although doping with other elements can increase its resistivity,too much doping can easily lead to precipitation of the second phase.Consequently,the improvement is not obvious.So we use the design method of high-entropy oxide for reference,driving the single-phase exist stably by entropy.By mixing multiple principal elements into the spinel transition metal oxide framework in an equal molar ratio,we design the high-entropy oxide（CoCrFeMnNi）₃O₄ with five principal elements.Using（CoCrFeMnNi）₃O₄ ceramic as the target,the epitaxial growth of（001）-oriented single crystal films is realized on different single crystal substrates by pulsed laser deposition.These films show good growth quality,and roughness of the surfaces is below 1 nm.The saturation magnetization of the film deposited on LaAlO₃substrate is 393 emu/cc,closing to that ofFe₃O₄ film,and the Curie temperature is above 400 K.Resistivity of the film at room temperature is 4.3×10³Ω·cm,which is1 to 3 orders of magnitude higher than that of common spinel transition metal oxide films.When using Bi（CoCrFeMnNi）O₃ target to deposit thin films,the perovskite phase decomposes into Bi₂O₃ phase and spinel phase（CoCrFeMnNi）₃O₄.Raising the growth temperature,Bi₂O₃ volatilizes and disappears,（111）-oriented spinel phase single crystal films are then obtained.Elemental analysis proves that only a small amount of Bi element exists at the interface between the film and the substrate.The saturation magnetization of the film deposited on LaAlO₃ substrate increases to 789emu/cc,and the Curie temperature is also above 400K.Besides,the film is still highly insulating at room temperature,with a resistivity of 3.6×10³Ω·cm.The experimental results show that the prepared single crystal high-entropy spinel oxide film（CoCrFeMnNi）₃O₄ is a kind of ideal ferromagnetic insulator material,which is of great significance to the development of subsequent magnetic devices.Meanwhile,it also provides a reference for the design and preparation of other new high-entropy oxide films.