具有条纹磁畴结构的NiFe薄膜的制备与磁各向异性研究

具有条纹磁畴结构的磁性薄膜表现出面内转动磁各向异性,对于解决高频电子器件的方向性问题起着至关重要的作用.本文采用射频磁控溅射的方法,研究了NiFe薄膜的厚度、溅射功率密度、溅射气压等制备工艺参数对条纹磁畴结构、面内静态磁各向异性、面内转动磁各向异性、垂直磁各向异性的影响规律.研究发现,在功率密度15.6 W/cm~2与溅射气压2 mTorr(1 Torr=1.33322×102Pa)下生长的NiFe薄膜,表现出条纹磁畴的临界厚度在250 nm到300 nm之间.厚度为300 nm的薄膜比250 nm薄膜的垂直磁各向异性场增大近一倍,从而磁矩偏离膜面形成条纹磁畴结构,并表现出面内转动磁各向异性.高溅射功率密度可以降低薄膜出现条纹磁畴的临界厚度.在相同功率密度15.6 W/cm~2下生长300 nm的NiFe薄膜,随着溅射气压由2 mTorr增大到9 mTorr,NiFe薄膜的垂直磁各向异性场逐渐由1247.8 Oe(1 Oe=79.5775 A/m)增大到3248.0 Oe,面内转动磁各向异性场由72.5 Oe增大到141.9 Oe,条纹磁畴周期从0.53μm单调减小到0.24μm.NiFe薄膜的断面结构表明柱状晶的形成是表现出条纹磁畴结构的本质原因,高功率密度下低溅射气压有利于柱状晶结构的形成,表现出规整的条纹磁畴结构,高溅射气压会导致柱状晶纤细化,面内转动磁各向异性与面外垂直磁各向异性增强,条纹磁畴结构变得混乱.

Preparation and magnetic anisotropy of NiFe film with stripe domains

Magnetic anisotropy is one of the most important fundamental properties of magnetic film.For the high-frequency applications,the magnetic anisotropy determines the ferromagnetic resonance frequency of magnetic film.Due to the directionality of conventional static magnetic anisotropy in magnetic film,the high-frequency device usually exhibits a remarkable angular dependent behavior.Only when the microwave magnetic field is perpendicular to the magnetic anisotropy,can the device work at the best performance.The magnetic film with a thickness beyond a critical value displays a stripe domain structure as well as an in-plane rotatable magnetic anisotropy,which can be an important strategy to solve the problem of magnetic field orientation dependent performance in high-frequency device.Thus, the fabrication,the magnetic anisotropy,the magnetic domain and the high-frequency behavior for magnetic film with stripe domain structure have received extensive attention.Previously,a lot of studies have qualitatively indicated that the different fabrication processes could change the critical thickness values of displaying stripe domains,the magnetic domains,and the magnetic anisotropies in many magnetic films.However,the quantitative investigation,especially regarding the magnetic anisotropy which determines the high-frequency behaviors of magnetic films,is less.NiFe alloys display excellent soft magnetic properties,which have been extensively applied to various spintronic devices.In addition, the stripe magnetic domain is discovered for the first time in NiFe film.In this work,we fabricate NiFe magnetic thin films by using radio frequency magnetron sputtering technique at room temperature and quantitatively study the effects of film thickness,sputtering power density and Ar pressure on the magnetic domain structure,in-plane static magnetic anisotropy,in-plane rotatable magnetic anisotropy and out-of-plane magnetic anisotropy.For NiFe films fabricated at a power density of 15.6 W/cm² and an Ar pressure of 2 mTorr (1 Torr=1.33322×10² Pa),the critical thickness values for the appearance of stripe domain structures in NiFe films are between 250 and 300 nm.The out-of-plane magnetic anisotropy field of 300 nm NiFe film is nearly twice as that of 250 nm NiFe film,which gives rise to the occurrence of stripe domain structure as well as the in-plane rotatable magnetic anisotropy.The high sputtering power density could reduce the critical thickness for the occurrence of stripe domains.For 300 nm NiFe film fabricated at a power density of 15.6 W/cm²,with Ar pressure increasing from 2 to 9 mTorr,the out-of-plane magnetic anisotropy field increases from 1247.8 to 3248.0 Oe (1 Oe=79.5775 A/m) and the in-plane rotatable magnetic anisotropy field increases from 72.5 to 141.9 Oe.Meanwhile,the stripe magnetic domain structure changes from well aligned to disordered state,and the corresponding wavelength of stripe domain is reduced from 0.53 to 0.24 μm.The cross-sectional characterizations of NiFe film indicate that the formation of columnar structure produces an out-of-plane magnetic anisotropy,giving rise to the appearance of stripe magnetic domain structures.The low Ar pressure is in favor of the formation of columnar structure in magnetic film under the high sputtering power density,which gives rise to the appearance of well aligned stripe magnetic domains.However,the high Ar pressure leads to a fibrous columnar structure,which enhances the out-of-plane magnetic anisotropy and reduces the critical thickness for the occurrence of stripe domains.Our investigation provides an important reference to fabricating magnetic films and controlling their static and rotatable magnetic anisotropies for the application in high-frequency devices.