Room Temperature Ferromagnetism And Phase Structure Manipulation Of Undoped ZrO₂ Thin Films

In the past decades,numerous work has been done on the field of diluted magnetic semiconductor?DMS?,particularly oxide?DMO?,which has evolved into a popular branch of materials science.The observation of room temperature ferromagnetism?RTFM?in undoped oxide made the mechanism of ferromagnetism origin more controversial.Experimental and theoretical research on undoped DMOs have shown that structural defect,surface defect,strain or intrinsic point defect in oxides may have a certain role to play in determining the magnetic order,but a lot of contradicting views still remain.Therefore,it is of much interest to clarify the physics behind.This work has been focusing on Zirconia?ZrO₂?that is predicted to be a promising DMO candidate.Previous research shows the magnetic property might be dependent on the phase structure of ZrO₂ films,which can be monoclinic,tetragonal and cubic.Both tetragonal and cubic are high temperature stabilized polymorphs,and are usually stabilized at room temperature?RT?by doping.Undoped ZrO₂ is much preferred in our case to study the intrinsic magnetic property though.Given that,phase structure manipulation of ZrO₂ without doping should be achieved firstly.Crystalline undoped ZrO₂ thin films are prepared by both pulsed electron beam deposition?PED?and DC magnetron reactive sputtering.By adjusting the deposition parameters,the phase structure of ZrO₂ thin films has been successfully manipulated.In particular by sputtering,the O2/Ar ratio during sputtering process is found to have a significant influence on tuning the phase structure.Specifically,a pure tetragonal phase can be obtained at an O2/Ar ratio of 2/18,while a pure monoclinic one at 4/16.Evidenced by the defect characterization,it is oxygen vacancy that helps the tetragonal undoped ZrO₂ stabilized at RT.Amorphous undoped ZrO₂ thin films prepared by electron beam evaporation are also used to carry out the phase structure manipulation.With the thickness ranging from 10 to 300 nm,the amorphous undoped ZrO₂ thin films are found to crystallize into tetragonal phase firstly at a certain temperature that decreases and then increases with the increase of thickness,and the tetragonal phase can be stabilized at RT.If the amorphous films are annealed at high temperature that is much higher than the crystalline point,the phase transition from tetragonal to monoclinic will occur,and a mixture will be obtained after cooling down to RT,of which the fraction of residual tetragonal phase decreases with the increase of thickness.For instance,after 1000 oC annealing,10 nm amorphous ZrO₂ film exhibits tetragonal,while 300 nm one exhibits monoclinic.RTFM has been observed in undoped ZrO₂ thin films,indicating it results from some intrinsic factors.With help of the analysis on both phase structure and defect,RTFM in undoped ZrO₂ thin films is observed to be positively dependent on the volume fraction of tetragonal phase,as well as on the amount of oxygen vacancy.Further validation experiments show that the RTFM in tetragonal ZrO₂ thin films can be manipulated by introducing or compensating the oxygen vacancy,but that monoclinic ZrO₂ thin films still exhibit nonmagnetic in spite of a similar level of oxygen vacancy with that in tetragonal.The ferromagnetism mechanism has been also discussed.Finally,it is concluded that the phase structure plays a crucial role in determining the magnetic order of undoped ZrO₂.It seems only tetragonal phase,rather than monoclinic,that can be ferromagnetic at RT.Meanwhile,the oxygen vacancy has an important role playing in manipulating the RTFM in tetragonal ZrO₂ thin films,and specifically,the RTFM in undoped tetragonal ZrO₂ thin films originates from,and can be enhanced by the singly charged oxygen vacancy?V_O⁺?.