退火温度对NZFO铁氧体薄膜磁性能的影响

訾振发, 朱剑博

(合肥师范学院物理与电子工程系,安徽合肥 230061)

退火温度对NZFO铁氧体薄膜磁性能的影响

訾振发, 朱剑博

(合肥师范学院物理与电子工程系,安徽合肥 230061)

采用化学溶液沉积法在Si(001)衬底上制备Ni0.7Zn0.3Fe2O4铁氧体薄膜,XRD谱表明样品具有单相的尖晶石结构;扫描电子显微镜结果表明样品平均颗粒尺寸随着退火温度的上升从10 nm增加到32 nm。NZFO铁氧体薄膜磁性能与退火温度有强烈的依赖关系,薄膜的矫顽力从退火温度为500℃时的25 Oe增加到900℃时的80 Oe,饱和磁化强度也由146 emu/cm3增加到283 emu/cm3,这对于现代电子器件微型化有着非常重要的意义。

NZFO铁氧体薄膜;化学溶液沉积法;矫顽力

1 Introduction

In the past decades,Nickel-zinc ferrites have been widely used as high-performance microw ave devices due to their high resistivity,high Curie temperature,strong mechanical hardness,excellent soft magnetic properties at high frequency, and chemical stability[1-4].Ni-Zn ferrite is a type of mixed spinel in which tetrahedral(A)sites are occupied by Zn2+and Fe3+ions,w hereas the octahedral(B)sites are occupied by Ni2+and Fe3+in the cubic spinel lattice.These two antiparallel sublattices fo rm a ferrimagnetic structure,in w hich Ni2+and Fe3+are coup led by superexchange interactions through the O2-ions.

A t p resent,bulk Ni-Zn ferrite components employed in discrete devices at micro wave frequencies are not compatible with the rapid developments of electronic applications towardsm iniaturation,high density,integration,and multifunction. Thus,mo re attention has been attracted to solve these difficulties in performing the required miniaturization fo r comp lex devices[5]. Ni-Zn ferrite filmsmay play an important role in facilitating the design and fabrication of devices such as micro-inductors,micro-transformers,and microwave nonreciprocal devices[6].The ferrite thin film s incorpo rated into magnetic integrated circuits are expected to rep lace the current surface mounting modules in the near future.

The successful grow th of magnetic Ni-Zn ferrite film s is an important step towards their future inco rpo ration as inducto rs and transfo rmers into integrated circuits operating at high frequency. Several A ttemp ts have been made by researchers to deposit Ni-Zn ferrite film s by a variety of techniques including alternative sputtering technology[7],pulsed-laser deposition[8],and spin-sp ray p lating[9].However,most of them cannot be eco-nomically applied on a large scale because they require high vacuum system,complicated experimental steps,and high reaction temperatures.Additionally,the higher coercivity in Ni-Zn ferrite film s compared with bulk materials,which leads mainly to eddy current loss,has become a barrier in the way of applications.

In this study,the chemical solution deposition (CSD)was app lied to synthesize NZFO ferrite thin film s on Si(100)substrates.A s a result,the annealing temperature for the crystallization is comparatively low(500C)as to obtain homogeneous film s with small grain size,and the coercivity of Ni-Zn ferrite film s is also low,which are required fo r fabrication of devices.U sing CSD method is useful to achieve the low-temperature fabrication of magnetic ferrite film s,which is an efficient w ay to fabricate integrated thin film devices.The composition Ni0.7Zn0.3Fe2O4(NZFO)was selected because itsmagnetic propertiesare superior to those of other composition in NixZn1-xFe2O4(0≤x≤1)system in p revious report[10].

2 Experimen tal procedure

Ni-Zn ferrite NZFO film s were prepared by the CSD method.The starting materials were high-purity nickel nitrate hexahydrate Ni(NO3)2· 6H2O,zinc nitrate hexahydrate Zn(NO3)2· 6H2O,and ferric citrate pentahydrate FeC6H5O7· 5H2O.The starting materials w ere dissolved in a mixture of ethylene glycol and 2-methoxyethanol at 70 C and stirred at this temperature fo r 30 min. The transparent solution with brow n color was shaken in ultrasonic cleaner 20 m in and stirred at room temperature for 20 h in order to get a well mixed solution. The total concentration of the metal ions was 0.2M.The film s were prepared by spin-coating method on Si(100)substrates using rotation speed of 4000 rpm and time of 50 s,and then the as-deposited filmsw ere baked at 300℃fo r 30 min in order to expel out the organics.The spinning-coating and dry procedures were repeated for four times in order to obtain film s with desired thickness.The dried filmswere annealed at selected at varying from 500 to 900℃in the air for 20 min in order to get crystallized film s.

The crystal structure was examined by Philips X’pert PRO x-ray diffractometer(XRD)with Curadiation at room temperature.Field emission scanning electronic microscopy (FE-SEM,FEI designed Sirion 200 type)was carried out to check the morphology and thickness.Surface root-mean roughness was estimated by atomic force microscope(AFM Park Scientific Instruments designed, Autop robe CP type). Magnetic measurements were conducted w ith a SQU ID magnetometer (M PM S,Quantum design).

3 Resultsand discussion

The room temperature XRD patterns of NZFO ferrite thin film s annealed at different temperatures are show n in figure 1.A ll of peaks can be indexed to a single-phase spinel structure with the space group ofFd3-m.With increasing the annealing temperature,the relative intensity of the peak (113)enhances gradually while the full width at half-maximum(FWHM)of the peak(113)reduces.This indicates NZFO ferrite grains grow larger and larger with increasing the annealing temperature.According to the low angle(113) peak central positions,the ferrite lattice parameter was estimated to be 8.362±0.002Å.

Figure 2(A)to(E)show the FE-SEM results for different NZFO film s annealed at different temperatures.It is clearly seen that the grain size increases with increasing the annealing temperature. The mean grain size is 10,15,18,23,and 32 nm fo r the film s annealed at 500,600,700,800,and 900℃,respectively.Moreover,it is seen that the grain size are rather homogenous.Figure 2(F) show s the cross-sectional SEM result of the film annealed at 500℃.It can be seen that the thicknesses of the film is about 200 nm.Additionally,it is observed that the film thickness is independent of the annealing temperature.

The surface mo rphologies of the NZFO film s have been characterized by AFM.The typical image is show n in figure 3(A)for the sample annealed at 800℃,which reveals a rather smooth surface.The surface roughness is in the rootmean square(RM S)range from 1.5 to 4.5 nm.The result of the surface roughness is show n in figure 3 (B)as a function of annealing temperature.The surface roughness is strongly dependent on the annealing temperature.That is,the surface roughness linearly increases with the annealing temperature.W hen the annealing temperature increases, NZFO ferrite grains w ith higher energy have enough mobility on the thermal substrate.Larger NZFO grains can enhance the surface roughness. The mean grain size estimated through Image Processing in PSI ProScan software from AFM image of the NZFO film annealed at 800℃is about 28 nm,which is slightly larger than that estimated from FE-SEM micrographs.

TheM(H)loop s at 300 K under external fields from-20 to 20 kOe,which are parallel to the film p lane,are show n in figure 4.It is seen that all NZFO ferrite film s exhibit hysteresis behavior in theM(H)curves,indicating the characteristic of ferrimagnetism,which confirm s the desired Ni-Zn ferrite film s were successfully obtained.

Figure 5(A)p resents the annealing temperature dependence of the coercivity(Hc).The value ofHcrefers to the strength of the magnetic field required to reduce the magnetization of the magnetic sample to zero,after the saturation magnetization of the sample has been reached.It is found that theHcis enhanced with the increasing annealing temperature.To understand Hc mechanism clearly,the critical size of a monodomain particle can be estimated by the following formula[11]

w hereσ,w=(2kB TC|K1||K1|,TC,M s,KB,aare thewall density energy,magnetocrystalline aniso tropy constant,Curie temperature,Boltzmann constant and,lattice constant,respectively.Fo r NZFO ferrite,M s=270 Gs,TC=860 K,A=8.345 ×10-8cm,and|K1|=6.5×104erg/cm3[12], which give the value ofDm～72.9 nm.The calculated value is larger than that of our sample. Therefore, our samples should exhibit the monodomain behavior.According to Herzer’s random anisotropy model[13],the grain size has a large influence onHc,that is,Hc decreases dramatically with a decrease in grain size.This can be understood by the fact that as the domain contains many grains,the magnetocrystalline aniso tropy is averaged over many grains and various orientations.The reduced anisotropy leads to a decrease incoercivity with decreasing grain sizes.

Figure 5(B)exhibits the dependence of the saturation magnetization on the annealing temperature(M s)w ith the app lied fields paralleling to the film surfaces.It is seen thatM sexhibits an increasing tendency with increasing the annealing temperature.The enhancement in magnetization can be attributed to the improvement of the crystallinity during annealing processing,leading to the change in degree of inversion parameter due to the increase in grain size[14,15],and the decreased spin disorder in the shell[16].

4 Conclusions

In conclusion,the CSD method was used to p repare NZFO ferrite thin film s.The effects of annealing temperature on the microstructure as w ell as the magnetic properties were investigated.The FE-SEM results showed that uniform NZFO film s with thicknesses of 200 nm can be obtained,while the grain size increases with increasing the annealing temperature.The excellent magnetic properties including room temperatureM sandHcindicate the Ni-Zn ferrite film s can be considered as a potential candidate for modern miniaturization of electronic devices.

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Effect of Annealing Temperature on Magnetic Properties of NZFO Film s

ZIZhen-fa, ZHU Jian-Bo
(Department of Physics and Electronic Engineering,Hefei Normal University,Hefei 230061,People’s Republic of China)

Nickel-zinc ferrite Ni0.7Zn0.3Fe2O4(NZFO)film s were fabricated on Si(001)substrate by a simple chemicalmethod.The microstructure and magnetic properties were systematically investigated.X-ray diffraction results show that all samples have a single-phase spinel structure.The results of field-emission scanning electronic microscopy show that the mean grain size increases from 10 to 32 nm with increasing the annealing temperature from 500 to 900℃.The magnetic properties of NZFO ferrite thin film s exhibit a strong dependence on the annealing temperature.The coercivity increases from 25 to 80 Oe and the saturation magnetization increases from 146 to 283 emu/cm3with increasing the annealing temperature, which is in favor of modern electronic device miniaturization.

NZFO ferrite film s;Chemical solution deposition;Coercivity

TM 277

A

1674-2273(2010)06-0020-04

2010-04-20

安徽省2009年度重点科研计划项目(09020204031);2010年安徽高校省级自然科学研究项目(KJ2010B435);合肥师范学院2010年度院级自然科学重点科研项目(2010kj03zd);国家自然科学基金项目(51002156)

訾振发(1980-),男,合肥师范学院物理与电子工程系教师,博士。